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Difference in linear expansion coefficients: Different metals have varying linear expansion coefficients. When two metals with significantly different such coefficients are welded together, it results in a complex state of high internal stresses in the weld joint, with one metal experiencing compressive stresses while the other experiences tensile stresses. Metal subjected to tensile stress can develop cracks, and it may even lead to the separation of the weld from the base metal. The problem becomes even more severe if the structure undergoes repeated heating and cooling cycles. This is exactly the case when welding austenitic stainless steels with other non-austenitic steels; therefore, when welding such dissimilar steels, special care must be taken to prevent the occurrence of the aforementioned defects. Preheating metals with a low linear expansion coefficient before welding, or inserting a metal with good plasticity between two metals with large differences in linear expansion coefficient to form a welded transition joint that serves as a buffer zone, are both effective methods. The linear expansion coefficient also changes with temperature; the figure below shows the curves depicting how the linear expansion coefficient of various steel grades varies with temperature. Image: Differences in thermal conductivity and specific heat capacity. The thermal conductivity and specific heat capacity of metals have a significant impact on the melting of the materials to be welded, the formation of the weld pool, the temperature field in the welding area, and the crystallization process of the weld metal. In metals with high thermal conductivity, the heat from the welding pool is quickly carried away by the unmelted base material, resulting in insufficient melting of the metal ； Increasing the cooling rate during cooling is equivalent to reducing the welding heat input. When the two metals differ significantly in this regard, it can cause the melting of the metals to be asynchronous, leading to poor formation of the molten pool and inadequate bonding between the metals. This results in unfavorable crystallization conditions in the weld, as well as poor weld quality and shape. To overcome this difference, during fusion welding, the heat source should generally be positioned on the side of the metal with better thermal conductivity, or the base material with high thermal conductivity should be preheated. For example, when welding pure copper with 18-8 austenitic stainless steel, since the thermal conductivity of copper is 20 times higher than that of 18-8 austenitic stainless steel, it is necessary to concentrate most of the heat from the heat source on the side of the pure copper that is to be welded, in order to ensure that the metals on both sides of the weld zone melt and solidify uniformly and simultaneously. For metals with a low specific heat capacity, this is equivalent to metals with a low thermal conductivity; the same welding heat input can melt more base metal. When the specific heat capacities of the two metals differ greatly, the metal with the lower specific heat capacity will melt more, which is equivalent to an increased welding heat input. The heat source location should be on the side of the material with a higher specific heat capacity. Difference in melting points: If the melting points of two metals differ significantly, it can lead to an uneven amount of melting for each metal. This may result in the metal with a lower melting point melting in excessive amounts, while the metal with a higher melting point melts in insufficient amounts; in extreme cases, one of the metals may not melt at all. As a result, fusion welding is difficult to use, but methods such as brazing and pressure welding can be employed. A large difference in melting point between the two base metals, or between the weld metal and one of the base metals, can cause the metal with the lower melting temperature to be damaged. Excessive shrinkage of metals with higher melting points can generate tensile stresses in another metal with a lower melting point that is in a weaker state; in severe cases, this can lead to cracks or even delamination. Applying one or more layers of material with a higher melting temperature to the surface of a metal with a high melting temperature can solve this problem; this method is known as interlayer cladding. When this barrier surfacing layer is welded to another metal with a lower melting temperature, the barrier layer serves to reduce the difference in melting points. Differences in electromagnetic properties: When welding dissimilar metals together, it sometimes happens that the welding arc deviates from its intended path, or the arc burns unevenly, resulting in poor weld quality. This is often caused by significant differences in the electromagnetic properties of the two metals, such as in the case of welding non-austenitic steels with other non-magnetic metals (aluminum, copper, etc.), or austenitic stainless steels with other types of steel. For heat sources with a low density but a large cross-sectional area (such as arc welding), this electromagnetic difference has not yet had a significant impact on the welding process ； For heat sources with a high density and a small cross-sectional area, this electromagnetic difference has a significant impact on the welding process. For example, when welding copper and low-carbon steel using an electron beam, if the electron beam is directed toward the joint between copper and steel or at the copper base material (in general, in copper-steel dissimilar metal welding, since copper has a much higher thermal conductivity than steel, the heat source should be positioned on the side of copper), it is observed that the electron beam shifts toward the side of the low-carbon steel base material. This phenomenon is caused by the differences in electromagnetic properties between the two metals.