New Rheocasting (the NRC process) is a recently developed semisolid processing route. There are two versions of this route. In one, molten alloy is poured directly into a mould and through careful temperature control during cooling a spheroidal semisolid microstructure is achieved, before the material in the mould is upended into a shot sleeve and hence forced into a die. Alternatively, the molten alloy is poured onto a cooling slope and thence into a mould before processing. The aim of the work described in this paper, and its companion, was to develop understanding of the microstructural development during the initial stages of this process i.e. in the mould before processing and with the cooling slope/mould combination. In the previous paper, an analogue system based on aqueous ammonium chloride has been used to visualise what happens when an alloy is poured into a tilted mould with a chill wall, which acts to mimic the mould and the cooling slope in the NRC process. In this companion paper, the results for pouring A356 aluminium alloy directly into a mould, and also via a cooling slope into a mould, are presented.

New Rheocasting (the NRC process) is a recently developed semisolid processing route. There are two versions of this route. In one, molten alloy is poured directly into a tilted mould and through careful temperature control during cooling a spheroidal semisolid microstructure is achieved, before the material in the mould is upended into a shot sleeve and hence forced into a die. Alternatively, the molten alloy is poured onto a cooling slope and thence into a mould before processing. The aim of the work described in this paper, and its companion, was to develop understanding of the microstructural development during the initial stages of this process i.e. in the mould before processing and with the cooling slope/mould combination. In this first paper, an analogue system based on aqueous ammonium chloride has been used to visualise what happens when an alloy is poured into a tilted mould with a chill wall. In the companion paper, the results for pouring A356 aluminium alloy directly into a mould, and also via a cooling slope into a mould are presented. Keywords Cooling slope; thixoforming; semisolid processing; thixotropic; transparent analogue; ammonium chloride; New Rheocasting. IntroductionIn the early 1970s, a group at MIT discovered that when a solidifying alloy was continuously stirred the viscosity fell to a much lower value than for the static case [1]. This was associated with the fact that, during stirring in the semisolid range, the dendritic microstructure was destroyed and the semisolid consisted of spheroids of solid in a liquid matrix. In this state, the material behaved thixotropically i.e. when sheared it flowed but when allowed to stand it thickened again; the viscosity was a function both of time at temperature and of shear rate. The behaviour was associated with the dynamic development of microscopic 'welds' between solid spheroids (agglomeration) and the break up of these welds under shear (disagglomeration) [2]. It became the basis of a new range of forming technologies termed collectively semisolid processing. The advantages over conventional die casting including less mould attrition, reduced porosity and hence higher part integrity, and near net shape (because there is less solidification shrinkage). A range of reviews are available [2][3][4][5][6].