Contact welding of aluminum - features and practical recommendations

05/31/2019 Author: VT-METALL

Issues discussed in the material:

• Why welding aluminum is difficult
• How to properly organize welding of aluminum and its alloys
• Is it possible to weld aluminum parts manually using electrodes?
• What modern aluminum welding methods are used in production?

Aluminum parts have high thermal conductivity and low weight. These properties of the material have made it very popular in various manufacturing fields. However, the technology for welding aluminum and its alloys is not so simple. It is necessary to take into account many different factors and characteristics of the material in order to perform welding work at a high level. In our article we will talk in more detail about what aluminum joining technologies exist and how they differ from each other.

Features of welding aluminum and its alloys

Sometimes during welding work on aluminum or aluminum alloys, difficulties arise that significantly affect the quality of the welds. Here are examples of the most common problems:

1. The weld pool is quite difficult to control due to the high fluidity of the material. Hence the need to use heat-dissipating pads.
2. Aluminum oxidizes easily, which causes a refractory film to form on droplets of molten metal. As a result, joining into a single seam becomes difficult. Properly organized reliable protection of the welding area from ambient air helps prevent the appearance of a film.
3. On the surface of aluminum products there is always an oxide film of Al2O3, which has a melting point of about +2040 °C, while the metal itself melts at a temperature of +660 °C.
4. Significant shrinkage of the material can cause deformation of the weld after it cools and solidifies.
5. A decrease in the mechanical characteristics of the material is possible due to the tendency to pore formation and cracks in the seam.
6. Due to the high thermal conductivity of aluminum, high operating current is required for welding work.

The listed difficulties are quite surmountable, so the popularity of various aluminum welding technologies does not decrease. This metal allows you to create very strong and reliable structures.

How can one explain the poor weldability of aluminum?

Welding aluminum and its alloys has some difficulties and difficulties that can affect the further quality and strength of the welds. This is explained by certain features of this metal:

• The main difficulty of heat treatment is that an oxide film is constantly formed on the surface of the metal. It melts at high temperatures - 2044 degrees. And the metal itself melts at a reduced temperature - at 660 degrees;
• During welding of aluminum, drops of welded metal are formed in the welded area, on which a coating of an oxide refractory film quickly forms. It is this that prevents a uniform connection. To prevent all these problems, aluminum welding must be performed by protecting the welded area from interaction with the surrounding air. Argon gas is used for these purposes;
• When melted, aluminum exhibits a high degree of fluidity, which is what creates difficulties in forming a weld pool. For this reason, the technology of welding aluminum alloys recommends the use of special pads with a heat-dissipating structure;
• Aluminum contains hydrogen in dissolved form, which begins to come out during the solidification of the metal. This can lead to the appearance of pores and crystallized cracks in the weld area. Many aluminum alloys contain a high level of silicon, which is what causes cracks to appear during cooling of parts;
• aluminum has a high coefficient of linear expansion. Because of this, severe shrinkage of the metal occurs during its solidification. All this leads to deformation of the joints of the parts that are used for welding;
• The process of welding aluminum alloys should be carried out only under the influence of high current discharges. This is explained by the fact that this metal has high thermal conductivity. But when welding other steels, currents with the lowest intensity are used - 1.2-15 times. This is due to the fact that, unlike aluminum, they have a higher melting point;
• Welding aluminum at home can be difficult because it is often impossible to determine the exact grade of alloy from which the parts being joined are made. This can greatly complicate the choice of welding modes and methods used to perform it.

Aluminum welding technology: preparation of materials and parts

To create a high-quality welded joint, it is necessary to take full care to ensure that the welding area is protected as much as possible from all kinds of contamination:

• For gas-electric welding of aluminum, a clean, dry, dust-free room should be allocated.
• The air speed should not exceed 0.2 m/sec.
• The workpieces and filler wire must be thoroughly cleaned using the methods provided by the technology.
• As shielding gases, only pure argon grade A according to GOST 10157–62 and high-purity helium (high purity) according to MRTU 51-04-23-64 can be used.
• Gas supply fittings, hoses and welding torch are thoroughly washed with alcohol before starting welding work and then periodically re-cleaned and washed as work proceeds.

The technology for high-quality cleaning of welding wire includes washing off the preservative lubricant with a solvent or hot water, and removing the oxide film by chemical treatment.

We recommend articles on metalworking

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To perform the above work, it is necessary to organize a special place, separate from the welding area. Chemical treatment technology involves several cleaning methods. The most popular chemical cleaning method is the following sequence:

1. Etching in a 5% solution of caustic soda NOH at a temperature of +60...+65 °C for 2-3 minutes;
2. Rinse in hot (+45…+50 °C) water, and then in cold running water.
3. Clarification in a 15–30% solution of nitric acid HN03 at a temperature of +60…+65 °C for 2-3 minutes;
4. Rinse in hot (+45…+50 °C) water, and then in cold running water.
5. Drying at a temperature not lower than +60° C until moisture is completely removed.

If you do not have the opportunity to immediately use the welding wire after drying, then store it in a specially designated place. A box or cabinet with tightly closing doors is suitable for this.

The treated wire can be stored for no more than 12 hours if it is a filler wire of small diameter (up to 1.6 mm) for working at low currents with a non-consumable electrode.

If we are talking about thicker wire (4-5 mm) for working with a consumable electrode at currents above 400 amperes, then a longer storage period is allowed (up to one and a half days). In this case, the quality of the weld will not be affected. It is only important to follow the rules for storing processed wire and aluminum welding technology. During work, the prepared wire should not be handled with hands without gloves, as this can lead to grease particles getting onto its surface.

It is advisable to carefully process the part itself and its edge before welding. When welding, preference is given to chemical processing of parts, the technology of which is given above. Small-volume products are processed completely, and large workpieces are completely degreased; only the edges and up to 10 cm of the surface from the joint are subjected to chemical treatment.

For small welded joints, the edges are cleaned with a scraper immediately before welding. It is also necessary to remove the oxide film in the area of ​​the conductor. This can be done using a scraper or steel wire brush.

If you do not have the opportunity to chemically treat a large part, you can clean the edges with steel wire brushes. In this case, it is advisable to wipe the surface with alcohol or acetone before and after processing the edge.

The bristles of a steel wire brush are made from stainless steel. It is best to use a brush with bristles no larger than 2 mm in diameter. It will allow you to process the edge better without leaving too deep scratches or defects. While processing the part, the brush is periodically washed in solvent.

Treated parts must be stored in a warm, dry place, covering the edges with a clean cover. In this form, it is possible to store parts for the same amount of time as processed filler wire.

If aluminum welding technology requires long-term work (installation, testing, etc.) between cleaning parts and welding, then in this case welding with a large-diameter consumable electrode is used. In addition, it is necessary to ensure that the edges are protected from contamination at all intermediate stages.

If welding is performed in several passes, then at each stage it is necessary to clean the surface of the seam and groove using brushes and acetone.

Aluminum not only conducts heat well, but also has a high heat of fusion (96 cal/g). This property is the basis of aluminum welding technology, since in order to create a high-quality weld, the direct impact of the welding arc on the entire area of ​​​​contact between the liquid and solid phases of the weld pool will be required.

If a non-consumable electrode is used for the connection, then a bath of liquid metal will form only in the arc burning zone. In this case, it is mainly formed due to the melting of the base metal (usually it contains no more than 30% of the filler material). It features a smooth transition to the base metal.

By joining aluminum with a consumable electrode, you will create a larger depression of the base metal due to a more concentrated arc. Accordingly, the size of the weld pool will be larger, it will contain more than 50% of the deposited metal. In this case, the peripheral part of the bath is not affected by the arc; therefore, non-fusion may occur.

It is important that the shape of the cutting of the edges makes it possible for those places where lack of fusion may occur to be re-melted with an arc when applying subsequent beads. Compliance with edge cutting technology allows you to achieve high quality welds. In any case, the best connection is obtained by performing double-sided welding.

If it is not possible to use the double-sided method, then it is necessary to take measures to prevent and eliminate defects at the root of the seam.

How to prepare metal for welding

Regardless of the conditions in which aluminum parts are welded - industrial or domestic, it is necessary to carefully prepare the edges of these parts. Such preparation is as follows.

• The surfaces of the parts to be welded (as well as the filler material) are cleaned of dirt, oil and grease. To degrease surfaces, they are treated with white spirit, acetone, aviation gasoline and any other solvent.
• Preparation also includes cutting the edges of the parts to be welded, which is performed if necessary. If welding of parts made of this metal is not carried out using coated electrodes, then the edges should be cut when the thickness of the parts being joined exceeds 4 mm. If electrodes are used for welding, then edge cutting is performed when the thickness of the parts exceeds 20 mm. If aluminum sheets with a thickness not exceeding 1.5 mm are to be welded, then their ends are flanged before making the connection.
• The oxide film must be removed from the surface of parts made of this metal before welding. For this procedure, a file or brush with stainless steel bristles is used, with the help of which the oxide film is removed from the edges of the workpieces being joined (25–30 mm wide).

Treatment of the welding site with a penetrating compound in order to identify defects and determine the location of the seam

Often, when welding aluminum at home, chemicals are used to remove the oxide film: caustic soda, gasoline. If the edges of the parts to be joined are treated with caustic soda, after such treatment it is necessary to rinse them with running water.

Aluminum electrode welding technology

The technology of welding aluminum with electrodes is used very rarely. This method is suitable where it is not possible to use special equipment. That is, it is more often used in the field or in small workshops where they cannot financially afford to purchase the necessary equipment. In this case, the use of electrodes can reduce both costs and time.

There are several brands of commercial electrodes:

• OK - electrodes for aluminum with an admixture of manganese or magnesium. It should be protected from moisture, so do not remove all the rods from the packaging.
• OZANA - there are two varieties here, which differ slightly in application depending on the type and alloy of the metal. Such rods are used for horizontal and vertical welding.
• OZA - consists entirely of aluminum and is similar in production to homemade rods. Used to connect aluminum alloy with silicon.
• UANA - due to their origin and properties, they are intended for welding aluminum alloys and can be deformed.
• EHF - used for welding in an environment where argon is used as protection. These electrodes consist entirely of tungsten.

Electrodes for connecting aluminum parts vary in cost, so when choosing the appropriate option, pay attention to the characteristics that are of paramount importance to you.

1.

Manual arc welding of aluminum with coated electrodes (MMA technology).

Manual joining technology using coated electrodes is used for non-critical structures made of pure aluminum and its alloys: AlSi, AlMg and AlMn. This method is only suitable for products less than 4mm thick.

The disadvantages of this method of joining material are:

• porosity and low strength of the seam, which implies low quality of the connection;
• a large amount of splashes of molten metal;
• poor detachability of the slag crust, which can cause corrosion.

To perform the work, a current of reverse polarity without transverse oscillations is required. It is important to correctly calculate the current using the following formula: 25–30 A per 1 mm of electrode.

If you want to achieve a high-quality connection, it is advisable to heat the parts to a certain temperature before starting welding. For thin and medium-thick parts, a temperature of +250…+300 °C is sufficient. Large products require temperatures up to +400 °C.

Don't forget that the optimal temperature may be specified by the electrode manufacturer. If you find such data, it is better to be guided by it.

2.

Manual arc welding with carbon electrodes.

Aluminum welding technology using carbon electrodes is most often used for non-critical structures. For this type of work you will need direct current of straight polarity.

For dimensional products whose thickness is more than 2.5 mm, it is necessary to cut the edges. The additive diameter should be in the range of 2–8 mm. Paste-like flux can be applied both to the rod and to the working surface.

3.

Manual arc welding with a tungsten electrode in inert gas (AC TIG technology).

This is a very popular method of joining materials, used when it is necessary to obtain a very strong connection with an excellent appearance. The technology of arc welding of aluminum with a tungsten electrode is based on the use of a rod with a diameter of 1.6–5 mm and an additive of 1.6–4 mm.

To perform work using this technology, a protective environment of helium or argon is required. The electric arc is supported by an alternating current source, which gives good results in destroying the oxide film.

• The angle between the electrode and the working surface should be 70–80°; between filler wire and electrode – 90°. Arc length – from 1.5 to 2.5 mm.
• The additive is fed in short reciprocating movements. Transverse movements of the electrode and filler rod are unacceptable.
• The burner moves after the rod.
• A copper and steel gasket should be placed under the aluminum product, which will act as a heat sink. This will prevent the formation of holes, especially when working with thin metal.
• The dimensions of the weld pool should be minimal.
• The argon supply starts 5–7 seconds before the arc is excited, and turns off 5–7 seconds after it breaks.

Selecting mode parameters

The non-consumable electrode welding method is used for products made of aluminum alloys with a thickness of up to 12 mm. When welding metal with a thickness of 1 to 6 mm, tungsten electrodes with a diameter of 1 to 5 mm are used. Welding current (A) is determined by the formula:

Ist=(60?65)de,

where de is the diameter of the electrode, mm

The arc is powered by an alternating current source with an oscillator, which helps destroy the oxide film. The open circuit voltage of the source must be increased. The reliability of gas protection of the arc and weld pool depends on the diameter and shape of the torch nozzle, the distance of the nozzle from the surface of the work being welded.

It is recommended to maintain the following ratios:

The length of the W-electrode protruding from the nozzle (outlet) should be 1-1.5 mm when welding butt joints, and 4-8 mm for T-joints and corner joints. The arc length is maintained within 1.5-3 mm. Welding speed is selected from 8 to 12 m/h.

It is advisable to use connections with flanged edges when welding metal with a thickness of 0.8-2 mm.

Technology of manual electric arc welding of aluminum

1. To perform welding work on aluminum, direct current with reverse polarity is required (swap the connectors on the inverter). It is important to observe the ratio of power and electrode diameter and regulate the current using the following formula: 30 amperes for each millimeter of diameter.
2. The parts to be welded must be preheated. For medium thickness, the workpiece is heated to +200…+300 °C. Large parts require higher heating (up to +400 °C).
3. Despite the usual arc formation, a feature of working with aluminum will be a higher burning rate of the electrodes. Accordingly, greater speed will be required when forming the seam.
4. Be sure to plan the length of the seam based on the melting of one electrode. The welding process cannot be interrupted. The slag crust that forms at the end of the weld will prevent the arc from being ignited again in this place.
5. Compared to welding steel blanks, it is prohibited to make lateral movements when joining aluminum parts.
6. When finishing work, immediately remove slag from the joint. Clean everything with a wire brush and rinse with hot water.

Welding aluminum with argon: technology, instructions, nuances of the process

According to the technology of welding in an argon environment, high demands are placed on both the welding machine and additional equipment that ensures proper storage and supply of consumables. All these parameters are of decisive importance when forming a weld.

Argon welding of aluminum and its alloys can be performed with the following equipment:

• a source of electric current to which the welding machine and all other equipment will be connected;
• a cylinder in which the protective gas argon is stored;
• a mechanism responsible for feeding filler wire into the welding zone.

The technology for performing welding work using argon at large industrial enterprises is well established. In this case, the shielding gas is supplied through a centralized network. Whole reels are formed from welding wire and installed on a semi-automatic welding machine. All work is performed on special workbenches, the surface of which is made of stainless steel.

A high-quality weld is obtained only when the parts being joined are thoroughly cleaned of various types of contaminants (grease, dirt, oil). Cleaning is done using a solvent. For sheet blanks with a thickness of more than 4 mm, edge cutting is required. In this case, welding work is carried out end-to-end. To remove the refractory oxide film from the surface of the product, it is necessary to treat the joint with a file or a wire brush. With a complex connection configuration, processing with a grinding machine is possible.

The technology of welding aluminum semi-automatically in an argon environment has a number of characteristic features. When performing work semi-automatically or with manual supply of the additive, you will need tungsten electrodes with a diameter of 1.5–5.5 mm. When forming a welding arc, the electrode must be positioned at an angle of 80° to the surface of the parts.

The manual feed of filler wire technology allows an angle of 90° relative to the electrode. In this case, the filler wire moves in front of the electrode. This is very clearly visible in the demonstration videos, which show the process of welding using argon.

Argon connection technology requires maintaining the arc length within 3 mm. In this case, transverse movements of the filler wire are not allowed.

It is advisable to connect thin aluminum sheets on a lining, which can be a steel sheet. This technology facilitates faster removal of heat from the work area, resulting in rare burnouts or leaks of molten metal. In addition, the lining allows you to save energy, significantly increasing the speed of welding work.

The technology of welding aluminum and its alloys using argon has a number of undeniable advantages over other methods of joining workpieces. First of all, this concerns low heating of the parts being connected. This is a very valuable quality when cooking workpieces of complex shapes.

When using an argon connection, a very strong weld is obtained with high uniformity of the material in a given zone, a minimum number of pores, impurities and foreign inclusions. A uniform penetration depth along the entire length of the weld is a very important indicator that distinguishes argon welding technology.

Of course, each technology has its drawbacks, and working with argon is no exception. The disadvantage of this method is the use of complex equipment. Only with proper configuration of the welding machine and additional equipment is it possible to achieve maximum efficiency of all operations, resulting in a high-quality weld.

The determining parameter when setting up all equipment to perform work in an argon or other protective environment is the speed and uniformity of the filler wire feed. If this parameter is violated, the additive is supplied intermittently, the welding arc is interrupted, and the consumption of shielding gas and electricity increases significantly.

Grades of aluminum alloys most used for welding

Aluminum alloys are classified into two groups: thermally hardenable and, accordingly, not thermally hardenable. Among the grades that cannot be thermally hardened, aluminum-magnesium alloys of the AMG grade are used for welding. Their chemical composition corresponds to GOST 4784, and the assortment of sheets corresponds to GOST 1946. See table:

Brand

With an increase in the percentage of magnesium to 7%, the weldability of metals deteriorates. With a magnesium content of up to 3%, the risk of cracking increases, but the weld becomes more dense.

To reduce the number of pores in the weld, filler wire is selected, in which the magnesium content is higher than in the metal being welded. Due to this, the porosity of the seam is reduced.

For welding high-strength structures, heat-treatable aluminum alloys - duralumin - are used. Duralumin grades D1, D16 and D19 are widely used in fusion welding. Alloy D20 belongs to the satisfactory weldability group of steels.

Magnesium-aluminum alloys with an aluminum content of up to 11% are welded satisfactorily by fusion welding. And with a welding method such as resistance welding, the weldability of these metals is good.

Semi-automatic aluminum welding technology

Another name for semi-automatic welding is MIG welding. The high productivity of this process is ensured by pulsed equipment, which generates a powerful high-voltage pulse. Under its influence, the oxide shell quickly collapses.

The operating technology of this equipment involves driving each particle of the molten metal rod into the welding area. Thanks to this, a high-quality weld is formed, characterized by high strength characteristics. This is an aluminum spot welding technology.

MIG/MAG connection in semi-automatic mode is made using rather expensive equipment, which not everyone can purchase. But modern craftsmen who are keen on welding have found a way out of this situation by modifying a standard semi-automatic machine used to join aluminum workpieces and stainless steel.

The principle of operation of such a device is similar to a MIG connection, although there are a number of technical features that have a significant impact on the quality of the result obtained:

• Pure aluminum and its alloys cannot be welded with high voltage current and straight polarity. The exact opposite option is used.
• To feed aluminum rods, special equipment is required. This is due to the fact that it is much softer than its steel counterparts and, accordingly, can bend. The special feeder is equipped with four rollers, a small sleeve and a Teflon gasket.
• Non-ferrous metal expands greatly when heated. This feature can cause the wire to get stuck in the feed tip area. Special tips with the Al brand will help prevent this. They can be replaced with standard tip models with an increased diameter.
• Material consumption and quality of the weld are determined by the type of filler wire. High melting rates of the bar may require a higher feed rate. Accordingly, the tip will need to be replaced too often.

Modern aluminum welding technologies

1.

Laser welding.

This is a purely production technology with very specific characteristics. Before starting welding work, it is important to very thoroughly clean the product from contamination.

Laser aluminum welding technology allows for amazing joint precision. In this case, the heat-affected zone is quite small, and the seam turns out to be very narrow. In addition, there are other advantages of this method:

• the ability to create seams of complex shapes;
• high level of productivity;
• efficiency of the welding process and (most often) its automation;
• environmentally friendly welding mode;
• welding can be carried out in any spatial position;
• The heat-affected zone is practically not exposed to thermal influence, which allows maintaining all the original properties.

You should also remember the disadvantages of this method:

• high cost of equipment and the entire process as a whole;
• the laser does not process thick-walled products well;
• designed to work with a narrow range of products.

2.

Plasma welding.

Modern technology for welding aluminum using plasma allows one to achieve very good results. This achieves a high concentration of energy and fairly deep penetration.

The technology is similar to the process of joining metals in an argon environment. During the welding process, the metal in the desired location is melted under the influence of plasma. All this action takes place in a protective cloud, thanks to which gases contained in the atmosphere do not enter the weld pool area.

The technology of welding aluminum using plasma involves the use of special equipment - a plasma welding machine. The design of the device is a connection of a plasma torch and a source of alternating and direct current with an inverse value. The plasma torch helps generate a plasma discharge.

There are several modifications of power supplies that differ from each other in current strength, no-load voltage, load duration, etc. Depending on these characteristics, the power consumption of the power source changes.

The plasma torch is equipped with special inlets through which plasma-forming and protective gas is supplied and the nozzle walls are cooled with liquid or air. The technology of the burner operation relies on the use of an electrode made of copper, refractory tungsten or hafnium.

There are four types of aluminum plasma welding:

1.

Welding using a consumable electrode.

To perform this process, you will need a special gas environment that will have protective properties. For these purposes, helium, argon, or a mixture of these gases is usually used. The technology involves the use of special tungsten electrodes and additives with a diameter of up to 2.5 mm. Welding work is carried out using reverse polarity current.

2.

Automatic arc welding.

To perform automatic arc welding, you will need a semi-open plasma submerged arc and a closed submerged arc. According to the technology, flux marked AN-A1 is used if parts made of technical aluminum are welded. Flux marked AN-A4 is intended for joining aluminum-magnesium alloys. A consumable split electrode is used for operation.

A layer of flux is necessary in order to prevent possible process disruptions and shunting. If welding is carried out manually, then the dimensions of the flux are determined based on the thickness of the elements. The average data is width 20–45 mm, thickness 7–15 mm.

3.

Manual arc.

Manual aluminum welding technology is used when different metals (aluminum, aluminum-silicon alloy, zinc or magnesium alloy) are joined. In this case, a thickness of parts of at least 4 mm is required. The current must be constant with reverse polarity and high speed.

Cutting the edges is necessary if their thickness is more than 1 cm. In the case of cutting the edges, the weld is formed end-to-end. The “overlap” technology is not used, since slag can penetrate into the joint, which will lead to destruction of the material. Preliminary heating of parts to +400 °C is required.

4.

Electron beam.

Electron beam technology involves the use of vacuum. Oxides are destroyed under the influence of metal vapors. Thus, thanks to the vacuum, the oxide decomposes and hydrogen is removed from the weld.

The quality of the connection is very high, the seams are even, the workpiece is not deformed, and the material in the joint area retains its strength characteristics.

Specifics

The inverter itself, used for welding aluminum workpieces, can be entry-level, household grade. Surface preparation, selection of welding materials and careful adherence to technology are of decisive importance .

Electrodes of the OZ series have excellent performance characteristics. But these qualities appear only at low humidity of the coating material. Therefore, before use, they must be calcined at a temperature of 120-140°C for 40 minutes. After calcination, the electrodes must be stored in an oven or in a special sealed case.

During work, you must be careful and careful. The high fluidity of the melt and its tendency to splash does not allow working in vertical and overhead positions . And in the lower welding position, it is recommended to use backing plates to prevent melt leakage.

To avoid thermal deformations during hardening, try to place the seams as far apart as possible.