Application of a Pore Fraction Hot Tearing Model to Directionally Solidified and Direct Chill Cast Aluminum Alloys
- PDF / 1,880,954 Bytes
- 9 Pages / 593.972 x 792 pts Page_size
- 62 Downloads / 181 Views
UCTION
THE direct chill (DC) casting process often involves defects such as hot tearing,[1,2] cold cracks,[3–5] butt curl,[6] macro/micro segregation,[7,8] and shrinkage porosity.[9] Hot tearing and shrinkage porosity in particular have been widely recognized in the literature for more than ten decades. Shrinkage porosity is caused by the volume change occurring during solidification combined with the restricted feeding of liquid to the final solidifying region. Hot tearing is a defect with similar formation mechanisms, but additionally requires tensile deformation of the mushy zone.[10] Hot tearing and porosity are known to be inter-related. For example, it has been shown that hot tears nucleate on small intergranular pores[11] and that a reduction in porosity significantly increases semi-solid ductility.[12] Several hot tearing criteria have been developed on the basis of different parameters, e.g., strain,[13] strain rate,[14,15] alloy composition, and solidification conditions.[1,16] The so-called RDG criterion[15] is a prominent RUIFENG DOU, Lecturer, is with the School of Mechanical Engineering, University of Science and Technology Beijing, Beijing, 100083 China. A.B. PHILLION, formerly Associate Professor with the School of Engineering, The University of British Columbia, Kelowna, BC, V1V 1V7, Canada, is now Associate Professor with the Department of Materials Science and Engineering, McMaster University, Hamilton ON, L8S 4L7, Canada. Contact e-mail: andre. [email protected] Manuscript submitted September 7, 2015. Article published online June 8, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
criterion based on the liquid pressure drop in the mushy zone that combines the effects of both tensile deformation perpendicular to the thermal gradient and shrinkage feeding on hot tear formation. The hot tearing predictor is given by the strain rate that satisfies the critical pressure drop DPcr required for hot tear formation. Although this criterion has achieved much recognition in the literature, it cannot distinguish between shrinkage porosity and hot tearing, especially for low thermal gradients.[17] The Niyama criterion, Ny, is commonly used as a qualitative predictor of solidification shrinkage porosity during metal casting processes.[18] Recently, Carlson and Beckermann[9] proposed a dimensionless form of the Niyama criterion, Ny*, that directly predicts shrinkage pore fractions. As with the RDG criterion, Ny* was developed based on the liquid pressure drop in the mushy zone, but, however, it neglects tensile deformation. Within the framework of Ny*, DPcr marks the point at which liquid flow ceases in the mushy zone. Thus, any solidification occurring after liquid feeding has ceased can result in shrinkage porosity. Monroe and Beckermann[17] later modified Ny* by adding a term related to strain rate perpendicular to the thermal gradient into the pressure drop equation. With the same method, i.e., tracking of solidification and deformation after liquid feeding cessation, both the shrinkage and deformation pore fractio
Data Loading...
