Laser welding of dissimilar materials
Using a novel joint design, laser welding of dissimilar materials can be transformed, according to scientists at The Welding Institute (TWI), Cambridge, UK. This could improve the quality of lightweight magnesium and aluminium alloy components used in the automotive sector.
Researchers sandwiched a magnesium layer between two aluminium sheets, and then focused a three-kilowatt neodymium:yttrium aluminium garnet laser beam on the surface of the top aluminium layer. The one-millimetre spot, which is positioned directly above holes cut out in the magnesium, melts the top and bottom aluminium sheets, spot welding them together and interlocking the magnesium layer in the middle.
‘Essentially, you do not melt weld magnesium to aluminium,’ says Dr Lien Nguyen, Project Leader of the Laser and Sheet Processes Group at TWI. ‘We transform the problem of welding dissimilar materials to that of welding similar substrates.’
Joining lightweight materials helps reduce the weight of vehicles which, in turn, aids fuel efficiency. However, conventional fusion welding techniques (arc, electron beam, resistance and laser) ‘result in low quality aluminium to magnesium welds’, says Nguyen.
‘In fusion welding, the materials are melted and then solidified to form the welds. When applied to magnesium and aluminium, the two metals melt, diffuse into one another and, due to their metallurgical properties, form brittle intermettalics.’
She adds, ‘By introducing holes in the middle material, we effectively only melt one type of material and interlock the middle layer’.
Tensile tests of the new joint have demonstrated shear stress that is 70% of that in conventional aluminium-to-aluminium welds. Further research will focus on applying a filler material to increase strength and avoid the sunken top beads where spot welding takes place.
Nguyen envisages the joint configuration being used to bond any pair of dissimiliar materials. The team is looking to industry to apply the technology.
Nigel Heath, Technical Specialist for Materials, Joining, Corrosion and Pre-development at Jaguar/ Land Rover in Warwick, UK, suggests that there might be a need for additional sealing after welding. ‘The biggest issue I have would be the potential for water ingress and consequential corrosion. The heat of the welding process would burn off any organic barrier coating or films used to isolate the two different metals.’