PVD coated hot work tool steels for tooling applications in semi-solid processing of steels

Semi-solid processing of metals combines the advantages of forging and casting for the shaping of metallic components. Having already matured into an industrial practice for Al and Mg alloys, this innovative forming technology could upgrade the market and provide lightweighting for forged steel parts. However, with process temperatures above 1250°C, the surface-to-interior temperature differentials in steel thixoforming dies are much larger than with Al and Mg. Combined with the erosive wear caused by abrasion and impact of the already solid particles in the slurries and high temperature oxidation of the die surface, these cyclic thermal stresses confer very specific requirements on tool materials. Hot work tool steel dies proved to be entirely inadequate when thixoforming steels, due basically to the limited temper resistance of the commercially available grades. Physical Vapour Deposition (PVD) hard coatings, shown to have beneficial impact on the lifetime of pressure die casting tools, could offer a solution. X32CrMoV33 hot work tool steel widely used in the manufacture of conventional forging dies was coated with PVD AlTiN and AlTiON in the present work. The performance of the coated samples was subsequently tested under thermal fatigue conditions encountered in the thixoforming of steels.