Metal composite panels often face a series of technical challenges during welding due to their unique double- or multi-layer structure. The most prominent of these is the welding compatibility issue caused by the difference in materials between the base layer and the cladding layer. The base layer is typically carbon steel or low-alloy steel, while the cladding layer is often made of corrosion-resistant and high-temperature-resistant materials such as stainless steel, titanium alloys, or copper alloys. Significant differences exist between the two in chemical composition, physical properties, and metallurgical characteristics, which can easily lead to a series of problems during welding.
The primary problem caused by material differences is the dilution and performance degradation of the weld joint. When the base layer and cladding layer are directly welded, the base metal easily melts into the cladding weld at high temperatures, diluting key alloying elements in the cladding layer, such as chromium and nickel in stainless steel. This results in decreased corrosion resistance of the weld and may even lead to the formation of a brittle martensitic structure, reducing joint toughness. The key to solving this problem lies in introducing a transition layer weld using high-alloy welding materials, such as nickel-based alloys, to compensate for the dilution of the cladding weld by the base layer, ensuring that the weld composition is similar to that of the cladding layer, thereby maintaining its corrosion resistance and mechanical properties.
Carbon migration is another common problem, especially in the welding of stainless steel composite panels. At high temperatures, carbon readily diffuses from the base layer to the cladding layer, forming a carburized layer in the cladding weld, reducing its corrosion resistance. Simultaneously, a decarburized layer forms near the fusion line of the base layer, weakening the joint strength. Effective measures to control carbon migration include optimizing the welding sequence, welding the base layer first to reduce the cladding layer's exposure time at high temperatures; employing low heat input welding methods, such as pulsed TIG welding, to reduce the width of the heat-affected zone; and adding strong carbide-forming elements, such as niobium and vanadium, to the transition layer welding material to fix carbon and inhibit its migration.
Changes in the properties of the heat-affected zone are also a concern when welding metal composite panels. Differences in the coefficients of thermal expansion and thermal conductivity of different materials lead to thermal stress during welding, easily inducing cracks, especially in cladding materials such as titanium alloys, which are sensitive to thermal stress and prone to cold cracking. To reduce thermal stress, segmented back-welding can be used to control welding deformation; post-weld local heat treatment, such as low-temperature tempering, can eliminate residual stress; and optimizing the bevel design to reduce the fusion ratio and lower heat input are also effective methods.
Residual stress management in welded joints is equally important. During dissimilar metal welding, the difference in linear expansion coefficients and elastic moduli can easily generate significant residual stress in the joint, affecting its fatigue strength and corrosion resistance. Using high-nickel-based welding electrodes with a linear expansion coefficient close to that of the base metal and good plasticity to weld the transition layer can concentrate thermal stress at the fusion line on the stainless steel side. Through the plastic deformation of the stainless steel, the adverse effects of thermal stress and thermal fatigue stress can be reduced.
Furthermore, during welding, care must be taken to prevent the cladding weld from melting into the base metal, avoiding overheating of the cladding layer and the precipitation of chromium carbide, which compromises corrosion resistance. This requires strict control of welding heat input, using low current and rapid welding, avoiding lateral electrode oscillation, and ensuring that the weld bead formed in a single pass is not too wide. In multi-layer welding, the previous layer should be allowed to cool to an appropriate temperature before welding the next layer to prevent overheating of the weld metal.
For welding metal composite panels, a customized welding process must be developed based on the specific material and structural characteristics. For example, when welding titanium-steel composite plates, niobium foil can be added between the titanium and steel as an interlayer to prevent the titanium and steel from fusing together and avoid the formation of brittle phases. At the same time, strengthening pre-welding preparation, such as thoroughly cleaning weld oxides and selecting appropriate welding materials, is also crucial to ensuring weld quality.
