Papers by Author: David H. StJohn

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: Grain refinement and hot tearing are important key factors affecting the quality of castings. There have been substantial advances in the understanding of both of these phenomena over the last two decades. The paper discusses strategies for obtaining the lowest cost grain refiner addition and provides an explanation for how the refinement of equiaxed grains leads to a reduction in hot tear susceptibility. However, it also provides a warning that adding more grain refiner may not be better for reducing hot tear susceptibility. Alloy factors affecting hot tearing are also discussed. Finally, a list of six key considerations is provided to help casthouse and foundry engineers when trying to optimise grain refinement and reduce hot tearing.

Abstract: Grain refinement of cast titanium alloys is believed to have many benefits. However, literature on how to control and manipulate β-grain size during the solidification of cast components is scarce. This paper discusses the current state of research in grain refining practices in cast titanium alloys.

Abstract: This paper outlines the current state of research into laser welding of titanium and its alloys for medical applications. The differences that exist between the medical and other industries are described and a direction for advancing research in this field is proposed.

Abstract: This paper will use a new method for predicting grain size and then apply it to various solidification environments to reveal which factors are dominant in determining the final grain size. This study will only focus on methods where the grain size is set during a solidification process. These processes include grain refinement by inoculation of the melt with grain refining particles, increasing the cooling rate, low superheat casting, ultrasonic treatment and the use of chill moulds. Each of these methods can control the grain size to some extent but in order to predict the outcome it needs to be understood how the alloy constitution, inoculant particle characteristics and the casting conditions affect the prime nucleation event for the formation of new grains. These methodologies are currently being used, or have potential to be used, for the production of fine spherical grained semi-solid slurries.

Abstract: This paper provides an overview of the key areas of magnesium research and development being undertaken internationally and consideration of the future challenges confronting the further implementation of magnesium components. This paper will also include a description of some of the technologies that the CAST Cooperative Research Centre has developed and licensed for commercialisation which address some of the future challenges. There will also be a brief overview of the extent of Australia’s commitment to magnesium alloy research through the CAST CRC, the Centre of Excellence for Design in Light Metals and the CSIRO including their Light Metals Flagship program.

Abstract: The transformation of Bocar Automotive Products from an aluminium products fabricator to an innovative manufacturer supplying automakers with accessory product systems has required the adaptation of forming processes and product capabilities to address the relatively small volumes in the Australian market, strategic change in the company’s target markets and cultural change in its internal operations. The strategy for developing new capability in the casting and forming of aluminium alloy products has taken place over 5 years beginning with the bending of extruded shapes, progressing into the forming of sheet aluminium alloys and the casting of bumper bar components, and finally into the integration of these processes into bumper systems for a number of 4 Wheel Drive models. This paper describes the product design, metal forming and production changes that Bocar has introduced and integrated with its marketing strategy to become a recognized innovator in the accessories market in Australia.

Abstract: Ti6Al4V alloys are considered difficult materials to machine, especially at high cutting speeds due to their low thermal conductivity and specific heat which causes high localized cutting temperatures at the tool-workpiece interface. For these reasons machining titanium alloys usually results in excessive tool wear and its low modulus promotes chatter. This regenerative vibration or chatter is a significant problem. In the investigation reported here, vibration monitoring has been used to optimise machining processes by correlating machining process parameters with vibration severity. Machining experiments were carried out under wet and dry machining conditions while vibrations were measured and analysed. The results have demonstrated that the application of a vibration monitoring system can be an important tool to increase machining speed.

Abstract: The CAST Cooperative Research Centre was established in 1993 as a joint venture between industry, research and government partners to carry out research and development in the area of light metals. Over the thirteen years since its formation CAST has developed a world class research program and, in addition to the significant application of outcomes by our industry partners, much of our research is being commercialised. In addition, CAST has developed a significant education and training program, a best practice technology transfer program for small and medium enterprises, consulting and design activities and established an alliance with Advanced Magnesium Technologies as a provider of their research and development needs. The development of CAST has been achieved by cooperation between all our partners who represent most of the light metals research groups and a significant proportion of light metals industries in Australia. The close involvement of the industry partners in all aspects of the operation of CAST has ensured we have a program of activities focused on delivering benefits to Australia’s light metals industry. This paper presents an overview of the magnesium research undertaken by CAST and other research groups such as the Centre of Excellence in Design of Light Metals and the CSIRO including their Light Metals Flagship program. Some highlights are new magnesium powertrain alloys, an alloy suitable for decorative and domestic applications called AM-lite, and new CSIRO casting technologies T-Mag and a twin roll strip casting process.

Abstract: Using modifications to the Rappaz-Drezet-Gremaud hot tearing model, and using empirical equations developed for grain size and dendrite arm spacing (DAS) on the addition of grain refiner for a range of cooling rates, the effect of grain refinement and cooling rate on hot tearing susceptibility has been analysed. It was found that grain refinement decreased the grain size and made the grain morphology more globular. Therefore refining the grain size of an equiaxed dendritic grain decreased the hot tearing susceptibility. However, when the alloy was grain refined such that globular grain morphologies where obtained, further grain refinement increased the hot tearing susceptibility. Increasing the cooling decreased the grain size and made the grain morphology more dendritic and therefore increased the likelihood of hot tearing. The effect was particularly strong for equiaxed dendritic grain morphologies; hence grain refinement is increasingly important at high cooling rates to obtain more globular grain morphologies to reduce the hot tearing susceptibility.