Dissimilar metals refer to metals composed of different elements, such as aluminum and copper, or alloys formed from the same base metals with distinct differences in their metallurgical properties, such as physical and chemical properties. They can be used as base metal, filler metal or weld metal.
Dissimilar material welding refers to the process of joining two or more materials with different chemical compositions, metallurgical structures and performance under specific process conditions.
The most common type of dissimilar metal welding is dissimilar steel welding, followed by dissimilar nonferrous metal welding, and steel and nonferrous metal welding.
Related Reading: Ferrous vs Non-Ferrous Metals
In terms of joint shape, there are three basic scenarios: the joining of two different base metals, the joining of the same base metal but with different filler metals (such as medium carbon quenched and tempered steel joined with austenitic welding materials), and the joining of composite metal plates.
Dissimilar material welding involves welding two different metals, resulting in a transition layer with different properties and microstructure than the base metal.
Welding dissimilar metals is much more complex than that of similar materials in terms of welding mechanism and technological operation due to significant differences in elementary properties, physical properties and chemical properties.
The main challenges in welding different materials are as follows:
1. The greater the difference in melting points between different materials, the more challenging welding will be.
This is because when the material with a low melting point reaches its molten state, the material with a higher melting point remains solid. At this point, the molten material can easily penetrate the grain boundary of the superheated zone, leading to the loss of low-melting point material, the burning or evaporation of alloying elements, and making the weld joint difficult to weld.
For example, when soldering iron and lead (which have very different melting points), the two materials not only cannot dissolve into each other in the solid state, but they also cannot dissolve into each other in the liquid state. The liquid metal separates into layers and crystallizes separately after cooling.
2. The greater the difference in linear expansion coefficients between different materials, the more challenging welding will be.
The greater the coefficient of linear expansion, the greater the rate of thermal expansion, the greater the contraction during cooling, and the greater the welding stress that will be produced when the molten pool crystallizes.
This type of welding stress is not easily eliminated, resulting in significant weld deformation.
Due to the different stress states of the materials on both sides of the weld, cracks can easily form in the weld and the heat-affected zone, and even lead to peeling of the weld metal and base metal.
3. The greater the difference in thermal conductivity and specific heat capacity between different materials, the more challenging welding will be.
The thermal conductivity and specific heat capacity of the material can negatively impact the crystallization conditions of the weld metal, severely thicken the grain structure, and impact the wettability of the refractory metal.
Therefore, it is important to choose a strong heat source for welding and position the heat source at an angle to the side of the base metal with good thermal conductivity.
4. The greater the difference in electromagnetic properties between different materials, the more difficult welding will be.
The greater the difference in electromagnetic properties between materials, the more unstable the welding arc will be, leading to a poorer quality weld.
5. The more intermetallic compounds are formed between different materials, the more difficult the welding will be.
The fragility of intermetallic compounds makes them prone to causing cracks or even fractures in the weld.
6. During welding of dissimilar materials, changes in the metallographic structure or the formation of new structures in the welding zone may result in deterioration of the performance of the welded joint, presenting significant difficulties in welding.
The mechanical properties of the fusion zone and the heat-affected zone of the joint are poor, with a notable decrease in plastic toughness.
This decrease in the tenacity of the joint and the presence of welding stress makes the welded joint of different materials prone to cracking, especially in the heat-affected zone.
7. The greater the oxidizability of different materials, the more challenging welding will be. For example, fusion welding of copper and aluminum can easily result in the formation of copper and aluminum oxides in the molten pool.
During cooling crystallization, the oxide at the grain boundary can reduce the intergranular bond strength.
8. When different materials are welded, it is difficult for the weld and the two base metals to meet the requirement of equal strength.
This occurs because metallic elements with low melting points are susceptible to burning and evaporation during welding, changing the chemical composition of the weld and reducing its mechanical properties, especially when welding dissimilar non-ferrous metals.
