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DOI 10.1007/s11595-009-2198-x
Effects of Thermomechanical Treatment on the Mechanical Properties and Microstructures of 6013 Alloy HE Lizi, ZHANG Haitao, CUI Jianzhong (Key Lab of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang 110004, China)
Abstract: The mechanical properties and microstructures of 6013 alloy after different thermomechanical treatments were investigated. The detailed dislocation configurations after deformation and morphologies of age hardening precipitates were examined through transmission electron microscopy (TEM). The experimental results show that the thermomechanical treatment can significantly enhance the strength of 6013 alloy, and has a similar influence trend on single and two-step aging behaviors. With the increasing deformation ratio, the peak-hardness (HVmax) increases, the time to HVmax shortens, and the density of tangled dislocation network increases. The aging precipitates become larger and inhomogeneous by applying thernomechanical treatment. Key words: 6013 alloy; thermomechanical treatment; aging; dislocation
1 Introduction Al-Mg-Si alloys have medium strength, excellent corrosion resistance, favorable formability and low density, and are widely used in both cast and wrought forms[1]. In recent years, many researches have been focused on improving the strength of Al-Mg-Si-Cu alloys with good corrosion resistance and formability. The precipitation hardened Al-Mg-Si-Cu alloy 6013 T6 was developed by Alcoa Aluminum Corporation and selected for replacement of the traditional 2024-T3 alloy in structural parts of aircraft such as wings and fuselage skins. It is demonstrated that it could replace the 2024 alloy providing improved corrosion resistance with good mechanical properties[2]. On the basis of 6013 alloy, many other Al-Mg-Si-Cu alloys have been developed[3-5]. It is well known that the performance of many aluminium alloys can be improved by combination plastic deformation and thermal treatments. Various commercial practices utilize cold working together with aging[6]. These include the T3 (solution heat treatment, cold working and natural aging), T8 (solution heat treatment, cold working and artificial aging), and T9/T10 (artificial aging and cold working) temper conditions. Deformation prior to aging might be used to minimize the possible (Received: Feb. 22, 2008 ; Accepted: May 9, 2008) HE Lizi(何立子): Assoc. Prof.; Ph D; E-mail:
[email protected] Funded by the National Key Technology R&D Program of China (No.2007BAE38B01)
deleterious effects of precipitate free zones, that is, the dislocations which result from cold working may act as nucleation sites for precipitation as well as mechanically strengthening the grain boundary region[7]. Many investigations[8-14] showed that proper control of the thermomechanical procedures could result in improved strength, fatigue resistance, high temperature stability, fracture toughness and/or stress corrosion resistance. The potential increased use of these new 6000 series alloys in demanding environment of aerospace industry, however, has created the desire for a more insight into the microstructures and mechanical properties of these alloys. The present work was aimed to investigate effects of thermomechanical treatment on the mechanical properties and microstructures of 6013 alloy.
2 Experimental The chemical composition, of 6013 alloy used are given in Table 1. The alloy was produced from a semi-continuously cast ingot with size of 200 mm× 1300 mm ×30 mm, using 99.97wt% pure aluminium, 99.99 wt% copper, 99.99wt% magnesium as well as master alloys of Al-20wt%Si, Al-10wt%Mn, Al-2.5wt%Cr, Al-5wt%Ti. The ingot was homogenized at 520 ℃ for 12 h, descaled and finally hot rolled at 450 ℃ with a reduction of 85%. Subsequent annealing was carried out at 415 ℃ for 2 h, following by furnace cooling and then cold rolled to 1.5 mm with a reduction of 65%. The
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thermomechanical treatment procedures for all specimens were divided into two types. The first type was solution heat-treatment (at 540 ℃ for 20 min, water quenched at room temperature), cold deformation (0%, 1%, 5%, 10%, 30% and 60%) and artificial ageing (at 170 ℃ for different times). The second type was solution heat-treatment (at 540 ℃ for 20 min, water quenched at room temperature), cold deformation (0%, 1%, 5%, 10% and 30%), preaging (at 140 ℃ for 22 h) and artificial aging (at 170 ℃ for different times). The solution heat-treatment was performed in a salt bath (NaNO3：KNO3=1). Artificial aging was done in a salt bath (NaNO2：KNO2=1). The values of Vickers hardness were obtained by using an HVA-5 low-load tester with a load of 5 kg and a holding time of 15 s. For one specimen, each hardness value was the average of seven measurements. Samples for optical microscopic tests were polished by standard technique and chemically etched with a solution of 2 mL HF, 3 mL HCl, 5mL HNO3 and 250 mL H2O. A Philip EM 420 transmission electron microscope (TEM) was used for high magnification observation. The thin foils were prepared by twin-jet thinning electrolytically in a solution of 30% nitric acid and 70% methanol at －25 ℃. Table 1 The chemical compositions of 6013 alloy/ wt% Cu Mg Si Mn Cr Ti Fe Al 1.65
1.1
0.80
0.35
0.15
0.02
0.22
Bal.
3 Results and Discussion 3.1 Effect of thermomechanical treatment on the single artificial age hardening behaviour of 6013 alloy The effects of deformation ratio in thermomechanical treatment on the single artificial age hardening behaviour of 6013 alloy are illustrated in Fig.1. The hardness of as-quenched 6013 alloy is 52HV. It can be seen that cold rolling deformation significantly increases the hardness of as-quenched alloy. With increasing the deformation ratio, the hardness of as-rolled alloy increases. Subsequent artificial aging further increases the hardness of the as-rolled alloy. The age hardening curves of 6013 alloy under all testing conditions have a similar shape, that is, the hardness increases gradually and reaches a maximum value (HVmax) and then decreases gradually. The change of HVmax with deformation ratio can be divided into two groups: A (1%-10%) and B (30%-60%). The HVmax in group A is lower than that in group B. The HVmax of 6013 alloy without deformation
increases 15% when deformation ratio is 30%. It also should be noted that rolling deformation can notably shorten the time to HVmax (tmax). With increasing deformation ratio, the tmax value decreases. The age hardening capability, HVmax－HVR, of 6013 alloy in single aging are listed in Table 2. The value of HVmax-HVR reaches a maximum when deformation ratio is 5%, and then decreases with the increasing deformation ratio. Table 2 Effect of deformation ratio on the value of HVmax-HVR of single artificial aged 6013 alloy Deformation ratio/%
1
5
10
30
60
HVmax-HVR/HV
67
74
65
51
43
Fig.1 Effect of the deformation ratio in the thermomechanical treatment on single age hardening of 6013 alloy at 170 ℃
3.2 Effect of thermomechanical treatment on the two-step artificial age hardening behaviour of 6013 alloy The effects of deformation ratio in thermomechanical treatment on the two-step artificial age hardening behaviour of 6013 alloy are shown in Fig.2. It can be seen that the HVmax of two-step artificial aged alloy is higher than that of single artificial aged alloy. A similar changing trend of HVmax and tmax with increasing deformation ratio in single aged alloy are also observed in two-step artificial aged alloy, that is, the HVmax value increases and the tmax value decreases. But the effect of deformation ratio on two-step ageing is less significant than that on single ageing. The age hardening capability of 6013 alloy during two-step ageing is listed in Table 3. The value of HVmax-HVR is lower than that of single aged alloy, and changes little with increasing deformation ratio. Table 3 Effect of deformation ratio on the value of HVmax-HVR of two-step aged 6013 alloy 1 5 10 30 Deformation ratio/% HVmax-HVR/HV
27
27
28
30
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Vol.24 No.2 HE Lizi et al: Effects of Thermomechanical Treatment…
Fig.2 Effect of the deformation ratio in the thermomechanical treatment on age hardening of 6013 alloy aged at 140 ℃/22 h+170 ℃
3.3 Microstructure analyses Fig.3 shows the dislocation morphologies of 6013 alloy after deformed at different deformation ratios. It can be seen that high dislocation density is introduced by deformation prior to aging. The dislocation density increases and takes the form of tangled networks as deformation increased (Fig.3(a)-Fig.3(d)). No identification in the precipitate structure can be revealed as deformation directly after quenching, because the precipitates are too fine to be visible in the bright field image.
Fig.3 The effect of deformation ratio in the thermomechanical treatment on microstructures of 6013-T6 alloy after solution treatment: (a) 0%, (b) 10%, (c) 30%, (d) 60%
Fig.4 shows the morphologies of precipitates of 6013 alloy at different aging conditions. The bright-field image reveals a strain constrast of needlelike precipitates
oriented along [100]Al and [010]Al directions together with some fine dots in the alloy aged at 170 ℃/7 h (Fig.4(a)). Most of dots should be the end-on sections of the needles along [100]Al. The selected area diffraction pattern shows faint streaks along [100]Al and [010]Al due to the needlelike precipitates. According to Dutta and Allen[15], these precipitates are designated as β". A noticeable modification in the precipitate structure is found to be developed by deformation as shown in Fig.4(b) and Fig.4(c). The needle-shape precipitates become larger in size and less homogeneous. This is also accompanied by a change in the dislocation configuration. The dislocations are present in cells of rather recovered interiors instead of the uniform tangled structure (Fig.3(c)). When deformation combines with two-step aging, the precipitates continue to grow up and distribute inhomogeneously. The above phenomena indicate that the deformation can not only effectively strengthen alloy but also change the morphology of age hardening precipitates significantly. It is well know that solute atoms and flaws have a strong interaction in aluminium alloys, and they contribute to instability of the structure, and to change the solute distribution and the diffusion velocity of solute atoms. By accelerating the effect of solute atom diffusion, deformation causes to form crowded dislocation tangles, dislocation blocks and precipitation hardening[16-18]. The pre-aging plays an important role, as it gives many uniformly distributed nuclei. The present results show that the strength of 6013 alloy is considerably increased by deformation prior to aging in agreement with previous findings[13,19]. The increase in strength can be attributed to observing high density dislocations and age hardening precipitates. Point defects and dislocations introduced by deformation greatly increase the nucleation rate of GP zones upon subsequent aging. When the deformation ratio exceeds 30%, the hardness of 6013 alloy remains unchanged (Fig.1), may be due to the coarser precipitate structure and the more recovered dislocation cell structure as shown in Fig.3(c). Increasing deformation produces higher density of dislocations, upon subsequent aging, will facilitate solute diffusion and thus accelerate the precipitate growth as previous report[19].
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Fig.4 The microstructures of the alloy under different conditions: (a) 170 ℃/7 h, (b) 30% deformation＋170 ℃/4 h, (c) 30% deformation+140 ℃/22 h＋170 ℃/2 h
4 Conclusion
Fatigue Behaviour of 2014 Al-alloy[J]. Bulletin of Materials Science, 2005, 28(2): 91-96 [9] T Y Kuo, H S Lin, H T Lee. The Relationship Between of
The thermomechanical treatment and the two-step artificial aging can effectively improve the strength of 6013 alloy. The peak-aged hardness has increased by 15% after a 30% reduction. The effect of thermomechanical treatment on the age hardening behaviour of single aged alloy is more significant than that of two-step artificial aged alloy.The thermomechanical treatment can significantly accelerate the precipitate growth.
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