Effect of Annealing Process on Microstructure and Properties of ... - Core

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Procedia Engineering 00 (2011) 000–000 Procedia Engineering 27 (2012) 895 – 902

Procedia Engineering www.elsevier.com/locate/procedia

2011 Chinese Materials Conference

Effect of annealing process on microstructure and properties of roll-casting AZ31B Mg alloy sheet Jianping Li, Zhipeng Lia*, Daheng Mao, Biao Zhang, Shi Chen State key Lab. of High Performance Complex Manufacturing, Central South University, Changsha 410083, China

Abstract The effect of annealing temperature and time on the microstructure and mechanical properties of AZ31B Mg alloy sheets were investigated. The specimens were deformed by general roll-casting and combining energy-field rollcasting (electromagnetic field and ultrasonic). The microstructure analysis showed that the re-crystallization temperature of combining energy-field Mg sheet was lower than that of general Mg sheet. It has been found that at the annealing of 250℃, no obvious re-crystallization occurred for the general Mg strip sheet, while the local recrystallization occurred for the Mg strip sheet with combining energy-field roll-casting. However, in the condition of annealing at 300℃, the specimens of roll-casting Mg strips fully re-crystallized with the holding time of 4h, and the specimens with compound energy-field Mg strip had fine grains and uniform microstructure, and the average grain diameter were 14~19μm, 8~13μm for the general Mg strip sheet and combining energy-field Mg strip sheet, respectively. When the specimens were annealed at 400℃, the grains became coarse within 1h, the average grains diameters were 20~25 μm and 28~33 μm respectively for the general Mg roll-casting sheet and combining energyfield Mg roll-casting sheet when annealing at 400℃ for 1h. The results of EDS showed that the precipitates amount of annealed Mg roll-casting sheets were reduced by annealing, the precipitates of general Mg roll-casting sheet with thicker size were enriched on the grain boundaries. The results of the mechanical properties indicated that the plastic deformation of roll-casting Mg sheets was improved significantly; Under the annealing condition of 300℃ × 4h, the values of HBS, σb, σ0.2 and δ of combining energy-field Mg strip were improved by 4.7%, 17.2%, 34.1%, 74.6% , compared to general Mg sheet.

© under responsibility responsibility of of Chinese Chinese Materials Materials © 2011 2011 Published Published by by Elsevier Elsevier Ltd. Ltd. Selection Selection and/or and/or peer-review peer-review under Research Society Open access under CC BY-NC-ND license. Research Society Keywords: combining energy-field; roll-casting; Mg alloy; annealing

1. Introduction * Corresponding author. Tel.: +86-15116186544. E-mail address: [email protected].

1877-7058 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Chinese Materials Research Society Open access under CC BY-NC-ND license. doi:10.1016/j.proeng.2011.12.536

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Based on the energy conversation and environment protection to the importance of human sustainable development, Mg alloy is paid to unprecedented attention with lightweight, high specific strength, easy to process and recycle etc. characters. At present, Mg alloy products are mainly obtained by casting or multi-pass rolling, which limit the development and application of Mg alloy because of low yield, high energy consumption and poor plastic deformation etc.[1-5]. To enhance the performances of Mg alloy, experts from domestic and foreign made a series of related researches. Koike [6] et al studied that grain refinement was an effective way to improve the mechanical properties of Mg alloy. Daheng Mao et al [7-8] studied that electromagnetic field roll-casting was beneficial to refine the grains. Jianping Li, Deming Gao et al [9-11] studied that roll-casting Mg alloy which was introduced by ultrasonic field can have finer and more uniform grains. Hence, roll-casting Mg alloy which is applied by electromagnetic field and ultrasonic field [hereinafter referred to as combining energy-field] will have finer and more uniform grains except for reducing segregation. However, roll-casting Mg sheets have still some problems, such as the non-uniform microstructure, the serious anisotropy and work-hardening, which greatly influence subsequent processing properties of Mg alloy. Consequently improving subsequent processing properties of Mg alloy which has become the hot topic at home and abroad. Wenpeng Yang et al [12-13] studied that annealing temperature and holding time were controlled reasonably could make a number of fine equiaxed grains, and finally the plastic deformation of Mg alloy was enhanced. Xu-yue Yang et al [14-15] studied that annealing was beneficial to eliminate the residual stress, original deformation microstructure and reduce work-hardening, and ultimately the subsequent processing performance of Mg alloy was improved. Heat treatment related articles of general roll-casting are very common, but heat treatment related articles of combining energyfield roll-casting have not yet been reported so far. Therefore the studies of heat treatment on the properties of combining energy-field roll-casting AZ31B Mg alloy that is very important to the application of alloy subsequent processing. 2. Experiment 2.1. Preparation of experimental materials First, we can charge mixture with industrial pure Mg, pure aluminium and pure zinc according to a certain proportion, and then the mixture was smelted [temperature was 695~705℃] in the resistance furnace [capacity was 200 kg] and held for half an hour. In order to prevent Mg melt be oxidized and burned, Ar was passed into the resistance furnace when smelting. And then the molten Mg was passed through the groove, head box [temperature maintained at 670~680℃], across the bridge, nozzle, etc.. Finally the molten Mg was flowed into the horizontal twin-roll-casting machine with Φ400mm×500mm, and the experiments of general roll-casting and combining energy-field roll-casting Mg alloy were respectively carried out, two kinds of roll-casting AZ31B Mg alloy sheets were successfully prepared which had bright surface and neat edge with width was 200mm and thickness was 4.8mm. AZ31B Mg alloy chemical composition was shown in Table 1: Table 1. Chemical composition of AZ31B Mg alloy [wt%] Al

Zn

Cu≤

Mn≤

Cl≤

Fe≤

Si≤

Mg

3.0~3.1

0.9~1.0

0.002

0.003

0.003

0.004

0.005

Bal.`

2.2. Experimental methods

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Two kinds of roll-casting AZ31B Mg sheets were annealed in the KSW-4D-C EAF temperature controller, the annealing processes as shown in Table 2. Took the metallographic specimens from annealed Mg sheets along the cross section, the longitudinal section and the normal section, then the microstructure of various annealed states was observed and compared under Leica DMI 5000M metallographic microscope after mosaicing, water milling, rough grinding, fine grinding, polishing, corrosion and etc. metallographic processes. Intercepted the tensile specimens of annealed Mg sheets selectively along horizontal, vertical and 45o direction according to GB/T 228-2002 ―Metallic Materials Tensile Testing Method‖, three specimens were taken in all directions, then the tensile mechanical properties were tested and made comparison in the WPL-250 static universal testing machine. Meanwhile the hardness was tested in the HW-187.5 Brinnell hardness testing machine and annealing temperature and holding time on the law of alloy hardness which was found out. In the end, further the precipitates of roll-casting Mg sheets were analyzed and compared. Table 2. Annealing processes of roll-casting Mg sheets Process combiningene rgy-field

NO.

Temp./℃

Time /h

1#

NO.

Temp./℃

Time/h

250

1,2,3,4

4#

250

1,2,3,4

#

300

1,2,3,4

6#

400

1,2,3,4

#

300

1,2,3,4

3#

400

1,2,3,4

2

Process

general

5

3. Results and discussions 3.1. The microstructure of roll-casting Mg sheets

Fig.1. The microstructure of roll-casting Mg sheets a. general Mg sheet b. combining energy-field Mg sheet

The microstructure of roll-casting Mg sheets is shown in Figure 1. Fig.1a showed that the chrysanthemum-like cellular dendrites of general Mg sheet were coarse, the grain structure was heterogeneous, and the part of the grain diameters were larger than 200μm. Fig.1b showed that most of the chrysanthemum-like cellular dendrites of combining energy-field Mg sheet were broken into granular, fibrous or massive, and the average grains diameter were about 30~40μm. We can conclude that under the oscillation and stirring of combining energy-field, primary α-phase dendrite arms were cut and flowed into the liquid phase, which increased the collision and friction between the liquid phase and nucleus, finally further new smaller nucleus were formed. So the combining energy-field roll-casting played the role of uniform refinement grains on the roll-casting Mg sheets. 3.2. Annealing on the microstructure

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Fig.2. The microstructure of Mg sheets at the annealing of 250℃×4h a. general Mg sheet b. combining energy-field Mg sheet

The microstructure of roll-casting Mg sheets annealed at 250℃ for 4h is shown in Figure 2. Fig.2a showed that at the annealing temperature of 250℃ within 4h, no obvious re-crystallization appeared in the general Mg sheet. Meanwhile, part of secondary cellular dendrites was chrysanthemum-like and lots of original grain deformation textures existed in the grains, and grain structure was also non-uniform, grain boundaries were blurred. Fig.2b shows that combining energy-field Mg sheet partially recrystallized, and majority of the chrysanthemum-like cellular dendrites were diffused into the matrix and formed the new nucleation which was fine but inhomogeneous. At the beginning of annealing, a large number of black short stick intermetallic compounds [mainly β [Mg (Al, Zn)]] enriched between the grain boundaries and cellular dendrites of roll-casting Mg sheets. All these can be explained that in short time, the re-crystallization of roll-casting Mg sheets was relatively slow, simultaneously owing to the hexagonal crystal structure of Mg alloy, which resulted in the slower diffusion rate ofβ phase, ultimately β phase could not be diffused and deposited with flake in the grains. But the re-crystallization temperature of combining energy-field Mg sheet was lower.


The microstructure of roll-casting Mg sheets annealed at 300℃ for 1h is shown in Fig. 3. The results in Fig.3 showed that under the annealing condition of 300℃×2h, general Mg sheet appeared the recrystallization, combining energy-field Mg sheet locally re-crystallized, and the grains size gradually were decreased within 4h. Fig.3b showed that holding for 4h, the combining energy-field Mg sheet had fine grains and uniform microstructure, which presented the very clear fine equiaxed crystals. These indicated that on the one hand, the combining energy-field Mg sheet fully re-crystallized, defect density and deformation energy storage were reduced, so grain microstructure became uniform. On the other hand, secondary phase was fully dispersed and dissolved into α-Mg matrix under the role of heat activation energy, the grain boundaries became narrow and clear. Fig.3a showed that holding for 4h, general Mg sheet whose grains size were relatively large, but still small amounts of β-phase failed to

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spread and residue at the grain boundaries, so segregation phenomenon appeared. The average grain diameter of general Mg sheet and combining energy-field Mg sheet were 14~19μm, 8~13μm.


The microstructure of roll-casting Mg sheets annealed at 400℃ for1h is shown in Figure 4. The results in Fig.4 showed that at the annealing of 400℃, roll-casting Mg sheets apparently re-crystallized in short time, grains began to be coarse, even annealing twin appeared. Fig.4b showed that holding for 1h, the grains diameter of the combining energy-field Mg sheet were up to 50μm, grain structure was inhomogeneous and annealing twin appeared. Fig.4a showed that the grains diameter of general Mg sheet were relatively small. These mainly due to the role of high-temperature activation energy, which accelerated the migration rate of new grain boundaries and phagocytized the surrounding original deformation microstructure, meanwhile distortion energy was consumed, finally speeded up the recrystallization grains growth rate. But the grains growth rate of the combining energy-field Mg sheet was faster. Holding for 1h, the average grain diameter of general Mg sheet and the combining energy-field Mg sheet were respectively 20~25μm，28~33μm. Combination of energy-field cast-rolling General cast-rolling

50 Combination of energy-field cast-rolling General cast-rolling

Size/μ ｍ

Size/μ ｍ

40

60

30

50 40 30

20 20

10

10

0

1

2 Time/h

3

4

0

1

2 Time/h

3

4

Fig.5. The grain size of Mg sheets by annealing a. at the annealing temperature of 300℃ b. at the annealing temperature of 400℃

The dependence of grain size on the annealing time was plotted in Figure 5. Fig.5 showed that no full re-crystallization of as roll-casting Mg sheets at the annealing of 250℃within 4h, at the annealing of 300℃, re-crystallization appeared in the roll-casting Mg sheets, and the grains size reduced with the annealing time. However, the grains appeared to coarsen at the annealing of 400℃. On the other hand, grain sizes of the combining energy-field Mg sheet were smaller than that of the general Mg sheet at the annealing of 300℃, but it was on the contrary at the annealing of 400℃. 3.3. EDS analysis

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The precipitate morphology and EDS of roll-casting Mg sheets are shown in Figure 6. The results in Fig.6 indicated that the annealed Mg sheets of general roll-casting and the combining energy-field rollcasting were analyzed separately on the JSM—6490LV energy disperse spectroscopy[EDS]. Aluminum and zinc which were added into the Mg alloy with Mg matrix formed intermetallic compounds as Mg17Al12 phase, MgZn2 phase and Mg17(AL, Zn)12 phase. EDS indicated that the precipitates amount of annealed roll-casting Mg sheet were both reduced as Mg17Al12 phase and Mg17(AL, Zn)12 phase. Judged from the morphology and size of precipitates, with the spreading of aluminium and zinc, the intermetallic compound phases of the combining energy-field Mg sheet were reduced, and granular precipitates were dispersed on the grain boundaries. However, precipitates of general Mg sheet with thicker size enriched on the grain boundaries.

Fig.6. Precipitate morphology and EDS of roll-casting Mg sheets (a). general Mg sheet (b). combining energy-field Mg sheet

3.4. Annealing on the mechanical properties The tensile tests of typical annealed AZ31B Mg alloy was carried out and the results are listed in Table 3. The results in Table 3 showed that the mechanical properties of annealed roll-casting Mg sheets were enhanced and anisotropy was weakened. At the annealing of 300℃×4h, the elongation of combining energy-field Mg sheet was increased by 167.1%; At this time, compared to general Mg sheet, tensile strength, yield strength and elongation of the combining energy-field Mg sheet were respectively improved by 17.2%, 34.1%, 74.6%. These indicated that on the one hand, combined energy-field Mg sheet fully re-crystallized and increased the grains mutual occlusion and more grain boundaries, which had a stronger impediment on the movement of dislocations, finally dislocation density was reduced; On the other hand, β-phase was dissolved into the α-Mg matrix, which had an effect of solid solution strengthening on the matrix. Finally the plastic deformation of roll-casting Mg sheets enhanced.

901


Table 3. Mechanical properties of typical annealed AZ31B Mg alloy annealing process

roll-casting process general Mg sheet

before annealing combining energy-field Mg sheet general Mg sheet

Direction

σb /MPa

σ0.2/MPa

δ/%

RD

228.1

156.1

2.6

45o

211.2

132.7

2.7

TD

202.9

128.2

2.5

RD

261.7

219.5

4.8

o

240.5

194.5

4.7

TD

236.2

193.3

4.5

RD

186.2

112.4

7.8

o

180.1

102.7

7.4

TD

168.6

94.9

6.3

45

45

300℃×4 h after annealing Combining energy-field Mg sheet 300℃×4 h

RD

213.6

152.1

13.2

45 o

210.1

136.8

12.5

TD

202.7

126.9

11.7

3.5. Annealing on the hardness 250℃ 300℃ 400℃

80

70

60

60

50

50

40 30 20 10 0

250℃ 300℃ 400℃

80

Hardness/HBS

Hardness/HBS

70

40 30 20 10

1

2

3 Time/h

4

0

1

2

3

4

Time/h

Fig. 7. The hardness of roll-casting Mg sheets by annealing a. general Mg sheet b. combining energy-field Mg sheet

The dependence of the hardness of annealed roll-casting Mg sheets on annealing time are shown in Figure 7. The results in Fig.7 showed that the HB hardness values of annealed roll-casting Mg sheets decreased, and the annealing temperature affected the hardness significantly but the holding time was not obvious. Under the same annealing conditions, the HB hardness value of combining energy-field Mg sheet was higher than that of general Mg sheet. The variation trend annealing within 4h was that, the 250 ℃ HB hardness value was the highest, these mainly due to low-temperature annealing recrystallization was slow, β-phase was not completely dissolved, thus pined the grain boundaries and inhibited the rotation of grain boundaries; The 400℃HB hardness value was the lowest and decreased after rising, it demonstrated that roll-casting Mg sheets completed the re-crystallization in short time when annealing at high temperature, grains began to be coarse with holding time passing, even emerged annealing twin, which caused work-hardening, these were consistent with the metallographic observations; The 300℃HB hardness value decreased after rising, anisotropy was weakened, it maybe secondary phase gradually was dissolved into the matrix, and segregation was reduced, thus which had an effect of solid solution strengthening on the matrix.

7

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4. Conclusions 1. Compared with roll-casting AZ31B Mg alloy strips, the re-crystallization temperature of AZ31B Mg strip with combining energy-field roll-casting was significantly reduced. 2. No obvious re-crystallization for the roll-casting Mg sheets occured under the low annealing temperature of (250), however, the grains began to coarsen under the high annealing temperature of (400) . However under the annealing condition of 300℃×4h, the combining energy-field Mg sheet had fine grains and homogeneous microstructure. 3. The precipitates amount of the roll-casting Mg sheets reduced by annealing. However, the precipitates of the general Mg sheet with thicker size enriched at the grain boundaries compared to the combining energy-field Mg sheet. 4. Mechanical properties of roll-casting Mg sheets were enhanced and anisotropy was weakened by annealing. Acknowledgements The work presented in this paper has been fully supported by National Natural Science Foundation of China (No.50974139) and the National Basic Research Program of China (973) (NO.2007CB613706). References [1] Avedesian M. Magnesium and Magnesium Alloys. USA: ASM International; 1999. [2] Aghion E, Bronfin B, Eliezer D. The role of the magnesium industry in protecting the environment. Materials Processing Technology 2001;117:381-385. [3] Mordike BL, Ebert T. Magnesium properties-applications-potential. Material Science and Engineering 2001;A302:37-45. [4] Manuel M, Louis G, Hector. Microstructural effects of AZ31 magnesium alloy on its tensile deformation and failure behaviors. Material Science and Engineering 2006;A418:341-356． [5] Zhang BP, Tu YF, Chen JY, Zhang HL. Preparation and characterization of as-rolled AZ31 magnesium alloy sheets. Materials Processing Technology 2007;184:102-107. [6] Koike J, Ohyama R, Kobayashi T. Grain boundary sliding in AZ31 magnesium alloys at room temperature to 523K. Materials Transactions 2005;44(4):445-451. [7] Li T, Mao DH, et al. Experimental study of electromagnetic roll-casting for AZ31B Mg alloy. Hot Working Technology 2010;15:1-4. [8] Xu GM, Bao WP, Cui JZ. Effect of magnetostatic field on the microstructure of magnesium alloys ZK60. Transactions of Nonferrous Metals Society of China 2003;13(6):1270-1273. [9] Li JP, Hu JB, Mao DH, et al. Experiment research on ultrasound roll-casting AZ31alloy sheet. Journal of Huazhong University of Science and Technology 2010;38(12):1-4. [10] Gao DM, Li ZJ, Han QY, et al. Effect of ultrasonic power on microstructure and mechanical properties of AZ91 alloys. Materials Science and Engineering 2009;502 (1-2):2-5 [11] Guo XW, Ding WJ, Lu C, et al. Influence of ultrasonic power on the structure and composition of anodizing coatings formed on Mg alloys. Surface and Coatings Technology;183(2-3):359-368. [12] Yang PW, Guo XF, Yang KJ. Effect of heat treatment on microstructure and microhardness of rapidly solidified ZK60 magnesium ribbons. Transactions of Materials and Heat Treatment 2010;31(4):1-5. [13] Kun YU, Li WX, Wang RC. Effects of heat treatment on microstructures and mechanical properties of ZK60 magnesium alloy. The Chinese Journal of Nonferrous Metals 2007;17(2):1-5. [14] Yang XY, Zhu YK, et al. Static re-crystallization behavior of hot-deformed magnesium alloy AZ31 during isothermal annealing. Transactions of Nonferrous Metals Society of China 2010;20:1269-1274. [15] Yoshihara S, Yamamoto H, Manabe K, Nishimura H. Formability enhancement in magnesium alloy deep drawing by local heating and cooling technique. Materials Processing Technology 2003;143-144:612-615.