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Refined thinking

Any engineer involved in engine or powertrain development will understand the problems that porosity can bring to structural alloys. While preventative design can help to address the issue, a recent project involving Brunel University, Jaguar Land Rover and Grainger & Worrall, highlights the potential for a new generation of grain refiners.

A term used by both engineers and end customers when discussing defects, porosity does not adequately describe the many ways in which the problem manifests itself. Used as a catch-all phrase, it can include shrinkage in the form of micro-pores, sponge type voids, large macro-voids.

Understanding the subtleties of various imperfections obviously helps to inform the process of casting design – which can reduce defects. While some faults can be fixed during the casting process, others can be reduced through design changes or a combination of both. By knowing the factors that can contribute to the different defects, design engineers can relocate porosity-prone areas to non-structural areas of the part, thus achieving acceptable levels of quality.

Looking at the higher end of the quality casting range, which includes high-performance parts for the aerospace, automotive and motorsport sectors, the ideal scenario is ‘zero porosity’ – as opposed to the highly challenging (and time-consuming) management of the condition. Using the latest CT scanning technology, we can examine individual cylinder heads used in Formula 1 racing and other mission critical applications.

Our motorsport team was the first in the UK to employ advanced CT scanning to gain a better understanding of a casting's integrity and geometric accuracy. Thanks to this ability to examine both the interior and exterior of the parts, we now see a detailed picture of how castings are behaving at every stage of manufacturing. Such an appreciation of the different geometries enables us to calculate differential contraction rates, rapidly validate and define evolutionary changes to tooling, creating more precise castings.

For several years now, Grainger & Worrall engineers have been interested in the use of additives which are employed in the casting process to reduce the level of shrinkage porosity. Titanium di-boride (TiB2) particles are good grain refiners of aluminum alloys and have traditionally been used extensively in aluminum foundries, however the grain refining effect of these particles are significantly reduced by one of the most common alloying additions for shape casting – silicon.

TiB2 is widely used in wrought aluminum casting, where alloying with silicon is far less common, but its use has spilled over into shape casting where high silicon contents significantly reduce its effectiveness. Without a viable, effective alternative, many foundries have continued to use it. This is a key challenge facing the casting industry, especially in the automotive, aerospace and other high-performance sectors as casting complexity continues to increase.

Our TSB-funded research into the use of a newly developed refiner, which we call NGR (Novel Grain Refiner), has not yet been fully validated, but the early results are exciting. Following our two-year project, working with Brunel and JLR, we’re confident that the NGR we’ve been testing can significantly reduce the presence of shrinkage porosity in the casting process.

While this could be heralded as a ‘game-changer’ for engine development programs, it would be fairer to describe NGR as one of a series of important, enhancements to the casting process that are required to support the ever more demanding design requirements and development schedules of structural automotive cast components.

The adoption of this new refiner leads to less shrinkage porosity, which is prevalent in certain types of geometries. Traditionally the use of additional feed metal on the casting, external to the final component geometry is used to control shrinkage porosity. The porosity forms in these ‘feeders’ as they are the last areas to solidify and then they are subsequently machined off. The use of NGR should significantly reduce the size of the feeders required and therefore lead to a leaner more material efficient casting process.

While the emergence of this new family of NGRs will enable much more rapid casting – thus reducing the time it takes to design and manufacture structural castings – it will also facilitate the development of complex geometries. This final point is of particular interest to engine designers, who will soon be able to realize many of the features previously restricted by the impact of porosity.

While we are still testing to validate the long-term structural integrity, repeatability and real-world performance of the NGR in cylinder head castings, the future looks bright for this innovation. Independent testing of the new alloy is yet to be undertaken, however, this significant development in grain refining looks set to fundamentally change the way that engines and other powertrain elements are developed in the next few years.

Working in partnership with Brunel University, JLR, ESI, AMG Superalloys and Innoval, Grainger & Worrall is confident that this type of funded research is crucial to the long-term competitive position of the UK’s engineering supply chain.

 

2 April 2015

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