High Pressure Die Casting of Aluminium and Magnesium Alloys.pdf

Introduction

When the metal is poured into the shot sleeve in the cold chamber high pressure die casting (HPDC) process, the superheat is quickly dissipated. The metal is therefore in a semi-solid state when it becomes exposed to the greater cooling rate in the die cavity. The microstructure eventually possesses a duplex grain structure which consists of fine grains and coarse externally solidified crystals (ESCs). In magnesium [1-3]  and aluminium HPDC [4], a large volume fraction of ESCs is typically present near the gate in the casting, whilst, further away, there is a significantly lower fraction of ESCs. The conditions for solidification and feeding are influenced, and the ability to successfully transmit the after-pressure can be affected. The mechanisms responsible for the inhomogeneous distribution of ESCs are described in [2,4]. There are great interests in further development of shot sleeve technology with  particular emphasis on reducing the amount of ESCs. It is therefore necessary to improve the understanding of the formation of ESCs and assess the solidification mechanisms in detail: The solidification of alloys begins when crystals are nucleated on suitable substrates [5]  at a sufficient undercooling of the melt. These crystals can evolve into dendrites, and eventually, the thermosolutal [6]  diffusion fields encapsulating the dendrites impinge on each other, and peripheral growth is substituted  by crystal coarsening [7]. In the dynamic nature of the casting process, crystals can be formed without the chemical addition of inoculants. The mechanisms are reviewed by Hutt and StJohn [8], and are briefly as follows: During the filling of a die, crystals are nucleated in the thermally undercooled region at the wall. The crystals possess a necked shape because solute diffusion at the root is restricted by the wall. This provides easy separation from the wall, by convection, as outlined by Ohno [9]. Chalmers [10] proposed that free chill crystals are formed in the thermally undercooled region adjacent to the die wall, whilst Southin [11] argued that heat radiation at the melt surface generates a sufficient thermal undercooling for nucleation and growth of crystals which detach by breaking off. Jackson [12] proposed that dendrite fragments can be formed by ripening phenomena, i.e. the dendrite dissolves at the root where the curvature is greatest. Winegard [13] explained that, in an alloy, (with a eutectic phase diagram), solute rejection at the crystal interface and simultaneous release of latent heat generates a constitutionally undercooled zone at the growth front in which suitable substrates can nucleate crystals.

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