Segregation band formation in Al-Si die castings
- PDF / 3,705,699 Bytes
- 11 Pages / 608.4 x 786 pts Page_size
- 102 Downloads / 192 Views
8/10/04
12:20 PM
Page 2881
Segregation Band Formation in Al-Si Die Castings CHRISTOPHER M. GOURLAY, HANS I. LAUKLI, and ARNE K. DAHLE Banded defects are often found in high-pressure die castings. These bands can contain segregation, porosity, and/or tears, and changing casting conditions and alloy are known to change the position and make-up of the bands. Due to the complex, dynamic nature of the high-pressure die-casting (HPDC) process, it is very difficult to study the effect of individual parameters on band formation. In the work presented here, bands of segregation similar to those found in cold-chamber HPDC aluminum alloys were found in laboratory gravity die castings. Samples were cast with a range of fraction solids from 0 to 0.3 and the effect of die temperature and external solid fraction on segregation bands was investigated. The results are considered with reference to the rheological properties of the filling semisolid metal and a formation mechanism for bands is proposed by considering flow past a solidifying immobile wall layer.
I. INTRODUCTION
IN many commercial casting processes, some solidification occurs during filling and, increasingly commonly, material is semisolid before filling begins. Therefore, the flow behavior of the solid and liquid phases in combination with solidification produces the as-cast microstructure. In these processes, there is an increasing solid fraction during flow and, while the dramatic increase in apparent viscosity with fraction solid is widely reported,[1] it is also important to consider the semisolid microstructures at different fraction solids. It is these structures that give rise to the mechanical response of the mushy zone, whether it be the apparent viscosity, the mechanism of flow, or the permeability of a solid network. Therefore, understanding the rheological behavior of semisolid alloys during flow is important in developing solidification models for commercial casting processes. The developing microstructure with increasing fraction solid gives rise to two points at which there are marked changes in mechanical response.[2] Material of fraction solid below the coherency point ( fsCh) has no mechanical strength[3] and flows as a suspension of solid crystals in liquid. The coherency point marks the point where stresses begin to be transmitted between solid particles, a solid network begins to form, and shear strength development initiates.[2] Coherency typically occurs in the fraction solid range 0.1 to 0.3.[3,4] On further cooling, dendritic reorientation in response to a stress becomes more difficult as there is less liquid and the semisolid strength increases until the dendrites are sufficiently large and branched that they interlock such that they can no longer reorient. This marks the second distinct change in mechanical response: the maximum packing fraction ( fsPk), which usually occurs in the fraction solid range 0.3 to 0.6.[3,5] Above this point, the increase in shear strength is much greater[3] and the structure often deforms by fracture of the
Data Loading...
