Thixocasting

Materials Science and Engineering A 479 (2008) 171–180

Thixocasting of an A356 alloy: Fluidity, porosity distribution and thermomechanical fatigue behavior
E. de Freitas a,∗ , M. Ferrante b , C.T. Ruckert a , W.W. Bose Filho a
a

University of Sao Paulo, Department of Materials, Aeronautic and Automotive Engineering, 13566-590 S˜ o Carlos, SP, Brazil a b Federal University of Sao Carlos,Department of Engineering Materials, 13569-905 S˜ o Carlos, SP, Brazil a Received 9 January 2007; received in revised form 13 June 2007; accepted 14 June 2007

Abstract An extensive set of experiments was performed on a semi-solid A356 alloy in order to assess its flow behavior, mechanical properties, microstructural evolution and porosity level. Three different microstructural conditioningtechniques (raw material preparation) were employed: deformation recrystallization, magnetohydrodynamic stirring and low temperature pouring. Measurement of microstructural parameters such as Al- particle size, shape factor, contiguity and entrapped liquid showed a relative equivalency among the various conditioning techniques. It was found that the strongest influence on semi-solid slurry fluidity isexerted by the mould temperature. Tensile properties and porosity levels were measured on a demonstration part produced with different slurry ingate velocities. Results showed similar strength levels among all thixocast samples, a strong correlation between elongation and pore volume fraction and porosity levels much lower than the typical figure for permanent mould or die cast Al–Si alloys. Finally,thermomechanical fatigue tests results were much more favorable to the semi-solid material when compared with the conventionally cast alloy, a result attributed to lower porosity, spheroidal shape of the Al- phase, and refined Si eutectic particles. © 2007 Elsevier B.V. All rights reserved.
Keywords: A356; Semi-solid; Microstructure; Mechanical properties

1. Introduction One of the goals of thetransport industry is weight reduction; indeed, it has been shown that replacement of steel by Al alloys can lead to a 20–30% saving which translates into fuel economy and lower tail pipe emissions [1]. These two final objectives can of course be met by improvements on combustion efficiency, but the use of light materials is expected to be much more effective. The weight of Al alloys in the averageAmerican car was forecasted to reach 135 kg by 2005, higher than that of polymers, and it is expected to rise to 270 kg by the year 2015, structure excluded [1]. Presently, Al alloys can be found in applications already close to saturation such as crankshafts, head cylinders, gearbox cases and heat exchangers. Thus, for further weight reduction the use of Al will have to be extended to suspensionparts, structural frames or space frames (body-in-white) and to
∗

Corresponding author. Present address: Department of Materials, Aeronautic and Automotive Engineering, University of Sao Paulo, Av. Trabalhador Sancarlense 400, 13566-590 S˜ o Carlos, SP, Brazil. Tel.: +55 16 3373 9590; a fax: +55 16 3373 9590. E-mail address: emersonfreitas@yahoo.com (E. de Freitas). 0921-5093/$ – see frontmatter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.06.037

both internal and external body panels. In the structural frame concept, the connecting nodes – components whose purpose is to keep together the various parts of the space frame – are highly stressed castings requiring good weldability and high elongation [2]. As for suspension, the strength level and fatigueproperties cannot be met by conventional casting processes while forging would be a too expensive technology for the car industry. Thixocasting, also known as semi-solid state (SSS) casting is a relatively new technology which combines the forming capabilities of die casting with the mechanical properties of forged products. Moreover, it has near net shape capability, which is a primary requirement for...