Effect of Cooling Rate on Mg<SUB>17</SUB>Al<SUB>12</SUB> Volume Fraction and Compositional Inhomogeneity in a Sand-Cast AZ91D Magnesium Plate

Abstract

Mg 17 Al 12 compound is a hardening phase in AZ91D alloy. Moreover, the concentration of Al and Zn has been found to be closely related to the corrosion performance of the magnesium alloy. The aim of this work was to study the effect of cooling rate on distribution of Mg 17 Al 12 compound and compositional inhomogeneity in an AZ91D magnesium sand-cast plate (200 Â 140 Â 20 mm 3 ). A copper chill block, which was placed at the end of mold cavity, was used to increase the cooling rate during solidification. A sand-cast plate was also produced, where no chill block being mounted at the end of mold cavity. The ''with chill block'' plate showed a rapid increasing in cooling rate with respect to distance from riser, as compared to the ''without chill block'' plate where almost no cooling rate fluctuation occurred. The volume fraction of Mg 17 Al 12 ( phase) in the ''without chill block'' plate was higher than that in ''with chill block'' plate. In the ''without chill block'' plate, volume fraction of Mg 17 Al 12 was about 13.9 vol% (near the riser) to about 19.3 vol% (close to the end of the plate). However, the ''with chill block'' plate was solidified in a higher cooling rate, leading to low volume fraction of the phase (13.4 vol%). Higher cooling rate also resulted in more severe compositional inhomogeneity in the sand-cast plate. The Al and Zn concentration in the ''with chill block'' plate showed a concave downward dependence against the distance to riser. Moreover, in the ''with chill block'' plate, concentrations of Al and Zn did not enrich at the position near chill face. Instead, the Al and Zn contents near the chill surface were well below the average value. This finding is in disagreement with previous studies.

Complex reaction behaviour of ceramic mould with the molten AZ91 alloy during investment casting

“…However, the β intermetallics are almost discontinuous in the present alloys, compared to the previous investigations [20,22]. Moreover, the volume fraction of these intermetallics is lower than that of the previous research studies (shown in Figure 6(g)) [20,22,23]. The reason is related to different cooling conditions.…”

“…However, the low thickness of the ceramic shell and also the small size of the mould in the present work significantly enhanced the cooling rates, leading to rapid solidification. Based on earlier investigations [22][23][24], higher cooling rates during solidification could be resulted in the harder diffusion of solutes, keeping more of them in the matrix and hindering the formation of β intermetallics. This could lead to a lower volume fraction of these compounds.…”

Study on microstructure of squeeze casting AZ91D alloy

“…Bakke et al 4 investigated it by casting in open cast iron moulds and found that the volume fraction of Mg 17 Al 12 in the region near mould wall was larger than that in the central regions. Cho et al 5 reported that the β volume fraction in ‘without chill’ plate was larger than that in the ‘with chill’ plate in sand cast. This phenomenon was explained by inverse segregation, as the melt enriched alloying elements tended to be drawn outwards to colder regions.…”