Migration of crystals during the filling of semi-solid castings
- PDF / 1,957,900 Bytes
- 14 Pages / 612 x 792 pts (letter) Page_size
- 93 Downloads / 182 Views
I. INTRODUCTION
IN many modern casting processes, the material injected into the die cavity is partially solid so that the die fills by semi-solid flow where the material behavior is generally non-Newtonian.[1,2] In the case of semi-solid casting processes such as rheocasting and thixocasting, the solid fraction and microstructure are carefully controlled before injection.[3] In these processes, a solid fraction of 50 pct is typically used. In cold-chamber high-pressure die casting (HPDC), solidification begins when superheated metal is poured into the shot sleeve and impinges on the relatively cold shot sleeve wall and plunger. When the plunger moves forward, a mixture of externally solidified crystals (ESCs) and melt is injected into the die cavity. The nature of solidification in the shot sleeve leads to a variable fraction of ESCs being injected into the die, so there is less control over the solid fraction injected in HPDC than in semi-solid casting. The ESCs have been regularly observed in the cold-chamber HPDC microstructures of both Al alloys[4,5,6] and Mg alloys,[7,8,9] with fractions of ESCs up to 20 pct being reported. In HPDC microstructures, ESCs are commonly compacted in the casting cross section. A typical example of this is shown in Figure 1(a), which is a HPDC aluminum casting made with A356.[6] It contains a surface layer with randomly oriented fine grains of 5 to 20 m and a central region containing a mixture of ESCs of 50 to 100 m and fine grains. Such HPDC microstructures are therefore often said to be made up of a bimodal distribution of grains.[5,7,8] In the bends of the castings, the ESCs tend to be compacted in the central region, as shown in Figure 1(b). Past work has found the H.I. LAUKLI, Research Scientist, formerly with the Department of Materials Technology, NTNU, N-7491 Trondheim, Norway, is with Hydro Aluminium RDM, N-6601 Sunndalsøra, Norway. Contact e-mail: [email protected] C.M. GOURLAY, Postgraduate Student, and A.K. DAHLE, Associate Professor, are with the CRC for Cast Metals Manufacturing, Division of Materials Engineering, The University of Queensland, Brisbane QLD 4072, Australia. Manuscript submitted April 2, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
ESCs to be partially spheroidized.[5] Furthermore, the inhomogeneous microstructure has been found to significantly improve corrosion resistance of high-pressure die-cast Mg-Al alloys,[10] and may also influence the wear and mechanical properties. The ESCs are not equivalent to the cold flakes,[12] which occasionally form due to the breaking up of the solid layer at the chamber wall, but instead are single-crystal dendrites. The reported terminology is somewhat inconsistent, and floating crystals,[6,9] externally solidified dendrites,[13] externally solidified grains,[14] and, in the case of gravity pour casting, intrinsic crystals[15] are occasionally used. While centered ESCs are commonly observed in semisolid-cast and HPDC components, the phenomenon has never been assessed in detail. Two brief explanations
Data Loading...
