New improved tool materials for short-run sheet-metal forming - Metal matrix composites and microstructural changes of metals

Abstract: The traditional sheet metal forming process, including cast steel die tool and conventional work materials, is fairly well established and analysed. Developments of new alternative materials make great demands upon new forming processes and manufacturing methods. A general goal for the manufacturing industries is to reduce the weight of metal parts, reduce batch sizes and reduce the lead-time of new products. This has lead to a demand for low weight/high strength sheet metal materials and tool materials suitable for short-run die tools. New interesting sheet metal materials are for example high strength aluminium, HSS (High Strength Steel), EHSS (Extra High Strength Steel) and possibly titanium. In common for these new sheet metals is that they are exposed to galling and also put new demands to the tool material due to increased tool wear. In spite of the development of new zinc alloys, the wear resistance compared to cast iron is low. Therefore zinc alloys are only used for prototype or short-run tools in sheet metal forming processes. The manufacturing industry, for example the automotive industry, is highly competitive and its customers expect numerous renewals and models. Therefore, the long-line production will be shortened and new products will be on the market more frequently. To reduce the costs of tool dies, alternative tool materials can be used. Zinc based tools are today used for prototype manufacturing in batches of 100-1,000 parts, depending on the complexity of the components. Future trends in the automotive industry are production series about 50,000-100,000 parts. With conventional tools about ten times more parts can be produced. This means that the total cost of production will be divided on a reduced number of parts. These conventional tools are often very expensive; therefore a reduction of the amount of parts will increase the costs of the product. The objective of this investigation was to find out if alternative tool materials can be used in a sheet metal forming process. The objective of this comparative investigation was to find out if commercial Zn-alloys can be used as a tool material and if microstructural changes of metals during its solidification can increase the mechanical properties suitable for a tool material. Investigations of the wear resistance of metal alloys have traditional been carried out with conventional methods, such as pin-on-disc or block-on-ring tests. However, these results can only present the wear behaviour of the alloys under a steady load. These tool materials therefore have to be directly investigated in forming process equipment. The conclusion of this comparative evaluation of the wear resistance of zinc alloys, Norzak2, ZA27 and ACuZinc5, is the importance of the primary phase. The primary phase in ACuZinc5 is a ƒÕ-phase surrounded by a ternary (ƒÑ+ƒÕ+ƒØ) eutectic. The ƒÕ-phase is harder and stronger than both the ƒØ-phase and the ƒÑ-phase. ACuZinc5 is full measure 9 times, Norzak2 1.8 times more abrasive resistant than ZA27. For the reinforced materials the fibres act as load carriers and therefore achieve less wear than the pure alloys. The ZA27/50 vol.-% S235JR-chip material is almost 300 times more wear resistant than the pure ZA27 alloy.