Fluidity of Aluminium Foundry Alloys

Abstract

The fluidity of an alloy plays a key role for the foundry and transport industries as it affects the quality and soundness of the cast products. Particularly, fluidity influences the reject rates, hence casting costs and the production of thinwalled, hence light components. Fluidity is a complex technological property and depends on many parameters. However, many aspects of this subject are still not fully understood. The motivation of the research presented in this doctoral thesis was, therefore, to fill this gap in knowledge. The study has aimed at understanding the influence of various parameters on the fluidity of aluminium foundry alloys and, in particular, Al-Si foundry alloys.

A literature review of previously reported results on fluidity was carried out. It was found that a lack of a highly reproducible test method as well as some contradictory results existed in the literature. Therefore, a new fluidity test method was developed. To study the accuracy and reproducibility of this test was one of the goals of this work. The new test method allowed a constant melt superheat, which is considered as one of the major factors affecting fluidity measurements, and a constant pouring velocity. It was found that the reproducibility of the new method was higher than previous methods.

The effect of casting temperature, and hence melt superheat, was assessed through a series of tests. A linear relationship between casting temperature and fluidity length was observed.

The effect of grain refiner on the fluidity of an A356 alloy was systematically investigated. The fluidity lengths without grain refiner and with three additions of Al-5wt%Ti-1wt%B master alloy were measured. The results showed that grain refinement reduced the grain size throughout the spiral somewhat, particularly at the tip, but there were no statistically significant effects on fluidity.

The effect of dissolved hydrogen was also investigated in this study. The hydrogen content was drastically increased by plunging pieces of wood beneath the surface of the molten metal. The fluidity of this melt was measured and compared to a melt with low hydrogen content. It was concluded that the difference in fluidity between the melts with different hydrogen levels was not significant.

The effect of minor alloying elements (Sr, Ti, Fe and Mg) on the fluidity of Al-7wt%Si alloys was investigated. The Design Of Experiment (DOE) technique and the Taguchi approach were used to design the experiments. The Analysis Of Variance (ANOVA) was performed to analyse the results. It was concluded that the addition of minor alloying elements to a major alloy system, e.g. Al-7wt%Si, does not significantly affect its fluidity and the melt superheat had a far greater impact on fluidity than the minor alloying elements.

The effect of mould coating on fluidity was studied on a commercial strip mould which consisted of a H13 die with five channels of different cross sectional areas. The coating was sprayed to achieve a thickness of 0.2mm. Fluidity measurements were performed on the uncoated and coated mould. It was concluded that mould coating significantly increases fluidity. In addition, fluidity measurements on the uncoated and coated mould were undertaken at two different melt superheats and it was found that coating the mould plays a more significant role at low melt superheats.

The effect of oxide content on fluidity was also investigated. Three alloys, namely a standard A356 alloy, the same alloy with 20% (A356+20%) and 50% (A356+50%) re-melted turning chips, were used and their fluidities compared. Qualitative analysis on the type of oxides present in the three alloys was carried out with a PoDFA test apparatus and the oxide level was quantified with optical microscopy analysis. The results showed that the addition of turning chips significantly increased the oxide content. Among the investigated alloys, A356 without turning chip additions showed the lowest oxide content and the highest fluidity. No significant differences in either oxide content or fluidity were found between the A356+20% and A356+50% melts.

Two fluidity test methods, a commercially available one and an experimentally developed test, were used for measuring the fluidity of Al-Mg-Si alloys. Although the two methods were different, they gave consistent results.

Numerical simulations of fluidity tests were carried out on an A356 alloy and the results showed that numerical simulation software can be a useful tool for predicting fluidity in aluminium foundry alloys.

These are the major findings achieved by this thesis work which contribute to improve our understanding of the effect of several key variables on fluidity. It is believed that these results will solve some of the problems currently encountered in foundries and improve their processes.