Determination of optimum regimes of combined laser-ultrasonic hardening and finishing of tool steel AISI D2

Abstract

To improve the physical and mechanical properties of working surfaces of products, a new method combined laser-ultrasonic surface hardening and finishing using a scanning laser beam ultrasonic multiple impact head was suggested. The surface layers of cold worked tool steel AISI D2 were hardened by laser heat treatment and by ultrasonic impact treatment. The laser transformation hardening process was implemented using a 1 kW solid state fiber-laser with scanning optics and PID closed loop temperature control. The ultrasonic deformation hardening process was carried out by means of a 0.3 kW ultrasonic generator and ultrasonic oscillatory system, which contained a piezoceramic transducer, step-like horn and the impact head with seven cylindrical pins. The single-path processes were studied. This paper is focused on experimental analysis of the effects of heating temperature (1200…1340 °C) and specimen feed rate (40…140 mm/min) used at the laser transformation hardening process on the hardening depth and surface hardness/microhardness of subsurface layer, as well as the effects of the vibration amplitude of ultrasonic horn (15…18 μm) and treatment duration (60…240 s) at ultrasonic impact peening on the surface roughness/waviness and hardness using response surface methods. The experimental plan is based on a miscellaneous design matrix method. The quadratic regression equations for predicting the studied output parameters were developed. The optimum regimes of the laser transformation hardening and ultrasonic impact peening processes were determined based on the highest surface hardness and minimum roughness/waviness. It was established that the process of laser transformation hardening should realize at heating temperature 1250...1300 °C and specimen feed rate 80...100 mm/min, and the ultrasonic impact peening process – at vibration amplitude of ultrasonic horn 18 mm and treatment duration 100...120 s.