Influence of Grain Refinement on Slurry Formation and Surface Segregation in Semi-solid Al-7Si-0.3Mg Castings

Abstract

This study aims to evaluate the effect of grain refinement on slurry formation and surface segregation in semi-solid castings produced by the Rheometalä process. The effect of two grain refiners, Al-8B and Al-5Ti-1B, on the slurry a-Al grain size, shape factor and solid fraction was evaluated. The results suggest that the addition of a grain refiner can affect the solid fraction obtained in the Rheometal TM process and, consequently, reduce the solute content near the casting surface. Grain refiner addition resulted in a larger fraction of a-Al grains £ 60 lm for the refined alloys compared with the unrefined alloy. Additionally, the growth of a-Al slurry globules was greater for the unrefined alloy compared with the refined alloy during solidification in the die-cavity. A more homogeneous and finer microstructure was observed near the surface in the grain-refined castings compared with the unrefined castings. Evidence of significant liquid penetration was identified in some a-Al globules, indicating that disintegration of a-Al globules may occur during the Rheometalä casting process.

On the Generation of Excess Solid Fraction in the RheoMetal Process

“…The RheoMetal process deviates significantly from equilibrium, as shown by Santos et al [9]. This deviation is more significant for the RheoMetal process than for the SEED and GISS processes [8].…”

“…The formation of the excess solid fraction in the RheoMetal process was analysed using a Couette type of flow to model the formation of the freeze-on layer on the EEM cylinder. The following conclusions were made using the data by Payandeh et al [10,11] and Santos et al [9]:…”

“…The data from the existing literature, from Santos et al [9] and Payandeh et al [10][11][12], will be re-examined.…”

“…This deviation is more significant for the RheoMetal process than for the SEED and GISS processes [8]. Santos et al [9] found that for the RheoMetal process at the slurry forming temperature, the solid fraction in a conventional A356 alloy was 0.31 ± 0.04, whilst the equilibrium value was 0.23, calculated using ThermoCalc TM . This 0.08 fraction solid deviation was typical in a 0.06 m radius ladle.…”

The key attributes of processing parameters on semi-solid metal casting: An Overview

“…The two or more materials namely "Matrix" and "Reinforcement" having different physical and chemical properties when mixed in a way that neither they fully fuse nor liquefy into one another, so despite they act jointly but remain dispersed are called composite [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Composites are mainly categorized into three groups as 1) Ceramic Matrix Composites (CMCs), a subgroup that consists of ceramic matrix and ceramic fibers [19][20][21][22][23][24][25][26].…”