Pulse shape optimization for defect prevention in pulsed laser welding of stainless steels

Abstract

A variety of stainless steels were exposed to pulsed Nd:YAG laser with the rectangular shape power to first investigate practical problems related to pulsed laser welding. Porosities, cracks and undercuts were found as weld defects. A large porosity was formed in a key-hole mode of deep penetration weld metal of any stainless steel. Solidification cracks were present in Type 310S with above 0.017%P and undercuts were formed in Type 303 with about 0.3%S. The conditions for the formation of porosity were determined in further detail in Type 316. With the objectives of obtaining a fundamental knowledge of formation and prevention of weld defects, the fusion and solidification behavior of a molten puddle was observed during laser spot welding of Type 310S through high speed video photographing technique and the behavior was estimated by using a heat-conduction and solidification theory model. It was deduced that cellular dendrite tips grew rapidly from the bottom to the surface, and consequently residual liquid remained at the grain boundaries in wide regions and enhanced the solidification cracking susceptibility. Several laser pulse shapes were investigated and optimum pulse shapes were proposed for the reduction and prevention of porosity and solidification cracking.A variety of stainless steels were exposed to pulsed Nd:YAG laser with the rectangular shape power to first investigate practical problems related to pulsed laser welding. Porosities, cracks and undercuts were found as weld defects. A large porosity was formed in a key-hole mode of deep penetration weld metal of any stainless steel. Solidification cracks were present in Type 310S with above 0.017%P and undercuts were formed in Type 303 with about 0.3%S. The conditions for the formation of porosity were determined in further detail in Type 316. With the objectives of obtaining a fundamental knowledge of formation and prevention of weld defects, the fusion and solidification behavior of a molten puddle was observed during laser spot welding of Type 310S through high speed video photographing technique and the behavior was estimated by using a heat-conduction and solidification theory model. It was deduced that cellular dendrite tips grew rapidly from the bottom to the surface, and consequently residual liqu...