For structural, application or economic reasons, many industries need to join different metal materials. By combining different metals it is possible to make better use of the best properties of each metal. Therefore, before starting any welding operation, the welder must determine the properties of each material, which include the melting point of the metal, thermal expansion, etc. The welding process is then selected to suit them according to the material properties.
Dissimilar metal welding is the process of welding two or more different materials (different in chemical composition, metallurgical organization or properties, etc.) under certain process conditions. In the welding of dissimilar metals, the most common is the welding of dissimilar steel, followed by dissimilar non-ferrous metals welding. When dissimilar metals are welded, a transition layer with different properties from the parent material is produced. Because of the significant differences in elemental properties, physical properties and chemical properties of dissimilar metals, the welding of dissimilar materials is much more technically complex than the welding of dissimilar materials.
What are the problems of dissimilar metals welding?
1. The greater the difference in the melting point of dissimilar materials, the more difficult it is to weld.
This is because the low melting point of the material reaches the melting state, the high melting point of the material is still in a solid state, when the melted material is easy to penetrate the grain boundaries of the superheated zone, which can cause the loss of low melting point materials, alloying elements burned or evaporated, making it difficult to weld the joint. For example, when welding iron and lead (melting point difference), not only the two materials in the solid state can not dissolve each other, but also in the liquid state can not dissolve each other, the liquid metal is distributed in layers, and after cooling each separate crystallization.
2. The greater the difference in the coefficient of linear expansion of dissimilar materials, the more difficult to weld.
The greater the coefficient of linear expansion of the material, the greater the thermal expansion rate, the greater the shrinkage during cooling, the melt pool crystallization will produce a large welding stress. This welding stress is not easy to eliminate, the result will produce a large welding deformation. As the material on both sides of the weld is subjected to different stress states, easily lead to cracks in the weld and heat-affected zone, and even lead to the stripping of weld metal and base material.
3. The greater the difference in 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 make the weld metal crystallization conditions deteriorate, the grains are seriously coarsened, and affect the wetting properties of refractory metals. Therefore, a strong heat source should be selected for welding, the location of the heat source should be biased toward the side of the base material with good thermal conductivity when welding.
4. The greater the difference between the electromagnetic properties of dissimilar materials, the more difficult to weld.
Because the greater the difference in electromagnetism of the material, 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.
Since intermetallic compounds have greater brittleness, easily lead to cracks and even fractures in the weld.
6. The stronger the oxidation of dissimilar materials, the more difficult it is to weld.
If welding copper and aluminum with fusion welding method, the melt pool is very easy to form copper and aluminum oxides. When cooling crystallization, the presence of oxides at the grain boundaries can make the intergranular bonding force is reduced.
7. When welding dissimilar materials, the weld and the two base metals are difficult to achieve the requirements of equal strength.
This is due to the low melting point of the metal elements easily burned and evaporated during welding, resulting in changes in the chemical composition of the weld, mechanical properties reduced, especially when welding dissimilar non-ferrous metals more significant.
The application of laser welding technology for dissimilar metal welding
1. Copper and steel laser welding
Copper and steel welding is typical of dissimilar materials welding. The melting point, thermal conductivity, coefficient of linear expansion and mechanical properties of copper and steel are very different, which is not conducive to the direct welding of copper and steel. Laser welding of copper and steel has become the current development trend based on the advantages of high thermal energy density, low amount of melted metal, narrow heat-affected zone, high joint quality and high productivity of laser welding. However, the absorption rate of copper for most industrial applications is relatively low, and copper is also prone to defects such as oxidation, porosity, and cracking during the welding process. Based on multi-mode laser laser welding process of copper and steel dissimilar metals to be further developed.
2. Aluminum and steel laser welding
Aluminum, steel melting point difference, easy to form metal parts compound of dissimilar materials, and aluminum, steel alloy with high reflectivity and high thermal conductivity characteristics, it is difficult to form a keyhole in the welding process, the welding requires a high energy density. Experiments found that by controlling the laser energy and material action time, can reduce the thickness of the interface reaction layer, effectively control the generation of intermediate phases.
3. Magnesium aluminum and magnesium aluminum alloy laser welding
Aluminum and its alloys have good corrosion resistance, high specific strength, good electrical and thermal conductivity and other advantages. Magnesium is lighter than aluminum, a non-ferrous metal, also has a high specific strength and stiffness and good seismic capacity. The main problem of magnesium and aluminum welding is that the base material itself is very easy to oxidation, heat transfer coefficient, easy to produce cracks and pores and other welding defects, and very easy to produce intermetallic compounds, which significantly reduces the mechanical properties of welded joints.
