Microstructurally inhomogeneous composites: Is a homogeneous reinforcement distribution optimal?

Modelling the semisolid processing of metallic alloys

Commercially extruded 7075 alloy (extrusion ratio of 16:1) has been used as a feedstock for thixoforming in order to investigate thixoformability of a high performance aluminium alloy. The microstructure in the semi-solid state consists of fine spheroidal solid grains surrounded by liquid. The results of thixoforming with one step, two-step and three-step induction heating regimes are presented. Typical defects in poorly thixoformed material (e.g. liquid segregation, impedance of flow by unrecrystallised grains and porosity) are shown alongside successfully thixoformed material (thixoforming temperature of between 615 and 618 °C with a three-step induction heating regime). The highest yield strength and elongation obtained for material thixoformed into a simple graphite die and heat treated to the T6 condition is 478 MPa and 6.9% elongation. For thixoforming at 615 °C into a tool steel die heated to 250 °C, the highest yield strength and elongation obtained are 474 MPa and 4.7% (ram velocity 2000 mm/s). These values (particularly for strength) are approaching those of 7075 in the wrought heat treated condition (505 MPa and 11% elongation).

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Thixoforming 7075 aluminium alloys

Microstructural characterization of in situ TiC/Al and TiC/Al-20Si-5Fe-3Cu-1Mg composites prepared by spray deposition

Computational mesomechanics of composites

A rheological investigation has been conducted using a concentric cylinder rheometer on a semi-solid sn-15% Pb alloy at solid fractions between 0.2 and 0.5. The results show anomalous flow behaviour in that, at low shear rates, the shear stress falls with increasing shear rate up to 250 s−1 under steady-state conditions. Possible explanations of this anomalous behaviour are discussed and it is concluded that it is a genuine feature of the bulk slurry and not an experimental artefact. It is further suggested to be a consequence of the rapid breakdown of the internal structure of solid particle clusters within the slurry which occurs at low shear rates.

Anomalous rheological behaviour of semi-solid alloy slurries at low shear rates

Thixoforming is a semisolid processing route capable of producing near net shape components with low levels of porosity in a single, rapid operation. It has potential for the processing of aluminium alloy based metal matrix composites (MMCs). In contrast with casting of fully liquid metal, the flow of material into the die in thixoforming is primarily laminar. Thus, a novel layered MMC structure, generated by spray forming, consisting of alternate lamellae of reinforced and unreinforced alloy, might be preserved in thixoforming. The laminated structure is intended to give an increase in toughness, compared to a homogeneously reinforced material. The material was produced by spray forming alternate layers of aluminium matrix alloy 2014 (Al–4Cu–0·8Si–0·8Mn–0·5Mg) and matrix alloy reinforced with ∼17 vol.%SiC particles. Following thixoforming, the layered structure was well preserved with good retention of layer integrity and arrangement. Impact measurements for estimating toughness (Charpy and Izod...

A.R.A. McLelland

Papers

Anomalous rheological behaviour of semi-solid alloy slurries at low shear rates

Thixoforming of a novel layered metal matrix composite

Thixoforming spray-formed aluminium/silicon carbide metal matrix composites