Iron-rich intermetallic phases and their role in casting defect formation in hypoeutectic Al-Si alloys

Iron is the most common and detrimental impurity in aluminum casting alloys and has long been associated with an increase in casting defects. While the negative effects of iron are clear, the mechanism involved is not fully understood. It is generally believed to be associated with the formation of Fe-rich intermetallic phases. Many factors, including alloy composition, melt superheating, Sr modification, cooling, rate, and oxide bifilms, could play a role. In the present investigation, the interactions between iron and each individual element commonly present in aluminum casting alloys, were investigated using a combination of thermal analysis and interrupted quenching tests. The Fe-rich intermetallic phases were characterized using optical microscope, scanning electron microscope, and electron probe microanalysis (EPMA), and the results were compared with the predictions by Thermocalc. It was found that increasing the iron content changes the precipitation sequence of the beta phase, leading to the precipitation of coarse binary beta platelets at a higher temperature. In contrast, manganese, silicon, and strontium appear to suppress the coarse binary beta platelets, and Mn further promotes the formation of a more compact and less harmful a phase. They are therefore expected to reduce the negative effects of the phase. While reported in the literature, no effect of P on the amount of beta platelets was observed. Finally, attempts are made to correlate the Fe-rich intermetallic phases to the formation of casting defects. The role of the beta phase as a nucleation site for eutectic Si and the role of the oxide bifilms and AIP as a heterogeneous substrate of Fe intermetallics are also discussed.

Development of a non-intrusive heat transfer coefficient gauge and its application to high pressure die casting Effect of the process parameters

A heat transfer coefficient gauge has been built, obeying particular rules in order to ensure the relevance and accuracy of the collected information. The gauge body is made out of the same materials as the die casting die (H13). It is equipped with six thermocouples located at different depths in the body and with a sapphire light pipe. The light pipe is linked to an optic fibre, which is connected to a monochromatic pyrometer. Thermocouples and pyrometer measurements are recorded with a data logger. A high pressure die casting die was instrumented with one such gauge. A set of 150 castings was done and the data recorded. During the casting, some process parameters have been modified such as piston velocity, intensification pressure, delay before switch to the intensification stage, temperature of the alloy, etc.... The data was treated with an inverse method in order to transform temperature measurements into heat flux density and heat transfer coefficient plots. The piston velocity and the initial temperature of the die seem to be the process parameters that have the greatest influence on the heat transfer. (c) 2005 Elsevier B.V. All rights reserved.

Dissolution, recovery and fade of Sr master alloys in Al-7Si-0.5Mg casting alloys

A number of commercial Al-Sr master alloys of differing Sr content and product form have been added to Al-7Si-0.5Mg casting alloy melts and held at constant temperature for periods up to 7 hours following the addition. The master alloys were added to achieve a specific Sr target level of 200 ppm, and the melts were held at various temperatures (most at 710ºC, but also 670, 690, 740 and 770ºC). A total of thirty six melt trials were conducted and during each trial chill-cast disc samples were taken throughout for subsequent chemical analysis. The Sr concentration versus time data of each trial has been considered in terms of Sr dissolution/recovery behaviour, as well as Sr loss/fade. Trends in the data are identified and discussed, and implications for industrial practices are suggested.