1. Problems in welding of dissimilar metals
Some inherent problems of dissimilar metal welding hinder its development, such as the composition and performance of the dissimilar metal fusion zone. Most of the damage to the dissimilar metal welding structure occurs in the fusion zone. Due to the different crystallization characteristics of the welds near the fusion zone, It is easy to form a transition layer with poor performance and varying composition.
In addition, due to a long time at high temperatures, the diffusion layer in this area will expand, which will further increase the unevenness of the metal. Moreover, when dissimilar metals are welded or after heat treatment or high-temperature operation, it is often found that the carbon on the low alloy side "migrates" through the weld boundary to the high alloy weld, forming a decarburized layer on both sides of the fusion line. And the carburization layer, the decarburization layer is formed on the low alloy side of the base material, and the carburization layer is formed on the high alloy weld side.
Obstructing and preventing 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, impact, bending, etc.) of the welded joint area of dissimilar metals are generally better than the performance of the base material to be welded, but the performance of the joint area is inferior to the base material at high temperature or after long-term operation at high temperature.
（2）There is a martensite transition zone between the austenite weld and the pearlite base metal. This zone has low toughness and is a high-hardness brittle layer. It is also a weak zone that leads to failure and destruction of components. It will reduce the welded structure. Reliability of use.
（3）Carbon migration during post-weld heat treatment or high-temperature operation will result in the formation of a carburized layer and a decarburized layer on both sides of the fusion line. It is generally believed that the reduction of carbon in the decarburized layer leads to a large change (generally deterioration) in the organization and performance of the area, which makes the area prone to early failure during service. Many high-temperature pipelines in service or high-temperature pipelines in tests fail in the decarburized layer.
（4）Failure is related to conditions such as time, temperature, and alternating stress.
（5）The 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 obvious difference between the metal on both sides of the weld and the alloy composition of the weld, during the welding process, the base metal and the welding material will melt and mix with each other. The uniformity of the mixing varies with the welding process. Change, and the degree of uniformity of mixing at different positions of the welded joints is also very different, which causes the inhomogeneity of the chemical composition of the welded joints.
（7）The inhomogeneity of the metallographic structure.
Due to the discontinuity of the chemical composition of the welded joint, after the welding thermal cycle, different structures appear in each area of the welded joint, and very complex structures often appear in some areas.
（8）Discontinuity of performance.
The difference in chemical composition and metallographic structure of welded joints brings about different mechanical properties of welded joints. The strength, hardness, plasticity, toughness, impact performance, high-temperature creep, and durability of each area along the welded joint are very different. This significant inhomogeneity makes the behavior of different areas of the welded joint under the same conditions very different, and weakened areas and strengthened areas appear, especially under high-temperature conditions, the dissimilar metal welded joints are in the service process Early failures often occur in the medium.
2. The characteristics of different welding methods when welding dissimilar metals
Most welding methods can be used for the welding of dissimilar metals, but the characteristics of dissimilar metals should still be considered when selecting the welding method and formulating process measures. According to the different requirements of the base material and the welded joint, fusion welding, pressure welding, and other welding methods are all used in the welding of dissimilar metals, but they all have their own advantages and disadvantages.
（1）Fusion welding
Fusion welding methods are commonly used in dissimilar metal welding. Commonly used fusion welding methods include electrode arc welding, submerged arc welding, gas-shielded arc welding, electroslag welding, plasma arc welding, electron beam welding, laser welding, etc. In order to reduce dilution, reduce the fusion ratio or control the melting amount of different metal base materials, usually, electron beam welding, laser welding, plasma arc welding, and other methods with a higher energy density of the heat source can be used.
In order to reduce the penetration depth, process measures such as indirect arc, swing welding wire, ribbon electrode, and additional non-current welding wire can be adopted. But in any case, as long as it is fusion welding, part of the base metal will always melt into the weld and cause dilution. In addition, intermetallic compounds, eutectic, etc. will also be formed. In order to alleviate such adverse effects, it is necessary to control and shorten the residence time of the metal in the liquid or high-temperature solid-state.
However, despite the continuous improvement and perfection of fusion welding methods and technological measures, it is still difficult to solve all the problems in welding of dissimilar metals, because there are many kinds of metals, performance requirements are diverse, and joint forms are different, and in many cases, it is still necessary. Use pressure welding or other welding methods to solve the welding problem of specific dissimilar metal joints.
（2）Pressure welding
Most pressure welding methods only heat the metal to be welded to a plastic state or even without heating and apply a certain pressure as the basic feature. Compared with fusion welding, pressure welding has certain advantages when welding dissimilar metal joints. As long as the joint form allows and the welding quality can meet the requirements, pressure welding is often a more reasonable choice.
During pressure welding, the surface of dissimilar metals can be melted or not, but due to the effect of pressure, even if there is molten metal on the surface, it will be squeezed and discharged (such as flash welding and friction welding), only a few cases After pressure welding, the metal that was once melted (such as spot welding) remains.
Since pressure welding is not heated or has a low heating temperature, it can reduce or avoid the adverse effects of thermal cycling on the properties of the base metal and prevent the production of brittle intermetallic compounds. Some forms of pressure welding can even squeeze the produced intermetallic compound out of the joint. In addition, there is no problem with weld metal properties changes caused by dilution during pressure welding.
However, most pressure welding methods have certain requirements for the joint form. For example, spot welding, seam welding, and ultrasonic welding must use lap joints; in friction welding, at least one workpiece must have a cross-section of a rotating body; explosive welding is only suitable for Larger area connections, etc. Pressure welding equipment is not yet popular. These undoubtedly limit the application range of pressure welding.
（3）Other methods
In addition to fusion welding and pressure welding, there are some methods that can be used for dissimilar metal welding. For example, brazing is a method of welding dissimilar metals between brazing material and base metal, but what is discussed here is a more special brazing method.
One method is called fusion welding-brazing, which is called fusion welding for low melting point base materials in dissimilar metal joints, and brazing for high melting point base materials. And usually, the same metal as the low melting point base material is used as the brazing filler metal. Therefore, the soldering process between the solder and the low melting point base metal is the fusion welding process of the same metal, and there is no special difficulty.
The brazing process between the brazing filler metal and the high melting point base metal does not cause the base metal to melt or crystallize, which can avoid many weldability problems, but the filler metal is required to wet the base metal well.
Another method is called eutectic brazing or eutectic diffusion brazing. This is to heat the contact surface of dissimilar metals to a certain temperature so that the two metals form a low-melting eutectic at the contact surface. The low-melting eutectic is liquid at this temperature and essentially becomes a kind of non-additional solder. Brazing method.
Of course, this requires the formation of a low-melting eutectic between the two metals. During diffusion welding of dissimilar metals, the intermediate layer material is added, heated under very low pressure to melt the intermediate layer material, or in contact with the welded metal to form a low melting point eutectic. All the intermediate layer materials are diffused into the base material and homogenized so that dissimilar metal joints without intermediate materials can be formed.
In this type of method, a small amount of liquid metal will appear during the welding process. Therefore, it is also called liquid phase transition welding, and their common feature is that there is no casting structure in the joint.
