New powder metallurgy (P/M) aluminum alloy and new extrusion process have been developed for cylinder liners in high-speed motorcycle engines. The new alloy composition is Al-26Si-0.8Mg (mass%).
Most commercialized high Si P/M aluminum alloys had a matrix of 2000 series and containing 3 - 5 mass % Cu which increases the hot extrusion resistance and lower the solidus temperature of the alloys. On the other hand, the new alloy matrix contains a small amount of Mg (0.8 mass%), and this composition resulted in a good extrusion surface, higher extrusion speed and higher recovery.
The new manufacturing process of powder billets has been developed. Rapidly solidified aluminum alloy powder prepared by the air-atomization technique was cold-isostatic- pressed (CIP) into a cylindrical shape (260mm in diameter and 420mm in length) and vacuum sintered. The sintered billet was transferred by mass production line conveyers without any breaking problems and quickly pre-heated by the induction heater before seamless extrusion. This new process enabled P/M billets to be treated as same process and time schedule as I/M (Ingot Metallurgy) billets. In the conventional powder extrusion, CIP billets were carefully transferred manually and they were heated up in a inert gas furnace for long hours. Also this process does not include the canning process and it has two advantages. Firstly, scalping process is not needed after extrusion, and secondly, the top end and the discard of the powder extrusions can be directly re-melted for an ingot of the same alloy for atomization without any composition adjustment. A novel P/M extrusion process including recycle was achieved by this billet making process in mass production scale.
Among the each manufacturing process, the sintering condition of the billet was carefully chosen. The higher the sintering temperature, the harder the sintered billet, but the hardness of the extruded tube decreased. At lower sintering temperatures, the hardness of the billet was not enough to be transferred by production line conveyer and blistering problem occurred after T6 heat treatment of the extruded tubes. Finally 560C was chosen as a standard sintering temperature.