Influence of a Laser Power Modulation on the Element Intermixture at Austenitic-ferritic Stainless Steel Welds

Summary: Tha aim of this research is an experimental study of the correlation between laser power modulation and corresponding weld seam properties. The main reasons to perform laser trials with power-modulated laser beam instead of continuous wave laser are to reduce the average energy consumption and to manipulate the element intermixture inside the weld seam. This manipulation is an increase in the element intermixture homogenity and an improvement in laser-metal thermal impact. The weld trials were carried out with two different spot sizes and three different modulations. Electon microprobe analysis, Vickers micro hardness measurment, sulfuric acid corrosion test and tensile analysis were performed to study weld seam properties.

By doubling the spot size from 300 μm to 600 μm, the weld seam element intermixture homogeneity decrese that was also proved by electron microprobe analysis. The reason is that by increasing the spot size, the average heat input decreases and leads to an element intermixing reduction. The transverse and top-view analysis of weld seams for each modulation shapte did not reveal any considerable effect on increasing intermixture homegeneity and all moduation shapes had similar impact on weld seam. The hardness profile across the weld seam mixing compunds reveald high Vickers values which indicate that the power-modulated laser welding can lead to an increase in overall weld seam hardness. In 600 μm spot size, laser power modulation exposed higher hardness values compared with non-modulated beam but this effect was not observed in 300 μm spot size. In both 300 and 600 μm spot sizes independent of power modulation, high pick values in similar ferritic butt-joints above expected base material detected. The tensile test over all similar butt-joint showed that the overall strength increases in welded regions since all specimens failed in base material. There was also a slight increase in tensile strength since all specimens failed in base materials. There was also a slight increase in tensile strength in 600 μm spot size copared to 300 μm spot size.

To sum up the idea, power-modulated laser beam welding can be employed instead of normal continuous wave beam welding by considering two aspects. First, the same or improved weld seam properties can be achieved by laser power modulation besides having less energy consumption. Second, element intermixture homogeneity and corrosion resistance can be increased in some modulation shapes compared with non-modulated beam which is demandes in energy laser beam weld process.