Excerpt from the article in the June 2005 issue of Acta Materialia by X.G. Hu, Q. Zhu, S.P. Madison, H.V. Atkinson, et. al.
Semi-solid die casting of relatively high solid-fraction aluminum alloys (0.5–0.7 fraction solid) can be used for the production of high quality industrial components. However, surface blistering during solution heat treatment can still be a problem and is associated with the entrapment of gas whether from air or from burned lubricant. Here the mechanism for formation of blisters is presented. The Reynolds number in the surface layer of the semi-solid flow is then analyzed to obtain the relationships with hydraulic diameter and flow velocity for different slurry temperatures. The hypothesis is that it is some flow instability at the flow front, even where the overall nature of the flow is essentially laminar, which is leading to the entrapment. The crucial finding is that if the Reynolds number is plotted against temperature there is a decrease followed by an increase. The position of this minimum is dependent on the ratio of fill velocity to the hydraulic diameter, v/D. Thus there is a ‘sweet spot’ in terms of temperature (i.e. fraction liquid), flow velocity and hydraulic diameter (i.e. die design) where the flow front has the maximum stability, giving maximum resistance to blister formation. This is in contrast with conventional wisdom which would suggest that low fractions liquid would give the most stable flow front. A rationale for this is presented in terms of the particle crowding at the relatively low fraction of liquid.
Experimental results with aluminum alloy 319s as an exemplar, and a die which has varying cross sectional dimensions, are presented and validate the hypothesis.Read More