Aluminum alloy welding heat input is easy to quickly lose to the base metal, and it is easy to cause aluminum and aluminum alloys to produce unfused defects.
The chemical properties of aluminum and aluminum alloys are very active, and it is easy to form a refractory oxide film on the surface (for example, the melting point of Al2O3 is about 2050°C, the melting point of MgO is about 2500°C), and the thermal conductivity of aluminum and aluminum alloy is very strong. Welding heat input is easy to quickly lose to the base metal, so it is easy to cause aluminum and aluminum alloys to produce unfused defects. The main problems of aluminum and aluminum alloys in welding production are as follows:
1. The specific heat capacity and thermal conductivity of aluminum are large
The specific heat capacity and thermal conductivity of aluminum are larger than that of steel. Therefore, the heat input of the welding process is lost due to the rapid conduction to the base metal. Therefore, when welding by fusion welding, a highly concentrated heat source is required for welding in order to obtain high-quality welding. For joints, it is sometimes necessary to use preheating process measures to achieve the fusion welding process; when welding aluminum and aluminum alloys by resistance welding, it is necessary to use an extra-high-power power source welding.
2. Larger linear expansion coefficient
The coefficient of linear expansion of aluminum and aluminum alloys is larger, about twice that of steel, and the volume shrinkage rate during solidification is about 6.5%. Therefore, the weldment is prone to large welding deformation.
3. The affinity of aluminum and oxygen is large
Aluminum has a high affinity for oxygen and is extremely easy to oxidize. During the welding process, aluminum and aluminum alloys oxidize on the welding surface to form a high-density (3.85g/cm3) oxide film (Al2O3) with a melting point of up to 2050°C. This oxide film will hinder the good combination of molten metals during the welding process, which is easy Cause slag inclusion.
4. Prone to pores
The most likely defect in the welding process of aluminum and aluminum alloy is the hydrogen hole. This is because there is always a certain amount of moisture in the space of the welding arc column, especially in the humid season or humidity. When welding in large areas, the hydrogen decomposed by the moisture in the arc column atmosphere will dissolve into the overheated molten pool metal. When solidified at low temperature, the solubility of hydrogen will change greatly and drop sharply. It cannot be precipitated before the pool is solidified, and hydrogen pores are formed when it remains in the weld.
Secondly, the moisture adsorbed in the oxide film of the welding wire and weldment is also an important cause of pores. The oxide film of Al-Mg alloy is not dense and has strong water absorption. Therefore, Al-Mg alloy has a greater tendency to produce pores than pure aluminum with dense oxide film.
5. No color change when aluminum and aluminum alloy are melted
When aluminum and aluminum alloy change from solid to liquid during welding, there is no obvious color change. Therefore, it is difficult for welders to control the heating temperature. In addition, due to the low strength of aluminum and aluminum alloys at high temperatures (the strength of aluminum is only 10 MPa at 370°C), it is easy to cause the weld pool to collapse or the weld pool metal to leak. Therefore, a backing plate must be added to the back of the weld during welding.
6. Welding hot cracks
During the welding of aluminum and aluminum alloys, the thermal cracks that appear in the weld metal and the near-joint area are mainly metal solidification cracks, and liquefaction cracks can also be seen in the near-joint area. The existence of fusible eutectic is an important cause of solidification cracks in aluminum and aluminum alloy welds. The coefficient of linear expansion of aluminum and aluminum alloys is twice that of steel. When welding under constrained conditions, greater welding stress is generated, which is also one of the reasons why aluminum and aluminum alloys have a greater tendency to crack.
7. Equivalent strength of welded joints
Except for Al-Zn-Mg alloys, energy-aging-strengthened aluminum alloys, whether they are welded in an annealed state or in an aging state, have a lower welding strength than the base metal without heat treatment after welding. Non-aging hardened aluminum alloys, such as A-Mg alloy, have the same strength as the base metal when welded in the annealed state; the welded joint strength is lower than the base metal when welded in the cold-worked state.
The unequal strength performance of aluminum and aluminum alloy welding indicates that the welded joint has softened to a certain extent or there is a weak link in performance. The weak link in the performance of this joint can exist in any area of the weld, the fusion zone or the heat-affected zone.
(1) Since the weld zone is a cast structure, the strength difference with the base metal may not be much, but the plasticity of the weld is generally not as good as the base metal. At the same time, the greater the welding heat input, the greater the tendency for the performance of the weld to decline.
(2) For the non-aging strengthened aluminum alloy in the fusion zone, the main problem in the fusion zone is the reduction of plasticity due to the coarsening of the crystal grains; when the aging strengthened aluminum alloy is welded, not only the grains are coarsened, but also may be caused by the liquefaction of the grain boundaries. crack. Therefore, the main problem in the weld fusion zone is the deterioration of plasticity.
(3) The post-welding performance of the non-aging strengthened aluminum alloy in the heat-affected zone and the energy-aging strengthened aluminum alloy is mainly the softening of the weld metal.
8. Corrosion resistance of welded joints
After aluminum and aluminum alloys are welded, the corrosion resistance of welded joints is generally lower than that of the base metal. The main reasons that affect the corrosion resistance of welded joints are as follows:
1) Due to the inhomogeneity of the welded joint structure, the electrode potential of each part of the welded joint is uneven. Therefore, the heat treatment before and after welding will affect the corrosion resistance of the joint.
2) A large amount of impurities, coarse crystal grains, and precipitation of brittle phases will significantly reduce the corrosion resistance. Therefore, the purity and compactness of the weld metal is one of the reasons that affect the corrosion resistance of the joint.
3) Welding stress also affects corrosion resistance.
The above are the main features of aluminum alloy welding.
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