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Abstract The manufacture of many products related to the automobile and airplane industries by semi-solid forming is nowadays increasing from environmental and economical points of view. Especially in the automobile industry, to upgrade engine capability and reduce engine weight, the5128
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aluminum engine piston has already been developed by researchers and industrial technicians. Currently, forming methods to manufacture engine piston are casting, squeeze casting and hot forging. These methods, however, include several casting defects and need post machining as well. In this study, aluminum engine piston will be manufactured by thixoforging according to forming variables. In addition, it is very important to find the effects of forming variables on the final product by thixoforging. In order to determine the effects, however, researchers and industrial
technicians have depended upon many experimental trials. In this study, a statistical approach for thixoforging has been adopted to improve experimental efficiency. Forming variables such as initial solid fraction, die temperature, and compression holding time were considered for manufacturing aluminum engine piston by thixoforging. Hardness and
microstructure characterization were carried out so that the optimal forming condition could be found by the statistical approach. The experiment to make aluminum engine piston was performed under the optimal condition found by the statistical approach.
Keywords Thixoforging . Aluminum automotive piston .Surface response analysis . Genetic algorithm .
Neural network decision maker
1 Introduction
There have been continuous efforts to develop new manufacturing processes using aluminum based alloy materials for an automotive engine piston, which is one of the most important automotive components. As the automotive engine pistons play an important role by transferring the explosive impact from the explosion chamber to the connecting rod, high thermal resistance and great structural strength is required to endure extremely high temperature and pressure. Gravity die-casting, squeeze casting, hot forging, and powder forging processes have been generally used for the manufacturing of automotive engine pistons of various specifications [1–3].
Among them, application of the gravity die-casting process is dominant enough to occupy over 90% of piston manufacturing in the automotive industry. However, as feed materials of the gravity die-casting process are handled at the completely molten state, its final product undergoes inhomogeneous solidification, which damages the integrity. Moreover, they have dendritic microstructures which lower the strength of material. Recently, the challenged hot forging process is suitable for high strength
products, because the workpiece experiences a significant amount of work hardening. However, its requirement for large forming load and the poor generosity on the product geometry could not be overlooked.
The thixoforging process is considered an effective manufacturing process that makes it possible to overcome the drawbacks of the conventional processes as mentioned above [1, 4]. Compared with the gravity die-casting
process, the superiority of the thixoforging technology lies in its low pore inclusion due to laminar flow behavior during forming [5]. From the microstructural viewpoint, the globularized structure of the preformed material in the thixoforging is also known to surpass that of the dendritic
cast structure. Moreover, low forming temperature of the thixoforging implies that the die set of the thixoforging experiences relatively low thermal load and, as a result, the die life is longer. In comparison with the hot forming process, the feed materials of the thixoforging are of high fluidity, in other words, easy to deform. In this way, not only the requisite forming load can be reduced by a noticeable amount, but also a product with complicated geometry can be formed. Especially for the automotive piston, the skirts portion has such thin walls that it requires