简介概要

Mechanical properties,microstructure and surface quality of Al-1.2Mg-0.6Si-0.2Cu alloy after solution heat treatment

来源期刊：Rare Metals2017年第7期

论文作者：Li-Zhen Yan Yong-An Zhang Bai-Qing Xiong Xi-Wu Li Zhi-Hui Li Hong-Wei Liu Shu-Hui Huang Gang Zhao

文章页码：550 - 555

摘要：The effect of solution heat treatment（SHT） on mechanical properties,microstructure and surface quality of Al-1.2Mg-0.6Si-0.2Cu-0.6Zn alloy was investigated by tensile test,Erichsen test,surface topography,scanning electron microscope（SEM） and electron back-scattered diffraction（EBSD）.The results indicate that with the increase in SHT temperature,yield strength and cupping test value（I_E） of the sheets increase greatly and reach a peak value,then decrease.Meanwhile,intermetallic compounds dissolve into matrix gradually.The grains grow up as SHT temperature increases,and abnormal grain growth leads to the surface defects after solution-treated above 560 ℃.Considering mechanical properties,I_E value,residual phases,grain size and surface quality of the sheets,SHT temperature for the alloy should not be higher than 550 ℃.

稀有金属(英文版) 2017,36(07),550-555

Mechanical properties,microstructure and surface quality of Al-1.2Mg-0.6Si-0.2Cu alloy after solution heat treatment

Li-Zhen Yan Yong-An Zhang Bai-Qing Xiong Xi-Wu Li Zhi-Hui Li Hong-Wei Liu Shu-Hui Huang Gang Zhao

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

Key Laboratory for Anisotropy and Texture of Materials(Ministry of Education),Northeastern University

收稿日期：11 September 2014

基金：supported by the National Program on Key Basic Research Project of China(No. 2012CB619504);the National Natural Science Foundation of China(No.51271037);

Mechanical properties,microstructure and surface quality of Al-1.2Mg-0.6Si-0.2Cu alloy after solution heat treatment

Li-Zhen Yan Yong-An Zhang Bai-Qing Xiong Xi-Wu Li Zhi-Hui Li Hong-Wei Liu Shu-Hui Huang Gang Zhao

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

Key Laboratory for Anisotropy and Texture of Materials(Ministry of Education),Northeastern University

Abstract：

The effect of solution heat treatment(SHT) on mechanical properties,microstructure and surface quality of Al-1.2Mg-0.6Si-0.2Cu-0.6Zn alloy was investigated by tensile test,Erichsen test,surface topography,scanning electron microscope(SEM) and electron back-scattered diffraction(EBSD).The results indicate that with the increase in SHT temperature,yield strength and cupping test value(I_E) of the sheets increase greatly and reach a peak value,then decrease.Meanwhile,intermetallic compounds dissolve into matrix gradually.The grains grow up as SHT temperature increases,and abnormal grain growth leads to the surface defects after solution-treated above 560 ℃.Considering mechanical properties,I_E value,residual phases,grain size and surface quality of the sheets,SHT temperature for the alloy should not be higher than 550 ℃.

Keyword：

Al-Mg-Si alloy; Solution heat treatment; Surface defects; Mechanical properties; Microstructure;

Author： Yong-An Zhang e-mail:zhangyongan@grinm.com;

Received： 11 September 2014

1 Introduction

Aluminum alloys have been considered as an alternative material to replace steels in automotive industry,owing to increasingly strict standards with respect to fuel efficiency,air pollution,recycling and safety [ 1, 2] .Generally speaking,automotive skin panels account for 30%total weight of a car [ 3, 4, 5] .The effect of weight reduction is ideal with substituting aluminum alloy skins for steel panels.Among all aluminum alloys,6000 series aluminum alloys are widely used for body panels owing to their medium strength,good corrosion resistance,excellent formability,weldability and easily recycling,besides age hardening Al-Mg-Si alloys which require complex-shaped designs with high strength.

The strength of 6000 series aluminum alloys depends on alloy composition,solid solubility and aging hardening treatment.As for automotive body panel,the bake hardening process is usually carried out at 170-180℃for 20-30 min.Consequently,solid solubility is an important issue for an Al-Mg-Si alloy.Solid solubility is proportional to solution heat treatment (SHT) temperature,and high solid solubility promotes the improvement of strength.The increase in SHT temperature also favors grain growth.If SHT temperature is too high,the probability of abnormal grain growth is increased,which leads to the different values of grain size and nonuniform microstructure.The inhomogeneous microstructure influences the mechanical properties and surface quality of the sheet.Surface quality is crucial from the perspective of aesthetics for automotive skin panels,because surface defects cannot be removed during paint baking process.Surface defects,including orange peeling,surface roughening,roping and ridging,easily appeared during sheet stamping process [ 6, 7, 8, 9, 10] .Investigations focused their attention on the effect of SHT on mechanical properties and microstructure of6082 alloys through experimentation and modeling [ 11] and high Si content Al-Si-Mg alloys [ 12, 13, 14] ,while some investigations focused on the external electric field applied during solution heat treatment of AA6022 [ 15, 16] and AA6111 [ 17] alloys.As usual,excessive amounts of Si are usually added to improve the precipitation hardening during the short paint baking process,and to sufficiently raise the dent resistance of the parts.Excessive amounts of Si detrimentally influence the formability [ 18] .Therefore,the present work investigated the effects of SHT temperature on mechanical properties,microstructure and surface quality of an Al-1.2Mg-0.6Si-0.2Cu-0.6Zn alloy with excessive amounts of Mg.

2 Experimental

Chemical composition of the alloy was Al-1.2Mg-0.6Si-0.2Cu-0.6Zn-0.1Mn-0.12Zr (wt%).The raw materials were melted by induction heating and then cast into iron molds to produce billets.The billets were homogenized at 430℃for 10 h,and subsequently at 550℃for 18 h,followed by air cooling to room temperature.The billets were hot rolled to a thickness of 6 mm and then cold rolled to the final gauge of 1 mm.These sheets were solution-treated for30 min at elevated temperatures (500,520,530,540,550,560 and 580℃) and brought to room temperature by a water quench.The sheets were held for 30 min at room temperature before pre-aging and after SHT.Finally,these sheets were pre-aged at 130℃for 10 min and then aged at170℃for 30 min.

Room temperature tensile tests were conducted with an Instron tensile testing machine after the sheet was pre-aged and aged.Tensile tests after pre-aging simulated stamping process of body panels,which were also used to measure the surface quality of sheets.Mechanical tests after aging were used to determine the dissolution of phases during SHT process.Gage length of the tensile test specimens was 60 mm.The tests were performed on the transverse direction of the sheet.Microstructure of the sheets was characterized using a field emission gun (FEG) scanning electron microscopy(SEM,JEOL JSM 7001) with accelerating voltage of20 kV.Surface profile and surface roughness were measured by three-dimensional confocal microscope phase shift (MicroXAM-3D) with a scanning area of3460μm×2573μm.Each sample was imaged with magnification of 2.5 times.Erichsen tests were carried out on Zwick/roell BUP 600 sheet-forming machine,in order to evaluate the formability of sheets.

3 Results and discussion

3.1 Mechanical properties of sheets after SHT

SHT time of 30 min was determined for the sheet in present work,which usually depended on used SHT time for Al-Mg-Si automotive skin panels [ 19, 20, 21, 22] .Figure 1 shows mechanical properties of the sheets as a function of SHT temperature for 30 min.In this case,yield strength increases rapidly,reaches a peak value at 550℃,and then decreases after SHT above 560℃.Cupping test value (I_E)exhibits the same tendency with strength,which is 8.27,8.45,8.80 and 7.75 mm for the sheet solution-treated at500,530,550 and 560℃,respectively.Elongation is more than 20%at temperature of 500-550℃,whereas it decreases to<20%for other samples.The sheets exhibit different strengths and elongations after SHT,which may favor different surface roughness and micros true tures.

3.2 Surface topography of sheets after SHT

As automotive body panels,elongation of the sheets is required for more than 25%to stamp after pre-aged.In order to simulate sheet metal-forming process and analyze the influence of grain size on surface quality,the sheets were stretched 25%along transverse direction in present work.Typical topographies and height profiles of specimens after solution-treated are presented in Fig.2.It can be apparently concluded that the sheets with normal surface quality are observed after solution-treated at the temperature of 500-550℃.The average surface roughness (R_a) is1.5μm for the sheet solution-treated at 500-550℃,as obtained in Fig.2a-d.However,the sheet presents the highest surface roughness after solution-treated above560℃,as shown in Fig.2e,f,and the average surface roughness (R_a) is 5.2μm.Thus,surface defects occur after the sheets were solution-treated above 550℃.As surface quality of the sheets depends on texture of grains and micro structure,it is necessary to analyze the micros tructure of the samples.

Fig.1 Mechanical properties of Al-1.2Mg-0.6Si-0.2Cu-0.6Zn sheets after SHT process

Fig.2 Surface topographies (a,c,e) and height profiles (b,d,f) of Al-1.2Mg-0.6Si-0.2Cu-0.6Zn sheets after solution-treated at a,b 500℃,c,d 550℃,and e,f 560℃

3.3 Micros tructure of sheets after SHT

Representative SEM images of sheets solution-treated for30 min at 500,530,550 and 560℃are presented in Fig.3.The typical SEM image shows numerous irregular-and rodlike precipitates in the matrix (Fig.3a),indicating that the precipitates do not dissolve at 500℃.Therefore,low solid solubility of alloying elements,such as Mg and Si,leads to the reduction in driving force for precipitates.This favors the lowest strength for the sheet at 500℃,as observed in Fig.1.We noted that Mg2Si phases begin to dissolve into Al matrix at 530℃ [ 23] .Figure 3b shows SEM image of the sheets solution-treated at 530℃.It can be seen that irregular-like intermetallic compounds exhibit in the matrix.The results indicate that most phases dissolve into matrix.Solid solubility of A1 matrix increases,leading to a rapid increase in strength,as shown in Fig.1.The phases dissolve completely above 550℃(Fig.3c,d),and the sheet exhibits peak strength andI_E value at 550℃.As phases dissolve into matrix,the grain boundary pinning is reduced.Thus,grains tend to grow up with the increase in SHT temperature.

Fig.3 SEM images of Al-1.2Mg-0.6Si-0.2Cu-0.6Zn sheets after solution-treated at a 500℃,b 530℃,c 550℃and d 560℃for30 min

Typical grain orientations of the sheets after solution treated are presented in Fig.4.Obviously,grain size increases as SHT temperature increases.Figure 4a,c shows more homogeneous grain size distribution and smaller average grain diameter in comparison with Fig.4e.Grains are almost perfectly equiaxed with about 20μm in diameter at 530℃(Fig.4a).Figure 4c contains fine,equiaxed grains with a uniform distribution and about 30μm in diameter.The sheets present the same fraction of high angle boundary after solution-treated at 530 and 550℃,which indicates that the sheets are recrystallized perfectly after solution-treated.Min and max values in Fig.4 represent the variation range of the grain boundary angle.Meanwhile,the main grain orientation is cube component at SHT temperature of higher than 530℃,as seen in Fig.4b,d.The fraction of cubic orientation is more than10%.During SHT process,recrystallization is required for an Al-Mg-Si alloy,which is helpful for the sheet to stamp during metal-forming process.Even though grains are perfectly equiaxed,abnormal grain growth is observed in Fig.4e,f.The average grain size is bigger than 50μm at560℃.The microstructure is nonuniform,leading to the surface defects of the sheet at 560℃(Fig.2e).It might be caused by the unpinning effect of grain boundary,owing to the complete dissolution of the precipitates,such as Mg2Si intermetallic compounds.Meanwhile,coarse grains are easy to produce nonuniform deformation and texture,which would promote surface defects.Elongation,strength and I_E value of the sheets decrease owing to the heterogeneous microstructure.

3.4 Discussion

SHT is thermal process in which the material is held at the SHT temperature for a given time until the alloying elements are fully taken into the solid solution.The key points influencing SHT process are SHT time,temperature and quenching rate,while the most important issue for SHT process is temperature.The rate of dissolution is proportional to SHT temperature,and it is thus clearly beneficial to carry out SHT as close as possible to the solidus of the alloy.If SHT temperature is below the dissolution temperature of phases (Fig.3a),the driving force of precipitates will decrease and also the age hardening effect.But the maximum temperature must never be higher than the solidus because of the risk of material melting which enables unwanted microstructural features to occur,such as the coarsening of grain and abnormal grain growth.The abnormal microstructure leads to the decrease in material performances.

Fig.4 EBSD analysis of Al-1.2Mg-0.6Si-0.2Cu-0.6Zn sheets after solution-treated at a,b 530℃,c,d 550℃,and e,f 560℃

During SHT process,precipitates dissolve into matrix,so Al matrix is supersaturated,and the crystal defects are also supersaturated.That gives a more homogeneous microstructure throughout the whole material and hence a more formable material.Figure 5 shows typical micro structural evolution of the material during SHT process [ 11] .At the start of SHT,the precipitates with different sizes provide barriers to the flow of the material(Fig.5a),leading to the increase in strength,while low dissolution of the precipitates causes the strength to decrease.In this case,the low solid solubility is the main issue affecting the strength.Therefore,the sheet presents the lowest strength at 500℃(Fig.1).With the increase in temperature,these precipitates dissolve into material,diffusing throughout the whole structure (Fig.5b,c).The solid solubility of matrix increases significantly,and the driving force of precipitates is also reinforced.The age hardening effect is enhanced,as seen in Fig.1.But grains grow abnormally at the temperature of above 560℃,owing to the decrease in pinning to the grain boundary.At this moment,abnormal grain growth is the dominating factor affecting the materials properties.It is indicated that the ductility and strength decrease (Fig.1) and the sheets present surface defects after SHT above 560℃(Fig.2).

Throughout the process of SHT,microstructure of the material is modified.This results in dynamic changes of mechanical properties of the material.The SHT process is actually controlled by atomic diffusion which depends on SHT temperature.According to Porter and Easterling [ 24] ,it is known that the diffusion process can be described as:

where D is diffusion parameter depending on temperature,which allows for the effect of increasing diffusion rate on increasing temperature,D₀ is diffusion coefficient,Q is activation energy,T is SHT temperature and R is universal gas constant.For the Al-1.2 Mg-0.6Si-0.2Cu-0.6Zn alloy,D₀ and Q are constants.Hence,from Eq.(1) it can be concluded that with the increase in SHT temperature,the diffusion rate of alloying elements increases,which in turn promotes the solid solubility.Therefore,the mechanical properties of the sheets increase with SHT temperature ranging from 500 to 550℃.

On the other hand,during SHT process,the grain size tends to increase with the increase in SHT temperature.From Hall-Petch relationship,it is known that the grain size and strength of the material can be described as:

whereσ_s is yield stress (MPa),d is grain size (m),σ_i is frictional stress (MPa) and k_y is a constant.Obviously,the yield strength of the sheets decreases as grain size increases.If SHT temperature is too high,solid solubility of the matrix increases,and grain size is bigger,which lead to the decrease in yield strength and ductility,as observed in Fig.1.Moreover,abnormal grain growth occurs at the temperature of above 560℃(Fig.4).The precipitates dissolve completely,and grain boundary pinning is reduced.At high temperature,the grains tend to easily coarsen and grow up.The nonuniform micro structure causes heterogeneous deformation during stamping of the sheet,resulting in surface defects,as seen in Fig.2e.

The yield strength,formability,grain size and surface quality are summarized in Table 1.Clearly,grain size(d) tends to increase as SHT temperature increases,whereas yield strength (YS) and I_E value firstly increase to a peak value,and then decrease.Surface roughness also increases and the sheets exhibit surface defect owing to abnormal grain growth at above 560℃.

下载原图

Table 1 Yield strength,grain size,formability and surface quality of Al-1.2Mg-0.6Si-0.2Cu-0.6Zn alloy after SHT process

Fig.5 Schematic diagram showing microstructural evolution:a at beginning of SHT,b intermediate SHT,and c close to finishing SHT

4 Conclusion

Al-1.2 Mg-0.6Si-0.2Cu-0.6Zn alloy was investigated through mechanical tests,Erichsen test,SEM and EBSD technique.The strength andI_E value of the sheets increase with SHT temperature ranging from 500 to 550℃,whereas strength,I_E value and ductility of the sheets decrease above 560℃.The average grain size tends to increase with the increase in SHT temperature,and grains grow abnormally above 560℃,leading to surface defects of sheets.Thus,SHT temperature for the alloy should not be higher than 550℃.

Acknowledgments This study was financially supported by the National Program on Key Basic Research Project of China (No.2012CB619504) and the National Natural Science Foundation of China (No.51271037).The authors wish to thank Dr.Xiao-Lei Han for EBSD work.