Nick R. Green

Comparison of Oxide Thickness of Aluminium and the Effects of Selected Alloying Additions

An experiment was undertaken to study the oxidation of liquid superpure aluminium (SP-Al) and alloys containing Mg, Si, Cu and Fe. The alloys were held at 750 °C, for a number of different holding times up to 7 hours. A comparison of the oxidation of SP-Al (superpure) held at 700 °C, 750 °C and 800 °C for 3 hours was also carried out. On observation of the samples using SEM and EDX the oxides of SP-Al and the Al-Mg alloy grew quickly (by 5.8 μm after 7 hours and 2 μm after 1 hour, respectively), in a manner reported in the literature. The other alloys had reduced rates of oxide growth, with thickness changes between 30 nm and 0.25 μm for Al-Si, Al-Cu and Al‑Fe alloys. Changes in holding temperature showed a thick oxide on samples held at 850 °C for 3 hours.

Focused Ion Beam Milling and Imaging: An Advanced Method to Detect Fine Inclusions in Cast Aluminium Alloys

The combined function of a FIB milling technique utilising beam sizes of under 10 nm coupled with a micromanipulator and FIB imaging enables analysis of the microstructure of samples and fabrication of TEM thin foils. This is accomplished at desired locations in the same chamber without moving the sample or any mechanical sawing and thinning. In this study, the FIB milling and imaging technique was used to examine the microstructure and chemical composition of fine inclusions in an Al alloy, which are generally difficult to detect by conventional optical and/or scanning electron microscopy due to their size and volume fraction. Examples are presented of fine particles in cast commercial purity aluminium and a melt conditioned AA 5754 alloy.

The Study of the Oxidation of a Ti–45Al–2Nb–2Mn–0.2B Alloy in the Investment Casting Using an Yttria Face Coat Mould

Investment casting is an economical method to manufacture near net-shape metal components. Due to the very high thermal and chemical inertness, yttria has been widely used as the mould face coat material for the investment casting of titanium alloy for many years. An investigation was undertaken to study the oxidation behaviour of TiAl alloy during casting in a mould using pure yttria as the face coat. This research shows that the TiAl alloy was still oxidized in the mould during casting when using yttria as the face coat. During high temperature casting, the yttria in the face coat was dissolved by high temperature molten metal flow. The oxygen from the yttria face coat diffused into TiAl and interacts with TiAl to form different microstructure and phases (e.g. precipitates such as oxygen enriched Ti3Al and Al2O3 phases). Meanwhile, the dissolved yttrium was then re-precipitated at the metal interfacial area as yttria inclusions after metal cool down.

In Situ Characterisation of Entrainment Defects in Liquid Al-Si-Mg Alloy

Entrainment defects occur frequently in aluminium alloy castings during mould filling, and are very detrimental to both mechanical properties and reliability. The formation mechanisms and influences of these defects have been discussed previously, but the behaviour of the defects in the liquid metal and their evolution during solidification has not been studied in detail. In this research, samples of Al-Si-Mg alloy A356 that contained entrainment defects were scanned using ultra-fast synchrotron-based X-ray tomographic microscopy at the TOMCAT beamline of the Paul Scherrer Institut. The samples were directly viewed at both room temperature and in a fully liquid state. The reconstructed images showed three different entrainment defect morphologies, namely, entrained pores, tangled double oxide films and closed cracks. The evolution of the morphology of the entrainment defects was studied to better understand morphological changes of the defects, and the relationship between entrainment defects and microporosity.

Enhanced sintering properties of yttria face coat by addition of B2O3, YF3, Al2O3 and ZrO2

Abstract Yttria is widely used as a key material in the manufacture of the mould face coat for the investment casting of Ti or titanium aluminide alloys, as it has the required thermal and chemical inertness. Because of the high melting temperature of yttria, moulds manufactured using an yttria face coat need high sintering temperatures to achieve a good surface finish, which increases the production cost of the mould. In this research, small amounts of sintering aids, B2O3, YF3, Al2O3 and ZrO2, were added into a simple yttria face coat and sintered at temperatures from 1000 to 1400°C to investigate the influence on the sintering properties of yttria. These experiments found that the sintering additive B2O3 showed the best performance in enhancing yttria face coat sintering over the test temperatures. Sintering aids such as YF3 only slightly altered the sintering properties of yttria at temperatures around 1000–1200°C, and powders such as YAZ were seen to be more suitable for sintering at temperatures around 1300–1400°C. It may be possible to use these sintering aids for the production of investment casting moulds.

The Response of Entrainment Defect Distribution to Varied Fluid Flow Parameters in the High Pressure Die Casting of Al Alloys

High Pressure Die Casting is an economical means of manufacturing complex thin walled parts in high volumes. However, the process is prone to high levels of surface turbulence and fluid break-up, which results in the entrainment of bifilm defects. A study was carried out on a commercial casting to identify how changes in melt velocity and mould geometry affected the distribution of damaging bifilm defects, and the distribution of fracture stress. The statistical analysis of tensile test data indicated that two distinct defect populations existed; one of which caused relatively few failures, but often accounted for a large reduction in fracture stress when present. Where this defect population was not significant, Weibull moduli of 20 to 30 were achieved, comparable to sand castings with well-designed running systems. SEM fractography was also performed on a selection of samples to determine the probable cause of failure. A comparison of the SEM analysis to the statistical results indicated that bi-film defects initiated the fracture of those samples that failed at significantly lower stresses. Additionally, it was demonstrated that the geometric distribution of these cases was strongly correlated to changes in fluid flow conditions, suggesting that targeted modifications to mould geometry could increase the in-service reliability of High Pressure Die Castings.

Modelling of the Effects of Entrainment Defects on Mechanical Properties in a Cast Al-Si-Mg Alloy

Entrainment defects such as double oxide films and entrapped bubbles occur frequently in aluminium alloy castings during the mould-filling process, and are very detrimental to both mechanical properties, and reproducibility of casting properties. In this study a modelling algorithm was used to predict the formation and distribution of entrainment defects in Al-Si-Mg alloy castings. The tensile strength of cast test bars was compared with either the number of defects, or the defect concentration within the bars obtained from the simulation. A general relationship between the mechanical strength of the cast test bars and the quantity of estimated defects was apparent.