Welding of dissimilar metals

Thread Content

I. 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 high 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 seam. It is generally believed that the decarburized layer undergoes significant changes (usually deterioration) in its microstructure and properties due to the reduction in carbon content; this makes the area prone to premature failure during service. The failure locations 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 leads to uneven chemical composition in the welded joint. 7. Heterogeneity of the microstructure. Due to the discontinuity in the chemical composition of the weld joint, after undergoing the welding thermal cycle, different microstructures appear in various regions of the weld joint; in some areas, extremely complex structural patterns often form. 8. Discontinuity in performance. Differences in the chemical composition and microstructure of the welded joint result in variations in its mechanical properties. 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 inhomogeneity causes different regions of the welded joint to exhibit vastly different behaviors under the same conditions, resulting in weakened and strengthened areas. Particularly at high temperatures, dissimilar metal welded joints often suffer from premature failure during service. II. 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 a high energy density of the heat source 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 electrode, and additional non-energized wire can be employed. But in any case, with fusion welding, some of the base metal always melts into the weld, causing dilution. Additionally, intermetallic compounds, eutectics, and the like 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 continuous improvements and refinements in fusion welding methods and process techniques, it remains difficult to resolve all the issues that arise when welding different types of metals. This is because there are a wide variety of metal types, 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 welded to a plastic state, or even without 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 requirements, 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 (as in spot welding). Since spot welding does not involve heating or uses a low heating temperature, it can reduce or eliminate the adverse effects of heat cycles on the mechanical properties of the base material, thereby preventing the formation of brittle intermetallic compounds. Certain forms of pressure welding can even force 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 workpiece must have a rotational body cross-section ； Explosive welding is only suitable for joining larger areas, etc. Welding equipment is not yet widely available either. These undoubtedly all limit the scope of application for pressure welding. 3. Others: Besides fusion welding and pressure welding, there are also some 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 metal is essentially a fusion welding process of the same type of metal, and there are 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 an external filler metal. Of course, this requires that a low-melting-point eutectic can be formed 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 completely into the base materials and become uniform. In this way, a joint between dissimilar metals without any interlayer material is created. In such methods, a small amount of liquid metal is produced 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 workpieces
a. From the perspective of equal strength, select an electrode that meets the mechanical properties of the base metal. Alternatively, considering the weldability of the base metal, one may opt for an electrode that does not ensure equal strength but offers good weldability. However, the structural form of the weld must be taken into account to ensure that both equal strength and equal stiffness requirements are met. b. Make its alloy composition meet or be close to that of the base material. c. When the base metal contains relatively 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 titanate-type electrodes. If the problem cannot be resolved, low-hydrogen sodium-type electrodes can be used. 2. Consider the operating conditions and performance requirements of the welded parts. a. 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. b. 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, as well as whether intergranular corrosion is a concern or not. c. When operating under wear conditions, it is necessary to distinguish whether it is normal wear or impact wear, and whether the wear occurs at normal temperature or high temperature. d. 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 weld cracks, and the welding position. a. For weldments with complex shapes or large thicknesses, the weld metal experiences high contraction stresses during cooling, which can 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. b. For weldments that cannot be flipped due to constraints, electrodes capable of welding in all positions should be used. c. 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. For certain steels (such as pearlitic heat-resistant steels), post-weld thermal stress relief is required; however, when heat treatment cannot be carried out due to equipment limitations (or structural constraints of the component itself). 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 welding electrodes with lower prices should be preferred over alkaline welding electrodes. Among acidic welding electrodes, those with titanium or titancalcium coatings are more expensive; given China’s mineral resources, welding electrodes with ferrotitanium coatings should be widely promoted.