Cooling slope casting to obtain thixotropic feedstock

Thixoforming, and related semi-solid processing routes for metallic alloys, require feedstock with a non-dendritic microstructure in the semi-solid state. The material then behaves in a thixotropic way in that, when it is sheared it flows and can be forced to fill a die and, when it is allowed to stand it thickens again. The New Rheocasting (the NRC process) is a recently developed semi-solid 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 semi-solid microstructure is achieved. The material in the mould is then 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 was to develop understanding of the microstructural development during the initial stages of this process. The results for pouring A356 aluminium alloy via a cooling slope into a mould are presented

Semisolid processing characteristics of AM series Mg alloys by rheo-diecasting

The official published version of this Article can be found at the link below - Copyright @ 2006 ASM InternationalAn investigation has been made into the solidification behavior and microstructural evolution of AM50, AM70, and AM90 alloys during rheo-diecasting, their processibility, and the resulting mechanical properties. It was found that solidification of AM series alloys under intensive melt shearing in the unique twin-screw slurry maker during rheo-diecasting gave rise to numerous spheroidal primary magnesium (Mg) particles that were uniformly present in the microstructure. As a result, the network of the beta-Mg17Al12 phase was consistently interrupted by these spheroidal and ductile particles. Such a microstructure reduced the obstacle of deformation and the harmfulness of the beta-Mg17Al12 network on ductility, and therefore improved the ductility of rheo-diecast AM alloys. It was shown that, even with 9 wt pct Al, the elongation of rheo-diecast AM90 still achieved (9 +/- 1.2) pct. Rheodiecasting thus provides an attractive processing route for upgrading the alloy specification of AM series alloys by increasing the aluminum (Al) content while ensuring ductility. Assessment of the processibility of AM series alloys for semisolid processing showed that high Al content AM series alloys are more suitable for rheo-diecasting than low Al content alloys, because of the lower sensitivity of solid fraction to temperature, the lower liquidus temperature, and the smaller interval between the semisolid processing temperature and the complete solidification temperature.This work is supported by the EPSR

Morphology and Orientation Selection of Non-Metallic Inclusions in Electrified Molten Metal

The effect of electric current on morphology and orientation selection of non-metallic inclusions in molten metal has been investigated using theoretical modelling and numerical calculation. Two geometric factors, namely the circularity (fc) and alignment ratio (fe) were introduced to describe the inclusions shape and configuration. Electric current free energy was calculated and the values were used to determine the thermodynamic preference between different microstructures. Electric current promotes the development of inclusion along the current direction by either expatiating directional growth or enhancing directional agglomeration. Reconfiguration of the inclusions to reduce the system electric resistance drives the phenomena. The morphology and orientation selection follows the routine to reduce electric free energy. The numerical results are in agreement with our experimental observations

Differential scanning calorimetry (DSC) and thermodynamic prediction of liquid fraction vs temperature for two high-performance alloys for semi-solid processing (Al-Si-Cu-Mg (319s) and Al-Cu-Ag (201))

There is a need to extend the application of semi-solid processing (SSP) to higher performance alloys such as 319s (Al-Si-Cu-Mg) and 201 (Al-Cu-Ag). The melting of these two alloys was investigated using differential scanning calorimetry (DSC) and thermodynamic prediction. The alloys had been processed by magneto-hydrodynamic (MHD) stirring before receipt to produce a microstructure suitable for SSP. The DSC results for the as-received MHD material were compared with those for material which has been taken through a complete DSC cycle and then reheated for a second DSC run. The effects of microsegregation were then analyzed. A higher liquid fraction for a particular temperature is found in the second DSC run than the first. Microstructural observations suggest this is because the intermetallics which form during the first cooling cycle tend to co-located. Quaternary and ternary reactions then occur during the second DSC heat and the co-location leads to enhanced peaks. The calculated liquid fraction is lower with 10 K/min DSC heating rate comparing with 3 K/min at a given temperature. The DSC scan rate must therefore be carefully considered if it is to be used to identify temperature parameters or the suitability of alloys for SSP. In addition, the starting material for DSC must represent the starting material for the SSP. With thermodynamic prediction, the equilibrium condition will provide better guidance for the thixoforming of MHD stirred starting material than the Scheil condition. The Scheil mode approximates more closely with a strongly microsegregated state

In situ observation of calcium oxide treatment of inclusions in molten steel by confocal microscopy

Calcium treatment of aluminum killed steel was observed in situ using high-temperature confocal scanning laser microscope (HT-CSLM). This technique along with a novel experimental design enables continuous observation of clustering behavior of inclusions before and after the calcium treatment. Results show that the increase in average inclusion size in non-calcium-treated condition was much faster compared to calcium-treated condition. Results also show that the magnitude of attractive capillary force between inclusion particles in non-treated condition was about 10−15 N for larger particles (10 µm) and 10−16 N for smaller particles (5 µm) and acting length of force was about 30 µm. In the case of calcium-treated condition, the magnitude and acting length of force was reduced to 10−16 N and 10 µm, respectively, for particles of all sizes. This change in attractive capillary attractive force is due to change in inclusion morphology from solid alumina disks to liquid lens particles during calcium treatment

Effect of ceramic particle size and applied pressure on time to complete infiltration of liquid aluminium into SiC powder compacts

Monitoring of pressurized advance of liquid aluminium into SiC particle compacts of mean particle sizes 12.8, 22.8 and 36.7 µm was carried out under applied pressures of 400 to 900 kPa. Infiltration time of liquid aluminium in the compact was recorded by computer logging the output from pairs of open ended wires with a potential difference between them. Results showed that time necessary to complete infiltration in the compacts decreased with increasing particle size and increasing applied pressure. Experimental results are in reasonably good agreement with model predictions. © 2000 Kluwer Academic Publishers.University of SheffieldThis work formed part of a PhD programme at the University of Sheffield supported by a scholarship for EC from the Turkish Government

Process optimisation in the semi-solid forming of hypereutectic Al/Si MMCs

A novel processing route for the fabrication of Al/high silicon MMCs is presented. The silicon size is much finer than can be achieved by casting, yet the materials can still be formed into a near-net shape. Initial properties of the MMCs are presented, and methods under investigation to optimise processing and improve properties are discussed