Variable beam intensity profile shaping for layer uniformity control in laser hardening applications

A circular laser beam with a variable intensity profile is used to manipulate the time dependent temperature field within the surface of a hypo-eutectoid steel with the aim of controlling the uniformity of the resultant hardened layer. A 3D thermal model with a moving surface heat source is coupled with a simple rate equation containing a temperature dependent time constant calculated using the initial grain size and carbon content of the steel. The transformation kinetics of the pearlite/ferrite → austenite conversion are modelled and used to calculate the resultant martensite fraction in the material surface and the martensite field is plotted to reveal the shape and extent of the hardened layer. The beam profiles used in the model are recreated in an experiment using a variable laser beam profile shaper. The beam shaper is capable of transforming the raw top-hat beam output of a laser into a thin annular shaped beam with a uniform central intensity feature; the power ratio of the central to annular intensity features can be varied. Microscopy and microhardness tests are used to characterise the shape and hardness of a cross-section of the hardened layer created by a single pass of the laser beam. Both the experiment and the model show that the uniformity of the hardened layer can be controlled by selection of the appropriate power ratio of central to annular intensities. It is found that there are optimal power ratios for maximum uniformity that increase with increasing processing speed.