Mechanical behavior of aluminum matrix composite during extrusion in the semisolid state
- PDF / 4,196,377 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 20 Downloads / 198 Views
I. INTRODUCTION
THE fabrication of parts from particle reinforced metal matrix composites is often based on standard foundry techniques. However, the distribution of reinforcing particles in the matrix is difficult to control due to the variable solidification rate at different locations in the part. The control of temperature is critical in order to limit the chemical reactions that occur between the ceramic particles and the liquid alloy. On the other hand, solid-state processing through plastic deformation usually results in fracture of the reinforcing particles, leads to severe wear of the tools, and requires high forming pressure. For these reasons, semisolid processing appears to be an interesting alternative for forming metal matrix composite materials. The ability to shape the material is favored by the thixotropic nature of the semisolid composite, where the viscosity is reduced with increasing shear rate and duration of shear.[1,2] This behavior is attributed to the role of ceramic particles which prevent metal particles from forming dendritic particles and coalescing in the semisolid slurry. Depending on the processing temperature, the behavior of the semisolid alloy can be described as a solidlike[3,4] or multiphase fluid. The transition between solid and liquid behavior can be explained by the degree of coherency corresponding to the condition where the particles and agglomerates begin to touch and form a more rigid network.[5] The pseudo-plastic nature of the semisolid material is mainly due to the solid phase structure at high solid volume fractions for dendritic alloys.[6] For the case of direct extrusion of semisolid alloys, it was observed that during the first phase of billet compression, a certain amount of liquid is expelled through the die. Two distinct modes of deformation have S. TURENNE and F. AJERSCH, Professors, are with the De´partement de Me´tallurgie et de Ge´nie des Mate´riaux, Ecole Polytechnique, Montreal, PQ, Canada H3C 3A7. N. LEGROS, Research Associate, is with the Industrial Materials Institute, National Research Council of Canada, Boucherville, PQ, Canada J4B GY4. S. LAPLANTE, formerly Graduate Student with the De´partement de Me´tallurgie et de Ge´nie des Mate´riaux, Ecole Polytechique, is Research Engineer, Alcan International Ltd., Centre de Recherche et de De´veloppement, Jonquie`re, PQ, Canada G7S 4K8. Manuscript submitted October 31, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
been identified:[7] (1) a compaction mode where some of the liquid squeezes out of the solid matrix and (2) a flow mode where the remaining liquid and solid flows homogeneously through the die. The solid fraction at the beginning of the flow mode is normally higher than predicted from the extrusion temperature[8] due to the loss of liquid. The behavior of the semisolid alloy can then be determined by the characteristics of the solid phase for low strain rate[4,9] if the liquid is free to escape at some boundaries. For squeezing flow experiments on semisolid metal matrix composites, it was sho
Data Loading...
