Magnesium alloys are considered to be in the vanguard of advanced materials. They are lightweight, with a density of 1.8 g/cm3, (approximately two thirds that of aluminium alloys) and have the benefits of high specific strength and stiffness, good damping characteristics, acceptable weldability, and excellent machinability and castability. Magnesium alloys also have superior shielding capabilities against electromagnetic interference (EMI), environmental stability, effective heat dissipation and are recyclable in a range of established foundry processes. Consequently, many manufacturers, particularly in the automotive and telecommunications components industry, are starting to examine the benefits of using magnesium, and as a result the demand for magnesium sheet has never been higher. Responding to this demand, scientists at the Commonwealth Scientific and Industrial Research Organisation (CSIRO), have developed a low-cost method for casting magnesium sheet that overcomes many of the drawbacks of current casting methods.
The Magnesium Sheet Production Process
Developed and patented by the Elaborately Transformed Metals Group at CSIRO, the new process has been extensively tested and developed into a production-scale caster. The CSIRO machine produces as-cast magnesium sheet of up to 600mm wide in commercial quantities, using systems specifically developed for handling molten magnesium. CSIRO has been developing this process since 2000 and has developed an exhaustive programme to demonstrate that the process is safe, reliable, low-cost, efficient and suitable for both continuous (large) and batch (small and medium) production, while producing good quality magnesium alloy sheet from a large range of conventional and new magnesium alloys.
Technical Problems that had to be Addressed to Enable Magnesium Sheet Production
Casting magnesium, particularly sheet, is not easy. Among the problems that distinguish magnesium casting from the more conventional aluminium casting process is the tendency of the magnesium melt to oxidise and catch fire, together with its tendency to freeze faster than aluminium wrought alloys. This is mainly due to its lower specific heat (1.4kJ/kg.K) and lower latent fusion heat (370kJ/kg) compared to aluminium. Another major problem is that magnesium alloys have larger freezing ranges than aluminium wrought alloys. Most commercial magnesium alloys have a freezing range between 50-130K, compared with 10‑20K for most wrought aluminium alloys. The main problems encountered when casting alloys with large freezing ranges is the formation of surface defects and internal segregation defects in the as-cast sheet. There is also the on-going requirement to reduce the operational cost of twin-roll casting.
Economics of the Magnesium Sheet Casting Process
CSIRO’s magnesium sheet casting plant demonstrates large savings on the capital cost of conventional production. If the total production is below 40,000-50,000 tonnes per year, CSIRO’s casting technology becomes feasible for a cost-competitive product.
Advantages of the CSIRO’s Magnesium Sheet Casting Process
In the conventional production of magnesium alloy sheet, the magnesium alloy material is supplied as slabs of up to 300mm by 1,000mm in cross section, and 2,000mm in length. These slabs are first homogenised (at 480°C for alloy AZ31) for several hours and then continuously hot rolled on a reversing hot mill to reduce the slab to a thickness of 5-6mm. The sheet metal is annealed at 340°C before each pass of 20% reduction in a final finish rolling mill. This latter part of the rolling process is time consuming and progressively conducive to lower metal yield. In contrast, the CSIRO casting method enables magnesium alloy strip to be produced directly from the melt with a thickness at near-net-shape, thereby reducing capital investment and operational costs.
Sheet Thicknesses Available Using the Sheet Casting Process
Commercial quality sheet samples of 100-600mm wide and 2.3-5mm thick have been successfully cast in standard alloys (AZ31, AZ61, AM60 and AZ91) along with new magnesium wrought alloys. These samples were rolled down to 0.5-0.6mm gauges using a unique finish-rolling schedule developed by CSIRO, specifically for cast magnesium alloy sheet.
The Effect of Rapid Cooling Rates
Cooling rates achieved by the new process range from 500°C per second to several thousands. This near-rapid solidification leads to homogeneity of microstructures, refined grain size and increased solubility Enhanced precipitation nucleation within the matrix is assisted by possible increased vacancy contents and a distribution of fine precipitates (<10nm) along sub-grain boundaries in the as-cast materials. All of these characteristics contribute to significant improvement in mechanical properties.
Comparative Properties Magnesium Sheet Produced Using the Sheet Casting Process
Mechanical tests have confirmed that magnesium sheet produced by the CSIRO process has equivalent or superior properties to conventional wrought magnesium AZ31B sheet, and product prototyping has highlighted the improved performance of the CSIRO magnesium sheet materials in press forming. This process will expand the use of magnesium sheet, exploiting applications that have, until now, been prohibitive in terms of cost.