Dissimilar metals refer to metals with different elements (such as aluminum, copper, etc.) or some alloys formed from the same basic metals (such as carbon steel, stainless steel, etc.) with significant differences in metallurgical properties, such as physical and chemical properties.
They can be used as base metal, filler metal or weld metal.
Welding of dissimilar materials refers to the welding process of two or more different materials (different chemical composition, metallographic structure and performance) under certain process conditions.
In the welding of dissimilar metals, the most common is dissimilar steel welding, followed by dissimilar non-ferrous metal welding and steel and non-ferrous metal welding.
From the joint form, there are three basic cases, namely, the joint of two different base metals, the joint of the same base metal but different filler metals (such as the joint of medium carbon quenched and tempered steel welded with austenitic welding materials), and the joint of composite metal plate.
The welding of dissimilar materials is to weld two different metals together, which will produce a transition layer with different properties and microstructure from the base metal.
Due to the significant differences in element properties, physical properties and chemical properties of dissimilar metals, the welding of dissimilar metals is much more complicated than that of the same materials in terms of welding mechanism and operation technology.
The main problems in welding dissimilar materials are as follows:
1. The larger the difference of melting point between different materials, the more difficult it is to weld.
This is because when the material with low melting point reaches melting state, the material with high melting point is still in solid state.
At this time, the melted material is easy to penetrate into the grain boundary of the overheated zone, which will cause the loss of low melting point materials, burning or evaporation of alloy elements, and make the welded joint difficult to weld.
For example, when welding iron and lead (the melting point is very different), not only the two materials can’t dissolve each other in the solid state, but also they can’t dissolve each other in the liquid state. The liquid metal is distributed in layers and crystallizes separately after cooling.
2. The larger the difference of linear expansion coefficient between different materials, the more difficult it is to weld.
The larger the coefficient of linear expansion is, the greater the thermal expansion rate is, the greater the shrinkage is during cooling, and the great welding stress will be produced when the molten pool crystallizes.
This kind of welding stress is not easy to eliminate, resulting in great welding deformation.
Due to the different stress state of the materials on both sides of the weld, it is easy to cause cracks in the weld and heat affected zone, and even lead to the peeling of the weld metal and the base metal.
3. The larger the difference between the thermal conductivity and specific heat capacity of different materials, the more difficult it is to weld.
The thermal conductivity and specific heat capacity of the material will deteriorate the crystallization condition of the weld metal, coarsen the grain seriously, and affect the wettability of the refractory metal.
Therefore, the strong heat source should be selected for welding, and the position of the heat source should be inclined to the side of the base metal with good thermal conductivity.
4. The larger the difference of electromagnetic properties between different materials, the more difficult it is to weld.
Because the greater the electromagnetic difference of materials, the more unstable the welding arc, the worse the weld.
5. The more intermetallic compounds formed between dissimilar materials, the more difficult it is to weld.
Because of the brittleness of intermetallic compounds, it is easy to cause cracks or even fracture in the weld.
6. In the welding process of dissimilar materials, because of the change of metallographic structure or the newly formed structure in the welding zone, the performance of the welded joint deteriorates, which brings great difficulties to the welding.
The mechanical properties of the fusion zone and heat affected zone of the joint are poor, especially the plastic toughness decreases obviously.
Due to the decrease of joint toughness and the existence of welding stress, the dissimilar material welded joint is prone to crack, especially in the welding heat affected zone.
7. The stronger the oxidizability of dissimilar materials, the more difficult it is to weld. If copper and aluminum are welded by fusion welding, it is easy to form copper and aluminum oxides in the molten pool.
During the cooling crystallization, the oxide existing in the grain boundary can reduce the intergranular bonding force.
8. When dissimilar materials are welded, it is difficult for the weld and the two base metals to meet the requirement of equal strength.
This is because the metal elements with low melting point are easy to burn and evaporate during welding, so that the chemical composition of the weld is changed and the mechanical properties are reduced, especially when welding dissimilar non-ferrous metals.