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

Abstract

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.

Investigations on the Specifics of Laser Power Modulation in Laser Beam Welding of Round Bars

“…An ultrasonic transceiver with a frequency of 75 MHz is used for the SAM investigations. A more detailed description is contained in [3].…”

“…Prior to a successful application of laser beam welding for round bars, specific process development is necessary. In previous investigations [3] weld pool bulging was found, which can effect weld defects. Keyhole gas can remain in the weld [4] and solidification cracks can be effected since the bulge solidifies last [3,5].…”

“…In previous investigations [3] weld pool bulging was found, which can effect weld defects. Keyhole gas can remain in the weld [4] and solidification cracks can be effected since the bulge solidifies last [3,5]. Power modulation is a promising approach for bulge reduction and keyhole stabilisation.…”

“…The weld pool structure and weld pool bulging can be recognised in the SAM micrographs' keyhole plane since the weld pool partly solidifies as a network of cracks, if the laser beam is shut down without ramping it down, cf. [3]. The bright lines are bent at different degrees depending on the power modulation, see Table 8.…”

“…In round bar welding, the linear welding speed decreases from the specimen surface to the centre and the energy per unit length increases in contrast to sheet welding [3]. Since power modulation in round bar welding is not state of the art the following two objectives will be investigated:…”