FINITE ELEMENT ANALYSIS OF GRAIN REFINEMENT OF BULK METAL BY MULTI-FORGING PROCESS

摘要

Grain refinement process can improve not only the strength, but also the forming characteristics of metals. In this paper, the free form forging process, which is broadly used in metal working factory, is considered to develop a multi-forging process, which can be served as a grain refinement process. Thereby, a bulk metal is plastically deformed repeatedly according to the principle of thermomecham''cal process for grain refinement. For each forging or deformation step, a flat punch is traveled onto the pre-heated material, so that the material height is to be halved. And the forged metal should then be folded or stacked after cutting and so prepared for the next deformation step, that the material will have the same height as in the previous step. However, before folding or stacking, the bulk metal must be quenched in water to hold the microstructure in the deformed metal. After folding or stacking, the material will be heated again to the forging temperature. In this paper, the commercial finite element code DEFORM™ is used to simulate such a grain refinement process in a plane strain state, in which a nickel alloy 718, 50 mm both in width and in thickness, is heated to 1000℃ and forged with a punch at 1000℃. For the input data, especially for recrystallization processes, some stated in literatures are applied in this study. The simulation results show that the quenched grains with an average size of 22 μm can be achieved just after one forging step on the nickel alloy whose grains are in 100μm initially. However, after two or three forging steps executed, an average quenched grain size in 28 μm or in 27 μm can only be realized, which is larger than that just after one forging step. It is also observed, that the alloy grain becomes larger after each reheating process to the forging temperature 1000℃, i.e. the heating processes make the grain larger. However, it is experienced as well in this paper, that the grain size will be finer, if the material temperature to be heated for the following forging step is not so high as before. For example, an average grain size in 16 μm can be attained, if a temperature 950℃ heated for the second forging step in stead. If the heating temperature for the third forging step is further down to 900℃, an average grain in 12 μm can be achieved. After three forging steps, a quenched grain in 9 μm can be obtained. If the alloy is further heated again up to 900℃, one can even have an average grain in 7 μm. It is therefore obvious, that the forging temperature and the reheating process after quenching are dominant to refine the grains with the multi-forging process introduced in this paper. Conclusively, the results presented in this paper have demonstrated the potential of the multi-forging process, which can be regarded as an alternative for grain refinement.