Surface turbulence and the tensile properties, reproducibility and reliability of cast irons – a study of entrainment defects

Nashwan, Zakareya (2023). Surface turbulence and the tensile properties, reproducibility and reliability of cast irons – a study of entrainment defects. University of Birmingham. Ph.D.

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Abstract

Mechanical properties, and their reproducibility and reliability, of several cast alloys are negatively influenced by surface turbulent filling as a result of entrainment defect formation. It is therefore important to examine the effect of entrainment defects and surface turbulent filling on the mechanical properties, reproducibility and reliability of cast irons. Resin-bonded sand castings were made of three types of cast irons, spheroidal graphite iron (SGI), lamellar graphite iron (LGI), and compacted graphite iron (CGI) in testbar moulds with three filling designs: quiescently-filled (QFM); surface turbulently-filled (TFM); and very (surface) turbulently-filled (VTFM). Subsequently the castings were tensile tested, and the fracture surfaces examined.

Regrading SGI, the tensile test showed a negative effect on surface turbulence. The ultimate tensile strength of the QFM samples had a Weibull modulus 92\% higher than TFM, and 212\% higher than VTFM. The \%Elongation of QFM samples had a Weibull modulus 63\% higher than TFM, and 131\% higher than VTFM. Samples with low tensile properties were found to contain entrainment defects that formed due to the folding over of surface films during casting. The entrainment defects were either oxide bifilms (magnesium as the main constituent), lustrous carbon bifilms, inflated bifilms (nucleated by shrinkage or gas pore), and bifilm entraining inclusions (sand, slag, or both). The magnesia bifilms were only a nanometer in thickness.

Regarding LGI, the tensile tests performed on TFM and QFM samples showed mixed evidence for the effect of surface turbulence on LGI casting. However, the Ultimate Tensile Strength of the VTFM samples had a Weibull modulus 60\% lower than the average between QFM and TFM Weibull modulus. Samples with low tensile properties were found to contain no evidence of entrainment defects that could be the cause of deterioration. Limited evidence of entrainment defects was found in LGI tensile testbars, either oxide bifilms (silicate as the main constituent), or bubble trail defects. Further, a reduced pressure test was employed to examine evidence of entrainment defects in LGI, and no evidence was found. LGI die cast in an oxygen-rich environment contained evidence of oxide bifilm formation.

Regarding CGI, the tensile test results showed mixed evidence of the effect of surface turbulence. The Ultimate Tensile Strength of QFM samples had a Weibull modulus that was on average 26\% higher than TFM, but surprisingly, VTFM had a Weibull modulus 108\% higher than QFM. The \%elongation values followed the same trend, QFM had a Weibull modulus on average 44\% higher than TFM samples, and VTFM had a Weibull modulus 120\% higher than QFM. Samples with low tensile properties were found to contain no evidence of entrainment defects that could explain the deterioration. Novel defects with plate structures were observed in several samples with low tensile properties that were cast in TFM. The defects were only observed in one melt.

Entrainment defects in cast iron behaved similarly to entrainment defects in aluminium by exhibiting the following: they could form from several constituents, they could act as nucleation sites for shrinkage and gas pores, they could entrain different types of inclusions, the wetted surfaces of the defect could act as a nucleation site for precipitates, and the entrapped atmosphere in the defect might gradually be consumed by reaction with the surrounding liquid metal. Finally, due to its stable oxide surface film, SGI is the main cast iron alloy similar to aluminium in regards to the negative effect of surface turbulence on casting quality.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):