Y. Rong, Worcester Polytechnic Institute, Worcester, MA; L. He, J. Kang, T. Huang, Tsinghua University, Beijing, China
Among cast aluminium alloys, 319 ranks as one of the commercially important alloys used in automotive applications, on account of its excellent casting characteristics and good mechanical properties. It is necessary for 319 aluminium alloy castings to be heat treated (in T6 condition) for the improvement of their mechanical properties. However, heat treatment needs not only cycle time but also energy cost. The heat treatment with residual heat of aluminum castings, immediately after their solidification, can reduce reheating, cycle time and energy consumption. In this study, step-shape castings were heat treated in T6 and the new integrated technique, and the microstructural evolution was followed by optical and scanning electron microscopy. After aging, test bars were prepared for tensile strength and hardness tesing at room temperature, to study the effect of heat treatment on the mechanical properties. The results showed that the similar microstructures can be obtained by both the new technique and the traditional technique, in which dispersed and thinner Si particles can be observed and Cu can be dissolved effectively. The tensile strength was increased by about 10%-15% and the hardness was also improved by the integrated technique. Compared to traditional T6 technique, about 50% of solution time can be saved at most to acquire the equal tensile strength.
Summary: Generally, it is necessary for 319 aluminium alloy castings to be heat treated (T6 condition) for the improvement of their mechanical properties. However, heat treatment needs not only cycle time but also energy cost. The heat treatment with residual heat of aluminum castings, immediately after their solidification, can reduce reheating, cycle time and energy consumption. The Consolidated Engineering Company has developed the “Sand Lion 3-in-1” system, in which decoring and removal of the sand from the casting, sand reclamation and solution heat treatment of the casting can be performed at the same time. The research on this integrated technology of foundry and heat treatment is carried out to understand the microstructure evolution and mechanical properties of the castings.
The effects of the heat treatment technology with residual heat of castings on the microstructure evolution of 319 aluminium alloy castings were experimentally investigated by using the methods of microscopic analysis, SEM and EDS. The results show that the shorter solution treatment time is needed to obtain the required mechanical properties and the coarsening of Si particles is restrained by the new technique. The effects of the new heat treatment technique on the thickness of castings were studied and the results show that there is almost no difference for the solutinized Cu content when the castings were solution treated for more than 3 hours, although there is big difference during the casting and the begining of solution treatment process.
The effects of solution treatment time in the new technique on the mechanical properties were investigated. The test bars , which were solution treated, quenched and artificial aged, were tested and the results showed that the tensile strength and hardness of castings are increased by about 15% and 10% respectively. Compared to traditional T6 technique, about 50% of solution time can be saved at most to acquire the equal tensile strength.
The shakeout and reclamation of the sand molds and cores was investigated and the results showed that they can be easily broken up and effectively reclaimed in the solution procedure of the new integrated technique.
The effects of T6 heat treatment with residual heat of casting on microstructure and mechnical properties in 319 aluminium alloy were studied in the paper and the reasons of the improvement on the mechanical properties were given. The results can help to ditermine the process parameters and improve the production efficiency.
