Adam Brown - Exploring Synchotron X-Ray Scattering for Identifying Nucleation in Casting

Metal casting is a 6000-year-old manufacturing process which remains vitally important for industry today. Despite thousands of years of practice we still do not completely understand exactly what happens when metals transform from liquid to solid. In this article Adam Brown of the Department of Engineering describes his research which will help reveal some of these fundamental processes. 

About My Research

Most metal casting processes, whether performed on simple, pure metals or complex multi-component alloys, make use of extra materials and additives which assist the process and improve the properties of the cast part. Some of the most important additions are grain refiners; small particles which are added to and distributed throughout the molten metal. They provide ideal sites for nucleation to occur – that is, the initial transformation of liquid metal to solid. In doing this, they help to develop an optimum grain structure in the solid metal, thus ensuring the desired material properties are obtained in the cast part. 

Grain refiners are very widely used in casting processes; however, the way in which liquid metal behaves around the particles is not fully understood. By looking at the interface between a refiner particle and the surrounding liquid metal, we can probe exactly what is happening as the system is cooled. We aim to explore exactly how refiner materials promote nucleation and improve our understanding of this important process.

Research Approach

Synchrotron facilities are large, complex and extremely powerful machines which generate very high intensity X-Ray beams by accelerating electrons around a circular path. The electrons shed energy when forced to travel in this circular motion; this energy is lost in the form of X-Rays which are focused into an intense beam. Much like a doctor can use X-Rays to look within the body of a patient, engineers and scientists can use X-Ray beams to probe deep within the structure of metals and other materials. In this project, we have used the UK’s national synchrotron facility - Diamond Light Source - located in Oxfordshire, and shown in the picture below.

Diamond Light Source, Oxfordshire, UK

A small sample, designed to represent a model of the refiner/liquid metal system in casting, is mounted on equipment which brings it into the path of the X-Ray beam. The beam scatters from the sample in a characteristic way, producing a pattern which is a representation of the structure inside. This scattering pattern is then monitored as the sample is heated and cooled to observe how the structures change at different temperatures, and specifically to observe how the liquid behaves as it begins to solidify.

Research Findings

Casting remains an extremely important process in modern manufacturing, with applications in aerospace, automotive and civil engineering to name a few. There is huge expertise in the UK, and particularly in the East Midlands, with the Rolls-Royce precision casting foundry in Derby. As the complexity of casting foundries and the parts to be manufactured increases, so does the desire to greater understand the fundamental processes within.

Grain refinement is a common practice in casting yet is in essence based on trial-and-error and experience rather than solid, scientific understanding. In aluminium casting, for example, a material called titanium diboride or ‘TiB2’ is the industry standard refiner; however, the science suggests that this material should not be an effective nucleating agent for aluminium, though it appears to be so. Similarly, recent research has suggested that the natural oxide layers which form on metal surfaces may have some grain refining capabilities, despite the science suggesting otherwise. This project represents a novel approach to investigating this problem; using synchrotron facilities allows the solidification to be monitored in-situ rather than simply looking at solidified samples.

In the current study we used aluminium as the model casting material, and aluminium oxide or ‘Al2O3’ as a model refiner. So far, we have discovered information about the temperatures required to instigate nucleation, the types of structure that form in the aluminium as it cools, and the strain present in the solid once solidified. Data analysis is ongoing and it is hoped that this will reveal further detail about the atomic structure of the aluminium when it is in the liquid state.

Further studies in the summer of 2013 will continue to look at Al/ Al2O3, as well as the Al/ TiB2 system discussed previously. Our results will thus have great relevance to industrial processes and allow a new perspective on nucleation in casting systems. In the longer term, the results and the techniques we have developed will help in the selection of new refiner or casting materials, and in the design of casting processes and foundries themselves. This will allow more efficient and cost-effective operation and greater control over the material properties which can be achieved in cast parts.

About Adam Brown

Adam BrownAdam Brown is a research student working towards completion of his doctoral degree in the Department of Engineering.

Adam is supervised by Professor Hongbiao Dong.

Department of Engineering
University of Leicester
University Road
Leicester
LE1 7RH

Adam will present his work at the Festival of Postgraduate Research 27 June 2013 - see Adam's Festival poster.

The Festival is open to all members of the University community and the public - book your place here.

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