Improvement of resistance spot weldability of Al-Fe-alloy-coated HPF steel sheets

The resistance spot weldability of Al-Fe-alloy-coated (AlFe-c) HPF steels manufactured by heat-treating Al-Si coated (AlSi-c) steel at 650 degrees C for 10 h was investigated. Since AlFe-c steel had thick oxides in surface layer, the transition resistance was higher and weldability was poorer than the conventional AlSi-c steels. During spot welding of AlFe-c steels, expulsion occurred at low current, so the weld could not grow sufficiently. For this reason, AlFe-c steels had no weldable current range. To improve the weldability, pre-pulse current pattern was applied. By removing the oxides in the coating layer during the pre-pulse step, the resistance of the weld part was reduced, and the expulsion was delayed, so the weldable current range of AlFe-c steel was improved.