Erik Tolf, Joakim Hedegård and Magnus Söderberg

Abstract

Resistance spot welding of ultra highstrength thin sheet steel was investigated to evaluate the influence of reduced cooling time on the joint properties. To be able to set the cooling time independently, the resistance spotwelding machine was upgraded with a timer system that can activate the electrode opening process prior to the hold time end. With the upgrade it was possible to reduce the cooling time to zero, that is, allowing the welding current to be switched on until the moment when the electrodes arecompletely released.

Compared to low strength steels, the fitup is more critical, especially in combination with reduced cooling time,for high strength steels due to higher yield strength and springback. To simulate bad fit-up, joints with misalignment were produced using plane coupons with a spacer inserted between the sheets.

Two and three-sheet joint combinationsof martensitic (M), dual phase (DP),transformation induced plasticity(TRIP), complex phase (CP) and hardened boron steels were investigated in this study. The joints were evaluated by static shear-, cross- and peel-tensiletesting as well as metallographicalexaminations and hardness measurements.

With knowledge of the materials weldability, and evaluation of the behaviour when welding with reduced cooling time, it was possible to develop optimised weld schedules and thereby increased productivity.

Based on the experiences from thewelding trials it is recommended that the programming of the hold time in thepower sources is modified in order to enable full flexibility in setting of the hold time / cooling time. The results showed that the cooling time could bereduced significantly without endangering the joint integrity.

A comparison between three commonly used electrode caps (Cap B, E & F) were performed to evaluate the influence of the electrode cap type on the current range in combination with reduced cooling time for ultra highstrength thin sheet steel.