Modification of Double Oxide Film Defects with the Addition of Mo to An Al-Si-Mg Alloy

Qi Chen,W D Griffiths

doi:10.1007/s11663-020-02038-w

Abstract

In this work, Mo was added into Al melt to reduce the detrimental effect of double-oxide film defect. An air bubble was trapped in a liquid metal (2L99), served as an analogy for double-oxide film defect in aluminum alloy castings. It was found that the addition of Mo significantly accelerated the consumption of the entrapped bubble by 60 pct after holding for 1 hour. 2 sets of testbar molds were then cast, with 2L99 and 2L99+Mo alloy, with a badly designed running system, intended to deliberately introduce double oxide film defects into the liquid metal. Tensile testing showed that, with the addition of Mo, the Weibull modulus of the Ultimate Tensile Strength and pct Elongation was increased by a factor of 2.5 (from 9 to 23) and 2 (from 2.5 to 4.5), respectively. The fracture surface of 2L99+Mo alloy testbars revealed areas of nitrides contained within bi-film defects. Cross-sections through those defects by Focused Ion Beam milling suggested that the surface layer were permeable, which could be as thick as 30 μm, compared to around 500 nm for the typical oxide film thickness. Transmission Electron Microscopy analysis suggested that the nitride-containing layer consisted of nitride particles as well as spinel phase of various form. The hypothesis was raised that the permeability of the nitride layers promote the reaction between the entrapped atmosphere in the defect and the surrounding liquid metal, reducing the defect size and decreasing their impact on mechanical properties.

Highlights

DOUBLE oxide film defects are thought to be one of the main defects in aluminum castings, responsible for both reduction and variation in mechanical properties
The average remaining bubble size for 2L99 alloy and 2L99+Mo alloy were 25 mm and 15 mm in height, respectively, which suggested that the Mo addition accelerated the consumption of the entrapped air in the liquid metal
SEM examination of the sample surface (Figure 5) showed that, for the experiment with 2L99 alloy, a continuous oxide layer on the sample surface was observed, and EDX analysis suggested that this oxide layer contained Mg and O in addition to Al, suggesting the oxide might be MgAl2O4, according to thermodynamic calculation.[19]

Summary

Introduction

DOUBLE oxide film defects are thought to be one of the main defects in aluminum castings, responsible for both reduction and variation in mechanical properties. This concept was initially raised by Campbell[1] who suggested that the oxidized surface film of a liquid Al alloy could be folded-over, dry-side to dry-side, due to surface turbulence of the liquid metal during the casting operation. After solidification, these doubled over oxide film defects would form a gas-filled crevice, captured in the casting and, create an initiation site for crack growth leading to the premature failure of the casting in service. If the liquid metal velocity was less than 0.5 msÀ1 the liquid metal would well up into the mold cavity and double

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