1、中文 3765 字 ,2400 单词 Effect of heat treatment on microstructure and tensile properties of A356 alloys PENG Ji-hua1, TANG Xiao-long1, HE Jian-ting1, XU De-ying2 1. School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China； 2. Institute of Nonferrous Meta
2、l, Guangzhou Jinbang Nonferrous Co. Ltd., Guangzhou 510340, ChinaReceived 17 June 2010； accepted 15 August 2010 Abstract Two heat treatments of A356 alloys with combined addition of rare earth and strontium were conducted. T6 treatment is a long time treatment (solution at 535 for 4h + aging at 150
3、for 15 h). The other treatment is a short time treatment (solution at 550 for 2h + aging at 170 for 2h). The effects of heat treatment on microstructure and tensile properties of the Al-7%Si-0.3%Mg alloys were investigated by optical microscopy, scanning electronic microscopy and tension test. It is
4、 found that a 2 h solution at 550 is sufficient to make homogenization and saturation of magnesium and silicon in (Al) phase, spheroid of eutectic Si phase. Followed by solution, a 2 h artificial aging at 170 is almost enough to produce hardening precipitates. Those samples treated with T6 achieve t
5、he maximum tensile strength and fracture elongation. With short time treatment (ST), samples can reach 90% of the maximum yield strength, 95% of the maximum strength, and 80% of the maximum elongation. Key words: Al-Si casting alloys； heat treatment； tensile property； microstructural evolution 1 Int
6、roduction The aging-hardenable cast aluminum alloys, such as A356, are being increasingly used in the automotive industry due to their relatively high specific strength and low cost, providing affordable improvements in fuel efficiency. Eutectic structure of A390 can be refined and its properties ca
7、n be improved by optimized heat treatment. T6 heat treatment is usually used to improve fracture toughness and yield strength. It is reported that those factors influencing the efficiency of heat treatment of Al-Si hypoeutectic alloys include not only the temperature and holding time, but also the a
8、s-cast microstructure and alloying addition. Some T6 treatment test method standards of A356 alloys are made in China, USA, and Japan, and they are well accepted. However, they need more than 4h for solution at 540 , and more than 6 h for aging at 150 , thus cause substantial energy consumption and
9、low production efficiency. It is beneficial to study a method to cut short the holding time of heat treatment. The T6 heat treatment of Al-7Si-0.3 Mg alloy includes two steps: solution and artificial aging； the solution step is to achieve (Al) saturated with Si and Mg and spheroidized Si in eutectic
10、 zone, while the artificial aging is to achieve strengthening phase Mg2Si. Recently, it is shown that the spheroidization time of Si is dependant on solution temperature and the original Si particle size. A short solution treatment of 30 min at 540 or 550 is sufficient to achieve almost the same mec
11、hanical property level as that with a solution treatment time of 6 h. From thermal diffusion calculation and test, it is suggested that the optimum solution soaking time at 540 is 2h. The maximum peak aging time was modeled in terms of aging temperature and activation energy. According to this model
12、, the peak yield strength of A356 alloy could be reached within 24h when aging at 170 . However, few studies are on the effect of combined treatment with short solution and short aging. In our previous study, it was found that the microstructure of A356 alloy could be optimized by the combination of
13、 Ti, B, Sr and RE, and the eutectic melting peak temperature was measured to be 574.4 by differential scanning calorimetry (DSC). In this study, using this alloy modified together with Sr and RE, the effect of different heat treatments on the microstructure and its mechanical properties were investi
14、gated. 2 Experimental Commercial pure aluminum and silicon were melted in a resistance furnace. The alloy was refined using Al5TiB master alloy, modified using Al-10Sr and Al-10RE master alloys. The chemical composition of this A356 alloy ingot (Table 1) was checked by reading spectrometer SPECTROLA
15、B. Before casting, the hydrogen content of about 0.25 cm3 per 100 g in the melt was measured by ELH-III (made in China). Four bars of 50 mm70 mm120 mm were machined from the same ingot and heat-treated according to Table 2. Followed the solution, bars were quenched in hot water of 70 . Samples cut f
16、rom the cast ingot and heat-treated bars were ground, polished and etched using 0.5% HF agent. Optical microscope Leica430 and scanning electric microscope LEO 1530 VP with EDS (Inca 300) were used to examine the microstructure and fractograph. To quantify the eutectic Si morphology change of differ
17、ent heat treatments, an image analyzer Image-Pro Plus 6.0 was used, and each measurement included 8001200 particles. Table 1 Chemical composition of A356 modified with Ti, Sr and RE (mass fraction, %) Si Cu Fe Mn Mg Ti Zn RE Sr 6.85 0.01 0.19 0.01 0.37 0.23 0.03 0.25 0.012 Table 2 Heat treatments in
18、 this study Treatment Solution Aging Temperature/C Holding time/h Temperature/C Holding time/h ST 5505 2 170 2 T6 5355 4 150 15 Tensile specimens were machined from the heat treated bars. The tensile tests were performed using a screw driven Instron tensile testing machine in air at room temperature
19、. The cross-head speed was 1mm/min. The strain was measured by using an extensometer attached to the sample and with a measuring length of 50 mm. The 0.2% proof stress was used as the yield stress of alloys. Three samples were tested for each heat treatment to calculate the mean value. 3 Results and
20、 discussion 3.1 Microstructural characterization of as-cast alloy The microstructure of as-cast A356 alloy is shown in Fig. 1(a). It is shown that not only the primary (Al) dendrite cell is refined, but also the eutectic silicon is modified well. By means of the image analysis, microstructure parame
21、ters of as-cast A356 alloy were analyzed statistically as follows: (Al) dendrite cell size is 76.1 m, silicon particle size is 2.2 m1.03 m (lengthwidth), and the ratio aspect of silicon is 2.13. The distributions of RE (mish metal rare earth, more than 65% La among them), Ti, Mg, and Sr in the area
22、shown in Fig. 1(b) are presented in Figs. 1(c)(f) respectively. It is shown that the eutectic silicon particle is usually covered with Sr, which plays a key role in Si particle modification； Ti and RE present generally uniform distribution over the area observed, although a little segregation of RE
23、is observed and shown by arrow in Fig. 1(d). It is suggested that because the refiner TiAl3 and TiB2 are covered with RE, the refining efficiency is improved significantly. In the as-cast alloy, some clusters of Mg probably indicate that coarser Mg2Si phases exist (arrow in Fig. 1(d). Ti solute can limit the growth of (Al) primary dendrite because of its high growth restriction factor. The impediment of formation of poisoning Ti-Si compound around TiAl3 and promotion of Ti(Al1xSix)3
