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Acta Metall Sin  2019, Vol. 55 Issue (11): 1395-1406    DOI: 10.11900/0412.1961.2018.00555
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Effects of Zn Addition on the Natural Ageing Behavior and Bake Hardening Response of a Pre-Aged Al-Mg-Si-Cu Alloy
ZHU Shang1,LI Zhihui1(),YAN Lizhen1,LI Xiwu1,ZHANG Yongan1,XIONG Baiqing1,2
1. State Key Laboratory of Nonferrous Metals and Processes, GRINMAT Engineering Institute Co. , Ltd. , Beijing 101407, China
2. GRINM Group Co. , Ltd. , Beijing 100088, China
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Abstract  

Al-Mg-Si(-Cu) alloys are widely used in automotive body panels because of their excellent combined performance of high strength-to-weight ratio, good formability and corrosion resistance. Zn additions to Al-Mg-Si(-Cu) alloys have been tested and shown to effectively affect the precipitation microstructure and enhance the age-hardening response. The present study investigates the natural ageing (NA) behavior and bake hardening response in the pre-aged Al-0.9Mg-0.8Si-0.2Cu (mass fraction, %) and Al-0.9Mg-0.8Si-0.2Cu-0.6Zn (mass fraction, %) alloys. The results are compared to clarify the effect of Zn addition. During NA after pre-ageing at 80 ℃ for 15 min (PA), cluster growth is the dominant process in the Zn-free and Zn-added alloys. Some Zn atoms are partitioned into the clusters under PA+NA condition. Partitioning of Zn may change the stability of clusters, increasing the growth rate of clusters. The yield strength of the two alloys increases with the increasing NA time. The smaller cluster spacing and larger cluster shear modulus lead to the higher yield strength in the Zn-added alloy during NA after PA. The prolonged NA inhibits the transformation of clusters to GP zones and β″ phases during bake hardening (BH) treatment at 170 ℃ for 30 min in the Zn-free and Zn-added alloys, resulting in the lower BH response. The Zn does not significantly partition into clusters or precipitates, and the majority of Zn remains in the Al matrix during BH treatment, prompting the transformation from solute clusters to GP zones and β″ phases. As a result, the yield strength of the Zn-added alloy after PA+NA+BH treatment is higher than that of the Zn-free alloy.

Key words:  Al-Mg-Si-Cu alloy      ageing      Zn addition      precipitate     
Received:  19 December 2018     
ZTFLH:  TG146.21  
Fund: National Key Research and Development Program of China(2016YFB0300802)
Corresponding Authors:  Zhihui LI     E-mail:  lzh@grinm.com

Cite this article: 

ZHU Shang,LI Zhihui,YAN Lizhen,LI Xiwu,ZHANG Yongan,XIONG Baiqing. Effects of Zn Addition on the Natural Ageing Behavior and Bake Hardening Response of a Pre-Aged Al-Mg-Si-Cu Alloy. Acta Metall Sin, 2019, 55(11): 1395-1406.

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https://www.ams.org.cn/EN/10.11900/0412.1961.2018.00555     OR     https://www.ams.org.cn/EN/Y2019/V55/I11/1395

AlloyMgSiZnCuMnZrFeAl
Zn-free0.900.820.0060.230.110.09<0.1Bal.
Zn-added0.920.780.6400.230.100.12<0.1Bal.
Table 1  Chemical compositions of the Zn-free and Zn-added Al-Mg-Si-Cu alloys (mass fraction / %)
Fig.1  Schematic of heat treatment process
Fig.2  Vickers hardness curves of the pre-aged Zn-free and Zn-added alloys during natural ageing (a) and subsequent bake-hardening treatment (b)

Treatment

Zn-free alloyZn-added alloy
YS / MPaUTS / MPaA50 / %YS / MPaUTS / MPaA50 / %
PA+NA 2 d12724424.513224925.2
PA+NA 120 d13925725.014526625.5
PA+NA 2 d+BH17928522.119429821.5
PA+NA 120 d+BH16427324.817327823.9
Table 2  Tensile properties of the pre-aged Zn-free and Zn-added alloys during natural ageing and subsequent bake-hardening treatment
Fig.3  Morphologies of precipitates observed by 3DAP in the Zn-free and Zn-added alloys during natural ageing after pre-ageingColor online(a) Zn-free alloy after PA+NA 2 d (The analyzed volume is 133 nm×75 nm ×77 nm)(b) Zn-added alloy after PA+NA 2 d (The analyzed volume is 160 nm×57 nm ×59 nm)(c) Zn-free alloy after PA+NA 120 d (The analyzed volume is 260 nm×40 nm ×41 nm)(d) Zn-added alloy after PA+NA 120 d (The analyzed volume is 279 nm×54 nm×55 nm)
Fig.4  Morphologies of precipitates observed by 3DAP in the pre-aged Zn-free and Zn-added alloys under different ageing conditionsColor online(a) Zn-free alloy after PA+NA 2 d+BH (The analyzed volume is 181 nm×103 nm×105 nm)(b) Zn-added alloy after PA+NA 2 d+BH (The analyzed volume is 286 nm×79 nm×81 nm)(c) Zn-free alloy after PA+NA 120 d+BH (The analyzed volume is 331 nm×79 nm×82 nm)(d) Zn-added alloy after PA+NA 120 d+BH (The analyzed volume is 328 nm×77 nm×77 nm)
Fig.5  Relationships between the size and shape of precipitates in the Zn-free (a, c, e, g) and Zn-added (b, d, f, h) alloys under different ageing conditions (x, y and z are the dimensions of each precipitate measured along three orthogonal axes of best-fit ellipsoid)(a, b) PA+NA 2 d (c, d) PA+NA 120 d(e, f) PA+NA 2 d+BH (g, h) PA+NA 120 d+BH
Fig.6  Number densities of precipitates in the pre-aged Zn-free (a) and Zn-added (b) alloys during natural ageing and subsequent bake-hardening treatment
Fig.7  Fractions of the total amounts of solute atoms incorporated in the precipitates of pre-aged Zn-free and Zn-added alloys under different ageing conditions (For example, 4.56% of the total amount of Mg in the Zn-free alloy after PA+NA 2 d was incorporated in clusters, so 95.44% of the Mg remained in the matrix. Different types of precipitates are distinguished in Fig.5)(a) Zn-free alloy after PA+NA 2 d and PA+NA 2 d+BH(b) Zn-free alloy after PA+NA 120 d and PA+NA 120 d+BH(c) Zn-added alloy after PA+NA 2 d and PA+NA 2 d+BH(d) Zn-added alloy after PA+NA 120 d and PA+NA 120 d+BH
AlloyAgeing treatmentPrecipitate typeYield strength / MPa

Zn-free

PA+NA 2 dCluser33.6
PA+NA 120 dCluser36.9

Zn-added

PA+NA 2 dCluser40.7
PA+NA 120 dCluser47.4
Table 3  Calculated yield strengths of the pre-aged Zn-free and Zn-added alloys during NA after PA (Clusters are identified in Figs.5a~d)
AlloyAgeing treatmentPrecipitate typeYield strength / MPa
Zn-freePA+NA 2 d+BHCluser28.6
GP zone15.9
PA+NA 120 d+BHCluser34.3
GP zone7.9
Zn-addedPA+NA 2 d+BHCluster32.1
GP zone21.8
β41.5
PA+NA 120 d+BHCluster46.4
GP zone15.7
β19.5
Table 4  Calculated yield strengths of the pre-aged Zn-free and Zn-added alloys during BH treatment after NA (Different types of precipitates are distinguished in Figs.5e~h)
AlloyAgeing treatmentAverage radius / nmMg/Si ratioMg/Zn ratio
Zn-freePA+NA 2 d1.031.12-
PA+NA 120 d1.051.04-
Zn-addedPA+NA 2 d1.081.236.17
PA+NA 120 d1.131.133.22
Table 5  Changes in the average radius and the Mg/Si ratio of clusters in the Zn-free and Zn-added alloys and the Mg/Zn ratio of clusters in the Zn-added alloy during NA after PA
Fig.8  Schematics of precipitation evolution in the pre-aged Zn-free (a) and Zn-added (b) alloys during NA and BH treatmentsColor online
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