Effect of friction stir welding on microstructure and wear properties of 7022 aluminium alloy

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 21, October 2014, pp Effect of friction stir welding on microstructure and wear properties of 7022 aluminium alloy H F Wang a,b *, J L Wang a, D W Zuo b, W W Song a & Xinglin Duan a,b a College of Mechanical & Electrical Engineering, Huangshan University, Huangshan , China b College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing , China Received 31 October 2013; accepted 1 July 2014 The 7022 aluminium alloy generally presents low weldability by traditional fusion welding. The development of the friction stir welding (FSW) has provided an alternative improved way of satisfactorily producing 7022 aluminium alloy welded components. The influence on microstructure of welding parameters and wear properties of 7022 aluminium alloy joints produced by FSW are analysed. From the investigation it is found that welded joints fabricated at a rotational speed of 400 rpm and a welding speed of 100 mm/min have better wear properties compared to traditional types of such joints. Keywords: Friction stir welding, Microstructure, Wear properties, 7022 aluminium alloy The 7022 aluminium alloy is divided into casting and deformation types, it has good mechanical properties, high electrical and thermal conductivities, good corrosion and wear resistances; deformation aluminium alloy can withstand pressure during processing, its mechanical properties are better than those of casting aluminium alloy for this purpose 1. It is mainly used in the manufacture of aviation components, general parts, door and window frames, etc. The widespread application of 7022 aluminium alloy is limited due to inadequate development of secondary processes such as machining and joining. Joining of 7022 aluminium alloy is still a challenge. Reports of 7022 aluminium alloy joining are rarely seen. The 7022 aluminium alloy is a high strength aluminium alloy, it is welded by traditional fusion welding which makes it difficult to meet out-turn product quality requirements. Researchers explored other methods and friction stir welding (FSW) was found to be a potential candidate for high strength aluminium alloy joining. In 1991, The Welding Institute (TWI) invented FSW. The rotating tool is non-consumable, harder than the base material, and plunged into the abutting edges of the plates to be joined under sufficient axial force and advanced along the line of the joint. The tool has two parts: shoulder and pin. The material around the pin is softened due to the frictional heat *Corresponding author (E-mal: wanghnfeng@163.com) generated by the tool rotation. Along the welding direction, the tool pushes plastically deformed material from the front to the back of the tool and then forges to complete the welding seam. The welding process is accomplished in the solid state 2-9. Some studies on FSW of high strength aluminium alloy were reported in the literature Fuller et al. 10 studied naturally aged 7050 and 7075 Al FSW, and thought that, in the range of aging times tested, transverse tensile strengths of 7050-T7651 and 7075-T651 Al alloy FSW increased with the faster tool travel speeds. Li et al. 11 analysed the effect of friction welding parameters on joints in 7075 Al and AZ31BMg, and obtained the best joints at a rotational frequency of 13 rad/s and a welding speed of 30 mm/min. Rajakumar et al. 12 fabricated square butt joints of AA7075-T6 by varying process parameters and tool parameters, and found that the joint fabricated at a tool rotational speed of 1400 rpm, a welding speed of 60 mm/min, an axial force of 8 kn, using a tool with a 15 mm shoulder diameter, a 5 mm pin diameter, and 45 HRC tool hardness yielded higher strength properties compared to other joints. Wang et al. 13 studied FSW of 5 mm 7075 aluminium alloy, and thought that the tensile strength was optimised when the rotational speed of the tool is 400 rpm at a welding speed of 40 mm/min. The aforementioned analysis shows that high strength aluminium alloy joining can be accomplished by FSW, and ensuing joint properties are good. The

2 558 INDIAN J ENG. MATER. SCI., OCTOBER 2014 authors trialled FSW technology to join 7022 aluminium alloy and evaluate the microstructure and wear properties of the butt joints thus formed. Experimental Procedure The chemical composition (% w/w) was: Si 0.35, Fe 0.41, Cu 1.43, Mg 2.52, Cr 0.15, Mn 0.1, Zn 5.43, and Al balance. The flat 7022 aluminium alloy plates (measuring 200 mm 100 mm 10 mm) were extrusion formed. The T6 heat treatment method was used. The welding area was cleaned by acetone and anhydrous alcohol. The butt welding of 7022 aluminium alloy was carried out automatically in an FSW machine built by China s FSW Centre (FSW-LM-A10). A tool made of tool steel with a threaded, tapering, pin profile was used. The joints were fabricated at tool rotational speeds of 300, 400, and 600 rpm, and welding speeds of 30, 50, and 100 mm/min. The axial force changed with tool rotational speed and welding speed. This is obtained by the experiment. The parameters were chosen on the basis of a need to avoid inducing defects in the welds. Typical FSW defects such as tunnels, porous material, and other flaws were observed in the welded plates. Specimens were cut from the welded plates to carry out microstructural and property characterisation. The specimen was prepared according to national standards. Images of the weld seam surface and macrostructure of the specimen were captured by digital camera. Before the microstructure was observed, the specimen was corroded using Kalle s Fig. 1 Macrostructure of the joint 600rpm, 50mm/min reagent. The microstructure was observed using an optical microscope (Shanghai Cai Kang Optical Instrument Co. Ltd (4XCE)) and a scanning electron microscope (HITACHI (s-4800)). The wear test was done using a steel ball-on-disc wear apparatus (Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences (HT-1000)) at room temperature without adding lubricant according to national standards. The Gr15 steel balls diameter was 6 mm, and the specimen was fixed on the disc. The test was carried out at a sliding rotational speed of 560 rpm, under normal force of 1.47 N and test duration of 5 min. A computer aided data acquisition system was used for friction coefficient data collection and polishing scratch width and depth measurement. The volumetric loss was computed by Eq. (1): 2π h r 2 2 Vw = (3h + 4 b ) (1) 6b where V w is the polishing scratch volumetric loss; h is the polishing scratch depth; b is the polishing scratch width; and r is the polishing scratch track radius. Wear test was done on the weld seam cross-section position. Results and Discussion Macrostructure and microstructure of welded joints The butt joining of 7022 aluminium alloy was successfully accomplished by FSW. The weld seam surface had no defects and presented a smooth quality surface finish. The weld seam surface had semi-circular features appearing as arc line shapes thereon. The various zones typically present in an FSW of 7022 aluminium alloy were visible. The macrostructure consisted of basic metal (BM), a heat affected zone (HAZ), a thermomechanically affected zone (TMAZ), and the nugget (N) (see Fig. 1): similar zones were observed in all such welded joints. Optical photomicrographs of the nugget was formed by the welding parameters for rotational speed 400 rpm and welding speed 30mm/min are shown in Fig. 2. Fig. 2 Optical photomicrographs in the nugget site (a) top, (b) middle and (c) bottom

3 WANG et al.: EFFECT OF FRICTION STIR WELDING ON 7022 ALLUMINIUM ALLOY 559 From top to bottom, the grain changed from coarse to fine. The main cause of this phenomenon was the top of the nugget became closed to the shoulder of the tool, and thus allowed greater frictional heat build-up, which led to grain growth at higher temperatures; the bottom of the nugget lay far from the shoulder, and was thus cooler because a portion of the frictional heat was exchanged, through air, by the workbench, which led to the grains in the bottom of the nugget suffering inhibited growth. In Fig. 2, the black particles (a strengthening phase) were found at the bottom of the nugget, there are few at the top of the nugget. This is due to high temperatures at the top of the nugget leading to the strengthening phase dissolving into the matrix. Optical photomicrographs of the same site in the nugget under different welding parameters are shown in Fig. 3. Figs 3(a) and (b) reveal that at a constant rotational speed, faster welding speeds produced finer grains: this was because the faster welding speeds resulted in minimal friction per unit time per unit distance, less frictional heating built-up, so grain growth was restricted. Figures 3(c) and (d) reveal that at a constant welding speed, larger rotational speeds generated greater frictional heat, and the grains grew. In Fig. 3, the grains appeared elongated, and as they were not typically isometric crystals, this indicated grain compression by the tool. The material in the TMAZ was affected by thermo-mechanical coupling. The material in the TMAZ changed to a plastic flow state at high temperatures, and the grain presented zonal distributions formed by the tool extrusion. Due to the pin surface of the tool being threaded, the material in the TMAZ generated plastic flow along the direction of the thread which led to the grains making a well-defined angle to the horizontal. The material close to the shoulder suffered greater frictional heat build-up, so recrystallisation grains could be seen herein, and the grains were larger than those in the nugget. Figure 4 shows that the flow directions of the material close to the nugget on the advancing side (AS) and retreating side (RS) were different. The material on the AS exhibited bottom-to-top flow, with an obvious boundary with the nugget present. The material on the RS exhibited a relative horizontal flow direction, and had a fuzzy boundary with the nugget. They were caused by the tool s rotational direction being the same as the material flow direction on the AS: the thread on the tool caused the material upward movement, the material flow direction was the same, thus generating a clear boundary. On the RS, the tool s rotational direction was not the same as the material flow direction (the force causing this came from the tool) and the BM and workbench acted on the material, the material flow direction became practically horizontal, thus causing the fuzzy boundary. The variation of the microstructure close to the shoulder in the TMAZ under different rotational speed conditions (600 rpm and 300 rpm) is shown in Fig. 5: isometric crystals were present on both the AS and RS with increasing tool rotation speed. More frictional heat was generated in the shoulder area, and Fig. 3 Optical photomicrographs of the nugget under different parameter (a) 300 rpm, 50 mm/min; (b) 300 rpm, 100 mm/min; (c) 600 rpm, 30 mm/min and (d) 300 rpm, 30 mm/min Fig. 4 Optical photomicrographs of TMAZ (300rpm, 50mm/min) (a) AS( 1500), (b) AS( 60), (c) RS( 1500) and (d) RS( 60)

4 560 INDIAN J ENG. MATER. SCI., OCTOBER 2014 with the tool s mechanical force, this led to the grains close to the shoulder in the TMAZ generating recrystallisation. With increasing welding speed, the welding time was reduced, faster heat dissipation ensued, less frictional heat build-up arose, and thus, recrystallisation was inhibited. This showed that the shoulder generated friction heat was larger than that of the pin of the tool, but the effect of the mechanical force was the same. The grain sizes in the TMAZs on the AS and RS were different, the grains on the AS were elongated, and larger than those on the RS; this was because the temperature on the AS was higher than that on the RS. Figure 6 reveals the microstructure in the HAZ under a rotational speed of 300 rpm and a welding speed 50 mm/min. The HAZ was only influenced by frictional heat, and no mechanical force, so the grain-generated plastic deformation was small, and each part s grain sizes were different in the HAZ because of differences in the heat distribution. In Fig. 6, the black reinforced phase was more prevalent on the AS than on the RS; this was caused by the temperature difference between the AS and RS. Sliding wear behaviour of welded joints The friction and wear SEM micrographs of specimens for various welding parameters are shown in Fig. 7. It can be seen that specimen friction and wear mechanism were mainly abrasive and adhesive wear, and the depth and width of the grinding crack under different parameters differed significantly. The cumulative sliding friction and cutting plastic deformation are shown in Fig. 7. Spherical particles were seen in the grinding crack, and a part of the spherical particles was crushed under frictional pressure and the edge of the grinding crack appeared furrowed which was caused by the uneven distribution of micro-hardness in the welding seam. From Fig. 7 it can be seen that wear extrusion peeling, and layer chipping were caused by the material s plastic extrusion. The average wear volume of the specimen under different process parameters is shown in Table 1. The wear volume in Table 1 is calculated according to Eq. (1). From Table 1, it can be seen that the microhardness can basically reflect the weld seam wear resistance, high hardness has good wearability, but the hardness is too high the wear resistance is reduced. That is the reason of the base metal of unwearable quality. The friction coefficient data are shown in Fig. 8. The friction coefficient trend followed that of the wear volume change. From Fig. 7, Table 1, and Fig. 8, Fig. 5 Optical photomicrographs of the TMAZ under different rotational speed (a) 300 rpm, 50 mm/min in AS, (b) 300 rpm, 50 mm/min in RS, (c) 600 rpm, 50 mm/min in AS and (d) 600 rpm, 50 mm/min in RS. Fig. 6 Optical photomicrographs of the HAZ under different rotational speed (a) 300 rpm, 50 mm/min in AS, (b) 300 rpm, 50 mm/min in RS

5 WANG et al.: EFFECT OF FRICTION STIR WELDING ON 7022 ALLUMINIUM ALLOY 561 Fig. 7 The friction and wear SEM micrographs of specimens (a) 300 rpm, 30 mm/min, (b) 300 rpm, 50 mm/min; (c) 300 rpm, 100 mm/min; (d) 400 rpm, 30 mm/min; (e) 400 rpm, 50 mm/min; (f) 400 rpm, 100 mm/min; (g) 600 rpm, 30 mm/min; (h) 600 rpm, 50 mm/min and (i) 600 rpm, 100 mm/min Table 1 Average wear volume of the specimen under different process parameters Specimen No. Rotational speed Welding speed Measurement times Average wear volume (mm 3 Average microhardness ) (r/min) (mm/min) (HV 0.2 ) a b c d e f g h i it can be seen that the specimen s resistance wear property was optimised for a tool rotational speed of 400 rpm and a welding speed of 100 mm/min. This was because these welding parameters produced finer isometric crystals than other welding parameters on the same welding area. These were the best FSW process parameters for an 7022 aluminium alloy. Fig. 8 Friction coefficient Conclusions FSW was successfully applied to the joining of 7022 aluminium alloy. The microstructure, tensile, micro-hardness, and sliding wear behaviour of the joints were analysed. The following conlusion can be drawn from this study:

6 562 INDIAN J ENG. MATER. SCI., OCTOBER 2014 (i) The microstructure of FSW 7022 aluminium alloy was divided into four zones: N, TMAZ, HAZ, and BM. (ii) The grain size changed from coarse to fine from top-to-bottom of the nugget. (iii) In the nugget, the rotational speed was constant, a faster welding speed produced finer grains; when the welding speed was constant, a higher rotational speed produced coarser grains. The grains were compressed by the tool, and appeared flat. (iv) In the TMAZ, the grains appeared in a zonal distribution influenced by the tool extrusion, and the grains were larger than those in the nugget. The flow directions of the material close to the nugget on the AS and RS were different. The TMAZ had an obvious boundary with the nugget on the AS, and a fuzzy boundary on its RS. (v) The microstructure close to the shoulder in the TMAZ for different rotational speeds can be seen as isometric crystals presented on both the AS and RS with increasing tool rotational speed. (vi) In the HAZ, the grain-generated plastic deformation was small, and each part s grain sizes were different. (vii) The welded joints friction and wear mechanisms were mainly abrasive and adhesive. The wear volume of the welded joint was inversely proportional to the micro-hardness. Acknowledgments This study is supported by National Natural Science Foundation of China (Grant No and Grant No ); the Science Foundation for Post Doctorate Research from Jiangsu Province of China (Grant No C) and the Starting Foundation for Talents from Huangshan University of China (Grant No.2013xkjq003). References 1 Wang H F, Zuo D W, Shao D L, et al. J Aeronaut Mater, 31 (1) ( Huang C P, Xing L, Ke L M & Liu G P, Trans China Weld Inst, 30(12) (2009) Khorrami M S, Kazeminezhad M & Kokabi A H, Mater Sci Eng A, 543 (2012) Yan D Y, Wu A P, Slivanus J & Shi Q Y, Mater Des, 32 (2011) Palanivel P, Mathews P K, Murugan N & Dinaharan I, Mater Des, 40 (2012) Rajakumar S & Balasubramanian V, Mater Des, 2012, 40, Metz D F & Barkey M E, Int J Fatigue, 43 (2012) Guo J, Gougeon P & Chen X G, Mater Sci Eng A, 553 (2012) Xue P, Ni D R, Wang D, Xiao B L & Ma Z Y, Mater Sci Eng A, 528 (2011) Fuller C B, Mahoney M W, Calabrese M & Micona L, Mater Sci Eng A, 527 (2010) Li D, Sun M H & Cui Z Q, Trans China Weld Inst, 38 (8) (2011) Rajakumar S, Muralidharan C & Balasubramanian V, Mater Des, 32 (2011) Wang X J & Sun G P, Aerospace Mater Technol, 38 (6) (2008)