Study on surface morphology and recast layer microstructure of medium thickness stainless steel sheets using high power laser cutting

With the rapid development of high-power lasers, efficiency of laser cutting and thickness of cutting material have been greatly increased. In this study, a 10 kW fiber laser was used to cut 18 mm thick 316L stainless steel sheets. The surfaces and cross sections of samples were observed by an optical microscope and a scanning electron microscope. It was found that the quality of laser cutting was closely related to its process parameters, and the most important defects during the laser cutting process included striation and dross. On this basis, effects of process parameters such as defocusing amount and cutting speed on the kerf morphology and microstructure of the recast layer were investigated. The cutting surface was clearly divided into four characteristic areas in the vertical direction; the surface morphology and the recast layer microstructure of each area were analyzed. The results of the nanoindentation test showed that microhardness of the recast layer increased significantly comparing with that of the substrate, which was due to the refined microstructure of the recast layer. The influence of the notch effect caused by the dross at the bottom of the cutting surface on the crack source is greater than that caused by the surface striation, so the crack source is more likely to be generated near the dross. The parameters for good cutting quality in this experiment are as follows: 10 bar N2 assist gas pressure, 9 kW of laser power, −15 mm of defocus, and 350 mm/min of cutting speed.