1 National Engineering Research Center of Die and Mold CAD, Shanghai Jiao Tong University, Shanghai 200030, China 2 Shanghai Aircraft Manufacturing Co., Ltd., Shanghai 201324, China
Cite this article:
Xifeng LI, Nannan CHEN, Jiaojiao LI, Xueting HE, Hongbing LIU, Xingwei ZHENG, Jun CHEN. Effect of Temperature and Strain Rate on Deformation Behavior of Invar 36 Alloy. Acta Metall Sin, 2017, 53(8): 968-974.
Since the thermal expansion coefficient of Invar 36 alloy is so low that it matches the composite materials well. It is very suitable as the material of composite material forming mould. Invar 36 alloy mould surface is usually produced by hot pressing technology. The hot pressing temperature and strain rate severely affect the quality of mould surface. In this work, the mechanical properties of Invar 36 alloy were studied in the temperature range from room temperature to 900 ℃ under different initial strain rates (8×10-5, 8×10-4 and 8×10-3 s-1) by using uniaxial tensile tests. The effect of temperature on the springback trend of thick Invar 36 alloy sheet by three-point bending tests at room temperature, 600 ℃ and 800 ℃ was investigated. The results indicate that the yield strength and ultimate tensile strength of Invar 36 alloy significantly decrease with increasing temperature. Meanwhile, the elongation firstly increases and then decreases with the increase of temperature. It reaches a peak value of 69.2% at 600 ℃ and increases by 55% than that at ambient temperature, which mainly results from the plasticity improvement by dynamic recrystallization. Invar 36 alloy at lower temperature (room temperature and 500 ℃) shows insensitive to the strain rate. Nevertheless, the strength and plasticity at 800 ℃ substantially decrease with decreasing strain rate. When the strain rate decreases from 8×10-3 s-1 to 8×10-5 s-1, the yield strength, ultimate tensile strength and elongation reduce by 38%, 47% and 50%, respectively. The three-point bending springback value decreases by 87.0% when the tested temperature increases from room temperature to 800 ℃.
Fig.2 Macroscopic fractographies of the specimens stretched at different temperatures
Fig.3 Engineering stress-strain curves of Invar 36 alloy stretched at different temperatures
Temperature
Yield strength
Ultimate tensile
Elongation
℃
MPa
strength
%
MPa
Room temp.
292
447
44.7
210
185
352
45.7
500
113
267
49.5
600
81
203
69.2
700
71
138
59.1
800
56
88
38.5
900
34
51
38.0
Table 1 Mechanical properties of Invar 36 alloy tested at different temperatures (8×10-4 s-1)
Fig.4 OM images of Invar 36 alloy as-received (a) and after tension at room temperature (b), 210 ℃ (c), 500 ℃ (d), 600 ℃ (e), 700 ℃ (f), 800 ℃ (g) and 900 ℃ (h)
Fig.5 OM images of longitudinal section near high-temperature tensile fracture of Invar 36 alloy at 600 ℃ (a), 700 ℃ (b) and 800 ℃ (c)
Fig.6 Effects of strain rates on engineering stress-strain curves of Invar 36 alloy at room temperature (a), 500 ℃ (b) and 800 ℃ (c)
Temperature / ℃
αf / (o)
αi / (o)
Δα / (o)
Ks / %
Room temperature
34.85
38.60
3.75
10.8
600
31.50
32.95
1.45
4.6
800
32.15
32.60
0.45
1.4
Table 2 Experimental data of three-point bending springback of thick Invar 36 alloy sheet at different temperatures
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