Electric arc welding of aluminum and its alloys is fraught with considerable difficulties, this is especially true when there is a need to weld aluminum and copper, aluminum and brass. The main difficulties are the physical and chemical properties of these materials. Contact welding of aluminum, as well as contact welding of this metal and copper alloys, removes some of the issues and is a more technological process, if necessary, to establish mass production.
Aluminum welding scheme.
The key point in the production of permanent aluminum joints is the preparation of welding surfaces before work. The gray color of aluminum is due to the presence of a dense oxide film on its surface. Its melting point exceeds 2000 ° C, while aluminum itself melts at a temperature of only 657 °. Already at 1800 ° aluminum boils. Therefore, it is difficult to remove oxides by burning.
Chemical properties of aluminum.
In addition, the oxide film on the surface of aluminum is chemically neutral. The solution of caustic soda helps to soften the oxides, you can still use gasoline. Such treatment itself can lead to corrosion of aluminum, so the solvents are thoroughly washed with plenty of water.
Instead of or in addition to chemical treatment, mechanical removal of oxides is used. With a small length of the welded surfaces do it with a brush on the metal. To remove the film on a large area using a power tool with nozzles. For demanding work and for repeatability of results, use scraper.
The oxide film on the surface of aluminum is restored fairly quickly, so its removal is best done immediately before the welding process. The time between cleaning and welding should not exceed 10 hours.Back to table of contents
Welding process technology
Only billets of relatively small thickness are subjected to contact bonding. With a thickness of parts more than 8 mm, this is already irrational. The greater the thickness of the workpieces, the greater the strength of the current necessary for high-quality heating of the metal in the contact zone. Since even with a very small thickness of the workpieces, the welding current is measured in thousands of amperes, for massive products the current can be hundreds of thousands of amperes. This causes problems with the cooling of the electrodes. The required compressive forces of the tool working bodies are also growing.
One of three types of resistance welding is used to join aluminum billets, it can be seam, butt and spot welding.
Aluminum spot welding machine.
In any of these types of thermomechanical welding processes, the approach of the atoms of the crystal lattice is performed using pressure during heating of the contact zone by means of an electric current passing through it.
Aluminum is a good conductor of current. The greatest resistance to current flow and the associated heating are created precisely at the junction of the surfaces to be connected. The surfaces of the parts heated to a plastic state are compressed. As a result of the high temperature, the material in the contact zone melts and a welding grain is formed.
Types of resistance welding differ in the method of application of compressive force and the seam geometry. In seam welding, surfaces are clamped between two rollers through which alternating welding current is passed. The current supply can be discrete or continuous. Hence, the seam itself can be continuous, hermetic, intermittent and even pinpoint. So connect the edges of large-section pipes, sheet materials. The thickness of the blanks at the same time does not exceed 3 mm. Roller electrodes are usually made of copper. Aluminum seam welding requires very powerful power sources with ionic type breakers.
Butt welding is suitable for connecting the ends of rods, profiles, pipes of small diameter. The blanks are clamped in the holders, welding current and simultaneous axial force is applied to them. At the same time, thorough preparation of surfaces is of great importance: cleaning, ensuring parallel edges and combining the axes of the workpieces. The magnitude of the welding current reaches 15 kA per square centimeter of the area of the ends of the workpieces.
The most common in the connection of sheet blanks received spot welding of aluminum. Due to the low electrical resistance of aluminum, heating and melting of the metal occur very quickly. The duration of exposure to one point takes only about 0.005 seconds or less. This requires a rapid approach of the tool squeeze edges. Otherwise, the compression will not keep pace with the melting of the metal. High discharge rate is achieved by using capacitors in the power supply circuit.
In this type of welding, electrodes are also made of copper. Copper and some of its alloys have high hardness and excellent electrical conductivity. Well suited for such purposes alloy EV. When spot welding with copper electrodes is performed, copper of the contact electrodes and the aluminum of the workpiece can be welded. Then you have to clean the ends of the electrodes with the removal of the surface layer of the metal.Back to table of contents
Dimensions of spot welded joints.
When spot welding aluminum billet welding of copper with aluminum is an undesirable phenomenon, but sometimes there is a need to obtain a permanent connection of two metals. Especially often this is required to ensure reliable electrical contact in the connection. With detachable joints, an insulating oxide film increases resistance at the point of contact.
Electric arc welding of copper and aluminum is associated with some difficulties. The melting points of these substances are different. With a large zone of aluminum melting, copper still remains in a crystalline state. These metals are mutually dissolved in each other. Due to the earlier melting of aluminum, the seam is formed from a solution of copper in aluminum. At high copper concentrations, the solution becomes brittle.
If it is difficult to find an alternative to electric arc welding in construction conditions, then in the factory, the joining of these metals is more often carried out using the same contact methods as when joining aluminum billets. In resistance welding, it is possible to ensure minimum mutual penetration of metals and prevent the embrittlement of the joint. Good results are achieved by the use of low temperatures in combination with a short exposure time.
Cladding, that is, the coating of aluminum sheets with pure copper, refers to the cold welding of these metals. The connection is obtained by rolling with an overlap of up to 70%. So, on the aluminum billet roll a layer of copper to 0.8 mm.
To obtain a cladding layer with a thickness of up to 2.5 mm, hot rolling with heating up to 450 ° C is used. The operation is performed in two stages to reduce the oxidation of copper cladding sheets.