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Heterogeneous metals refer to metals composed of different elements (such as aluminum, copper, etc.), or certain alloys formed from the same base metal but with significant differences in metallurgical properties such as physical and chemical characteristics (such as carbon steel, stainless steel, etc.). They can be used as base metal, filler metal, or weld metal. Welding of dissimilar materials refers to the process of welding two or more different materials (with differences in chemical composition, microstructure, properties, etc.) under certain processing conditions. In the welding of dissimilar metals, the most common case is the welding of dissimilar steels, followed by the welding of dissimilar non-ferrous metals and the welding of steel with non-ferrous metals. From the perspective of joint types, there are also three basic situations: joints between two different metal bases, joints where the base metals are the same but the filler metals differ (such as joints formed by using austenitic welding materials to weld medium-carbon quenched and tempered steel), and welded joints of composite metal sheets. Welding of dissimilar materials involves joining two different metals together, which necessarily results in the formation of a transition layer whose properties and structure differ from those of the base material. Due to the significant differences between dissimilar metals in terms of elemental properties, physical properties, and chemical properties, welding dissimilar materials is much more complex than welding similar materials, both in terms of welding mechanisms and operational techniques. The main problems in welding dissimilar materials are as follows: 1. The greater the difference in melting points between the dissimilar materials, the more difficult it is to weld them together. This is because when materials with low melting points reach their melting state, those with high melting points remain in a solid state. At this point, the melted material can easily penetrate into the grain boundaries of the superheated area, leading to the loss of the low-melting-point materials as well as the burnout or evaporation of alloying elements, thereby making it difficult to form a proper weld joint. For example, when welding iron and lead (which have a large difference in melting points), not only do the two materials not dissolve into each other in their solid state, but they also do not dissolve into each other in their liquid state as well; the liquid metals separate into layers, and upon cooling, they crystallize separately. 2. The greater the difference in linear expansion coefficients between different materials, the more difficult it is to weld them together. The greater the linear expansion coefficient of a material, the higher its thermal expansion rate, and the greater its contraction upon cooling; this results in significant welding stresses during crystal formation in the molten pool. This type of welding stress is difficult to eliminate, resulting in significant welding deformation. Due to the different stress conditions experienced by the materials on either side of the weld, cracks can easily form in the weld and the heat-affected zone, and this may even lead to the separation of the weld metal from the base material. 3. The greater the difference between the thermal conductivity and specific heat capacity of different materials, the more difficult it is to weld them together. The thermal conductivity and specific heat capacity of the material can deteriorate the crystallization conditions of the weld metal, causing severe grain coarsening and affecting the wettability of refractory metals. Therefore, a powerful heat source should be used for welding, and the position of the heat source during welding should be placed on the side of the base material with better thermal conductivity. 4. The greater the difference in electromagnetic properties between different materials, the more difficult it is to weld them together. Because the greater the difference in the electromagnetic properties of the materials, the more unstable the welding arc becomes, and the poorer the quality of the weld. 5. The more intermetallic compounds formed between different materials, the more difficult it is to weld them. Due to the high brittleness of intermetallic compounds, they can easily cause cracks in the welds, or even lead to their failure. 6. During the welding of dissimilar materials, changes in the microstructure of the welded area or the formation of new microstructures can degrade the properties of the welded joint, posing significant challenges to the welding process. The mechanical properties of the joint fusion zone and heat-affected zone are poor, especially with a significant decline in plasticity and toughness. Due to the decrease in the plasticity and toughness of the joint as well as the presence of welding stresses, welded joints of dissimilar materials are prone to cracking, especially in the welding heat-affected zone, where cracking and even fracture can occur. 7. The stronger the oxidizing property of dissimilar materials, the more difficult it is to weld them. When welding copper and aluminum using fusion welding methods, oxides of copper and aluminum are very likely to form in the molten pool. During cooling and crystallization, the oxides present at the grain boundaries can reduce the intergranular bonding strength. 8. When welding dissimilar materials, it is difficult for the weld and the two base metals to achieve equal strength. This is because metal elements with low melting points are prone to burnout and evaporation during welding, which alters the chemical composition of the weld and reduces its mechanical properties, especially when welding different types of non-ferrous metals.