Porosity formation in AlSi9Cu3 alloy castings : the influence of iron, strontium, sodium, antimony and bismuth

Otte, Matthew Otho. (2001). Porosity formation in AlSi9Cu3 alloy castings : the influence of iron, strontium, sodium, antimony and bismuth PhD Thesis, School of Engineering, The University of Queensland.

       
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Author Otte, Matthew Otho.
Thesis Title Porosity formation in AlSi9Cu3 alloy castings : the influence of iron, strontium, sodium, antimony and bismuth
School, Centre or Institute School of Engineering
Institution The University of Queensland
Publication date 2001-01-01
Thesis type PhD Thesis
Total pages 218
Language eng
Subjects 091207 Metals and Alloy Materials
Formatted abstract
Aluminium-silicon based foundry alloys have become increasingly popular engineering materials, particularly in the transportation industry. Aluminium casting alloys are used in automotive applications such as engine blocks, cylinder heads and wheels to address concerns about vehicle weight. Lightweight aluminium based castings allow designers to trim vehicle weight while maintaining performance requirements.

The use of secondary foundry alloys, produced from recycled materials, also results in cost savings to manufacturers. However, these alloys have greater tolerances for impurity elements than comparable primary foundry alloys. In order to reliably produce high quality castings from secondary alloys, it is necessary to have an understanding of the effect of impurity elements on the castability of the alloy.

Iron is a major impurity element in secondary aluminium alloys such as AlSi9Cu3, and is difficult to economically remove from melts or to reduce the concentration to low levels. Iron is soluble in liquid aluminium, but segregates strongly during solidification to form various iron-containing intermetallics. Industrial experience and previous research has indicated that the β-Al₅FeSi phase is associated with damaging porosity-related defects or reject castings.

Bismuth may also be present as an impurity element in secondary alloys. Until recently, little attention was paid to the effect of bismuth on aluminium-silicon alloys. However, recent work has indicated that bismuth may influence the morphology of the eutectic silicon phase and may also influence porosity formation in these alloys.

Minor additions of sodium, strontium or antimony to aluminium-silicon alloys are made to modify the eutectic silicon phase from acicular plates to a fibrous or lamellar morphology. It is common practice in industry to add these elements in order to achieve enhanced mechanical properties provided by the modified morphology of the brittle silicon phase. However, anecdotal evidence and previous research indicates that the use of these elements may also be accompanied by changes in porosity content, or morphology, in aluminium-silicon castings.

The mechanisms of porosity formation associated with both the presence of impurity levels of kon or bismuth, and the use of sodium, strontium and antimony to modify the eutectic is not well understood. The production of high quality, sound aluminium-silicon castings requires a deeper understanding of the role these elements play in the development of the microstructure during solidification and subsequent porosity formation.

An experimental program was carried out in which AlSi9Cu3 alloy castings containing these elements were produced using an industrially relevant casting process. A series of unmodified AlSi9Cu3 castings were produced with a range of iron content from 0.10 to 1.32%. An additional series of castings were produced to examine individually the influence of strontium, sodium, antimony and bismuth at 270, 70, 1400, and 750 ppm, respectively, in AlSi9Cu3 alloys containing 0.6 and 1.0% iron.

The porosity content of the castings was examined by pyknometry. Thermal analysis and metallographic investigations were also performed to build an understanding of the role of composition on the development of the microstructure during solidification.

Iron additions to unmodified and modified AlSi9Cu3 alloy resulted in an increase in total porosity content of the castings. In the unmodified alloy, total porosity increased from 0.9 to 1.3 vol.% and the alloy exhibited a critical iron content, above which sponge-like shrinkage porosity defects became large and erratic. The critical iron content was found to be 0.7% iron and corresponded to the solidification of the Al-Si- β ternary eutectic. Above this composition, β particles associated with the Al- β binary eutectic were found to cause localised porosity defects, particularly in areas of marginal cooling or feeding conditions. The critical iron content was found to have shifted to greater concentrations of iron by faster cooling rates and improved feeding.

Compared to unmodified alloys, strontium-modification was found to increase the porosity content in AlSi9Cu3 alloy castings, whereas sodium-modification produced significantly lower levels. Castings containing antimony and bismuth were found to have porosity levels closer to the unmodified conditions. Total porosity in the AlSi9Cu3 castings containing 0.6% iron with the Sr, Na, Sb, and Bi additions was 1.22, 0.78, 1.14 and 0.90 vol. %, respectively, compared to 1.07 vol. % for the unmodified castings. Total porosity in the castings containing 1.0% iron with the Sr, Na, Sb, and Bi additions was 1.51, 0.89, 1.25 and 1.17 vol. %, respectively, compared to 1.35 vol. % for the unmodified castings.

The changes in porosity content were also accompanied by a change in the morphology and distribution of pores in the casting. Porosity in the Sr, Na, and Sb containing castings consisted of compact, isolated, intergranular pores rather than sponge-type porosity, whereas castings containing Bi revealed a mixture of both morphologies.

A mechanism for porosity formation in the experimental alloys was developed which includes the role of β-Al₅FeSi particles and the morphology of the aluminium-silicon eutectic growth front, which is influenced by the presence of sodium, strontium, antimony and bismuth. Both the β particles and the eutectic growth front are suggested to have important implications on the permeability of the mushy zone and efficiency of feeding mechanisms. The industrial implications of the experimental results and the proposed mechanism of porosity formation are discussed and avenues for future research are suggested.

Keyword Alloys

Document type: Thesis
Collection: UQ Theses (RHD) - UQ staff and students only
 
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