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Problems in welding dissimilar metals Some inherent problems associated with welding dissimilar metals hinder its development. These include the composition and properties of the fusion zone; failures in welded structures made of dissimilar metals usually occur in this fusion zone. Due to the different crystallization characteristics of the welds in the areas adjacent to the fusion zone, it is easy for transition layers with poor properties and altered compositions to form. Furthermore, due to being exposed to high temperatures for an extended period, the diffusion layer in this area expands, which further increases the unevenness of the metal. Moreover, during the welding of dissimilar metals, or after welding followed by heat treatment or operation at high temperatures, it is common to observe carbon from the low-alloy side migrating into the high-alloy weld across the weld boundary, resulting in a decarburized layer on the low-alloy side of the base material and a carburized layer on the high-alloy weld side. The obstacles and barriers to the use and development of dissimilar metal structures are mainly manifested in the following aspects: 1. At room temperature, the mechanical properties (such as tensile strength, impact resistance, bending strength, etc.) of the weld zone in dissimilar metal joints are generally superior to those of the base materials being welded; however, at high temperatures or after prolonged operation at such temperatures, the properties of the joint zone become inferior to those of the base materials. 2. There is a martensitic transition zone between the austenitic weld and the pearlitic base metal; this zone has low toughness and is a layer of high hardness and brittleness. It represents a weak point that can lead to failure of the component, and it reduces the reliability of the welded structure. 3. Carbon migration during post-weld heat treatment or high-temperature operation can result in the formation of carburized and decarburized layers on either side of the weld joint. It is generally believed that the decarburized layer causes significant changes in the microstructure and properties of that region due to the reduction in carbon content (usually leading to deterioration), which makes that region prone to premature failure during service. The failure sites of many high-temperature pipelines in service or under testing are concentrated in the decarburized layer. 4. Failure is related to conditions such as time, temperature, and alternating stress. 5. Post-weld heat treatment cannot eliminate the residual stress distribution in the joint area. 6. Inhomogeneity of chemical composition. When welding dissimilar metals, due to the significant differences in the alloy compositions of the metals on either side of the weld and those of the weld itself, both the base material and the filler metal melt and mix with each other during welding. The degree of mixing varies depending on the welding process, and it also differs greatly in different parts of the welded joint, which results in uneven chemical composition in the welded joint. 7. Heterogeneity of the microstructure. Due to the discontinuity in the chemical composition of the welded joint, after undergoing the welding heat cycle, different microstructures appear in various regions of the joint; often, extremely complex microstructural patterns are formed in certain areas. 8. Discontinuity in performance. Differences in the chemical composition and microstructure of the welded joint result in varying mechanical properties of the welded joint. There are significant differences in strength, hardness, plasticity, toughness, impact resistance, high-temperature creep, and endurance across various regions of the welded joint. This significant unevenness causes different regions of the welded joint to exhibit very different behaviors under the same conditions, resulting in areas of weakened strength and areas of enhanced strength. Especially under high-temperature conditions, dissimilar metal welded joints often experience premature failure during service. 2. Characteristics of welding dissimilar metals using different welding methods. Most welding methods can be used for welding dissimilar metals, but when selecting a welding method and formulating process parameters, the characteristics of welding such metals must still be taken into account. Depending on the different requirements of the base material and the welded joint, fusion welding, pressure welding, and other welding methods are all used in welding dissimilar metals, each with its own advantages and disadvantages. 1. Fusion welding: The fusion welding method is widely used in welding dissimilar metals. Common fusion welding methods include shielded metal arc welding, submerged arc welding, gas shielded arc welding, electroslag welding, plasma arc welding, electron beam welding, laser welding, etc. To reduce dilution, lower the fusion ratio, or control the amount of melting of different metal bases, methods with higher heat source energy density such as electron beam welding, laser welding, and plasma arc welding are commonly used. To reduce the penetration depth, process measures such as indirect arc, oscillating wire, strip electrodes, and additional non-energized wires can be employed. But in any case, as long as it is fusion welding, some of the base material always melts into the weld, causing dilution; in addition, intermetallic compounds, eutectics, and similar substances are also formed. To mitigate such adverse effects, it is necessary to control and shorten the residence time of the metal in a liquid state or in a high-temperature solid state. However, despite the continuous improvement and refinement of fusion welding methods and process techniques, it remains difficult to resolve all the problems that arise when welding different metal types. This is because there are a great variety of metals, with diverse performance requirements and different joint configurations; in many cases, pressure welding or other welding methods must be employed to address the welding challenges associated with specific dissimilar metal joints. 2. Pressure welding: Most pressure welding methods involve heating the metals to be joined only to a plastic state, or even without any heating at all, with the application of certain pressure being the key feature. Compared to fusion welding, pressure welding has certain advantages when joining dissimilar metals; as long as the joint configuration permits it and the welding quality meets the required standards, using pressure welding is often a more reasonable choice. During pressure welding, the interface between different metals can melt or it may not; however, due to the applied pressure, even if metal melts at the surface, it is forced out as a result (as in flash welding and friction welding). In only a few cases does the melted metal remain after pressure welding (such as in spot welding). Since spot welding does not involve heating or uses a low heating temperature, it can reduce or avoid the adverse effects of thermal cycling on the mechanical properties of the base material, thereby preventing the formation of brittle intermetallic compounds. Certain forms of pressure welding can even squeeze the intermetallic compounds that have formed out of the joint. Furthermore, there is no issue of changes in the weld metal properties due to dilution during pressure welding. However, most pressure welding methods have certain requirements regarding the type of joint; for example, spot welding, seam welding, and ultrasonic welding require lap joints ； During friction welding, at least one of the workpieces must have a cross-section of a rotating body ； Explosive welding is only suitable for joining larger areas, etc. Soldering equipment is not yet widespread either. These undoubtedly all limit the scope of application for pressure welding. 3. Other methods: In addition to fusion welding and pressure welding, there are also some other methods that can be used for welding dissimilar metals. For example, brazing is a method of welding dissimilar metals together using a filler metal; however, what is discussed here is a more specialized form of brazing. There is a method called fusion welding – brazing, in which fusion welding is used for the side of the dissimilar metal joint with the lower melting point material, while brazing is used for the side with the higher melting point material. Moreover, the filler metal is usually the same metal as the base material with a low melting point. Therefore, the process of fusion welding between the filler metal and the low-melting-point base material is a welding process of the same metal, with no special difficulties. Between the filler metal and the high-melting-point base material, a brazing process takes place; the base material does not melt or crystallize, which allows many welding-related problems to be avoided. However, it is required that the filler metal can wet the base material well. Another method is called eutectic brazing or eutectic diffusion brazing. This involves heating the contact surfaces of different metals to a certain temperature, causing the two metals to form a eutectic with a low melting point at those contact surfaces; this low-melting-point eutectic remains in a liquid state at this temperature, effectively constituting a brazing method that does not require the use of external filler metal. Of course, this requires the formation of a eutectic with a low melting point between the two metals. In the diffusion welding of dissimilar metals, an interlayer material is used; heating at very low pressures causes this interlayer material to melt, or it comes into contact with the metals to be welded to form a eutectic with a low melting point. The thin layer of liquid that forms is then held at a constant temperature for a certain period of time, allowing the interlayer material to diffuse entirely into the base materials and become uniform. In this way, a joint between dissimilar metals without any interlayer material is created. These methods all result in a small amount of liquid metal during the welding process. Therefore, it is also known as liquid-phase transition welding; their common feature is the absence of cast structure in the joint. III. Precautions for welding dissimilar metals 1. Consider the physical, mechanical properties, and chemical composition of the welded parts. (1) Based on the principle of equal strength, select electrodes that meet the mechanical properties of the base material; or, taking into account the weldability of the base material, use electrodes that do not provide equal strength but have good weldability. However, the structural form of the weld should be considered to ensure equal strength and stiffness. (2) Make its alloy composition meet or be close to that of the base material. (3) When the base metal contains high levels of harmful impurities such as C, S, and P, welding electrodes with good crack resistance and porosity resistance should be selected. It is recommended to use calcium titanium oxide-type electrodes. If the problem cannot be resolved, low-hydrogen sodium-type electrodes can be used. 2. Consider the operating conditions and performance of the welded parts. (1) When subjected to dynamic and impact loads, in addition to ensuring strength, high requirements are placed on impact toughness and elongation; therefore, low-hydrogen, calcium-titanium, and iron-oxide type electrodes should be selected from the outset. (2) For those exposed to corrosive media, appropriate stainless steel welding electrodes must be selected based on the type and concentration of the medium, the operating temperature, and whether intergranular corrosion is a concern. (3) When operating under wear conditions, it is necessary to distinguish between general wear and impact wear, as well as wear at normal temperature versus high temperature. (4) When operating under non-ambient conditions, welding electrodes that ensure mechanical properties at low or high temperatures should be selected. 3. Consider the complexity of the shape of the welded assembly, its stiffness, the condition of the welding cracks, and the welding position. (1) For weldments with complex shapes or large thicknesses, the weld metal experiences high shrinkage stresses during cooling, which can easily lead to cracks; therefore, welding electrodes with strong crack resistance must be used, such as low-hydrogen electrodes, highly ductile electrodes, or iron-oxide electrodes. (2) For weldments that cannot be flipped due to constraints, electrodes capable of welding in all positions must be used. (3) For weldments where the welding area is difficult to clean, use acidic electrodes with strong oxidizing properties that are not sensitive to scale and oil stains, in order to avoid defects such as pores. 4. Consider the equipment at the welding site. In areas where there is no DC welder, it is not advisable to use electrodes that require a DC power supply; instead, electrodes compatible with both AC and DC power supplies should be chosen. Certain steels (such as pearlitic heat-resistant steels) require heat stress relief after welding, but when heat treatment is not possible due to equipment limitations (or inherent structural constraints). Welding electrodes made of non-base metal materials (such as austenitic stainless steel) should be used, eliminating the need for post-weld heat treatment. 5. Consider improving the welding process and protecting workers’ health. Where both acidic and alkaline electrodes can meet the requirements, acidic electrodes should be used as much as possible. 6. Consider labor productivity and economic viability. When the performance is the same, acidic electrodes with lower prices should be preferred over basic electrodes. Among acidic electrodes, those with titanium or titancalcium coatings are more expensive; given China’s mineral resources, electrodes with ferrotitanium coatings should be widely promoted.