Weldability Evaluation and Nugget Formation Mechanism in Three Sheets Spot Welding of High Strength Steels

摘要

The current trends of automotive industry are to reduce the weight of vehicle to increase fuel efficiency and also to improve the crash worthiness. These two properties hinder the improvement of each others. In the purposes of optimizing these two properties, three sheets spot welding of advanced high strength steel is introduced in automobile industry. The present study focused on the weldability of three sheets spot welding. Nugget growth mechanism and expulsion criteria were also evaluated. Weldability of (GA SGARC (0.7t) + Al-HPF (l.2t) + DP 590 (l.4t)) and (GA SGARC (0.7t) + TRIP 780 (l.2t) + DP 590 (1.4t)) was determined. Though based on button dia Al-HPF combination shows better weldable current range than TRIP combination but on the basis of weld strength both combinations having weldable current range of 1.5kA.The more reduction in weldable current range for Al-HPF combination compared to the TRIP combination attributed to the high button diameter for TRIP combinations. In earlier cycle of welding, heat generation for TRIP combination was occurring mostly in interface of thick-thick sheet and nugget growth rate was high in sheet direction. For Al-HPF combination significant amount of heat was generated in both interfaces due to the resistive Al-Si coating layer and nugget growth rate was high in electrode direction. Melting was observed at thick-thick sheet interface as well as geometric centre of middle sheet of Al-HPF combination. The expulsion was noticed in thick-thick sheet interface for both combinations. The higher temperature in thick-thick sheet interface for TRIP combination cause earlier expulsion in TRIP combination.