Casting semi-solid metal. (Aluminum).

The 23 Aluminum Div. papers and panels offered information on a wide variety of topics, including semi-solid metalcasting (SSM), furnace technology, fluxes and eutectic solidification,

In his presentation "SLC, A Novel New & Economical Approach to Semi-Solid Metal Casting" (02-180), J. Jorstad, J.L.J. Technologies, Inc., discussed the sub-liquidus casting (SLC) process, a slurry approach that uses normal foundry ingot and requires no processing equipment extraneous to the casting machine or processing time outside of the normal diecasting cycle. This improves the economics of using SSM to make high-integrity die castings.

Jorstad found that SSM is able to make castings with thin walls, details, complexity and dimensional control akin to conventional die castings, while using appropriate alloys and providing part integrity suitable for structural applications. However, the highcost of MHD-stirred billet for the process has deterred the widespread use of SSM components.

Significant cost reductions via slurry approaches are major current thrusts, according to Jorstad. Success with such approaches should increase the SSM market. The cost-saving economics of the process include energy conversion during parts manufacturing, long tool life and minimized secondary machining through net-shape casting.

Jorstad concluded that the SLC concept marries the SSM slurry process with grain-refined foundry alloys and simple molten metal processing and temperature controls. It offers all the characteristics of SSM made from MHD-stirred billet, but at a cost similar to conventional high-pressure diecasting.

R. DasGupta, SPX Contech Division, presented "Alloy Characterization for the New UBE Rheocasting Process" (02-032). The presentation, co-authored by M. Lukasson, Foundry Institute at RWTH-Aachen, and D. Apelian, Metal Processing Institute/WPI, discussed the advantages of rheocasting, a process that is achieved by solidifying the required alloy to a semisolid state in a special container (steel/ ceramic). While the conventional SSM process requires the reheating of billets to a semi-solid state, rheocasting produces "slurry on demand."

According to DasGupta, the UBE process contains all of the advantages of the thixocasting process, including lightweight thin walls, uniform microstructure and composition, nearly no porosity and high mechanical properties.

In addition, the presentation showed that rheocasting is not as sensitive to variations in process parameters, such as reheating power input. The amount of liquid phase that provides for the viscosity of the slug is directly proportional to the fraction of the solid. DasGupta deduced that the flowability of the rheocasting slurry is better than the thixocast slurry of the same composition due to the lack of entrapped liquid.

Rheocasting also is more cost effective than thixocasting because:

* common foundry alloys can be used;

* an expensive billet reheating furnace isn't required;

* scrap can be recycled directly.

DasGupta cited that the major future concern is to establish a knowledgeable database about the process parameters for different alloys. The mechanism controlling the process in the cooling and reheating step also must be studied.

The presentation, "Effect of Magnesium, Iron and Copper on Eutectic Solidification of Hypoeutectic Aluminum Silicon Alloys" (02-088), by G. Heiberg, L. Arnberg and M. Raanes, Norwegian Univ. of Science and Technology, A. L. Dons, SINTEF, A. K. Dahle and K. Nogita, Univ. of Queensland, attempted to give a snapshot of eutectic growth interface in aluminum by quenching the samples during growth. They found that magnesium, iron and copper affect the morphology of the interdendritic eutectic growth surface.

The authors found that directional solidification can be carried out with a pure aluminum-silicon alloy, a pure aluminum-silicon-magnesium alloy and an aluminum-silicon-copper alloy. A356 with and without strontium modification also was directionally solidified. Other findings from the study include:

* in pure binary aluminum-silicon alloy, the growth interface is irregular, but flat, In the other alloys, a eutectic mushy zone with inter-eutectic pockets and channels of quenched liquid were observed;

* the formation of the eutectic mushy zone has been linked to the presence of magnesium, iron and/or copper in the alloys;

* the type and amount of alloying elements affect the extent to which a eutectic mushy zone forms;

* the instability of the eutectic front occurs because of constitutional undercooling caused by accumulation of solute ahead of the eutectic growth front.

Other presentations from the division looked at fluxes and porosity formation.