Joining dissimilar materials

Joining dissimilar materials is a topic of extensive interest in manufacturing - particularly in the automotive sector, where the benefits of using lightweight materials such as magnesium and aluminium alloys can help to reduce vehicle body weight.

Fusion welding magnesium and aluminium alloys, however, results in undesirable brittle intermetallics which limit the resulting weld quality. Consequently, there's a drive to improve existing techniques and develop new methods for joining such dissimilar lightweight materials.

Now, the first stage of the development of one new method has been successfully completed by TWI's Laser and Sheet Processes Group.

The joining technique welds two sheets of the same material and interlocks the second material between them. In the trials at TWI, holes were first machined in a magnesium sheet which was then sandwiched between two aluminium sheets.

Using a 3kW Nd:YAG laser, a spot weld was produced by focusing a 1mm diameter laser spot on the surface of the top aluminium sheet, directly above the position of the hole in the magnesium sheet.

The laser beam fused together the two aluminium sheets and the central magnesium sheet became interlocked between the top and bottom sheets.

The resulting joints were tensile tested; the best of these achieved a shear stress of 70 per cent of the shear stress of Al/Al welds. Further work in this area aims to increase the strength of the joint by the use of a filler material to avoid the typical sunken top bead, as shown below.

Researchers at TWI claim that the use of laser processing to form a mechanical joint shows promise: the method is not restricted to aluminium/magnesium, but can be applied to any pair of dissimilar materials.