An investigation was carried out into the variability of tensile properties of A201 aluminium alloy investment castings. Fracture surfaces and microstructures of tensile specimens were examined by scanning electron and light microscopy. Chemical analysis of phases was carried out using energy dispersive x-ray analysis. Casting composition variations were measured using spark emission spectroscopy. The main factors that contributed to the variation of properties were incipient grain boundary melting during heat treatment, the macro segregation of chemical alloying elements and interdendritic porosity that had not sealed during the hot isostatic pressing (HIPping) process. Simple step plate castings were produced to investigate the effect of changes in section thickness on the local segregation of alloying elements. The compositional variations occur due to the flow of solute rich liquid through the dendrite interstices in the mushy zone. An investigation was carried out into the effect of titanium diboride particles on the formation of macro segregation and interdendritic porosity. It was found that by increasing the concentration of titanium diboride particles above 2.7 +/- 0.2 % the tendency for macro segregation was reduced to negligible levels. The formation of surface connected pores, as revealed by dye penetrant examination, was also reduced when this. concentration was exceeded. The change in the solidification behaviour is due to a lack of coherent dendrite formation in the mushy zone. This results in solidification contraction being compensated for by mass feeding rather than interdendritic fluid flow of liquid through the dendrite interstices. An additional effect of the presence of the particles was to prevent the formation of a low melting point'phase containing copper, magnesium and silver. This reduced the susceptibility of the alloy to grain boundary incipient melting during solution heat treatment. In the light of the findings of the experimental work the concept of casting reliability is discussed. The importance of preventing the variability of tensile properties by robust alloy design is highlighted. The future design and development of reliable components with regard to process specification and property prediction is discussed.
|Creators||Butler, Simon Andrew|
|Publisher||University of Birmingham|
|Source Sets||Ethos UK|
|Type||Electronic Thesis or Dissertation|
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