Effect of aluminium sheet surface conditions on feasibility and quality of resistance spot welding

A study investigating the effect of sheet surface condition on resistance spot welding (RSW) of aluminium has been carried out at the University of Warwick. This concentrates on two commercial automotive aluminium alloys; AA5754 and AA6111, used for structural and closure applications respectively. The results show the marked effect that surface condition can have on the RSW process. For AA5754 sheet incomplete removal of a ‘disrupted surface layer’ prior to surface pretreatment is shown to have a detrimental effect on the RSW process. Another potential influence is the solid wax lubricant used to assist metal forming. This lubricant leads to unpredictable changes in contact resistance, which affects the process stability. In practice, a welding schedule devised to displace the lubricant prior to the main weld pulse can provide a solution. For AA6111 closures the final surface topography can influence the RSW process. Standard ‘mill’ and electro discharge textured (EDT) finish sheet surfaces were examined and preliminary results suggest that both are suitable for welding. The successful application of RSW of aluminium sheet requires careful consideration of the sheet surface condition, and this requires close collaboration between material suppliers and automotive manufacturers, to ensure that optimum process parameters are applied at all stages of the manufacturing.