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Enhanced microstructure homogeneity and mechanical properties of AZ31 magnesium alloy by repetitive upsetting
Materials Science and Engineering A 540 (2012) 115–122
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Enhanced microstructure homogeneity and mechanical properties of AZ31 magnesium alloy by repetitive upsetting W. Guo a,b , Q.D. Wang a,b,∗ , B. Ye a,b , M.P. Liu a,b , T. Peng a,b , X.T. Liu a,b , H. Zhou a,b a b
National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, Shanghai 200240, China State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 31 August 2011 Received in revised form 7 January 2012 Accepted 24 January 2012 Available online 1 February 2012 Keywords: AZ31 magnesium alloy Severe plastic deformation Repetitive upsetting Microstructure Mechanical properties
a b s t r a c t An as-cast AZ31 Mg alloy in the form of thick plate (100 mm diameter and 20 mm height) with an initial grain size ∼200 m was processed by a novel severe plastic deformation technique, repetitive upsetting (RU). Experiments were carried out for 1, 3, and 5 deformation passes at 250, 300, and 350 ◦ C through route A and route B. Finite element analysis of the deformation process indicates homogeneous deformation during RU. With the number of RU passes, a finer grain size and more uniform microstructure are obtained along with significant improvement in both strength and ductility. A homogeneous grain structure with an averaged grain size of ∼2.6 m through route A and ∼1.6 m through route B are achieved after 5 passes at 350 ◦ C. A tensile strength of 304 MPa and elongation of 28% are obtained in route B RU processing. AZ31 alloy exhibits a mean grain size of 1.3 m and yield strength of 294 MPa and tensile strength of 354 MPa after 5 RU passes at 250 ◦ C as compared to initial 43 MPa and 174 MPa, respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Refining grain is often believed to be the most effective means to attain both the desired strength and ductility at room temperature [1]. These fine grains also provide the possibility for superplastic behavior at relatively lower temperature and higher strain rate. Among different methods in achieving fine grained materials, severe plastic deformation (SPD) is of more attractive by imposing intense strain on a sample without any change of its dimensions between passes. It has successfully fabricated bulk ultrafine-grained (UFG) materials without any porosity or contamination [2] in various shapes. Equal-channel angular pressing (ECAP) [3–5], cyclic extrusion compression (CEC) [6–8] and twist extrusion (TE) [9] have been developed for processing rods materials. High pressure torsion (HPT) [10], accumulative roll-bonding (ARB) [11] and repetitive corrugation straightening (RCS) [12] are utilized for preparing sheet materials. In addition, multi-directional forging (MDF) has been used for producing quadrangular samples [13]. However, these SPD technologies have some limitations and they are not able to process the material in the form of thick plates. In this report, we propose a novel SPD technique, repetitive upsetting (RU) process, for fabricating thick plates.
∗ Corresponding author at: National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, Shanghai 200240, China. Tel.: +86 21 54742715; fax: +86 21 34202794. E-mail address: [email protected] (Q.D. Wang). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2012.01.111
We analyze the deformation behavior in RU with finite element (FE) commercial software DEFORM considering realistic material behavior and processing parameters. We study the evolution of microstructure and mechanical properties of AZ31 alloy during RU processes at 250, 300 and 350 ◦ C for 1, 3 and 5 passes. Compared to other SPD methods, the RU process has the following characteristics: (1) larger deformation strain is imposed in each pass; (2) it allows to process less ductile materials due to the compressive deformation; (3) more uniform deformation is achieved because the transversal flow in a material is more abundant and multidirectional deformation is involved; (4) it is capable of producing large fully dense materials with UFG structure for real structural applications. 2. Principles of repetitive upsetting method The repetitive upsetting (RU) method can be used to process thick plates under cylindrical or rectangular configuration. The schematic representation of the RU process is shown in Fig. 1 for processing cylindrical thick plates. The corresponding graphic view of the RU apparatus is shown in Fig. 2. The apparatus consists of an upper rectangular cuboid chamber die, a lower cylindrical chamber die, and a rectangular cuboid plunger for external loading. The length of the upper chamber is equal to the diameter of the lower chamber. The upper chamber perpendicularly connects the lower chamber at the center of cylindrical lower chamber along the radial direction. A well-lubricated cylindrical billet is placed into the upper chamber and pressed into the lower chamber by
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Fig. 3. The compression stress–strain curves of as-cast AZ31 alloy at strain rates of 0.01, 0.1 and 1 s−1 at 350 ◦ C.
Fig. 1. The schematic representation of RU processing for fabricating cylindrical thick plates.
the plunger. During this upsetting process, the billet is simultaneously subjected to three different deformation modes, i.e. shear, extrusion, and compression. The shear strain is introduced at the connection of the upper and lower chamber. The extrusion and compressive strains are introduced by the pressure of upsetting deformation between the plunger and the lower chamber. When the cylindrical billet has the same dimension as the lower chamber, the dimension of the billet is kept constant but normal direction changes 90◦ from horizontal to vertical after upsetting. When the dimension of the billet remains unchanged, the upsetting can be repetitively applied (Fig. 1), and as a result, this repetitively process is called repetitive upsetting (RU). During RU, arbitrarily high shear strain is accumulated resulting in microstructure refinement. The uniform and multi-directional deformation can be achieved by rotating the billet 90◦ along either X-axis (route A) or Yaxis (route B) in the radial direction in Fig. 1 between each pass due to the stress direction and deformation path change. Deformation ability is also remarkably improved especially for Mg alloys because of compressive deformation characteristics in the RU process. 3. Experimental procedures A commercial AZ31 alloy with a nominal composition of 3 wt.% Al and 1 wt.% Zn was sectioned into cylindrical samples with a diameter of 100 mm and a height of 20 mm. RU was conducted on AZ31 round plate at 350 ◦ C and a speed of 4 mm/s for 1, 3, and 5 passes in an H-13 tool steel chamber die through route A and route B, respectively. RU was also carried out at 250, 300, and 350 ◦ C at a speed of 4 mm/s for 5 passes through route A. Graphite on the wall of the die was used as lubricant to reduce the friction between the billets and the die wall. Flat dog-bone tensile specimens (10 mm gage length, 3.5 mm gage width and 2 mm gage thickness) were created by electrical discharge machining. The specimen was machined from the center portion of X–Y plane with the gage length direction parallel to the Y-axis. Tensile testing was conducted at an initial strain rate of 8.3 × 10−4 s−1 at room temperature. Metallographic samples were mechanically polished with 200 and 400 grid emery paper, 9 and 1 m diamond suspensions and 0.05 m colloidal silica as final polishing. After thorough cleaning, the samples were etched for 15 s with a solution of 1 g oxaldehyde, 1 ml nitric acid, 1 ml acetic acid and 150 ml water before optical microscopy observation. 4. Finite element modeling
Fig. 2. The graphic view of RU die set-up for processing cylindrical AZ31 plates.
The repetitive upsetting process was simulated with threedimensional (3D) model using the commercial FE code DEFORM.
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Fig. 4. FE simulation of billet deformation process in the first RU route A pass at 350 ◦ C: von Mises strain contour plot of deformed billet at different steps.
During RU process, a flat-faced plunger was applied to push the AZ31 billet through the chamber die for a total displacement of 80 mm in the plunger at a constant speed of 4 mm/s at 350 ◦ C. The chamber die geometry was taken with chamber angle ˚ = 90◦ and corner angle = 0◦ . The round billet had a diameter of 100 mm and a height of 20 mm. The chamber die and plunger were modeled with rigid bodies. The billet was modeled as elastic-plastic material with Young’s modulus (E) of 45 GPa and Poisson’s ratio () of 0.35. The stress and strain relationship of the plastic deformation of AZ31 alloy at different strain rates were tabulated using the testing data generated by Gleeble 3500 thermo-simulator (Fig. 3). The billet was meshed with 6476 four node tetrahedron elements. A friction
Fig. 5. Comparison of finial sample shape obtained by (a) simulation and (b) experiment after the first RU route A pass at 350 ◦ C.
Fig. 6. Microstructure of the as-cast AZ31 alloy with an average grain size of ∼200 m.
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Fig. 7. Microstructures of the AZ31 alloy after RU route A at 350 ◦ C viewed from direction X (left column) and direction Y (right column) for (a, b) 1, (c, d) 3, and (e, f) 5 passes.
coefficient between the billet and the chamber die was assumed to be 0.3. 5. Results and discussion 5.1. Deformation behavior Repetitive upsetting of a round billet with a diameter of 100 mm and a height of 20 mm was conducted at 350 ◦ C and a nominal speed of 4 mm/s. The RU process was simulated by finite element software DEFORM for total displacement of 80 mm. Effective (von Mises) strain contour plots of deformed billet at different steps are shown in Fig. 4. In the initial step with a displacement around 2 mm, most region of the billet does not see strain except the top surface in contact with the plunger. At step 100 with a displacement of 20 mm, the top and the bottom of the billet are equally compressed to form a flatten surface. The compression in vertical
direction leads to the side surface flattening with constraint in the lateral direction due to volume conservation. The observation in Fig. 4b is typical during the billet filling the upper die by taking the shape of the upper chamber die. After the upper chamber die is filled, the billet is forced to flow through the 90◦ die-chamber inter-section into the lower chamber exposing a large amount of strain as shown in Fig. 4c. The material splits into two flows in the lower chamber and each flow is similar to a typical equal channel angular extrusion (ECAE) with a typical shear strain of 1.15 [14]. The material of the billet flows in the lower chamber and fills the bottom cylinder. When the material is in contact with the wall of the lower chamber, the material is forced to flow upwards exposing even higher strain and fills the remaining space in the lower chamber as shown in Fig. 4d. This suggests that it will be different in the next RU pass when the sample is rotated along X-axis (route A) or Y-axis (route B). It also suggests that more uniform microstructure can be achieved through route B. On
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Fig. 8. Microstructures of the AZ31 alloy after RU route B at 350 ◦ C viewed from direction Y for (a) 1, (b) 3, and (c) 5 passes.
average, the total strain induced into the billet is larger than that for ECAE. Results of FE simulations of one-pass upsetting are consistent with the experimental observations by comparison the final shape obtained from simulation and experiment in Fig. 5. This provides some indication on the validity of the FE simulation. 5.2. Microstructural evolution during RU processes The initial coarse grain structure of AZ31 is shown in Fig. 6 with a typical grain size around 200 m. The microstructural evolution of AZ31 alloy subjected to 1, 3 and 5 passes of RU route A at 350 ◦ C is shown in Fig. 7. Sampling was taken from both radial direction X and direction Y normal to the upsetting direction. After one pass, the average grain size is drastically refined to about 20 m in width viewed from direction X and 30 m in width viewed from direction Y. Several parallel shear bands are formed due to intense flow localization between columnar shaped large grains. The formation of shear band is to satisfy the requirements of grain compatibility due to limited active slip systems in Mg alloys [15]. Within the bands much finer grains ∼1 m and ultrafine grains have formed due to the occurrence of dynamic recrystallization. Stress concentrations within these shear bands may promote active slip process for further dynamic recrystallization resulting in the formation of uniform microstructures in Mg alloys [16]. During the first pass, there exists a few minor of coarse grains with a large volume fraction and a majority amount of fine and ultrafine grains, or an inhomogeneous bimodal grain structure. Coarse grains continue to refine, and finer and ultrafine grains grow with number of RU passes. After 3 RU passes the billet still exhibits a bimodal grain structure consisting of fine grains of ∼1–5 m and coarse grains of about ∼10–20 m in size. Similar grain distribution was observed in a ZKQX Mg alloy processed by conventional extrusion [17]. Although there is uniform strain flow during RU as shown in Fig. 4c and d, some grains with preferred orientations are deformed and refined first, leaving regions of less deformed and coarser grains due to the anisotropic
deformation of Mg alloys [18]. A general observation is that the coarse grains continue to refine and fine and ultrafine grains continue to grow. The bimodal grain structure is inherently transitional and eventually homogeneous microstructure shall be formed when deformation spreads to all the grains. After 5 RU route A passes, the microstructure becomes somewhat uniform with an average grain size of ∼2.6 m as shown in Fig. 7. Therefore, the grain refinement of AZ31 alloy is achieved by a combination of mechanical shearing and subsequent continuous recovery, recrystallization and growth of subgrain cells during SPD [19]. Similar observation of microstructure evolution is shown in Fig. 8 by RU route B for 1, 3, and 5 passes. During RU route B, the strain flow in the subsequent pass is in orthogonal to the previous pass and effectively breaks the columnar grain structure as shown in the 1st pass in Fig. 8. After 3 passes, the coarse grains are remarkably refined in route B with an averaged grain size about 2.7 m as shown in Fig. 8 compared to that in route A. More uniform microstructure with a finer grain size around 1.6 m is obtained after 5 RU route B passes. Microstructures of AZ31 alloy after 5 passes of route A RU at 250, 300 and 350 ◦ C are shown in Fig. 9. At 250 ◦ C, the AZ31 alloy has a bimodal grain structure of 40% (area fraction) relatively large grains (∼3–5 m) and 60% finer grains of ∼0.5–1 m. The grain size on average is determined to be ∼1.3 m. More homogeneous microstructure is obtained at 300 ◦ C with an equiaxed grain size of ∼1.8 m. Most uniform structure is achieved at 350 ◦ C although grain growth is more prominent and the measured grains size is about ∼2.6 m. Grain sizes of AZ31 alloy in the range of 1–3 m after 5 passes of RU route A are consistent with those obtained by ECAP after 4 passes at ◦ C in the literature [20,21]. The homogeneity in microstructure after 5 passes of RU route A increases when processing temperature increases from 250 to 300 to 350 ◦ C. This effect may be attributed to the significant drop in the normal anisotropy with temperature and a much more uniform deformation is achieved [22]. It suggests a higher processing temperature is better for homogeneous microstructure although the notable grain growth prohibits choosing higher processing temperature.
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Fig. 9. Microstructures of the AZ31 alloy after 5 RU route A passes at (a) 250 ◦ C, (b) 300 ◦ C, and (c) 350 ◦ C viewed from direction X.
Deformation temperature plays an important role in the formation of UFG structures by influencing the dislocation density, evolvement of dislocation cells and subgrain structures. The grain size of AZ31 alloy subjected to SPD is determined by the dynamic recrystallization process. Aforementioned results indicate that the higher deformation temperature in the range of present work, the larger the average grain size due to excessive grain growth, and the more equiaxed grain shape and homogeneous microstructure. The most uniform grain structure of ∼1.6 m is achieved by route B at 350 ◦ C and the finest average grain size of ∼1.3 m is obtained by route A at 250 ◦ C. The obtained grain size is consistent with an earlier report [18,23]. Xia et al. reported that a uniform structure with an average grain size of ∼3 m was formed in an AZ31 Mg alloy after 4 passes of ECAP at 200 ◦ C. When the same alloy was pressed at 150 ◦ C, it showed a bimodal structure with a mean grain size of ∼1.4 m. Lapovok et al. [23] also reported that a homogeneous structure was formed in a ZK60 Mg alloy after ECAP at 300 ◦ C but only bimodal grain distribution was attained when pressing at 200 ◦ C.
5.3. Mechanical properties enhancement with RU passes The room temperature tensile stress–strain curves of AZ31 alloy after 0, 1, 3 and 5 RU route A passes at 350 ◦ C are shown in Fig. 10a. Variation of yield strength (YTS), tensile strength (UTS) and elongation with pass number is correspondingly plotted in Fig. 10b. A general observation is that both YTS and UTS increase with the pass number. After 5 passes at 350 ◦ C, the tensile strength is 292 MPa and elongation is about 15.1%. Compared to ECAP processing, the strength and ductility after 5 RU route A passes at 350 ◦ C are similar to those obtained from ECAP at 200 ◦ C via route Bc for 8 passes [18]. After 1 pass, the yield strength considerably increases from 43 MPa to 127 MPa. This strength increase may be attributed to large internal stress and high dislocations density by the severe plastic strain during upsetting. The remarkable strength increase is continuously being observed after 3 RU route A passes and the
Fig. 10. Room temperature mechanical properties of the AZ31 alloy processed by RU route A at 350 ◦ C: (a) tensile stress–strain curves and (b) variation of the YTS, UTS and elongation with processing pass number.
W. Guo et al. / Materials Science and Engineering A 540 (2012) 115–122
Fig. 11. Room temperature mechanical properties of the AZ31 alloy processed by RU route B at 350 ◦ C: (a) tensile stress–strain curves and (b) variation of the YTS, UTS and elongation with processing pass number.
yield and tensile strength is 195 MPa and 286 MPa, respectively. The saturation of strength increase starts after more than 3 RU passes and a yield and tensile strength after 5 passes is measured 226 MPa and 292 MPa, respectively. A significant work hardening observed in the tensile loading of AZ31 alloy is possibly related to non-basal cross-slip dislocation [24]. There is a significant increase in yield strength but not in tensile strength after 5 passes compared to that after 3 passes. This may be attributed to the enhancement of mechanical properties from bimodal grain structure [25] formed after 3 passes since the averaged grain size after 5 passes of RU route A is much finer. Ductility increases with RU passes except a drop in the 1st pass. The reduction in elongation after the 1st pass is attributed to the inhomogeneous microstructure with coexisting of very large grains and ultrafine grains. It is prone to fail along the interface of two distinct regions with different size of grains. Similar tensile stress–strain curves of route B RU process at 350 ◦ C after 1, 3, and 5 passes are shown in Fig. 11. The enhancement of both yield strength and tensile strength with the number of passes is also observed. After 3 passes, the yield and tensile strengths are 209 MPa and 280 MPa, respectively, and after 5 passes are 246 MPa and 304 MPa, respectively. At the same time, remarkable elongation of 28% is achieved. The much improved strength also suggests that route B is more effective in grain refinement compared to route A. The strength enhancement from grain refinement is consistent with the common recognition that SPD is efficient grain refinement method for Mg alloys. However, the changing trend of strength in the RUed AZ31 alloy is not the same as an ECAPed AZ31 alloy [18], where strength initially decreased and then improved as the numbers of passes increased. For the ECAPed AZ31 alloy, the texture softening was more significant and it resulted in strength decrease at the initial several passes. Then the grain refining effect
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Fig. 12. Room temperature mechanical properties of the AZ31 alloy processed by 5 RU route A passes at 250, 300 and 350 ◦ C: (a) tensile stress–strain curves and (b) variation of the YTS, UTS and elongation with processing pass number.
dominated as the texture stabilized at the subsequent passes. In comparison, for the RUed AZ31 alloy, continuous improvement of the strength is dominated by the progressively refined grain size with increasing pass number. The tensile stress–strain curves of AZ31 alloy after 5 passes of RU route A at 250, 300 and 350 ◦ C are shown in Fig. 12a, and the yield and tensile strength and elongation are summarized in Fig. 12b. The mechanical properties are strongly influenced by RU temperature. The yield strength decreases from 294 MPa to 232 MPa to 226 MPa when processing temperature increases from 250 ◦ C to 300 ◦ C to 350 ◦ C. Similar trend for the tensile strength is observed and a maximum of 354 MPa is achieved at 250 ◦ C. On the other hand, the elongation increases with processing temperature and 15.1% elongation is obtained at 350 ◦ C. These observations are consistent with the microstructural evolution with RU process temperature. 6. Conclusions The as-cast AZ31 alloy with grain size ∼200 m was processed through repetitive upsetting (RU) route A and B at temperatures 250–350 ◦ C for 1, 3, and 5 passes. The following main conclusions are reached: (1) The 3D simulation by DEFORM portrays the detailed material flow in the RU process and indicates a homogeneous deformation. (2) The initial as-cast coarse microstructure with a grain size ∼200 m is significantly refined to inhomogeneous bimodal microstructure with columnar coarse grains and a majority
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amount of fine and ultrafine grains between columnar grains in the first pass. The inhomogeneous microstructure leads to decrease in elongation. (3) At 350 ◦ C a homogeneous grain structure is achieved after 5 passes with an averaged grain size of ∼2.6 m through route A and ∼1.6 m through route B. The microstructure initially is heterogeneous with a “bimodal” grain size distribution but becomes increasingly uniform with further passes. A tensile strength of 304 MPa and elongation of 28% are obtained in route B RU processing. (4) For the material processed at 350 ◦ C, both the yield and tensile strengths increase up to 5 passes through continuous grain refinement. Decreasing the processing temperature leads to a decrease in mean grain size and microstructural homogeneity. Finest mean size of ∼1.3 m is obtained and maximum YTS and UTS of 294 MPa and 354 MPa are achieved after 5 passes at 250 ◦ C. Acknowledgments The present study was supported by the National Natural Science Foundation of China (NSFC) under Grant No. 50674067 and No. 51074106, National Basic Research Program of China under Grant No. 61330, Key Hi-Tech Research and Development Program of China under Grant No. 2009AA033501, the Science and Technology Commission of Shanghai Municipality under Grant No. 09JC1408200, International Cooperation Fund of Shanghai Science and Technology Committee under Grant No. 06SR07104.
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Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Enhanced microstructure homogeneity and mechanical properties of AZ31 magnesium alloy by repetitive upsetting W. Guo a,b , Q.D. Wang a,b,∗ , B. Ye a,b , M.P. Liu a,b , T. Peng a,b , X.T. Liu a,b , H. Zhou a,b a b
National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, Shanghai 200240, China State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 31 August 2011 Received in revised form 7 January 2012 Accepted 24 January 2012 Available online 1 February 2012 Keywords: AZ31 magnesium alloy Severe plastic deformation Repetitive upsetting Microstructure Mechanical properties
a b s t r a c t An as-cast AZ31 Mg alloy in the form of thick plate (100 mm diameter and 20 mm height) with an initial grain size ∼200 m was processed by a novel severe plastic deformation technique, repetitive upsetting (RU). Experiments were carried out for 1, 3, and 5 deformation passes at 250, 300, and 350 ◦ C through route A and route B. Finite element analysis of the deformation process indicates homogeneous deformation during RU. With the number of RU passes, a finer grain size and more uniform microstructure are obtained along with significant improvement in both strength and ductility. A homogeneous grain structure with an averaged grain size of ∼2.6 m through route A and ∼1.6 m through route B are achieved after 5 passes at 350 ◦ C. A tensile strength of 304 MPa and elongation of 28% are obtained in route B RU processing. AZ31 alloy exhibits a mean grain size of 1.3 m and yield strength of 294 MPa and tensile strength of 354 MPa after 5 RU passes at 250 ◦ C as compared to initial 43 MPa and 174 MPa, respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Refining grain is often believed to be the most effective means to attain both the desired strength and ductility at room temperature [1]. These fine grains also provide the possibility for superplastic behavior at relatively lower temperature and higher strain rate. Among different methods in achieving fine grained materials, severe plastic deformation (SPD) is of more attractive by imposing intense strain on a sample without any change of its dimensions between passes. It has successfully fabricated bulk ultrafine-grained (UFG) materials without any porosity or contamination [2] in various shapes. Equal-channel angular pressing (ECAP) [3–5], cyclic extrusion compression (CEC) [6–8] and twist extrusion (TE) [9] have been developed for processing rods materials. High pressure torsion (HPT) [10], accumulative roll-bonding (ARB) [11] and repetitive corrugation straightening (RCS) [12] are utilized for preparing sheet materials. In addition, multi-directional forging (MDF) has been used for producing quadrangular samples [13]. However, these SPD technologies have some limitations and they are not able to process the material in the form of thick plates. In this report, we propose a novel SPD technique, repetitive upsetting (RU) process, for fabricating thick plates.
∗ Corresponding author at: National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, Shanghai 200240, China. Tel.: +86 21 54742715; fax: +86 21 34202794. E-mail address: [email protected] (Q.D. Wang). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2012.01.111
We analyze the deformation behavior in RU with finite element (FE) commercial software DEFORM considering realistic material behavior and processing parameters. We study the evolution of microstructure and mechanical properties of AZ31 alloy during RU processes at 250, 300 and 350 ◦ C for 1, 3 and 5 passes. Compared to other SPD methods, the RU process has the following characteristics: (1) larger deformation strain is imposed in each pass; (2) it allows to process less ductile materials due to the compressive deformation; (3) more uniform deformation is achieved because the transversal flow in a material is more abundant and multidirectional deformation is involved; (4) it is capable of producing large fully dense materials with UFG structure for real structural applications. 2. Principles of repetitive upsetting method The repetitive upsetting (RU) method can be used to process thick plates under cylindrical or rectangular configuration. The schematic representation of the RU process is shown in Fig. 1 for processing cylindrical thick plates. The corresponding graphic view of the RU apparatus is shown in Fig. 2. The apparatus consists of an upper rectangular cuboid chamber die, a lower cylindrical chamber die, and a rectangular cuboid plunger for external loading. The length of the upper chamber is equal to the diameter of the lower chamber. The upper chamber perpendicularly connects the lower chamber at the center of cylindrical lower chamber along the radial direction. A well-lubricated cylindrical billet is placed into the upper chamber and pressed into the lower chamber by
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Fig. 3. The compression stress–strain curves of as-cast AZ31 alloy at strain rates of 0.01, 0.1 and 1 s−1 at 350 ◦ C.
Fig. 1. The schematic representation of RU processing for fabricating cylindrical thick plates.
the plunger. During this upsetting process, the billet is simultaneously subjected to three different deformation modes, i.e. shear, extrusion, and compression. The shear strain is introduced at the connection of the upper and lower chamber. The extrusion and compressive strains are introduced by the pressure of upsetting deformation between the plunger and the lower chamber. When the cylindrical billet has the same dimension as the lower chamber, the dimension of the billet is kept constant but normal direction changes 90◦ from horizontal to vertical after upsetting. When the dimension of the billet remains unchanged, the upsetting can be repetitively applied (Fig. 1), and as a result, this repetitively process is called repetitive upsetting (RU). During RU, arbitrarily high shear strain is accumulated resulting in microstructure refinement. The uniform and multi-directional deformation can be achieved by rotating the billet 90◦ along either X-axis (route A) or Yaxis (route B) in the radial direction in Fig. 1 between each pass due to the stress direction and deformation path change. Deformation ability is also remarkably improved especially for Mg alloys because of compressive deformation characteristics in the RU process. 3. Experimental procedures A commercial AZ31 alloy with a nominal composition of 3 wt.% Al and 1 wt.% Zn was sectioned into cylindrical samples with a diameter of 100 mm and a height of 20 mm. RU was conducted on AZ31 round plate at 350 ◦ C and a speed of 4 mm/s for 1, 3, and 5 passes in an H-13 tool steel chamber die through route A and route B, respectively. RU was also carried out at 250, 300, and 350 ◦ C at a speed of 4 mm/s for 5 passes through route A. Graphite on the wall of the die was used as lubricant to reduce the friction between the billets and the die wall. Flat dog-bone tensile specimens (10 mm gage length, 3.5 mm gage width and 2 mm gage thickness) were created by electrical discharge machining. The specimen was machined from the center portion of X–Y plane with the gage length direction parallel to the Y-axis. Tensile testing was conducted at an initial strain rate of 8.3 × 10−4 s−1 at room temperature. Metallographic samples were mechanically polished with 200 and 400 grid emery paper, 9 and 1 m diamond suspensions and 0.05 m colloidal silica as final polishing. After thorough cleaning, the samples were etched for 15 s with a solution of 1 g oxaldehyde, 1 ml nitric acid, 1 ml acetic acid and 150 ml water before optical microscopy observation. 4. Finite element modeling
Fig. 2. The graphic view of RU die set-up for processing cylindrical AZ31 plates.
The repetitive upsetting process was simulated with threedimensional (3D) model using the commercial FE code DEFORM.
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Fig. 4. FE simulation of billet deformation process in the first RU route A pass at 350 ◦ C: von Mises strain contour plot of deformed billet at different steps.
During RU process, a flat-faced plunger was applied to push the AZ31 billet through the chamber die for a total displacement of 80 mm in the plunger at a constant speed of 4 mm/s at 350 ◦ C. The chamber die geometry was taken with chamber angle ˚ = 90◦ and corner angle = 0◦ . The round billet had a diameter of 100 mm and a height of 20 mm. The chamber die and plunger were modeled with rigid bodies. The billet was modeled as elastic-plastic material with Young’s modulus (E) of 45 GPa and Poisson’s ratio () of 0.35. The stress and strain relationship of the plastic deformation of AZ31 alloy at different strain rates were tabulated using the testing data generated by Gleeble 3500 thermo-simulator (Fig. 3). The billet was meshed with 6476 four node tetrahedron elements. A friction
Fig. 5. Comparison of finial sample shape obtained by (a) simulation and (b) experiment after the first RU route A pass at 350 ◦ C.
Fig. 6. Microstructure of the as-cast AZ31 alloy with an average grain size of ∼200 m.
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Fig. 7. Microstructures of the AZ31 alloy after RU route A at 350 ◦ C viewed from direction X (left column) and direction Y (right column) for (a, b) 1, (c, d) 3, and (e, f) 5 passes.
coefficient between the billet and the chamber die was assumed to be 0.3. 5. Results and discussion 5.1. Deformation behavior Repetitive upsetting of a round billet with a diameter of 100 mm and a height of 20 mm was conducted at 350 ◦ C and a nominal speed of 4 mm/s. The RU process was simulated by finite element software DEFORM for total displacement of 80 mm. Effective (von Mises) strain contour plots of deformed billet at different steps are shown in Fig. 4. In the initial step with a displacement around 2 mm, most region of the billet does not see strain except the top surface in contact with the plunger. At step 100 with a displacement of 20 mm, the top and the bottom of the billet are equally compressed to form a flatten surface. The compression in vertical
direction leads to the side surface flattening with constraint in the lateral direction due to volume conservation. The observation in Fig. 4b is typical during the billet filling the upper die by taking the shape of the upper chamber die. After the upper chamber die is filled, the billet is forced to flow through the 90◦ die-chamber inter-section into the lower chamber exposing a large amount of strain as shown in Fig. 4c. The material splits into two flows in the lower chamber and each flow is similar to a typical equal channel angular extrusion (ECAE) with a typical shear strain of 1.15 [14]. The material of the billet flows in the lower chamber and fills the bottom cylinder. When the material is in contact with the wall of the lower chamber, the material is forced to flow upwards exposing even higher strain and fills the remaining space in the lower chamber as shown in Fig. 4d. This suggests that it will be different in the next RU pass when the sample is rotated along X-axis (route A) or Y-axis (route B). It also suggests that more uniform microstructure can be achieved through route B. On
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Fig. 8. Microstructures of the AZ31 alloy after RU route B at 350 ◦ C viewed from direction Y for (a) 1, (b) 3, and (c) 5 passes.
average, the total strain induced into the billet is larger than that for ECAE. Results of FE simulations of one-pass upsetting are consistent with the experimental observations by comparison the final shape obtained from simulation and experiment in Fig. 5. This provides some indication on the validity of the FE simulation. 5.2. Microstructural evolution during RU processes The initial coarse grain structure of AZ31 is shown in Fig. 6 with a typical grain size around 200 m. The microstructural evolution of AZ31 alloy subjected to 1, 3 and 5 passes of RU route A at 350 ◦ C is shown in Fig. 7. Sampling was taken from both radial direction X and direction Y normal to the upsetting direction. After one pass, the average grain size is drastically refined to about 20 m in width viewed from direction X and 30 m in width viewed from direction Y. Several parallel shear bands are formed due to intense flow localization between columnar shaped large grains. The formation of shear band is to satisfy the requirements of grain compatibility due to limited active slip systems in Mg alloys [15]. Within the bands much finer grains ∼1 m and ultrafine grains have formed due to the occurrence of dynamic recrystallization. Stress concentrations within these shear bands may promote active slip process for further dynamic recrystallization resulting in the formation of uniform microstructures in Mg alloys [16]. During the first pass, there exists a few minor of coarse grains with a large volume fraction and a majority amount of fine and ultrafine grains, or an inhomogeneous bimodal grain structure. Coarse grains continue to refine, and finer and ultrafine grains grow with number of RU passes. After 3 RU passes the billet still exhibits a bimodal grain structure consisting of fine grains of ∼1–5 m and coarse grains of about ∼10–20 m in size. Similar grain distribution was observed in a ZKQX Mg alloy processed by conventional extrusion [17]. Although there is uniform strain flow during RU as shown in Fig. 4c and d, some grains with preferred orientations are deformed and refined first, leaving regions of less deformed and coarser grains due to the anisotropic
deformation of Mg alloys [18]. A general observation is that the coarse grains continue to refine and fine and ultrafine grains continue to grow. The bimodal grain structure is inherently transitional and eventually homogeneous microstructure shall be formed when deformation spreads to all the grains. After 5 RU route A passes, the microstructure becomes somewhat uniform with an average grain size of ∼2.6 m as shown in Fig. 7. Therefore, the grain refinement of AZ31 alloy is achieved by a combination of mechanical shearing and subsequent continuous recovery, recrystallization and growth of subgrain cells during SPD [19]. Similar observation of microstructure evolution is shown in Fig. 8 by RU route B for 1, 3, and 5 passes. During RU route B, the strain flow in the subsequent pass is in orthogonal to the previous pass and effectively breaks the columnar grain structure as shown in the 1st pass in Fig. 8. After 3 passes, the coarse grains are remarkably refined in route B with an averaged grain size about 2.7 m as shown in Fig. 8 compared to that in route A. More uniform microstructure with a finer grain size around 1.6 m is obtained after 5 RU route B passes. Microstructures of AZ31 alloy after 5 passes of route A RU at 250, 300 and 350 ◦ C are shown in Fig. 9. At 250 ◦ C, the AZ31 alloy has a bimodal grain structure of 40% (area fraction) relatively large grains (∼3–5 m) and 60% finer grains of ∼0.5–1 m. The grain size on average is determined to be ∼1.3 m. More homogeneous microstructure is obtained at 300 ◦ C with an equiaxed grain size of ∼1.8 m. Most uniform structure is achieved at 350 ◦ C although grain growth is more prominent and the measured grains size is about ∼2.6 m. Grain sizes of AZ31 alloy in the range of 1–3 m after 5 passes of RU route A are consistent with those obtained by ECAP after 4 passes at ◦ C in the literature [20,21]. The homogeneity in microstructure after 5 passes of RU route A increases when processing temperature increases from 250 to 300 to 350 ◦ C. This effect may be attributed to the significant drop in the normal anisotropy with temperature and a much more uniform deformation is achieved [22]. It suggests a higher processing temperature is better for homogeneous microstructure although the notable grain growth prohibits choosing higher processing temperature.
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Fig. 9. Microstructures of the AZ31 alloy after 5 RU route A passes at (a) 250 ◦ C, (b) 300 ◦ C, and (c) 350 ◦ C viewed from direction X.
Deformation temperature plays an important role in the formation of UFG structures by influencing the dislocation density, evolvement of dislocation cells and subgrain structures. The grain size of AZ31 alloy subjected to SPD is determined by the dynamic recrystallization process. Aforementioned results indicate that the higher deformation temperature in the range of present work, the larger the average grain size due to excessive grain growth, and the more equiaxed grain shape and homogeneous microstructure. The most uniform grain structure of ∼1.6 m is achieved by route B at 350 ◦ C and the finest average grain size of ∼1.3 m is obtained by route A at 250 ◦ C. The obtained grain size is consistent with an earlier report [18,23]. Xia et al. reported that a uniform structure with an average grain size of ∼3 m was formed in an AZ31 Mg alloy after 4 passes of ECAP at 200 ◦ C. When the same alloy was pressed at 150 ◦ C, it showed a bimodal structure with a mean grain size of ∼1.4 m. Lapovok et al. [23] also reported that a homogeneous structure was formed in a ZK60 Mg alloy after ECAP at 300 ◦ C but only bimodal grain distribution was attained when pressing at 200 ◦ C.
5.3. Mechanical properties enhancement with RU passes The room temperature tensile stress–strain curves of AZ31 alloy after 0, 1, 3 and 5 RU route A passes at 350 ◦ C are shown in Fig. 10a. Variation of yield strength (YTS), tensile strength (UTS) and elongation with pass number is correspondingly plotted in Fig. 10b. A general observation is that both YTS and UTS increase with the pass number. After 5 passes at 350 ◦ C, the tensile strength is 292 MPa and elongation is about 15.1%. Compared to ECAP processing, the strength and ductility after 5 RU route A passes at 350 ◦ C are similar to those obtained from ECAP at 200 ◦ C via route Bc for 8 passes [18]. After 1 pass, the yield strength considerably increases from 43 MPa to 127 MPa. This strength increase may be attributed to large internal stress and high dislocations density by the severe plastic strain during upsetting. The remarkable strength increase is continuously being observed after 3 RU route A passes and the
Fig. 10. Room temperature mechanical properties of the AZ31 alloy processed by RU route A at 350 ◦ C: (a) tensile stress–strain curves and (b) variation of the YTS, UTS and elongation with processing pass number.
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Fig. 11. Room temperature mechanical properties of the AZ31 alloy processed by RU route B at 350 ◦ C: (a) tensile stress–strain curves and (b) variation of the YTS, UTS and elongation with processing pass number.
yield and tensile strength is 195 MPa and 286 MPa, respectively. The saturation of strength increase starts after more than 3 RU passes and a yield and tensile strength after 5 passes is measured 226 MPa and 292 MPa, respectively. A significant work hardening observed in the tensile loading of AZ31 alloy is possibly related to non-basal cross-slip dislocation [24]. There is a significant increase in yield strength but not in tensile strength after 5 passes compared to that after 3 passes. This may be attributed to the enhancement of mechanical properties from bimodal grain structure [25] formed after 3 passes since the averaged grain size after 5 passes of RU route A is much finer. Ductility increases with RU passes except a drop in the 1st pass. The reduction in elongation after the 1st pass is attributed to the inhomogeneous microstructure with coexisting of very large grains and ultrafine grains. It is prone to fail along the interface of two distinct regions with different size of grains. Similar tensile stress–strain curves of route B RU process at 350 ◦ C after 1, 3, and 5 passes are shown in Fig. 11. The enhancement of both yield strength and tensile strength with the number of passes is also observed. After 3 passes, the yield and tensile strengths are 209 MPa and 280 MPa, respectively, and after 5 passes are 246 MPa and 304 MPa, respectively. At the same time, remarkable elongation of 28% is achieved. The much improved strength also suggests that route B is more effective in grain refinement compared to route A. The strength enhancement from grain refinement is consistent with the common recognition that SPD is efficient grain refinement method for Mg alloys. However, the changing trend of strength in the RUed AZ31 alloy is not the same as an ECAPed AZ31 alloy [18], where strength initially decreased and then improved as the numbers of passes increased. For the ECAPed AZ31 alloy, the texture softening was more significant and it resulted in strength decrease at the initial several passes. Then the grain refining effect
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Fig. 12. Room temperature mechanical properties of the AZ31 alloy processed by 5 RU route A passes at 250, 300 and 350 ◦ C: (a) tensile stress–strain curves and (b) variation of the YTS, UTS and elongation with processing pass number.
dominated as the texture stabilized at the subsequent passes. In comparison, for the RUed AZ31 alloy, continuous improvement of the strength is dominated by the progressively refined grain size with increasing pass number. The tensile stress–strain curves of AZ31 alloy after 5 passes of RU route A at 250, 300 and 350 ◦ C are shown in Fig. 12a, and the yield and tensile strength and elongation are summarized in Fig. 12b. The mechanical properties are strongly influenced by RU temperature. The yield strength decreases from 294 MPa to 232 MPa to 226 MPa when processing temperature increases from 250 ◦ C to 300 ◦ C to 350 ◦ C. Similar trend for the tensile strength is observed and a maximum of 354 MPa is achieved at 250 ◦ C. On the other hand, the elongation increases with processing temperature and 15.1% elongation is obtained at 350 ◦ C. These observations are consistent with the microstructural evolution with RU process temperature. 6. Conclusions The as-cast AZ31 alloy with grain size ∼200 m was processed through repetitive upsetting (RU) route A and B at temperatures 250–350 ◦ C for 1, 3, and 5 passes. The following main conclusions are reached: (1) The 3D simulation by DEFORM portrays the detailed material flow in the RU process and indicates a homogeneous deformation. (2) The initial as-cast coarse microstructure with a grain size ∼200 m is significantly refined to inhomogeneous bimodal microstructure with columnar coarse grains and a majority
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amount of fine and ultrafine grains between columnar grains in the first pass. The inhomogeneous microstructure leads to decrease in elongation. (3) At 350 ◦ C a homogeneous grain structure is achieved after 5 passes with an averaged grain size of ∼2.6 m through route A and ∼1.6 m through route B. The microstructure initially is heterogeneous with a “bimodal” grain size distribution but becomes increasingly uniform with further passes. A tensile strength of 304 MPa and elongation of 28% are obtained in route B RU processing. (4) For the material processed at 350 ◦ C, both the yield and tensile strengths increase up to 5 passes through continuous grain refinement. Decreasing the processing temperature leads to a decrease in mean grain size and microstructural homogeneity. Finest mean size of ∼1.3 m is obtained and maximum YTS and UTS of 294 MPa and 354 MPa are achieved after 5 passes at 250 ◦ C. Acknowledgments The present study was supported by the National Natural Science Foundation of China (NSFC) under Grant No. 50674067 and No. 51074106, National Basic Research Program of China under Grant No. 61330, Key Hi-Tech Research and Development Program of China under Grant No. 2009AA033501, the Science and Technology Commission of Shanghai Municipality under Grant No. 09JC1408200, International Cooperation Fund of Shanghai Science and Technology Committee under Grant No. 06SR07104.
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