Thermal Stress Model Predicts Die Life Results Confirm Experience with Heated Dies Tests conducted by CDA over the past several years show that there are two secrets to achieving acceptable die life when die-casting copper motor rotors: Use a nickel-base superalloy as the die material. Preheat the die to approximately 650 C (1202 F) before injecting molten copper, and maintain that temperature between shots. A superalloy — and there are several viable choices — resists softening and oxidation at the high operating temperatures involved, while preheating the die and keeping it hot reduces thermal stresses that rise abruptly each time the die is filled. Reduced stress, in turn, delays the onset of the alligator-skin-like heat checking that marks die failure. CDA found that applying these practices extends die life to at least several thousand shots, long enough to make the process commercially viable. The pertinent question, of course, is how long can die life actually be extended: Is it 5,000 shots? 10,000? 50,000? Companies considering the production of copper motor rotors need a reasonable estimate to predict costs. The fact that a European manufacturer is currently in large-scale serial production (Update, Vol.4 Issue 1) of copper rotors suggests that even longer life than that experienced in CDA's tests might be attainable. Unfortunately, die life testing using actual rotors is made very expensive by the need for large numbers of laminations and arbors, not to mention the cost of the tooling itself. CDA therefore chose to simulate tests with a computerized model of the die-casting process. Modeling Program Prof. Jerald Brevick of the Ohio State University (OSU) Department of Mechanical Engineering supervised the study. The OSU researchers first calculated the surface temperature of the die at the time molten copper (at 1200 C, 2192 F) is introduced and for several minutes thereafter. This portion of the study required thermal mapping, a task accomplished with ANSYS, a commercial analysis program. Dies would be assigned preheat temperatures of 200 C, 350 C and 650 C (392 F, 662 F and 1202 F), respectively, as a check on CDA's experience that increasing die temperature increases die life. As might be expected, cooling times increased with increasing die preheat temperatures. MORE >>

Update: Copper Motor Rotor is published monthly by the Copper Development Association Inc. to provide current information of interest to those in the electric motor and related industries. CDA assumes no responsibility or liability in connection with this publication and makes no warranties of any kind with respect to the information contained herein.