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Stabilization of 7xxx aluminium alloys
Journal of Alloys and Compounds 740 (2018) 167e173
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Stabilization of 7xxx aluminium alloys a, * € Johannes A. Osterreicher , Georg Kirov a, Stephan S.A. Gerstl b, Ermal Mukeli c, a Florian Grabner , Manoj Kumar a, 1 a
LKR Light Metals Technologies Ranshofen, Austrian Institute of Technology, Postfach 26, 5282 Ranshofen, Austria ScopeM Scientific Center for Optical and Electron Microscopy, ETH Zürich, Auguste-Piccard-Hof 1, 8093 Zürich, Switzerland c Magna Steyr Fahrzeugtechnik AG & Co KG, Liebenauer Hauptstrasse 317, 8041 Graz, Austria b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 May 2017 Received in revised form 29 November 2017 Accepted 1 January 2018 Available online 3 January 2018
High strength Al-Zn-Mg(-Cu) alloys are rarely used in the automotive industry due to pronounced natural ageing after quenching and resulting poor formability at room temperature. Furthermore, the paint bake response is often suboptimal. To overcome these challenges, we performed various low temperature pre-ageing treatments followed by storage at room temperature and a short high temperature heat treatment simulating industrial paint baking. The intermediate and final mechanical properties were assessed and the nanostructure, precipitation, and local chemistries of the samples were studied by atom probe tomography and differential scanning calorimetry. It was found that a properly designed stabilization treatment can inhibit the natural ageing process in AA7021 resulting in a stable condition with a lower hardness than samples naturally aged for longer than approximately one week. Atom probe tomography revealed that the stabilization leads to co-precipitation of Mg and Zn (indicating GP-zones) hindering the further increase in hardness over the course of three weeks. Furthermore, the paint bake response of AA7075 was improved resulting in yield strength only 2% short of a peak aged sample. © 2018 Published by Elsevier B.V.
Keywords: Atom probe tomography Heat treatment Differential scanning calorimetry Al-Zn-Mg-Cu Paint bake Automotive
1. Introduction B-pillars and other key automotive parts require a high specific strength to satisfy the roof crush and side impact test standards while keeping the weight low. High strength Al alloys of the 7xxx series could fulfil these requirements; however, mainly due to their poor formability at room temperature, 7xxx alloys are still mostly used in the aircraft industry and up to this time have only found limited use as automotive parts [1,2]. The final mechanical properties of the parts are a measure for process chain's quality. It was found in many investigations that the final strength of the parts was much lower than the sheet's maximum possible strength (i.e., T6). The main reasons were lower quenching rates than the critical cooling rate and incomplete ageing during paint baking. In one investigation, AA7075-T6 sheet was processed via various routes, however, it was found that the initial as-received mechanical properties of the sheet could not be
* Corresponding author. € E-mail address: [email protected] (J.A. Osterreicher). 1 Current address: EBNER Industrieofenbau GmbH, Ebner-Platz 1, 4060 Leonding, Austria. https://doi.org/10.1016/j.jallcom.2018.01.003 0925-8388/© 2018 Published by Elsevier B.V.
matched [3]. Kumar el al. [1] investigated the warm forming of AA7020-T6; it was found that as a result of warm forming and paint baking, the yield strength and ultimate tensile strength were reduced by 9% and 13%, respectively. To recover the strength, an intermediate heat treatment before paint baking was proposed. Such an approach was investigated by Bardelcik et al. [4] who studied the effect of an intermediate heat treatment of 100 C for 3 h on paint bake response of AA7075. Although paint bake response was improved, the mechanical properties achieved by a peak ageing were not matched. In a patent application [5] a variety of pre-ageing treatments with the intent to increase the yield strength after paint baking are disclosed. It is demonstrated that by applying two- or three-step heat treatments, mechanical properties similar to the peak aged state can be achieved in much shorter time. In this paper, different intermediate heat treatments were applied on sheet material of AA7021 and AA7075 in T4 temper. Such heat treatments are commonly called stabilization due to the fact that thereafter the precipitation of hardening phases should not occur during storage at RT (i.e., no, or reduced, natural ageing). To assess the efficacy of the stabilization treatments, the natural ageing behaviour of as-quenched and stabilized samples was studied over 22 days. In the automotive industry, a material with
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defined mechanical properties before paint baking is vital for the design of joining operations such as self-pierce riveting; furthermore, the paint bake response should be equal or better as in the non-stabilized material. Therefore, the effect of stabilization on the intermediate and final mechanical properties (i.e., after paint baking or peak ageing) is reported. The effects of the various heat treatments on the nano- and microstructure were investigated by means of atom probe tomography (APT) and differential scanning calorimetry (DSC).
2. Experimental AA7075-T6 and an AA7021-T4 type alloy (Structurlite® 400, provided by Aleris) with a thickness of 2 mm each were used; the chemical compositions in weight percent as determined by optical emission spectroscopy (for AA7075) and provided by Aleris (for the AA7021 type alloy) are shown in Table 1. Although the Mg and Zn contents of the AA7021 type alloy slightly exceed the standard composition limits of AA7021 (i.e., a maximum Mg and Zn content of 1.8% and 6.0%, respectively) it will be referred to as AA7021 below. Both sheets were heat treated according to the schemes shown in Fig. 1. For the stabilization heat treatment, the sheets were held at 90 C, 120 C, and 150 C for 1 h, 2 h, 4 h, and 5 h. The samples were stored for different times at RT to determine the influence of natural ageing (na) and then subjected to a paint baking (pb) heat treatment of 185 C/20 min or a peak ageing (pa or T6) heat treatment of 120 C/24 h. DSC and APT measurements (for AA7021) as well as hardness and tensile tests were carried out to study the precipitation evolution, nano- and microstructure, and the resulting mechanical properties. Vickers micro-hardness (HV0.1) was measured five times per sample using a load of 100 g. Tensile tests were conducted in triplicate on a Zwick Z100 materials testing machine according to EN ISO 6892-1. Specimens were cut parallel to the rolling direction. They had a width of 12.5 mm, a reduced section length of 75 mm and a gauge length of 50 mm. APT specimens were produced by cutting matchstick shaped samples with edge dimensions of 0.5 mm 0.5 mm 10 mm, mechanically polishing them to an equal width of approximately 0.3 mm, and electropolishing a sharp tip with a solution of 10% perchloric acid in ethanol using an ElectroPointer device at room temperature [6]. The potential was gradually decreased from 20 V to 5 V AC during the electropolishing process which took approximately 15e20 min per sample. The samples were then inspected, sharpened, and cleaned in a scanning electron microscope equipped with a Ga focussed ion beam (FEI FIB-SEM Helios 600i). The FIB was operated at 30 kV for initial annular milling and 5 kV for final cleaning to minimize Ga implantation. APT analysis was performed on a Cameca LEAP 4000X HR atom probe system at a temperature of 34 K. The measurements were performed in voltage pulse mode with a pulse fraction of 0.2, a pulse frequency of 200 kHz, and a detection rate of 0.5%. For reconstruction of APT datasets, the software IVAS 3.6.14 was used. Differential scanning calorimetry (DSC) experiments were carried out using a Netzsch-DSC 204 F1 differential scanning calorimeter with a heating rate of 10 K min 1.
3. Results and discussion 3.1. Influence of stabilization on the mechanical properties The response of the solution heat treated and quenched sheets to the stabilization heat treatments is shown in Fig. 2; the hardness values of the two alloys increase with stabilization heat treatment time. The hardness of all samples was monitored for 22 days after stabilization and it was found that for all stabilization variants no significant changes in hardness occurred, with the exception of 90 C/1 h and 120 C/1 h for AA7075 where, after 22 days, an increase in HV0.1 from 139 ± 2 to 151 ± 2 (Fig. 3) and 142 ± 2 to 157 ± 3, respectively, was found. It can be concluded that all other stabilization treatments are suitable to inhibit the natural ageing process in the alloys in the surveyed time frame of 22 days. In Fig. 3 the hardness evolution due to natural ageing over the course of 22 days is shown for the sheets in W-temper as well as in peak aged condition (for reference). As expected, there is a considerable increase in hardness over time for both alloys in Wtemper. Furthermore, the natural ageing behaviour of the samples stabilized at 90 C for one or two hours are shown; the abovementioned increase in hardness for AA7075 stabilized at 90 C/ 1 h can be seen. Stabilizing AA7021 at 90 C/1 h yields a stable condition with hardness lower than 125 HV0.1 whereas the hardness after 22 days of natural ageing was 130 ± 1 HV0.1. This is of great interest because the lower hardness is beneficial for subsequent manufacturing operations. For example, we found that proper self-pierce riveting of the sheets is only possible up to a hardness of approximately 125 € llhoff). This value is HV0.1 (using a standard C H4 type rivet by Bo surpassed by natural ageing of AA7021 after approximately one week while the stabilized sheet maintains a lower hardness. It has been reported for other 7xxx series alloys that GP(II)-zones are only formed by ageing above 70 C while GP(I)-zones are formed at RT as well as at higher temperatures [7,8]. Therefore, a possible explanation for the observed stabilization by pre-ageing at 90 C could be the formation of stable GP(II)-zones which inhibit subsequent formation of GP(I)-zones and the associated increase in hardness at room temperature. However, others have reported that no GP(II)zones were found after ageing AA7050 for 1.5 h at 120 C [9]. We performed atom probe tomography to obtain further insight into these early stage precipitation processes (see section 3.3). For AA 7075, a stable condition with hardness lower than 125 HV0.1 or lower than the hardness after three weeks of natural ageing could not be achieved. Since 90 C for 1 h and 120 C for 1 h yielded unstable conditions a reduction of ageing time seems not a promising approach. However, stabilizing at lower temperature for longer time might be worth exploring. Fig. 4 shows the results of tensile tests performed on sheets in different tempers. While the effect of one week of natural ageing is clearly visible for both alloys, the strength of the stabilized samples did not change. Furthermore, there is no significant difference in the mechanical properties whether the stabilization treatment was applied directly after quenching or after one week of natural ageing. Expectedly, the values for the latter samples stayed constant also after another week of natural ageing following stabilization (data not shown). Likewise, after paint baking no difference could be
Table 1 Chemical composition of the used alloys (in wt. %). Alloys
Al
Si
Fe
Cu
Mn
Mg
Zn
Cr
Ti
Zr
Others
AA7075-T6 AA7021-T4
rest rest
0.19 max. 0.25
0.11 max. 0.40
1.50 max. 0.16
0.04 max. 0.10
2.64 1.60e2.10
6.06 6.00e6.80
0.18 max. 0.05
0.04 max. 0.1
0.02 max. 0.18
max. 0.03 max. 0.15
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Fig. 1. Schematics of solution heat treatment (SHT) and (a) stabilization heat treatment procedure, (b) paint bake response after storage, (c) paint bake response after stabilization and (d) paint bake response and peak ageing response after delayed stabilization.
Fig. 2. Response of AA7021 and AA7075 to the stabilization heat treatment at different temperatures.
found between samples stabilized directly or after 1 week. For AA7021, the stabilization treatments had no significant influence on the final mechanical properties after paint baking or peak ageing. However, it can be seen that stabilization at 150 C for 2 h directly yields mechanical properties very similar to those of the paint baked samples. Generally, yield strength and ultimate tensile strength (UTS) increase and elongation decreases for the stabilization variants due to the occurrence of precipitation hardening processes. However, this is not the case for the elongation of the sample stabilized at 90 C for 2 h which show even slightly higher elongation, giving further evidence that the ductility of the material is retained by stabilization at 90 C. For AA7075, the stabilization treatments resulted in an
improved paint bake response: the yield strength of the sample stabilized at 120 C for 2 h and subjected to paint baking is significantly higher than that of the non-stabilized sample (508 ± 1 MPa versus 460 ± 8 MPa). Furthermore, it is only 2% lower than the yield strength of the samples peak aged for 24 h (518 ± 1 MPa). This could help improve the suitability of the alloy for high strength automotive parts where excessive ageing times are not practicable. For comparison, in a recent patent application [5] the highest reported yield strength (507 MPa) was achieved after a stabilization treatment of 110 C for 6 h (i.e., triple the duration suggested in this paper) and subsequent paint baking. It was found by Hansen et al. [10] that the microstructure resulting from two-stage ageing treatments of an Al-Zn-Mg-Zr
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Fig. 3. Natural ageing behaviour of peak aged (pa), stabilized, and as quenched (W) samples. Error bars represent standard deviation.
alloy is governed mainly by nucleation in the first stage. In particular, the formation of GP(II)-zones during low temperature preageing (70e150 C) and their subsequent direct transformation to s’ during a second ageing step at higher temperature is proposed to lead to a high density of s’ precipitates with low internal order. Such a microstructure is reported to give the highest strength. However, it was also found that the direct transformation of GP(II) to s’ occurs between 120 and 170 C [10,11] whereas the paint bake temperature used in this paper is 185 C. A possible explanation is that the direct transformation could take place also at temperatures higher than 170 C in AA7075. 3.2. Characterization by differential scanning calorimetry The impact of the stabilization treatments on the DSC traces of the samples are given in Fig. 5 (a) and (c) for AA 7021 and AA7075, respectively. There is no significant difference in the DSC signals of the samples whether a stabilization treatment was applied directly after quenching (data only shown for 120 C/2 h) or after one week. This is in line with the results of the tensile tests (Fig. 4) where no difference was found either. The peaks of the DSC curves were interpreted in accordance with [12,13]. Thus, the exothermic event (1) around 85 C in the curve of the as quenched (W) sample of AA7021 (shown in both Fig. 5 a and b) can be associated with the formation of GP-zones from the initial super-saturated solid solution; the endothermic peak 2a is due to their dissolution. The exothermic peaks 3b and 4 can be linked with the formation of the s’ and the equilibrium s (MgZn2) phase, respectively. Finally, the dissolution of all secondary Mg-Zn-precipitates results in the broad endothermic event between ~290 C and ~380 C (6). The as quenched (W) sample of AA7075 (Fig. 5 c and d) exhibits equivalent peaks. However, peak 1 is much more pronounced due to the alloy's higher content of alloying elements. An additional peak (5) occurs which could be linked to the formation of the T phase (Mg3Zn3Al2) since it has been reported that in alloys with Mg to Zn ratios from 1:2 to 1:3, s is formed at lower temperatures and the T phase at higher temperatures [14,15]. The DSC results of AA7021 samples subjected to the stabilization treatments (Fig. 5a) at 90 C and 120 C are very similar to the curve of the as quenched (W) sample. However, peak 1 is missing indicating that the formation of GP-zones has already occurred in these
samples before measurement. Interestingly, the sample stabilized at 90 C exhibits the peak linked to dissolution of GP-zones around 85 C (2a) similar to the W-temper sample while the samples stabilized at 120 C feature the peak at a slightly higher temperature (2b). This could indicate that the GP-zones that form during stabilization at 120 C are larger than in the case of stabilization at 90 C. For the different stabilization treatments of AA7075 (Fig. 5c) only the treatment at 90 C exhibits a strong GP-zone dissolution peak 2a as well as peak 3b linked to precipitation of s’. It can be concluded that all other stabilization treatments lead to significant s’ precipitation. The curves of AA7021 and AA7075 stabilized at 150 C (Fig. 5a and c, respectively) exhibit neither of the peaks 1, 2a/b, or 3b. Yet, they feature the endothermic peak 3a which is linked to the dissolution of s’. This indicates that the stabilization at 150 C should rather be considered as a regular artificial ageing treatment leading to the precipitation of significant amounts of the main hardening phase s’ and associated increase in tensile strength. In fact, the curves are very similar to those of the peak aged (pa) and stabilized plus paint baked (pb) samples given in Fig. 5 (b) and (d). Comparing the curves of the pa and stabilized plus pb samples, it can also be seen that peak 3a (dissolution of s’) is less pronounced for the stabilized plus pb curves. Since s’ is the main hardening phase this could explain the lower mechanical properties of the stabilized plus pb samples for AA7021. For AA7075, however, the stabilized plus pb sample exhibited mechanical properties very similar to the pa sample and the significant difference in the DSC curve is surprising. Further research is needed to explore the precipitation status of stabilized plus paint baked AA7075. 3.3. Atom probe tomography APT of a sample of AA7021 naturally aged for 2723 min (~45 h) and a sample stabilized for one hour at 90 C and subsequently naturally aged for 1955 min (~33 h) was performed. For each sample, a measurement with more than 7 million detected ions was achieved and chosen for in-depth analysis. Since crystallographic poles can lead to artifacts in APT [16,17], representative regions with approximately equal volumes away from the poles are shown in Fig. 6 with iso-concentration surfaces delimiting regions of locally increased Mg and Zn concentrations. Despite the
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Fig. 4. Mechanical properties of AA7021 (a, b, and c) and AA 7075 (d, e, and f) after different heat treatments and natural ageing (na) intervals of 1 week (1 w). W stands for W temper, stab. for stabilization, pa for peak ageing (120 C, 24 h), and pb for paint baking (185 C, 20 min).
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Fig. 5. DSC curves of AA7021 (a and b) and AA7075 (c and d) after different heat treatments and natural ageing intervals of 1 week (1 w). pa designates peak ageing (120 C, 24 h) and pb is paint baking (185 C, 20 min).
Fig. 6. Atom probe tomography of AA7021 naturally aged for ~45 h (a,b) and AA7021 stabilized at 90 C for 1 h and naturally aged for ~33 h (c, d). Separate iso-concentration surfaces of Mg and Zn concentration (a,c) or a combined Mg and Zn isosurface (b,d) are given. Mg and Zn atoms are depicted as points in the same color as their respective isosurfaces. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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relatively low temperature and short time used for the stabilization heat treatment, its effect is clearly visible: there are more regions with increased Mg and Zn concentration in the stabilized sample than in the sample that was only naturally aged. This is especially true for regions of significantly increased Zn concentrations which are scarce in the only naturally aged sample. For the stabilized sample, the combined Mg and Zn iso-concentration surfaces reveal a strong correlation of Mg and Zn agglomeration. This could indicate that the formation of GP-zones, which have been reported to have Zn:Mg ratios in the range of 0.8e1.5 [8], is more advanced in the stabilized sample. This finding is in good agreement with the DSC measurements where the GP-zone formation peak was not found for stabilized samples (cf. section 3.2). In contrast, the naturally aged sample mainly contains regions of increased Mg concentration which could be described as Mg-rich clusters or earlier stages of GP(I)-zones [9]. 4. Conclusions The natural ageing process in AA7021 can be inhibited by a stabilization heat treatment of 90 C for one hour. The resultant hardness does not significant change for at least three weeks, and it is lower than the hardness resulting from natural ageing times longer than one week. Therefore, the material remains suitable for manufacturing operations such as self-pierce riveting for a longer time frame. We propose that such a treatment should be applied directly after cooling from solution heat treatment or an elevated temperature forming process. It is suggested that the stabilization occurs mainly due to the precipitation of GP-zones which inhibit further hardness increase during storage for the studied time period. Evidence for the co-precipitation of Mg and Zn during the stabilization treatment, indicating the presence of GP-zones, has been obtained by APT. The co-precipitation was less pronounced in a naturally aged sample which mainly contained regions with increased Mg concentration and a lower degree of Zn enrichment. Further research on the exact mechanisms that inhibit natural ageing is encouraged. For AA7075, the application of suitable stabilization heat treatments can improve paint bake response. Thus, yield strengths very similar to the peak aged state can be achieved; in this paper, a decrease of only 2% was observed. This provides a way of addressing the common problem that paint baked parts do not match the possible peak aged strength. The stabilization can be applied either directly after cooling from solution heat treatment or an elevated temperature forming process or later in the process chain. The results for both alloys can contribute to improve forming process chains and/or simplify logistics in the automotive industry, potentially leading to an increased use of 7xxx alloys. Funding This work was supported by the Austrian Research Promotion Agency (FFG) [grant no. 843537]; the Austrian Federal Ministry for
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Transport, Innovation and Technology (bmvit); and the State of Upper Austria.
Acknowledgements The authors would like to thank Magna Steyr Fahrzeugtechnik AG & Co KG for supporting this research and for the permission to publish this article, as well as Aleris Aluminum Duffel BVBA for providing some of the material used. We would like to thank Robin €ublin of ETH Zürich for help with microstructural inScha vestigations and the technical staff at LKR for their invaluable assistance.
References [1] M. Kumar, N. Sotirov, C.M. Chimani, Investigations on warm forming of AW7020-T6 alloy sheet, J. Mater. Process. Technol. 214 (2014) 1769e1776, https://doi.org/10.1016/j.jmatprotec.2014.03.024. [2] A. Smeyers, B. Schepers, W. Braunschweig, A. Bürger, K. Vieregge, S. Khosla, A. Wise, 7xxx grades for automotive applications, Alum. Int. Today (2011). ~ a, L. Galdos, R. Ortubay, J. Mendiguren, X. Agirretxe, Room [3] E.S. de Argandon temperature forming of AA7075 Aluminum alloys: W-Temper process, Key Eng. Mater. 651e653 (2015) 199e204, https://doi.org/10.4028/www.scientific.net/KEM.651-653.199. [4] A. Bardelcik, A. Bouhier, M.J. Worswick, Three point bend performance of solutionized, die quenched and heat treated AA7075 beam members, in: Alum. Alloy, Trans Tech Publications, 2014, pp. 431e436, https://doi.org/ 10.4028/www.scientific.net/MSF.794-796.431. 2014-ICAA14. [5] R.G. Kamat, H. Mousavi-Anijdan, R. Kulkarni, M.A. Salgado-Ordorica, Reduced Aging Time of 7xxx Series Alloy, 2016. [6] D.J. Larson, T.J. Prosa, R.M. Ulfig, B.P. Geiser, T.F. Kelly, Local Electrode Atom Probe Tomography: a User's Guide, Springer, 2013. [7] L.K. Berg, D. Schryvers, L.R. Wallenberg, GP-zones in Al e Zn e Mg alloys and their role in artificial ageing, Acta Mater. 4 (2001) 3443e3451. [8] K. Stiller, P.J. Warren, V. Hansen, J. Angenete, J. Gjønnes, Investigation of precipitation in an AleZneMg alloy after two-step ageing treatment at 100 and 150 C, Mater. Sci. Eng. A 270 (1999) 55e63. [9] G. Sha, A. Cerezo, Early-stage precipitation in AleZneMgeCu alloy (7050), Acta Mater. 52 (2004) 4503e4516, https://doi.org/10.1016/ j.actamat.2004.06.025. [10] V. Hansen, O.B. Karlsen, Y. Langsrud, J. Gjùnnes, Precipitates, zones and transitions during aging of Al e Zn e Mg e Zr 7000 series alloy, Mater. Sci. Technol. 20 (2004) 185e193, https://doi.org/10.1179/026708304225010424. [11] T. Engdahl, V. Hansen, P.J. Warren, K. Stiller, Investigation of fine scale precipitates in Al e Zn e Mg alloys after various heat treatments, Mater. Sci. Eng. A 327 (2002) 59e64. [12] X.J. Jiang, J. Tafto, B. Noble, B. Holme, G. Waterloo, Differential scanning calorimetry and electron diffraction investigation on low-temperature aging in Al-Zn-Mg alloys, Metall. Mater. Trans. A 31 (2000) 339e348, https:// doi.org/10.1007/s11661-000-0269-x. [13] M. Nicolas, A. Deschamps, Characterisation and modelling of precipitate evolution in an Al-Zn-Mg alloy during non-isothermal heat treatments, Acta Mater. 51 (2003) 6077e6094, https://doi.org/10.1016/S1359-6454(03)004294. chet, Influence of quench and heating rates on the ageing [14] A. Deschamps, Y. Bre response of an AleZneMge(Zr) alloy, Mater. Sci. Eng. A 251 (1998) 200e207, https://doi.org/10.1016/S0921-5093(98)00615-7. [15] L.F. Mondolfo, Structure of the aluminium: magnesium: zinc alloys, Metall. Rev. 16 (1971) 95e124, https://doi.org/10.1179/mtlr.1971.16.1.95. €ffler, S. Pogatscher, [16] P. Dumitraschkewitz, S.S.A. Gerstl, P.J. Uggowitzer, J.F. Lo Atom probe tomography study of As-quenched AleMgeSi alloys, Adv. Eng. Mater. (2017) 1e5, https://doi.org/10.1002/adem.201600668. [17] E.A. Marquis, F. Vurpillot, Chromatic aberrations in the field evaporation behavior of small precipitates, Microsc. Microanal. (2008) 561e570.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Stabilization of 7xxx aluminium alloys a, * € Johannes A. Osterreicher , Georg Kirov a, Stephan S.A. Gerstl b, Ermal Mukeli c, a Florian Grabner , Manoj Kumar a, 1 a
LKR Light Metals Technologies Ranshofen, Austrian Institute of Technology, Postfach 26, 5282 Ranshofen, Austria ScopeM Scientific Center for Optical and Electron Microscopy, ETH Zürich, Auguste-Piccard-Hof 1, 8093 Zürich, Switzerland c Magna Steyr Fahrzeugtechnik AG & Co KG, Liebenauer Hauptstrasse 317, 8041 Graz, Austria b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 May 2017 Received in revised form 29 November 2017 Accepted 1 January 2018 Available online 3 January 2018
High strength Al-Zn-Mg(-Cu) alloys are rarely used in the automotive industry due to pronounced natural ageing after quenching and resulting poor formability at room temperature. Furthermore, the paint bake response is often suboptimal. To overcome these challenges, we performed various low temperature pre-ageing treatments followed by storage at room temperature and a short high temperature heat treatment simulating industrial paint baking. The intermediate and final mechanical properties were assessed and the nanostructure, precipitation, and local chemistries of the samples were studied by atom probe tomography and differential scanning calorimetry. It was found that a properly designed stabilization treatment can inhibit the natural ageing process in AA7021 resulting in a stable condition with a lower hardness than samples naturally aged for longer than approximately one week. Atom probe tomography revealed that the stabilization leads to co-precipitation of Mg and Zn (indicating GP-zones) hindering the further increase in hardness over the course of three weeks. Furthermore, the paint bake response of AA7075 was improved resulting in yield strength only 2% short of a peak aged sample. © 2018 Published by Elsevier B.V.
Keywords: Atom probe tomography Heat treatment Differential scanning calorimetry Al-Zn-Mg-Cu Paint bake Automotive
1. Introduction B-pillars and other key automotive parts require a high specific strength to satisfy the roof crush and side impact test standards while keeping the weight low. High strength Al alloys of the 7xxx series could fulfil these requirements; however, mainly due to their poor formability at room temperature, 7xxx alloys are still mostly used in the aircraft industry and up to this time have only found limited use as automotive parts [1,2]. The final mechanical properties of the parts are a measure for process chain's quality. It was found in many investigations that the final strength of the parts was much lower than the sheet's maximum possible strength (i.e., T6). The main reasons were lower quenching rates than the critical cooling rate and incomplete ageing during paint baking. In one investigation, AA7075-T6 sheet was processed via various routes, however, it was found that the initial as-received mechanical properties of the sheet could not be
* Corresponding author. € E-mail address: [email protected] (J.A. Osterreicher). 1 Current address: EBNER Industrieofenbau GmbH, Ebner-Platz 1, 4060 Leonding, Austria. https://doi.org/10.1016/j.jallcom.2018.01.003 0925-8388/© 2018 Published by Elsevier B.V.
matched [3]. Kumar el al. [1] investigated the warm forming of AA7020-T6; it was found that as a result of warm forming and paint baking, the yield strength and ultimate tensile strength were reduced by 9% and 13%, respectively. To recover the strength, an intermediate heat treatment before paint baking was proposed. Such an approach was investigated by Bardelcik et al. [4] who studied the effect of an intermediate heat treatment of 100 C for 3 h on paint bake response of AA7075. Although paint bake response was improved, the mechanical properties achieved by a peak ageing were not matched. In a patent application [5] a variety of pre-ageing treatments with the intent to increase the yield strength after paint baking are disclosed. It is demonstrated that by applying two- or three-step heat treatments, mechanical properties similar to the peak aged state can be achieved in much shorter time. In this paper, different intermediate heat treatments were applied on sheet material of AA7021 and AA7075 in T4 temper. Such heat treatments are commonly called stabilization due to the fact that thereafter the precipitation of hardening phases should not occur during storage at RT (i.e., no, or reduced, natural ageing). To assess the efficacy of the stabilization treatments, the natural ageing behaviour of as-quenched and stabilized samples was studied over 22 days. In the automotive industry, a material with
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defined mechanical properties before paint baking is vital for the design of joining operations such as self-pierce riveting; furthermore, the paint bake response should be equal or better as in the non-stabilized material. Therefore, the effect of stabilization on the intermediate and final mechanical properties (i.e., after paint baking or peak ageing) is reported. The effects of the various heat treatments on the nano- and microstructure were investigated by means of atom probe tomography (APT) and differential scanning calorimetry (DSC).
2. Experimental AA7075-T6 and an AA7021-T4 type alloy (Structurlite® 400, provided by Aleris) with a thickness of 2 mm each were used; the chemical compositions in weight percent as determined by optical emission spectroscopy (for AA7075) and provided by Aleris (for the AA7021 type alloy) are shown in Table 1. Although the Mg and Zn contents of the AA7021 type alloy slightly exceed the standard composition limits of AA7021 (i.e., a maximum Mg and Zn content of 1.8% and 6.0%, respectively) it will be referred to as AA7021 below. Both sheets were heat treated according to the schemes shown in Fig. 1. For the stabilization heat treatment, the sheets were held at 90 C, 120 C, and 150 C for 1 h, 2 h, 4 h, and 5 h. The samples were stored for different times at RT to determine the influence of natural ageing (na) and then subjected to a paint baking (pb) heat treatment of 185 C/20 min or a peak ageing (pa or T6) heat treatment of 120 C/24 h. DSC and APT measurements (for AA7021) as well as hardness and tensile tests were carried out to study the precipitation evolution, nano- and microstructure, and the resulting mechanical properties. Vickers micro-hardness (HV0.1) was measured five times per sample using a load of 100 g. Tensile tests were conducted in triplicate on a Zwick Z100 materials testing machine according to EN ISO 6892-1. Specimens were cut parallel to the rolling direction. They had a width of 12.5 mm, a reduced section length of 75 mm and a gauge length of 50 mm. APT specimens were produced by cutting matchstick shaped samples with edge dimensions of 0.5 mm 0.5 mm 10 mm, mechanically polishing them to an equal width of approximately 0.3 mm, and electropolishing a sharp tip with a solution of 10% perchloric acid in ethanol using an ElectroPointer device at room temperature [6]. The potential was gradually decreased from 20 V to 5 V AC during the electropolishing process which took approximately 15e20 min per sample. The samples were then inspected, sharpened, and cleaned in a scanning electron microscope equipped with a Ga focussed ion beam (FEI FIB-SEM Helios 600i). The FIB was operated at 30 kV for initial annular milling and 5 kV for final cleaning to minimize Ga implantation. APT analysis was performed on a Cameca LEAP 4000X HR atom probe system at a temperature of 34 K. The measurements were performed in voltage pulse mode with a pulse fraction of 0.2, a pulse frequency of 200 kHz, and a detection rate of 0.5%. For reconstruction of APT datasets, the software IVAS 3.6.14 was used. Differential scanning calorimetry (DSC) experiments were carried out using a Netzsch-DSC 204 F1 differential scanning calorimeter with a heating rate of 10 K min 1.
3. Results and discussion 3.1. Influence of stabilization on the mechanical properties The response of the solution heat treated and quenched sheets to the stabilization heat treatments is shown in Fig. 2; the hardness values of the two alloys increase with stabilization heat treatment time. The hardness of all samples was monitored for 22 days after stabilization and it was found that for all stabilization variants no significant changes in hardness occurred, with the exception of 90 C/1 h and 120 C/1 h for AA7075 where, after 22 days, an increase in HV0.1 from 139 ± 2 to 151 ± 2 (Fig. 3) and 142 ± 2 to 157 ± 3, respectively, was found. It can be concluded that all other stabilization treatments are suitable to inhibit the natural ageing process in the alloys in the surveyed time frame of 22 days. In Fig. 3 the hardness evolution due to natural ageing over the course of 22 days is shown for the sheets in W-temper as well as in peak aged condition (for reference). As expected, there is a considerable increase in hardness over time for both alloys in Wtemper. Furthermore, the natural ageing behaviour of the samples stabilized at 90 C for one or two hours are shown; the abovementioned increase in hardness for AA7075 stabilized at 90 C/ 1 h can be seen. Stabilizing AA7021 at 90 C/1 h yields a stable condition with hardness lower than 125 HV0.1 whereas the hardness after 22 days of natural ageing was 130 ± 1 HV0.1. This is of great interest because the lower hardness is beneficial for subsequent manufacturing operations. For example, we found that proper self-pierce riveting of the sheets is only possible up to a hardness of approximately 125 € llhoff). This value is HV0.1 (using a standard C H4 type rivet by Bo surpassed by natural ageing of AA7021 after approximately one week while the stabilized sheet maintains a lower hardness. It has been reported for other 7xxx series alloys that GP(II)-zones are only formed by ageing above 70 C while GP(I)-zones are formed at RT as well as at higher temperatures [7,8]. Therefore, a possible explanation for the observed stabilization by pre-ageing at 90 C could be the formation of stable GP(II)-zones which inhibit subsequent formation of GP(I)-zones and the associated increase in hardness at room temperature. However, others have reported that no GP(II)zones were found after ageing AA7050 for 1.5 h at 120 C [9]. We performed atom probe tomography to obtain further insight into these early stage precipitation processes (see section 3.3). For AA 7075, a stable condition with hardness lower than 125 HV0.1 or lower than the hardness after three weeks of natural ageing could not be achieved. Since 90 C for 1 h and 120 C for 1 h yielded unstable conditions a reduction of ageing time seems not a promising approach. However, stabilizing at lower temperature for longer time might be worth exploring. Fig. 4 shows the results of tensile tests performed on sheets in different tempers. While the effect of one week of natural ageing is clearly visible for both alloys, the strength of the stabilized samples did not change. Furthermore, there is no significant difference in the mechanical properties whether the stabilization treatment was applied directly after quenching or after one week of natural ageing. Expectedly, the values for the latter samples stayed constant also after another week of natural ageing following stabilization (data not shown). Likewise, after paint baking no difference could be
Table 1 Chemical composition of the used alloys (in wt. %). Alloys
Al
Si
Fe
Cu
Mn
Mg
Zn
Cr
Ti
Zr
Others
AA7075-T6 AA7021-T4
rest rest
0.19 max. 0.25
0.11 max. 0.40
1.50 max. 0.16
0.04 max. 0.10
2.64 1.60e2.10
6.06 6.00e6.80
0.18 max. 0.05
0.04 max. 0.1
0.02 max. 0.18
max. 0.03 max. 0.15
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Fig. 1. Schematics of solution heat treatment (SHT) and (a) stabilization heat treatment procedure, (b) paint bake response after storage, (c) paint bake response after stabilization and (d) paint bake response and peak ageing response after delayed stabilization.
Fig. 2. Response of AA7021 and AA7075 to the stabilization heat treatment at different temperatures.
found between samples stabilized directly or after 1 week. For AA7021, the stabilization treatments had no significant influence on the final mechanical properties after paint baking or peak ageing. However, it can be seen that stabilization at 150 C for 2 h directly yields mechanical properties very similar to those of the paint baked samples. Generally, yield strength and ultimate tensile strength (UTS) increase and elongation decreases for the stabilization variants due to the occurrence of precipitation hardening processes. However, this is not the case for the elongation of the sample stabilized at 90 C for 2 h which show even slightly higher elongation, giving further evidence that the ductility of the material is retained by stabilization at 90 C. For AA7075, the stabilization treatments resulted in an
improved paint bake response: the yield strength of the sample stabilized at 120 C for 2 h and subjected to paint baking is significantly higher than that of the non-stabilized sample (508 ± 1 MPa versus 460 ± 8 MPa). Furthermore, it is only 2% lower than the yield strength of the samples peak aged for 24 h (518 ± 1 MPa). This could help improve the suitability of the alloy for high strength automotive parts where excessive ageing times are not practicable. For comparison, in a recent patent application [5] the highest reported yield strength (507 MPa) was achieved after a stabilization treatment of 110 C for 6 h (i.e., triple the duration suggested in this paper) and subsequent paint baking. It was found by Hansen et al. [10] that the microstructure resulting from two-stage ageing treatments of an Al-Zn-Mg-Zr
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Fig. 3. Natural ageing behaviour of peak aged (pa), stabilized, and as quenched (W) samples. Error bars represent standard deviation.
alloy is governed mainly by nucleation in the first stage. In particular, the formation of GP(II)-zones during low temperature preageing (70e150 C) and their subsequent direct transformation to s’ during a second ageing step at higher temperature is proposed to lead to a high density of s’ precipitates with low internal order. Such a microstructure is reported to give the highest strength. However, it was also found that the direct transformation of GP(II) to s’ occurs between 120 and 170 C [10,11] whereas the paint bake temperature used in this paper is 185 C. A possible explanation is that the direct transformation could take place also at temperatures higher than 170 C in AA7075. 3.2. Characterization by differential scanning calorimetry The impact of the stabilization treatments on the DSC traces of the samples are given in Fig. 5 (a) and (c) for AA 7021 and AA7075, respectively. There is no significant difference in the DSC signals of the samples whether a stabilization treatment was applied directly after quenching (data only shown for 120 C/2 h) or after one week. This is in line with the results of the tensile tests (Fig. 4) where no difference was found either. The peaks of the DSC curves were interpreted in accordance with [12,13]. Thus, the exothermic event (1) around 85 C in the curve of the as quenched (W) sample of AA7021 (shown in both Fig. 5 a and b) can be associated with the formation of GP-zones from the initial super-saturated solid solution; the endothermic peak 2a is due to their dissolution. The exothermic peaks 3b and 4 can be linked with the formation of the s’ and the equilibrium s (MgZn2) phase, respectively. Finally, the dissolution of all secondary Mg-Zn-precipitates results in the broad endothermic event between ~290 C and ~380 C (6). The as quenched (W) sample of AA7075 (Fig. 5 c and d) exhibits equivalent peaks. However, peak 1 is much more pronounced due to the alloy's higher content of alloying elements. An additional peak (5) occurs which could be linked to the formation of the T phase (Mg3Zn3Al2) since it has been reported that in alloys with Mg to Zn ratios from 1:2 to 1:3, s is formed at lower temperatures and the T phase at higher temperatures [14,15]. The DSC results of AA7021 samples subjected to the stabilization treatments (Fig. 5a) at 90 C and 120 C are very similar to the curve of the as quenched (W) sample. However, peak 1 is missing indicating that the formation of GP-zones has already occurred in these
samples before measurement. Interestingly, the sample stabilized at 90 C exhibits the peak linked to dissolution of GP-zones around 85 C (2a) similar to the W-temper sample while the samples stabilized at 120 C feature the peak at a slightly higher temperature (2b). This could indicate that the GP-zones that form during stabilization at 120 C are larger than in the case of stabilization at 90 C. For the different stabilization treatments of AA7075 (Fig. 5c) only the treatment at 90 C exhibits a strong GP-zone dissolution peak 2a as well as peak 3b linked to precipitation of s’. It can be concluded that all other stabilization treatments lead to significant s’ precipitation. The curves of AA7021 and AA7075 stabilized at 150 C (Fig. 5a and c, respectively) exhibit neither of the peaks 1, 2a/b, or 3b. Yet, they feature the endothermic peak 3a which is linked to the dissolution of s’. This indicates that the stabilization at 150 C should rather be considered as a regular artificial ageing treatment leading to the precipitation of significant amounts of the main hardening phase s’ and associated increase in tensile strength. In fact, the curves are very similar to those of the peak aged (pa) and stabilized plus paint baked (pb) samples given in Fig. 5 (b) and (d). Comparing the curves of the pa and stabilized plus pb samples, it can also be seen that peak 3a (dissolution of s’) is less pronounced for the stabilized plus pb curves. Since s’ is the main hardening phase this could explain the lower mechanical properties of the stabilized plus pb samples for AA7021. For AA7075, however, the stabilized plus pb sample exhibited mechanical properties very similar to the pa sample and the significant difference in the DSC curve is surprising. Further research is needed to explore the precipitation status of stabilized plus paint baked AA7075. 3.3. Atom probe tomography APT of a sample of AA7021 naturally aged for 2723 min (~45 h) and a sample stabilized for one hour at 90 C and subsequently naturally aged for 1955 min (~33 h) was performed. For each sample, a measurement with more than 7 million detected ions was achieved and chosen for in-depth analysis. Since crystallographic poles can lead to artifacts in APT [16,17], representative regions with approximately equal volumes away from the poles are shown in Fig. 6 with iso-concentration surfaces delimiting regions of locally increased Mg and Zn concentrations. Despite the
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Fig. 4. Mechanical properties of AA7021 (a, b, and c) and AA 7075 (d, e, and f) after different heat treatments and natural ageing (na) intervals of 1 week (1 w). W stands for W temper, stab. for stabilization, pa for peak ageing (120 C, 24 h), and pb for paint baking (185 C, 20 min).
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Fig. 5. DSC curves of AA7021 (a and b) and AA7075 (c and d) after different heat treatments and natural ageing intervals of 1 week (1 w). pa designates peak ageing (120 C, 24 h) and pb is paint baking (185 C, 20 min).
Fig. 6. Atom probe tomography of AA7021 naturally aged for ~45 h (a,b) and AA7021 stabilized at 90 C for 1 h and naturally aged for ~33 h (c, d). Separate iso-concentration surfaces of Mg and Zn concentration (a,c) or a combined Mg and Zn isosurface (b,d) are given. Mg and Zn atoms are depicted as points in the same color as their respective isosurfaces. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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relatively low temperature and short time used for the stabilization heat treatment, its effect is clearly visible: there are more regions with increased Mg and Zn concentration in the stabilized sample than in the sample that was only naturally aged. This is especially true for regions of significantly increased Zn concentrations which are scarce in the only naturally aged sample. For the stabilized sample, the combined Mg and Zn iso-concentration surfaces reveal a strong correlation of Mg and Zn agglomeration. This could indicate that the formation of GP-zones, which have been reported to have Zn:Mg ratios in the range of 0.8e1.5 [8], is more advanced in the stabilized sample. This finding is in good agreement with the DSC measurements where the GP-zone formation peak was not found for stabilized samples (cf. section 3.2). In contrast, the naturally aged sample mainly contains regions of increased Mg concentration which could be described as Mg-rich clusters or earlier stages of GP(I)-zones [9]. 4. Conclusions The natural ageing process in AA7021 can be inhibited by a stabilization heat treatment of 90 C for one hour. The resultant hardness does not significant change for at least three weeks, and it is lower than the hardness resulting from natural ageing times longer than one week. Therefore, the material remains suitable for manufacturing operations such as self-pierce riveting for a longer time frame. We propose that such a treatment should be applied directly after cooling from solution heat treatment or an elevated temperature forming process. It is suggested that the stabilization occurs mainly due to the precipitation of GP-zones which inhibit further hardness increase during storage for the studied time period. Evidence for the co-precipitation of Mg and Zn during the stabilization treatment, indicating the presence of GP-zones, has been obtained by APT. The co-precipitation was less pronounced in a naturally aged sample which mainly contained regions with increased Mg concentration and a lower degree of Zn enrichment. Further research on the exact mechanisms that inhibit natural ageing is encouraged. For AA7075, the application of suitable stabilization heat treatments can improve paint bake response. Thus, yield strengths very similar to the peak aged state can be achieved; in this paper, a decrease of only 2% was observed. This provides a way of addressing the common problem that paint baked parts do not match the possible peak aged strength. The stabilization can be applied either directly after cooling from solution heat treatment or an elevated temperature forming process or later in the process chain. The results for both alloys can contribute to improve forming process chains and/or simplify logistics in the automotive industry, potentially leading to an increased use of 7xxx alloys. Funding This work was supported by the Austrian Research Promotion Agency (FFG) [grant no. 843537]; the Austrian Federal Ministry for
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Transport, Innovation and Technology (bmvit); and the State of Upper Austria.
Acknowledgements The authors would like to thank Magna Steyr Fahrzeugtechnik AG & Co KG for supporting this research and for the permission to publish this article, as well as Aleris Aluminum Duffel BVBA for providing some of the material used. We would like to thank Robin €ublin of ETH Zürich for help with microstructural inScha vestigations and the technical staff at LKR for their invaluable assistance.
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