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Microstructure investigations on 2A10 aluminum alloy bars subjected to electromagnetic impact upsetting
Author’s Accepted Manuscript Microstructure investigations on 2A10 aluminum alloy bars subjected to electromagnetic impact upsetting Xu Zhang, Junjia Cui, Junrui Xu, Guangyao Li www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(17)30806-7 http://dx.doi.org/10.1016/j.msea.2017.06.039 MSA35177
To appear in: Materials Science & Engineering A Received date: 26 March 2017 Revised date: 8 June 2017 Accepted date: 10 June 2017 Cite this article as: Xu Zhang, Junjia Cui, Junrui Xu and Guangyao Li, Microstructure investigations on 2A10 aluminum alloy bars subjected to electromagnetic impact upsetting, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.06.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure investigations on 2A10 aluminum alloy bars subjected to electromagnetic impact upsetting
Xu Zhang1,2, Junjia Cui2,3, Junrui Xu4, Guangyao Li2,3*
1
College of Mechanical and Electrical Engineering, Hunan University of Science and Technology, Xiangtan, 411100, China
2
State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, 410082, China 3
Joint Center for Intelligent New Energy Vehicle, Shanghai, 200092, China
4
School of Mechanical Engineering, Xiangtan University, Xiangtan 411105, China
*
Corresponding author at: State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, 410082, China. Tel.: +86 731 88664001; fax: +86 731 88822051. [email protected]
1
Abstract In this work, electromagnetic impact upsetting (EIU) process was proposed. And EIU and quasi static upsetting (QSU) experiments were performed with Φ6mm-2A10 aluminum alloy bars. Microstructure evolutions for both processes were revealed by metallographic observations under the varying engineering strains. Results showed that the most deformations for the EIU process significantly concentrated in narrower adiabatic shear bands (ASBs) comparing with the QSU process. The ASBs initialized in the diagonal points of samples and gradually extended to the central with the continuous deformations. And four ASBs encountered and integrated in the central. The high strain rate contributed to deformation concentration and the forming of the adiabatic shear bands. In addition, hardness values for the EIU process were higher than that of the QSU process, demonstrating that the strengthening effect of the strain rate was notable. TEM observations presented that the ASBs with the highest hardness contained many lamellar sub-grains and second phase particles (Al2Cu). The high strain rate caused that rotated sub-grains converted into dynamic recrystallization grains within ASBs. Plentiful dislocation tangles and strengthening phases accounted for highest hardness values within the ASBs.
Keywords: electromagnetic impact upsetting; 2A10 aluminum alloy; high strain rate; microstructures
2
1. Introduction The effective and green manufacturing technology was an urgent need in industrial fields. The proposed electromagnetic impact upsetting (EIU) is a dynamic plastic deformation (DPD) process, in which specimens are subjected to the high speed loading. Metals are formed under the high strain rate. This process is parallel to DPD tests, such as Split Hopkinson Pressure Bar test [1], drop hammer impact [2] and the electromagnetic riveting [3-5]. Mechanical properties of deformed alloys significantly depend on microstructural characteristics such as deformation texture, grain size, and microstructural morphology. However, the high strain rate (generally higher than 102 s-1) has an important effect on deformations, which makes the analysis of the EIU process become more complex than that of quasi-static compressions. Moreover, deformations with the high strain rate can be considered as an adiabatic process, in which adiabatic shear bands (ASBs) are the most important deformation behaviors. ASB is a narrowband region where shear deformation highly concentrated, and microstructure evolution within it is a determinant factor for material properties and forming quality. At present, the deformation mechanism under high strain rate has been investigated by many researchers. As reported, the two main phenomena (grain refinement and dynamic recrystallization) were found in ASBs. Severe plastic deformation (SPD) is an effective route to refining coarse grain size into micrometer or nanometer. Many studies [6-8] demonstrated that grains size in ASBs was nanometer scale and the ultrafine grains resulted from dynamic recrystallization mechanism. Andrade et al. [9] and Meyers et al. [10] observed the fine equiaxed grains in the ASBs of copper under the high speed impact deformation, indicating the possibility of dynamic recrystallization. Researches on titanium alloys with the cold DPD showed that the recrystallized structures existed in ASBs [11-15]. Additionally many similar results had been also reported in the ASBs of other materials including tantalum [16-17], steel [18]. Dynamic recrystallization during hot forming is usually 3
considered to be a nucleation and growth process of recrystallized grains. Time and temperature play a vital role in the recrystallization process. However, there are three characteristics different from conventional forming methods during the DPD process: approximate adiabatic process (almost all plastic works are converted to heats), very short forming time (quick cooling rate) and deformation localizations. What is the mechanism of dynamic recrystallization with high strain rate has aroused extensive concern among many researchers. Hines [19] proposed a model that recrystallized grains were developed from rotated sub-grains, and this rotated mechanism was to coordinate the high strain rate deformations. Studies by Owolabi et al. [20] confirmed this model. In addition, some results [21-22] presented that the rotation mechanism led to the significant misorientations among sub-grains and made them convert into recrystallized grains. Moreover, numerous studies about electromagnetic riveting indicated that adiabatic shear bands were important characteristics in upsetting process of rivets. It was observed by Yu et al. [23] that grains of TA1 titanium alloy rivet were remarkably refined in ASBs. But the change of other grains size was negligible. Cao et al. [24] reported the width of ASBs existed in rivet heading was generally 10 ~ 102 microns. Choo et al. [25] found ASBs were root of microcrack generation in 7075-T73 aluminum alloy rivet after EMR. Deng et al. [26] stated dynamic recrystallized grains were found in ASBs of titanium alloy rivet. Above all, microstructures in ASBs were the key factor to determine quality and fatigue life of workpieces after high speed impact forming. However, the detailed investigations on microstructure evolution in ASBs with the electromagnetic impact upsetting have not been reported. Moreover, the comparing investigation on microstructures and mechanical properties between electromagnetic impact upsetting process and quasi static upsetting process had been little investigated. In this work, Electromagnetic impact upsetting (EIU) and quasi static upsetting (QSU) tests for 2A10 aluminum alloy were performed at ambient temperature. Microstructure and mechanical property difference between the EIU process and QSU process was revealed by the comparing method. The microstructure evolutions were characterized by optical microscopy (OM), and 4
transmission electron microscope (TEM). The dislocation density was obtained by XRD (X-ray diffraction). Micro-hardness values were measured to investigate mechanical property distribution. 2. Experimental material and procedure The specimens used in this work were annealed 2A10 aluminum alloy bars with the height of 9 mm and diameter of 6 mm. The compositions of the specimens were listed in table 1. The materials were similar to 2024 aluminum alloy and had relatively high strength and good plasticity. Consequently, 2A10 aluminum alloy were commonly used as rivet materials. The original microstructure of as-received specimens was shown in Fig. 1. It could be seen that grains size was uniform and around 50 micrometers. EIU experiments were conducted using a 48 kJ electromagnetic forming machine (EFM) at the Hunan University. The schematic of the machine and experiment setup is demonstrated in Fig. 2. The discharge principal of electromagnetic forming can be described as a typical RLC circuit (the system resistance R, the capacitance C and the inductance L). The capacitors are charged by adjusting charging parameters and stored electrical energy. The energy is discharged by closing the switch and produces high amplitude changing currents in coil, around which a rapidly changing electromagnetic field is generated. Meanwhile, eddy currents are induced in diver plate (copper sheet with high conductivity) which is exposed to alternating magnetic fields. And then these circulating eddy currents induce magnetic fields. The two fields (coil magnetic fields and eddy magnetic fields) generate a repulsive force and push the punch to impact onto specimen. The deformation of samples for the EIU process was controlled by discharge energies. In general, repeated EIU experiments should be performed to determine the relationship between the specimen height and discharge energies. All EIU experiments were carried out by discharge energy 3 kJ and the deformation height was controlled using rigid gaskets. This could ensure that all experiments had initial loading conditions. Consequently, it was approximately considered that the deformation results with different heights were discretization results of the whole sequential EIU process. Microstructures on specimens with different heights were observed to reveal microstructure 5
evolution during the EIU process. In order to compare two upsetting methods, same deformations along the height of specimens were achieved. Firstly, four EIU specimens with different heights were obtained. The height after the EIU process was measured to determine compression displacements of the QSU process. The QSU experiments were carried out by using a universal Instron 8801 electronic testing machine with a compressing velocity of 2 mm/min and the predetermined compression displacement. The deformation process and subsequent processing of specimens were shown in Fig. 3. Specimens were split along its axis and mechanically polished. The chemical etchant used on the OM specimens was the Keller solution of 2.5 ml HNO3, 1.5 ml HCL, 1 ml HF and 95 ml H2O. The metallographic microstructures of tested specimens were examined by a Leica microscope. TEM (Transmission Electron Microscope) specimens were taken in the ASBs to reveal microstructure evolution. The selected specimens were mechanically polished to the thickness of 80 μm, and were thinned by a twin-jet electro-polishing device with an etching solution of 30% nitric acid and 70% ethyl
alcohol.
All
TEM
observations
were
performed
using
a
TecnaiF2F30
transmission electron microscope. The XRD (X-ray diffraction) tests were conducted with Rigaku D/MAX 2500 X to measure dislocation density. 3. Results 3.1 Deformation results The engineering strains along the axis of samples were calculated by the reduction of height, namely ε = (H0 - H)/H0, where H0 and H were the initial and final height of specimens, respectively. Fig. 4 showed deformation results for the electromagnetic impact upsetting (EIU) and quasi static upsetting (QSU). For the EIU process, shear deformation bands initiated at the four diagonal points of sample (ε =32.4%). And the shear bands gradually extended to the center of the deformed sample with the increase of axial engineering strain. Four shear bands intersected in the center under the engineering strain 46.4%. All shear bands increased as a whole and evolved to up and down two parabola-shapes for the higher strain. Whether the lower strain or the higher strain for the 6
QSU process, the shear deformation bands distributed along the two diagonals. And the width of shear bands was relatively larger than that of the EIU. The intersecting zone (namely the center of samples) of two diagonal bands had the larger deformation. The impact velocity in the electromagnetic upsetting was tested using a high speed video camera. The deformation process of upset specimens and impact velocity were shown in Fig. 5. The whole upsetting process endured about 1 millisecond and the peak impact velocity was 4.75 m/s. It was noted that the loading velocity of the EIU process was significantly higher than that of the QSU process, and the EIU process could be classified as a dynamic plastic deformation. Table 2 showed the contrastive results between the EIU and QSU with the same deformation. The discharge energy 3 kJ was employed for the EIU, giving rise to 57.3%. Consequently, average strain rate was estimated by dividing the engineering strain and loading time. The average strain rate approximately reached to the 103 s-1. 3.2 Metallographic structure for the EIU process and QSU process Metallographic microstructures for the EIU process and QSU process were shown in Fig. 6 and Fig. 7, in which the full section shape of specimens were obtained through assembling numerous metallographic pictures. In order to observe clearly the microstructure evolution in different zones after forming, partial enlargements of metallographic figures were obtained. It could be seen from the two process methods that shear bands initiated and spread along diagonal direction of specimens, and the shear bands (upper and lower) encountered in the center. The width of shear bands for EIU was remarkably less than that of QSU, which embodied distinctive characteristic (adiabatic shear bands) under the high strain rate. The deformation microstructure of specimens could be divided into four zones (small deformation zone (SDZ), free deformation zone (FDZ), central deformation zone (CDZ) and shear band zone (SBZ)), as presented in Fig. 8. For microstructures in the SDZ, both upsetting processes had small deformation and grains still kept original equiaxial morphology. The plastic flow of metals in this zone was limited by the friction effect between the end surfaces and punch (or base plate), causing that most metals was almost in the static state. So only some 7
grains close to shear bands were distorted and deformed, and the deformation degree was more severe closer to shear bands. For microstructures in the FDZ, the metal in this zone could freely flow along the radial direction of samples and was in a state of two-direction compressive stress and one-direction tensile stress. Grains were remarkably elongated along the radial direction and broken down. The grain deformations for EIU process was larger comparing with that of the QSU process. For the microstructures in the CDZ and SBZ, the characteristic of shear deformation was very significant and the metal flow lines distributed along shear bands. The shear deformation unavoidably occurred due to transitional effect between the static zones (SDZ) and free deformation zones (FDZ). In addition, shear bands of samples under the EIU were narrower than that of the sample with QSU. The shear deformations within the shear band zone for the EIU were also more severe. The grains were distorted and elongated to the fiber-like microstructures (Fig. 9(b)), and the morphology of these grains was unable to identify. It was illustrated that the plastic deformation concentrated highly in the shear band zones, and other zones had smaller deformations comparing with shear zones. Grains (in the FDZ and SDZ) close to shear bands were driven to deform and flowed within the SDZ, causing that the boundaries of shear bands were not smooth. In addition, some second phase particles (the white particles) existed in adiabatic shear bands. The actual number of particles was more than that of observed particles. From the morphology of voids, these micro-voids were positions of the falling second phase particles during SEM sample preparations. Mechanical polishing and chemical etching caused that small amounts of particles fell off during metallographic specimen preparations. The maximum deformations also located in shear zones for the QSU process. However, some incomplete broken grains were mingled with deformed microstructures. The strain rate for the EIU process was up to 2000 s-1, causing the isothermal state was converted into adiabatic state during the upsetting process. Consequently the heats generated from plastic works were unable to transfer quickly to the ambient, and concentrated in the heat source area. The shear bands with larger deformations had more heats from plastic works, leading to significant temperature rise. At the same time, the work hardening effect was more 8
notable with the increase of the deformation degree. The severe plastic deformation could highly concentrate in some partial zones when the softening effect suppressed work hardening effect. And then the mutual interaction mechanism between plastic deformation and temperature rise accounted for the formation of narrow shear bands. So the shear bands resulted from deformation concentrations under the adiabatic state, and were also called as adiabatic shear bands (ASBs). It could be seen that the width of shear deformation bands under the QSU process was approximately 600μm. However, the width of adiabatic shear bands under the EIU process was only 150μm (Fig. 9). This illustrated that the high strain rate contributed to deformation concentrations and the forming of the adiabatic shear bands. 3.3 Micro-hardness Mechanical properties of upset specimens were significantly determined by microstructures after
the
upsetting
deformations.
Previous
microstructure
analysis
demonstrated
that
inhomogeneous plastic flow resulted in the non-uniform distribution of microstructures. And then this greatly affected the distribution law of mechanical properties. Five measured points in the three routes (as shown in Fig. 6 and Fig. 7) were selected to test micro-hardness values. Testing processes were performed using a HVS-1000 Digital Micro Vickers Hardness Tester with a steel ball of 10μm diameter, a load of 500 g and holding time of 30s. Fig. 10 showed the hardness results on the measured routes for electromagnetic impact upsetting and quasi static upsetting. The original hardness value of undeformed 2A10 aluminum alloy bars was 110 HV. It could be seen that the hardness values (comparing with the original value) increased due to work hardening effect. It is well known that the hardness values presented the work hardening degree (namely deformation degree). Hardness values on the Route 2 (direction along shear bands) for both upsetting process were significantly higher than that on the Route 1 (in the SDZ) and Route 3 (in the FDZ). The hardness values were the minimum for microstructures in the SDZ. However, the hardness in the SDZ was still slightly improved in spite of the smaller deformations. Hardness values for 9
microstructures in the FDZ were in between the SDZ and the SBZ. The hardness distribution law accorded with microstructure distribution mentioned above, demonstrating that deformation degree notably affected mechanical properties of deformed specimens. For the measured Route 1, the hardness gradually increased from the end surface of the specimen to the central, revealing that the deformation degree close to shear bands was higher due to the driving effect of shear deformation. The same changing trend was obtained on the measured Route 3. But the fluctuation range among five measured points for the QSU process was smaller than that of the EIU process. Metallographic structure analysis had demonstrated that the deformation in the FDZ was relatively smaller and more uniform during the QSU process. For the measured Route 2, the zones close to the central were the intersecting locations of upper and lower adiabatic shear bands for the EIU process. These zones for the QSU process were mixed microstructures between the shear bands and microstructures with larger deformations under the triaxial compressive stress. Consequently, the relatively higher deformation degree in these zones caused the higher hardness values of deformed microstructures. And the average hardness on the Route for the EIU process and the QSU process was improved by 66.4% and 50% (relative to the original hardness 110 HV), respectively. Moreover, hardness values in every measured route of the EIU process were higher than that of the QSU process, indicating that the hardening effect of strain rate was very notable during the electromagnetic impact upsetting. 4. Discussions 4.1 Sub-grain morphology in the adiabatic shear bands Previous analysis demonstrated that the deformations concentrated in adiabatic shear bands during electromagnetic impact upsetting. However, the general morphology of the ASBs was difficultly observed through metallographic structures due to smaller width of them. The microstructure evolution needed to be characterized by the transmission electron microscope (TEM). Fig. 11 showed that the lamellar substructures within adiabatic shear bands. This Figure was 10
composed of many TEM pictures to observe sub-grain morphology. It could be seen that many lamellar structures distributed along the direction of the adiabatic shear band, and were similar to the fiber-like deformation structures (as shown in Fig. 6). The average width of lamellar structures was about 0.8μm and much smaller than the average size of the original undeformed grains (50μm). Slip deformations along the same shear direction caused those lamellar sub-grains conformably distributed in ASB. The neighboring lamellar substructures had coincident diffraction patterns, illustrating that all lamellar structures were sub-grains generated from the same original grain. In addition, some more slender strip microstructures were mingled among lamellar substructures. The width of slender strips was very small and around 100 nm. The diffraction patterns showed the low angle boundary between strips and lamellar sub-grains, demonstrated that these strips belonged to sub-grains. No twins with the substructures could illustrate dislocation slips were the main deformation mechanism during the EIU process with high strain rate. During the plastic deformation, the external forces reached to critical shear stress which was the driving stress of the slip system activation. The metals would be dislocated along the direction of slip systems, and then was refined to sub-grains. The slip planes of crystals were also subjected to the normal force at the same time of withstanding the tangential forces. Consequently the normal forces could cause the rotation of the formed substructures, and the slip planes were bent. As shown in Fig. 11, the boundaries among lamellar substructures were not straight and slightly bent. 4.2 Dislocation patterns in the adiabatic shear bands There were some dislocation cells within adiabatic shear bands besides lamellar sub-grains. Fig. 12 presented the dislocation pattern in the ASBs. These dislocation cells distributed along shear bands and the size of them was not uniform. Among these dislocation cells, the incomplete dislocation cell had only partial formed cell walls. And the dislocations had not been ordered in the incomplete cell walls. The 2A10 aluminum alloy with the face-centered cubic crystal structure had greatly stacking fault energies. The dislocation motions were affected by the time effect during the high speed deformation, causing that the boundaries of dislocation cell could spread insufficiently. 11
The duration of impact stress pulse was very short during the electromagnetic impact upsetting. Dislocations were unable to reach equilibrium in the short period. So the shapes of the formed dislocation cells were irregular and some cell walls had not been completely formed. Fig. 12(b) showed the dislocation patterns in the cell walls. It could be seen that plenty of dislocations tangled each other. The dislocation density gradually increased as the deformations increased during the EIU process. The dislocation motions were hindered by the dislocation tangles and other dislocations. The dislocation strengthening from strong obstacle effect caused that the microstructures in the ASBs were remarkably hardened. So the hardness values in the adiabatic shear bands were obviously higher than that in other zones (as shown in Fig. 8). These dislocation cells were considered as the original states before the formed lamellar sub-grains. The dislocation cells generated by dislocation motions were also elongated under the severe shear stress. Furthermore, the adiabatic temperature rise was significant within the ASBs during the high speed impact process. ZHANG et al [27] reported the maximum temperature rise was 250 °C in the adiabatic shear bands of 2A10 aluminum alloy rivet during the electromagnetic riveting. The higher temperature rise could promote the dislocation cell walls to transform into sub-grain boundaries. Consequently, the elongated dislocation cells were converted into the lamellar sub-grains. In addition, the nanometer strips in the Fig. 11 were generated from some relatively smaller dislocation cells. These smaller dislocation cells were elongated and distributed along the ASBs. At the same time, the cells were subjected to extrusion effect from other larger cells. So these smaller dislocation cells finally evolved into the slender strip sub-grains, and the nanometer sub-grains were mixed among the lamellar sub-grains. Fig.12(b) depicted a very high density of dislocations tangled each other. However, the dislocation morphology could not be observed, causing that dislocation density could not be quantified by counting method. In this work, XRD tests were employed to calculate the whole dislocation density with Williamson-Hall (WH) method [28]. The dislocation density can be calculated with the widening model of diffraction peaks. The peak widening caused by the changing 12
of crystalline interplanar spacing can be described:
e,hkl = 2etan hkl
(1)
Where hkl is the position of diffraction peak (hkl) and e is average microstrain. The peak widening caused by the grain refining in coherently diffracting domains can be described:
D,hkl =
(2)
Dcos hkl
Where D is the size of grains, and is the length of diffracting wave (0.15418nm). It is assumed that the peak widening is changing in a linear relationship with the grain refining and microstrain. The widening value ( hkl ) of diffracting peaks can be described:
h k l=
hkl
coshkl
e, hkl
=
D, hkl
sin hkl 1 2e D
(3)
(4)
When the grain size is in micron range, the 1/D can be ignored. Consequently, the equation (4) can be simplified as:
hkl
coshkl
2e
sinhkl
(5)
Fig. 13 showed (111), (200), (220), (311) and (420) diffraction peaks. According to above equations, the relationship between hkl
cos hkl
and
2 sin hkl
could be obtained (as presented in
Fig. 13). So the microstrain e could be achieved by the linear fitting, and the e value was 0.004. When crystal lattice aberrance is only resulted from dislocation density variation, the dislocation density can be described as:
e2 14.4 2 b
(6)
Where b is the Burgers vector of materials (0.286 nm in Al) [29]. Consequently, the whole dislocation density could be calculated as 1.96×1014 m-2. 13
The dislocations creating rotational boundaries have a lower energy per unit line length of dislocations [30]. The high dislocation density resulted in higher distortion energy, which was stored in adiabatic shear bands after EIU. This would significantly contribute to the formation of rotational boundaries. Fig. 14 showed dynamic recrystallization grains inside ASBs for the EIU process. It could be seen that there were some equiaxed grains within shear bands and the size of these grains was about 150 nm. Nano rings of diffraction patterns demonstrated that microstructures in ASBs were in the nanoscale. The previous hardness analysis of ASBs presented the recrystallization did not reduce the mechanical strength. It could be illustrated that the dynamic recrystallization mechanism was different from conventional crystallization models. In addition, it could be seen in Fig. 14(a) that these recrystallization grains still contained many dislocations. It could be regarded that recrystallization mechanism accorded with rotational dynamic recrystallization [19]. The energy stored could provide sufficient energy for rotations of sub-grain boundaries. The forming time (1ms seen in Fig. 5) during EIU was sufficient to complete rotations of sub-grain boundaries. 4.3 The second phase particles in the adiabatic shear bands 2A10 aluminum alloy bars (chemical compositions were shown in Table 1) were one of Al-Cu alloy, and inevitably had some second phase particles. Many second phase particles (as shown in Fig. 15(a)) were scattered in adiabatic shear bands. The diffraction peak results of the energy spectrum analysis were presented in Fig. 15(c). And the Quantification results on chemical compositions of the second phase particles were shown in Table 3. According to the ratio of Al element and Cu element, these second phases were classified as the Al2Cu strengthening phase. The Al2Cu strengthening phase was significantly responsible for improving the strength of alloys. Consequently, the ASBs including these second phase particles were strengthened and had very high hardness values. In addition, it could be seen in Fig. 15(b) that a small amount of Al2Cu particles distributed outside ASB. The matrix materials were most aluminum elements and fewer cuprum elements (Fig. 15(d) and Table 4). There are more particles within the ASB than that 14
outside the ASB. This illustrated that severe plastic deformation during the high speed EIU contributed to the formation of these particles. 5. Conclusions The mechanical properties and the corresponding evolved microstructure of 2A10 aluminum alloy bars investigated during the electromagnetic impact upsetting and quasi static upsetting. The following conclusions could be expressed: Comparing with the quasi static upsetting, the severe plastic deformations highly concentrated in the adiabatic shear bands during the electromagnetic impact upsetting. The ASBs initialized in the diagonal points of specimens and extended to the center with increase of the strains. The high strain rate contributed to the forming of ASBs, and caused that the hardness values for the electromagnetic impact upsetting were higher than that of the quasi static upsetting. The adiabatic shear bands composed of many lamellar sub-grains with the thickness of 0.8μm. These lamellar sub-grains were developed from the elongated dislocation cells under the severe shear stress. And some more slender strip sub-grains were mingled among lamellar sub-grains. The width of strip sub-grains was about 100 nm and an eighth of sub-grains. The adiabatic shear bands contained very high dislocation density. And the dislocation motions were hindered by the dislocation tangles and other dislocations. The dislocation strengthening effect caused that microstructures within the ASBs were remarkably hardened, and the hardness values was significantly higher than that of other zones. In addition, the abundant strengthening phases (Al2Cu particles) also accounted for the highest hardness values in the ASBs. The high strain rate during the EIU caused that rotated sub-grains converted into dynamic recrystallization grains within ASBs. The size of recrystallization grains was about 150nm, and the hardness values did not reduce due to dynamic recrystallization.
Acknowledgment This paper was financially supported by the National Natural Science Foundation of China (No. 15
51405149) and the State Key Program of National Natural Science Foundation of China (No. 61232014). The authors would like to take this opportunity to express their sincere appreciation.
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[19] J.A. Hines, K.S. Vecchio, S.A. Ahzi, Met. And Mat. Trans A. 29 (1998): 191-203. [20] G.M. Owolabi, A.G. Odeshi, M.N.K. Singh, Mater. Sci. Eng A. 457 (2007) 114-119. [21] D.S. Hong, N.R. Tao, X. Huang, K. Lu, Acta Mater. 58(2010) 3103-3116. [22] S.N. Medyanik, W.K. Liu, S.F. Li, J. Mech. Phys. Solids. 55(2007) 1439-1461. [23] H.P. Yu, J.H. Deng, C.F. Li, J. Harbin Eng Univer. 32(2011) 378-383. [24] Q.L Zhang, Z.Q. Cao, L.G Qin, Y.Q Chen, Rare. Metal. Mat. Eng. 42 (2013) 1832-1837. [25] P.G Reinhall, S. Ghassaei, V. Choo, J. Vib., Acoust., Stress, and Reliab. 110(1988) 65-69. [26] J.H. Deng, C. Tang, M.W. Fu, Y.R. Zhan, Mater. Sci. Eng A. 591 (2014) 26-32. [27] X. Zhang, H.P Yu, C.F Li, Int. J. Adv. Manuf. Technol. 73(2014) 1751-1763. [28] G.K. Williamson, R.E. Smallman, Philos. Mag. 1(1956) 34-46. [29] W. Woo, T. Ungár, Z. Feng, E. Kenik, B. Clausen, Metall. Mater. Trans. A, 41(2010) 1210-1216. [30] D.A. Hughes, N. Hansen, D.J. Bammann, Scripta. Mater, 48(2003) 147-153.
17
Fig. 1 The original microstructure of as-received specimens Fig. 2 The schematic of electromagnetic impact upsetting Fig. 3 The deformation process and subsequent processing of specimens Fig. 4 The deformation results for the EIU and QSU: (a) Electromagnetic impact upsetting (EIU), (b) Quasi static upsetting (QSU) Fig. 5 The impact velocity during electromagnetic impact upsetting Fig. 6 Metallographic microstructure of deformed specimen with electromagnetic impact upsetting Fig. 7 Metallographic microstructure of deformed specimen with quasi static upsetting Fig. 8 The schematic diagram of deformation microstructure distribution Fig. 9 The SEM observations for the EIU process: (a) The microstructures of central deformation zone (CDZ), (b) Partially enlarged view of ASB Fig. 10 The results of hardness measurements on the measured Routes: (a) The measured Route 1, (b) The measured Route 2, (c) The measured Route 3, (d) The average hardness on each route Fig. 11 The lamellar substructures within adiabatic shear bands Fig. 12 The dislocation pattern in adiabatic shear bands: (a) Dislocation cells (red marked arrows represented complete dislocation cells, the blue marked arrow represented an incomplete dislocation cell), (b) Dislocation tangles in the cell wall Fig. 13 The Williamson-Hall plots for the diffraction patterns of EIU specimens Fig. 14 The dynamic recrystallization grains (marked arrows) in adiabatic shear bands: (a) TEM bright field image, (b) TEM dark field images Fig. 15 The second phase particles in adiabatic shear bands: (a) Second phase particles (red arrows) inside ASB, (b) Second phase particles (red arrows) outside ASB, (c) The results of the particle energy spectrum, (d) The results of the matrix energy spectrum
18
Table 1 Chemical compositions of the investigated materials (wt. %) Fe
Si
Mn
0.20
0.25
Cu
0.30-0.50 3.90-4.50
Mg
Zn
Ti
Al
0.15-0.30
0.10
0.15
remaining
Table 2 The contrasted results between the EIU and QSU Deformation time /s Final height /mm Engineer strain /%
Loading velocity
EIU
1.0e-3
3.84
57.3
4.75m/s (peak)
QSU
216
3.80
57.8
2mm/min
Table 3 Quantification results on chemical compositions of the second phase particles Element Weight /% Atomic /% Uncertainty /% Detector Correction k-Factor Al
41.99
63.03
0.14
0.92
1.030
Cu
58.00
36.96
0.20
0.99
1.601
Table 4 Quantification results on chemical compositions of the matrix Element Weight /% Atomic /% Uncertainty /% Detector Correction k-Factor Al
95.22
97.91
0.26
0.92
1.030
Cu
4.77
2.08
0.07
0.99
1.601
Highlights
The electromagnetic impact upsetting is proposed for the upsetting of 2A10 aluminum alloy bars.
The microstructure distribution laws of deformed specimens were investigated by comparing with the quasi static upsetting.
The effect of microstructure distribution on the forming properties was investigated.
Microstructure evolution in adiabatic shear bands was revealed by the TEM observations.
19
Fig. 1
20
Fig. 2
21
Fig. 3
22
(a)
(b) Fig. 4
The impact velocity (m/s)
5 4
0ms
3 0.3ms 2 0.6ms
1 0
1.0ms
-1 -2
0.0
0.2
0.4
0.6
Deformation time /ms 23
0.8
1.0
Fig. 5
Fig. 6
24
Fig. 7
25
Fig. 8
26
(a)
(b) Fig. 9
27
Electromagnetic impact upsetting Quasi static upsetting
150
Electromagnetic impact upsetting Quasi static upsetting
200
Hardness /HV
Hardness /HV
120 90 60 30 0
160 120 80 40
1
2
3
4
0
5
1
2
(a) 200
Electromagnetic impact upsetting Quasi static upsetting
200
Average hardness /HV
Hardness /HV
120 80 40
1
2
4
5
(b)
160
0
3
Measured points on the Route 2
Measured points on the Route 1
3
4
Electromagnetic impact upsetting Quasi static upsetting
160 120
5
80 40 0
Route 1
Measured points on the Route 3
(c)
Route 2
(d) Fig. 10
28
Route 3
Fig. 11
29
(a)
(b) Fig. 12
20.0k
Intensity (counts)
Measured values Fitting
0.040
(420)
-1
16.0k
hklcos hkl/nm
0.045
(111)
12.0k 8.0k
(200)
(311)
0.035
0.025 0.020
(220)
(200)
0.030
(111) 4
5
6
7
8
9
-1
sinhkl/nm
4.0k
(220)
(311) (420)
0.0 40
50
60
2Degree Fig. 13
30
70
80
(a)
(b) Fig. 14
31
(a)
(b)
(c)
(d) Fig. 15
32
Graphical Abstract
33
PII: DOI: Reference:
S0921-5093(17)30806-7 http://dx.doi.org/10.1016/j.msea.2017.06.039 MSA35177
To appear in: Materials Science & Engineering A Received date: 26 March 2017 Revised date: 8 June 2017 Accepted date: 10 June 2017 Cite this article as: Xu Zhang, Junjia Cui, Junrui Xu and Guangyao Li, Microstructure investigations on 2A10 aluminum alloy bars subjected to electromagnetic impact upsetting, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.06.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure investigations on 2A10 aluminum alloy bars subjected to electromagnetic impact upsetting
Xu Zhang1,2, Junjia Cui2,3, Junrui Xu4, Guangyao Li2,3*
1
College of Mechanical and Electrical Engineering, Hunan University of Science and Technology, Xiangtan, 411100, China
2
State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, 410082, China 3
Joint Center for Intelligent New Energy Vehicle, Shanghai, 200092, China
4
School of Mechanical Engineering, Xiangtan University, Xiangtan 411105, China
*
Corresponding author at: State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, 410082, China. Tel.: +86 731 88664001; fax: +86 731 88822051. [email protected]
1
Abstract In this work, electromagnetic impact upsetting (EIU) process was proposed. And EIU and quasi static upsetting (QSU) experiments were performed with Φ6mm-2A10 aluminum alloy bars. Microstructure evolutions for both processes were revealed by metallographic observations under the varying engineering strains. Results showed that the most deformations for the EIU process significantly concentrated in narrower adiabatic shear bands (ASBs) comparing with the QSU process. The ASBs initialized in the diagonal points of samples and gradually extended to the central with the continuous deformations. And four ASBs encountered and integrated in the central. The high strain rate contributed to deformation concentration and the forming of the adiabatic shear bands. In addition, hardness values for the EIU process were higher than that of the QSU process, demonstrating that the strengthening effect of the strain rate was notable. TEM observations presented that the ASBs with the highest hardness contained many lamellar sub-grains and second phase particles (Al2Cu). The high strain rate caused that rotated sub-grains converted into dynamic recrystallization grains within ASBs. Plentiful dislocation tangles and strengthening phases accounted for highest hardness values within the ASBs.
Keywords: electromagnetic impact upsetting; 2A10 aluminum alloy; high strain rate; microstructures
2
1. Introduction The effective and green manufacturing technology was an urgent need in industrial fields. The proposed electromagnetic impact upsetting (EIU) is a dynamic plastic deformation (DPD) process, in which specimens are subjected to the high speed loading. Metals are formed under the high strain rate. This process is parallel to DPD tests, such as Split Hopkinson Pressure Bar test [1], drop hammer impact [2] and the electromagnetic riveting [3-5]. Mechanical properties of deformed alloys significantly depend on microstructural characteristics such as deformation texture, grain size, and microstructural morphology. However, the high strain rate (generally higher than 102 s-1) has an important effect on deformations, which makes the analysis of the EIU process become more complex than that of quasi-static compressions. Moreover, deformations with the high strain rate can be considered as an adiabatic process, in which adiabatic shear bands (ASBs) are the most important deformation behaviors. ASB is a narrowband region where shear deformation highly concentrated, and microstructure evolution within it is a determinant factor for material properties and forming quality. At present, the deformation mechanism under high strain rate has been investigated by many researchers. As reported, the two main phenomena (grain refinement and dynamic recrystallization) were found in ASBs. Severe plastic deformation (SPD) is an effective route to refining coarse grain size into micrometer or nanometer. Many studies [6-8] demonstrated that grains size in ASBs was nanometer scale and the ultrafine grains resulted from dynamic recrystallization mechanism. Andrade et al. [9] and Meyers et al. [10] observed the fine equiaxed grains in the ASBs of copper under the high speed impact deformation, indicating the possibility of dynamic recrystallization. Researches on titanium alloys with the cold DPD showed that the recrystallized structures existed in ASBs [11-15]. Additionally many similar results had been also reported in the ASBs of other materials including tantalum [16-17], steel [18]. Dynamic recrystallization during hot forming is usually 3
considered to be a nucleation and growth process of recrystallized grains. Time and temperature play a vital role in the recrystallization process. However, there are three characteristics different from conventional forming methods during the DPD process: approximate adiabatic process (almost all plastic works are converted to heats), very short forming time (quick cooling rate) and deformation localizations. What is the mechanism of dynamic recrystallization with high strain rate has aroused extensive concern among many researchers. Hines [19] proposed a model that recrystallized grains were developed from rotated sub-grains, and this rotated mechanism was to coordinate the high strain rate deformations. Studies by Owolabi et al. [20] confirmed this model. In addition, some results [21-22] presented that the rotation mechanism led to the significant misorientations among sub-grains and made them convert into recrystallized grains. Moreover, numerous studies about electromagnetic riveting indicated that adiabatic shear bands were important characteristics in upsetting process of rivets. It was observed by Yu et al. [23] that grains of TA1 titanium alloy rivet were remarkably refined in ASBs. But the change of other grains size was negligible. Cao et al. [24] reported the width of ASBs existed in rivet heading was generally 10 ~ 102 microns. Choo et al. [25] found ASBs were root of microcrack generation in 7075-T73 aluminum alloy rivet after EMR. Deng et al. [26] stated dynamic recrystallized grains were found in ASBs of titanium alloy rivet. Above all, microstructures in ASBs were the key factor to determine quality and fatigue life of workpieces after high speed impact forming. However, the detailed investigations on microstructure evolution in ASBs with the electromagnetic impact upsetting have not been reported. Moreover, the comparing investigation on microstructures and mechanical properties between electromagnetic impact upsetting process and quasi static upsetting process had been little investigated. In this work, Electromagnetic impact upsetting (EIU) and quasi static upsetting (QSU) tests for 2A10 aluminum alloy were performed at ambient temperature. Microstructure and mechanical property difference between the EIU process and QSU process was revealed by the comparing method. The microstructure evolutions were characterized by optical microscopy (OM), and 4
transmission electron microscope (TEM). The dislocation density was obtained by XRD (X-ray diffraction). Micro-hardness values were measured to investigate mechanical property distribution. 2. Experimental material and procedure The specimens used in this work were annealed 2A10 aluminum alloy bars with the height of 9 mm and diameter of 6 mm. The compositions of the specimens were listed in table 1. The materials were similar to 2024 aluminum alloy and had relatively high strength and good plasticity. Consequently, 2A10 aluminum alloy were commonly used as rivet materials. The original microstructure of as-received specimens was shown in Fig. 1. It could be seen that grains size was uniform and around 50 micrometers. EIU experiments were conducted using a 48 kJ electromagnetic forming machine (EFM) at the Hunan University. The schematic of the machine and experiment setup is demonstrated in Fig. 2. The discharge principal of electromagnetic forming can be described as a typical RLC circuit (the system resistance R, the capacitance C and the inductance L). The capacitors are charged by adjusting charging parameters and stored electrical energy. The energy is discharged by closing the switch and produces high amplitude changing currents in coil, around which a rapidly changing electromagnetic field is generated. Meanwhile, eddy currents are induced in diver plate (copper sheet with high conductivity) which is exposed to alternating magnetic fields. And then these circulating eddy currents induce magnetic fields. The two fields (coil magnetic fields and eddy magnetic fields) generate a repulsive force and push the punch to impact onto specimen. The deformation of samples for the EIU process was controlled by discharge energies. In general, repeated EIU experiments should be performed to determine the relationship between the specimen height and discharge energies. All EIU experiments were carried out by discharge energy 3 kJ and the deformation height was controlled using rigid gaskets. This could ensure that all experiments had initial loading conditions. Consequently, it was approximately considered that the deformation results with different heights were discretization results of the whole sequential EIU process. Microstructures on specimens with different heights were observed to reveal microstructure 5
evolution during the EIU process. In order to compare two upsetting methods, same deformations along the height of specimens were achieved. Firstly, four EIU specimens with different heights were obtained. The height after the EIU process was measured to determine compression displacements of the QSU process. The QSU experiments were carried out by using a universal Instron 8801 electronic testing machine with a compressing velocity of 2 mm/min and the predetermined compression displacement. The deformation process and subsequent processing of specimens were shown in Fig. 3. Specimens were split along its axis and mechanically polished. The chemical etchant used on the OM specimens was the Keller solution of 2.5 ml HNO3, 1.5 ml HCL, 1 ml HF and 95 ml H2O. The metallographic microstructures of tested specimens were examined by a Leica microscope. TEM (Transmission Electron Microscope) specimens were taken in the ASBs to reveal microstructure evolution. The selected specimens were mechanically polished to the thickness of 80 μm, and were thinned by a twin-jet electro-polishing device with an etching solution of 30% nitric acid and 70% ethyl
alcohol.
All
TEM
observations
were
performed
using
a
TecnaiF2F30
transmission electron microscope. The XRD (X-ray diffraction) tests were conducted with Rigaku D/MAX 2500 X to measure dislocation density. 3. Results 3.1 Deformation results The engineering strains along the axis of samples were calculated by the reduction of height, namely ε = (H0 - H)/H0, where H0 and H were the initial and final height of specimens, respectively. Fig. 4 showed deformation results for the electromagnetic impact upsetting (EIU) and quasi static upsetting (QSU). For the EIU process, shear deformation bands initiated at the four diagonal points of sample (ε =32.4%). And the shear bands gradually extended to the center of the deformed sample with the increase of axial engineering strain. Four shear bands intersected in the center under the engineering strain 46.4%. All shear bands increased as a whole and evolved to up and down two parabola-shapes for the higher strain. Whether the lower strain or the higher strain for the 6
QSU process, the shear deformation bands distributed along the two diagonals. And the width of shear bands was relatively larger than that of the EIU. The intersecting zone (namely the center of samples) of two diagonal bands had the larger deformation. The impact velocity in the electromagnetic upsetting was tested using a high speed video camera. The deformation process of upset specimens and impact velocity were shown in Fig. 5. The whole upsetting process endured about 1 millisecond and the peak impact velocity was 4.75 m/s. It was noted that the loading velocity of the EIU process was significantly higher than that of the QSU process, and the EIU process could be classified as a dynamic plastic deformation. Table 2 showed the contrastive results between the EIU and QSU with the same deformation. The discharge energy 3 kJ was employed for the EIU, giving rise to 57.3%. Consequently, average strain rate was estimated by dividing the engineering strain and loading time. The average strain rate approximately reached to the 103 s-1. 3.2 Metallographic structure for the EIU process and QSU process Metallographic microstructures for the EIU process and QSU process were shown in Fig. 6 and Fig. 7, in which the full section shape of specimens were obtained through assembling numerous metallographic pictures. In order to observe clearly the microstructure evolution in different zones after forming, partial enlargements of metallographic figures were obtained. It could be seen from the two process methods that shear bands initiated and spread along diagonal direction of specimens, and the shear bands (upper and lower) encountered in the center. The width of shear bands for EIU was remarkably less than that of QSU, which embodied distinctive characteristic (adiabatic shear bands) under the high strain rate. The deformation microstructure of specimens could be divided into four zones (small deformation zone (SDZ), free deformation zone (FDZ), central deformation zone (CDZ) and shear band zone (SBZ)), as presented in Fig. 8. For microstructures in the SDZ, both upsetting processes had small deformation and grains still kept original equiaxial morphology. The plastic flow of metals in this zone was limited by the friction effect between the end surfaces and punch (or base plate), causing that most metals was almost in the static state. So only some 7
grains close to shear bands were distorted and deformed, and the deformation degree was more severe closer to shear bands. For microstructures in the FDZ, the metal in this zone could freely flow along the radial direction of samples and was in a state of two-direction compressive stress and one-direction tensile stress. Grains were remarkably elongated along the radial direction and broken down. The grain deformations for EIU process was larger comparing with that of the QSU process. For the microstructures in the CDZ and SBZ, the characteristic of shear deformation was very significant and the metal flow lines distributed along shear bands. The shear deformation unavoidably occurred due to transitional effect between the static zones (SDZ) and free deformation zones (FDZ). In addition, shear bands of samples under the EIU were narrower than that of the sample with QSU. The shear deformations within the shear band zone for the EIU were also more severe. The grains were distorted and elongated to the fiber-like microstructures (Fig. 9(b)), and the morphology of these grains was unable to identify. It was illustrated that the plastic deformation concentrated highly in the shear band zones, and other zones had smaller deformations comparing with shear zones. Grains (in the FDZ and SDZ) close to shear bands were driven to deform and flowed within the SDZ, causing that the boundaries of shear bands were not smooth. In addition, some second phase particles (the white particles) existed in adiabatic shear bands. The actual number of particles was more than that of observed particles. From the morphology of voids, these micro-voids were positions of the falling second phase particles during SEM sample preparations. Mechanical polishing and chemical etching caused that small amounts of particles fell off during metallographic specimen preparations. The maximum deformations also located in shear zones for the QSU process. However, some incomplete broken grains were mingled with deformed microstructures. The strain rate for the EIU process was up to 2000 s-1, causing the isothermal state was converted into adiabatic state during the upsetting process. Consequently the heats generated from plastic works were unable to transfer quickly to the ambient, and concentrated in the heat source area. The shear bands with larger deformations had more heats from plastic works, leading to significant temperature rise. At the same time, the work hardening effect was more 8
notable with the increase of the deformation degree. The severe plastic deformation could highly concentrate in some partial zones when the softening effect suppressed work hardening effect. And then the mutual interaction mechanism between plastic deformation and temperature rise accounted for the formation of narrow shear bands. So the shear bands resulted from deformation concentrations under the adiabatic state, and were also called as adiabatic shear bands (ASBs). It could be seen that the width of shear deformation bands under the QSU process was approximately 600μm. However, the width of adiabatic shear bands under the EIU process was only 150μm (Fig. 9). This illustrated that the high strain rate contributed to deformation concentrations and the forming of the adiabatic shear bands. 3.3 Micro-hardness Mechanical properties of upset specimens were significantly determined by microstructures after
the
upsetting
deformations.
Previous
microstructure
analysis
demonstrated
that
inhomogeneous plastic flow resulted in the non-uniform distribution of microstructures. And then this greatly affected the distribution law of mechanical properties. Five measured points in the three routes (as shown in Fig. 6 and Fig. 7) were selected to test micro-hardness values. Testing processes were performed using a HVS-1000 Digital Micro Vickers Hardness Tester with a steel ball of 10μm diameter, a load of 500 g and holding time of 30s. Fig. 10 showed the hardness results on the measured routes for electromagnetic impact upsetting and quasi static upsetting. The original hardness value of undeformed 2A10 aluminum alloy bars was 110 HV. It could be seen that the hardness values (comparing with the original value) increased due to work hardening effect. It is well known that the hardness values presented the work hardening degree (namely deformation degree). Hardness values on the Route 2 (direction along shear bands) for both upsetting process were significantly higher than that on the Route 1 (in the SDZ) and Route 3 (in the FDZ). The hardness values were the minimum for microstructures in the SDZ. However, the hardness in the SDZ was still slightly improved in spite of the smaller deformations. Hardness values for 9
microstructures in the FDZ were in between the SDZ and the SBZ. The hardness distribution law accorded with microstructure distribution mentioned above, demonstrating that deformation degree notably affected mechanical properties of deformed specimens. For the measured Route 1, the hardness gradually increased from the end surface of the specimen to the central, revealing that the deformation degree close to shear bands was higher due to the driving effect of shear deformation. The same changing trend was obtained on the measured Route 3. But the fluctuation range among five measured points for the QSU process was smaller than that of the EIU process. Metallographic structure analysis had demonstrated that the deformation in the FDZ was relatively smaller and more uniform during the QSU process. For the measured Route 2, the zones close to the central were the intersecting locations of upper and lower adiabatic shear bands for the EIU process. These zones for the QSU process were mixed microstructures between the shear bands and microstructures with larger deformations under the triaxial compressive stress. Consequently, the relatively higher deformation degree in these zones caused the higher hardness values of deformed microstructures. And the average hardness on the Route for the EIU process and the QSU process was improved by 66.4% and 50% (relative to the original hardness 110 HV), respectively. Moreover, hardness values in every measured route of the EIU process were higher than that of the QSU process, indicating that the hardening effect of strain rate was very notable during the electromagnetic impact upsetting. 4. Discussions 4.1 Sub-grain morphology in the adiabatic shear bands Previous analysis demonstrated that the deformations concentrated in adiabatic shear bands during electromagnetic impact upsetting. However, the general morphology of the ASBs was difficultly observed through metallographic structures due to smaller width of them. The microstructure evolution needed to be characterized by the transmission electron microscope (TEM). Fig. 11 showed that the lamellar substructures within adiabatic shear bands. This Figure was 10
composed of many TEM pictures to observe sub-grain morphology. It could be seen that many lamellar structures distributed along the direction of the adiabatic shear band, and were similar to the fiber-like deformation structures (as shown in Fig. 6). The average width of lamellar structures was about 0.8μm and much smaller than the average size of the original undeformed grains (50μm). Slip deformations along the same shear direction caused those lamellar sub-grains conformably distributed in ASB. The neighboring lamellar substructures had coincident diffraction patterns, illustrating that all lamellar structures were sub-grains generated from the same original grain. In addition, some more slender strip microstructures were mingled among lamellar substructures. The width of slender strips was very small and around 100 nm. The diffraction patterns showed the low angle boundary between strips and lamellar sub-grains, demonstrated that these strips belonged to sub-grains. No twins with the substructures could illustrate dislocation slips were the main deformation mechanism during the EIU process with high strain rate. During the plastic deformation, the external forces reached to critical shear stress which was the driving stress of the slip system activation. The metals would be dislocated along the direction of slip systems, and then was refined to sub-grains. The slip planes of crystals were also subjected to the normal force at the same time of withstanding the tangential forces. Consequently the normal forces could cause the rotation of the formed substructures, and the slip planes were bent. As shown in Fig. 11, the boundaries among lamellar substructures were not straight and slightly bent. 4.2 Dislocation patterns in the adiabatic shear bands There were some dislocation cells within adiabatic shear bands besides lamellar sub-grains. Fig. 12 presented the dislocation pattern in the ASBs. These dislocation cells distributed along shear bands and the size of them was not uniform. Among these dislocation cells, the incomplete dislocation cell had only partial formed cell walls. And the dislocations had not been ordered in the incomplete cell walls. The 2A10 aluminum alloy with the face-centered cubic crystal structure had greatly stacking fault energies. The dislocation motions were affected by the time effect during the high speed deformation, causing that the boundaries of dislocation cell could spread insufficiently. 11
The duration of impact stress pulse was very short during the electromagnetic impact upsetting. Dislocations were unable to reach equilibrium in the short period. So the shapes of the formed dislocation cells were irregular and some cell walls had not been completely formed. Fig. 12(b) showed the dislocation patterns in the cell walls. It could be seen that plenty of dislocations tangled each other. The dislocation density gradually increased as the deformations increased during the EIU process. The dislocation motions were hindered by the dislocation tangles and other dislocations. The dislocation strengthening from strong obstacle effect caused that the microstructures in the ASBs were remarkably hardened. So the hardness values in the adiabatic shear bands were obviously higher than that in other zones (as shown in Fig. 8). These dislocation cells were considered as the original states before the formed lamellar sub-grains. The dislocation cells generated by dislocation motions were also elongated under the severe shear stress. Furthermore, the adiabatic temperature rise was significant within the ASBs during the high speed impact process. ZHANG et al [27] reported the maximum temperature rise was 250 °C in the adiabatic shear bands of 2A10 aluminum alloy rivet during the electromagnetic riveting. The higher temperature rise could promote the dislocation cell walls to transform into sub-grain boundaries. Consequently, the elongated dislocation cells were converted into the lamellar sub-grains. In addition, the nanometer strips in the Fig. 11 were generated from some relatively smaller dislocation cells. These smaller dislocation cells were elongated and distributed along the ASBs. At the same time, the cells were subjected to extrusion effect from other larger cells. So these smaller dislocation cells finally evolved into the slender strip sub-grains, and the nanometer sub-grains were mixed among the lamellar sub-grains. Fig.12(b) depicted a very high density of dislocations tangled each other. However, the dislocation morphology could not be observed, causing that dislocation density could not be quantified by counting method. In this work, XRD tests were employed to calculate the whole dislocation density with Williamson-Hall (WH) method [28]. The dislocation density can be calculated with the widening model of diffraction peaks. The peak widening caused by the changing 12
of crystalline interplanar spacing can be described:
e,hkl = 2etan hkl
(1)
Where hkl is the position of diffraction peak (hkl) and e is average microstrain. The peak widening caused by the grain refining in coherently diffracting domains can be described:
D,hkl =
(2)
Dcos hkl
Where D is the size of grains, and is the length of diffracting wave (0.15418nm). It is assumed that the peak widening is changing in a linear relationship with the grain refining and microstrain. The widening value ( hkl ) of diffracting peaks can be described:
h k l=
hkl
coshkl
e, hkl
=
D, hkl
sin hkl 1 2e D
(3)
(4)
When the grain size is in micron range, the 1/D can be ignored. Consequently, the equation (4) can be simplified as:
hkl
coshkl
2e
sinhkl
(5)
Fig. 13 showed (111), (200), (220), (311) and (420) diffraction peaks. According to above equations, the relationship between hkl
cos hkl
and
2 sin hkl
could be obtained (as presented in
Fig. 13). So the microstrain e could be achieved by the linear fitting, and the e value was 0.004. When crystal lattice aberrance is only resulted from dislocation density variation, the dislocation density can be described as:
e2 14.4 2 b
(6)
Where b is the Burgers vector of materials (0.286 nm in Al) [29]. Consequently, the whole dislocation density could be calculated as 1.96×1014 m-2. 13
The dislocations creating rotational boundaries have a lower energy per unit line length of dislocations [30]. The high dislocation density resulted in higher distortion energy, which was stored in adiabatic shear bands after EIU. This would significantly contribute to the formation of rotational boundaries. Fig. 14 showed dynamic recrystallization grains inside ASBs for the EIU process. It could be seen that there were some equiaxed grains within shear bands and the size of these grains was about 150 nm. Nano rings of diffraction patterns demonstrated that microstructures in ASBs were in the nanoscale. The previous hardness analysis of ASBs presented the recrystallization did not reduce the mechanical strength. It could be illustrated that the dynamic recrystallization mechanism was different from conventional crystallization models. In addition, it could be seen in Fig. 14(a) that these recrystallization grains still contained many dislocations. It could be regarded that recrystallization mechanism accorded with rotational dynamic recrystallization [19]. The energy stored could provide sufficient energy for rotations of sub-grain boundaries. The forming time (1ms seen in Fig. 5) during EIU was sufficient to complete rotations of sub-grain boundaries. 4.3 The second phase particles in the adiabatic shear bands 2A10 aluminum alloy bars (chemical compositions were shown in Table 1) were one of Al-Cu alloy, and inevitably had some second phase particles. Many second phase particles (as shown in Fig. 15(a)) were scattered in adiabatic shear bands. The diffraction peak results of the energy spectrum analysis were presented in Fig. 15(c). And the Quantification results on chemical compositions of the second phase particles were shown in Table 3. According to the ratio of Al element and Cu element, these second phases were classified as the Al2Cu strengthening phase. The Al2Cu strengthening phase was significantly responsible for improving the strength of alloys. Consequently, the ASBs including these second phase particles were strengthened and had very high hardness values. In addition, it could be seen in Fig. 15(b) that a small amount of Al2Cu particles distributed outside ASB. The matrix materials were most aluminum elements and fewer cuprum elements (Fig. 15(d) and Table 4). There are more particles within the ASB than that 14
outside the ASB. This illustrated that severe plastic deformation during the high speed EIU contributed to the formation of these particles. 5. Conclusions The mechanical properties and the corresponding evolved microstructure of 2A10 aluminum alloy bars investigated during the electromagnetic impact upsetting and quasi static upsetting. The following conclusions could be expressed: Comparing with the quasi static upsetting, the severe plastic deformations highly concentrated in the adiabatic shear bands during the electromagnetic impact upsetting. The ASBs initialized in the diagonal points of specimens and extended to the center with increase of the strains. The high strain rate contributed to the forming of ASBs, and caused that the hardness values for the electromagnetic impact upsetting were higher than that of the quasi static upsetting. The adiabatic shear bands composed of many lamellar sub-grains with the thickness of 0.8μm. These lamellar sub-grains were developed from the elongated dislocation cells under the severe shear stress. And some more slender strip sub-grains were mingled among lamellar sub-grains. The width of strip sub-grains was about 100 nm and an eighth of sub-grains. The adiabatic shear bands contained very high dislocation density. And the dislocation motions were hindered by the dislocation tangles and other dislocations. The dislocation strengthening effect caused that microstructures within the ASBs were remarkably hardened, and the hardness values was significantly higher than that of other zones. In addition, the abundant strengthening phases (Al2Cu particles) also accounted for the highest hardness values in the ASBs. The high strain rate during the EIU caused that rotated sub-grains converted into dynamic recrystallization grains within ASBs. The size of recrystallization grains was about 150nm, and the hardness values did not reduce due to dynamic recrystallization.
Acknowledgment This paper was financially supported by the National Natural Science Foundation of China (No. 15
51405149) and the State Key Program of National Natural Science Foundation of China (No. 61232014). The authors would like to take this opportunity to express their sincere appreciation.
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17
Fig. 1 The original microstructure of as-received specimens Fig. 2 The schematic of electromagnetic impact upsetting Fig. 3 The deformation process and subsequent processing of specimens Fig. 4 The deformation results for the EIU and QSU: (a) Electromagnetic impact upsetting (EIU), (b) Quasi static upsetting (QSU) Fig. 5 The impact velocity during electromagnetic impact upsetting Fig. 6 Metallographic microstructure of deformed specimen with electromagnetic impact upsetting Fig. 7 Metallographic microstructure of deformed specimen with quasi static upsetting Fig. 8 The schematic diagram of deformation microstructure distribution Fig. 9 The SEM observations for the EIU process: (a) The microstructures of central deformation zone (CDZ), (b) Partially enlarged view of ASB Fig. 10 The results of hardness measurements on the measured Routes: (a) The measured Route 1, (b) The measured Route 2, (c) The measured Route 3, (d) The average hardness on each route Fig. 11 The lamellar substructures within adiabatic shear bands Fig. 12 The dislocation pattern in adiabatic shear bands: (a) Dislocation cells (red marked arrows represented complete dislocation cells, the blue marked arrow represented an incomplete dislocation cell), (b) Dislocation tangles in the cell wall Fig. 13 The Williamson-Hall plots for the diffraction patterns of EIU specimens Fig. 14 The dynamic recrystallization grains (marked arrows) in adiabatic shear bands: (a) TEM bright field image, (b) TEM dark field images Fig. 15 The second phase particles in adiabatic shear bands: (a) Second phase particles (red arrows) inside ASB, (b) Second phase particles (red arrows) outside ASB, (c) The results of the particle energy spectrum, (d) The results of the matrix energy spectrum
18
Table 1 Chemical compositions of the investigated materials (wt. %) Fe
Si
Mn
0.20
0.25
Cu
0.30-0.50 3.90-4.50
Mg
Zn
Ti
Al
0.15-0.30
0.10
0.15
remaining
Table 2 The contrasted results between the EIU and QSU Deformation time /s Final height /mm Engineer strain /%
Loading velocity
EIU
1.0e-3
3.84
57.3
4.75m/s (peak)
QSU
216
3.80
57.8
2mm/min
Table 3 Quantification results on chemical compositions of the second phase particles Element Weight /% Atomic /% Uncertainty /% Detector Correction k-Factor Al
41.99
63.03
0.14
0.92
1.030
Cu
58.00
36.96
0.20
0.99
1.601
Table 4 Quantification results on chemical compositions of the matrix Element Weight /% Atomic /% Uncertainty /% Detector Correction k-Factor Al
95.22
97.91
0.26
0.92
1.030
Cu
4.77
2.08
0.07
0.99
1.601
Highlights
The electromagnetic impact upsetting is proposed for the upsetting of 2A10 aluminum alloy bars.
The microstructure distribution laws of deformed specimens were investigated by comparing with the quasi static upsetting.
The effect of microstructure distribution on the forming properties was investigated.
Microstructure evolution in adiabatic shear bands was revealed by the TEM observations.
19
Fig. 1
20
Fig. 2
21
Fig. 3
22
(a)
(b) Fig. 4
The impact velocity (m/s)
5 4
0ms
3 0.3ms 2 0.6ms
1 0
1.0ms
-1 -2
0.0
0.2
0.4
0.6
Deformation time /ms 23
0.8
1.0
Fig. 5
Fig. 6
24
Fig. 7
25
Fig. 8
26
(a)
(b) Fig. 9
27
Electromagnetic impact upsetting Quasi static upsetting
150
Electromagnetic impact upsetting Quasi static upsetting
200
Hardness /HV
Hardness /HV
120 90 60 30 0
160 120 80 40
1
2
3
4
0
5
1
2
(a) 200
Electromagnetic impact upsetting Quasi static upsetting
200
Average hardness /HV
Hardness /HV
120 80 40
1
2
4
5
(b)
160
0
3
Measured points on the Route 2
Measured points on the Route 1
3
4
Electromagnetic impact upsetting Quasi static upsetting
160 120
5
80 40 0
Route 1
Measured points on the Route 3
(c)
Route 2
(d) Fig. 10
28
Route 3
Fig. 11
29
(a)
(b) Fig. 12
20.0k
Intensity (counts)
Measured values Fitting
0.040
(420)
-1
16.0k
hklcos hkl/nm
0.045
(111)
12.0k 8.0k
(200)
(311)
0.035
0.025 0.020
(220)
(200)
0.030
(111) 4
5
6
7
8
9
-1
sinhkl/nm
4.0k
(220)
(311) (420)
0.0 40
50
60
2Degree Fig. 13
30
70
80
(a)
(b) Fig. 14
31
(a)
(b)
(c)
(d) Fig. 15
32
Graphical Abstract
33