L. SONG et al.: MICROSTRUCTURE, TEXTURE AND MECHANICAL PROPERTIES ... 839–844 MICROSTRUCTURE, TEXTURE AND MECHANICAL PROPERTIES OF A FRICTION-STIR-PROCESSED Mg-Al-Ca-Mn-Zn ALLOY MIKROSTRUKTURA, TEKSTURA IN MEHANSKE LASTNOSTI TORNO-VRTILNO PROCESIRANE Mg-Al-Ca-Mn-Zn ZLITINE Leipeng Song 1 , Yang Zhang 1 , Yalin Lu 1,2* , Xingcheng Li 2 , Jian Wang 1 , Jiangtao Wang 2 1 School of Materials Engineering, Jiangsu University of Technology, No. 1801 Zhongwu Road, Changzhou, Jiangsu 213001, China 2 Key Laboratory of Advanced Materials Design and Additive Manufacturing of Jiangsu Province, No. 1801 Zhongwu Road, Changzhou, Jiangsu 213001, China Prejem rokopisa – received: 2019-04-28; sprejem za objavo – accepted for publication: 2019-06-27 doi:10.17222/mit.2019.090 Microstructure, texture and mechanical properties of a friction-stir-processed (FSPed) Mg-1Al-0.3Ca-0.3Mn-0.6Zn (w/%) alloy were investigated in the present work. During FSP, complete dynamic recrystallization (DRX) takes place in the coarse primary -Mg matrix and a considerable grain refinement is achieved. Furthermore, DRXed grain sizes increase with an increase of the rotation speed from 1000 min –1 to 1600 min –1 , which is attributed to the integrated effect of the strain rate and thermal input. After FSP, a strong {0001} basal texture is formed, with the intensity ranging from 83.64 to 105.19. Compared to the base metal, the elongation is significantly enhanced, by 158 %, while the ultimate tensile strength and yield strength are reduced. The variation in the mechanical properties is mainly due to the grain refinement and strong {0001} basal texture obtained with FSP. Keywords: Mg-Al-Ca-Mn-Zn alloy, friction-stir processing, microstructure, texture V ~lanku avtorji opisujejo raziskave mikrostrukture, teksture in mehanskih lastnosti torno-vrtilnega postopka (FSP) Mg-1Al-0,3Ca-0,3Mn-0,6Zn (v mas. %) zlitine. Med FSP-postopkom je pri{lo do popolne dinami~ne rekristalizacije (DRX) v grobi primarni matrici -Mg in do znatnega udrobljenja mikrostrukture. Nadalje se je velikost dinami~no rekristaliziranih zrn pove~ala s pove~anjem hitrosti vrtenja orodja iz 1000 min –1 na 1600 min –1 , kar so pripisali celovitemu u~inku hitrosti defor- macije in vnosa toplote. Med izvedbo FSP-postopka je pri{lo do tvorbe mo~ne {0001} bazalne teksture, z intenziteto med 83,64 in 105,19. V primerjavi z osnovno zlitino je raztezek FSP-zlitine mo~no narasel (za pribli`no 158 %), medtem ko sta se natezna trdnost in meja te~enja zni`ali oz. zmanj{ali. Sprememba mehanskih lastnosti je predvsem posledica udrobljenja mikrostrukture in nastanka mo~ne {0001} bazalne teksture zaradi izvedbe FSP-postopka. Klju~ne besede: Mg-Al-Ca-Mn-Zn zlitina, torno-vrtilni postopek varjenja, mikrostruktura, tekstura 1 INTRODUCTION Magnesium alloys are recognized as environment- friendly materials with favorable properties including low density, high specific strength and stiffness, which make them fascinating candidates for applications in the fields of transportation and aerospace. 1,2 Friction-stir processing (FSP) attracted much attention in the past decades. 3,4 During FSP, dynamic recrystallization takes place in the nugget zone, leading to a significant grain refinement. The grain refinement plays the key role in enhancing the strength, which can be described with the Hall-Patch formula. 5,6 According to the previous referen- ces, the majority of FSPed Mg alloys show a moderate increase in the strength, while a fraction of them exhibit diametrically opposite results. 7,8 Besides the grain size, the texture was also found to have a significant influence on the comprehensive performance of wrought magne- sium alloys. 9,10 FSW leads to an enormous reduction in the grain-boundary misorientation of AZ31 alloys due to the low symmetry of the hexagonal close-packed (HCP) structure and limitations of the {0001} texture. 11 And the role of the texture in deteriorating the yield strength (YS) of FSPed magnesium alloys was also investigated. 12–13 T. Nakata et al. 14-15 reported that a complex micro- alloyed Mg-1.3Al-0.3Ca-0.4Mn (w/%) alloy with merits of low cost shows favorable mechanical properties after an extrusion, and a further addition of the Zn element was beneficial to the mechanical properties. However, there has been limited investigation on the FSP of the Mg-Al-Ca-Mn-Zn alloy. In the present study, a Mg-1Al-0.3Ca-0.3Mn-0.6Zn (w/%) alloy was subjected to FSP with different rotational speeds. The purpose of the present study was to investigate the variations of the microstructure, texture and mechanical properties of the FSPed Mg-1Al-0.3Ca-0.3Mn-0.6Zn (w/%) alloy. Materiali in tehnologije / Materials and technology 53 (2019) 6, 839–844 839 UDK 620.1:67.017:669.017.13 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(6)839(2019) *Corresponding author's e-mail: luyalin@163.com (Yalin Lu) 2 EXPERIMENTAL PART The alloy selected for the present investigation was Mg-1Al-0.3Ca-0.3Mn-0.6Zn in w/%, fabricated from high-purity Mg, Al, Zn (99.9 w/%) and Mg-20 w/%Ca, Mg-5 w/% Mn master alloys using a vacuum induction furnace with an argon atmosphere. Plates with a thick- ness of 8 mm were cut from the ingots. Then solid-solu- tion treatment was carried out at 410 °C for 24 h and followed by water quenching. The solid-solutioned base metal was designated as BM. During FSP, a tool with a shoulder of 15 mm in diameter and a cylindrical screw pin of 4 mm in root diameter and 4 mm in length were utilized. The tool tilt angle was kept constant at 2.5° and the tool plunge depth was 5 mm. FSP was carried out at different rotational speeds (1000, 1300 and 1600) min –1 , while the travel speed was kept as 60 mm/min. Microstructure obser- vation was conducted on the BM and FSPed alloy using an optical microscope (OM). The texture variation in the SZ of the FSPed alloy was examined using electron backscatter diffraction (EBSD). Tensile tests were implemented on a universal testing machine with a strain rate of 1×10 –3 s –1 along the processing direction. At least three specimens were processed for each condition to obtain the mean values of mechanical properties. Ten- sile-fracture morphologies were examined using scanning electron microscopy (SEM). 3 RESULTS AND DISCUSSION Figure 1 reveals the macrostructure of the transversal cross-section of an FSPed specimen. RS denotes the retreating side and AS denotes the advancing side. Based on macrostructural observations, the typical zones of an FSPed Mg alloy, including the stir zone (SZ), the thermo-mechanically affected zone (TMAZ) and BM are marked. Obvious boundaries between the violently stirred region and BM are observed, indicating the difference in the microstructure. L. SONG et al.: MICROSTRUCTURE, TEXTURE AND MECHANICAL PROPERTIES ... 840 Materiali in tehnologije / Materials and technology 53 (2019) 6, 839–844 Figure 2: Microstructure of the BM and the central area of the SZ of the FSPed specimens at different rotational speeds: a) BM, b) 1000 min –1 , c) 1300 min –1 and d) 1600 min –1 Figure 1: Cross-sectional macrostructure of an FSPed specimen Figure 2 shows the microstructure of the BM and the central area of the SZ of the FSPed specimens at diffe- rent rotational speeds. As shown in Figure 2a, the BM contains coarse -Mg grains with the mean grain size of ~75 μm. After FSP, a complete DRX took place in the coarse primary -Mg matrix and fine equiaxed grains were obtained. Compared to the BM, the grains of the FSPed specimens became considerably refined. A quan- titative analysis of the DRXed grain size was conducted following the EBSD analysis. Figure 3 shows the inverse pole-figure maps of the FSPed specimens at different rotational speeds obtained with EBSD. Black lines represent high-angle grain boun- daries (HAGB) with a misorientation angle larger than 15° and white lines represent low-angle grain boundaries (LAGB) with a misorientation angle smaller than 15° but larger than 2°. The EBSD analysis also confirmed that a complete DRX took place in these specimens and the DRXed grain size increased with an increase of the rotational speed from 1000 min –1 to 1600 min –1 . The mean grain sizes of the FSPed alloy were 2.64 μm for 1000 min –1 , 3.22 μm for 1300 r/min, and 5.20 μm for 1600 min –1 . In addition, the rotation of the DRXed grains took place during FSP, showing a visibly preferential orientation. DRX during FSP is promoted by the thermo-mechanical effect. The strain rate during FSP is greatly related to the rotational speed. 16 Besides the strain rate, the thermal input also has a significant influence on the microstructure evolution during FSP. 17 When the feed speed is kept constant, the increased rotational speed is accompanied by a great thermal input, which can induce a coarsening of DRXed grains. There- fore, the variation in the microstructure is the interac- tional outcome of the strain rate and thermal input. Figure 4 shows the {0001} pole figures of the FSPed specimens at different rotational speeds, where ND, PD and TD denote the normal direction, the processing direction and the transverse direction, respectively. As shown in Figure 4, a strong {0001} basal texture is formed in the FSPed specimens, with the texture inten- sity ranging from 83.64 to 105.19. Figure 5 shows the effect of the rotational speed on the tensile properties of the FSPed specimens and Table 1 exhibits detailed data of the tensile properties. The ultimate tensile strength (UTS), yield strength (YS) and elongation (EL) of the BM are 164 MPa, 82 MPa and 14.3 %, respectively. As shown in Figure 5, the strain-stress curve of the BM shows slip characteristics L. SONG et al.: MICROSTRUCTURE, TEXTURE AND MECHANICAL PROPERTIES ... Materiali in tehnologije / Materials and technology 53 (2019) 6, 839–844 841 Figure 4: Pole figures of the FSPed specimens at different rotational speeds: a) 1000 min –1 , b) 1300 min –1 , c) 1600 min –1 Figure 3: Inverse pole-figure maps of the FSPed specimens at diffe- rent rotational speeds: a) 1000 min –1 , b) 1300 min –1 , c) 1600 min –1 at a high YS and low EL, attributed to the initial grain orientation with a low Schmid factor for the basal slip or extension twinning. After FSP, the EL of the specimens increases dramatically, by 158 %. On the other hand, the YS has a different rate reduction, especially at 1600 min –1 . The YS at 1600 min –1 decreases by 40 % in comparison with the BM. The UTS of the FSPed speci- mens is also on the decline. However, the UTS and YS of the FSPed specimens decrease with an increase in the rotational speed. Table 1: Tensile properties of BM and FSPed specimens at different rotational speeds Specimens UTS /MPa YS /MPa Elongation /% BM 164 82 14.3 FSP-1000 min –1 143 79 37.4 FSP-1300 min –1 133 67 37.0 FSP-1600 min –1 129 53 36.8 Figure 6 shows SEM images of the tensile-fracture morphologies. As shown in Figure 6a), the tensile fracture of the BM is characterized by a typical inter- granular fracture including a cleavage fracture (indicated by a white arrow) and a large hollow (surrounded by a black frame) due to the separation of coarse grains. The orientation of the cleavage plane is related to the angle between the grain and the tensile axis. It can be seen in Figure 6b to 6d) that the FSP fracture surface is much flatter than the BM fracture surface, characterized by a fine grain fracture due to the unidirectional crack propa- gation along the banded structure. As shown in Figure 5 and Table 1, the application of FSP to the Mg-1Al-0.3Ca-0.3Mn-0.6Zn (w/%) alloy leads to an increase in the EL of about 158 % in compa- rison to the BM. As shown in Figure 2, compared to the BM, the grain sizes of the FSPed alloys are considerably L. SONG et al.: MICROSTRUCTURE, TEXTURE AND MECHANICAL PROPERTIES ... 842 Materiali in tehnologije / Materials and technology 53 (2019) 6, 839–844 Figure 5: Effect of rotational speed on the tensile properties of FSPed specimens Figure 6: SEM images of the tensile-fracture morphologies: a) BM, b) 1000 min –1 , c) 1300 min –1 and d) 1600 min –1 refined. It is well accepted that the grain refinement can activate non-basal slipping in magnesium alloys, which can enhance the plastic deformability, and then the EL increases accordingly. More importantly, the pole figures from Figure 4 show that after FSP, the FSPed alloy exhibits an extremely strong {0001} basal texture. The observed strong distinction in the intensity of texture components suggests that the dominant plastic deforma- tion mode is the slip, while the deformation in the direction is minor. It is also reported that a texture modification significantly improves the EL of magne- sium alloys due to the formation of a strong {0001} basal texture, moving the easy basal-slip system to the preferred orientation. 18 A similar report on a texture causing an improvement of the ductility was provided by M. Vargas. 19 However, the strength of the BM declines after FSP, which is not consistent with the Hall-Petch formula. A previous study of FSPed AZ31 suggested that soft orientation was the main reason of this phenomenon. 20 W. Yuan et al. 21 pointed out that the texture would affect the Hall-Petch relationship since soft orientation of a basal texture would cause a decrease in the strength, while hard orientation of a basal texture would make it difficult for the basal slip system to become activated. During FSP, a complete DRX and simultaneous grain rotation contribute to the soft orientation of the basal texture, causing a decrease in the UTS and YS. More- over, a strong {0001} texture can also weaken the work- hardening ability and result in a low UTS (Figure 5). Therefore, the effect of grain refinement on the strength is counteracted by the effect of the texture in some FSPed samples. However, as for FSPed alloys, since the effect of the texture on the strength is almost the same in this study, the difference in the grain size shows an obvious influence on the YS of FSPed specimens and the FSPed specimen at 1600 min –1 with the largest grain size exhibits the lowest YS among the FSPed specimens. 4 CONCLUSIONS In this study, the microstructure, texture and mecha- nical properties of an Mg-1Al-0.3Ca-0.3Mn-0.6Zn alloy after FSP are systematically investigated. The main con- clusions are as follows: 1) During FSP, a complete DRX takes place in the coarse primary -Mg matrix and a considerable micro- structural refinement is achieved. DRXed grain sizes increase with an increase of the rotational speed from 1000 min –1 to 1600 min –1 , which is attributed to the interactional outcome of the strain rate and thermal input. 2) After FSP, a strong {0001} basal texture is formed in the FSPed specimens, with the texture intensity ranging from 83.64 to 105.19. The texture intensity peaks of {0001} turn by approximately 30° from the ND towards the PD. 3) The EL of the FSPed specimens is significantly enhanced, by 158 %, while the YS and UTS are reduced. The variation in the mechanical properties is due to the grain refinement and strong {0001} basal texture obtained with FSP. Acknowledgment This research was funded by the National Natural Science Foundation of China (No. 51601076), Major Project of the Natural Science Foundation from the Jiangsu Higher Education Institutions (no. 17KJA430005, No. 18KJA430007), the Natural Science Foundation of the Jiangsu Higher Education Institutions (No. 16KJB430013) and Postgraduate Research & Practice Innovation Program of the Jiangsu Province (No. SJCX18_1043). 5 REFERENCES 1 A. Y. Zhang, R. Kang, H. C. Pan, L. Wu, H. C. Pan, H. B. Xie, Q. Y. Huang, Y. J. Liu, Z. R. Ai, L. F. Ma, Y. P. Ren, G. W. Qin, A new rare-earth-free Mg-Sn-Ca-Mn wrought alloy with ultra-high strength and good ductility, Mater. Sci. Eng. A, 754 (2019), 269–274, doi:10.1016/j.msea.2019.03.095 2 Y. F.Wang, F. Zhang, Y. T. Wang, Y. B. Duan, K. J. Wang, W. J. Zhang, J. Hu, Effect of Zn content on the microstructure and me- chanical properties of Mg-Gd-Y-Zr alloys, Mater. Sci. Eng. A., 745 (2019), 149–158, doi:10.1016/j.msea.2018.12.088 3 I. Charit, R. S. Mishra, Effect of friction stir processed micro- structure on tensile properties of an Al-Zn-Mg-Sc alloy upon subsequent aging heat treatment, J. Mater. Sci. Technol., 34 (2018), 214–218, doi:10.1016/j.jmst.2017.10.021 4 Y. Y. Jin, K. S. Wang, W. Wang, P. Peng, S. Zhou, L. Y. Huang, T. Yang, K. Qiao, B. Zhang, J. Cai, H. L. Yu, Microstructure and mechanical properties of AE42 rare earth-containing magnesium alloy prepared by friction stir processing, Mater. Charact., 150 (2019), 52–61, doi:10.1016/j.matchar.2019.02.008 5 Y. L. Lu, Y. Zhang, M. Q. Cong, X. C. Li, W. T. Xu, L. P. Song, Microstructure and mechanical properties of extruded AZ31-xCaO alloy, Materials, 11 (2018), 1–14, doi:10.3390/ma11081467 6 Y. Zhang, L. P. Song, X. Y. Chen, X. P. Li, Effect of Zn and Ca addition on microstructure and strength at room temperature of as-cast and as-extruded Mg-Sn alloy, Materials, 11 (2018), 1–14, doi:10.3390/ma11091490 7 Y. X. Huang, Y. B. Wang, X. C. Meng, L. Wan, J. Cao, L. Zhou, J. C. Feng, Dynamic recrystallization and mechanical properties of friction stir processed Mg-Zn-Y-Zr alloys, J. Mater. Process. Technol., 248 (2017), 331–338, doi:10.1016/j.jmatprotec.2017. 06.021 8 F. Khan MD, G. M. Karthik, S. K. Panigrahi, G. D. Janaki, Friction stir processing of QE22 magnesium alloy to achieve ultrafine- grained microstructure with enhanced room temperature ductility and texture weakening, Mater. Charact., 147 (2019), 365–378, doi:10.1016/j.matchar.2018.11.020 9 Q. Shang, D. R. Ni, P. Xue, B. L. Xiao, K. S. Wang, Z. Y. Ma, An approach to enhancement of Mg alloy joint performance by additional pass of friction stir processing, J. Mater. Process. Technol., 264 (2019), 336–345, doi:10.1016/j.jmatprotec.2018.09.021 10 N. Xu, Q. N. Song, Y. F. Bao, {10-12} twinning assisted micro- structure and mechanical properties modification of high-force friction stir processed AZ31B Mg alloy, Mater. Sci. Eng. A., 745 (2019) 4, 400–403, doi:10.1016/j.msea.2018.12.127 L. SONG et al.: MICROSTRUCTURE, TEXTURE AND MECHANICAL PROPERTIES ... Materiali in tehnologije / Materials and technology 53 (2019) 6, 839–844 843 11 S. Mironov, T. Onuma, Y. S. Sato, H. Kokawa, Microstructure evolution during friction-stir welding of AZ31 magnesium alloy, Acta Mater., 100 (2015), 301–312, doi:10.1016/j.actmat.2015.08.066 12 S. H. Chowdhury, D. L. Chen, S. D. Bhole, X. Cao, P. Wanjara, Friction stir welded AZ31 magnesium alloy: microstructure, texture, and tensile properties, Metall. Mater. Trans. A., 44 (2013)1 , 323–336, doi:10.1007/s11661-012-1382-3 13 R. L. Xin, B. Li, A. L. Liao, Z. Zhou, Q. Liu, Correlation between texture variation and transverse tensile behavior of friction- stir-processed AZ31 Mg alloy, Metall. Mater. Trans. A, 43 (2012)7, 2500–2508, doi:10.1007/s11661-012-1080-1 14 T. Nakata, C. Xu, R. Ajima, K. Shimizu, S. Hanaki, T. T. Sasaki, L. Ma, K. Hono, S. Kamado, Strong and ductile age-hardening Mg-Al-Ca-Mn alloy that can be extruded as fast as aluminum alloys, Acta Mater., 130 (2017), 261–270, doi:10.1016/j.actmat.2017.03.046 15 M. Z. Bian, T. T. Sasaki, B. C. Suh, T. Nakata, S. Kamado, K. Hono, A heat-treatable Mg-Al-Ca-Mn-Zn sheet alloy with good room temperature formability, Scr. Mater., 138 (2017), 151–155, doi:10.1016/j.scriptamat.2017.05.034 16 C. I. Chang, C. J. Lee, J. C. Huang, Relationship between grain size and Zener-Holloman parameter during friction stir processing in AZ31 Mg alloys, Scr. Mater., 51 (2004) 6, 509–514, doi:10.1016/ j.scriptamat.2004.05.043 17 M. Abbasi Gharacheh, A. H. Kokabi, G. H. Daneshi, B. Shalchi, R. Sarrafi, The influence of the ratio of "rotational speed/traverse speed" ( /v) on mechanical properties of AZ31 friction stir welds, Int. J. Mach. Tools. Manuf., 46 (2006) 15, 1983–1987, doi:10.1016/ j.ijmachtools.2006.01.007 18 W. Yuan, R. S. Mishra, Grain size and texture effect on deformation behavior of AZ31 magnesium alloy, Mater. Sci. Eng. A, 558 (2012), 716–724, doi:10.1016/j.msea.2012.08.080 19 M. Vargas, S. Lathabai, P. J. Uggowitzer, Y. Qi, D. Orlov, Y. Estrin, Microstructure, crystallographic texture and mechanical behaviour of friction stir processed Mg-Zn-Ca-Zr alloy ZKX50, Mater. Sci. Eng. A, 685 (2017), 253–264, doi:10.1016/j.msea.2016.12.125 20 F. Y. Hung, C. C. Shih, L. H. Chen, T. S. Lui, Microstructures and high temperature mechanical properties of friction stirred AZ31–Mg alloy, J. Alloy. Compd., 428 (2007) 1–2, 106–144, doi:10.1016/ j.jallcom.2006.03.033 21 W. Yuan, R. S. Mishra, B. Carlson, R. K. Mishra, R. Verma, R. Kubic, Effect of texture on the mechanical behavior of ultrafine grained magnesium alloy, Scr. Mater., 64 (2011) 6, 580–583, doi:10.1016/j.scriptamat.2010.11.052 L. SONG et al.: MICROSTRUCTURE, TEXTURE AND MECHANICAL PROPERTIES ... 844 Materiali in tehnologije / Materials and technology 53 (2019) 6, 839–844