ECAP-An alternative route for producing AlSiCu in SSM processing
In thixoforming processes control of the solid-to-liquid transition and grain size of the mush metal is essential. If eutectic melting is avoided, subsequent melting of the primary phase should be controllable. To achieve this, the sensitivity of the liquid fraction, dfL/dT, at the desired liquid fraction, fL, exclusively for the primary phase should be as low as possible (b0.03 °C−1). In addition, the smaller the grain size, the better the rheological behaviour. However, for aluminium alloys with a high silicon content, current commercial processes such as continuous casting result in materials with a large grain size. This paper therefore proposes a new method based on equal channel angular pressing (ECAP) for producing aluminium-copper alloys with a low silicon content. The alloys investigated were Al5Si2.8Cu, Al6Si2.8Cu and Al7Si2.8Cu. To evaluate the semisolid behaviour of these alloys after ECAP processing as well as the effect of heat treatment on globularization of the alloys, they were heated up to 590, 580 and 575 °C, respectively, to achieve a solid fraction of 40% and subsequently soaked for 0, 30, 90 and 210 s and cooled rapidly. The structures of the three alloys had an excellent response to the recovery and recrystallization mechanisms, and refined microstructures ideal for thixoforming were produced. Grain and globule sizes of less than 80 μmand a circularity shape factor of more than 0.60were obtained. All the alloys exhibited good morphological stability in the semisolid state without significant changes in size or shape, indicating homogeneous apparent viscosity and, consequently, homogeneous die filling. The findings of this study suggest that ECAP is a promising route for the production of semisolid raw material for use in thixoforming.