Effect of Fe, Sr, P, Ti and Undercooling on the Precipitation of β-Al5FeSi in A319.2 Type Alloys

Abstract

The present work was performed on an A319.2 alloy with different amounts of Fe. The melt was poured from two temperatures, i.e. 750 and 950 °C. The results show that increasing the melt superheat leads to an increase in the undercooling, e.g., 1.2 and 3 °C for melt temperatures of 750 and 950 °C, respectively. The presence of oxide films accelerates the precipitation of the β-Al5FeSi iron intermetallic phase platelets. The amount of oxide films increases with the increase in the melt temperature, coupled with the addition of Sr and P. Strontium has a strong tendency to break down the length of the β-platelets, e.g., 160 and 109 µm, for non-modified and Sr-modified alloys containing 1.2% Fe, respectively. In the absence of Sr and for a melt temperature of ~950 °C, P precipitates in the form of (Al, P)O2 oxide films. These films act as nucleation sites for the precipitation of β-platelets much finer (60 µm) than those obtained for samples poured at 750 °C. The addition of P to the 319 alloy modified with Sr results in the precipitation of (Al, P, Sr)2O5, which significantly reduces the beneficial effect of Sr. Titanium precipitates in the form of Ti(Al, Si)3 platelets at 740 °C. Relatively fine β-platelets (~130 µm) can precipitate along the long sides of the Ti–rich platelets. Iron has a harmful effect on the alloy strength due to the precipitation of dense β-platelets. The addition of Sr improves the alloy mechanical properties through fragmentation of β-platelets, especially at low content of Fe. Finally, phosphorus has an abnormal effect on the three tensile parameters, and the Sr–P interaction diminishes the beneficial effect of Sr.