简介概要

Quench sensitivity of novel Al-Zn-Mg-Cu alloys containing different Cu contents

来源期刊：Rare Metals2020年第12期

论文作者：Jin-Sheng Chen Xi-Wu Li Bai-Qing Xiong Yony-An Zhang Zhi-Hui Li Hong-Wei Yan Hong-Wei Liu Shu-Hui Huang

摘要：The effect of copper content on quench sensitivity in novel Al-Zn-Mg-Cu alloys containing high zinc content was investigated by Jominy end quench test.Electrical conductivity and hardness test,temperature collecting,and transmission electron microscopy（TEM）technique were adopted for the properties and microstructure characterization of three alloys with different copper contents.The results indicate that the electrical conductivity of all three alloys increases with the increase of distance from the quenched end,while the hardness shows an opposite trend.If the dropping of 10% hardness is defined as the critical evaluation standard of quenching,the depth of quenched layer of Alloys Ⅰ,Ⅱ,and Ⅲ are 70,55,and 40 mm,respectively.The precipitation behavior on grain boundaries of three alloys is similar except for a little difference in size,while the size of precipitates in grains of Alloy Ⅲ with higher copper content is larger than those of the other two alloys at the same location.Considering all results,the stability of the supersaturated solid solution of Alloy Ⅲ is lower than those of the other two alloys,meaning that Alloy Ⅲ shows the highest quench sensitivity.Higher copper content leads to higher quench sensitivity in novel Al-Zn-Mg-Cu alloys with the same content of magnesium,zinc,and other trace elements.

详情信息展示

稀有金属(英文版) 2020,39(12),1395-1401

Quench sensitivity of novel Al-Zn-Mg-Cu alloys containing different Cu contents

Jin-Sheng Chen Xi-Wu Li Bai-Qing Xiong Yony-An Zhang Zhi-Hui Li Hong-Wei Yan Hong-Wei Liu Shu-Hui Huang

State Key Laboratory of Nonferrous Metals and Processes,General Research Institute for Nonferrous Metals

作者简介：*Xi-Wu Li,e-mail:lixiwu@grinm.com;

收稿日期：26 October 2016

基金：financially supported by the National Key Research and Development Program of China (No. 2016YFB0300803);the National Natural Science Foundation of China (No.51274046);the National Key Basic Research Program (No.2012CB619504);

Quench sensitivity of novel Al-Zn-Mg-Cu alloys containing different Cu contents

Jin-Sheng Chen Xi-Wu Li Bai-Qing Xiong Yony-An Zhang Zhi-Hui Li Hong-Wei Yan Hong-Wei Liu Shu-Hui Huang

State Key Laboratory of Nonferrous Metals and Processes,General Research Institute for Nonferrous Metals

Abstract：

The effect of copper content on quench sensitivity in novel Al-Zn-Mg-Cu alloys containing high zinc content was investigated by Jominy end quench test.Electrical conductivity and hardness test,temperature collecting,and transmission electron microscopy(TEM)technique were adopted for the properties and microstructure characterization of three alloys with different copper contents.The results indicate that the electrical conductivity of all three alloys increases with the increase of distance from the quenched end,while the hardness shows an opposite trend.If the dropping of 10% hardness is defined as the critical evaluation standard of quenching,the depth of quenched layer of Alloys Ⅰ,Ⅱ,and Ⅲ are 70,55,and 40 mm,respectively.The precipitation behavior on grain boundaries of three alloys is similar except for a little difference in size,while the size of precipitates in grains of Alloy Ⅲ with higher copper content is larger than those of the other two alloys at the same location.Considering all results,the stability of the supersaturated solid solution of Alloy Ⅲ is lower than those of the other two alloys,meaning that Alloy Ⅲ shows the highest quench sensitivity.Higher copper content leads to higher quench sensitivity in novel Al-Zn-Mg-Cu alloys with the same content of magnesium,zinc,and other trace elements.

Keyword：

Al-Zn-Mg-Cu alloy; Copper content; Quench sensitivity; Jominy end quench test; Precipitates;

Received： 26 October 2016

1 Introduction

Al-Zn-Mg-Cu alloys (7xxx series) are widely applied in aerospace and transportation for their good performance in mechanical properties,hot workability,fatigue durability,and stress corrosion cracking (SCC) resistance [ 1, 2, 3] .But with the rapid development of modern aerospace industries,structural components should be bigger and lighter to meet the requirements of various situations [ 4, 5, 6] .That means it is quite necessary to develop a novel Al-Zn-MgCu alloy with higher strength and toughness.Increasing the content of alloying elements is an effective way,but it will increase the supersaturated solid solubility meanwhile [ 7, 8] .

Al-Zn-Mg-Cu alloys are typical aging hardened alloys,whose properties depend on the amount,size,and distribution of precipitates formed during the aging treatment [ 1, 9, 10] .In general,precipitates mainly refer to Mg(Zn,Cu,Al)2 phase,which will also be influenced by the solution and quenching treatment.After suitable solution treatment,soluble phases could dissolve into the matrix to form the supersaturated solid solution [ 11] .However,because of the high supersaturated solid solubility of novel Al-Zn-Mg-Cu alloys,precipitates are more likely to form in the quenching stage,which will cause insufficient precipitates in the aging process.Fast quenching may suppress quenched-induced precipitates to some extent,but residual stress will increase at the same time,and properties of alloys are deteriorated [ 12, 13] .Therefore,it is significant to study the mechanism of quench sensitivity of novel AlZn-Mg-Cu alloys with high alloying elements content.In this paper,the effect of copper content on quench sensitivity was investigated by Jominy end quench test.

2 Experimental

The composition of novel Al-Zn-Mg-Cu alloys is detailed in Table 1.Self-made hot-rolled plates with the cross section of 102 mm×26 mm were cut into bars ofΦ24 mm×150 mm along the rolling direction.In order to reflect the actual quenching of thick plates production,the bar was surrounded by 7085 aluminum alloy with size ofΦ48 mm×150 mm.The experiment was carried out by a set of self-made Jominy end quench test equipment.The bar was solution heat-treated at 470℃for 6 h in the Muffle furnace with forced air convection and then transferred to the quenching system within 5 s [ 14, 15, 16, 17] .The room temperature water was chosen as the quenchant to cool down the bar from its bottom end.The free height of spraying water was kept at 200 mm during the whole process for the need of constant pressure on the bar.And the distance between the bar and the nozzle was 20 mm.

Aiming to master cooling curves during the quenching,several K thermocouples were inserted into the core of the bar at distance of 5,10,25,40,60,80,100,115,and135 mm from the quenched end.And temperature changes would be collected by a computer-driven MX 100 data logging system.

After quenching,the bar was cut into two halves by a wireelectrode cutting at low temperature.The electrical conductivity was measured every 10 mm by a WD-Z eddy current conductivity meter immediately with the half.The microstructure was examined with transmission electron microscope (TEM,JEOL 2010FX) at 200 kV.The TEM samples were processed into small foil ofΦ3 mm×0.05 mm from the as-quenched half,and then thinned by a twin-jet polisher with a 33%nitric acid in methanol at-25℃

[ 18, 19, 20] .The other half was aging heat-treated at 120℃for24 h,whose Vickers hardness was performed every 5 mm on a WOLPERT 430SVD meter subsequently.

3 Results and discussion

3.1 Electrical conductivity and hardness test

The samples were polished before the electrical conductivity and hardness test to reduce effect of oxide layer and rough surface.The results are shown in Fig.1.Obviously,the electrical conductivity of all three alloys increases with the increase of distance from the quenched end and then begins to flatten out when the distance reaches 80 mm.In detail,the differences between the maximum electrical conductivity and the minimum one of AlloysⅠ,Ⅱ,andⅢare 1.5,1.4,and 1.7 MS·m^-1,respectively.

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Table 1 Chemical composition of three alloys (wt%)

The difference in electrical conductivity is related to the stability of the supersaturated solid solution [ 21, 22] .The lower the supersaturated solid solution is,the more easily the quenched-induced precipitates form during the slow quenching,which will reduce the degree of distortion of the lattice.As a consequence,the blocking effect on the movement of conduction electrons is reduced,which will cause the increase of the electron free path and electrical conductivity.In this experiment,the difference in electrical conductivity reflects the different stability of the supersaturated solid solution of three alloys.AlloyⅢwith higher Cu content shows the lowest stability of the supersaturated solid solution,while that of AlloysⅠandⅡis at almost the same level.

On the contrary,the hardness of alloys in T6 temper decreases with the increase of the distance from the quenched end and tend to be stable from the distance of80 mm,as shown in Fig.2.The hardness of AlloyⅢis the highest of three alloys at the location of 5 mm,but also the lowest one at the location of 140 mm.So,AlloyⅢshows the highest difference between the maximum hardness and the minimum,while AlloyⅠwith the lowest Cu content shows the lowest difference.If the dropping of 10%hardness is defined as the critical evaluation standard of quenching,the depth of quenched layer of AlloysⅠ,Ⅱ,andⅢare 70,55,and 40 mm,respectively.

Considering the results of electrical conductivity and hardness measurement,AlloyⅢwith the highest Cu content shows the highest quench sensitivity.

Fig.1 Electrical conductivity profiles of AlloysⅠ,Ⅱ,andⅢusing Jominy end quench test

Fig.2 Hardness profiles of AlloysⅠ,Ⅱ,andⅢusing Jominy end quench test

3.2 Cooling curves

Taking all influencing factors of quenching into account,the cooling rate is an important one which should be an appropriate value neither too high nor too low to balance the mechanical properties and residual stress [ 23, 24] .Figure 3 shows the temperature changes at different distances from the quenched end of the bar and average cooling rates in the quenching sensitive temperature range(450-250℃ [ 25, 26] ).In order to simulate the one-dimensional cooling in factories,the bar holder was surrounded with asbestos to prevent from thermal convection with air environment during the quenching.The highest cooling rate appears at the quenched end directly touched by the spraying water,which emerges different degrees of decline with the increase of the distance from the quenched end.Cooling curves show a rapid decline in the beginning and then gradually tend toward stable when the temperature is below 100℃.

In Fig.3b,the average cooling rates of the alloys at 5,10,25,40,60,80,100,115,and 135 mm from the quenched end in the quench sensitive temperature range(450-250℃ [ 25] ) are 29.20,15.27,7.96,4.28,2.59,1.97,1.72,1.63,and 1.52 K·s^-1,respectively.The farther the distance is,the more the time is needed to cool down through the quenching sensitive temperature range,which actually reflects the cooling law from the surface to the core during the quenching of thick and ultra-thick plates.In view of the depth of layer,the theoretical quenching rates of AlloysⅠ,Ⅱ,andⅢshould be no less than 2.26,3.07,and4.78 K·s^-1,respectively.

3.3 TEM characterization

The bright-filed (BF) TEM images and selected area electron diffraction (SAED) patterns in<100>_Al,<110>_Al,and<112>_Al zone axes for AlloyⅢare shown in Fig.4.The main strong diffraction spots were indexed,and it is easy to know that they are from Al matrix.The diffraction spots from spherical Al₃Zr are found at{100}and{110}positions in<100>Al,<110>Al,and<112>Al projections.Besides,the diffraction spots at outside of{220}positions in<110>Al projections come from the non-coherentηprecipitates.

The quenched-induced precipitates morphology and distribution of AlloyⅠat different locations from the quenched end are shown in Fig.5.At the location of 5 mm,there are some fine lamellar precipitates (ηphase) along the grain boundaries and some spherical Al₃Zr in grains with no precipitates on its surface.The boundary precipitates become remarkably coarser and longer at the location of 40 mm,whose size further increases at 100 mm.However,there are only a few precipitates in grains at the location of 100 mm.Results indicate that the supersaturated solid solution of AlloyⅠis stable enough to suppress the precipitation during the quenching.

Figure 6 illustrates the precipitation behavior at different locations from the quenched end of AlloyⅡ.The precipitation behavior on grain boundaries is the same as that of AlloyⅠ.But at the location of 40 mm,except spherical Al₃Zr particles,there are plenty of heterogeneous precipitates nucleating on the surface of Al₃Zr particles in grains.And heterogeneous precipitates are found to further grow up to the size of 500 nm at the location of 100 mm.

Fig.3 a Cooling curves and b average cooling rates in quench sensitive temperature range (450-250℃)

Fig.4 SAED patterns in a<100>_Al,b<11 0>_Al,and c<112>_Al zone axes for AlloyⅢ

Fig.5 BF-TEM images of precipitates of AlloyⅠat different locations from quenched end:a,b 5 mm;c,d 40 mm;e,f 100 mm

Figure 7 displays the quenched-induced precipitates morphology of AlloyⅢ.Similarly,the precipitation behavior on grain boundaries of AlloyⅢis the same as that of AlloyⅡ.However,a small amount of precipitates are discovered in grains at the location of 5 mm.And precipitates grow up with the increase of distance from the quenched end,whose size is about 655 nm at the location of 100 mm.

The comparison of precipitation behavior of three alloys is detailed in Table 2.It is easy to find that they all have similar precipitation behavior on grain boundaries,except for a little difference in size.But the situation in grains is quite different,that is plenty of precipitates of AlloysⅡandⅢappear at the location of 40 mm and further grow up at100 mm.And the size of precipitates of AlloyⅢis larger than that of AlloyⅡat the same location.Different precipitation behaviors reflect the different stabilities of the supersaturated solid solution of alloys [ 27, 28, 29, 30] .The higher the stability of the supersaturated solid solution is,the harder the precipitates form during the quenching.Accordingly,the stability of the supersaturated solid solution of AlloyⅢis lower than those of the other two alloys,and that of AlloyⅠis the highest.It also could be understood that AlloyⅢshows higher quench sensitivity,and AlloyⅠshows the lowest one.

Fig.6 BF-TEM images of precipitates of AlloyⅡat different locations from quenched end:a,b 5 mm;c,d 40 mm;e,f 100 mm

Fig.7 BF-TEM images of precipitates of AlloyⅢat different locations from quenched end:a,b 5 mm;c,d 40 mm;e,f 100 mm

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Table 2 Comparison of precipitation behavior of three alloys

4 Conclusion

The depth of quenched layer of AlloysⅠ,Ⅱ,andⅢare 70,55,and 40 mm,respectively.Higher copper content promotes precipitation during quenching process,so that the depth of quenched layer will be reduced,and higher cooling rate would be necessary to ensure properties of alloys.All three alloys show similar precipitation behavior on grain boundaries,except for a little difference in precipitate size.Besides,different precipitation behaviors in grains give evidence to the conclusion that the stability of the supersaturated solid solution of AlloyⅢis the lowest,which is consistent with the results of electrical conductivity and hardness test.In this study,AlloyⅢwith the highest copper content shows the highest quench sensitivity,indicating that higher copper content leads to higher quench sensitivity in novel Al-Zn-Mg-Cu alloys with the same content of magnesium,zinc,and other trace elements.