MMC Piston Alloys
In a previous article on the subject of advanced metals, we reported on the properties and uses of aluminium metal matrix composite materials as used in the modern racing engine. As can be seen from the table in that article, metal matrix composites have a lot to offer the designer of racing engines. The obvious areas where we might seek to use stiff lightweight alloys are the parts which we need to accelerate and change direction at high frequency, and the largest and most important of these, is the piston.
Metal matrix composite pistons are not a new concept, but haven’t been widely taken up. The natural arena for development of new piston alloys would have been Formula One, but this avenue of development has been closed for the time being. The ‘trickledown’ of technology to those formulae without the large budgets that are the preserve of Formula One is an important part of the progress which has been made in racing engine design. However, even without this helping hand, MMC materials have continued to be developed and applied to engines.
Recently, Race Engine Technology magazine’s Wayne Ward discussed the subject of MMC piston materials with Dr. Jonathan Silk from metal matrix composite specialists AMC-MMC in England. The company has more than one piston alloy available, but where technical regulations do not specifically prohibit it, AMC225XE is their material of choice. This contains small particulate reinforcement in the form of ceramic particles of silicon carbide. Silk reports that the particulate reinforcement particles are very small (typically 3 microns) and this means that the alloy can be forged without difficulty to ‘near net shape’ and can be machined with similar ease to a high level of surface finish. The accompanying pictures, supplied courtesy of AMC-MMC, shows a sectioned motorsport piston forging in 225XE of the popular box-bridge type with stiffening ribs which tie together the two piston pin bosses. A further picture shows the microstructure of the material with the silicon carbide particles within the aluminium alloy matrix.
As can be seen from the table of properties presented in the aforementioned RET Monitor article, the mechanical properties of the material are very impressive, and it ticks many of the boxes that we would ask for in terms of an improved piston alloy, being stiffer and less dense than the current favourite piston alloy, 2618. It also has good thermal conductivity which is something else which is valued by piston designers. Silk states that AMC225XE is able to offer greater fatigue strength than 2618 at all temperatures.
Whilst AMC225XE is banned under the current Formula One engine regulations, it is designed to have a specific stiffness which just comes within the chassis limit of 40 GPa /(g/cc) and so still finds widespread use in Formula One and other areas of motorsport. Thankfully this means that 225XE remains available to racing engine manufacturers outside of the Formula One regulations. Dr Silk stated that the material is currently used for a range of piston applications and also that further developments of these alloys continue which should yield materials of even greater benefit to the piston designer.
Written by Wayne Ward.