Comparison of weld quality with normal and 30° incident laser beams

Abstract

The thermal cycle during laser welding of high carbon steels typically has a rapid cooling rate of about 104 K/s; in high carbon steels this results in a microstructure comprising ferrite matrix and hard martensite colonies [1,2]. Such high cooling rates can lead to deterioration of the weld quality due to hardness discontinuities between the fusion and heat affected zones. The hardness induced in the welded joint is dependent on the cooling process, itself being dependent on the laser parameters and weld geometry. The weld quality and hardness characteristics were improved by implementing an angular welding technique; this ameliorated the poor characteristics associated with rapid cooling. Two different angles of incidence (0°,30°) for welding were compared as were effects of the pulse length and pulse repetition frequency (PRF) on the mechanical and microscopic properties of the material. The gauge plates (0.88 mm) that were welded had a nominal composition of 0.85 wt % C, 0.4 wt % Si, 1.1 wt % Mn, 0.4 wt % Cr, 0.25 wt % V and 0.4 wt % W. The welding was done with Lumonic’s MS830 Nd:YAG laser, operating at 1.06 pm. The beam was delivered via a fibre optic system which was robotically manipulated. The welds were produced with a constant power of 200 watts and an argon shielding gas pressure of 5 x 104 Pa. The effect of varying the pulse length and PRF was quantified by measuring the hardness transverse to the weld direction, tensile strength, aspect ratio, weld volume formation rate and examining the phase transformation. Figures 1 and 2 show the hardness profiles of the weld for different pulse lengths and PRF, for the flat and 30° welding configurations, respectively. For both geometries, the hardness profiles decreased with increasing pulse length and PRF, however, the hardness gradients were lower for the 30° welding configuration. The hardness profile was dependent on the thermal distribution around the fusion and heat affected zones. Because of the rapidity of cooling for the normal weld geometry, the main weld region consisted of a martensitics structure [3], and the grain structure was coarser and less dense in the fusion zone. For the 30° welding configuration, a slower cooling rate was achieved, leading to a less brittle weld. The grain structure was typically fine and granular, and the structure was completely modified at the fusion zone. Additionally, a lower aspect ratio was obtained; this was due to the wider weld width produced with this geometry. Benefits of welding at 30° include: improved microstructure and reduced peak hardness profiles, greater weld width, higher tensile strength and greater weld volume formation rate. Ultimately, a higher welding speed was achieved.