Controlling microstructure for Al-Si casting

To reduce CO2 and energy fuel consumption in the automotive industry, increasing demands on thin walled light weight casting part, e.g. cylinder heads, are required. For this purpose, high performances Al- Si alloys are widely used to produce the thin-walled light weight structures due to its excellent filling ability and high strength. The metallurgical challenge is to ensure the filling and feeding during solidification with little flaws. This is generally achieved through a controlled setting of the microstructure with a grain refinement of Al primary dendrites and a subsequent refinement of the Al-Si eutectic. The current understanding of the fundamental metallurgical processes is not suitable to ensure safe process control with decreasing wall thickness and increasing influence of trace elements that are introduced via recycled processes. In particular, for the filling and supply of thin-walled structures, a mass feeding of fine Al dendrites is desirable, but it becomes a problem when the main alloying element Si increases from about 3 wt.%. Si poisoning to the well-known grain refiner (TiB2) occurs, however, the mechanisms are not scientifically clarified. Also, for the refinement of the Al-Si eutectic by Sr addition, there are unwanted interactions. Although a trace Sr addition can modify the eutectic Si, but it also causes a coarse grain of the Al-Si eutectic. In contrast to the primary Al, the eutectic grain size can be several millimeters, in the order of magnitude of the wall thickness. Furthermore, the eutectic grains determine the residual melts distribution in large parts, which results in pores and other casting defects. The limiting step to understand the different interactions is the ignorance of the detailed nucleation mechanisms. The aim of this work is therefore to investigate nucleation mechanisms with innovative electron-microscopic methods and to incorporate the understanding with the practical problems in foundries. Controlled solidification experiments will be performed to elucidate the grain refinement of primary Al and grain formation of the eutectic. Focused Ion Beam (FIB) will be used to position the samples, which will be further examined atomically. Finally, these findings will be used to develop new concepts for improved process controls and transfer the obtained know-how to possible applications.

Al alloy is light and strong. It has been widely used in automotive and aerospace industry. It was regarded as the second metal next to steel. Especially, Al-Si based alloy has been widely used due to its excellect castability (higher fluidity and lower hot tear tendency) and mechanical properties. More than 90% foundry Al alloy is based on Al-Si based alloy. In order to produce high quality casting part by Al-Si based alloy, at least three most important melting treatments (degassing, grain refinement of -Al and modification of eutectic Si) have been widely used to control the solidification microstructure and thereby improve mechanical properties. In terms of grain refinement of -Al, the standard Al-5Ti-1B grain refiner has been widely used due to it higher grain refinement potency and its massive production. But, for Al-Si based alloy, when the Si concentration is higher than 3 wt.%, the grain size increases and the so-call Si poisoning occurs. Avoiding or reducing Si poisoning is the first research object of this FWF project. In terms of the modification of eutectic Si, the standard Al-10Sr or Na have been widely used. Eutectic Si can be well-modified, but, due to the interactions between the nucleation site (i.e. AlP) of eutectic grains and the modifier (Sr, Na) of eutectic Si by forming Sr3P2 and Na3P and thereby reducing the nucleation sites for the nucleation of eutectic grains, coarse eutectic grains form. Due to the fact that the formation of pores or porosities occus along eutectic grain boundaries during the last stage of the solidification, the formation of coarse eutectic grains results into the formation of large pores or porosities along eutectic grain boundaries, which is very harmful to mechanical properties, in particular to dynamic fatigue properties. Achieving a simultaneous modification of eutectic Si and refinement of eutectic grains is the second research object of this FWF project. In this FWF project, one effective grain refinement metholodogy for Al-7Si-0.3Mg based alloys by using Al-2.2Ti-1B grain together with Al-10Ta master alloys has been developed, which can refine the grain size down to 150 m, avoiding or reducing the so-called Si poisoning. This is a novel observation and completely different from the exsited grain refinement theory. In addition, one effective modification metholodogy for Al-7Si-0.3Mg based alloys by using Al-5Eu master alloys with the controlled addition of P has also been developed, which can achieve a simultaneous modification of eutectic Si and refinement of eutectic grains and thereby optimize the size and size distribution of pores and porosities. This is also a novel observation and completely different from the exsited modification theory. The obtained results in this FWF project have been used for some applications.