||In this study the effects of P on a hyper-eutectic Al-17 Si alloy were thoroughly studied by using a binary Al-P alloy modifier, instead of other commercially available chemical modifiers such as Cu3P, P-bearing flux and Al-Cu-P alloys. A variety of melt treatments which have profound influences on the refinement of primary silicon in the Al-Si alloy were examined. These include: adding different amount of P, mechanical stirring to speed up the P treatment, melt holding treatment, re-melting, melt over-heating treatment and P dilution treatment.
The results of a prolonged melt holding (61 h) test of an Al-0.1% P alloy showed that coarsening of AlP particles takes place during the melt holding treatment. This practically sweep off the majority of fine AlP particles (particles size below 3μm) and causes an increase in the average AlP particles size. Observations of the change in the AlP particles size indicate that the reason for the fading of P refinement in hyper-eutectic Al-Si alloys is most likely due to the coarsening of fine AlP particles in the course of melt holding.
According to the above theory, one can avoid the P fading by limiting the occurrence of coarsening of AlP particles. This can be achieved simply by restricting the P content of the melt to its P solubility. The results of P fading tests performed in Al-17 Si alloy melts containing 80, 120, 150, 200 and 400 ppm P showed that there is no fading observed in the Al-Si alloy melts which contains 80-150 ppm P. But when adding 200-400 ppm P to the alloy melt, fading occurs in about 8 hours. In a prolonged fading test of Al-17 Si alloy containing 120 ppm P, it was shown that the average primary Si particles remained nearly unchanged at about 25μm. Re-melting treatment of the prolonged fading tested Al-17 Si-120 ppm P alloy was found having little effects on the particle size of the primary Si.
In the studies of P fading tests in A390-100 ppm P and A390-120 ppm P alloys, it was found that the A390 alloy with higher P content (120ppm) exhibits a better modification effect in the early stage of melt holding period (0 – 3 hours) and a more distinct fading behavior in a long period od holding (after 8 hours). Reducing the P content to 100 ppm can effectively minimize and defer the occurrence of P fading in the A390 alloy.