Many industries require the joining of dissimilar metallic materials for structural, application or economical reasons. Combining different metals can take better advantage of the best properties of each metal. Therefore, before starting any welding operation, the welder must determine the properties of each material, this includes the metal's melting point, thermal expansion, etc. Then choose the welding process suitable for them according to the material characteristics.

 

Dissimilar metal welding refers to the process of welding two or more different materials (different in chemical composition, metallographic structure or 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. When dissimilar metals are welded, a transition layer with different properties from the base metal will be produced. Since dissimilar metals have significant differences in elemental properties, physical properties, and chemical properties, compared with welding of the same material, the welding of dissimilar materials is much more complicated in operation technology.

 

1. The greater the difference between the melting points 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 is easy to penetrate into the grain boundary of the superheated zone, which will cause the loss of low melting point materials, alloy elements burning or evaporation. Make welded joints difficult to weld. For example, when welding 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 crystallizes separately after cooling.

2. The greater the difference between the linear expansion coefficients of dissimilar materials, the more difficult it is to weld.

A material with a larger linear expansion coefficient has a larger thermal expansion rate and a larger shrinkage when cooling, and a 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.

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 deteriorate the crystallization conditions of the weld metal, severely coarsen the grains, and affect the wettability of the refractory metal. Therefore, a strong heat source should be selected for welding, and the position of the heat source should be biased towards the side of the base metal with good thermal conductivity during welding.

4. The greater the electromagnetic difference between dissimilar materials, the more difficult it is to weld.

Because the greater the difference in the electromagnetic properties of the material, the more unstable the welding arc and the worse the weld.

5. The more intermetallic compounds formed between dissimilar materials, the more difficult it is to weld.

Due to the high brittleness of intermetallic compounds, it is easy to cause cracks or even fractures in the weld.

6. The stronger the oxidation of dissimilar materials, the more difficult it is to weld.

If copper and aluminum are welded by fusion welding, oxides of copper and aluminum are easily formed in the molten pool. During cooling and crystallization, the oxides present at the grain boundaries can reduce the intergranular bonding force.

7. When dissimilar materials are welded, it is difficult for the weld seam and the two base metals to meet the requirements of equal strength.

This is because metal elements with low melting points are easy to burn and evaporate during welding, which changes the chemical composition of the weld and reduces the mechanical properties, especially when welding dissimilar non-ferrous metals.

 

 

1. Laser welding of copper and steel

Copper and steel welding is a typical dissimilar material welding. The melting point, thermal conductivity, linear expansion coefficient and mechanical properties of copper and steel are very different, which is not conducive to the direct welding of copper and steel. Based on the advantages of laser welding, such as high heat energy density, less molten metal, narrow heat-affected zone, high joint quality, and high production efficiency, laser welding of copper and steel has become the current development trend. However, the absorption rate of copper for most industrial applications of laser light is relatively low, and copper is prone to defects such as oxidation, pores, and cracks during the welding process. The laser welding process of copper and steel dissimilar metals based on multimode lasers needs to be further developed.

2. Laser welding of aluminum and steel

The melting points of aluminum and steel are very different, and it is easy to form dissimilar materials of metal compound. Moreover, aluminum and steel alloys have the characteristics of high reflectivity and high thermal conductivity, so it is difficult to form keyholes in the welding process, and high energy density is required for welding. The experiment found that by controlling the laser energy and the action time of the material, the thickness of the interface reaction layer can be reduced, and the formation of the intermediate phase can be effectively controlled.

3. Magnesium-aluminum and magnesium-aluminum alloy laser welding

Aluminum and its alloys have the advantages of good corrosion resistance, high specific strength, good electrical conductivity and thermal conductivity. Magnesium is a non-ferrous metal that is lighter than aluminum, and it also has higher specific strength and specific stiffness and good shock resistance. The main problem of magnesium-aluminum welding is that the base metal itself is extremely easy to oxidize, has a large thermal conductivity, is prone to welding defects such as cracks and pores, and easily produces intermetallic compounds, which significantly reduces the mechanical properties of the welded joint.

The above is the welding application of laser welding machine in dissimilar metal materials. Laser welding of dissimilar metal materials has expanded from dissimilar steel to non-ferrous metals and their alloys, especially for magnesium-aluminum alloys and titanium-aluminum alloys. Laser welding has made progress, and welded joints with certain penetration depth and strength have been obtained.