Magnesium alloy VSC Machine successfully begins trial operation at GM China Advanced Technical Center
|Magnesium alloy VSC machine. Click to enlarge.|
The GM China Advanced Technical Center (ATC) in Shanghai announced the operation of its magnesium alloy Vertical Squeeze Casting (VSC) machine—the first designed for developing next-generation magnesium castings. GM said that this achievement marks a breakthrough in its lightweight materials research.
Squeeze casting is a process in which molten metal is introduced to casting cavities with minimum turbulence and then solidifies under very high pressure (typically above 100 MPa) within closed dies. The absence of turbulent metal flow, aided by the high applied pressure, can suppress gas porosity, notes Ohio State University Professor Alan Luo in a 2013 review of magnesium casting technologies.
Most automotive magnesium die castings are currently produced by the cold chamber die casting process, in which molten magnesium is fed into a shot cylinder and then then injected fast (5–10 m/s) by a plunger into the cavity, where it solidifies into a net shape part under high pressure (35–140 MPa).
Typical parts include instrument panel beam, radiator support, engine cradle, seat frame, engine block, transmission case and oil pan. Fig. 6(a) is the first high-volume one-piece die cast magnesium instrument panel (IP) beam introduced in 1996 by General Motors (GM) for its full-size van (GMC Savana and Chevrolet Express). A 12.3-kg part with a nominal thickness of 4 mm, this was the world’s largest magnesium die casting, which provided 32% mass saving compared to the steel design and significant performance improvements (improved crashworthiness and reduced vibration) and cost savings due to parts consolidation (25 parts in the magnesium design vs. 67 parts in steel). The advance of the magnesium die casting technology in the last decade has resulted in more efficient IP designs in recent GM models achieving even greater mass savings (40–45%) and part consolidation. Fig. 6(b) shows a 6.9-kg magnesium IP beam casting for Buick LaCrosse.—Luo (2013)
Despite the high productivity, the biggest drawback of the conventional high-pressure die casting process (magnesium or aluminum) is the high porosity level due to entrapped gases resulting from the injection of molten metal at very high velocities during die casting, Luo noted.
In addition to the absence of turbulent metal flow in squeeze casting, the tendency toward shrinkage porosity in the process is also reduced by using a bare minimum of superheat in the melt during casting, according to Luo.
This is possible in squeeze casting because melt fluidity, which requires high casting temperatures, is not necessary for die fill, the latter being readily achieved by the high pressure applied. In heavy sections of the casting, which are particularly prone to the incidence of shrinkage porosity, the applied pressure squirts liquid or semi-liquid metal from hot spots into incipient shrinkage pores to prevent pores from forming. Alloys with wide freezing ranges accommodate this form of melt movement very well, resulting in sound castings with a minimum of applied pressure.—Luo (2013)
GM says that its VSC system is designed to improve casting integrity through the application of high squeeze pressure during the casting process. More importantly, its fully enclosed magnesium melting and transferring system can significantly improve the performance and quality of the castings by maintaining the molten metal surface essentially isolated from oxygen which would otherwise produce oxide inclusions.
The expected benefits of squeeze castings also mean we can use castings to replace some forged components at lower cost. Our efforts to promote magnesium alloy applications will not only meet customers’ needs for better fuel economy and better performance at an affordable cost, but also take advantage of the ready availability of magnesium in China.—Jeff Wang, Lab Group Manager of the materials research team at GM China Science Lab
China now accounts for more than 80% of global magnesium output.
GM says that its new casting technology will make it easier to manufacture vehicle parts from magnesium, which is lighter than aluminum, and which will therefore help improve a vehicle’s fuel economy. Magnesium alloy is the lightest metal currently available for the mass production of automotive materials.
A part made from magnesium alloy is 30% lighter than a part made from aluminum alloy. It is estimated that a vehicle’s fuel economy will improve 7% for every 150-kilogram (331-lb) reduction in weight.
The VSC machine was jointly designed by GM’s lightweight materials research teams in Detroit and Shanghai. It was built in collaboration with an equipment manufacturer in China.
The VSC machine in located in the micro-foundry and formability lab, which is part of the Advanced Materials Lab in the GM China Advanced Technical Center (ATC) that was opened in 2011. The ATC also has mechanical testing facilities, microstructure analysis, metallography and electrochemistry labs, and is focused on developing cost-effective lightweighting technologies.
In May 2013, GM and its Pan Asia Technical Automotive Center (PATAC) joint venture in Shanghai received the 2013 International Magnesium Association (IMA) Awards of Excellence for the development and mass production of a magnesium-intensive decklid. (Earlier post.)
Alan A. Luo (2013) Magnesium casting technology for structural applications, Journal of Magnesium and Alloys, Volume 1, Issue 1, Pages 2-22 doi: 10.1016/j.jma.2013.02.002