The development of effective grain refiner for the production of high performance light metal castings

Aluminium-Silicon (Al-Si) alloys are important alloys as they are lightweight (2.7 g/cc), easily recycled, corrosion resistant, durable and environmentally friendly. In comparison with steel (7.8 g/cc), they have higher strength to weight ratio. 90% of aluminium shape castings are based on these Al-Si alloys and currently, ~ 5 million tonnes per annum are being used in automotive industry. The average car manufactured in 2006 contained between 110-145 kg of Aluminium, compared to ~1000 Kg of Steel. Life cycle analysis suggest that, a 100 kg weight reduction through substituting Steel with Aluminium parts would result in a saving of 900 litres of petrol during the car's life span (200,000 km) and 2,000Kg of CO2 savings (10 g/km). The big problem for the light metal industry in increasing the further use of these lightweight Al-Si alloys with respect to replacing steel in the transport sector is the lack of ductility and yield strength associated with the large grain structure, defects and large intermetallic particles. Since Al-Si alloys are difficult to grain refine, widespread adoption is currently constrained due to excessive casting defects and inadequate mechanical properties. Higher Si levels in these alloys results in large grains, which make casting difficult, thus resulting in very high rejection rates, up to 60% for high value critical components.

Ti based grain refiners used for non-cast (wrought) Al alloys are less effective in cast Al-Si alloys due to reaction between Si and Ti. Brunel has invented an effective grain refiner that can revolutionize the foundry casting process for these Al-Si casting alloys, significantly reducing the casting defects and producing superior mechanical properties that, despite (deliberately) limited publicity, is already generating significant market interest within the materials supply chain. The Brunel grain refiner (BGR) improves castability of a wide range of aluminium alloys; eliminates hot-tearing, a common problem in castings; it improves much needed tensile strength and ductility; improves homogeneity in mechanical properties across the component; it enables thin-wall cast structures; and it tolerates high impurity levels (aluminium scrap can now be manufactured with superior properties). Additionally, it is expected to enable the casting of large and complex shaped structures using sand moulds and reduce the high (up to 60%) rejection rate during manufacture of high value components.

The unique selling points of the Brunel Grain refiner are that it is highly effective for bulk castings and easily employable by the industry. Currently there exists no effective grain refiner for Al-Si castings and previously difficult to produce high performance bulk castings without defects can now be easily processed.

Delivering benefits to a wide range of casting techniques, it should enable the production of castings with superior properties, allowing aluminium to replace a growing proportion of steel parts in the automotive sector. This will put the UK in the forefront of metallurgical technological developments. Very substantial environmental benefits are foreseen due to light-weighting and concomitant reductions in energy consumed and CO2 emitted.

Brunel has developed a novel grain refiner for aluminium-silicon alloys that refines both primary aluminium and silicon eutectic phases, which results in several advantages over current methods of grain refinement. The impact on casting technology would be significant. The Brunel Grain Refiner (BGR) provides fine grain structure at wide range of cooling rates, better fluidity during casting owing to the lack of large grains interfering with flow, less porosity with higher ductility, strength and toughness. The advantages in the casting operation would include; better process yield through reducing variation from melt to melt; less material used within runners and biscuits, meaning less internal recycling and dross loss; and less extremes in tooling temperature in die casting leading to higher tool life.

Weight savings through the use of high performance Aluminum-Silicon castings, fabricated with BGR technology, are expected to create new market opportunities in casting new automotive components that have never been cast before. The current market size for automotive Al castings in UK, EU and worldwide are 0.07 mt, 2.8 mt and 5.2 mt respectively. Market size for BGR technology is estimated to be up to £ 3 billion p.a.. Since adapting BGR would require no capital equipment investment for foundries, and little if any process changes, the market for casting is projected to be doubled in ten years. Volume car makers have identified that there is a huge potential for replacing steel parts, with Al provided the Al-Si castings are produced with higher strength and ductility, in an economical way. BGR has this potential due to the simple nature of the manufacturing process.

Current rejection rate during manufacturing process varies between 5% and 60% depending on nature of applications. This high rejection rate for high value castings should be much reduced. Considering that BGR offers, reduced porosity and macro-defects, the rejection rate during manufacturing would be minimal (<5%).

Impact on academic would be through academic publications, news letters, magazines focussed on light metals industry work, presentation and exhibitions at international Aluminium conferences.

By 2020, it is expected that there would be 1.2 billion cars worldwide. A 100 Kg weight reduction can result in 2.4 billion tons of CO2 savings (2 tonnes/car lifespan) by 2030, thus benefiting the planet's environment.

BGR allows direct recycling of aluminium alloy scrap that contains higher levels of impurities, so that post consumer Al alloy scrap as a valuable materials resource can be directly applicable for lightweight body & powertrain applications. This will make the UK less dependent on imports of expensive primary metal and contribute significantly to a lower environmental impact by re-balancing the flow of materials through the UK economy and reducing landfill and emissions.