Dry forming of low alloy steel materials by full forward impact extrusion with self-lubricating tool coatings and structured workpieces

Abstract

In cold bulk metal forming manufacturing processes high quality components are produced in large quantities. The high process loads lead to wear of the tools, which is reduced by lubricants based on mineral oils and phosphates. As these lubricants are questionable for ecological and economic reasons as well as to human health, their substitution is necessary. However, their absence in the tribological system leads to unknown contact conditions. This research project focuses on dry full forward impact extrusion of steel. The aim of the project is to control the increased process loads, such as contact normal stress, thermal loads and relative speed by a tool-sided and a workpiece-sided approach. The tool-sided approach is the development of self-lubricating coatings, which are deposited by physical vapor deposition (PVD) technology. Taking into account the load collective during dry full forward impact extrusion, the coatings must exhibit high wear resistance and simultaneously friction-reducing properties. For this purpose, coating systems consisting of the wear-resistant hard coating CrAlN were developed, which are modified with tribo-active elements Mo, W and S, which form friction reducing reaction layers MoS2 or WS2 under tribological load. The workpiece-sided approach is based on the surface structuring of workpieces by shot peening using various peening pa-rameters and peening media. This allows the adjustment of the real contact surface between tool and work-piece in order to partially exceed the yield limit of the workpiece before volume forming. The influence of self-lubricating tool coatings and structured workpieces on the friction and wear behavior was investigated in a first step by analogy model tests. These findings were then transferred to the full forward impact extrusion process in order to achieve an industrial process under dry tribological boundary conditions. In unlubricated industrial field trials, it was shown for the first time that a dry full forward extrusion process of steel can be achieved by applying self-lubricating tool coatings and the structured surfaces. Analyses of the interaction between the tool coating and the workpiece surface reveal a friction reduction during the first stroke with rougher surfaces, but an increase of the wear compared to smoother surfaces. Additionally, finite element method was used to simulate the dry forming process. Subsequently, a more complex two-shouldered die geometry was considered, which leads to significantly higher tool loads. Based on the field trials, dry full forward impact extrusion of steel was achieved for the first time. Subsequent wear investigations show that coatings have significantly reduced wear.