Effect of Silicon on the Thixoformability of Al-Si-Cu Alloys Davi Munhoz Benati and Euge ˆ nio Jose ´ Zoqui (Submitted March 27, 2014; in revised form May 14, 2014; published online June 10, 2014) The thixoformability of new Al-Si-Cu alloys was evaluated and characterized by their microstructural and rheological behavior. Alloys Al1Si2.6Cu, Al2Si2.6Cu, Al4Si2.6Cu, and Al7Si2.6Cu were produced with the addition of Al5Ti1B grain refiner alloy. The materials were heat treated under two controlled conditions: holding times of 0, 30, 90, and 210 s and solid fraction of 45 and 60%. The evaluation of the microstructure and semisolid behavior was characterized by globule size, shape factor (SF), minimum stress to flow, maximum stress, and apparent viscosity. The heat treatment times promoted the globularization of solid phase particles to achieve better apparent viscosity results for the alloys treated for 210 s. Both 45 and 60% solid fraction showed no significant differences in terms of SF, but the alloys containing lower solid fraction showed better performance for apparent viscosity. Better working ranges for these new Al-Si-Cu alloys were determined reaching average strain of 0.5 MPa and apparent viscosity of 10 5 Pa s. Keywords aluminum alloys, grain refinement, phase transforma- tion, semisolid processing, thixoforming 1. Introduction When thixotropic materials are sheared they flow, when allowed to stand they thicken up again; their viscosity is shear and time dependent. Spencer et al. (Ref 1) first discovered that semisolid alloys, whose microstructure in the semisolid state consists of spheroids of solid surrounded by liquid, exhibit behavior similar to thixotropic materials. This particular microstructure is a requirement for semisolid processing. When such materials are allowed to stand, the spheroids agglomerate and viscosity increases, when these materials are sheared the agglomerates break apart and the viscosity decreases, allowing the shaping, given that it flows as soon as it is sheared displaying a viscosity similar to that of heavy machine oil or tooth paste. This behavior was investigated and well described by Flemings (Ref 2) and Kirkwood (Ref 3). In this scenario, despite the development of a wide variety of alloys based on iron (Ref 4, 5), magnesium (Ref 6, 7), and aluminum (Ref 8-10), only the aluminum-silicon A356 and A357 alloys, as well as the magnesium alloys AZ91 and AM50, have been extensively used for thixoforming processes (Ref 11). However, both these alloys are casting alloys, fitted to the thixoforming purpose. Thus, for semisolid processing there is a lack of specific alloys with small grain size, good fluidity, or low viscosity—vital metallurgical characteristics that lead to the best semisolid behavior (Ref 2, 3). Therefore, the main goal of this work is to evaluate the thixoformability of Al-Si-Cu, particularly the Al1Si2.6Cu, Al2Si2.6Cu, Al4Si2.6Cu, and Al7Si2.6Cu alloys (in wt%), developing new semisolid materials (SSM) to fill some of these gaps, especially for thixoforging operations that use higher solid fractions, thereby helping to improve thixoforming processes. Copper content target was set at 2.6 wt% because this composition corre- sponding to 50% of the maximum amount of copper in solid solution at high temperature, i.e., at the semisolid range the copper is expected to be completely dissolved. In addition, these alloys could be easily submitted to T4/T5/T6 heat treatment to improve their mechanical properties after thixo- forming. The maximum magnesium content was set at 0.45 wt%, as it is the expected amount in the traditional A356 and A357 alloys. The main objective is to produce new SSM raw materials by the simplest and lowest possible cost method (consequently the grain refining procedure), which could also produce improved final mechanical properties, in comparison with the Al-Si alloys, hence the 2.6 wt%Cu, investigating the hypoeutectic range from the composition that indicates total silica solubili- zation (no eutectic phase) to the conventional and traditional silicon content of the Al7Si (A356/A357) alloys, which results in the eutectic composition of 50%. 2. Experimental Procedure The methodology to evaluate the thixoformability of the new Al-Si-Cu alloys is based on four steps: (a) thermodynamic analysis of the solid to liquid transition of the alloy; (b) fab- rication of the alloys and characterization of its initial microstructure; (c) evaluation of the microstructure stability at the semisolid range, and (d) their results in the rheological behavior following a methodology developed in previous papers (Ref 12-15). The thermodynamic evaluation used Thermo-Calc Ò simulation software. For the microstructure, the conventional metallography was used to characterize the globule size (GS) and its shape factor (SF), while the rheological behavior was characterized in terms of stress/strain and apparent viscosity/shear rate by compression tests. The Davi Munhoz Benati and Euge ˆnio Jose ´ Zoqui, Materials and Manufacturing Department, Faculty of Mechanical Engineering, University of Campinas – UNICAMP, Campinas, SP 13083-860, Brazil. Contact e-mails: dmbenati@fem.unicamp.br and zoqui@fem.unicamp.br. JMEPEG (2014) 23:3165–3179 ÓASM International DOI: 10.1007/s11665-014-1103-8 1059-9495/$19.00 Journal of Materials Engineering and Performance Volume 23(9) September 2014—3165