Aluminum and alu alloy have characteristics of low melting point, good thermal conductivity, easy oxidation, low temperature strength, and substantially no discoloration of solid-liquid transition, which brings a certain degree of difficulty to welding.
The biggest problem is that the surface of the aluminum alloy weldment has dense aluminum oxide film with melting point of 2050 ° C, while pure aluminum is only 660 ° C. During the welding process, this layer of insoluble oxide film tends to cause slag inclusions in the weld. Since the oxide film is not electrically conductive, affect the stability of the welding arc. In addition, the oxide film adsorbs a certain amount of crystal water to cause pores in the weld. Therefore,must remove the oxide film before soldering, but the aluminum will continue to oxidize at high temperatures during the soldering process.
Hence, measures must be taken to destroy and remove the oxide film in order for the solder to proceed smoothly. For example, gas welding powder for gas welding and AC welding for TIG welding.
Problem analysis-aluminum and alu alloy
If aluminum and alu alloy ues the welding wire to wire uniformly , the cracking tendency of the weld metal is very serious. In order to prevent crystal cracking, an Al-Si welding wire which can form a plurality of fusible eutectic and fluidity can be used. But the strength of the weld bead is lower than that of the base metal. During the welding, the “overaged” softening of the heat affected zone is prone to occur. And for the aluminum alloy which cannot be recovered after the softening of the weld, it is preferable to use annealing or welding in a solid solution state. Then heat treatment after welding. If post-weld heat treatment is not allowed, an energy concentrated welding method and a small welding heat input should be used to reduce the loss of joint strength.
During the welding process, the grain boundary melting occurs in the section of the heat-affected zone exceeding 548 °C. This fusible eutectic is not the original of the parent metal. But caused by the fusible eutectic segregation under unbalanced welding heating conditions. However, under the action of the welding stress, the liquefied grain boundary pulled easily apart to form a liquefaction crack. Even if there is no crack in the grain boundary, the presence of the brittle eutectic on the grain boundary embrittles the heat-affected zone, which reduces the cold bend angle and plasticity of the welded joint.
Solve problem (一）
There are three main types of filler wire used for welding aluminum and aluminum alloys.
The first is a special welding wire. Which is specially used for the base material which is the same or similar to its composition . If there is no ready-made wire, a narrow strip can be cut from the base metal as a filler metal.
The second is the same welding wire. Which is an Al-Si welding wire with a silicon content of 5%. This kind of welding wire can produce a large amount of (α+β) eutectic in welding. It has good fluidity and has good healing effect, so it can be improved. The crack resistance of the weld can applied in various aluminum alloys other than the Al-Mg alloy.
The third is a special welding wire. Which is a welding wire specially smelted for welding various hard aluminum and super-hard aluminum. Composition of the welding wire is very similar to that of the base metal. Generally, added a modifier to the base material, for example: Ti, Zr, V, D, etc., and appropriate adjustment of the content of alloying elements (Cu, Mg, Zn, etc.). In order to achieve grain refinement, reduce the crystallization temperature range or increase the number of eutectic.
Compared with general-purpose welding wire, special welding wire has good crack resistance and high strength and plasticity. Common aluminum and its alloy welding wire are: pure aluminum welding wire (SAl1070, SAl1450, SAl1200, etc.); Magnesium-aluminum alloy welding wire (SAl5554, SAl5556, SAl5654, SAl5183, etc.); The alu-copper alloy welding wire (SAl2319, etc.); And aluminum-manganese alloy ( SAl3103, etc.; As well as alu-silicon alloy wire (SAl4043, SAl4047, etc.).
Solve problem (二）
TIG welding is one of the main methods for welding aluminum and aluminum alloys. It is the most common one. This welding power source uses AC power, which has high thermal efficiency and tungsten carrying capacity. And can broke it by “cathode crushing”. The function “clears the oxide film on the surface of the welded portion of the aluminum plate. So that the we can welding smoothly, and a bright and beautiful weld surface is obtained.
In the welding process, the position angles of the welding torch, the welding wire and the workpiece should be kept approximately the same. Due to the size of the workpiece: the form of the joint; The welding parameters; Technique of the welder, etc.; The angle of the welding gun and the workpiece are slightly different throughout the welding process.
Manual tungsten argon arc welding generally adopts the left welding method. The welding gun should move linearly forward and evenly and maintain a constant arc length.Without butt welding, the arc length is about 0.5 to 2.0 mm. Added the butt welding, the arc length is about 4 to 7 mm. When the welding torch moves, it can stop intermittently. When reached a certain penetration depth , adding welding and then move forward.