Laser Grooving of Surface Cracks on Hot Work Tool Steel

Repair welding (RW) with wire cladding is a common procedure to extend the in-service life of heavily loaded tool parts for die and mould industries 1-5 . The main advantages of repair welding are short downtime and cost efficiency compared to production of new tool part. Repair welding in general lowers the tool cost in the final part and enables higher added value to the die casting and injection moulding industry. The in-service life of die casting dies and injection moulding tools is correlated with the thermo-mechanical loads during production. The production of 300000 castings is a common series for die-casting industry and 1000000 mouldings for injection moulding industry. The in-service tool life is affected by (a) thermal fatigue, which causes heat marks on the surface of the die, (b) corrosion and soldering of aluminium to the die surface, (c) erosion due to melt flow, and (d) catastrophic failures 6-9 . Generally the tool steels are considered as non-weldable or hard to weld materials due to its high carbon and alloying element composition. A repair welding of dies and mould tools, which are made of tool steels, are done using tungsten inert gas (TIG) welding or a new technology called laser cladding by wire (LCW), which is a new alternative technology for repairing of small failures. For a quality laser RW a clean welding surface is necessary beside suitable welding wire. In many cases when RW is done on surfaces of the dies and moulds, the cracked surface must be removed and/or cleaned. The cracks are usually filled with oil or grease and/or the casting materials and oxides 6-8 . To prepare the surface for LCW a laser grooving (LG) or laser beam machining (LBM) could be used. The LBM has many variants: drilling, cutting and grooving, turning and milling, and micromachining 10, 11 . The mechanism of material removal during LBM includes three different stages i.e.: (i) melting, (ii) vaporisation and chemical degradation, at which chemical bonds are broken, which causes the materials to degrade. When a high energy laser beam is focused on work surface the thermal energy is absorbed. That heats and transforms the work volume into a molten, vaporised or chemically changed state that can easily be removed by flow of high pressure assisted gas jet. This jet accelerates the transformed material and ejects it from machining zone 10, 11 . The effectiveness of LG depends on thermal properties and partly on the optical properties of the grooved material, whereas less on mechanical properties. The materials that exhibit a high degree of brittleness i.e. hardness and have low thermal diffusivity and conductivity are well suited for laser machining, since they are not so easily machined conventionally 10, 11 . Another benefit of LG is that there are no cutting forces generated by the laser, therefore any machine vibration, tool ware or mechanical damage is induced to the material. The main obstacles in LG are the changing of groove geometry (GG) with LG param-