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Metal Casting Technologies : June 2010
22 www.metals.rala.com.au ack in 2001, carmaker General Motors began a research project which aimed to demonstrate and enhance the feasibility and benefits of using magnesium alloys in place of aluminium in structural powertrain components. The idea was to achieve at least 15 per cent mass reduction in cast components. Together with Chrysler, Ford and a host of other organisations, GM has now completed it study and recently released its results which show that magnesium is ready for ready for cost- effective, mass reduction of major powertrain components. The final engine achieved nearly twice the original target, with a mass reduction for the magnesium components of 28 per cent. Cost analysis demonstrated that this mass reduction cost less than US$4 per pound (US$1.79/kg). The research team was able to physically test the magnesium components and assembled engines. With the exception of the bulkhead failure in the DTS engine, all other engine tests were successful. Metallurgical analysis and FEA (finite-element analysis) results confirmed the team's hypothesis about the failure of the insert/magnesium interface. They concluded that the thermal expansion mismatch problem could be avoided through redesign. The Magnesium Powertrain Cast Components (MPCC) project team's results indicate that the anticipated technical issues for the magnesium-intensive engine (creep behaviour and bolt load loss, corrosion, CTE (coefficient of thermal expansion) mismatch, and NVH) do not appear to be "show stoppers" in the implementation of magnesium engine components. However, field performance and robustness have yet to be demonstrated. Over the course of this project, the research teams' collaborations yielded considerable valuable information about creep-resistant magnesium alloys, their castability, designing with them, and the cost factors entering into achieving cost- effective mass reduction. Approach and accomplishments The MPCC project team's vision was of a magnesium-intensive engine that is cost-effective, light weight, and meets the manufacturability and durability requirements of the automotive industry. The approach taken was the redesign of an aluminium production engine (2.5L Ford Duratec) to a magnesium-intensive version; that is, to convert the cylinder block, bedplate, oil pan, and front engine cover to magnesium. All other parts of the engine were production carryover. The design, materials testing, tooling design and casting of the parts, and both component and assembled engine dynamometer testing were accomplished in prior years. Component and engine testing were undertaken. The cylinder block passed thermal cycling and thermal soak testing; the head gasket passed pulsator testing: assembled engines with the full complement of magnesium components (block, oil pan and front engine cover) passed hot and cold scuff tests and a 672 hour coolant durability test; and an engine with magnesium oil pan and front engine cover passed a 675 hour high speed durability test. FEA analysis was completed and the results confirmed the failure mechanism of the bulkheads during break-in operation of the engine for the Deep Thermal Shock Engine Test. The apparent cause of failure was little or no load transfer at the iron insert/magnesium interface due to the absence of a metallurgical bond between the insert and the magnesium. This problem can be overcome in future designs and is thus not a showstopper for the magnesium-intensive engine. Completed analysis of the coolant and teardown analysis of magnesium-intensive engine which was subjected to the 672 hour Coolant Corrosion Engine Test. The coolant protected the magnesium cylinder block very well and demonstrated that coolant corrosion is not an obstacle for the magnesium- intensive engine. An NVH (noise, vibration, and harshness) assessment was carried out, of the magnesium-intensive engine, the GM testing of magnesium parts a success By Bob R. Powell - General Motors Research & Development Center, USA. Email: email@example.com B LIGHT METALS R&D THE APPARENT CAUSE OF FAILURE WAS LITTLE OR NO LOAD TRANSFER AT THE IRON INSERT/MAGNESIUM INTERFACE DUE TO THE ABSENCE OF A METALLURGICAL BOND BETWEEN THE INSERT AND THE MAGNESIUM