Influence of process-related heat accumulation of laser beam welded 1.7035 round bars on weld pool shape and weld defects

The risk of weld defects increases when laser beam welding of round bars is performed in a rotational process. The reason is heat accumulation, which changes process conditions. The analysis of weld pool shape and weld defects in the course of a weld seam is essential for being able to evaluate the overall weld quality and to set up control measures. This study focuses on laser beam welding of round bars with partial welds and various welding speeds. The experiments are carried out with 1.7035 round bars of 30 mm diameter. For partial welds, a laser beam power of 6 kW and welding speed of 1 m/min are used for welding paths of 1/4, 1/2, and 3/4 of the circumference. Welding with various speeds is conducted with 0.5, 1.0, and 1.5 m/min and a constant energy per unit length of 240 kJ/m. The specimens are evaluated by metallographic microsections and scanning acoustic microscopy. The investigations reveal three major weld defects resulting from a gradient in linear welding speed between the specimen surface and the center and from heat accumulation due to specimen geometry. Porosity and hot cracks form under the surface and the weld root bulges, which also result in hot cracks. The weld depth increases to its final weld depth after approximately 1/8 of the circumference.