An investigation on the laser welding process parameter effects on the failure mode of tailor welded blanks during the formability testing

Abstract

The ability to produce high-quality weld with the least distortion and heat affected zone width by the Nd:YAG laser welding process has made it one of the best candidates for steel tailor welded blanks (TWBs). The effect of process parameters including laser peak power, pulse duration, welding speed, and thickness ratio on the mechanical properties and failure modes of TWBs were investigated. TWBs was made of St12 carbon steel sheets with different thickness of 0.5 mm, 1 mm, and 1.5 mm. The weld zone hardness profile in all samples followed a similar pattern, in that the maximum hardness was in the weld metal and gradually fell throughout the heat affected zone (HAZ) until it reached the hardness of the base metal at the end of the HAZ and the hardness of weld metal increased as the average joint thickness increased. Based on the distribution of hardness in different regions of the weld zone a simple model was proposed for failure mode. Weld metal failure mode (WFM) happened as a result of weld metal thickness reduction caused by incomplete weld penetration or excessive penetration. The heat input per unit volume of weld in thin sheet (Hin) was defined as a criterion that determines failure mode. Therefore, a qualitative criterion was proposed for the prediction of failure mode. Based on this simple model, there is an appropriate limit for Hin for all thickness ratios at which the failure mode is base metal failure mode (BFM). The WMF happens at values lower and higher than this appropriate value range. The experiments approved that BFM and the highest formability and joint strength were obtained when 11J/mm3<Hin<37J/mm3.