Forming limit of magnesium alloy at elevated temperatures for precision forging

Workability of magnesium alloy (ZK60) is investigated by the upsettability test and the results are applied to precision backward extrusion at elevated temperatures. In the upsettability test, cylindrical billets are compressed with concentrically grooved tools at a temperature range of 100-400 degreesC to obtain the critical reductions in height due to cracking, and the flow curves. In backward extrusion, the container is heated to the same temperature as the billet, and the cups are formed with a mechanical press without using lubricant at temperatures from 100 to 300 degreesC. The finite element simulation is conducted to determine the limit of forging with two fracture criteria. It is found that the billet is brittle at temperatures lower than 200 degreesC. A transient phenomenon is observed at 200 degreesC: the specimen is fractured at a small strain in the upsettability test, while the cups are extruded without cracks at extrusion ratios greater than 3.7. The Mg alloy shows an excellent workability at temperatures from 250 to 400 degreesC. Temperatures higher than 400 degreesC are not suitable for forming of the Mg alloy because of heavy oxidation. The fracture criterion expressed by the maximum tensile stress is effectively used for predicting the forming limit of the Mg alloy at temperatures between 100 and 200 degreesC. (C) 2002 Elsevier Science Ltd. All rights reserved.