Surface micro hardness profilometry of raster scanned laser bent thin metal sheets

Abstract

Surface microhardness Vickers profile measurements of laser formed thin metal sheets was performed in order to understand the effect on the integrity of the material of the process parameters, the sheet original geometry and the resulting turn radius. The laser bent specimens were fabricated using a 60 W CO2 laser modulated via a scanning system. Graphite coated AISI302 stainless steel plates (25 mm x 55 mm), 0.3 mm thick, were bent using different scanning patterns, changing the scan width, traverse speed and scan step. The as-received material corresponded to a cold rolled wrought sheet having an average surface microhardness value of 475 Hv and a yield point of 1320 MPa. After 15 passes all the specimens bent towards the heat source and showed a turn radius with very smooth corners. Resulting average microhardness Vickers, for three scan steps 0.5, 0.75 and 1, indicated that for specimens with resulting low turn radius (~3 mm) the average surface microhardness corresponded to 325 Hv, while for specimens showing higher turn radius (~ 30 mm) the average microhardness reached a plateau of 450 Hv. Additionally, average surface microhardness behaved logarithmically from 250 Hv up to 470 Hv with the product of the traverse speed (mm/s) and the scan width (mm), ranging respectively from 25 mm2/s up to 112.5 mm2/s. It can also be concluded that the surface hardness level measured directly over a laser raster line and over the gap between raster lines presented a maximum drop of 15% for the former. Finally, due to the finite longitudinal size of the sheets (25 mm), the zone near the entrance of the laser beam showed, for a scan step of 1, a 3 % up to 22 % higher hardness value when compared with the values measured at the exit zone, for different combinations of scan width and traverse speed parameters. This is due to the heat build up in the sheet as the laser front approaches the exit of the sheet; nonetheless, some specimens did show the opposite trend.