Papers by Author: Matthew S. Dargusch

Abstract: Cellular material such as aluminium foam has been considered as a potential material for energy absorption upon impact and blast loadings. One of the most important properties that contribute to this feature is the densification strain. At high impact velocity, prediction of the densification strain from quasi-static engineering stress-strain curve has been found inadequate. Furthermore, theoretical prediction using the equation proposed by Reid et al. always over-predicts the dynamic crushing stress. Formation of the shock wave at high impact velocity is believed to further increase the densification level of the foam. However, this effect is disregarded when determining the densification strain quasi-statically. The present study aims to address this issue by determining the densification strain experimentally from impact tests. Forty cylindrical aluminium foams with three different lengths were used as projectiles and were fired towards a rigid load cell by using a gas gun. The peak forces generated from the impact were recorded and analysed. The experimental densification strains were determined physically by measuring the deformation of the foam projectiles after the tests. It is concluded that, at high impact velocity, the densification strain varies with the initial impact velocity. Therefore an appropriate value of densification strain needs to be used in the equation of dynamic crushing stress for a better approximation.

Abstract: Ti-6Al-4V machining chips have been recycled into fully dense material by a solid-state process based on equal channel angular pressing (ECAP). The as-recycled material possessed a refined microstructure and contained a series of oxide layers associated with the boundary of each machining chip. The chip boundaries were observed to dissolve into interstitial solid solution following static annealing. A series of static heat treatments within the temperature range 700 to 1000°C, for durations of between 1 and 20 hours, were performed. The effect of thermomechanical processing on microstructure and mechanical properties is discussed and compared to the as-recycled and commercially available materials.

Abstract: In this study the effects of different aging heat treatments on the properties and microstructure of a high strength, high toughness metastable β Ti, BTi-6554 (Ti-6Cr-5Mo-5V-4Al), have been compared. An initial β phase solution treatment was followed by aging at moderate temperatures in the α/β dual phase zone by either step aging directly from the solution treatment temperature or by quenching to room temperature prior to the aging heat treatment. The differing heat treatment methods have significant effects on the microstructure and mechanical properties.

Abstract: The effect of HIPping on the mechanical properties and microstructure of a cast Ti-6Cr-5Mo-5V-4Al alloy have been investigated. In the cast condition, the alloy predominantly consists of coarse β grains. In comparison to the HIPped samples, it exhibits poor mechanical properties, in particular low ductility, due to large amounts of remnant porosity. HIPping at 740°C substantially reduces the amount and size of porosity, while HIPping at 820°C and 900°C virtually eliminates all porosity, significantly improving the mechanical properties.

Abstract: Pulsed electric field has been effectively used to control and modify the solidification process in castings. In this study, a pulsed electric field at 8 volts and 19 kHz has been applied to Fe-P, Fe-S, and Fe-Si binary alloys, and microstructure and elements distribution of the three alloy samples have been inspected. The results indicated that P migrated from cathode to anode while S and Si migrated from both electrodes to the center under the pulsed electric field during the solidification process.

Abstract: This paper presents a feasible machining test to measure, compare and predict the machinability of different titanium alloys. A drilling test was developed and investigated on the two most common grades of titanium, commercial purity and Ti6Al4V. The experiments and analysis revealed that tool wear followed a characteristic pattern for all machining conditions investigated. When machining Ti6Al4V, tool life was shorter and cutting forces higher compared with commercial purity Ti. Paradoxically, despite the more difficult machining, Ti6Al4V samples had better surface integrity than commercial purity samples. A procedure was developed that could be incorporated into a real-time process monitoring device to warn of imminent tool failure.

Abstract: The microstructures of laser welds of the near-β titanium alloy TLM were studied for substrates in the as hot rolled, solution treated and solution treated and aged conditions. Irrespective of substrate condition the fusion zone consists of a cellular β structure. Trends in bulk hardness of the various conditions seem to be carried through into the weld consistently across the profile. The hardness in the fusion zone remained similar to that of the bulk. However, a drop in hardness occurs in the HAZ. Lath that formed around the hardness indent of the fusion zone and HAZ suggest that the weld undergoes a martensitic transformation to α" during deformation.

Abstract: Grain refinement of titanium alloys during solidification is believed to have many benefits for processing and properties. Recent work has emphasized the importance of solute elements in grain refining cast titanium and it was demonstrated that the growth restriction factor is useful for predicting the grain refining effectiveness of solute elements in titanium. Despite oxygen being the major impurity element present in titanium alloys and having been previously identified as a theoretical growth restricting solute, its effect as a β-grain refiner is still unexplored. This paper investigates the effect of oxygen on the grain size in cast titanium alloys.

Abstract: The elastic response of a metastable β titanium alloy, Ti-25Nb-3Zr-3Mo-2Sn, is characterised. The effects of processing and heat treatment on phase composition and mechanical behaviour are reported. The alloy exhibits pseudoelastic behaviour which is related to the reversible formation of the stress induced α" phase. The solution treated condition displays the greatest pseudoelasticity and the hot rolled condition the least. Tensile deformation reduces the modulus of the alloy. This is due to increases in the relative proportions of the low modulus α" phase.

Abstract: A metastable β titanium alloy, BTi-6554 (Ti-6Cr-5Mo-5V-4Al) has been developed for structural applications in aircraft because of its high strength, high toughness, and good fatigue properties. This paper reports on the effect of heat treatment on microstructure and microhardness of the alloy. It has been shown that in the as hot rolled condition, the alloy consists of a single β phase. Heat treatment between 450-750°C results in the precipitation of α laths, while exposure to temperatures between 700-800°C results in the gradual transformation of the α phase back to β phase with larger grain sizes resulting from higher heat treatment temperatures.