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1、热处理对a356 铝合金组织结构和力学性能的影响 毕业外文翻译名师资料合集（完整版）资料(可以直接使用，可编辑 优秀版资料，欢迎下载）Effect of heat treatment on microstructure andtensile properties of A356 alloysPENG Ji-hua1, TANG Xiao-long1, HE Jian-ting1, XU De-ying21. School of Materials Science and Engineering, South China University of Technology,Guangzhou 51
2、0640, China;2. Institute of Nonferrous Metal, Guangzhou Jinbang Nonferrous Co. Ltd., Guangzhou 510340, ChinaReceived 17 June 2021; accepted 15 August 2021AbstractTwo heat treatments of A356 alloys with combined addition of rare earth and strontium were conducted. T6 treatment is a long time treatmen
3、t (solution at 535 for 4h + aging at 150 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 elec
4、tronic microscopy and tension test. It is 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
5、. Those samples treated with T6 achieve the 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 p
6、roperty; microstructural evolution1 IntroductionThe 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 A39
7、0 can be refined and its properties can 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 temper
8、ature and holding time, but also the as-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 caus
9、e substantial energy consumption and 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 an
10、d Mg and spheroidized Si in eutectic 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 suffi
11、cient to achieve almost the same mechanical 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 activa
12、tion energy. According to this model, 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
13、be optimized by the combination of 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
14、mechanical properties were investigated.2 ExperimentalCommercial 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 b
15、y reading spectrometer SPECTROLAB. 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
16、 hot water of 70. Samples cut from 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
17、 Si morphology change of different 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, %)SiCuFeMnMgTiZnRESr6.850.010.190.010.370.230.030.250.012Table 2 Heat tr
18、eatments in this study TreatmentSolutionAgingTemperature/CHolding time/hTemperature/CHolding time/hST550521702T65355415015Tensile 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. The
19、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 discus
20、sion3.1 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 parameters of
21、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 shown in
22、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 is observ
23、ed 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
24、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 film covering TiB2 are very important in Al-Si alloy refining. For Al-Si alloys, the effect of RE on the refining efficien
25、cy of Ti and B can be contributed to the following causes: preventing refiner phases from poisoning; retarding TiB2 phase to amass and sink; promoting the Ti(Al, Si)3 compound growth to cover the TiB2 phase. In this work, with suitable addition of Re and Sr, the microstructure of A356 alloy was opti
26、mized.Especially, eutectic Si is modified fully, which is beneficial to promote Si to spheroidize further during solution treatment.3.2 Microstructural evolution during heat treatmentThe microstructures of A356 alloys treated with solution at 550 for 2 h and ST treatment are presented in Figs. 2(a)
27、and (b) respectively, while those treated with solution at 535for 4 h and T6 treatment are presented in Figs. 2(c) and (d), respectively. From Fig. 1 and Fig. 2, after different heat treatments, the primary (Al) has been to some extent and the eutectic silicon has been spheroidized further. Both ST
28、and T6 treatments produce almost the same microstructure. The eutectic Si particle distribution and statistical mean aspect ratio of eutectic Si particle are shown in Fig. 3. After only solution at 535 for 4 h and 550 for 2 h, the mean aspect ratios of Si are 1.57 and 1.54 respectively. After being
29、treated by ST and T6, those aspect ratios of Si do not vary greatly, and they are 1.49 and 1.48, respectively.After solution or solution + aging in this study, the friction of eutectic Si particles with aspect ratio of 1.5 is 50%.The eutectic melting onset temperature of Al-7Si-Mg was reported to be
30、 more than 560 .550 is below the liquid+solid phase zone. During solution, two steps occur simultaneously, i.e., the formation of Al solution saturated with Si and Mg, and spheroidization of fibrous Si particle. The following model predicts that disintegration and spheroidization of eutectic silicon
31、 corals are finished at 540 after a few minutes (max): (1)where denotes the atomic diameter of silicon; symbolizes the interfacial energy of the Al/Si interface; is the original radius of fibrous Si; Ds is the inter-diffusion coefficient of Si in Al; and T is the solution temperature. When the Ds va
32、riation at different temperatures is taken into account, it is plausible to suggest that max at 550 is less than max at 540. From Fig. 2(a), it is actually proved that spheroidization of eutectic Si particle could be finished within 2 h when solution at 550 .In a selected area of A356 alloy treated
33、with only solution at 550 for 2 h (Fig. 4(a), the distribution of element Mg is presented in Fig. 4(b). Because there is no cluster of Mg in Fig. 4(b), it means a complete dissolution of Si, Mg into Al dendrite during this solution. From the microstructure of A356 alloy treated with T6 (Fig. 5(a), t
34、he distribution of Mg is shown in Fig. 5(b).Fig. 1 SEM images (a, b), and EDS mapping from (b) for Ti (c), La (d), Mg (e) and Sr (f) in as-cast alloy Fig. 2 Microstructure of A356 alloy with different heat treatments: (a) Solution at 550 C for 2 h; (b) ST treatment; (c) Solution at 535 C for 4 h; (d
35、) T6 treatmentFor A357 alloy with dendrite size of 240 m, uniform diffusion and saturation of Mg in Al could be finished at 540 C within 2 h. In this study, the cell size of primary (Al) is less than 100 m. It is reasonable that those solutions treated at 535 C for 4 h and 550 C for 2 h, can achieve
36、 (Al) solid solution saturated with Mg and Si because diffusion route is short, even at a higher solution temperature.Fig. 3 Statistic analysis of eutectic Si in A356 alloy with different heat treatmentsFig. 4 SEM image (a) and EDS mapping (b) of Mg distribution in alloy after only solution at 550fo
37、r 2hFig. 5 SEM image (a) and EDS mapping of Mg (b) in alloy after heat treatment with T6During aging, Si and Mg2Si phase precipitation happened in the saturated solid solution of (Al) according to the sequence in the Al-Mg-Si alloys with excess Si 21. The needle shaped Mg2Si precipitation was observ
38、ed to be about 0.5 m in length and less than 50 nm in width, and the silicon precipitates were mainly distributed in (Al) dendrites and few of them could be observed in the eutectic region 22. Because of the small size, these precipitations could not be observed by SEM in this study. However, it is
39、plausible to suggest that the distribution of Mg in dendrite Al cell zone and eutectic zone is uniform (Fig. 4(b) and 5(b). According to the study by ROMETSCH and SCHAFFER 15, the time to reach peak yield is 24 h and 1214 h at 170 C and 150C, respectively. From 150 to 190 C of aging temperature, the
40、 peak hardness varies between HB110 and HB120. Hence, it is believed that aging at 170 C for 2 h produces almost the same precipitation hardening as aging at 150 C for 15 h.3.3 Tensile properties of A356 alloysThe tensile mechanical properties of A356 alloys are given in Table 3. Due to the microstr
41、ucture optimization of A356 alloy by means of combination of refining and modification, tensile strength and fracture elongation can reach about 210 MPa and 3.7% respectively. Using T6 treatment in this study, strength and elongation can be improved significantly. For those samples with T6 treatment
42、, the tensile strength and ductility present the maximum values. 90% of the maximum yield strength, 95% of the maximum ultimate strength, and 80% of the maximum elongation can be reached for samples treated by ST treatment. However,T6 treatment spends about 19 h, while ST treatment takes only about
43、4 h. Fractographs of samples treated with T6 are presented in Fig. 6. The dimple size is almost similar with different heat treatments, indicating that the size and spacing of eutectic silicon particle vary little with different heat treatments. Shrinkage pore, microcrack inside the silicon particle
44、 and crack linkage between eutectic silicon particles were observed on the fracture surfaces.Table 3 Tensile properties of A356 alloys with different heat treatmentsHear treatmentb/MPa0.2/MPa/%As-cast2103.7ST2471785.6T62551857.0It is well known that shrinkage pores have a great effect on the tensile
45、 strength and ductility of A356 alloys. In-situ SEM fracture of A356 alloy indicates the fracture sequence as follows 4: micro-crack initiation inside silicon particle; formation of slipping band in the Al dendrite; linkage between the macro-crack and micro-crack, and the growth of crack. During ten
46、sile strain, inhomogeneous deformation in the microstructure induces internal stresses in the eutectic silicon and Fe-bearing intermetallic particles. Although the full modification of eutectic Si particle was reached in this study, those samples treated with T6 treatment do not perform as well as e
47、xpected. The main reason is probably due to the higher gas content (0.25 cm3 per 100 g Al).Our next step is to develop a new means to purify the Al-SI alloys to further improve their mechanicalproperties.Fig. 6 Fractographs of samples with different heat treatments: (a), (b) T6; (c), (d) ST4 Conclus
48、ions1) The solution at 535 C for 4 h and the solution at 550 C for 2 h can reach full spheroidization of Si particle, over saturation of Si and Mg in (Al). The heat treatments of T6 and ST produce almost the same microstructure of A356 alloy.2) After both T6 and ST treatments, the aspect ratio of eutectic Si particle will be reduced from 2.13 to less than 1.6, and the friction of eutectic Si particles with aspect ratio of 1.5 is 50%.3) The T6 treatment can make the maximum strength and fracture elongation for A356 alloy. After ST tre