The effect of transition metal additions on double oxide film defects in Al alloy castings

Chen, Qi (2017). The effect of transition metal additions on double oxide film defects in Al alloy castings. University of Birmingham. Ph.D.

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Abstract

This work investigated the effect of transition metal additions on the double oxide film defects in Al alloys.
A bubble trapping experiment was initially conducted, which deliberately trapped an air bubble inside the aluminium melt for a period of time in three different Al alloys (Commercial purity aluminium, 2L99(Al-7Si-0.35Mg) and Al-5Mg alloy), as an analogy of the consumption of the entrapped atmosphere in double oxide film defects in castings. Several elements, namely, Mo, Ti, Zr, Hf, Sc were selected and added into the aluminium melt. The result suggested that the three different alloys behaved differently with regard to the consumption of the entrapped bubble and the different oxide/nitride layers formed. Only the addition of Sc and Mo altered the structure of the oxide surface and promoted the consumption of the air in the trapped bubble in the 2L99 aluminium alloy melt.
Sand casting was subsequently conducted for 2L99 alloy with different element additions. Mo and W were found to improve the Weibull moduli of the UTS. Statistical analysis confirmed that such improvement was significant. For the castings with Mo addition, a nitride was found in some double oxide film defects, on the fracture surface of the tensile testbars. This was unusual, as bi-film defects in aluminium castings usually have a short solidification time and do not have enough time to consume the majority of their entrapped oxygen. The formation of the nitride on the surface of bi-film defects in +Mo castings, suggested that the majority of the oxygen was depleted and a reaction was going on between nitrogen and liquid aluminium. The formation of the permeable nitride surface layer on the bifilm defect might promote the consumption of the entrapped gas. This should lead to reductions in the bi-film size and an improvement in mechanical properties. For +W casting, W containing intermetallic compound might be nucleated on the sides of the bifilm and drag the bi-film to the bottom of the casting, resulting in a clean melt and improving mechanical properties.
The effect of Si modifier addition on the bi-film defect (Na, Sr and Ba) in 2L99 sand castings was also investigated, which suggested the addition of modifiers resulted in a reduction in the mechanical properties of the 2L99 castings while the bi-film defect content in the casting was high but significantly improved the Weibull moduli of the UTS of 2L99 castings while the bi-film defect content was reduced. The results suggested that the modifier addition tended to aggravate the effect of bi-film defects on mechanical properties by increasing the defect size.
Ti and Mn additions into 2L99 alloy were found to not significantly affect the mechanical properties of the casting. A porous oxide structure was found on only one of the fracture surfaces (out of ten) of Ti containing testbar examined, which hardly affect the mechanical properties of the casting. For the +Mn casting, the composition and structure of the oxide was not affected by the addition. The reduction of the mean value of the UTS for both element additions could be due to bi-film defects being introduced during master alloy preparation, master alloy addition and during degassing before casting.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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