Microstructure and selected properties of Monel 400 alloy after laser heat treatment and laser boriding using diode laser

摘要

This study concentrates on describing effects of laser heat treatment of Monel 400 and laser alloying its surface with boron. Surfaces without and with initial boron layers of two different thicknesses (100 and 200 mu m) were processed using diode laser. Laser beam power density was constant and equal to 178.3 kW/cm(2). To determine the influence of laser beam scanning velocity on final properties of treated surfaces, laser beam scanning velocity was set on four different values: 5, 25, 50, and 75 m/min. Microstructures of pure Monel 400 and Monel 400 alloyed with 100 mu m boron content are composed of dendrites. Areas laser alloyed with 200 mu m boron layer contain mainly nickel borides. Boron addition in Monel 400 surface results in microhardness increase in which the level depends on boron content and the laser beam scanning velocity. Increasing the thickness of initial boron layer and speeding up the laser beam lead to obtain higher microhardness. On the other hand, areas laser alloyed with 200 mu m boron layer using laser beam scanning velocity equal to 75 m/min contain deep cracks which propagate from the surface through the produced layer. Furthermore, it was found that the depths of laser heat-treated areas depend significantly on the boron content. As the result of differences in thermal properties between Monel 400 and boron, depth of re-melted zones in some conditions does not lower with increasing laser beam scanning velocity.