Vol.11 No.1 January 214 Effect of homogenizing treatment on microstructure and conductivity of 775 aluminum alloy prepared by low frequency electromagnetic casting *Wang Gaosong, Zhao Zhihao, Guo Qiang and Cui Jianzhong Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang 11819, China Abstract: The heat treatment process has great effects on microstructure and conductivity of ingots. In this study, the ingots of high strength 775 aluminum alloy were prepared by low frequency electromagnetic casting (, and the effect of different homogenization processes (single-step homogenization at 465 for different holding times and three-step homogenization on the microstructure and conductivity of 775 aluminum alloy were studied by means of metallographic microscopy, electrical conductivity test, differential thermal analysis and X-ray diffraction phase analysis. For comparison, the ingot by conventional direct casting ( under the same conditions was also prepared. Results show that the non-equilibrium eutectic phases with low melting point in the ingot dissolve continuously into the matrix as the holding time of single-step homogenization increases. The endothermic peak of non-equilibrium phases can not be completely eliminated through a 24 h single-step homogenization, but can be eliminated after a three-step homogenization (2 /2 h + 46 /6 h + 48 /12 h. Meanwhile, the homogenization has a better effect on the ingot than the conventional ingot. Under the same homogenizing conditions, the grains of ingot are characterized by a lower content of low melting point phases and the ingot shows higher electrical conductivity than ingot. Key words: ; heat treatment; 775 aluminum alloy; microstructure CLC numbers: TG146.21 Document code: A Article ID: 1672-6421(2141-39-7 Recently, the considerable commercial and military interest in Al-Zn-Mg-Cu alloys has been developed significantly. The Al-Zn-Mg-Cu alloys have high element contents and wide solidification range. However, the structure with more precipitates at the grain boundary and larger size grains is obtained than other aluminum alloys when the conventional casting is used to produce the ingot. Moreover, such structure cannot be completely eliminated in the subsequent processing and heat treatment, resulting in the degradation of performance in toughness and corrosion resistance [1-5]. Based on the principle of Vive s CREM (Casting, Refining, Electromagnetic [6], low frequency electromagnetic casting ( was developed by professor Cui [7-9] and his research team with an AC induction coil arranged around the conventional casting mold and the * Wang Gaosong Male, born in 1983, Ph.D. His research interests mainly focus on alloy design and heat treatment process development of aluminum alloys. E-mail: wgs2424@163.com Received: 213-5-1 Accepted: 213-11-12 application of a low frequency current (<5 Hz. This new technique has been applied to prepare both round and rectangular ingots of aluminum alloys. The process showed better grain refining effect than the CREM process [6]. Improved surface quality and reduced macrosegregation were also achieved with the application of process [1-11]. However, the previous research was mainly focused on the effect of process on the as-cast microstructure and conductivity. The research on the evolution of microstructure and conductivity of the ingot prepared with process in the subsequent heat treatment process [12-14] is quite limited. It is necessary to carry out a deeper research on this subject, because the heat treatment process has big effect on microstructure and conductivity of the ingots, and the conductivity is a good way to judge corrosion-resistance. In this study, the ingots of high strength 775 aluminum alloy were prepared by casting, and for comparison, the ingots by conventional casting were also prepared under the same conditions. The homogenizing treatment 39
Research & Development Vol.11 No.1 January 214 was carried out for the ingots and the evolution of microstructure and conductivity was systematically studied. - 2. Peak: 48. 36 Onset point: 478. 19 Enthalpy: 7. 8343 Jg -1 1 Experimental procedure The 775 aluminum alloy (Al-5.6%Zn-2.5%Mg-1.6%Cu-.23%Cr-.1%Ti, by wt. ingots with 15 mm in diameter were fabricated with conventional and casting, respectively. The specific casting parameters are listed in Table 1. The disc-shaped samples with 3 mm in thickness were cut along the cross section of the ingot, and then put into the homogenizing furnace to conduct single- and three-step homogenization. Table 1: Specific parameters of casting 775 aluminum alloy ingot Casting temp. Withdrawal Water flow Field coils rate rate current 72 1 mm min -1 6 L min -1 12 A (2Hz Figure 1 shows the DSC curves of the as-cast microstructure of the ingots with two casting processes. The first peak on the curve appears at 478 indicating the melting point of the phases. Therefore the temperature of homogenization cannot exceed 478 in order to avoid the overheat during the homogenization process. The temperature of single homogenization is set at 465 with the consideration of the measurement error and control precision of the furnace. So the single-step homogenization was carried out at 465 for 6, 16 and 24 h, respectively, and the three-step homogenization was achieved by 2 /2 h + 46 /6 h + 48 /12 h. The metallurgical microstructure was observed using a Leica DMI5 optical microscope (OM. The phase analysis was conducted by means of SISCIA58. metallographic analysis software to qualitatively and quantitatively investigate the degree of solubility of the dendrite network and the area fraction of residual phases. Using Sigmascope conductivity instrument made in Fisher Company, the average value of electrical conductivity was obtained from 5 tests for each sample. The precipitates were analyzed by Pw34/6 XRD from Netherlands, with a scanning rate of 6 min -1 and a ( -1 Heat flow W g - 4. - 6. - 8. - 1. Peak: 48. 34 Onset point: 478. 41 Enthalpy: 7. 3836 J g 36 4 44 48 52 562 6 64 Temperature ( measurement range of 5 to 11. DSC analysis was performed using MDSC Q1 differential scanning calorimeter made in U.S.A, with a heating rate of 1 min -1. 2 Results Fig. 1: DSC curves of as-cast samples with two casting processes 2.1 As-cast microstructure Figure 2 shows the microstructure of 775 aluminum alloy ingots fabricated by the two casting processes. It can be seen that many non-equilibrium eutectic phases present in the ascast microstructure, and the ingot has more coarse nonequilibrium eutectic phases than the ingot. The ingot was characterized by fine and homogeneous grains. The dendrite arm spacing of alloy was about 35 μm, which is larger than that of alloy (25 μm. The area percentage of non-equilibrium eutectic phases at the grain boundary is 3.16% for ingot and 3.8% for ingot. 2.2 Microstructure of ingots after different homogenization Figure 3 shows the area percentage of the residual phases after different homogenization treatment. After singlehomogenization, the area fraction of the residual phases decreases as the homogenizing time prolongs. The area of residual phases is the smallest after the three-step (a (b 4
Vol.11 No.1 January 214 (c Fig. 2: Microstructures of 775 aluminum alloy: (a and c; (b and d Area fraction ( % 4. 35. 3. 25. 2. 15. 1. 5. 465 / h 465 / 6h 465 / 16 h 465 / 24 h 2 / 2h+ 46 / 6h + 48 / 12 h Fig. 3: Area percentage of residual phase in 775 alloy under different homogenization conditions homogenization. Figure 4 illustrates the microstructures of the ingots after different homogenization treatment. After 6 h homogenization, part of the light gray net-shaped eutectics and dark bulk phases of alloy begins to dissolve into the matrix [Fig. 1(a and (b, Fig. 4(a and (b]. As the holding time increases, the non-equilibrium low melting eutectics gradually dissolves, the dendrite network becomes sparse, and the size of residual phases gradually decreases [Fig. 4(c to (f]. The net-shaped eutectics have basically dissolved after threestep homogenization, with only a few dark bulk-like phases left [Fig. 4(g and (h]. There is no overheating in matrix and the area fraction of residual phases after three-step homogenization is even lower than that after 24 h single-step homogenization, indicating the higher efficiency of the three-step homogenization. Compared the homogenized microstructure of ingot with ingot, the ingot shows lower area fraction of residual phases with the same homogenizing time. 2.3 DSC curve analysis after different homogenization Figure 5 shows the DSC curves of the ingots after different homogenizations. The enthalpy of endothermic peak decreases gradually as the homogenizing time increases. The enthalpy of endothermic peak of ingot is smaller than that of ingot. The endothermic peak at 48 disappears gradually after homogenizing for 16 h, and a new endothermic peak at about 49 appears. Therefore, all the non-equilibrium eutectic phases with low melting point dissolve into their matrix. Moreover, there are new endothermic phases separated out, and the new endothermic peak exists even after 24 h homogenization. All the endothermic peaks disappear after the three-step homogenization, indicating again the higher efficiency of the three-step homogenization. 2.4 X-ray diffraction analysis Figure 6 illustrates the X-ray diffraction curves of ascast and homogenized alloys. The Al 2 MgCu phase appears only after single-step homogenization. Since the cooling rate of semi-continuous casting is fast, before Zn and Cu element separate out, they have solved into the non-equilibrium solid phases. Thus, the non-equilibrium solid phases can be expressed as Mg(Al, Zn, Cu 2 phase. During homogenizing process, Zn diffuses from Mg(Al, Zn, Cu 2 phase to the matrix and leads to the transformation of Mg(Al, Zn, Cu 2 to Al 2 CuMg(S. Meanwhile, ingot exhibits a broader peak of η-(mgzn 2 phase than ingot. 2.5 Effect of homogenization on conductivity Figure 7 shows the histogram of conductivity of the ingots vs different homogenization treatments. The as-cast ingot shows the lowest conductivity. The conductivity of samples increase with extending the homogenizing time for single-step homogenization, and the sample after three-step homogenization has the highest conductivity. During homogenization, the nonequilibrium eutectic phase gradually dissolves into the matrix, and then precipitates from the matrix during cooling process. With prolonging the homogenizing time, more non-equilibrium eutectic phase will be dissolved, and more non-equilibrium eutectic phase will 41
& Development Research Vol.11 No.1 January 214 (a (b (c (e (f (g (h Fig. 4: Microstructures of 775 aluminum alloy ingot after different homogenization treatment: 465 /6 h, (a; 465 /6 h, (b; 465 /16 h, (c; 65 /16 h, ; 465 /24 h, (e; 465 /24 h, (f; three-step homogenization, (g; three-step homogenization, (h 42
Vol.11 No.1 January 214-2. - 4. - 6. - 8. - 1. (a Peak: 48. 26 Onset Point: 478. 17 Enthalpy: 4. 8216 J g -1 Peak: 48. 86 Onset Point: 478. 62 Enthalpy: 4. 631 J g -1-2. - 4. - 6. - 8. - 1. (b Peak: 495. 26 Onset Point: 49. 46 Enthalpy: 1. 34 J g -1 Peak: 496. 35 Onset Point: 491. 63 Enthalpy:. 8547 J g -1 36 4 44 48 52 56 6 64 Temperature ( 36 4 44 48 52 56 6 64 Temperature ( - 2. (c Peak: 494. 31 Onset Point: 491. 26 Enthalpy:. 5492 J g -1-2. - 4. - 6. - 8. - 1. Peak: 495. 18 Onset Point: 493. 73 Enthalpy:. 362 J g -1-4. - 6. - 8. - 1. 36 4 44 48 52 56 6 64 Temperature ( 36 4 44 48 52 56 6 64 Temperature ( Fig. 5: DSC curves of 775 aluminum alloy after different homogenization: 465 /6 h (a, 465 /16 h (b, 465 /24 h (c, 2 /2 h + 46 /6 h + 48 /12 h (a (b MgZn 2 MgZn2 Al MgCu 2 2 4 6 8 2 4 6 8 (c MgZn2 Al MgCu 2 MgZn2 Al MgCu 2 2 4 6 8 2 4 6 8 43
Research & Development Vol.11 No.1 January 214 MgZn 2 2 4 6 8 Fig. 6: X-ray diffraction results of 775 aluminum alloy ingot when as-cast and after different homogenizations: 465 / h (a, 465 /6 h (b, 465 /16 h (c, 465 /24 h, 2 /2 h + 46 /6 h + 48 /12 h (e Electrical conductivity ( % ACS 35 3 25 2 15 1 5 465 / h 465 / 6h 465 / 16 h 465 / 24 h 2 / 2h+ 46 / 6h + 48 / 12 h Fig. 7: Effect of homogenizing time on electrical conductivity of alloy precipitate from the matrix during cooling, which is helpful to reduce the lattice distortion and improve the conductivity of the alloys. Meanwhile, the conductivity of ingot is higher than that of ingot. This is because the amount of non-equilibrium eutectic phases of ingot is less than that of ingot without homogenization treatments. 3 Discussion During casting process, under the influence of electromagnetic field, a forced convection is formed in the melt. The forced convection makes the aluminum melt with low temperature near to the wall of the mold flows to the center of the mold during the casting. Therefore under the influence of forced convection, the temperature field becomes more uniform. The relatively uniform melt temperature is beneficial to forming plenty of homogeneous grain nuclei in the melt, and therefore reduces grain remelting caused by local overheating and increases the number of effective grain nuclei. The grains are closely connected with each other before the growth stage of dendrite, and then they form a fine and homogeneous microstructure with a globular shape. During solidification of ingot, the low-melting-point eutectics will form at the grain boundary due to the high content of alloying element of the 775 aluminum alloy and the nonequilibrium solidification of casting process. The application of electromagnetic field can somehow improve the solid solubility [7] and then lead to the decrease of the fraction of low-melting point eutectics at the grain boundary. Thus, ingot acquires less low-melting point eutectics at the grain boundary than ingot. The refinement of original microstructure has effect on the structure change and heat treatment in the following process. The dissolution rate and dissolubility of the second phase of ingots was higher than those of ingot under the same conditions during homogenization, which was ascribed to the less nonequilibrium second phase in the original as-cast microstructure of ingot. The homogenization process is mainly based on the atomic diffusion process within the grain. The solute atoms diffuse from a higher content at the grain boundary to the grains. The homogenization process has almost come to an end when the composition becomes uniform. The dynamic equation of homogenization is as follows: (1 where, T is the absolute homogenization temperature, K; P=R/Q, R is the gas constant, J (mol K -1, Q is the diffusion activation energy, kj mol -1 ; G = 4.6/ (4π 2 D, D is the coefficient which is unrelated to the temperature, cm 2 s -1 ; t is the homogenizing time, h; L is the dendrite arm space, μm. As shown in Equation (1, when the temperature of homogenization is constant, the time for homogenization is shortened as the dendrite arm space reduces, meaning that the refinement of original as-cast microstructure can improve the diffusion speed of solute atoms during homogenization. The ingot acquires finer grains, and the atom diffusion rate of ingot is higher than that of ingot, which makes the ingot better homogenization. 4 Conclusions (1 The as-cast microstructure of ingot is characterized by finer grains, more homogeneous distribution and smaller area percentage of nonequilibrium phases at the grain boundary compared with 44
Vol.11 No.1 January 214 the ingot. (2 The three-step homogenization has a better effect than single homogenization, as it can completely eliminate the endothermic peak of non-equilibrium phases. Many MgZn 2 phases present in the ingot with three-step homogenization. (3 The ingot acquires more uniform microstructure than the ingot, and obtains better homogenizing effect under the same homogenization conditions. The enthalpy of low-melting point phases of the ingot is smaller than that of ingot after different homogenization. (4 The conductivity of the ingot is consistently higher than that of the conventional ingot under the same homogenization condition. The ingot has higher conductivity after three-step homogenization than single homogenization. References [1] Zuo Y B, Cui J Z, Dong J, et al. Effect of low frequency electromagnetic field on the constituents of a new super high strength aluminum alloy. Journal of Alloys and Compounds, 25, 42: 149-155. [2] Dong J, Cui J Z, and Ding W J. Theoretical discussion of the effect of a low-frequency electromagnetic vibrating field on the as-cast microstructures of Al-Zn-Mg-Cu-Zr ingots. Journal of Crystal Growth, 26, 295:179-187. [3] He Yongdong, Zhang Xinming, Cao Zhiqiang. Effect of minor Sc and Zr addition on grain refinement of as-cast Al-Zn-Mg-Cu alloys. China Foundry, 29, 6(3: 214-218. [4] Wagner J A, Shrenoy R N. The effect of Copper, Chromium and Zirconium on the microstructure and mechanical properties of Al-Zn-Mg-Cu alloys. Metallurgical and Materials Transactions A, 1991, 22(1: 289-2812. [5] Wang Shuang, Zuo Yubo, Cui Jianzhong. Microstructures and constituents ofsuper-high strength aluminum alloy ingots made through process. China Foundry, 27, 4(4: 28-283. [6] Vives C. Solidification of Tin in the presence of electric and magnetic field. Journal of Crystal Growth, 1986, 76: 17-184 [7] Dong J, Cui J Z, Yu F X, et al. Effect of low-frequency electromagnetic casting on the castability, microstructure, and tensile properties of direct-chill cast Al-Zn-Mg-Cu alloy. Metallurgical and Materials Transactions A, 24, 35: 2487-2494. [8] Zhang B J, Cui J Z, Lu G M. Effect of electromagnetic frequency on microstructures of continuous casting aluminum alloys. Journal of Materials Science and Technology, 22, 18(5: 1-3. [9] Zhang B J, Cui J Z, Lu G M. Effect of electromagnetic field on macrosegregation of continuous casting 775 aluminum alloy. Transactions of Nonferrous Metals Society of China, 22, 12(4: 545-548. [1] Hao H, Zhang X G, Hou X G. Technological investigation of electromagnetic casting for double-ingot. Transaction of Nonferrous Metals Society of China, 2, 1(5: 59-594. [11] Wang Z F, Cui J Z. Analysis of magnetic field for low-frequency casting of hollow ingot. In Proc. of the 2nd Asian Workshop on Electromagnetic Processing of Materials, Northeastern University Press, Shenyang, China, May 25: 299-35. [12] Lyman C E and Vander Sande J B. A Transmission electron microscopy investigation of the early stages of precipitation in an Al-Zn-Mg-Cu Alloy. Metallurgical and Materials Transactions A, 1976, 7(7: 1211-1214. [13] Lorimer G W. The mechanical of phase transformation in crystalline solids. Inst Metals, 1968, 5(4: 36-39 [14] Jin Y, Li C Z. A Microstructural Analysis of 75 Aluminum Alloy. Acta Metall Sin, 1991, 27(5: A317-323. This work was financially supported by the National Natural Science Foundation of China (youth (No. 51436 and the Fundamental Research Funds (N12392. 45