简介概要

Static softening behaviors of 7055 alloy during the interval time of multi-pass hot compression

来源期刊：Rare Metals2013年第3期

论文作者：Liang-Ming Yan Jian Shen Jun-Peng Li Bai-Ping Mao

文章页码：241 - 246

摘要：Multipass plain strain compression test of 7055 alloy was carried out on Gleeble 1500D thermomechanical simulator to study the effect of interval time on static softening behavior between two passes. Microstructural features of the alloy deformed with delay times varying from 0 to 180 s after achieving a reduction of ～52 % in the 13 stages was investigated through TEM and EBSD observations. The 14th pass of peak stresses after different delay times were gained. The peak stress decreases with the interstage delay time increasing, but the decreasing trend is gradually slower. Static recovery, metadynamic recrystallization, and/or static recrystallization can be found in the alloy during two passes. The recovery and recrystallization degree increases with longer interstage delay time. The static recovery is the main softening mechanism. Subgrain coalescence and subgrain growth together with particle-stimulated nucleation are the main nucleation mechanisms for static recrystallization.

Rare Metals 2013,32(03),241-246+6

Static softening behaviors of 7055 alloy during the interval time of multi-pass hot compression

Liang-Ming Yan Jian Shen Jun-Peng Li Bai-Ping Mao

School of Materials Science and Engineering, Innermongolia University of Technology

Department of Processing Business, General Research Institute for Nonferrous Metals

Science and Technology Department, Aluminum Corporation of China Limited

作者简介：Liang-Ming Yan e-mail:yanliangming@126.com;

收稿日期：19 June 2012

基金：financially supported by the Natural Science Foundation of Inner Mongolia (No. 2011bs0802);Research Fund for the Higher Education of Inner Mongolia (No. NJZY11075);

Static softening behaviors of 7055 alloy during the interval time of multi-pass hot compression

Abstract：

Multipass plain strain compression test of 7055 alloy was carried out on Gleeble 1500D thermomechanical simulator to study the effect of interval time on static softening behavior between two passes. Microstructural features of the alloy deformed with delay times varying from 0 to 180 s after achieving a reduction of ～52 % in the 13 stages was investigated through TEM and EBSD observations. The 14th pass of peak stresses after different delay times were gained. The peak stress decreases with the interstage delay time increasing, but the decreasing trend is gradually slower. Static recovery, metadynamic recrystallization, and/or static recrystallization can be found in the alloy during two passes. The recovery and recrystallization degree increases with longer interstage delay time. The static recovery is the main softening mechanism. Subgrain coalescence and subgrain growth together with particle-stimulated nucleation are the main nucleation mechanisms for static recrystallization.

Keyword：

7055 aluminum alloy; Hot compression; Microstructure; Interval time;

Received： 19 June 2012

1 Introduction

In warm and hot deformations of metals,the hardening and softening phenomena take place.The softening process is caused by recovery,recrystallization[1,2],and forced or spontaneous changes of strain paths[3,4].The softening during the deformation is defined as a dynamic process taking place during interval in deformation or when deformation is finished as a static or metadynamic process.Multipass processing is very common in hot deformation,in which static softening between passes plays an important role,as it affects the forming load of each pass as well as the amount of microstructure evolution[4,5].The control of such operations is possible only when the knowledge of microstructural changes,and softening mechanisms taking place during the interstage duration is known and can be predicted.Interpass softening is classified mainly on the basis of two or three metallurgical phenomena:static recovery,metadynamic recrystallization,and/or static recrystallization(SRX)[1,2]Several researchers have studied the interpass softening behavior of various materials by softening fraction[5–8]They have also investigated the softening rule through changing various parameters,including strain,strain rate,and temperature.However,the results lack the microstructure explanation.Lin et al.[9]studied the influence of deformation temperature on microstructure and the peak stress of Al–Zn–Mg–Cu–Cr aluminum alloy during multipass compression The more the temperature decline between adjacent stages the larger the peak stress fall is.The microstructure evolution during deformation presents dynamic recovery,dynamic recrystallization,static recovery,and recrystallization in the interval time.Li et al.studied SRX in the 7050 aluminum alloy during slow cooling after hot compression[10].SRX takes place in the 7050 samples deformed to a reduction of 80%during slow cooling after hot compression,of which subgrain coalescence and PSN are the main nucleation mechanisms.However,in these investigations,effect of the interval time on static softening behavior has been taken into account.Despite large amount of efforts invested into static softening behavior on various materials,the static softening mechanism of 7055aluminum alloy still needs to be further investigated by microstructural observation.

The 7055 aluminum alloy,a typical Al–Zn–Mg–Cu series aluminum alloy,which belongs to precipitation strengthening alloy,is widely used due to its high strength and low density,and has become one research focus[11].In this study,multipass plain strain compression(PSC)test was carried out to modeling conventional industrial rolling process.The 14th pass of peak stresses were studied after13 passes compression and then holding temperature for different time.Microstructures were analyzed through the microstructure observation of the alloy compressed by 13passes,holding the same temperature for different times,and then quenched afterward.

2 Experimental

The 7055 aluminum alloy with the composition(wt%)of aluminum–7.87 zinc–2.16 magnesium–2.05 copper–0.12zirconium–0.06 iron–0.04 silicon was manufactured by semicontinuous casting.The homogenized ingot was machined to dimensions of 10 mm 9 20 mm 9 15 mm.Multipass PSC tests were performed on Gleeble 1500D thermomechanical simulator with parameters drafted according to the industry rolling process of 7000 serials aluminum alloys.Graphite pieces were used as lubricant to reduce friction between the anvils and specimen during the hot-compression tests.The process is shown in Table 1,and the 14 passes of compression process are the same as that in Table 1[12].As can be seen in Table 1,the majority of tests were conducted at different interstage delay time to study the effect of holding time on softening.The interrupted deformations were conducted with delay times varying from 0 to 240 s after achieving reduction of*52%.The specimens were heated to the desired temperature with the temperature remaining constant during hot compression and adjusting to the temperature of next pass during each interval.The specimens were deformed through 13 passes compression to a maximum reduction of52%,and then water quenched after holding at 0,90,and180 s,respectively,for microstructure observation.In order to research the influence of delay times on next pass peak stress,30,90,180,and 240 s were chosen as the interval between the 13th and the 14th pass.Microstructures of longitudinal section of the deformed specimens were observed by transmission electron microscopy(TEM)and electron backscattering diffraction(EBSD).The foil TEM samples were prepared by cutting the longitudinal section of the deformed specimen.The disks were ground to a thickness of about 50 lm and then were subjected to twin-jet electropolishing.TEM observations were made on a JEM-2000FX transmission electron microscope operated at 180 k V.Orientation data in the form of EBSD maps were obtained using an HKL technology channel5 EBSD system interfaced to a Shimadzu SSX-550 SEM.

3 Results and discussion

3.1 Influence of passes interval on peak stress

Peak stresses of the 14th pass compression after different interstage delay time from 30 to 240 s are shown schematically in Fig.1.The peak stress in the 14th pass varies depending on the interstage delay time and various metallurgical factors that affect the static recovery and recrystallization behavior of the material.It is found that the peak stress upon reloading(in the 14th stage)decreases as the delay time is increased from 30 to 240 s.The peak stress decreases rapidly as the delay time increases from 30to 90 s,but the decreasing trend is gradually slower with delay time increasing.

In the first stage of the holding time,due to the high deformation store energy,static recovery,metadynamic recrystallization,and/or SRX that immediately happened,the softening rate is large.Due to high alloy solute in 7055aluminum alloy,the second phase formation enhances with holding time increasing,which stabilizes the microstructure,so that the softening rate after 90 s delay time is lower than that on the first stage of the holding time.

Table 1 Process employed for multipass compression tests 下载原图

Table 1 Process employed for multipass compression tests

Fig.1 Peak stress of the 14th pass after different inter-stage delay time during multiple hot compression

3.2 Influence of passes interval on microstructure

During single-pass hot deformation,7055 aluminum alloy undergoes restoration by the mechanisms of dynamic recovery and dynamic recrystallization[13,14].Between passes in high-temperature multipass deformation,three distinct softening mechanisms,namely,static recovery metadynamic recrystallization,and SRX,may also occur[6,10],so different interstage delay time has different influence on the next stage of peak stress in Fig.1.Since there is relation between microstructure and flow stress,it is very necessary to obtain microstructure after different intervals.EBSD maps of the alloys with interstage delay time of 90 and 180 s after 13 passes compression is shown in Fig.2.In Fig.2a,b,grains and subgrains with different orientations are shown with dissimilar colors For these orientation imaging microscopy(OIM)maps in Fig.2a,b,a step size of 0.7 lm was used to cover a relatively large area of the samples and,meanwhile obtain the information of the low angle grain boundaries(\5°misorientation)in detail.In the EBSD maps HAGBs([15°misorientation)are depicted as coarse black lines,and low angle boundaries(10°–15°misorientation)have been depicted as fine black,pink(8°–10°misorientation),red(3°–5°misorientation),and blue lines(2°misorientation).The map display the grains elongation along the deformation direction,and there present many new HAGBs([15°misorientation)and LAGBs(10°–15°misorientation)along the original grain boundaries.It is proved that the recrystallization occurs,and the size of recrystallized grains reaches 10 lm.In comparison,the alloy with interstage delay time of 180 s has larger recrystallized grains size than that of 90 s.

Fig.2 EBSD analysis of 7055 alloy after 13 passes compression and intermittent time of 90 and 180 s:a EBSD map of 90 s,b EBSD map of180 s,c OIM image of 90 s,and d EBSD map of 180 s

Fig.3 Examples of boundary misorientation distributions for 7055 alloy compressed 13 passes and an interstage delay time of a 90 s and b 180 s

Fig.4 TEM images of 7055 alloy samples deformed after 13 passes compression with different cooling conditions:a water quenched and b intermittent time of 180 s and then water quenched

Examples of typical boundary misorientation distributions for the samples with different delay time are shown in Fig.3.The histograms show the relative frequency of boundary misorientations in 2°.However,due to orientation noise in the EBSD measurements leading to an overrepresentation of very low angle boundaries,the misorientations\2°have been omitted.An important feature of these results is that the fraction of high angle boundaries is much larger in the sample with an interstage delay time of180 s than that of 90 s.The low angle boundaries are still dominated after interstage delay time of 180 s,with only a few boundaries exceeding 15°in misorientation.It suggests that recovery is the dominated softening mechanism during the interstage delay.

Transmission electron microscopy observations were taken to further investigate the softening behaviors of the7055 alloy during the interstage delay.Figure 4 shows the TEM images of the 7055 aluminum alloys compressed by13 passes and then held for different time.Figure 4a shows the substructure of the 7055 alloy undergoing 13 passes compression,which was water quenched to squeeze as hotdeformed microstructure.Figure 4b shows the microstructure of the alloy held for 180 s and then quenched after hot compression.It can be seen that the microstructures are gradually changed with the increase of the interstage delay time.A mass of subgrains in the elongated original grains and high-density dislocation wall can be observed in Fig.4a.It proves that a part of grains or/and subgrains are deformed during the 13th pass compression.The reduction of the 13th pass is 10%,and the total reduction reaches52%.However,the sample tested in Fig.4a has higher dislocation density than that of the sample deformed at350°C[13].In Fig.4b,recrystallized grains or subgrains with a mean size of 4 lm,in which dislocation density is decreased due to static softening between passes,are developing.They possess larger misorientation rather than the adjacent small subgrains,which suggests the trace of subgrain coalescence during static softening.

Figure 4 also shows the transformation of the precipitated phases.It can be seen that there are more precipitated phases in the sample with an interstage delay time of 180 s than that of the sample compressed and water quenched.There is enough high deformation stored energy for the second phase to precipitate in the sample compressed by 13passes,but much time is necessary for solute atom diffusion.The 100–200 nm ball-shape precipitated phases can be clearly seen.The precipitation will be strengthened by the strain[9].These precipitations at elevated temperature are mainly the E-phase particles and can hinder the growth of the recrystallization grains and stabilize the structures[15].Recovery and recrystallization consume the deformation energy of the deformed sample,which decreases the peak stress.Due to the precipitated phases,however,the peak stress reduction is slight,with interstage delay time increasing,as shown in Fig.1.

3.3 Static softening mechanism during passes interval

In the alloys with a high stacking fault energy and high solute content,such as aluminum alloys,the microstructures after deformation at ambient or elevated temperatures can usually be described as a cellular or subgrain structure[16].The substructures were rearranged constantly by the migration of the low angle grain boundaries,merging into neighboring subgrains,annihilating,and decomposing.In regions with large orientation gradient,it is easy for subgrains to grow,and they may also lead to the initiation of recrystallization[13].These structures are inherently unstable.They might change in the interval between passes by static recovery and/or recrystallization,and some grain coarsening may take place when recrystallization is completed.Microstructure evolution of the 7055 alloy undergoes a complex process during the passes interval,which makes the deformation resistance reduce in the following deformation pass in Fig.1.

The microstructure contrast between Figs.2 and 4clarifies that the growth of some new dynamically recrystallized grains is not sufficient enough in the hot deformed7055 alloy.Furthermore,the residual dislocation developed by deformation at high temperatures provides the driving force for SRX.Therefore,SRX may take place during passes interval.The growing subgrains may continue to grow up into adjacent subgrains by subgrain growth and coalescence due to the energy gradient during passes interval[2].Therefore,the size of the statically recrystallized grains is bigger with inter-stage delay time increasing,and the rate of recrystallization increases,as is shown in Figs.2 and 3.The fact that traces of subgrain growth and coalescence can be observed in Fig.4b indicates that subgrain growth and coalescence are important for the nucleation of recrystallization.A green point at the triple grain boundaries,which is about 2–10 lm in size can be observed in Fig.2a,b.This is the coarse second phase that formed during casting[17].Recrystallization structures can be detected near these particles.It indicates that particle-stimulated nucleation(PSN)of recrystallization possibly occurs in the 7055 alloy during interval of passes The coarse second phase and the addition strain in the surrounding matrix produce a highly dense substructure suitable for nucleation.

4 Conclusion

Multipass plain strain hot-compression(PSC)test of 7055alloy was carried out to investigate the effect of interval time on static softening behavior between two passes.The next pass of peak stress decreases with the interstage delay time increasing,but the decreasing trend is gradually slower.Static recovery,metadynamic recrystallization and/or SRX can be found in the alloy during two passes and the recovery and recrystallization rate increases with more interstage delay time.However,static recovery is the main softening mechanism.Subgrain coalescence and subgrain growth together with PSN may be the main nucleation mechanisms for static recrystallization between two passes.

Acknowledgments This study was financially supported by the Natural Science Foundation of Inner Mongolia(No.2011bs0802)and Research Fund for the Higher Education of Inner Mongolia(No.NJZY11075).