Papers by Author: Mark A. Gibson

Abstract: The development of creep resistant high pressure die casting (HPDC) alloys is one of the main focuses in magnesium research. Alloying elements like rare earths, calcium, strontium or scandium added to the necessary aluminium for die casting have already been introduced. Newly developed barium containing magnesium alloys with three levels of alloying additions were processed via HPDC and their compression creep response at 200 °C was evaluated. DieMag633 (Mg-6Al-3Ba-3Ca) displays the best creep resistance followed by DieMag422 (Mg-4Al-2Ba-2Ca) and then DieMag211 (Mg-2Al-1Ba-1Ca). Stress exponents from tests at different applied stresses were calculated. The creep tests were also accompanied by microstructural investigations and mechanical property evaluation.

Abstract: This paper reports on the results of the castability of three MRI alloys (153A, 153M and 230D). MRI153A was found to cast best, with castings produced rated with a quality approaching AZ91. MRI230D produced the next best castings, whilst MRI153M showed the worst castability across a range of conditions. However, these alloys showed a tendency to build-up oxide in the melt transfer tube leading to melt transfer problems. This was particularly severe in MRI230D.

Abstract: The application of moderate cooling rates to metal alloys of certain composition can generate metals that exhibit an amorphous microstructure on a bulk scale. This phenomenon is related to the avoidance of the nucleation of the competing crystalline phases associated with the alloy during solidification. This work describes the devitrification behaviour of the bulk glass forming Mg60Cu29Gd11 system through the use of a number of analytical techniques including DSC, laser confocal microscopy, SEM and XRD. Attention is drawn to the correlation between the more common analytical techniques and the observation of phase transformations on the surface of the metal, evident using a laser scanning confocal microscope fitted with a heating stage.

Abstract: Sheets of a Mg60Cu29Gd11 alloy were produced by twin roll casting with all operational variables, except roll speed, being kept constant. As a function of the roll speed, the structure of the as-cast sheet changed from being crystalline to fully amorphous and then back to crystalline. Through careful selection of the casting speed that is suitable for the selected alloy system and with which the exit temperature of the sheet remains within the supercooled liquid region, a malleable sheet with no surface defects is produced. This work shows that twin roll sheet casting is a viable process for the production of magnesium-based bulk amorphous sheet in a continuous manner and on an industrial scale.

Abstract: A significant research effort within the CSIRO Light Metal Flagship is aimed at developing new processes for the manufacture of (semi-finished) titanium products based on a powder metallurgy approach. The main driver for considering alternative processing and consolidation techniques to conventional ingot metallurgy is improved techno-economics associated with a reduction in processing steps and increased productivity via rapid consolidation of parts. In this respect, CSIRO has developed a process to manufacture sheet products utilising direct powder rolling; the process consists of cold rolling the powder feedstock to a green strip, which is then rapidly heated and hot rolled to consolidate the material completely. The work reported here was an investigation into the feasibility of fabricating Ti-6Al-4V strip by a blended elemental powder metallurgy route. The development of microstructures occurring during the processing and heat treatment steps has been studied. The generic roles of some process, material and heat treatment variables on the tensile properties and homogeneity of the final material have been assessed and are discussed in this paper.

Abstract: The impact properties of high-pressure die cast Mg-RE alloys were investigated. It was found that, for rare earth contents between 2-4 wt.%, the Mg-La and Mg-Nd alloys performed better than the Mg-Ce alloys in un-notched tests. The notched results appear to be related to the amount of intermetallic. In contrast, the un-notched results indicate that at some compositions the Mg-La alloys out-performed the other alloys when compared to the amount of intermetallic. It was apparent that a lamellar eutectic structure can improve the un-notched impact properties of Mg-RE based alloys even when this is not evidenced in tensile test or notched impact results.

Abstract: This paper presents an analysis, based on microstructural observations by transmission electron microscopy, of the influence of chemical composition on creep resistance of high pressure die casting (HPDC) Mg-4RE-0.6Zn alloys. The improved creep resistance, by increasing the Nd/La ratio of the rare earth (RE) mixture, is shown to be associated with the supersaturation of Nd solute in the -Mg matrix, evidenced by the formation of Nd-rich precipitates on dislocations after a thermal ageing treatment. The result indicates that solid solution/precipitation strengthening of the -Mg matrix is important to the creep resistance of HPDC Mg-RE based alloys.

Abstract: The suitability of a twin-roll cast (TRC) age-hardenable alloy for wrought applications is explored. A Mg-4Zn (wt.%) alloy, 3mm thick, was cast using the TRC route. Deviating from the traditional practice of homogenization followed by age-hardening in die/sand cast parts, the TRC sheet, cut into small strips, was hot rolled and annealed after homogenization. They were then deep drawn and subsequently age-hardened. The rollability, mechanical properties and microstructure of the alloy at different stages of processing and after forming, are presented and discussed.

Abstract: Major research and development efforts both in CSIRO and elsewhere aim to develop processes for commercial production of low cost titanium powder. These processes could reduce the current cost of titanium, but the major savings are expected to come by enabling powder metallurgical processes which transform these powders into products. Powder metallurgical (PM) processes (e.g. CIP/HIP) are well established for discrete components, but technologies for continuous production of “mill” product are immature. New processes will be needed for the economic manufacture of mill product (e.g. sheet and tube) from the large quantities of low-cost powder which will become available when the emerging powder production processes enter commercial production. The paper will present a process for the production of commercial purity (CP) sheet by direct rolling of powder. It is novel in that it avoids both the use of binders and densification via batch sintering. A roll compacted green sheet is rapidly heated under a controlled atmosphere before being consolidated to nominally 100% density by hot rolling. Following conventional batch annealing, strip samples exhibit properties approaching those of commercial wrought sheet of an equivalent grade. In order to achieve this, a number of key variables including powder chemistry, morphology and particle packing, the roll compaction and hot rolling parameters needed to be understood and closely controlled.

Abstract: Extrusion is a way to produce near net shape components from CP grade titanium powders of optimum density with minimum porosity and acceptable mechanical properties. Chemically pure, hydride/dehydride titanium powders were cold pre-compacted and extruded at 850oC under an argon atmosphere. The extrusion stress required was ~450MPa. To characterize the extrusions, the porosity distribution, qualitative microstructure and tensile properties were evaluated and compared with conventional extruded wrought titanium. Extrusion occurred after the green billets were upset to ~100% of theoretical density and adequate lubrication was applied to the die. The resultant product was 100% dense with a narrow band of surface porosity and exhibited an equiaxed microstructure of similar magnitude to the starting material. The tensile properties of the bars were observed to be significantly superior to conventionally extruded CP titanium bar products, a result associated with the much finer average grain size. Outcomes from this study have assisted in the identification of a number of key characteristics important to the extrusion of titanium from pre-compacted CP titanium powders, allowing the elimination of canning and hot isostatic pressing (HIPping) of billets prior to extrusion as per conventional PM processes.