The latest development work in high precision castings has shown how a new novel grain refiner, originally developed for the wrought aluminum industry, is superseding existing processes. Keith Denholm, head of engineering at Grainger & Worrall, explores its use in automotive castings.
Porosity is a term used by both engineers and end customers when discussing defects, but it 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 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, individual cylinder heads used in F1 and other mission-critical applications can be examined. Grainger & Worrall’s 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 and get a detailed picture of how castings are behaving at every stage of manufacture.
For several years Grainger & Worrall engineers have been interested in the use of additives 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.
Grainger & Worrall’s TSB-funded research into the use of a newly developed refiner, NGR (novel grain refiner), has not yet been fully validated, but the early results of a two-year project with Brunel University and Jaguar Land Rover are exciting. NGR is 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, prevalent in certain types of geometries. The use of NGR should significantly reduce the size of the feeders required and lead to a leaner, more material-efficient casting process. While the emergence of this new family of NGRs will enable much more rapid casting, it will also facilitate development of complex geometries, of particular interest to engine designers, who will soon be able to realize many of the features previously restricted by the impact of porosity.
In terms of tangible benefit to OEMs, NGR reduces distribution of integrity in complex castings. Aside from significantly improving integrity, it is much more uniform and directly relates to mechanical property efficiency. Engineers can design components around a much more accurate property, which in turn could lead to reduced design safety factor tolerances. Ultimately, this could lead to more reliable, stronger lightweight aluminum castings.
Independent testing of the new alloy is yet to be undertaken, but this significant development in grain refining could fundamentally change the way engines and other powertrain elements are developed in the next few years.
September 29, 2015
