A review of the use of high power diode lasers in surface hardening

Laser surface hardening, although possible for a number of years, is a technology which is still in its infancy. The process involves the use of high intensity laser radiation to rapidly heat the surface of a steel into the austenitic region. Due to high rates of heat transfer, steep temperature gradients are set up which result in rapid cooling by conduction. This causes the transformation from austenite to martensite without the need for external quenching. Other mechanisms exist for the thermal hardening of some non-ferrous alloys. Until recently, the widespread use of lasers for materials processing has been hindered by the size, complexity and high investment cost of the laser systems. These molecular and solid-state laser systems are now beginning to give way to a new generation of rapidly evolving lasers called high power diode lasers or HPDLs. The wavelength of the emitted radiation allows high metallic absorption, which when coupled with favourable spatial and temporal beam profiles allows the HPDL to achieve a high efficiency. The following review paper is a synopsis of the fundamentals of laser hardening, outlining some of its benefits compared with conventional hardening techniques. A selective review of the experimental research carried out in this area is presented. Particular reference is made to hardening using HPDLs. A description of the construction and operation of HPDLs is also presented with emphasis on the technical and economic factors which make them advantageous for surface hardening applications.