Correlation process-microstructure-properties in High Pressure Die Casting Aluminum-Silicon alloys

Nowadays automotive industry and, in general, transportation one increasingly needs light components in order to reduce total weights and therefore limit harmful emissions and fuel consumption. The diecasting process, on one hand, is a versatile and high productivity process but, on the other, the amount of defects found in the castings is high and sometimes compromising the characteristics of the final product. Thus the reduction or, at least, the limitation of defects is capital in order to obtain much more reliable components, especially in the automotive field. For this reason it is important to know how the process works and investigate all the variables and parameters that may have consequences on the products and may compromise their mechanical characteristics, or worse may cause disease. This implies an increasingly detailed knowledge of process, castings’ microstructure and properties. The paper presents the results of analyses carried out on a reference casting specifically designed to generate as many kinds of defects as possible, at different levels of severity. The purpose of such a casting is to make the evaluation of defects quality level simpler, to generate defects of different degree through controlled variations of process parameters and to make the process monitoring easier and more effective by means of in-mold sensors. The paper also presents analyses and results carried out on industrial production diecastings, adopting the same way of proceeding optimized thanks to the reference casting mentioned above. The possible correlations between process parameters, microstructure and mechanical properties have been deduced analyzing all data collected through the equipments installed on the die casting machine, as in-mold pressure, temperature, plunger velocity/displacement sensors and thermo-cameras which monitor the temperature variation of both movable and fixed mold during the entire cycle, and also by means of analyses performed on the diecastings: X-ray NDT, metallographic inspection, tensile, three points bending, hardness tests and Scanning Electron Microscopy investigations. The main defects revealed are: porosities (caused both by the presence of gas or shrinkage during solidification), oxide films, incomplete filling of the casting and cracks, presence of segregation bands and ESCs, presence of flash and deformation of the casting. All of them have been detected, analyzed and related to one or more causes.