Materials Science and Engineering A 406 (2005) 63–73 Semi-solid behavior of new Al–Si–Mg alloys for thixoforming M. Paes, E.J. Zoqui ∗ Faculty of Mechanical Engineering, The State University of Campinas, UNICAMP, P.O. Box 6122, CEP 13083-860 Campinas, SP, Brazil Received in revised form 3 June 2005; accepted 15 July 2005 Abstract There is a lack of specific alloys for thixoforming processes. Usually alloys from conventional casting processes were fitted to the thixo- forming purpose, e.g. A356, A357. The aim of this study is to produce new Al–Si–Mg alloys specially tailored for thixoforming purpose. The as-rheocast: Al–7 wt%Si–0.5 wt%Mg, Al–4 wt%Si–0.5 wt%Mg, Al–1 wt%Si–0.5 wt%Mg, Al–7 wt%Si–1 wt%Mg, Al–4 wt%Si–1 wt%Mg and Al–1 wt%Si–1 wt%Mg were produced by grain refining. Thus, the effect of Si and Mg content on structure and rheological behavior of these new alloys was analyzed. To simulate the semi-solid behavior at a high temperature, the samples were re-heated to the semi-solid state to produce 45% solid fraction and their structural evolution was analyzed after 0, 90 and 210 s at this solid fraction. At the same temperature and holding time, the samples were submitted to a compression test using a parallel platen viscosimeter. Results showed that the grain/globule size increased as the Si and Mg content increased. Al–7 wt%Si–0.5 wt%Mg, the Al–4 wt%Si–0.5 wt%Mg and the Al–1 wt%Si–0.5 wt%Mg are suitable for thixoforming purposes. Al–Si alloys with 1.0 wt%Mg showed low apparent viscosity values. © 2005 Elsevier B.V. All rights reserved. Keywords: Semi-solid material; Rheological behavior; Aluminum–silicon–magnesium alloys; Grain refining; Morphological evolution 1. Introduction Al–Si and Al–Si–Mg rheocast alloys have been used widely for thixoforming processes. A356 and A357 are the most common alloys used to obtain near net shape products [1,2]. However, all these alloys are casting alloys, fitted to the thixoforming purpose. Thus, there is a lack of specific alloys for semi-solid processing that present good fluidity, low viscosity and small grain size, i.e. vital metallurgical characteristics that lead to the best semi-solid behavior [3,4]. Different methods have been used to produce alloys with non-dendritic structures, such as electromagnetic stirring, partial re-melting and ultra-refining using inoculants [3–5]. The present work used the ultra-refining method to obtain rheocast alloys. By adding an Al–Ti–B master alloy, during the casting, it is possible to promote heterogeneous nucle- ation of -aluminum [5,6]. Lee et al. [7] showed that the maximum grain refining is achieved by adding 0.05 wt%Ti in aluminium–silicon alloys. Above this value, an increase in ∗ Corresponding author. Tel.: +55 19 37883296; fax: +55 19 32893722. E-mail address: zoqui@fem.unicamp.br (E.J. Zoqui). titanium content has no effect on the grain size. Apart from this, it was observed that the grain refining is more efficient in low silicon alloys. For high silicon alloys, there is an inter- action between the silicon and titanium, reducing thus the refining efficiency. Despite this, for these alloys, there is a reduction in the solidification range as the silicon content increases. These factors also reduce the refining effect [7]. The thixoforming process depends upon the re-heating of the semi-solid raw material. Thus, it is necessary to know the microstructural evolution during the re-heating to the semi- solid range [8,9]. It is possible to correlate the macro and microstructure evolution with the rheological behavior and the final mechanical properties. Depending on the method to produce the non-dendritic structure, only a few seconds can be necessary to achieve the best globularisation [9]. Gener- ally, the most refined structure with the most globular shape of the primary phase, in the semi-solid range, exhibits the best semi-solid behavior. So, it is necessary to correlate the struc- ture at the temperature and holding time with its apparent viscosity. This parameter is very important to characterize whether a new alloy as suitable for being used in a thixo- forming processes. The apparent viscosity can be determined 0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.07.018