Automotive Pistons: Material, Forming & Machining, Heat Treatment and Surface Treatment

The most common production method of aluminum piston blank is metal mold gravity casting. In particular, the current metal mold is produced by CNC machine tools, which can ensure high dimensional accuracy of the blank, high productivity, and low cost. For the complex piston cavity, the metal core can be divided into three, five, or seven parts to split the mold, which is relatively complex and not durable. This gravity-casting method sometimes produces defects such as hot cracks, pores, pinholes, and porosity of the piston blank.

In the reinforced engine, a forged aluminum alloy piston can be used. Its grain size is fine, the metal streamline is well distributed, the strength is high, the metal structure is fine, and the thermal conductivity is good. So the temperature of the piston is lower than that of gravity casting. The piston has large elongation and good toughness, which is beneficial for relieving stress concentration. However, hypereutectic Al-Si alloy containing more than 18% silicon is not suitable for forging due to its brittleness, which often causes large residual stress in the piston during forging. Therefore, the forging process, especially the final forging temperature and heat treatment temperature, must be appropriate. The cracks of the forged piston during use are mostly caused by residual stress. Forging has strict requirements on the structure and shape of pistons and high cost.

Liquid die forging is pouring a certain amount of liquid metal into the metal mold, pressurizing it with a punch, making the liquid metal fill the mold cavity at a much lower speed than in die casting, and crystallize and solidify under the pressure, so as to obtain a product with the dense structure without shrinkage cavity, shrinkage porosity, and other casting defects. This process has the characteristics of casting and forging.