Aluminum alloy welding method

The thermal efficiency of oxyacetylene gas welding is low, the welding heat input is not concentrated, the welding of aluminum and aluminum alloy requires flux, and the residue needs to be removed after welding, so the quality and performance of the joint are not high. Because the gas welding equipment is simple, does not need power supply, and is convenient and flexible to operate, it is often used to weld aluminum alloy components with low quality requirements such as thin plates and small parts, and can also repair aluminum alloy components and aluminum castings.

(1) joint form of gas welding

When gas welding aluminum alloy, lap joint and T-joint are not suitable. It is difficult to clean the residual flux and welding slag flowing into the gap, so butt joints should be used as far as possible. In order to ensure that the weldment is fully welded and does not collapse or burn through, slotted backing plates, generally stainless steel or pure copper, can be used. Good reverse forming can be obtained by welding with backing plates and welding productivity can be improved.

(2) Selection of gas welding flux

In gas welding of aluminum alloy, in order to make the welding process go smoothly and ensure the weld quality, flux is needed to remove the oxide film and other impurities on the aluminum surface.

Gas welding flux (also known as gas agent) is a kind of flux in gas welding. Its main function is to remove the oxide film formed on aluminum surface during gas welding, improve the wettability of base metal and promote the compactness of weld structure. Flux must be used in gas welding of aluminum alloy. Generally, before welding, the flux is directly sprinkled on the groove of the workpiece to be welded, or dipped in the welding wire and added to the molten pool.

Aluminum alloy flux is chloride salt of potassium, sodium, calcium, lithium and other elements, which is a powdered compound prepared by crushing, sieving and a certain proportion. For example, aluminum cryolite (Na3AlF6) can melt alumina at 1000℃, and potassium chloride can transform refractory alumina into fusible aluminum chloride. This flux has low melting point and good fluidity, and can also improve the fluidity of molten metal and make the weld well formed.

(3) Selection of welding nozzle and flame

Aluminum alloy has strong oxidation and gas absorption. In gas welding, in order to prevent aluminum from being oxidized, neutral flame or weak carbonization flame (carbonization flame plus excessive acetylene) should be used to protect aluminum molten pool from being oxidized in reducing atmosphere. It is forbidden to use oxidation flame, because the use of oxidation flame with strong oxidation will make aluminum strongly oxidized and hinder the welding process; However, if there is too much acetylene, free hydrogen may dissolve into the molten pool, which will lead to porosity and weld looseness in the weld.

(4) positioning weld

In order to prevent the weldment from changing size and relative position during welding, the weldment needs spot welding before welding. Because of the large linear expansion coefficient of aluminum, fast heat conduction and large heating area of gas welding, the locating weld is denser than that of steel.

The filler wire used for tack welding is the same as that used for product welding. Before tack welding, a layer of gas agent should be coated in the gap between welds. The flame power of tack welding is slightly higher than that of gas welding.

(5) Gas welding operation

When welding steel, the heating temperature can be judged from the color change of steel. But when welding aluminum, there is no such convenience. Because there is no obvious color change in the process of aluminum alloy from room temperature heating to melting, it is difficult for operators to control the welding temperature. However, the welding opportunity can be grasped according to the following phenomena:

1) When the surface of the heated workpiece changes from bright white to dim silvery white, the oxide film on the surface wrinkles, and the metal at the heating place fluctuates, it indicates that it is about to reach the melting temperature and can be welded;

2) When the end of the welding wire is dipped in flux and the heated part can fuse with the base metal, welding can be carried out when it reaches the melting temperature;

3) When the edge of the base metal falls, the base metal can be welded when it reaches the melting temperature.

The left welding method can be used for gas welding thin plates, and the welding wire is located in front of the welding flame. This welding method is beneficial to prevent the molten pool from overheating, and the metal grains in the heat affected zone grow up and burn through, because the flame is directed at the cold metal with incomplete penetration. When the base metal thickness is greater than 5㎜, the correct welding method can be adopted. The advantages of this method are that the welding wire is behind the welding torch and the flame points to the weld, so the heat loss is small, the penetration depth is large and the heating efficiency is high.

When gas welding thin parts with thickness less than 3㎜, the inclination angle of welding gun is 20 ~ 40; When gas welding thick parts, the inclination angle of welding torch is 40 ~ 80, and the included angle between welding wire and welding torch is 80 ~100. When gas welding aluminum alloy, try to weld the joint at one time, and don't pile up the second layer, because the second layer will cause slag inclusion in the weld.