Wear of surface treated dies for aluminium extrusion -: A case study

Dies for extrusion of metals and alloys, e.g. aluminium, zinc, brass and copper, are exposed to severe thermal, chemical and mechanical conditions. Extrusion dies are usually made from hot work tool steels such as AISI H13. In order to reduce wear they are almost always surface treated by various forms of nitriding. Surface coating by physical vapour deposition (PVD) or chemical vapour deposition (CVD) is currently being introduced to further improve the wear resistance. The present study aims at elucidating the life limiting wear mechanisms of gas nitrided tools, and duplex treated tools. The latter has been plasma nitrided and subsequently PVD coated by CrN in the same process. The duplex coating is included for its potential to effectively reduce the wear of extrusion dies. Each tool was investigated by SEM to characterise the wear pattern and to identify the wear mechanisms. Hardness profilometry and chemical analysis were performed on metallographic cross-sections to reveal any tempering, plastic deformation and chemical degradation of the tool bearing surface layers. It is concluded that the duplex treatment is superior to the conventional nitriding. The reason is that the superficial PVD-layer shows a combination of high resistance to mechanical wear and chemical resistance to corrosion by hot aluminium. Eventually, the PVD-layer fractures and delaminates due to fatigue. When the nitrided substrate is exposed, the wear accelerates significantly. It is estimated that duplex treatment prolongs the tool life at least five times as compared to conventional nitriding.