简介概要

Texture evolution and mechanical properties of Al-Mg-Si alloys at different intermediate annealing temperatures

来源期刊：Rare Metals2019年第10期

论文作者：Yong Li Qi-Pan Wang Guan-Jun Gao Jia-Dong Li Zhao-Dong Wang Guang-Ming Xu

文章页码：937 - 945

摘要：Intermediate annealing treatment produces different effects on the state of particles in Al-Mg-Si alloy sheets,thereby affecting their recrystallization textures and formability.To improve the formability of the sheets,the effects of different intermediate annealing temperatures on the texture evolution and mechanical properties of these sheets for automotive applications were studied using optical microscope（OM）,scanning electron microscope（SEM） and tensile tests.The results reveal that intermediate annealing temperature has a significant influence on the recrystallization textures and average plastic strain ratio（r）.After solution treatment,all the alloy sheets possess similar recrystallization texture components comprising of cube_ND {100}<310> and P {011}<122> orientations,whereas a characteristic strong cube-oriented {100} <001>texture is observed only in the alloy annealed at a temperature of 380℃.However,in comparison with the alloy not annealed,and the alloy annealed at 550℃,the alloy annealed at 380℃ possesses a lower average r value.Furthermore,the relationship between textures and r value was analyzed by using the Taylor full constraints model in this study.

稀有金属(英文版) 2019,38(10),937-945

Texture evolution and mechanical properties of Al-Mg-Si alloys at different intermediate annealing temperatures

Yong Li Qi-Pan Wang Guan-Jun Gao Jia-Dong Li Zhao-Dong Wang Guang-Ming Xu

State Key Laboratory of Rolling and Automation,Northeastern University

作者简介：*Guan-Jun Gao e-mail:wwwgaoguanjun@126.com;

收稿日期：22 April 2019

基金：financially supported by the National Natural Science Foundation of China (No.51790485);

Texture evolution and mechanical properties of Al-Mg-Si alloys at different intermediate annealing temperatures

Yong Li Qi-Pan Wang Guan-Jun Gao Jia-Dong Li Zhao-Dong Wang Guang-Ming Xu

State Key Laboratory of Rolling and Automation,Northeastern University

Abstract：

Intermediate annealing treatment produces different effects on the state of particles in Al-Mg-Si alloy sheets,thereby affecting their recrystallization textures and formability.To improve the formability of the sheets,the effects of different intermediate annealing temperatures on the texture evolution and mechanical properties of these sheets for automotive applications were studied using optical microscope(OM),scanning electron microscope(SEM) and tensile tests.The results reveal that intermediate annealing temperature has a significant influence on the recrystallization textures and average plastic strain ratio(r).After solution treatment,all the alloy sheets possess similar recrystallization texture components comprising of cube_ND {100}<310> and P {011}<122> orientations,whereas a characteristic strong cube-oriented {100} <001>texture is observed only in the alloy annealed at a temperature of 380℃.However,in comparison with the alloy not annealed,and the alloy annealed at 550℃,the alloy annealed at 380℃ possesses a lower average r value.Furthermore,the relationship between textures and r value was analyzed by using the Taylor full constraints model in this study.

Keyword：

Al-Mg-Si alloys; Intermediate annealing; Texture evolution; Mechanical property;

Received： 22 April 2019

1 Introduction

Aluminum automotive sheets are widely used to meet the increasing demands of weight reduction in automotive industries and reduce fuel consumption and vehicle emissions,due to their high strength-to-weight ratio,excellent corrosion resistance and age-hardening potential [ 1, 2, 3, 4] .Among aluminum alloys,heat-treatable Al-Mg-Si alloys exhibit good formability and bake hardenability,thereby making them suitable for automotive body panels.However,the deep drawability of aluminum alloy sheets is far inferior to that of steel;hence,the drawability is required to be further improved [ 5] .

The plastic strain ratio (r,Lankford value) which is used to evaluate the formability of aluminum sheets is closely related to their crystallographic textures [ 6] .Generally,the higher the r value of an aluminum sheet is,the better its formability is.Some research results [ 6, 7] have further revealed that r mainly depends on the specific texture components and their corresponding volume fractions.The precipitated particle states of Al-Mg-Si alloy sheets play a critical role in influencing the recrystallization textures.Studies reveal that coarse particles tend to develop particle stimulated nucleation (PSN) during solution treatment,thereby resulting in weak recrystallization textures,which are composed of characteristic occupations of cubeND and P orientations.Moreover,fine particles effectively promote the formation of typical cube-recrystallization textures [ 8, 9] .In general,the state of particles in alloy sheet is closely related to the intermediate annealing treatment [ 10] .Therefore,it is necessary to study the effect of intermediate annealing on the recrystallization textures of Al-Mg-Si alloy sheets to improve their formability.

Texture evolution during thermomechanical processing is relatively complicated,and a large number of investigations have been made.Although some studies have independently studied the effects of hot rolling,cold rolling and solution treatment on the final recrystallization textures [ 11, 12, 13] ,a comprehensive focus on intermediate annealing effect is required.Accordingly,in the present study,three groups of Al-Mg-Si alloy sheets at different intermediate annealing temperatures were selected,and the effect of intermediate annealing on the microstructure,texture and mechanical properties of these sheets was studied.

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Table 1 Different intermediate annealing treatments of Al-Mg-Si alloy sheets

2 Experimental

The starting material used in this study was a 30-mm-thick,100-mm-wide and 200-mm-long ingot with a chemical composition of Al-1.6Mg-1.4Si-0.15Fe-0.1 Cr-0.02Mn-0.03Zn-0.02Ti (wt%).After face milling,the ingot was homogenized at 470℃for 5 h and subsequently at 540℃for 16 h in an air circulation furnace and then hot-rolled from a thickness from 30 mm to 5 mm.Subsequently,the hot-rolled sheet was pided into three groups denoted as A1,A2 and A3.They were subjected to different intermediate annealing treatments,as listed in Table 1.After intermediate annealing,the sheets were cold-rolled to a final thickness of 1 mm at room temperature (RT).Subsequently,all sheets were completely recrystallized and solution treated in an air circulation furnace at 560℃for10 min followed by water quenching.Finally,these sheets were immediately pre-aged at 100℃for 3 h and placed at RT for 2 weeks (T4P state).

The micros truc ture of the alloy sheets was observed using the Imager M2m ZEISS optical microscope (OM).The particle morphology in the alloy matrix was characterized using a Zeiss Ultra 55 field emission scanning electron microscope (FESEM) equipped with energy-dispersive spectrometer (EDS) systems.The textures of longitudinal sections after intermediate annealing and solution treatment were also characterized using Zeiss Ultra 55FESEM equipped with an electron backscattered diffraction (EBSD) system.Specimens for texture observation were mechanically ground and electropolished in a10 vol%perchloric acid in alcohol solution at 25 V for30 s.The mechanical properties were determined at room temperature (RT) using an Instron-4206 electronic universal testing machine.

3 Results and discussion

3.1 Microstructures

The micros truc tures of the Al-Mg-Si alloy sheets after the different intermediate annealing treatments are presented in Fig.1.The highest density of particles is observed in Alloy A1 with no intermediate annealing (Fig.1a).In Alloys A2and A3 with intermediate annealing,the density of precipitates in the matrix decreases with the increase in intermediate annealing temperature,and some of the particles have dissolved back into the matrix,as shown in Fig.lb,c.Furthermore,these particles in the three groups of sheets are arranged along the rolling direction (RD).

Figure 2 shows the inverse polar figures (IPFs) of the Al-Mg-Si alloy sheets after the different intermediate annealing treatments.Clearly,the grain structures in the three alloy sheets are relatively different.A typical rolled structure with highly elongated grains is observed in Alloy A1 (Fig.2a).In contrast,Alloys A2 and A3 exhibit obvious recrystallized structures (Fig.2b,c).Compared to Alloy A3 comprising of completely equiaxed grains,Alloy A2 displays a two-state structure consisting of slightly elongated grains and equiaxed recrystallized grains.

Fig.1 OM images of Al-Mg-Si alloy sheets after intermediate annealing treatment of alloys:a A1,b A2 and c A3

Fig.2 IPFs of Al-Mg-Si alloy sheets after intermediate annealing treatment of alloys:a A1,b A2 and c A3

Fig.3 SEM images of Al-Mg-Si alloy sheets after intermediate annealing treatment of alloys:a A1,b A2 and c A3;d enlarged SEM images of A1;EDS spectra of e white and f black particles (indicated by red arrows in d)

Figure 3 shows the distributions of particles in the AlMg-Si alloy sheets after the different intermediate annealing treatments.It is observed that most of the particles appearing in the matrix are about 1μm in size with two main types:black and white,as shown in Fig.3a-c.According to EDS analysis (Fig.3d-f),both the black and white particles contain Al,Mg and Si.These particles are most probably the stable Mg2Si phases,which are often observed in 6xxx alloys [ 13, 14] .Moreover,the density of the Mg2Si particles decreases significantly with an increase in the intermediate annealing temperature.The existence of these particles will affect the recrystallization textures during the subsequent solution treatment.

Fig.4 OM images of the Al-Mg-Si alloy sheets after the solution treatment of alloys:a A1,b A2 and c A3

Fig.5 EBSD analysis and IPFs of alloys after solution treatment:a A1,b A2 and c A3;recrystallized grain size distribution in alloys:d A1,e A2 and f A3

The microstructures of the Al-Mg-Si alloy sheets after the solution treatment are shown in Fig.4.No difference in the density of particle distribution is evident.Because the temperature of solution heat treatment exceeds 540℃,most of the soluble phase has dissolved into the matrix.Hence,most of the remaining particles in the three alloy groups are most probably Mg2Si,due to the existence of excessive Mg in the Al-Mg-Si alloy sheets,thereby reducing the solubility of the Mg2Si particles (Fig.4a-c).It is noteworthy that the number of particles that dissolve back into the matrix during the solution treatment is different in Alloys A1,A2 and A3.

Figure 5 shows IPFs and grain size distribution of the Al-Mg-Si alloy sheets after the solution treatment.Evidently,the recrystallized grain structures in the three alloy groups are almost equiaxed (Fig.5a-c);however,the grain size and uniformity are quite different.According to the statistical calculations (Fig.5d-f),the average grain sizes of Alloys A1,A2 and A3 are 10.35,13.78 and 12.11μm,respectively.Furthermore,the homogeneity of the recrystallized grain size distribution decreases from Alloys A1through A2 to A3.Therefore,the different intermediate annealing treatments affect the recrystallized grains after the solution treatment.

3.2 Texture evolution

Figure 6 shows the orientation distribution functions(ODFs) of the Al-Mg-Si alloy sheets after the different intermediate annealing treatments.The texture components and intensities in the three alloy groups are quite different.A typical copper-type rolling texture is observed in Alloy A1 (Fig.6a).This is attributed to a low hot-rolling exit temperature.In this texture,most orientations are concentrated along theβ-fiber running through the Euler angle space from the copper orientation{112}<111>through the S orientation{123}<634>to the brass orientation{011}<211> [ 9, 15] .The intensities of the copper,S and brass orientations are 7.4,4.2 and 6.6,respectively.In total,the corresponding content of deformed texture is 63 vol%.On the contrary,Alloys A2 and A3 exhibit weak recrystallization textures (Fig.6b,c).However,some retained rolling texture components,particularly R-oriented textures,are observed in the two alloys.

Fig.6 ODFs of Al-Mg-Si alloy sheets after intermediate annealing treatment of alloys:a A1,b A2 and c A3

Fig.7 Final recrystallization textures of Al-Mg-Si alloy sheets after solution treatment of alloys:a A1,b A2 and c A3

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Table 2 Intensities and content of recrystallization texture compo-nents of three alloy sheets

The final recrystallization textures in the Al-Mg-Si alloy sheets after the solution treatment are shown in Fig.7.Clearly,the recrystallization textures of the three alloys are slightly different.This may be attributed to the various recrystallization mechanisms during the solution treatment.The detailed intensities and contents of the specific texture components of the three sheets are listed in Table 2.Results show that the three alloy groups possess similar recrystallization texture components comprising of cubeND{001}<310>and P{011}<122>orientations(Fig.7a-c).In contrast,a cube-oriented{001}<100>texture is observed only in Alloy A2,as shown in Fig.7b.

3.3 Mechanical property characterization

Figure 8 shows r values calculated by 10%deformation in different directions,and the average r and absoluteΔr values of the Al-Mg-Si alloy sheets in T4P state.Obviously,the r value of Alloy A2 is lower than that of the other two alloys at 45°;however,it is slightly higher at 0°and 90°,as shown in Fig.8a.The average r andΔr values are calculated as follows [ 7, 16] :

Fig.8 a Values of r in three different directions;b calculated average r and absolute Ar values of Al-Mg-Si alloy sheets

Fig.9 Engineering stress-strain curves of Al-Mg-Si alloy sheets in T4P state of alloys:a A1,b A2 and c A3

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Table 3 Mechanical properties of three alloy sheets in different directions

where r₀,r₄₅ and r₉₀ are r values measured at 0°,45°and90°,respectively,to RD.Results reveal that the average r value of alloy A2 is slightly lower than that of the other two alloys (Fig.8b).However,the average r values of all the sheets are greater than 0.77 (Fig.8),indicating relatively good deep drawability.Furthermore,the absolute Ar value of Alloy A2 is much lower than that of Alloys A1and A3,indicating that Alloy A2 possesses the lowest planar anisotropy.

Figure 9 shows the stress-strain curves of the Al-Mg-Si alloy sheets in the T4P state.For the inpidual alloys,the mechanical properties in the three directions are quite different.The detailed mechanical properties of the three sheets in the three directions are summarized in Table 3.Results reveal that the elongation,yield strength (YS) and ultimate tensile strength (UTS) of the three alloy groups are relatively different in the three directions.This indicates that they are anisotropic (Fig.9a-c).However,the yield strength of Alloy A2 is the lowest,which is very beneficial for the final deep drawing operations.It is concluded that Alloy A2 possesses better comprehensive properties,including quite good formability and the lowest planar anisotropy and yield strength.

3.4 Discussion

Four nucleation sites,namely cube bands,deformation zones around large particles,grain boundaries and occasionally,shear bands,are very important to determine the final recrystallization texture [ 17, 18, 19] .For Al-Mg-Si alloy sheets,the evolution of final recrystallization textures is mainly determined by the competition between PSN and the nucleation at cube bands.In the present study,the P and cube_ND textures are observed in Alloys A1,A2 and A3after the solution treatment (Fig.7a-c),indicating the occurrence of PSN.PSN often occurs in the deformation region around large particles and easily produces weak textures comprising typical occupations of P and cubeND orientations [ 15, 20] .Coarse particles (>1μm) promote recrystallization by PSN,whereas fine particles (<1μm)effectively retard PSN to enable the cube orientation to prevail in the recrystallization textures [ 21] .For Alloys A1,A2 and A3,a large number of particles larger than 1μm exist in the matrix,as shown in Fig.1a-c.Thus,the three alloy groups possess similar recrystallization texture components comprising of cube_ND and P orientations.

After solution treatment,a rather strong cube-orientated texture is observed only in Alloy A2 (Fig.7b).This is probably due to the strong effect of nucleation at cube bands.It is generally assumed that small particles can effectively develop characteristic cube-oriented grains by nucleating at cube bands with a band-like structure by exerting significant pinning force [ 22] .Furthermore,it has been found that the nucleation of cube grains is also associated with small volumes of cube subgrains present in the deformed structure due to the metastability of the cubeoriented grains [ 23, 24] .The presence of many small particles in Alloy A2 results in the formation of strong cuberecrystallization textures (Fig.3b).

It is well known that various texture components correspond to different average plastic strain ratio (r) and planar anisotropy (Δr) values.In general,the r values of polycrystalline materials are calculated using the following formula [ 9, 21, 25] :

where r_j is the r value of a single crystal in the jth orientation,and V_j is the volume fraction of crystals in the jth orientation.Equation (3) reveals that the final formability of the three alloys can be interpreted by a superposition of the characteristic r value provided by the inpidual texture components.

In the present study,although the three alloy groups possess quite high average r values,the average r value of Alloy A2 is lower than that of Alloys A1 and A3 (Table 3).PSN is the main reason for the high average r value of the alloy sheets [ 26] .According to the contribution of r value to formability,the P and cubeND orientations enhance the formability,while the cube orientation does not contribute to improving r value of the alloy sheet [ 27, 28] .In Alloy A2,the strong cube-recrystallization textures result in a low average r value (Fig.8b).Thus,the PSN effect should be enhanced by controlling the intermediate annealing treatment to improve the formability of Al-Mg-Si alloy sheets.

To further study the effect of textures on r value,planar anisotropy of the r value was predicted using the Taylor full constraints model.Figure 10 shows the comparisons of the experimental and predicted r values of the alloy sheets in T4P state.The results reveal that although the predicted r values are higher than those measured experimentally,the basic tendency of the variation in the r values of the three sheets is well reproduced using the predictions (Fig.10ac).This clearly indicates the strong effect of crystallographic texture on plastic anisotropy [ 10] .The comparisons of the experimental and predicted average r andΔr values of the alloy sheets in the T4P state are summarized in Table 4.Both the predicted average r and absoluteΔr values are higher than those measured experimentally;however,the trend of variation in the average r and absoluteΔr values is the same.The higher predicted r values can be attributed to the application of the Taylor full constraints model or other factors such as the different recrystallized grain sizes and morphologies [ 29] .In summary,the recrystallization texture components and volume fraction show good correspondence with the r values of the alloy sheets.

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Table 4 Experimental and predicted average r andΔr values of alloy sheets

Fig.10 Comparison of experimental and predicted r values of alloy sheets in different directions for alloys:a A1,b A2 and c A3

4 Conclusion

In this study,the influences of different intermediate annealing temperatures on the texture evolution and mechanical properties of Al-Mg-Si alloy sheets were studied using OM,SEM and tensile tests.It is found that intermediate annealing treatment strongly affects the texture evolution and average r values of the Al-Mg-Si alloy sheets.After solution treatment,all the alloy sheets possess the same recrystallization texture components comprising of cube_ND and P orientations,whereas a rather strong cubeoriented texture is observed only in alloy A2 annealed at a temperature of 380℃.However,in comparison with Alloy A1 not annealed and Alloy A3 annealed at 550℃,Alloy A2 possesses a lower average r value.Furthermore,the relationship between the textures and r values was further analyzed by using the Taylor full constraints model,and the results indicate that the recrystallization textures have a relatively good correspondence with r values.