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Optimization of the pre-aging treatment for an AA6022 alloy at various temperatures and holding times
Journal of Alloys and Compounds 647 (2015) 238e244
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Optimization of the pre-aging treatment for an AA6022 alloy at various temperatures and holding times Lipeng Ding a, Yang He a, Zhang Wen a, Pizhi Zhao b, c, Zhihong Jia a, *, Qing Liu a a
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China Department of Fabrication Process and Technology for Aluminum Alloys, Suzhou Research Institute for Nonferrous Metals, Suzhou 215026, China c Department of Fabrication Process and Technology for Aluminum Alloys, CHINALCO Research Institute of Science and Technology, Beijing 100082, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 February 2015 Received in revised form 26 May 2015 Accepted 28 May 2015 Available online 10 June 2015
The effect of pre-aging at four different temperatures and three holding times on the natural aging and the bake hardening response of an AA6022 alloys were studied by Vickers microhardness measurement, tensile test, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM). It was revealed that pre-aging immediately after quenching is effective in suppressing the natural aging and improving the bake hardening response (BHR), which is attributed to the readily formation of Cluster(2) during pre-aging treatment as well as depressing of Cluster(1). The optimum pre-aging treatments were exploited as: 80 C for 8 h; 100 C for 3 h; 130 C for 20 min and 170 C for 5 min. By considering the practical process requirement pre-aging at 100 C for 3 h gives a better BHR without impairing the formability in T4P temper, which has the potential to be used in industrial production. © 2015 Elsevier B.V. All rights reserved.
Keywords: AA6022 alloy Pre-aging Natural aging Bake hardening response
1. Introduction 6xxx AleMgeSi alloys have been widely used in automotive body panels as main substitute materials for weight reduction. These alloys have a combination of good formability, good corrosion resistance and remarkable strengthening potential due to the formation of a large number of nano-sized, semi-coherent, metastable precipitates during the paint bake cycle [1e3]. Therefore, manipulating the heat treatment process is essential to achieve the optimum properties. Nowadays, the commercial 6xxx alloys including AA6016, AA6111 and AA6022 alloy have been developed for automotive body application. In order to be widely used in industrial production, 6xxx alloys would have low yield strength and good formability in T4 temper while high yield strength after paint bake treatment for in-service dent resistance [4,5]. However, a relatively low temperature and short duration of the most commercial paint bake processes, typically at 170e180 C for 20e30 min, could not fully exploit the age hardening potential of these alloys. Besides, for mainly practical reasons, these alloys are usually stored a period at room temperature (RT), referred to as natural aging (NA), before they are given
* Corresponding author. E-mail address: [email protected] (Z. Jia). http://dx.doi.org/10.1016/j.jallcom.2015.05.188 0925-8388/© 2015 Elsevier B.V. All rights reserved.
stamping and final paint-baking. During RT storage the strength of the alloys increases significantly and the bake hardening response (BHR) is dramatically suppressed [6e8]. Hence, suppressing the detrimental effect of natural aging and enhancing precipitation kinetics are essential for the development of AleMgeSi alloys. The most common practice is to employ pre-aging treatment shortly after the solution treatment, to create b00 -nuclei that could readily transform into b00 phase upon paint baking [9,10]. The preaging treatments have been widely investigated and used in industrial production. Birol [11] revealed that pre-aging in a wide temperature range (60e200 C) was effective in improving the BHR of the twin-roll cast 6016 sheet. A pre-aging treatment between 140 C and 180 C for <10 min were claimed to be optimum for this alloys as it offered a nice BHR without impairing the formability in T4 temper. By far, most pre-aging treatments investigated in lab are focused on the high temperature and short holding time process (about 160e200 C for 3e10 min) because of their high efficiency. However, in the modern industrial production, most manufacturers are prone to employ a low temperature and long holding time preaging treatment (about 80e90 C for 8e10 h). Since the pre-aging at high temperature has two weaknesses: firstly, the inside and outside temperature of the coil couldn't be kept even during such a short pre-aging time. Secondly, stability of the materials performance couldn't be guaranteed. The present work was undertaken to investigate the pre-aging treatments in a wide temperature
L. Ding et al. / Journal of Alloys and Compounds 647 (2015) 238e244
range and holding time, to compare the characteristics of low and high temperature pre-aging treatments and explore the suitable pre-aging treatment for industrial production. 2. Experimental procedure The AleMgeSi alloy sheet used in the present investigation was a typical AA6022 alloy with a composition of Al-1.11%Si-0.62%Mg0.06%Cu-0.16%Fe-0.06%Mn (wt.%). The alloy was cast into a slab ingot from high-purity Al(99.9%), high-purity Mg (99.9%), Al10wt.% Si and Al-49.5wt.% Cu master alloys by using an induction furnace. The chemical composition of the alloy was measured by optical emission spectroscopy. The ingot was homogenized at 560 C for 4 h in an air furnace which was heated up with a heating rate of 50 C/min from a room temperature, and subsequently naturally cooled. The ingot was then hot and cold rolled to 1 mm thick sheet. Specimens were cut from the cold rolled sheet, and solution heat treated at 560 C for 30 min in an muffle furnace, and water-quenched to room temperature, and then immediately carried out pre-aging at 80 C, 100 C, 130 C, 170 C for different holding times in the oil baths, respectively. Several sheet samples without pre-aging (T4 temper) were also prepared for comparison. All samples were then held at room temperature for two weeks (NA). A simulated paint bake treatment referring to an artificial aging at 180 C for 30 min was applied to the natural aged samples. The schematic heat treatment flow of the three-step aging process is presented in Fig. 1. Hardness of the samples after different heat treatments was measured using a MH-5L microhardness tester with a 500 g force and a 10 s loading time. Ten indentations were made to obtain an average hardness value of each alloy. The tensile properties of the sheet samples were measured before and after the paint bake treatment using an SHIMADZUAG-X10KN computer controlled test machine. The sample with 25 mm gauge length was stretched along the rolling direction with a tensile rate of 10 mm/min. For each condition, three parallel specimens were tested in order to verify the experimental results. Precipitation behavior was analyzed by differential scanning calorimetry (DSC) operated in an argon atmosphere using a METTLER-1100LF system ranging from 20 C to 500 C with heating rates of 5 C/min, 10 C/min and 20 C/ min. Microstructure investigations were carried out on a Zeiss Libra 200FE transmission electron microscopy. Specimens were prepared by electropolishing using a Struers TenuPol-5 machine. The electrolyte was consisted of 30% HNO3 in methanol and kept at a temperature in a range of 25 C to 35 C. A combination of the TEM bright-field images with corresponding thickness measurements were taken for the purpose of quantifying the average
Fig. 1. Schematic diagram of various heat treatments in 6022 alloy.
239
precipitate cross-section, length, number density and volume fraction. All the images were acquired along <100>Al zone axes. To reduce the effect of nonuniform distribution of the precipitates, ten images from different grains were taken and analyzed for each specimen. A Gatan parallel electron energy loss spectrometer (EELS) was used to determine the thickness of the investigated area. The measurement error could be controlled at a reasonable range as the total numbers of the measured precipitates were roughly 2500. A full description of the error analysis and statistic methodology has been given elsewhere [12]. 3. Results and discussions 3.1. Hardness results The hardness evolution of the sample without pre-aging during two weeks RT storage and paint bake treatment is shown in Fig. 2. The hardness of T4 temper sample increased from 56.7HV to 84.3HV during two weeks RT storage, and rose up to 107.4HV after paint bake treatment, which is lower than the paint baked sample without NA (116.3HV). So it was evident that natural aging could significantly increases the T4 temper hardness and reduces the BHR after the paint-bake cycle, which poses detrimental effects on the formability and in-serve rent resistant after paint-bake cycle. Hardening during natural aging implies forming of atomic cluster. It has been reported that immediately after quenching, Mg, Si atoms quickly trap vacancies to form the Cluster(1) [13], which grows up gradually and strengthens the alloys during RT storage. As a result, the precipitation of b00 phase would be retarded at the paint bake stage because the concentrations of quenched-in vacancies and solute elements have been consumed by the formation of Cluster(1). In addition, Cluster(1) is hardly dissolved at paint bake temperature due to its high thermal stability. Therefore, to modifying the early clustering behavior is important. The hardness curves of the samples with various pre-aging temperatures and holding times after the two weeks RT storage and paint bake treatment are shown in Fig. 3. For each pre-aging temperature, different holding times were applied to find out an optimum time, which could give good properties in both T4 temper and paint-bake cycle. It was evident that nearly all the pre-aging treatments between 80 C-170 C were effective in suppressing the natural aging and improving BHR. Pre-aging at 80 C required at least 8 h holding time, while 5 min holding time was enough for pre-
Fig. 2. Vickers hardness evolution of the sample without pre-aging.
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Fig. 3. Vickers hardness curves of the sample with pre-aging of different temperatures and holding times during the two weeks RT storage and paint bake treatment. (a) pre-aging at 80 C; (b) pre-aging at 100 C; (c) pre-aging at 130 C; (d) pre-aging at 170 C).
aging at 170 C. Accordingly, 3 h at 100 C and 20 min at 130 C were selected as the optimum process in these pre-aging temperature as shown in Fig. 3 (b and c). During the pre-aging treatment, another nanocluster called “Cluster(2)” was formed. A. Serizawa et al. [13] clarified the characteristics of Cluster(1) and Cluster(2) using a three dimensional atom probe technique (3DAP). They reports that Cluster(1) formed at room temperature is stable at the paint bake temperature, which is the main reason for detrimental effect of natural aging. Cluster(2) formed above 70 C, which has a Mg/Si ratio similar to the b00 phase. Cluster(2) could readily transform into the b00 phase during paint bake treatment, leading to obvious improvement of BHR. On the other hand, The formation of Cluster(2) relaxes the supersaturation of vacancies and atomic atoms, which greatly suppresses the formation of Cluster(1), and as a result, suppresses a deteriorate effect of natural aging. In each of these preaging temperatures, shorter holding time means less Cluster(2) formed which produces a lower T4P temper hardness and poor BHR [14]. While a longer holding time produces more Cluster(2), resulting in a rather high T4P hardness without obvious hardness improvement after paint bake treatment. Thus the negative effects of natural aging could be effectively suppressed by a suitable preaging treatment with a right holding time. The reasonable preaging process in the present work could be chosen as: 80 C for 8 h; 100 C for 3 h; 130 C for 20 min and 170 C for 5 min.
Changing the holding time in different pre-aging temperatures produces rather different results as shown in Fig. 3. Varying the holding time from 6 h to 10 h and 2 he4 h in pre-aging at 80 C and 100 C respectively would not make much difference in T4P temper hardness, while variation from 3 min to 10 min in pre-aging at 170 C results in a total different T4P temper hardness. In pre-aging at 130 C, changing the holding time for 10mine30 min results in less obvious hardness change than pre-aging at 170 C in T4P temper. So the formation of Cluster(2) is more sensitive to the high pre-aging temperature. It has been known that in the mass industrial production, the holding time of pre-aging treatment could hardly accurate controlled, thus the stability of materials performance could not guaranteed during such a short holding time. It is fair to conclude that pre-aging at low temperature has wider process window, which is benefit for the industrial manufacturing. From the above described pre-aging treatments, it could be found that pre-aging at low temperature such as 80 C, 100 C has lower T4P temper hardness (80.2HV, 81.3HV) compared to preaging at high temperature (83.7HV at 170 C). While after the paint bake treatment, pre-aging at higher temperature (122.3HV at 170 C) has higher paint-baked hardness than the low temperature pre-aging (116.5HV at 80 C, 118.3HV at 100 C). Pre-aging at 130 C has a slightly higher paint-baked hardness than pre-aging at 100 C. So it could be deduced that with an increase of pre-aging
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temperature, more Cluster(2) is formed during the pre-aging treatment (less related to holding time), which means higher hardness in both T4P temper and paint-bake cycle. 3.2. Tensile results The yield strength changes of the selected pre-aged samples before and after the paint bake treatment are shown in Fig. 4. The samples without pre-aging showed the highest yield strength of 156.8 MPa before paint bake treatment, while the yield strength after paint bake treatment was only 170.8 MPa, giving a very low BHR of 14 MPa, which failed to meet the required in-service dent resistance. After introducing pre-aging treatment, the yield strength before the paint bake treatment reduced generally, while the yield strength after paint bake treatment increased significantly, which are in agreement with the hardness results shown in Fig. 3. With an increase of pre-aging temperature, the yield strength before and after the paint bake treatment both increased gradually. Pre-aging at 80 C had a lower T4P temper yield strength (140.1 MPa) and BHR (of 97.8 MPa). Pre-aging at 170 C showed the highest BHR (about 123.4 MPa), but the rather high T4 temper yield strength (156.7 MPa) could risks its formability at the stamping process, pre-aging at 130 C also showed a higher T4P temper yield strength without obvious increase of BHR. So pre-aging at 100 C is supposed to be the ideal process as it offers a lower T4 temper yield strength (145.6 MPa) and relatively higher paint baked strength (250.8 MPa). For the industrial application, pre-aging at low temperature such as 80 C and 100 C is reasonable because of low sensitive to the holding time, which guarantees the stability of materials. 3.3. DSC analysis Fig. 5 show DSC curves obtained at a heating rate of 10 C/min of these selected samples after pre-aging and RT storage. In contrast to the sample without pre-aging, a dissolution trough between 180 and 200 C, which came from the reversion of the Cluster(1), was largely reduced or disappeared in the pre-aging samples, suggesting that pre-aging had effectively suppressed the formation of Cluster(1) [11]. The exothermic peak between 240 C-260 C, re00 ported as the precipitation of b phase [15], shifted to lower temperatures and became smaller with increasing pre-aging temperature. The smaller b00 peak indicates that a part of the
Fig. 4. Yield strength change of samples under different pre-aging condition.
Fig. 5. The DSC curves of samples under different pre-aging condition.
precipitation activities responsible for this exothermic peak has taken place prior to DSC heating. Cluster(2) formed during these pre-aging treatments could greatly promote the precipitation of b00 phase during paint bake treatment [16]. Evaluating the activation energy of precipitation reactions is important to understand the transformation kinetics of the pre-aged samples. According to the modified AvramieJohnsoneMehl (AJM) equation, the effective activation energy of b00 precipitation can be determined [17]. The AJM method can be written as follow:
dY F Q ¼ Ink0 In dT f ðYÞ RT
(1)
where Y, F, k0, Q and R represent the volume fraction of precipitate, the heating rate, the growth parameter, the activation energy of the reaction and the gas constant, respectively. Fig. 6 shows the plot of In[(dY/dT)(F/f(Y) as a function of T1/103 (K1) according to Eq. (1). Based on the slop of the plots (found by least square fitting), the activation energy E for the b00 phase is calculated as 132.4 kJ mol1, 110.3 kJ mol1, 106.5 kJ mol1, 96.4 kJ mol1, 63.8 kJ mol1 for the non-pre-aging, 80 C 8 h, 100 C 3 h, 130 C 20 min and 170 C 5 min, respectively. The value of non-pre-aging sample is higher
Fig. 6. The relationship between In[(dY/dT)(F/f(Y) and T1/103 (K1) for the peak of b00 phase derived from DSC measurements with different pre-aging process.
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than 115 kJ mol1 reported by Matsuda [18] because of the negative effect of natural aging. It is evident that pre-aging greatly decreases the activation energy of b00 by the formation of Cluster(2). With increasing pre-aging temperature, the activation energy becomes smaller. More Cluster(2) is formed with an accompanying increase of yield strength in both T4P temper and paint bake treatment. However, the higher T4P yield strength in pre-aging at 170 C is unfavorable for its formability. Therefore, the pre-aging temperature and holding time would be seriously regulated to control the precipitation of Cluster(2), which is essential to obtain a good BHR without impairing the formability in T4P temper. 3.4. Microstructure investigations TEM bright-field images of the non-pre-aging, pre-aging at 100 C for 3 h, pre-aging at 170 C for 5 min samples before and after paint baking are shown in Fig. 7. All the images were acquired along <100>Al zone axes. In the T4 temper, no cluster or precipitates is observed in the non-pre-aging sample, because of the small size and high coherency of the Cluster(1) with the matrix. In the pre-aged samples (100 C for 3 h and 170 C for 5 min), tiny dotlike particles with high density are observed, which could be referred as Cluster(2). Besides, the pre-aging at 170 C has a slightly higher number density than pre-aging at 100 C, which contributes to the higher yield strength after paint bake treatment. TEM bright-field images of the above samples after paint bake treatment are presented in Fig. 7(def). In the sample without preaging, plenty of fine dot-like precipitates were homogeneously distributed in the Al matrix, and some needle-like precipitates were also clearly observed. While in the pre-aged samples (pre-aging at 100 C for 3 h, pre-aging at 170 C for 5 min), large number of dot-
like precipitates and needle-like precipitates were observed, and most of the dot-like precipitates were larger than those in non-preaging sample. HRTEM and corresponding Fast Fourier Transforms (FFT) patterns of these dot-like precipitates in non-pre-aging sample are shown in Fig. 8. Most of these dot-like precipitates are very tiny (about 2 nm), the FFT image shows neither extra reflection nor diffuse scattering, suggesting that these precipitates are fully coherent with the matrix and do not have any distinct structure, which can thus be identified as GP zones, reported by M. Murayama [9]. Several bigger dot-like precipitates (about 3e4 nm) were also clearly observed. According to the unit cell structure and orientation relationship of these precipitates revealed in Fig. 7, b00 phase could be identified and most of these dot-like precipitates are needles viewed end-on [19]. So it is evident that most of the GP zones in the non-pre-aging samples have not transformed into b00 phase during paint bake treatment. This indicates that the precipitation of b00 phase is suppressed by natural aging, resulting in a poor BHR. While in the pre-aged samples, nearly all the GP zones were disappeared and large numbers of b00 precipitates were formed. It means that the precipitation of b00 phase had been accelerated by the introduced pre-aging. A quantification of precipitates as determined from the TEM images of the pre-aged samples is given in Table 1. The volume fraction and the number density of the b00 precipitate are higher for pre-aging at 170 C than for pre-aging at 100 C, in agreement with the results from the hardness measurements and tensile tests shown in Figs. 3 and 4. Although pre-aging at 170 C gives a good BHR after paint bake treatment, the higher T4P strength would make the stamping operation rather difficult. Besides, a rather sensitive holding time in this temperature could not guarantee the stability of material performances. Pre-aging at low temperature (80 C and 100 C) is
Fig. 7. TEM bright-field images of various pre-aging samples before and after paint bake treatment. (a, d) non-pre-aging sample; (b, e) pre-aging at 100 C for 3 h; (c, f) pre-aging at 170 C for 5 min).
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Fig. 8. HRTEM and corresponding FFT pattern of the precipitates after paint bake treatment in the non-pre-aging sample. (a and b) GP zones, (c and d) b00 phase.
Table 1 Statistical distribution of precipitates in pre-aged samples after paint bake treatment.
100 C,3 h 170 C,5 min
Cross-section(nm2)
Length(nm)
Number density
Volume fraction (%)
3.4 ± 0.2 4.2 ± 0.3
10.5 ± 2.5 12.2 ± 5.5
(3.73 ± 0.4)*e-6 (3.96 ± 0.4)*e-5
0.18 ± 0.05 0.24 ± 0.1
somewhat weak in BHR but its low T4P temper strength is favorable for stamping process. Pre-aging at 100 C has a combination of low T4P strength and high BHR compared with others pre-aging treatments, and the holding time is enough to stabilize the properties, so this process has a potential to be used in mass industrial production.
3. The optimum pre-aging time in each temperature could be chosen as: 80 C for 8 h; 100 C for 3 h; 130 C for 20 min and 170 C for 5 min. Considering the requirement of wide window during process, pre-aging at 100 C for 3 h seems a better preaging treatment process, which gives a good BHR without impairing the formability in T4P temper.
4. Conclusions
Acknowledgments
1. Pre-aging in a wide temperature range (80e170 C) is effective in suppressing the natural aging and improving the bake hardening response of AleMgeSi alloys, by suppressing the formation of Cluster(1) in T4P temper and promoting the formation of Cluster(2) which can be readily transformed into b00 strengthening phase during subsequent paint bake treatment. 2. The formation of Cluster(2) is sensitive to a pre-aging temperature. Pre-aging at 170 C for 5 min has higher yield strengths both in T4P temper and paint bake treatment in comparison to the low temperature and long holding time pre-aging, but the over higher T4P temper yield strength is unfavorable to its formability.
This work was financially supported by International Science & Technology Cooperation Program of China (Grant No. 2014DFA51270) and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51421001). References [1] G.A. Edwards, K. Stille, G.L. Dunlop, M.J. Couper, Acta Mater. 46 (1998) 3893e3904. [2] S. Pogatscher, H. Antrekowitsch, H. Leitner, T. Ebner, P.J. Uggowitzer, Acta Mater. 59 (2011) 3352e3363. [3] W. Yang, L. Huang, R. Zhang, M. Wang, Z. Li, Y. Jia, R. Lei, X. Sheng, J. Alloys Compd. 514 (2012) 220e233. [4] Y. Birol, M. Karlík, Mater. Sci. Technol. 21 (2005) 153e158.
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[5] Y. Birol, J. Mater. Sci. 45 (2010) 6727e6731. [6] J.D. Bryant, Metall. Mater. Trans. A 30 (1999) 1999e2006. [7] A. Cuniberti, A. Tolley, M.V.C. Riglos, R. Giovachini, Mater. Sci. Eng. A 527 (2010) 5307e5311. [8] L. Cao, P.A. Rometsch, M.J. Couper, Mater. Sci. Eng. A 559 (2013) 257e261. [9] M. Murayama, K. Hono, Acta Mater. 47 (1999) 1537e1548. [10] F. De Geuser, W. Lefebvre, D. Blavette, Philos. Mag. Lett. 86 (2006) 227e234. [11] Y. Birol, Mater. Sci. Eng. A 391 (2005) 175e180. [12] C.D. Marioara, S.J. Andersen, H.W. Zandbergen, R. Holmestad, Metall. Mater. Trans. A 36 (2005) 691e702. [13] A. Serizawa, S. Hirosawa, T. Sato, Metall. Mater. Trans. A 39 (2008) 243e251.
[14] M. Torsæter, H.S. Hasting, W. Lefebvre, C.D. Marioara, J.C. Walmsley, S.J. Andersen, R. Holmestad, J. Appl. Phys. 108 (2010) 073527. [15] Y. Takaki, T. Masuda, E. Kobayashi, T. Sato, Mater. Trans. 55 (2014) 1257e1265. [16] A. Gaber, M.A. Gaffar, M.S. Mostafa, E.F.A. Zeid, J. Alloys Compd. 429 (2007) 167e175. [17] K.S. Ghosh, N. Gao, Acta Metall. Sin. 21 (2011) 1199e1209. [18] K. Matsuda, S. Ikeno, H. Gamada, K. Fujii, Y. Uetani, T. Sato, Metall. Mater. Trans. A 29 (1998) 1161e1167. [19] X. Wang, S. Esmaeili, D.J. Lloyd, Metall. Mater. Trans. A 37 (2006) 2691e2699.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Optimization of the pre-aging treatment for an AA6022 alloy at various temperatures and holding times Lipeng Ding a, Yang He a, Zhang Wen a, Pizhi Zhao b, c, Zhihong Jia a, *, Qing Liu a a
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China Department of Fabrication Process and Technology for Aluminum Alloys, Suzhou Research Institute for Nonferrous Metals, Suzhou 215026, China c Department of Fabrication Process and Technology for Aluminum Alloys, CHINALCO Research Institute of Science and Technology, Beijing 100082, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 February 2015 Received in revised form 26 May 2015 Accepted 28 May 2015 Available online 10 June 2015
The effect of pre-aging at four different temperatures and three holding times on the natural aging and the bake hardening response of an AA6022 alloys were studied by Vickers microhardness measurement, tensile test, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM). It was revealed that pre-aging immediately after quenching is effective in suppressing the natural aging and improving the bake hardening response (BHR), which is attributed to the readily formation of Cluster(2) during pre-aging treatment as well as depressing of Cluster(1). The optimum pre-aging treatments were exploited as: 80 C for 8 h; 100 C for 3 h; 130 C for 20 min and 170 C for 5 min. By considering the practical process requirement pre-aging at 100 C for 3 h gives a better BHR without impairing the formability in T4P temper, which has the potential to be used in industrial production. © 2015 Elsevier B.V. All rights reserved.
Keywords: AA6022 alloy Pre-aging Natural aging Bake hardening response
1. Introduction 6xxx AleMgeSi alloys have been widely used in automotive body panels as main substitute materials for weight reduction. These alloys have a combination of good formability, good corrosion resistance and remarkable strengthening potential due to the formation of a large number of nano-sized, semi-coherent, metastable precipitates during the paint bake cycle [1e3]. Therefore, manipulating the heat treatment process is essential to achieve the optimum properties. Nowadays, the commercial 6xxx alloys including AA6016, AA6111 and AA6022 alloy have been developed for automotive body application. In order to be widely used in industrial production, 6xxx alloys would have low yield strength and good formability in T4 temper while high yield strength after paint bake treatment for in-service dent resistance [4,5]. However, a relatively low temperature and short duration of the most commercial paint bake processes, typically at 170e180 C for 20e30 min, could not fully exploit the age hardening potential of these alloys. Besides, for mainly practical reasons, these alloys are usually stored a period at room temperature (RT), referred to as natural aging (NA), before they are given
* Corresponding author. E-mail address: [email protected] (Z. Jia). http://dx.doi.org/10.1016/j.jallcom.2015.05.188 0925-8388/© 2015 Elsevier B.V. All rights reserved.
stamping and final paint-baking. During RT storage the strength of the alloys increases significantly and the bake hardening response (BHR) is dramatically suppressed [6e8]. Hence, suppressing the detrimental effect of natural aging and enhancing precipitation kinetics are essential for the development of AleMgeSi alloys. The most common practice is to employ pre-aging treatment shortly after the solution treatment, to create b00 -nuclei that could readily transform into b00 phase upon paint baking [9,10]. The preaging treatments have been widely investigated and used in industrial production. Birol [11] revealed that pre-aging in a wide temperature range (60e200 C) was effective in improving the BHR of the twin-roll cast 6016 sheet. A pre-aging treatment between 140 C and 180 C for <10 min were claimed to be optimum for this alloys as it offered a nice BHR without impairing the formability in T4 temper. By far, most pre-aging treatments investigated in lab are focused on the high temperature and short holding time process (about 160e200 C for 3e10 min) because of their high efficiency. However, in the modern industrial production, most manufacturers are prone to employ a low temperature and long holding time preaging treatment (about 80e90 C for 8e10 h). Since the pre-aging at high temperature has two weaknesses: firstly, the inside and outside temperature of the coil couldn't be kept even during such a short pre-aging time. Secondly, stability of the materials performance couldn't be guaranteed. The present work was undertaken to investigate the pre-aging treatments in a wide temperature
L. Ding et al. / Journal of Alloys and Compounds 647 (2015) 238e244
range and holding time, to compare the characteristics of low and high temperature pre-aging treatments and explore the suitable pre-aging treatment for industrial production. 2. Experimental procedure The AleMgeSi alloy sheet used in the present investigation was a typical AA6022 alloy with a composition of Al-1.11%Si-0.62%Mg0.06%Cu-0.16%Fe-0.06%Mn (wt.%). The alloy was cast into a slab ingot from high-purity Al(99.9%), high-purity Mg (99.9%), Al10wt.% Si and Al-49.5wt.% Cu master alloys by using an induction furnace. The chemical composition of the alloy was measured by optical emission spectroscopy. The ingot was homogenized at 560 C for 4 h in an air furnace which was heated up with a heating rate of 50 C/min from a room temperature, and subsequently naturally cooled. The ingot was then hot and cold rolled to 1 mm thick sheet. Specimens were cut from the cold rolled sheet, and solution heat treated at 560 C for 30 min in an muffle furnace, and water-quenched to room temperature, and then immediately carried out pre-aging at 80 C, 100 C, 130 C, 170 C for different holding times in the oil baths, respectively. Several sheet samples without pre-aging (T4 temper) were also prepared for comparison. All samples were then held at room temperature for two weeks (NA). A simulated paint bake treatment referring to an artificial aging at 180 C for 30 min was applied to the natural aged samples. The schematic heat treatment flow of the three-step aging process is presented in Fig. 1. Hardness of the samples after different heat treatments was measured using a MH-5L microhardness tester with a 500 g force and a 10 s loading time. Ten indentations were made to obtain an average hardness value of each alloy. The tensile properties of the sheet samples were measured before and after the paint bake treatment using an SHIMADZUAG-X10KN computer controlled test machine. The sample with 25 mm gauge length was stretched along the rolling direction with a tensile rate of 10 mm/min. For each condition, three parallel specimens were tested in order to verify the experimental results. Precipitation behavior was analyzed by differential scanning calorimetry (DSC) operated in an argon atmosphere using a METTLER-1100LF system ranging from 20 C to 500 C with heating rates of 5 C/min, 10 C/min and 20 C/ min. Microstructure investigations were carried out on a Zeiss Libra 200FE transmission electron microscopy. Specimens were prepared by electropolishing using a Struers TenuPol-5 machine. The electrolyte was consisted of 30% HNO3 in methanol and kept at a temperature in a range of 25 C to 35 C. A combination of the TEM bright-field images with corresponding thickness measurements were taken for the purpose of quantifying the average
Fig. 1. Schematic diagram of various heat treatments in 6022 alloy.
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precipitate cross-section, length, number density and volume fraction. All the images were acquired along <100>Al zone axes. To reduce the effect of nonuniform distribution of the precipitates, ten images from different grains were taken and analyzed for each specimen. A Gatan parallel electron energy loss spectrometer (EELS) was used to determine the thickness of the investigated area. The measurement error could be controlled at a reasonable range as the total numbers of the measured precipitates were roughly 2500. A full description of the error analysis and statistic methodology has been given elsewhere [12]. 3. Results and discussions 3.1. Hardness results The hardness evolution of the sample without pre-aging during two weeks RT storage and paint bake treatment is shown in Fig. 2. The hardness of T4 temper sample increased from 56.7HV to 84.3HV during two weeks RT storage, and rose up to 107.4HV after paint bake treatment, which is lower than the paint baked sample without NA (116.3HV). So it was evident that natural aging could significantly increases the T4 temper hardness and reduces the BHR after the paint-bake cycle, which poses detrimental effects on the formability and in-serve rent resistant after paint-bake cycle. Hardening during natural aging implies forming of atomic cluster. It has been reported that immediately after quenching, Mg, Si atoms quickly trap vacancies to form the Cluster(1) [13], which grows up gradually and strengthens the alloys during RT storage. As a result, the precipitation of b00 phase would be retarded at the paint bake stage because the concentrations of quenched-in vacancies and solute elements have been consumed by the formation of Cluster(1). In addition, Cluster(1) is hardly dissolved at paint bake temperature due to its high thermal stability. Therefore, to modifying the early clustering behavior is important. The hardness curves of the samples with various pre-aging temperatures and holding times after the two weeks RT storage and paint bake treatment are shown in Fig. 3. For each pre-aging temperature, different holding times were applied to find out an optimum time, which could give good properties in both T4 temper and paint-bake cycle. It was evident that nearly all the pre-aging treatments between 80 C-170 C were effective in suppressing the natural aging and improving BHR. Pre-aging at 80 C required at least 8 h holding time, while 5 min holding time was enough for pre-
Fig. 2. Vickers hardness evolution of the sample without pre-aging.
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Fig. 3. Vickers hardness curves of the sample with pre-aging of different temperatures and holding times during the two weeks RT storage and paint bake treatment. (a) pre-aging at 80 C; (b) pre-aging at 100 C; (c) pre-aging at 130 C; (d) pre-aging at 170 C).
aging at 170 C. Accordingly, 3 h at 100 C and 20 min at 130 C were selected as the optimum process in these pre-aging temperature as shown in Fig. 3 (b and c). During the pre-aging treatment, another nanocluster called “Cluster(2)” was formed. A. Serizawa et al. [13] clarified the characteristics of Cluster(1) and Cluster(2) using a three dimensional atom probe technique (3DAP). They reports that Cluster(1) formed at room temperature is stable at the paint bake temperature, which is the main reason for detrimental effect of natural aging. Cluster(2) formed above 70 C, which has a Mg/Si ratio similar to the b00 phase. Cluster(2) could readily transform into the b00 phase during paint bake treatment, leading to obvious improvement of BHR. On the other hand, The formation of Cluster(2) relaxes the supersaturation of vacancies and atomic atoms, which greatly suppresses the formation of Cluster(1), and as a result, suppresses a deteriorate effect of natural aging. In each of these preaging temperatures, shorter holding time means less Cluster(2) formed which produces a lower T4P temper hardness and poor BHR [14]. While a longer holding time produces more Cluster(2), resulting in a rather high T4P hardness without obvious hardness improvement after paint bake treatment. Thus the negative effects of natural aging could be effectively suppressed by a suitable preaging treatment with a right holding time. The reasonable preaging process in the present work could be chosen as: 80 C for 8 h; 100 C for 3 h; 130 C for 20 min and 170 C for 5 min.
Changing the holding time in different pre-aging temperatures produces rather different results as shown in Fig. 3. Varying the holding time from 6 h to 10 h and 2 he4 h in pre-aging at 80 C and 100 C respectively would not make much difference in T4P temper hardness, while variation from 3 min to 10 min in pre-aging at 170 C results in a total different T4P temper hardness. In pre-aging at 130 C, changing the holding time for 10mine30 min results in less obvious hardness change than pre-aging at 170 C in T4P temper. So the formation of Cluster(2) is more sensitive to the high pre-aging temperature. It has been known that in the mass industrial production, the holding time of pre-aging treatment could hardly accurate controlled, thus the stability of materials performance could not guaranteed during such a short holding time. It is fair to conclude that pre-aging at low temperature has wider process window, which is benefit for the industrial manufacturing. From the above described pre-aging treatments, it could be found that pre-aging at low temperature such as 80 C, 100 C has lower T4P temper hardness (80.2HV, 81.3HV) compared to preaging at high temperature (83.7HV at 170 C). While after the paint bake treatment, pre-aging at higher temperature (122.3HV at 170 C) has higher paint-baked hardness than the low temperature pre-aging (116.5HV at 80 C, 118.3HV at 100 C). Pre-aging at 130 C has a slightly higher paint-baked hardness than pre-aging at 100 C. So it could be deduced that with an increase of pre-aging
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temperature, more Cluster(2) is formed during the pre-aging treatment (less related to holding time), which means higher hardness in both T4P temper and paint-bake cycle. 3.2. Tensile results The yield strength changes of the selected pre-aged samples before and after the paint bake treatment are shown in Fig. 4. The samples without pre-aging showed the highest yield strength of 156.8 MPa before paint bake treatment, while the yield strength after paint bake treatment was only 170.8 MPa, giving a very low BHR of 14 MPa, which failed to meet the required in-service dent resistance. After introducing pre-aging treatment, the yield strength before the paint bake treatment reduced generally, while the yield strength after paint bake treatment increased significantly, which are in agreement with the hardness results shown in Fig. 3. With an increase of pre-aging temperature, the yield strength before and after the paint bake treatment both increased gradually. Pre-aging at 80 C had a lower T4P temper yield strength (140.1 MPa) and BHR (of 97.8 MPa). Pre-aging at 170 C showed the highest BHR (about 123.4 MPa), but the rather high T4 temper yield strength (156.7 MPa) could risks its formability at the stamping process, pre-aging at 130 C also showed a higher T4P temper yield strength without obvious increase of BHR. So pre-aging at 100 C is supposed to be the ideal process as it offers a lower T4 temper yield strength (145.6 MPa) and relatively higher paint baked strength (250.8 MPa). For the industrial application, pre-aging at low temperature such as 80 C and 100 C is reasonable because of low sensitive to the holding time, which guarantees the stability of materials. 3.3. DSC analysis Fig. 5 show DSC curves obtained at a heating rate of 10 C/min of these selected samples after pre-aging and RT storage. In contrast to the sample without pre-aging, a dissolution trough between 180 and 200 C, which came from the reversion of the Cluster(1), was largely reduced or disappeared in the pre-aging samples, suggesting that pre-aging had effectively suppressed the formation of Cluster(1) [11]. The exothermic peak between 240 C-260 C, re00 ported as the precipitation of b phase [15], shifted to lower temperatures and became smaller with increasing pre-aging temperature. The smaller b00 peak indicates that a part of the
Fig. 4. Yield strength change of samples under different pre-aging condition.
Fig. 5. The DSC curves of samples under different pre-aging condition.
precipitation activities responsible for this exothermic peak has taken place prior to DSC heating. Cluster(2) formed during these pre-aging treatments could greatly promote the precipitation of b00 phase during paint bake treatment [16]. Evaluating the activation energy of precipitation reactions is important to understand the transformation kinetics of the pre-aged samples. According to the modified AvramieJohnsoneMehl (AJM) equation, the effective activation energy of b00 precipitation can be determined [17]. The AJM method can be written as follow:
dY F Q ¼ Ink0 In dT f ðYÞ RT
(1)
where Y, F, k0, Q and R represent the volume fraction of precipitate, the heating rate, the growth parameter, the activation energy of the reaction and the gas constant, respectively. Fig. 6 shows the plot of In[(dY/dT)(F/f(Y) as a function of T1/103 (K1) according to Eq. (1). Based on the slop of the plots (found by least square fitting), the activation energy E for the b00 phase is calculated as 132.4 kJ mol1, 110.3 kJ mol1, 106.5 kJ mol1, 96.4 kJ mol1, 63.8 kJ mol1 for the non-pre-aging, 80 C 8 h, 100 C 3 h, 130 C 20 min and 170 C 5 min, respectively. The value of non-pre-aging sample is higher
Fig. 6. The relationship between In[(dY/dT)(F/f(Y) and T1/103 (K1) for the peak of b00 phase derived from DSC measurements with different pre-aging process.
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than 115 kJ mol1 reported by Matsuda [18] because of the negative effect of natural aging. It is evident that pre-aging greatly decreases the activation energy of b00 by the formation of Cluster(2). With increasing pre-aging temperature, the activation energy becomes smaller. More Cluster(2) is formed with an accompanying increase of yield strength in both T4P temper and paint bake treatment. However, the higher T4P yield strength in pre-aging at 170 C is unfavorable for its formability. Therefore, the pre-aging temperature and holding time would be seriously regulated to control the precipitation of Cluster(2), which is essential to obtain a good BHR without impairing the formability in T4P temper. 3.4. Microstructure investigations TEM bright-field images of the non-pre-aging, pre-aging at 100 C for 3 h, pre-aging at 170 C for 5 min samples before and after paint baking are shown in Fig. 7. All the images were acquired along <100>Al zone axes. In the T4 temper, no cluster or precipitates is observed in the non-pre-aging sample, because of the small size and high coherency of the Cluster(1) with the matrix. In the pre-aged samples (100 C for 3 h and 170 C for 5 min), tiny dotlike particles with high density are observed, which could be referred as Cluster(2). Besides, the pre-aging at 170 C has a slightly higher number density than pre-aging at 100 C, which contributes to the higher yield strength after paint bake treatment. TEM bright-field images of the above samples after paint bake treatment are presented in Fig. 7(def). In the sample without preaging, plenty of fine dot-like precipitates were homogeneously distributed in the Al matrix, and some needle-like precipitates were also clearly observed. While in the pre-aged samples (pre-aging at 100 C for 3 h, pre-aging at 170 C for 5 min), large number of dot-
like precipitates and needle-like precipitates were observed, and most of the dot-like precipitates were larger than those in non-preaging sample. HRTEM and corresponding Fast Fourier Transforms (FFT) patterns of these dot-like precipitates in non-pre-aging sample are shown in Fig. 8. Most of these dot-like precipitates are very tiny (about 2 nm), the FFT image shows neither extra reflection nor diffuse scattering, suggesting that these precipitates are fully coherent with the matrix and do not have any distinct structure, which can thus be identified as GP zones, reported by M. Murayama [9]. Several bigger dot-like precipitates (about 3e4 nm) were also clearly observed. According to the unit cell structure and orientation relationship of these precipitates revealed in Fig. 7, b00 phase could be identified and most of these dot-like precipitates are needles viewed end-on [19]. So it is evident that most of the GP zones in the non-pre-aging samples have not transformed into b00 phase during paint bake treatment. This indicates that the precipitation of b00 phase is suppressed by natural aging, resulting in a poor BHR. While in the pre-aged samples, nearly all the GP zones were disappeared and large numbers of b00 precipitates were formed. It means that the precipitation of b00 phase had been accelerated by the introduced pre-aging. A quantification of precipitates as determined from the TEM images of the pre-aged samples is given in Table 1. The volume fraction and the number density of the b00 precipitate are higher for pre-aging at 170 C than for pre-aging at 100 C, in agreement with the results from the hardness measurements and tensile tests shown in Figs. 3 and 4. Although pre-aging at 170 C gives a good BHR after paint bake treatment, the higher T4P strength would make the stamping operation rather difficult. Besides, a rather sensitive holding time in this temperature could not guarantee the stability of material performances. Pre-aging at low temperature (80 C and 100 C) is
Fig. 7. TEM bright-field images of various pre-aging samples before and after paint bake treatment. (a, d) non-pre-aging sample; (b, e) pre-aging at 100 C for 3 h; (c, f) pre-aging at 170 C for 5 min).
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Fig. 8. HRTEM and corresponding FFT pattern of the precipitates after paint bake treatment in the non-pre-aging sample. (a and b) GP zones, (c and d) b00 phase.
Table 1 Statistical distribution of precipitates in pre-aged samples after paint bake treatment.
100 C,3 h 170 C,5 min
Cross-section(nm2)
Length(nm)
Number density
Volume fraction (%)
3.4 ± 0.2 4.2 ± 0.3
10.5 ± 2.5 12.2 ± 5.5
(3.73 ± 0.4)*e-6 (3.96 ± 0.4)*e-5
0.18 ± 0.05 0.24 ± 0.1
somewhat weak in BHR but its low T4P temper strength is favorable for stamping process. Pre-aging at 100 C has a combination of low T4P strength and high BHR compared with others pre-aging treatments, and the holding time is enough to stabilize the properties, so this process has a potential to be used in mass industrial production.
3. The optimum pre-aging time in each temperature could be chosen as: 80 C for 8 h; 100 C for 3 h; 130 C for 20 min and 170 C for 5 min. Considering the requirement of wide window during process, pre-aging at 100 C for 3 h seems a better preaging treatment process, which gives a good BHR without impairing the formability in T4P temper.
4. Conclusions
Acknowledgments
1. Pre-aging in a wide temperature range (80e170 C) is effective in suppressing the natural aging and improving the bake hardening response of AleMgeSi alloys, by suppressing the formation of Cluster(1) in T4P temper and promoting the formation of Cluster(2) which can be readily transformed into b00 strengthening phase during subsequent paint bake treatment. 2. The formation of Cluster(2) is sensitive to a pre-aging temperature. Pre-aging at 170 C for 5 min has higher yield strengths both in T4P temper and paint bake treatment in comparison to the low temperature and long holding time pre-aging, but the over higher T4P temper yield strength is unfavorable to its formability.
This work was financially supported by International Science & Technology Cooperation Program of China (Grant No. 2014DFA51270) and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51421001). References [1] G.A. Edwards, K. Stille, G.L. Dunlop, M.J. Couper, Acta Mater. 46 (1998) 3893e3904. [2] S. Pogatscher, H. Antrekowitsch, H. Leitner, T. Ebner, P.J. Uggowitzer, Acta Mater. 59 (2011) 3352e3363. [3] W. Yang, L. Huang, R. Zhang, M. Wang, Z. Li, Y. Jia, R. Lei, X. Sheng, J. Alloys Compd. 514 (2012) 220e233. [4] Y. Birol, M. Karlík, Mater. Sci. Technol. 21 (2005) 153e158.
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