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a large forming load to fill the deep and thin cavity.
Therefore, thixoforging is considered as an alternative to remove the filling defect and can be classified as an outstanding technology for products of complex shape, such as the automotive engine piston.
In this study, aluminum automotive piston is manufactured by thixoforging. In addition, the process parameters that influence the thixoforging process of aluminum automotive engine piston are found by a statistical method and the correlation equations between the process parameters
and the quality of product are approximated through the surface response analysis. To solve the multi-objective optimization problem arising from the two response surface equations, the concept of neural network (NN)
decision maker, which can effectively reflect the supervisor’s own decisions on the optimization procedure, are also introduced. Finally, the genetic algorithm is adopted in order to reach a credible global solution and also to insert the neural network decision maker for objective evaluation.
2 Manufacture of the aluminum automotive piston by thixoforging
A schematic drawing and photograph of the experimental equipment for the piston are shown in Fig. 1.The body of the die set is pided into two parts, the upper and the lower parts. During the forming stage, the two parts are assembled by the clamping force and then separated for ejection. The ejecting part is designed to push the horizontal section portion on the sidewall of the piston. The heat-treated (HRC55) high-temperature tool steel, SKD61, is used as the die material and the cartridge heaters are equipped into the die set for preheating. A hydraulic press is used for the experiments whose load capacity is 200 ton. The feed materials are the commercial A356 rods of Pechiney Co. (USA) that are microstructurally globularized by electro-magnetically stirring. The size of the initial billet is 76 mm in diameter and 38 mm in
height. The 50 kWinduction heater is used to heat the billet as step heating. Figure 2 shows a photograph of the installed die set for the thixoforging.
3 Optimization of the process parameters
In the case of forming thin walled portions of the automotive piston by thixoforging, it is difficult to obtain uniform mechanical properties through the whole portion of the product due to solid-liquid segregation and nonuniform distribution of solidification [6]. To solve this problem, it is necessary to control the liquid fraction, the die temperature, and the compression holding time [7]. At the same time, to maximize the work hardening effect that is the main characteristic of the forging process, it is also necessary to analyze the process and to find the optimal
condition of the process parameters. Although there have been continuous efforts to analyze the thixoforging process, the difficulties in theoretical analysis have not been totally overcome. It is also troublesome to find optimal conditions of the process parameters by simulation techniques because its coverage is confined to just one or
very few factors through time-consuming computation.
3.1 Application of surface response analysis
3.1.1 Selection of input factors and target factors
In the thixoforging process, liquid fraction of the feed materials is closely related to hardness of the final products. Low liquid fraction leads to high strength by inducing the work hardening effect during deformation; this could yield the filling defects due to its low fluidity.
On the contrary, large contents of liquid fraction have been shown to improve the filling incompleteness, but this could lead to the reduction of the work hardening effect that means degradation of strength. A liquid-solid separation phenomenon that spoils uniformity of the products is also influenced by the contents of the liquid phase in the feed materials. The low die temperature that is responsible for the uneven solidification and the compression holding time are assumed to influence both uniformity and strength of the products.