Eight Problems in Welding of Dissimilar Materials
Dissimilar metals refer to metals of different elements (such as aluminum, copper, etc.). Or it refers to certain alloys (such as carbon steel, stainless steel, etc.) formed with the same base metal that differ significantly from metallurgical properties, such as physical properties, chemical properties, etc. They can be used as base metal, filler metal or weld metal.
Welding of dissimilar materials refers to the process of welding two or more different materials (referring to different chemical compositions, metallographic structures and properties) under certain technological 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 point of view of the joint form, there are also three basic situations.
1) Joints of two different metal base materials
2) Joints with the same base metal but different filler metals (such as joints of medium carbon quenched and tempered steel welded with austenitic welding consumables, etc.)
3) Welded joints of clad metal plates, etc.
Welding of dissimilar materials is that when two different metals are welded together, a transition layer with different properties and structures from the base metal is bound to be produced. Because dissimilar metals have significant differences in element properties, physical properties, chemical properties, etc., compared with the welding of the same material, the welding of dissimilar materials is much more complicated than the same material in terms of welding mechanism and operation technology. Therefore, various problems often occur when welding dissimilar materials.
The main problems existing in the welding of dissimilar materials are as follows:
- The greater the difference in melting point of dissimilar materials, the more difficult it is to weld.
This is because when the material with a low melting point reaches a molten state, the material with a high melting point is still in a solid state. At this time, the melted material easily penetrates into the grain boundary of the superheated zone, which will cause the loss of the low melting point material, the burning or evaporation of alloy elements, Makes the welded joint difficult to weld. For example, when soldering iron and lead (the melting point is very different), not only the two materials cannot dissolve each other in the solid state, but also cannot dissolve each other in the liquid state. The liquid metal is distributed in layers, and each crystallizes separately after cooling.
- The greater the difference between the linear expansion coefficients of dissimilar materials, the more difficult it is to weld.
The larger the coefficient of linear expansion, the larger the coefficient of thermal expansion and the greater the shrinkage during cooling, and the large welding stress will be generated when the molten pool crystallizes. This welding stress is not easy to eliminate, resulting in a large welding deformation. Due to the different stress states of the materials on both sides of the weld, it is easy to cause cracks in the weld and the heat-affected zone, and even lead to the peeling of the weld metal and the base metal.
- The greater the difference between the thermal conductivity and specific heat capacity of dissimilar materials, the more difficult it is to weld.
The thermal conductivity and specific heat capacity of the material will deteriorate the crystallization conditions of the weld metal, severely coarsen the grains, and affect the wetting properties of the refractory metal. Therefore, a strong heat source should be selected for welding, and the position of the heat source should be biased to the side of the base metal with good thermal conductivity during welding.
- The greater the difference in the electromagnetic properties of dissimilar materials, the more difficult it is to weld.
Because the greater the difference in the electromagnetic properties of the materials, the more unstable the welding arc and the worse the weld.
- The more intermetallic compounds formed between dissimilar materials, the more difficult it is to weld.
Due to the greater brittleness of intermetallic compounds, it is easy to cause cracks or even fractures in the weld.
- During the welding process of dissimilar materials, due to the change of the metallographic structure of the welding zone or the newly formed structure, the performance of the welded joint is deteriorated, which brings great difficulties to the welding.
The mechanical properties of the joint fusion zone and heat-affected zone are poor, especially the obvious decrease of plastic toughness. Due to the decrease of joint plastic toughness and the existence of welding stress, the welded joints of dissimilar materials are prone to cracks, especially the welding heat affected zone is more prone to cracks or even fractures.
- The stronger the oxidation of dissimilar materials, the more difficult it is to weld.
For example, when copper and aluminum are welded by fusion welding, oxides of copper and aluminum are easily formed in the molten pool. During cooling crystallization, the oxides present at the grain boundaries can reduce the intergranular bonding force.
- When welding dissimilar materials, it is difficult for the weld seam and the two base metals to achieve the same strength requirements.
This is because the metal elements with low melting point are easily burned and evaporated during welding, which changes the chemical composition of the weld and reduces the mechanical properties, especially when welding dissimilar non-ferrous metals.