抄録
To measure the flow stress of metals under forming conditions, i.e. at large strains, high strain rates and elevated temperatures, a method based on the ring compression test is proposed. In this method, a ring-shaped specimen is compressed between two flat tools with and without lubrication. The coefficient of friction between the specimen and the tools is determined from the change in inner diameter, and then the load and reduction in height measured in the test are converted into the average flow stress and average equivalent strain by taking the friction into account. The calibration curves for determining the coefficient of friction, the average equivalent strain and the average flow stress are derived from the results of rigid-plastic finite element calculation. This method is simple to carry out and does not require control of friction during compression. Ring compression tests for certain kinds of lubricants, materials and strain rates are carried out. The method is confirmed to provide flow curves within an error of 5%, and is effective especially for measuring the flow stress of heat-resistant materials such as Ti alloys and Ni base superalloys which are formed at elevated temperatures. As an example, the flow curves of Ti-6Al-4V alloy measured by the proposed method are demonstrated.
