Development of welding of parts from aluminium alloys

Contents

Contents

Introduction

1. Status of the question

1.1 Classification of aluminium-based parts

1.2 Weldability of aluminium alloys

1.3 Characteristics of alloy for manufacturing of gearbox cover

1.4 Characteristic defects for housing parts

1.5 Analysis of repair welding methods

2. Welding Process Development

2.1 Preparatory operation

2.2 Repair operation

2.3 Control Operation

3. Project safety and environmental friendliness

3.1 Workplace Description

3.2 Hazardous and harmful production factors

3.3 Measures to develop safe working conditions

3.4 Electrical Safety Assurance

3.5 Ensuring fire safety

3.6 Engineering grounding calculation

3.7 Environmental expertise of the developed facility

3.8 Safety of the facility in emergency situations

4. Project business case

4.1 Calculation of the part recovery time norm

4.2 Calculation of equipment operating time fund

4.3 Calculation of process cost of compared variants

4.4 Workshop Cost

4.5 Factory Cost

4.6 Full Cost

4.7 Calculation of Capital Costs for Process Execution

4.8 Calculation of economic efficiency

Conclusion

List of literature used

Summary

In this diploma project, the technology of restoring the cover of the AL9 alloy reduction gear with a three-phase arc was developed, the issues of safety and environmental friendliness of the project were worked out and economic efficiency was calculated.

Introduction

Aluminum is the most common non-ferrous metal in terms of content in the earth's crust, as well as in terms of production and scale of use. Aluminium alloys occupy the second place after iron by volume of application. This is due to the low weight and good mechanical properties of aluminum alloys.

Aluminium and its alloys are increasingly used in industry and construction. Aluminum alloys are the dominant structural material in the aircraft industry. They are widely used in the designs of rockets and artificial satellites of the Earth. They make skin for buses, trolleybuses, hulls of passenger and freight cars for bulk materials. The use of aluminum in high-speed trains gives great advantages. The use of aluminum alloys in the automotive industry is promising, as it saves fuel. It is also advantageous to use aluminum in units of freight, construction machines. Although aluminum alloys are 4-6 times more expensive than steel, and the manufacture of parts from them costs 20-30% more than steel, the use of 1 kg of aluminum allows you to reduce the weight of the machine by 2.25 kg.

Due to the higher cost of equipment and spare parts for it and a sharp decrease in the purchasing power of these goods, the restoration of worn-out parts is the most affordable way to keep the fleet of cars in working condition. In economically developed countries, refurbished parts predominate in the spare parts market, they are 1.5... 2.5 times cheaper than new ones, and in terms of resource, as a rule, they are not inferior to them.

The problem of restoring aluminum alloy products is becoming more and more urgent. Therefore, the current task is to develop resource-saving and relatively cheap technologies for manufacturing new and repairing failed equipment.

The purpose of the diploma project is to reduce the cost and increase the quality and productivity of the restoration process by developing a highly efficient repair welding of parts from aluminum alloys.

1. Status of the question.

Classification of aluminium-based parts

Currently, for mechanical engineering, a lot of parts are cast from aluminum alloys. Basically, these are the body parts of the car. The body parts include: covers of the gas distribution mechanism, piston; valve covers; engine cylinder block heads; engine units; distribution gear covers; oil and water pump housings; crankcase - clutches, gearbox, transfer box, steering gear; engine cooling radiators, fuel tanks, etc.

More details, in this diploma work we will try to restore the cover of the power take-off reduction gear, which is included in the hinged ground-cutting equipment of BGM 10.

Gearbox cover protects rotating parts of the unit from mechanical damages during operation. During operation, the part is exposed to dynamic and impact loads from the external environment (impacts of soil particles, stones, etc.). As a result, various chips, coughs and cracks occur on the working surface of the cover. In this regard, it is necessary to repair the covers using highly efficient and resource-saving methods of repair welding.

1.2. Weldability of aluminium alloys and their peculiarities

Aluminum and its alloys play an important role in modern industry. This is due to the fact that most industrial aluminum alloys have a number of unique properties: a combination of high mechanical properties (high specific strength) and physical properties (low density, high electrical conductivity, high thermal conductivity, which is 3-3.5 times higher than steel ).

The main areas of application of aluminum alloys are transport (aviation, shipbuilding, car building), construction (general-purpose metal structures) and the packaging industry. They make containers for the chemical and food industries, rocket and ship hulls, pistons, gearboxes and covers, airplanes, cars, dishes and much more. [12] All types of melting welding are used for aluminium and its alloys.

Aluminum alloys are approximately divided into several groups: 1) aluminum; 2) double alloys of Al - Mn, Al - Mg and Al - Cu with an admixture of magnesium and manganese; 3) triple alloys Al - Mg - Si; 4) triple alloys Al - Zn - Mg with Cu impurity; 5) complex alloying and 6) casting alloys. Properties and process weldability of these alloys are different. [6]

By weldability is meant a set of properties that determine the possibility of obtaining welded joints of a certain quality in a given welding method. The easier the quality joints are, the higher the weldability of the alloy. The multifaceted concept of "weldability" includes the tendency of alloys to form cracks, porosity, mechanical properties of welded joints, corrosion resistance, etc. During melting welding, weldability depends on the chemical composition of the alloy and its structure, which is created as a result of metallurgical conversion of the ingot. Among the physicochemical characteristics of the metal, the presence of an oxide film, chemical composition, thermal conductivity, melting point, density have the greatest effect on weldability. [11]

Impurities have a great influence on the weldability of the metal. [4] Magnesium, manganese, copper, silicon, zinc, less commonly lithium, nickel, titanium, zirconium, beryllium are used as main alloying elements for aluminium. Most alloying elements form with aluminum solid solutions of limited solubility, as well as intermediate phases with aluminum and among themselves (for example,

Mg2Si, CuAl2). [13]

The main impurities of technical aluminum are iron, silicon and copper. Iron dissolves slightly in solid aluminum. It can be present in aluminum as an independent phase FeAl3. Iron is a harmful impurity in aluminum and in most of its alloys. It reduces the corrosion resistance of aluminum, dramatically reduces ductility and electrical conductivity, but slightly increases strength. In some aluminum alloys, iron in combination with nickel is a useful impurity. Silicon is a common impurity in aluminum. In a number of aluminum alloys, silicon is specially added along with copper, magnesium, zinc, manganese and other elements. The resulting compounds (CuA12, Mg2Si, CuMgAl2, etc.) effectively strengthen aluminum alloys. Silicon has the same effect on aluminum properties as iron. Similarly, copper acts on aluminum, which also dissolves limited in it. With sulfur, phosphorus and carbon, aluminum reacts only at high temperature. [13]

Manganese and chromium increase corrosion resistance and enhance the aging effect of the alloy, in addition, manganese eliminates the harmful effect of iron. [13] The addition to aluminum of alloying elements such as silicon, copper, magnesium, manganese, vanadium, titanium improves the strength properties and somewhat reduces the plastic properties of the alloy. Alloying aluminum with small additions of elements, the specific gravity of which is significantly higher than the specific gravity of aluminum, several times increases the strength properties of the alloy, almost not increasing its specific gravity, which is very important when creating strong and light ship structures. [4]

The less impurities in the aluminum alloy, the generally higher its ductility. [11]

Among the most important factors determining the weldability of aluminum alloys and the choice of optimal welding conditions are their activity in interaction with atmospheric gases. When welding by melting, it is necessary to take into account the ability of the metal to absorb gases both in the solid state at high temperatures and in the liquid state at significant overheats. [13]

The reaction of aluminum with oxygen at elevated temperature is characterized by the formation of a refractory oxide film A12O3, whose melting point is 2054 ° C, i.e., significantly higher than the melting point of aluminum. The existence of a liquid oxide-liquid aluminum system under welding conditions is unlikely. Aluminum is easily oxidized in air and at normal temperature. [13] This film has a strength of up to 200 MPa and a density of 3.6 g/cm3, it is heavier than aluminum. When heated, the metal under the film melts before it. Breaking into unalloyed pieces during welding, the film sinks in the welding bath, forming inclusions in the weld metal. [12] Aluminium oxide film prevents arc excitation and proper formation of welding seam, reduces mechanical properties of metal. [4] Therefore, during welding it is necessary to crush and crush the oxide film.

Nitrogen does not dissolve in solid aluminum, but at a temperature of about 900 ° C, i.e., when liquid aluminum is overheated by 200-250 ° C, forms a stable refractory chemical compound AlN with it. Aluminum nitride may be present on the surface of the metal as a thin film or present in the melt as solid inclusions since its melting point is 2427 ° C. [13] Nitrides degrade metal properties; for this reason, nitrogen cannot be used as a protective gas in the arc welding of aluminum and its alloys. [12]

The main reason for the appearance of pores in aluminum and its alloys is considered to be the limited solubility of hydrogen in solid metal and its greater solubility in a liquid state (Figure 1.1). The source of hydrogen in arc welding is moisture, the vapors of which are dissociated under the action of high arc temperature, forming atomic (H) hydrogen, which is absorbed by metal by adsorption, diffusion and dissolution. It is now generally accepted by K. Fuss that if the gases do not form a solid solution with the metal, then with slow cooling they are released, and with fast gases remain in the alloy and solidify with it. Pores remaining during the crystallization of the seam have a significant effect on the subsequent operability of the weld joint. According to many researchers, pores, regardless of their size, are harmful. Pores and shells have a significant effect on the corrosion resistance of the welded joint. Each pore and zone around it are corrosive centers from which the destruction of metal spreads. It is necessary (in various ways) to ensure that the quantity and quality of pores in the weld metal approximates the quantity and quality of pores in the base metal. [6] Therefore, before welding, it is necessary to carefully prepare all welding materials and the surface of the welded parts, preventing moisture, the main supplier of hydrogen, from entering the welding zone. Moisture decomposing can also increase metal oxidation in the welding bath. During welding, it is desirable to reduce the cooling rate of the liquid metal so that more hydrogen emitted from the metal can reach the surface of the welding bath. To do this, the metal can be heated to a temperature of 150... 300 ° C before welding. However, heating can reduce the mechanical properties of the weld joint. To control porosity in the welding zone, an oxidizing atmosphere can be created by adding, for example, up to 1.5% oxygen to argon. [12]

Thermal conductivity characterizes the rate of heat removal from the heating source. The thermal conductivity of aluminum and its alloys is 3-3.5 times higher than steel. Alloying and accompanying impurities reduce the thermal conductivity of the metal. When the base metal is melted, this is often more important than the melting point. The melting point of pure aluminium is 660 ° C, i.e. 2 times lower than the melting point of steel. The melting point of the alloys is even lower. However, despite the lower melting point, the addition of the same amount of heat as for steel (or more) is required to produce an aluminum weld due to the high heat loss in the thickness of the metal to be welded. This is especially evident at low process speeds. Therefore, for welding aluminum, powerful heat sources are required, allowing large amounts of energy (concentrated heating sources) to be released in a small area. [11]

Corrosion resistance of aluminium and its alloys is determined by the presence of a dense oxide film on the surface of the articles. Aluminum is completely non-toxic, which determines its wide use in the food industry. It is very resistant in oxidative environments. In this regard, it is used in vessels for transporting and producing nitric acid and the like. As a rule, the less impurities in a technical metal, the higher its corrosion resistance. Aluminum and its alloys are completely unsuitable for work in an alkaline medium. [11]

The coefficient of linear expansion of aluminum is twice as high as that of iron. So, when welding aluminum alloys, the deformation and warping of parts will be more than on steels. Molten aluminum has a high fluid flow, which makes it difficult to form a seam during welding with through penetration of edges: burns are easily formed, a float is not uniformly formed. [12]

The most important indicator of the weldability of aluminum alloys is the ability not to form "hot cracks" during welding. Alloys that are extremely sensitive to hot crack formation are considered non-welded. Use in welded structures is not recommended.

Alloys not hardened by heat treatment have low level of alloying. Their mechanical strength is relatively low, but they are well welded and corrosion resistant. These are alloys of the aluminum-manganese system (Amz, AmcS), aluminum-magnesium (from Amg1 to Amg6, D12, etc.); these include technical aluminum (A99, A5). Blanks from these alloys are discharged in annealed and cold-deformed state.

Welding of aluminum as well as Amtz alloy containing less than 0.2% silicon and 0.2% iron presents great difficulties due to the tendency of the weld metal to form cracks. Prevention of crack formation is achieved by selection of chemical composition of welding wire. [4]

Aluminium-magnesium Amg alloys subjected to welding contain magnesium in an amount of up to 7%. With an increase in magnesium content, weldability worsens (with the exception of Amg6 alloy), while in alloys with a magnesium content of up to 3%, the risk of cracking increases, but the weld metal is more dense. [6]

Heat-hardened alloys (quenching followed by aging) typically have a higher degree of doping. Their strength is higher, but they are worse welded (some do not weld at all) and often have low corrosion resistance. These are aluminum-magnesium-silicon alloys (aircraft - AD31, AD33, AD35, AVh and AV), aluminum-copper (most belong to duralumins - D1, D16, AK41, 1201), aluminum-zinc (with additions of other elements - 1911, 1915, 1925, 1950).

Avials are well welded, however, using an additive material; welding them by fusing edges is not recommended.

Duralumins belong to non-welded alloys. The only welded aluminum-copper alloy (alloy 1201) and its foreign analogues. The alloy of aluminum with copper - duralumin - due to low corrosion properties and poor weldability did not find wide use in shipbuilding. [4]

Triple alloys of aluminum with zinc and magnesium are welded well only if the content of these alloying elements in total does not exceed 7-7.5%. Weldable include the domestic alloy 1915 and its foreign analogues.

For casting alloys, welding is used only for repair purposes and also for correction of casting defects. The complexity of welding is different and depends on the alloy grade. The alloys of the Al-Si system (AL2, AL4, AL9, AK7 (AL98), etc.) are well welded, some grades of the Al-Si-Cu system (AL5, AK5M7 (AL10V), AK7M2 (AL14V), etc.) and the rest grades are poorly welded. In all these alloys, iron admixture averages about 1%. [6] The casting alloys contain significant amounts of silicon, copper, zinc. The total content of alloying elements is 10-15% or more. [11]

Of all the casting alloys, aluminum and silicon alloys (silumins) became the most widespread. Due to the small crystallization interval and insignificant shrinkage, the silumin does not crack during welding. The risk of cracking in silicon alloys increases as volume shrinkage increases as the liquid bath solidifies. [6]

Among the main features of welding aluminum and its alloys by any method are: the need to remove the oxide film from the surface of welded products, thorough preparation for welding, preheating, etc. The main difficulties of welding aluminium and its alloys are given in the work [15].

The difficulty of welding aluminum and its alloys is as follows:

1. High heat capacity, thermal conductivity, latent heat of melting require higher and more concentrated heat deposition than when welding steel. So, when arc welding aluminum requires a current of 1.2... 1.5 times more than when welding steel, despite the lower melting point of aluminum.

2. Aluminum is readily oxidized in the solid and molten state. The dense refractory oxide film Al2O3 (Tpl = 2050 ° C) prevents fusion of the welding bath with the base metal and forms non-metallic inclusions in the weld metal. Before welding, remove the oxide film from the surface of the base and additive metal mechanically or by etching. In the welding process, the oxide film is removed by cathode sputtering, by using fluxes and coatings of electrodes that dissolve or destroy it by transferring it to a volatile compound.

3. Low aluminum strength at high temperatures (Fig.1.2, a) and high fluid flow contribute to the "failure" of the welding bath. To prevent dips and burns, liners made of graphite or steel are usually used.

4. The high solubility of the gases in the molten state contributes to the formation of pores during crystallization. The main reason for the appearance of pores in aluminum welds is the presence of hydrogen (Fig.1.2, b). Hydrogen dissolved in liquid metal should be released from it in the amount of 90... 95% of its volume, but this is hindered by the oxide film and the low diffusion coefficient of hydrogen in aluminum. The pores are preferably located within the seam near the fusion boundary and at the surface of the seam. The fight against gas porosity is the primary task of technologists. To prevent porosity, the oxide film, moisture and fat contaminants are removed from the surface of the welded materials, inert gases are dried, heating and mechanical action on the liquid metal of the welding bath (ultrasonic oscillations, magnetic mixing) are used during welding.

5. Aluminum alloys with a large effective crystallization interval tend to form hot cracks. The weld metal is prone to crack formation due to the coarse columnar structure, the release of low-melting eutectic grains along the boundaries, the development of significant internal deformations and stresses due to high casting shrinkage. On alloys of increased strength (for example, doped with zinc and magnesium), the appearance of cold cracks caused by residual stresses and intermetallic precipitation is possible.

6. Significant shrinkage of the weld metal and high coefficient of linear expansion lead to large residual deformations and warping of the structure. To reduce warpage, it is recommended to use rigid devices made of materials with low thermal conductivity.

7. When arc welding with a non-consumable tungsten electrode, it is possible to contaminate the weld with tungsten inclusions. During welding, electrodes made of pure tungsten EVH, made of tungsten with lanthanum oxide (EVL) or yttrium oxide (EVI1) according to GOST 2394980 are used.

8. Welding of naked or thermally hardened aluminium alloys reduces the strength of the weld joint compared to the strength of the base metal.

The welding technology (welding type, welding materials, welding technique) is selected depending on the main weldability indicator (or combinations of several indicators) for each particular material. [9]

1.3. Characteristics of alloy for manufacturing of gearbox cover.

AL9 is an aluminum casting alloy. It relates to alloys of the AlSi system, or silumines, which are the most common casting aluminum alloys. AL9 is a special silumin, since in addition to silicon it contains in a small amount other alloying components (Mg, Cu, Mn, etc.).

The chemical composition of the alloy in accordance with GOST 268575 is presented in Table 1.1. [3]

To increase mechanical properties by 15-20%, yttrium must be added to alloy AL9 in the range of 0.08-0.20%, while the iron impurity content in the alloy should not exceed 0.30%.

The AK7h alloy (AL9), which is neither modified before casting nor artificially aged (the castings are only hardened), is quite widely used due to the combination of satisfactory strength, high ductility and good casting properties. [8]

1.4 Characteristic defects for housing parts.

During operation, the housing parts are subjected to chemical, thermal and corrosive effects of gases and coolant, mechanical loads from alternating pressure of gases, dynamic loads, vibration, contact loads, influence of abrasive medium, etc. For this class of parts, the main types of wear are corrosion-mechanical and molecular-mechanical, which are characterized by the following phenomena: molecular setting, transfer of material, destruction of emerging bonds, tearing out particles and the formation of products of chemical interaction of metal with aggressive elements of the medium.

During operation of the machines in the housing parts, the following characteristic defects may appear:

- mechanical damages - base damages; cracks in walls and planes of connectors, surfaces for bearings and on support surfaces; nicks of mounting, welding or butt surfaces; fragments and holes of crankcase parts; hairpin debris; clogging or tearing of thread; blankings falling out;

- violation of geometric dimensions, shape and mutual arrangement of surfaces - wear of mounting and working surfaces, threads; cavitation wear of the holes through which the cooling liquid passes; misalignment, non-perpendicular, non-cylindrical and non-circular holes; warping or deformation of machined mounting, welding or butt surfaces.

1.5. Analysis of repair welding methods.

The choice of one or another method of repair welding is determined by the specific purpose - restoring the original dimensions of the worn-out gearbox cover .

When choosing the optimal welding method, a number of requirements should be taken into account: the quality of the weld, its appearance, productivity and other economic aspects of the process, its versatility, deformation of the product, local conditions. [11]

For aluminum and its alloys, all types of melting welding are used. Automatic and semi-automatic arc welding with a non-melting and melting electrode in the medium of inert protective gases, automatic arc welding using a flux (open or closed arc), electroslag welding, manual arc welding with a melting electrode, electron beam welding have found the most application. [15]

Based on the analysis of literary data, it can be concluded that it is better to use arc welding for repair work.

Manual arc welding with coated electrodes

Manual arc welding with coated electrodes (Fig. 1.3) is carried out for technical aluminum, aluminum-manganese and aluminum-magnesium (with a magnesium content of up to 5%) alloys, silumines with a metal thickness of more than 4 mm. It is possible to weld metal up to 20 mm thick without edge preparation, but it is recommended to perform preparation with a thickness of 10 mm. [15] Welding is performed by direct current of reverse polarity. For manual welding of aluminum, heating is necessary (for metal of medium thickness - up to 250300 ° C, for large thickness - up to 400 ° C), which allows you to obtain the required penetration at moderate welding currents. When welding large-sized structures, preheating of only individual sections is often used. The composition of the bar and coating is selected depending on the composition of the metal to be welded, the requirements for seams, the conditions for stable arc burning, etc. During welding, the molten slag coating floats over the liquid metal of the welding bath, protecting it from air. When the weld metal interacts with the slag, exchange reactions occur, as a result of which the coating elements can pass into the seam, and harmful impurities are removed from the seam. Refining, alloying and modification of the weld metal is based on these metallurgical processes. During manual welding with coated electrodes, protection of the welding bath from interaction with atmospheric gases is carried out due to slag and gases formed during melting of the coating. [13]

Electrode rod is made of wires of composition close to that of base metal. Electrode coatings have significant electrical resistance. In case of arc breaks, crater and electrode end are covered with slag film preventing its re-ignition. Therefore, welding is recommended to be performed at high speeds, without oscillating the end of the electrode, continuously within one electrode. When making multi-layer seams, careful cleaning of slag and oxides is required before applying each layer. The resulting welds have satisfactory mechanical properties. A significant disadvantage of manual arc welding with a coated electrode is the low productivity of the process and the dependence of the quality of the weld on the practical skills of the welder. [15]

The method of manual arc welding with a coated electrode is used less and less every year, since welded joints have low corrosion properties, especially in contact with aggressive media, and less stable mechanical properties than in welding using inert gases. [11]

Welding in inert gases is the most common welding method used for the manufacture of welded structures from aluminum alloys of responsible purpose. Welding is performed by a non-consumable tungsten electrode (mechanized and manual) and a melting electrode (semi-automatic and automatic). [13] Argon and helium are used as protective gas for welding aluminium and its alloys. Nitrogen is not suitable for this purpose, since it forms chemical compounds with aluminum. Helium is rarely used in welding due to its scarcity and high cost, therefore the most common use of argon is. [4]

Welding in protective gases is distinguished by the following advantages:

• High performance (2-3 times higher than conventional arc welding)

• possibility of welding in any spatial positions, good protection of the welding zone from oxygen and nitrogen of the atmosphere, absence of the need to clean the seam from slags and grind the seam during multilayer welding;

• small zone of thermal influence;

• relatively small product deformations;

• possibility to observe the process of seam formation;

• Availability of mechanization and automation.

Disadvantages of this welding method are the need to take measures to prevent blowing of the protective gas jet during welding, the use of gas equipment, and in some cases the use of relatively expensive protective gases. [9]

Welding with melting electrode

Melting electrode welding is an economical method of welding aluminum and its alloys with a thickness of more than 4 mm. Reliable destruction of the oxide film in this method of automatic and semi-automatic welding in argon is achieved only by feeding the arc with a direct current of reverse polarity. The mechanism for removing the oxide film in this case consists in breaking and spraying it with heavy positive ions bombarding the cathode (the so-called cathode spraying effect is used). [13] When direct polarity DC is used, the amount of molten electrode metal increases by 25... 30%, but arc stability is dramatically reduced and spattering metal losses are increased. AC cannot be used due to unstable arc burning. When welding with a melting electrode, a weld is formed due to the penetration of the main metal and the melting of additional metal - electrode wire (Figure 1.4). Therefore, the shape and dimensions of the seam, among other things (welding speed, spatial position of the electrode and article, etc.), also depend on the nature of the melting and transfer of the electrode metal to the welding bath. [15]

The disadvantage of the method of welding aluminum with a melting electrode is a slight decrease in mechanical properties compared to welding with a non-melting electrode. The decrease in weld strength is due to the fact that the electrode metal, passing through the arc gap, overheats to a greater extent than the additive wire when welded with the non-consumable electrode. [13] When welded with a melting electrode together with an electrode wire, hydrogen and an oxide film are inserted into the seam, so the quality of the seam is worse than when welded with a non-melting electrode, where the surface of the additive wire can be less. [12]

The advantages of this welding method include good mixing of the welding bath and therefore better cleaning of the seam from oxide inclusions, as well as high productivity. In one pass, you can weld metal 16 mm thick, and in two passes - up to 30 mm thick. [13] The economic feasibility of melting electrode welding increases as the thickness of the metal increases, the deep penetration of which provides the process with high productivity. [11]

Non-consumable electrode welding

Non-consumable electrode welding is suitable for aluminium and its alloys up to 12 mm thick. Manual welding with a tungsten metal electrode with a thickness of up to 10 mm is carried out without preliminary heating; if the thickness is more than 10 mm, pre-heating of the edges with a gas flame to a temperature of 200-250 ° С is required. [4]

The electric arc burns between the product and the noncompatible tungsten electrode. The additive metal is introduced into the welding bath as required regardless of the welding current (Figure 1.5). [11]

Welding of aluminium and its alloys is performed at alternating current and direct current (only reverse polarity). In the case of direct polarity, the arc burns relatively quietly, but the bath is covered with an oxide film that prevents the metal from melting. When the arc is burned at the reverse polarity, the oxide film is destroyed due to the so-called cathode sputtering, so that the molten metal bath has a clean mirror surface. It should be borne in mind that when welding with direct current at the reverse polarity, the tungsten electrode is intensively burned. Due to the limited permissible current strength, welding with a tungsten electrode of aluminum and its alloys on direct current is almost not carried out. Alternating current is generally used for this purpose. [4]

During argon arc welding at alternating current, one difficulty has to be encountered - the occurrence of a constant component in the circuit. This phenomenon in the welding of aluminum alloys leads to contamination of the welding bath, poor fusion of the welded edges and deterioration of the formation of the seam. Therefore, during argon arc welding at alternating current, the DC component is reduced or completely eliminated by sequentially including the active resistance capacitor battery (sometimes shunted by a semiconductor valve) in the welding circuit. [13]

When manually performing welds on aluminum with a non-consumable electrode, special requirements are placed on the welding technique. The angle between the filler wire and the electrode shall be about 90 ° (Figure 1.6). The additive is fed by short reciprocating movements. Transverse oscillations of the tungsten electrode are unacceptable. The arc length usually does not exceed 1.5-2.5 mm, and the distance from the protruding end of the tungsten electrode to the lower section of the torch tip at butt joints is 1-1.5 mm, at taurus (angular) - 4-8 mm. To reduce the risk of oxidation, the dimensions of the welding bath shall be minimal. Welding of metal up to 10 mm thick is usually carried out by the so-called "left" method, which allows to reduce overheating of the welded metal. [13]

Argon arc welding with a non-consumable electrode is most versatile and operational, i.e. welding is possible in various spatial positions and constrained conditions. [11]

However, in argon arc welding with a non-consumable electrode, already with an edge thickness of more than 2...... 3 mm, several passes are required, and with a thickness of more than 10 mm, edge preparation is required .

Welding with three-phase arc noncompatible

electrode in argon medium

To improve process performance, a more concentrated heat source is needed. As such a source, a three-phase arc is used, which is a flare of alternately burning three arcs: an independent arc between two tungsten electrodes and two dependent arcs burning between each of the electrodes and the welded product (Fig. 1.7). [12]

A large penetration ability of a three-phase arc allows you to weld aluminum parts up to 30 mm thick in one pass without cutting edges on a corrosion-resistant steel lining. At the same time, the porosity of the weld metal is sharply reduced, since welding is carried out without an additive metal, due to the surface of which the amount of hydrogen entering the melting zone usually increases.

When welding with a three-phase arc of a metal of large thickness, when the melting ability should be maximum, it is necessary that the current in the product is greater than in the electrodes. Conversely, when a minimum melting capacity of the arc is required, for example in a surfacing, the current in the article can be set to be less than the current in the electrodes. In addition, the depth of penetration of the base metal can be controlled by arranging the electrodes relative to the seam axis. Their successive arrangement causes an increase in the depth of penetration and a decrease in the width of the seam, and the transverse - depth of penetration reduces, and the width of the seam increases. The power source of the three-phase arc can be two single-phase transformers connected by an open triangle, or a special three-phase welding transformer. [12]

Three-phase welding using

regulation of heat deposition

The most suitable and effective method for repairing the gearbox cover is three-phase argon arc welding using heat deposition control. This method increases the efficiency of the welding process compared to conventional argon arc welding. Unlike the three-phase welding method described above, an additive wire is not connected to the middle phase of the power supply (Figure 1.8). This connection scheme allows a wide range of control of the thermal mode of the welding process, since part of the welding current flowing through the article is redistributed to the welding wire, that is, the arc burns both on the article and on the additive. As a result, the additive material is heated and to a greater extent, it is cleaned from the oxide film due to the cathode sputtering mechanism.

Using a three-phase arc with controlled heat deposition, it is possible to weld products with a wall up to 1 mm thick, automatically adjusting the heat deposition in the product and preventing burning. The disclosed method can be implemented with both manual and automatic welding of any available surfaces. Automatic control of heat deposition during welding results in a reduction of the level of residual deformations by about half and increases the amount of built-up metal by 2 times compared to traditional surfacing methods.

The technology of repair welding with a three-phase arc has some difference from the usual argon-arc non-consumable electrode. If the single-phase arc is excited on the part above or near the defect, the three-phase arc is excited most often outside the part by touching the coal by the torch electrodes or by discharging the oscillator. Gradually bringing the three-phase flare to a defective place on the part allows you to choose the most rational brewing scheme and start warming up the intended area, without disturbing the shape of the casting surface near the defect. In the process of brewing, it is important to ensure the necessary thermal mode of the welding bath in order to avoid excessive floats and undercuts. Typically, the bath temperature is controlled by feeding the additive material and moving the arc to less heated areas. With a three-phase arc, it is possible to adjust the heat setting by increasing the length of the arc without the risk of its breaking. This allows the seams to be laid through various cavities, ribs, protrusions without interrupting the process. This is especially important when repairing casts of complex configuration, where different ribs and tides drastically change the thermal situation near the welding bath. [14]

The brewing of the defect with a three-phase arc can be carried out without any preparation, with the exception of drilling of the ends of the cracks. The three-phase arc seams have a smaller width and a smoother surface which generally does not require post-treatment compared to the other arc welding seams. [14]

The best seam formation is achieved by welding with a noncompatible electrode. Melting electrode welding approaches this process, especially automatic welding. A much worse weld is produced by arc welding with coated electrodes. In some cases, the surface of such seams has to be mechanically aligned. The deformation of articles, which is greatest during manual argon arc welding with a non-consumable electrode, is slightly less during manual welding with coated electrodes. Local preheating in most cases increases residual deformations. The least deformation is provided by single-pass welding with a melting electrode.

Porosity and mechanical properties of the weld metal depend on the method, mode and technique of welding, which determine the linear energy and the degree of protection of the liquid bath from air, moisture, and impurities. Gas saturation is practically the lowest in argon arc welding with a non-consumable electrode. This has a positive effect on mechanical properties. When welded with a melting electrode, there are more voids in the seam than when welded with non-melting, since the pores not only arise in the bath, but also develop from vapor-gas bubbles entering with droplets of electrode metal.

The strongest and most ductile joints are obtained by welding with a non-consumable electrode with an alternating current in argon. They are slightly inferior to the compounds made by the melting electrode in argon; the least strength and ductility have compounds obtained by coated electrodes.

Multi-pass (including sub-weld) seams usually have lower mechanical properties than single-pass, with the exception of seams of very large sections, the crystallization of which occurs slowly during single-pass welding.

The choice of welding method in many cases depends on the thickness of the metal and the type of connection, aluminum sheets with a thickness of less than 3 mm are difficult to butt weld with a melting electrode due to the need to supply a thin soft electrode wire to the arc.

Currently, single-pass double-sided welding with a melting electrode in argon is used for metal up to 16 mm thick. The maximum thickness of sheets welded in one pass by a single-phase arc with a non-consumable electrode is 8-15 mm, and three-phase - 20-30 mm. If it is necessary to weld the metal of greater thickness with a melting electrode, it is resorted to multilayer seams with edge preparation. The smallest thickness of the metal welded butt to the non-consumable electrode is 1 mm. For smaller thicknesses, edge flanging is used.

Manual welding justifies itself only in case of one-time or small-scale production of products, since for this it is not advisable to purchase special equipment. In mass production and especially mass production, the mechanization of welding fully justifies itself. For a metal with a thickness of 10-20 mm, the cost of automatic welding with a melting electrode in argon is lowest. This method is approached by an automatic non-consumable electrode and a semi-automatic melting electrode in argon. Manual welding methods are the least economical. The cost of manual argon arc welding is highest. As the metal thickness decreases, the advantages of welding with a non-consumable electrode increase. On the contrary, as the thickness increases, the economic advantages of using a melting electrode increase. [11]

Analysis of the welding methods used in the repair of aluminum casting showed that the three-phase arc most fully meets the requirements for the heat source when brewing casting defects. The thermal power of the three-phase arc can vary from 500 to 5500 W at a higher heat concentration than that of the single-phase arc. Cathode sputtering of alumina with a three-phase arc is more intense than with power equivalent single-phase alternating current arcs. In addition, the three-phase arc does not fade when the burner is removed from the surface of the article. At this moment, a low-power interelectrode arc burns, which illuminates the repair zone, facilitates excitation of dependent arcs and in some cases provides local heating. This gives it wide technological versatility. [14]

All the described advantages of the proposed method will make it possible to increase quality and efficiency of process of repair welding of worn-out cover of reduction gear box.

To use three-phase welding technology for our case, the following tasks were solved in the diploma project:

1. Development of technology of reduction gear box recovery from AL9 alloy with three-phase arc.

2. Selection of equipment and calculation of modes for implementation of the proposed technology.

3. Project safety and environmental friendliness.

4. Economic calculation of the proposed technology.

Welding Process Development

2.1. Preparatory operation

When welding aluminum and its alloys, the cleanliness of the surface of the welded edges and the additive metal is crucial, so that before welding, it is necessary to very carefully clean the metal from the preservation coating, fats, moisture, aluminum oxide film and other contaminants. [4]

The main obstacle to welding aluminum and its alloys is oxide films. They cover the surface of the joined edges and wires. The melting point of alumina is 2050 ° C. Aluminum oxide films in the metal reduce its strength and other performance. In the natural conditions of production and storage, aluminum is covered with a layer of oxide that protects it from corrosion. In air, the ground surface is immediately coated with a new layer of oxide, the thickness of which is reduced almost within a few days, reliably protecting the metal from further oxidation. The natural protective film has a considerable thickness and its removal during welding is very difficult. Therefore, the surface of the connected parts and wire is cleaned from the oxide layer immediately before welding and an artificial oxide layer is created on it, which remains sufficiently thin for 8-16 hours. The resulting thin alumina layer is relatively easily removed by an electric arc during welding. [11]

Welded parts are usually degreased in a 10% aqueous solution of alkali NaOH heated to a temperature of 60-70 ° C. As a result of interaction with alkali, the oxide film is released from the surface for 2-3 minutes. After that, the alkali residues and reaction products are washed off the surface of the parts with cold water. To remove moisture from the surface of the article, it is dried with compressed air.

Wire cleaning is especially careful, since the presence of oxides and contaminants on its surface has a greater effect on the quality of the seam than the presence of oxides on the base metal. Moreover, the smaller the diameter of the wire, the greater the effect of the oxide film and impurities. The surface of the wire is cleaned only chemically. It is desirable to immerse the welding wire in solutions not in bays, but to extend sequentially through all solutions, supplying directly to the cutting machine. [4]

For cleaning the surface of aluminum welding wire, the following treatment is recommended: etching in 10% alkali solution NaOH for 5-10 minutes at T = 60-70 ° C, washing in cold water, drying with compressed air .

Final cleaning of the welded edges on the width of 15-25 mm is carried out immediately before welding with a metal brush. It is desirable to use stainless steel brushes (wire diameter is not more than 0.15 mm). It is not recommended to use sandwiches or paper, since they are quickly clogged with chips that leave large scratches on the metal surface. Small particles of stone and paper are clogged into scratches, which during welding can lead to the formation of pores. The brushed steel edges are carefully wiped with dry, clean rags to remove powdery substances. The edges thus prepared are suitable for welding within 5- 6 hours. After the specified period, the edges are cleaned again. [4]

The brewing (repair) of cracks without appropriate preparation can cause their instantaneous spread even under minor loads and temperature reduction. Before cutting it is necessary to examine carefully a crack, to precisely define its ends (borders of a crack are well shown when heating by their gas burner up to the temperature of 100150 °C), then to zasverlit a drill with a diameter of 5 mm. Drill so that the center of the hole coincides with the end of the crack or is 3-5 mm further than the crack. Crack handling and metal extraction shall be performed by chopping with a cogwheel. Dead cracks shall be cut to a depth exceeding the depth of their occurrence by at least 3 mm.

2.2. Repair operation

Repair of the gearbox cover includes the following operations: installation of the part on the welding table; setting parameters of welding modes; degreasing of the welding zone (acetone) and directly brewing the defect.

Welding materials

Tungsten electrodes are used as noncompatible electrodes for argon arc welding. The best results in welding are the use of electrodes not made of pure tungsten, but electrodes with activating additives. The addition of tungsten in the manufacture of electrodes 1.5... 2% lanthanum oxide increases their resistance and allows the use of increased welding currents on the 15% (Fig. 2.1). Characteristics of EVL electrodes are given in Table 2.1.

The presence in casting alloys of increased amounts of silicon, copper, magnesium causes the need to weld them with highly alloyed wires. For the welding of the AL9 alloy, a SWAC5 wire is usually used. [11] The chemical composition of the wire is given in Table 2.2.

To protect the molten metal of the welding bath and wire during welding of aluminum and its alloys, high-grade argon is used according to GOST 10157-79 (Table 2.3). Argon is supplied in cylinders under a pressure of 15 MPa, in each cylinder there is 6.2 m3 of argon gas. The cylinder for storing argon is painted gray, the inscription is green.

Welding equipment

Arc power sources (UDAR300, TIRE-300, etc.) used for repair welding and surfacing of articles from light alloys differ in narrow technological capabilities and do not provide regulation of mode parameters during welding. In this regard, it was necessary to develop a set of universal equipment that is reliable and durable in design, with a sufficiently high thermal power and stability of the welding arc, providing the ability to smoothly and in a large range adjust the welding current, brew craters, operate in welding and heating mode and have remote control of a power source. In addition, when working on this equipment, it is necessary to ensure the convenience and simplicity of setting and controlling welding parameters, as well as compliance with ergonomic requirements and safety.

The institute has developed a series of power supplies that provide a stable three-phase process. One of them - the source of UDGT315 - was used as a base in the development of a set of universal equipment. This source has a steep external volt-ampere characteristic, which is set by a welding transformer with an increased scattering flow. Welding current is controlled in the range 135-550 A by changing the distance between primary and secondary windings. The DC component is suppressed by means of a capacitor battery connected to the product circuit.

To expand technological capabilities, the plant was additionally equipped with a thyristor current regulator VD1 - VD2 (Fig. 2.2), which is included in the welding circuit of the product. The control circuit provides smooth opening and closing of thyristors in an adjustable period of time and allows changing the welding current from zero to nominal. The control direction is set from the control panel and, if necessary, can be changed at any time in the process.

For the convenience of control and easy adjustment of the power supply of the ISP and brewing of a certain type of defects, two remote control panels - manual (ECR) and foot (NPU) are included in the set of the welding station.

The first allows you to adjust the power supply before welding: set the nominal value of the welding current, turn on and off the power supply, and control the value using a digital indication.

The second is a device with two pedals and serves to control the source during welding. It allows you to light and dampen the arc, adjust the welding current and switch the power supply to heating mode (i.e., turn off the middle phase). The need for a foot control panel is due to the fact that repair welding is usually carried out manually when the operator holds the burner and the additive rod, and visibility is limited by the protective mask.

The tool for brewing defects is the welding two-electrode burner RGT6 developed at the institute for welding (current up to 350 A) in any spatial position with illumination and local heating of the defective area. At the same time reliable protection of welding bath from air impact is provided.

The developed set of equipment for repair welding of products from light alloys, consisting of a power source, two control panels and a welding torch, has wide technological capabilities, is universal, easy to control. [5]

Characteristics of UDGT315 unit are given in Table 2.4.

The additive wire is supplied manually. An electrode holder is used to attach the wire, supply welding current to it and manipulate the wire. The electric holder must be light (not more than 0.5 kg) and convenient, have reliable insulation, do not heat during operation, provide fast and reliable wire fastening. We will use a passivation electric holder of type ED31 (Fig. 2.3). [16] The welding wire supplying the current to the electrode holder shall be flexible enough to facilitate handling of the wire. Characteristics of the electrode holder ED31 are given in Table 2.5.

Reduction gear cover is installed on welding table and fixed by means of pressing devices.

Welding Mode Parameters

Selection of parameters by repair three-phase welding mode with application of heat deposition control, given in Table 2.6, was selected according to literature [11].

Brewing defect

In order to brew a defect, you need to perform the following actions:

• close the filler wire on the part;

• spread the welding bath;

• move the filler wire to the welding bath;

• weld the roller.

In order to adjust the amount of weld metal, it is necessary to alternate the switching during welding of the filler wire between the welding torch and the article (Fig. 2.4).

2.3. Control operation

Welding defects that reduce the quality of structures must be detected and eliminated. To do this, it is necessary to carry out quality control of finished products. External inspection is the cheapest, fastest and rather informative method of control.

By external inspection, visible defects can be found - burning, non-burning, undercuts, cracks, surface pores, tungsten inclusions, craters, benevolent colors, reinforcement of seams and the like. [13]

Weld seam and area of adjacent base metal at the distance of not less than 20 mm from seam after cleaning from slag, splashes and contaminants are inspected on finished products. The quality of the seam is evidenced by the consistency of its geometric dimensions, appearance, uniformity of the flakiness of the seam, as well as the color of the surface of the product. [12] There shall be no reduction in the actual size of the seam compared to the specified (nominal) size. [16]

The prepared weld joints examine with the naked eye or with use of a magnifying glass 4...10 multiple increases. Dimensions of welds are controlled by measuring tool or special templates. [13] In addition to the universal measuring tool, templates are often used to control the dimensions of the seam (Fig. 2.5). Inspection of joints not available for direct observation is carried out using optical instruments - endoscopes. Medical bronchoscopes, periscopes or flexible bundles are usually used - fiber optics. [12]

The possibility of detecting defects is affected by illumination, individual visual properties, brightness, color, angular dimensions and sharpness of the defect outline, as well as the contrast between the defect and the surface of the product. [12]

To detect internal defects, we use an ultrasonic control method. To do this, lubricate the part surface with technical oil and install UD212 flaw detector on it. By moving the transducer from the end to the middle of the part, we receive signals on the screen. By them, we determine the types of defects and the class of their defects. The defectoscope modes are set depending on the part thickness.

Project safety and environmental friendliness

3.1. Description of workplace, equipment and process operations

The proper organization of the welder's workplace contributes not only to increasing the productivity and quality of welding, but also to ensuring safe working conditions, reducing injuries and accidents. [10]

Welding of small articles (size less than 1 m) should be carried out in separate cabins on metal tables. The dimensions of the cabin must be at least 2 × 2 m2. The cabin walls are made 1.8-2 m high. For better ventilation, a clearance of 150-200 mm is left between the floor and the lower trim of the wall. As the material for the cabin walls, thin iron can be used, as well as plywood, tarpaulin impregnated with a fire-resistant composition, or other fire-resistant materials. The cab frame is made of metal pipes or angle steel. The doorway of the cabin is usually covered with a tarpaulin curtain fixed on the rings. [10]

To paint the cabin walls, it is recommended to use zinc whitefish, yellow crown, titanium whitefish, which absorb ultraviolet rays well. It is not recommended to paint the welding cabins in dark colors, as the overall illumination of the welding place deteriorates. [10]

The passages between the individual equipment and the passages on each side of the rack or table for manual welding shall be at least 1 m. The distance between the wall or column and the welding equipment shall be at least 1 m.

In the welding area there should be a cabinet for storing argon cylinders, a fire extinguisher is required.

Description of process operations and equipment used is given in Table 3.1.1.

3.2. Hazardous and harmful production factors

A dangerous factor is an impact on a person, which under certain conditions leads to injury or other sudden sharp deterioration in health.

A harmful factor is an effect on a person that under certain conditions leads to illness or a decrease in performance.

During the repair welding of the gearbox cover, the following hazardous and harmful production factors act on the worker:

• harmful substances;

• ultraviolet radiation;

• infrared radiation;

• visible radiation;

• electric current;

• noise;

• splashes and sparks of molten metal;

• pressurized systems.

Microclimate

Meteorological conditions, or microclimate, in laboratory conditions are determined by the following parameters:

1. air temperature t (° C);

2. relative humidity φ (%);

3. speed of air movement at the workplace (m/s).

The required microclimate of the welding shop is given in Table 3.2.1.

Noise

Noise causes great damage, harmful effects on the human body, thereby reducing labor productivity. Noise is any unwanted sound for a person. Noise appears when dynamically unbalanced units and tools work. Noise levels in welding shops shall be within the limits regulated by the "Sanitary Standards for Design of Industrial Enterprises" (SN 245-71). [10]

Lighting

Light is one of the most important conditions for the existence of a person, because it affects the state of his body. Properly organized lighting stimulates the processes of nervous activity and increases performance. With insufficient lighting, a person works less productively, gets tired quickly, and the likelihood of erroneous actions increases, which can lead to injuries.

Hygienic requirements for production lighting, based on the psychological characteristics of the perception of light and its effect on the human body, can be reduced to the following:

• the spectral composition of light produced by artificial sources should approach sunlight;

• The illumination level shall be sufficient to comply with hygienic standards;

• uniformity and stability of the level of illumination at the workplace shall be ensured;

• Lighting shall not create glasses in the workplace. [2]

Danger of poisoning

Welding works are accompanied by air pollution of the working zone by a welding aerosol, which includes oxides of various metals and gases that have a harmful effect on the human body. [16]

Welding with a non-consumable electrode in an argon medium of all electric arc welding methods is most favorable from a hygienic point of view. The content of dust in the welding zone, both in the manual and in the mechanized method, does not exceed 2-2.5 mg/m3; concentrations of manganese oxides are 10 times lower than the maximum permissible level. Nitrogen oxides and carbon monoxide are not found even in samples taken near the welding arc. The concentration of dust during welding with lanthanized (tungsten rod with 1.5% lanthanum additive) electrodes is even lower. Lanthanum belongs to the group of rare earth elements and does not cause persistent irreversible changes when entering the body. The gross dust emission when using lanthanized electrodes for welding aluminum and aluminum alloys does not exceed, according to the averaged data, 1.15-1.94 g/h during continuous welding. The concentration of dust in the breathing zone of the welder is significantly below the permissible limit. [10]

When welded with a non-consumable tungsten electrode in an argon environment, ozone is the main hazard.

Ozone is a gas formed in places of electric discharges, under the influence of ultraviolet rays, during electric arc welding in the medium of protective inert gases. The maximum permissible concentration of ozone in the working area shall not exceed 0.1 mg/m3. Exceeding the permissible concentration has a harmful effect on the respiratory organs. The most characteristic signs of poisoning are dry mouth, congestive pains, cough, burning in the stomach. Ozone toxicity is significantly increased when nitrogen oxides are present in the air: their joint effect on the body is many times stronger than separately. [16]

Poisoning with emitted toxic dust and gases occurs only when their amount in the breathing zone of the welder exceeds the maximum permissible concentration (MPC) given in Table 3.2.2. [13]

Poisoning with protective gases used for welding occurs due to their displacement of oxygen, the content of which in the air of the working room of the welder will become less than 19% (by volume). Such poisoning can occur when working in enclosed, poorly ventilated rooms. Argon, like heavy gas, accumulates in the lower part of the enclosed rooms. This should be taken into account when installing suction pumps for local and general ventilation. [13]

Harmful substances. Classification and general safety requirements GOST 12.1.00776.

Notes: 1. By the degree of exposure to the human body, harmful substances are divided into four classes: 1st - extremely dangerous, 2nd - high dangerous, 3rd - moderately dangerous, 4th - low dangerous. 2. Preferred aggregate states of substances in production conditions are indicated: A - aerosols, P - vapors or gases.

Risk of eye damage and burns

Welding with an open electric arc is accompanied by the release of powerful radiant and thermal energy. Such energy can cause eye damage and burns to unprotected parts of the body. The brightness of the unprotected electric arc exceeds 16,000 styles. Normal human vision can painlessly perceive the brightness of no more than one stylb. Not only visible light rays have a harmful effect, but also invisible ultraviolet and infrared rays. They cause inflammation of the mucosa of the eyes, if they act for 10... 30 sec. at a distance of up to 1 m from the source of radiation, and more than 30 sec. - in a radius of up to 5 m. The result of radiant energy is sharp pain in the eyes, light fear, electro-ophthalmia. [16]

Electro-ophthalmia begins after a small latent period lasting several hours. Then there is incision and pain in the eyes, the sensation of a foreign body, light fear, lacrimation, headache, accompanied by insomnia. These phenomena are due to the effect of ultraviolet rays on the mucous membrane of the eyes. Sometimes the process captures the cornea of the eyes. Frequent repetition of the disease of electroophthalmia leads to a decrease in corneal sensitivity, chronic conjunctivitis, and increased eye fatigue. Electro-ophthalmia is more often observed in auxiliary workers than in welders. Infrared radiation due to heat exposure can cause turbidity of the lens. Such cases of occupational diseases were not found in welders of machine-building plants. [10]

On unprotected parts of the body, radiant and thermal energy causes redness and burns of varying degrees. The degree of burn depends on the distance from the source of radiation to unprotected parts of the body. [16]

Burns may be from spattering molten metal and slag. Particularly intense spraying can be achieved by welding and welding with an AC electric arc. [16]

Danger of electric shock

When the body touches unprotected current-carrying parts of welding transformers, rectifiers, converters, electrical wires and other equipment under voltage, electric shock is possible. Safe for humans is an electric current of less than 1 mA. As the current intensity increases, the risk of damage increases dramatically (Table 3.2.3).

A very significant factor determining the current passing through the body is its resistance. The main element that has significant resistance to the flow of electric current is the human skin. The resistance of the skin drops sharply when it is moistened, the contact area with the current-carrying parts increases, if there are wounds in the contact zone, etc.

The danger of a severe outcome increases if people suffering from heart disease, internal secretion, nervous system, tuberculosis, high sweating, and drunk persons are exposed to electric current.

Explosion hazard

Inert gases are delivered to the welding site in metal cylinders, the explosion of which can only be associated with violation of safety rules during their storage and transportation. All cylinders intended for flammable operations and transport of inert gases shall have a distinctive colour and inscriptions. Residual gas pressure in cylinders must be not less than 0.049 MPa (0.5 kgf/cm2). [13]

3.3. Measures to develop safe working conditions

All welding and other fire operations shall be performed in accordance with the requirements of the "Safety Rules for Working with Tools and Devices."

Persons under 18 years of age who have undergone special training and verification of theoretical knowledge, practical skills, knowledge of safety instructions and fire safety rules and who have a "Welder's Certificate" are allowed for electric welding work. Electrical welders shall have an Electrical Safety Group not lower than II. All welders shall be subject to an annual Occupational Safety Instruction Knowledge Check. [16]

Welding shops and areas shall be heated so that the temperature in the working area of the premises in winter is not lower than + 16 ° C. The limit temperature, at which outdoor work is allowed, it is from 20 to 30 ° С. At a temperature of 20 to 25 ° С, workers should be provided with the possibility of heating in the immediate vicinity of the workplace for 10 minutes every hour. At a temperature of 25 to 30 ° С, the working day is reduced by 1 hour (except for work caused by a natural disaster or accident). [1]

Lighting in production buildings and open areas can be carried out by natural and artificial light. In case of insufficient natural lighting, combined lighting is used, when artificial lighting lamps are used in daylight. Natural lighting can be carried out through windows in side walls (side), through upper light openings (aeration lights) or simultaneously through lights and windows (combined). [2]

Local plenum ventilation is carried out by air showers (air flow of certain parameters directed at a person). Local exhaust ventilation is performed, as a rule, in the form of exhaust cabinets. [2]

Personal protective equipment

To protect the respiratory organs at small concentrations of gases in the breathing zone, the anti-malignant respiratory devices ShB1, "Petal," "Astra2" can be used. The tissue of these restaurators is characterized by good dust-retaining properties, has a low mass (BB1 weighs 10 g) and low respiratory resistance (3-3.5 mm of water. article).

To protect the eyes of welders from the blinding visible part of the radiation spectrum, ultraviolet and infrared rays in glasses, shields and masks, protective filters according to GOST 12.4.12083 should be used.

To protect the head of electric welders, protective helmets made of current-conducting materials should be provided.

To protect the hearing organs from noise, it is recommended to use individual protective equipment as per GOST 12.1.02980.

Electric welders should be given protective suits, sleeves, special shoes. Workwear must be durable, fire resistant, light, breathable, non-electrically conductive, with low shrinkage. In case of increased danger of electric shock, welders should be given dielectric gloves, pebbles and mats.

Safety Rules for Welding

in the environment of protective gases [1]:

1. Current-carrying parts of oscillator shall be protected by casing

from dielectric material.

2. If the oscillator housing is metal, it must be grounded.

The casing shall be provided with a cover locked so that at

when it was opened, the oscillator was automatically disconnected from

supply mains.

3. In case of sparking between the gas-electric burner housing and

welding by welded part or welding table must be stopped until torch failure is eliminated (cleaning of nozzle, replacement of insulating washer, etc.).

4. Pipes for gas and water cooling of electric welding machines must be solid and do not pass water and gas at the points of connection of pipes with the nozzle.

5. In gas-electric burners operating with water cooling,

water outlet shall be visible. If water supply is stopped

electrical welding should be stopped immediately.

6. Burners for welding in the environment of protective gases shall not have from

covered current-carrying parts, and their handles must be covered with dielectric and heat-insulating material and equipped with a shield for

protection of welder's hands against burns.

7. Voltage of gas heater coming from the cylinder to the reduction gearbox,

shall not exceed 36 V AC and 48 V DC.

8. Stationary automatic installations for welding in the environment of protective, gases must have gas suction devices.

9. Argon is stored and transported in cylinders under excess pressure of 150 atm. It is necessary to handle cylinders during their transportation, storage and operation in accordance with the Rules of Gosgortekhnadzor.

3.4. Ensuring electrical safety

To avoid electric shock, the following requirements and conditions must be met:

1. Housings of electrical machines and transformers, welder's work table and all metal non-current-carrying parts of the device shall be reliably grounded. To connect the grounding wire on the electric welding equipment, a bolt with a diameter of 5... 8 mm should be provided, located in an accessible place, and the inscription "Earth" near the bolt. Grounding is performed by copper wire with cross-section not less than 6 mm2 or iron rod with cross-section not less than 12 mm2.

2. Connection and disconnection of electric welding devices, monitoring of their serviceable condition and repair should be carried out by electricians. Welders shall not perform these operations.

3. Wires from electric welders to the electrode holder shall be reliably insulated and protected from mechanical damage and high temperature action. During operation, it is necessary to monitor the state of insulation of wires and repair damaged places immediately.

4. The handle of the electrode holder must be made of current-conducting and fire-resistant material.

5. The connection of the wire to the electrode holder must be secure and insulated .

6. In mobile welders the return wire shall be insulated. This requirement does not apply to cases where the welded article itself is a return wire.

7. You can correct the electrical circuit only when the circuit breaker is turned off.

8. Welders must be disconnected from the mains for the duration of their movement.

9. Welders containing storage capacitors shall have automatic discharge devices when accessed.

10. The welder shall disconnect the equipment from the network after completion of the work or at temporary separation from the workplace.

In case of electric shock, it is necessary to provide first aid to the victim before the doctor arrives: (a) without touching the victim, turn off the primary circuit current, or disconnect the victim from the voltage, taking measures to prevent himself from getting under voltage; b) if the victim does not show signs of life, start the production of artificial respiration, providing access to fresh, clean air. Artificial respiration is performed before the arrival of medical care. [1]

3.5. Fire Safety

Fire safety requirements shall comply with

PPB 0193, SNiP 0188, GOST 12.1.00491, SSBT "Fire safety. General requirements. "

The causes of the fire during welding can be sparks and drops of molten metal and slag, careless handling of the burner flame in the presence of combustible materials near the welder's workplace. Fire hazards should especially be taken into account on construction and installation sites and during repair work in premises not adapted for welding.

To prevent fires, observe the following fire prevention measures:

• It is impossible to store flammable or flammable materials near the welding site, as well as perform welding work in rooms contaminated with oiled rags, paper, wood waste, etc.;

• It is forbidden to use clothes and hoses with traces of oils, fats, gasoline, kerosene and other combustible liquids;

• It is impossible to perform welding and cutting of structures freshly painted with oil paints until they are completely dried;

• It is forbidden to perform welding of electric devices and pressure vessels;

• It is impossible to perform welding and cutting of liquid fuel tanks without special preparation;

• When performing temporary welding work in the premises, wooden floors, flooring and pavements shall be protected against ignition by sheets of asbestos or: iron;

• You must constantly have fire extinguishers, sandboxes, shovels, buckets, fire hoses, etc. - and monitor their serviceability, as well as maintain a fire alarm in order;

• When welding works are completed, it is necessary to turn off the welding apparatus, as well as make sure that there are no burning or smoldering objects.

Fire extinguishing agents - water, foam, gases, steam, powder compositions, etc. Special water pipes are used to supply water to fire extinguishing plants. The foam is a concentrated emulsion of carbon dioxide in an aqueous solution of mineral salts containing a foaming agent. When extinguishing a fire with gases and steam, carbon dioxide, nitrogen, flue gases, etc. are used.

Do not use water and foam fire extinguishers when extinguishing kerosene, gasoline, oil, burning electrical wires. Sand, carbon dioxide or dry fire extinguishers should be used in these cases. [16]

3.6. Ground Engineering Calculation

The purpose of the calculation is to determine the number and dimensions of earthing electrodes.

Source Data:

• 380 V grounded unit voltage;

• maximum allowable value of resistance of ground conductor RD = 4 Ohm;

• type of grounding device - remote line with location of grounding elements;

• earthing conductor - pipe 3 m long, 50 mm diameter;

• connecting rod with diameter of 10 mm;

• soil type - chernozem;

• specific resistance of soil ρ=30 Ohm · m.

1. Resistance to current spreading from one ground conductor R1:

3. Horizontal steel rods are used to connect vertical electrodes. The length of the connecting electrode at the location of the earthing electrodes is unlikely to be determined by the formula:

3.7. Environmental expertise of the developed facility

In accordance with the Constitution of the Russian Federation, measures are taken to protect and rational use of land and its subsoil, water resources, plant and animal life, to preserve the purity of air and water, ensure the reproduction of natural wealth and improve the human environment. These activities are grouped into sections: protection and use of water resources, protection of the air basin, protection and rational use of land, protection and use of mineral resources.

Protection and use of water resources provide for the construction of facilities for the extraction of water from reservoirs, waste water treatment, the creation of recycled water supply systems in order to reduce irretrievable water losses, etc. In welding, many enterprises use a recycled water supply system. Water used for cooling welding equipment is repeatedly used after its natural cooling.

Protection of the air pool provides for measures to neutralize substances harmful to humans and the environment, emitted with waste gases: the construction of treatment plants in the form of wet and dry dust collectors for chemical and electrical cleaning of gases, as well as for the capture of valuable substances, waste disposal, etc .

The activity of the enterprise should not violate the normal working conditions of other enterprises and organizations, worsen the living conditions of the population. To this end, measures are also envisaged to combat production noise, vibrations, effects of electric and magnetic fields. Noise generated by welding equipment shall be minimal.

Welding arc power supplies, as well as a number of electrical devices used in welding machines and semi-automatic machines, interfere with radio and television reception. In order to eliminate this phenomenon, noise protection devices are installed in all types of welding equipment causing such interference. [16]

3.8. Facility safety in emergency and emergency situations

During welding operations in the argon environment, the most fire- and explosive object is a cabinet with cylinders containing argon. Storage and operation of cylinders shall comply with safety precautions. In case of safety violation, an explosion and fire are possible.

In case of fire, it is necessary to eliminate the source of fire, evacuate people and, if there are victims, provide medical assistance. As a rule, the occurrence of a fire in the building is accompanied by the release of a large amount of smoke, darkening the room and making it difficult to evacuate and extinguish the fire. In addition, smoke has suffocating properties and is especially dangerous.

Therefore, in order to avoid emergency and emergency situations, workers must undergo an annual knowledge check of the labor protection instruction.

Project business case

Introduction

In the economic section of the diploma project, the costs of materials, electricity, wages are determined, the cost of a unit of product is calculated (according to the basic and design options), the annual economic effect of the project being developed is calculated. Also in this section, the percentage of reduction of the cost of the restored product (gearbox cover) and the reduction of labor input are calculated.

As a basic option, we will adopt a common method of reduction - argon arc welding with a non-consumable electrode with the supply of additive wire. This method has significant disadvantages affecting the weldability of the weld layer with the base metal, also leading to the softening of the base metal in the repair zone and warping of the part, obtaining a satisfactory welding quality is difficult and requires significant energy and material costs.

To eliminate all these shortcomings, Togliatti State University developed a method and equipment for the repair welding of aluminum alloy products with a three-phase arc with the connection of additive wire to the middle phase.

4.1. Calculation of the part recovery time norm

4.3. Calculation of process cost of compared variants

4.3.1 Costs of basic and auxiliary materials and semi-finished products:

4.4. Shop cost

4.5. Factory Cost

4.6. Total Cost

4.7. Capital Cost Calculation for Process Execution

4.8. Calculation of economic efficiency

4.8.1. Labour Intensity Reduction Indicator:

Conclusion

In the course of solving the tasks of the diploma project, a technology for restoring the gearbox cover was developed, equipment was selected, measures were developed to create safe working conditions for workers.

Due to a decrease in labor intensity by 15.38% during the implementation of the design technology of argon-arc surfacing with a three-phase arc with a controlled heat input, the cost of surfacing decreases by 16.2%. Notional annual savings are assumed in the amount of 130,000 rubles. To implement this technology, additional capital investments in the amount of 24,630 rubles are required, which will pay off in 0.1 years. The annual economic effect, taking into account additional capital investments, will be 121900 rubles. Therefore, the developed technology is cost effective and can be used in repair work.

Report

Aluminum is the most common non-ferrous metal in terms of production and scale of use. Aluminium alloys are increasingly used in industry and construction.

Due to the high cost of aluminum parts, restoring worn parts is the most affordable way to keep the machine operational. In this regard, the development of relatively cheap repair technologies for failed equipment is an urgent task.

The purpose of the diploma project is to reduce the cost and improve the quality of the process of restoring parts from aluminum alloys.

From aluminum alloys, very many parts of machines from the "body" class are made. For example: gas distribution mechanism covers, piston; engine cylinder block heads, engine blocks; water and oil pump housings, fuel tanks, gearbox covers, etc.

It is the cover of the gearbox in this thesis that we will consider in more detail and try to restore it.

Cover of power take-off reducer of ground-cutting machine BGM10 protects rotating parts of the unit from mechanical damages during operation. The part is subjected to dynamic and impact loads (stones). As a result, chips, cracks, and coughs occur on the working surface of the cover. In this regard, the need for repair arises.

Most of the listed parts, as well as the gearbox cover, consist of aluminum casting alloy AL9. this is silumin (system SiAl), but in addition to silicon contains alloying elements such as Mg, Cu, Mn. AL9: Si = 68%, Mg = 0.2-0.4%

This alloy is widely used due to its satisfactory strength, high ductility, and good casting properties.

Of all the recovery methods discussed, the most suitable and effective for repairing the gearbox cover is three-phase argon arc welding using heat deposition control.

The three-phase arc is a flare of alternately burning three arcs: between two electrodes and between each electrode and the part. In our method, an additive wire is not connected to the middle phase of the power supply. Since the arc burns both on the article and on the additive → part of the current flowing through the article is redistributed to the additive wire → the thermal mode of the welding process can be adjusted. As a result, the additive material is heated → cleaned from the oxide film due to cathode sputtering.

+

- adjustable heat setting → does not allow burning

- reduction of residual deformations

- the seams have a smaller width and a smoother surface → no further treatment is required

- the arc does not go off when the burner is removed from the product, since the interelectrode arc burns → it illuminates the repair zone and facilitates excitation of dependent arcs.

All the described advantages of the method will improve the quality and efficiency of the repair welding process of the gearbox cover.

So, before you repair the part directly, you need to prepare it.

- at first the detail is degreased in 10%m NaOH alkali solution → for a stravleniye of an oxide film

- alkali residues are washed with water

- drying to remove moisture

- cleaning of welded edges is performed with a metal brush → remove powdered substances. (that is, the edges are suitable for welding 5-6 hours → again)

- inspect the crack, determine its ends, drill (so that the center of the hole coincides with the end of the crack) - to avoid crack propagation.

Repair of the cover includes the following operations:

- installing part on welding table

- setting of welding parameters and modes

- degreasing of the area of acetone welding

- direct brewing of defect (SWAC5 wire: base Al, Si = 4.56%, Fe = 0.6%, Zn + Sn = 0.1%, Cu = 0.2%)

Welding defects that reduce the quality of structures must be detected and eliminated. To do this, it is necessary to carry out quality control of finished products.

With the help of external inspection, you can detect visible defects - burning, non-blowing, surface pores, cracks.

To detect internal defects, we use an ultrasonic control method. With its help, you can detect pores, voids, microcracks, craters. (f = 2.5MHz; α=40º)

During the work of the diploma project, an analysis of harmful factors was made, proposals were made to improve working conditions and fire safety issues were considered.

The cost-effectiveness was also calculated and it was revealed that when restoring the gearbox cover using the proposed method, the cost price was reduced by 16%, and notionally annual savings were assumed in the amount of 13,000. additional investments will pay off in 0.1 years.

Therefore, the developed technology is cost effective and can be used in repair work.

List of literature used

Verkhovenko L.V., Tukin A.K. Welder's Handbook. - Mn., "Higher School," 1977. — 366 pages.

Gorina L.N. Ensuring safe working conditions at work. - Tutorial. - Togliatti: TolPI, 2000. — 68 pages.

GOST 268575 - Aluminium foundry alloys: Grades, specifications and test methods. - M.: Standards Publishing House, 1980.

Jivaga I.I. Electric arc welding of non-ferrous metals and alloys. - L.: Sudpromgiz, 1961. — 139 pages.

Yeltsov V.V., Karelin V.I., Kondrashov S.V. Set of universal equipment for the repair welding of products from light alloys. - Welding industry, 1984. — №9. — page 3536.

Klyachkin Y.L. Welding of non-ferrous metals and their alloys. - M.: Engineering, 1964. — 335 pages.

Kudinova G.E. Methodological manual of laboratory work for students of the 3rd year of specialty. 1206. - Togliatti: TSU, 2005. — 8 pages.

Metal science and thermal treatment of non-ferrous metals and alloys. Textbook for universities/Kolachev B.A., Elagin V.I., Livanov V.A. - 3rd ed., Converted. and supplement - M.: MISIS, 2001. — 416 pages.

Nikolaev A.A. Electrogas welder: Training manual for vocational schools. - Rostov N/A: Phoenix Publishing House, 2000. — 320 pages.

Labor protection during welding in mechanical engineering. - M.: Engineering, 1978. — 144 pages.

Rabkin D.M., Ignatiev V.G., Dovbishchenko I.V. Arc welding of aluminum and its alloys. - M.: Mechanical Engineering, 1982. — 95 pages.

Welding and cutting of materials: Textbook/M.D. Bannov, Yu.V. Kazakov, M.G. Kozulin and others; Ed. Yu.V. Kazakova. - M.: Publishing Center "Academy," 2000. — 400 pages.

Handbook on Non-ferrous Metals Welding/Gurevich S.M.; Otv. ed. Zamkov V.N. - 2nd ed., Redesign. and additional - Kiev: Sciences. Dumka, 1990. — 512 pages.

Stolbov V.I., Pechenkina V.A., Masakov V.V. Zavarka defects of aluminum casting with a three-phase arc. - Welding industry, 1978. — №10. — page 1920.

Technology and equipment of welding by melting and thermal cutting: Textbook for universities. - 2nd ed. corrected and supplemented/A.I. Akulov, V.P. Alekhine, S.I. Ermakov and others ./Ed. A.I. Akulova. - M.: Engineering, 2003. — 560 pages .

Chernyshov G.G. Welding business: Welding and cutting of metals: Textbook for beginning professional. Education/G.G. Chernyshov. - 2nd ed., Erased. - M.: Publishing Center "Academy," 2003. — 496 pages.