Experimental investigation of the transient and steady state rheological behaviour of Al /Si alloys in the mushy state D. Brabazon *, D.J. Browne, A.J. Carr Department of Mechanical Engineering, University College Dublin, Belfield, Dublin 4, Ireland Received 1 July 2002; received in revised form 18 December 2002 Abstract In order to properly model and control the semi-solid processing of metallic alloys, their thixotropic behaviour requires proper characterisation. In particular, the effects of shear rate, shear time, temperature and rest time on the rheology of such slurries needs to be understood and quantified. A purpose-built high temperature Searle rheometer was used to determine the rheological behaviour of aluminium alloy slurries at shear rates from 3.1 to 124.8 s 1 , periods of shear of up to 60 min for each shear rate, and periods of rest (no stirring) of up to 60 min for Al  /4wt.%Si and A356 alloys. Continuous cooling rheometry was used to determine the coherency point of the alloys. Isothermal fractions solid of 0.36 for Al  /4%Si and 0.33 for A356 were investigated. Isothermal tests were used to follow the temporal evolution of viscosity, which was found to be significantly different for both alloys, particularly at low shear rates. Steady state viscosity values that were determined over a range of shear rates indicated severe pseudoplastic behaviour, as measured by a viscosity  /shear rate power law index less than  /1. This work confirms that this finding is an actual rheological feature and not an artefact of a particular measurement device. A study using shear rate jumps determined the isostructural behaviour of the alloys by discounting equipment inertial effects. It is shown that peak stress or apparent viscosity is a better indicator of slurry thixotropy than the hysteresis loop from shear ramping experiments, and the work also shows that the effect of agglomeration on fluidity is an important parameter to measure as it has consequences for thixoforming. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Aluminium alloys; Apparent viscosity; Rheology; Semi-solid metal; Thixotropy; Rheocasting 1. Introduction Conventional liquid casting processes face many potential problems. These include gas entrapment dur- ing turbulent filling operations, hot tearing of the solidifying metal, and shrinkage porosity formation. Hot tearing (cracking of the stressed solidifying metal) occurs during the solidification of alloys as a result of the dendritic network limiting the alloy’s ductility in the semi-solid state [1]. Shrinkage porosity can also be produced due to the reduced feedability of liquid metal through the dendritic network. These problems are greatly alleviated in alloys with spheroidal primary phase morphologies. Under the action of shear these alloys can fill all areas of a die cavity in a laminar manner, reducing gas entrapment, shrinkage porosity, and hot tearing [2,3]. It is the rheology of the metal slurries with this spheroidal solid morphology that promote their use in semi-solid forming (SSF) opera- tions. It is, therefore, important to understand the rheology of these fluids. The exact viscosity of a semi-solid metal alloy is dependent on a number of parameters and may be represented by the following equation [4  /6]. h f ( ˙ g; t s ; T a ; ˙ T ; C o ; h) (1) where h represents the viscosity, ˙ g is the shear rate, t s is the period of shear, T a is the temperature of the semi- solid alloy, ˙ T is the cooling rate to T a , and C o is the alloy composition. Higher solute contents [5] and cool- ing rates [6] have each been found to cause a more dendritic microstructural morphology and hence higher viscosities for a given fraction solid. The apparent viscosity of the slurry at a particular point in time * Corresponding author. Tel.:  /353-1-7008213; fax:  /353-1- 7005345. E-mail address: dermot.brabazon@dcu.ie (D. Brabazon). Materials Science and Engineering A356 (2003) 69  /80 www.elsevier.com/locate/msea 0921-5093/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5093(03)00158-8