Applications of the MC-DC casting technology to 6xxx series automotive aluminium alloys

Abstract

Environmental problems, such as global warming due to the ozone layer depletion related to greenhouse gas (GHG) emissions from fossil fuels have been drawing attention in recent years, and attempts are being made in various fields. Efforts in the field of automobiles are being made to decrease CO2 emissions by improving fuel efficiency through producing vehicle bodies with reduced weight, as well as electric cars, fuel-cell vehicles, etc. The characteristic properties of aluminium, high strength to weight ratio, good formability, good resistance to corrosion and recycling potential make it the perfect candidate to substitute heavier materials (steel) in the car to meet the need for weight reduction in the automotive industry. The 6xxx series alloy has been the most commonly used for extrusion products due to its light weight, good extrudability, strong corrosion resistance, high strength with good machining performance and weldability. Semi-continuous direct-chill (DC) casting is a well-established method and the most commonly used in wrought alloy extrusion billet manufacturing. For aluminium billets produced by direct-chill (DC) casting a fine and uniform microstructure is always desirable. A novel direct chill (DC) casting process, melt conditioned direct chill (MC-DC) casting process, has been developed for production of high- quality aluminium alloy billets. In the MC-DC casting process, a high shear device is submerged in the sump of the DC mould to provide intensive melt shearing, which in turn, disperses potential nucleating particles, creates a macroscopic melt flow to uniformly distribute the dispersed particles, and maintains a uniform temperature and chemical composition throughout the melt in the sump. Experimental results have shown that the MC-DC casting process can produce aluminium alloy billets with a microstructure that is comparably refined to those produced by other casting methods, while also demonstrating a reduction in cast defects. This work focuses on extending current knowledge of MC-DC casting process and address the capabilities and the effect of intensive melt shearing in DC cast billets on thermomechanical processing of 6xxx series wrought aluminium alloys, and mechanical properties to serve the ever-increasing demands in the automotive industry with regards to light-weighting and reducing carbon footprints in general. The study found that the Melt Conditioned Direct Chill (MC-DC) casting process demonstrated an ability to achieve grain sizes comparable to traditional DC casting methods, indicating its potential for controlling microstructural characteristics. Additionally, MC-DC casting showed some improvement in the distribution and morphology of Fe-bearing intermetallics, contributing to a more uniform microstructure than what is typically observed in conventional DC-GR casting. The mechanical properties of MC-DC cast alloys, particularly in the 6xxx series, suggested possible enhancements in tensile strength and fatigue resistance, which could make them suitable for certain safety-critical applications in the automotive industry. However, the MC-DC V3 variant experienced challenges in crash testing scenarios, highlighting the need for further process optimization and a deeper investigation into the relationship between microstructure and mechanical performance under dynamic stress conditions. While MC-DC casting may reduce reliance on chemical grain refiners, suggesting a possible greener production approach, further research is necessary to fully understand its impact on supply chains and production costs. Overall, the study suggests that MC-DC casting has potential as a promising innovation in aluminium alloy production, with opportunities to enhance efficiency and sustainability in specific applications.