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Effect of tool shoulder geometry on Microstructure and mechanical properties of pure copper in friction stir welding

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https://www.eduzhai.net American Journal of M aterials Science 2013, 3(5): 136-141 DOI: 10.5923/j.materials.20130305.05 Influence of Tool Shoulder Geometry on Microstructure and Mechanical Characterization of Friction Stir Welded Pure Copper L. Suvarna Raju*, A. Kumar Department of M echanical Engineering, National Institute of Technology, Warangal, A.P., India Abstract The influence of tool shoulder geometry on microstructure and mechanical characterizat ion of friction stir welded copper was studied. The different tool shoulder geometries such as flat, concave and convex with square tool pin profile were used to fabricate the joints. The experimental results revealed that the defect free joints could be obtained by using three different tool shoulders. Fro m the investigation, it is found that the jo ints made with concave shoulder with square tool pin profile resulted in better mechanical propert ies compared to flat and convex tool shoulder geometries. Dry ice treatment was carried out on the joints made by concave shoulder with square tool pin profile and its mechanical p roperties were studied. Fro m the result, it is observed that the joints made with concave shoulder with square tool pin profile with dry ice treat ment exhib ited better mechanical properties than without dry ice treat ment. The observed results were correlated with the microstructure and fracture features. Keywords Frict ion St ir Welding, Tool Shoulder Geo metries, Microstructure, Mechanical Properties, Fracture Features, Dry Ice 1. Introduction Frict ion st ir weld in g (FSW ) is a so lid st at e jo in in g technique invented by The Welding Institute(TWI), United Kingdom in 1991[1]. FSW is a continuous process, in which the rotating tool is plunged into the materia l at high rotating speed. The heat generated by the frict ion creates a p lasticized region around the immersed portion of the tool. The shoulder of the tool is pressed against the surface of the materia l, thus g en erat es frict ion al h eat and th e p in p ro v id es so me additional heat to the work piece as we ll as in preventing the plasticized material being expelled fro mthe weld. The tool is traversed along the joint line, forcing the plasticized material to coalescence behind the tool to form a solid phase joint. Copper and its alloys are the most important engineering materials due to their good ductility, corrosion resistance, electrical and thermal conductivity[2]. Weld ing of copper is usually difficult by conventional fusion welding processes because copper has high thermal d iffusivity (401 W/m. K) which is about 10 to100 times higher than steels and nickel alloys. FSW is energy efficient, environ mental friendly, less distort ion , faster weld ing speeds than t rad it ional fus ion weld ing techniques and to join materials that are d ifficu lt to * Corresponding author: rajumst@gmail.com (L. Suvarna Raju) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved fusion weld[3] and also this technique generates no fumes or requires any shielding gases[4]. Ho wever, copper is used in containment canisters for nuclear waste has been manufactured via FSW process[5]. Fabrication of backing plates of copper alloys was used for the sputtering equipments[6]. Dry ice is the solid form of carbon dio xide (CO2). It closely resemb les normal water ice, but possesses different properties. It contains no water, non toxic, non flammable. When energy is applied, dry ice is directly converts into its gaseous state without liquefying. The FSW tool shoulders are usually designed to produce majority of the heat through friction and material deformation to the surface and subsurface of the work p iece and it also produces the downward forging act ion necessary for the weld consolidation. The design of the shoulder and of the pin is very impo rtant for the quality of the weld. The p in of the tool generates the heat and stirs the material being welded but the shoulder plays an important part by providing additional frictional heat as we ll as preventing the plasticized material being escaped fro m the weld zone (WZ). The plasticized material is extruded fro m the leading edge to the trailing edge of the tool but is trapped by the shoulder which moves along the weld to produce a smooth surface fin ish and also tool shoulders restricts metal flow upward. Shoulder geometry (shape) is one of the most important parameter in FSW for the heat generation and flow o f the plasticized material. Zhang et al[7] conducted a test on influence of shoulder size on the material deformat ion in FSW of American Journal of M aterials Science 2013, 3(5): 136-141 137 alu min iu m and based on numerical results, they concluded that the stir zone of the welds can be enlarged by increasing the shoulder size using the same pin dia meter. Ka ilas et al[8] investigated on the role of tool design in influencing the mechanis m for the format ion of friction stir welds in alu minu m alloy 7020 and reported that the 6mm diameter pin and the corresponding diameter o f 15 mm for the shoulder produced defect free weld with the same pin profile with crit ical value ratio as 2.5[shoulder diameter (D)/ p in diameter (d)]. E.T.Akin labi[9] studied the effect of shoulder size on we ld properties of dissimila r meta l frict ion stir we lds and reported that the tool having 18 mm shoulder diameter with threaded pin profile showed higher efficiency co mpared to 15 mm and 25 mm shoulder diameter tools with the weld ing condition at 950 rp m and 50 mm/ min respectively. Fro m the reported literature, it was observed that the influence of tool shoulder geometries such as flat, concave and convex and also the effect of dry ice treatment on microstructure and mechanical properties of frict ion stir welded copper was not studied. Hence, the present study is aimed to investigate the influence of different tool shoulder geometries such as flat, convex and concave with square pin profile tools and also the effect of dry ice treatment using concave shoulder with square pin profile tool on microstructure and mechanical properties of copper weld ments. 2. Experimental Procedure The base metal (BM ) sheets of 3mm th ick pure copper was welded by butting two plates and stirring them together with a rotating tool assembly by using vertical milling machine (Make HMT FM-2, 10hp, 3000 rp m). Fig.1 is the schematic sketch showing the main principle of FSW process. were presented in Table 2. Table 1. Mechanical propert ies of base mat erial Mat erial UT S (MP a) YS (MP a) % El Micro hardness Impact Toughness Grain Size (Hv) (J) (µm) P ure copper 260 231 31 110 18 21 Table 2. Welding parameters and tool dimensions Process parameter Rotational speed (RPM) Welding speed (mm/min) Axial force (KN) Tool shoulder diameter, D (mm) Pin diameter, d (mm) D/d ratio of tool Shoulder geometries Pin length, L (mm) Tool t ilt angle, θ (degrees) Value 900 40 5 24 8 3 Flat, Concave, Convex 2.8 3 The trial experiments were conducted on FSW of copper by varying tool rotation speed and welding speed. The optimu m rotational speed and weld ing speeds are found to be 900 rp m and 40 mm/ min respectively, wh ich resulted in better mechanical properties. Hence, these welding parameters were kept constant[11] and by vary ing the tool shoulder geometries such as flat (FSSQ), convex (CXSQ) and concave (CCSQ) shaped with square pin profile tools (Fig.2) were used to fabricate the joints. Figure 2. Typical photographsof tools (a) FSSQ (b) CXSQ and (c) CCSQ tool pin profiles Figure 1. Schematic sketch of Friction Stir Welding showing the various characteristic regions Mechanical properties of the BM are presented in Table 1. H-13 tool steel is selected as tool materia l because of its high strength at elevated temperature, thermal fatigue resistance and high wear resistance[10]. The weld ing parameters and tool dimensions used to fabricate the joints in FSW process The jo ints were found to be defect free. The welding was carried out at roo m temperature. The intensive cooling was performed by pouring granulated dry ice (-30℃) on both sides of the surface plates fabricated by using CCSQ during the welding process. The surface morphology of FSW joints were shown in Fig.3. The specimens for metallographic examination were sectioned to the required size fro m the FSW jo ints in 138 L. Suvarna Raju et al.: Influence of Tool Shoulder Geometry on M icrostructure and M echanical Characterization of Friction Stir Welded Pure Copper transverse to the welding direction. The specimens were etched with a solution of 100 ml distilled water, 15 ml HCl and 2.5g ferric chloride. The microstructure of the weld zone (WZ) and the unaffected BM were examined with optical microscope (Model: Nikon, Make: Epiphot200). The grain size was measured by the mean linear intercept method. The fractured surfaces of the tensile and impact test specimens were analysed using a scanning electron microscope (SEMHitachi, SU 6600) to study the fracture surface morphology and to establish the nature of the fracture. of the specimens are clearly specified in these images, which represent a great reduction in the grain size and format ion of equiaxed grains in CCSQ co mpared to FSSQ and CXSQ. This is due to severe stirring and h igher heat generation in a concave shoulder contact surface. The high amount of plastic deformation and frictional heat generation between tool and BM takes place during stirring action. Figure 3. Surface morphologies of the of FSW joints made by different tool geometries (a) FSSQ (b) CXSQ (c) CCSQ[without dry ice treatment] and (d) CCSQ [with dry ice treatment] Specimens for tensile testing were taken in transverse to the weld direct ion and machined as per ASTM E8/ E8M-11 standards. Tensile test was conducted using computer controlled universal testing machine (Model: Autograph, Make: Sh imat zu) with a cross head speed of 0.5 mm/ min. Specimens for impact testing were taken in transverse to the weld direct ion and machined as per ASTM A 370 standards. The charpy ‘V’ notch impact test was conducted at room temperature using pendulum type impact testing mach ine. The amount of energy absorbed in fracture was recorded and the absorbed energy is defined as the impact toughness of the material. Specimens were cut at the middle of the joints in transverse direction for conducting microhardness survey. Microhardness test was carried out using Vickers digital microhardness tester (Model: Autograph, Make: Shimatzu) with a 10 g load for 10 s duration. The microhardness was measured at an interval o f 0.15 mm across the WZ, Thermo-Mechanical Affected Zone (TMAZ), Heat-Affected Zone (HAZ) and BM. 3. Results and Discussion 3.1. Microstructure Studies FSW is well known for its severe plastic deformat ion process. The stirring action was observed at the weld centre and thus produces finer grains. The d iscrepancy reflected significantly, d ifferent microstructure in the WZ produced by various shoulder geometries such as FSSQ, CXSQ and CCSQ are as shown in Fig.4 (a-e). Differences in grain size Figure 4. Microstructure of weld joints made by various shoulder geometries (a) FSSQ (b) CXSQ (c) CCSQ[without dry ice treatment] and (d) CCSQ[with dry ice treatment] and (e) Base metal This is attributed to the mechanism of constant dynamic recrystalization (DRX). The same phenomenon was observed by M. A issani et al[12]. The DRX usually occurs in WZ and the region is refined and reduces the grain size[13]. The average gra in size of 5.3, 6.1 and 3.5µm was observed in the WZ of all the joints fabricated by FSSQ, CXSQ and CCSQ respectively and the average grain size of BM is 21µm. All the weld ments resulted in a significant decrease in the grain size in the WZ as compared to BM. This is due to occurrence of higher DRX, in which the former grains of the BM are heavily strained and then recrystallized to a fine grain structure. DRX is of great industrial interest due to the new grains being smaller than the initial grains and thereby improving mechanical p roperties at roo m temperature. The TMAZ consists of a slightly elongated grain structure; it is due to the annealing affect[13, 11]. In the HAZ slightly coarse grains were observed when compared with the BM due to the heat generation during the welding process[14] Fig.4d shows the microstructure of the joint fabricated by CCSQ with dry ice treatment. It consists of very fine equiaxed grains co mpared to the joint fabricated by CCSQ without dry ice treat ment. This is due to the application o f the intensive cooling on the joints during the welding so that the grain refinement in the WZ results from a DRX that American Journal of M aterials Science 2013, 3(5): 136-141 139 proceeded during the welding process. The average grain size of 3.1 µm was observed in the WZ of the joint fabricated by CCSQ with dry ice treat ment. 3.2. Mechanical Properties Mechanical properties of the joints fabricated by three shoulder geometries with square pin profile tool we re shown in Tab le 3. The material serves as the start of the reservoir for the forging action of the shoulder and forward movement of the tool forces new material into the cavity of the shoulder, pushing the existing material to flow around the pin and forms defect free jo int, wh ich effects the grain refinement during the welding process. The tool shoulder essentially performs the ro le of the “lid on the pot”, which prevents the escape of softened plasticized material as the tool is rotated and forced along the joint[15]. The jo int fabricated by CCSQ profile tool with d ry ice t reat ment exh ibited superior tensile properties with a joint efficiency of 97% ([ult imate tensile strength of the weld / ultimate tensile strength of the base metal] X 100) co mpared to the other joints. This is due to the compressive forging force on the weld, thorough mixing of the material and also proper material flow takes place which is due to the presence of the concavity and influence of the cooling on the plates during the welding. Hence, the welds obtained with higher stirring and mixing action of tool lead to h igh strength in WZ wh ile simu ltaneously keeping a notable extent of ductility. Impact toughness of the FSW joints were evaluated. Fro m the results, it is observed that the joint made by CCSQ profile tool with dry ice treat ment exb itted better impact toughness compared to CCSQ without dry ice treatment. This is due to the formation of fine grain structure in the WZ. Microhardness survey was carried out on all the welded joints. The various hardness profiles of weld jo ints were presented in Fig.5. The hardness reduction was observed in the WZ, even though the grain size in the WZ is smaller than that of the BM except in the WZ of the joints fabricated by CCSQ with dry ice treatment. This phenomenon has been reported previously and was believed that the annealing effect was larger than the grain refine ment on the mechanical properties of the copper weld ments. The hardness survey at both the sides of the WZ, TMAZ and HAZ were presented. The transition between TMAZ and HAZ showed the lower hardness value, this is due to difference between microstructure of TMAZ and HAZ[11]. The hardness of the WZ is also influenced by annealing softening and grain refinement in pure metals[16]. Ho wever, at the WZ the highest hardness of 117 HV has been observed for the joints fabricated by CCSQ profile tool with dry ice treat ment compared to the joints made by CCSQ tool without dry ice treatment. The microhardness in the WZ increased with decreasing recrystallized grain size. Th is is due to the application of intensive cooling and format ion of finer grains in the WZ. The further refinement of the grains in the WZ, the fine g rain strengthening effect enhanced significantly and exceeded the annealing softening effect, which increased the hardness of the WZ than that of BM. Table 3. Mechanical propert ies of FSWed copper with different t ool shoulder geomet ries Tool Shoulder geometries FSSQ CXSQ CCSQ CCSQ(with dry ice t reatment ) UT S (MP a) 218 209 236 252 YS (MP a) 182 177 202 219 % El 16 14.3 23.1 27 Micro hardness (Hv) 105 98 107 117 Impact Toughness (J) 16 14 16 17 Joint Efficiency (%) 85 81 91 97 Figure 5. Microhardness distribution of FSW joints made by different shoulder geometries with square tool pin profiles and the Base metal 140 L. Suvarna Raju et al.: Influence of Tool Shoulder Geometry on M icrostructure and M echanical Characterization of Friction Stir Welded Pure Copper 3.3. Fractography The fractured surfaces of the tensile and impact specimens of the BM were shown in Fig.6 (a&b ). It is observed that the fracture surface images of pure copper contain large voids and fine dimples which indicate ductile fracture. The fractured surfaces of tensile and impact specimens made by different tool shoulder geomet ries with square pin profile were studied and presented in Fig.7 (a-d) and Fig.8 (a-d) respectively. All the jo ints failed at the retreating side (RS) during tensile test. It can be found that all the specimens were fractured at the locations where hardness value is lowest, which matches well with the hardness measurement. Weld made by CCSQ profile tool with dry ice treat ment exh ibited superior ductility as co mpared with the other jo ints made by CCSQ profile tool without dry ice treatment. This is due to presence of tiny shallow dimples and also some large dimples resulted from micro dimp les coalescence. It could be attributed to the high plastic deformation which indicates more intense ductile fracture. to formation of fine equiaxed grains in the WZ. • The weld joint fabricated using a CCSQ tool p in profile with dry ice treat ment possesses 97% joint efficiency as compared to the joints made by CCSQ tool pin profile with out dry ice treat ment. This is due to the influence of the cooling on the plates during the weld ing. • The smaller grain size was observed in the microstructure of WZ using a CCSQ (3.1 µm) tool pin profile with dry ice treatment compared to other joints such as FSSQ (5.3 µm), CXSQ (6.1 µm) and CCSQ (3.5 µm) without dry ice treat ment due to the continuous DRX. • The weld jo ints fabricated by CCSQ tool p in profile with dry ice t reatment possesses higher hardness than CCSQ tool pin profile with out dry ice treatment, this is due to intensive cooling and presence of smaller grains in the WZ. • Weld joints made by CCSQ profile tool with dry ice treatment exhib its superior ductility compared to other joints. This is attributed to the presence of tiny shallow dimp les and some large dimp les resulted from micro dimples co ales cen ce. Figure 6. Fracture surface of the tensile and impact specimens of base metal: (a) tensile (b) impact Figure 8. Fracture surface of impact specimens of weld joints (a) FSSQ (b) CXSQ (c) CCSQ[without dry ice treatment] and (d) CCSQ[with dry ice t reatment ] ACKNOWLEDGEMENTS Figure 7. Fracture surface of tensile specimensof weld joints (a) FSSQ (b) CXSQ (c) CCSQ[without dry ice treatment] and (d) CCSQ [with dry ice t reatment ] 4. Conclusions The authors would like to thank the authorities of National Institute of Technology (NIT)-Warangal, NIT-Calicut and Defence Metallurg ical Research Laboratory (DM RL) Hyderabad and also one of author (L.Suvarna Raju) thankful to the Principal and the management of KITS, Hu zurabad for their constant support during this work. The influence of various tool shoulder geometries with square pin p rofiles on microstructure and mechanical properties of friction stir welded copper was investigated. The main conclusions were drawn as fo llo ws: • The weld jo ints made by CCSQ tool pin profile with dry ice treat ment resulted in better mechanical properties than CCSQ tool pin profile with out dry ice t reat ment; this is due REFERENCES [1] Thomas W M , Nicholas E D, Needham J C, M urch M G, Templesmith P, Dawes C J, Great Britain Patent Application No. 9125978.8 December (1991). [2] Chinese Welding Society, Welding Handbook, 2nd ed., vol. 2, American Journal of M aterials Science 2013, 3(5): 136-141 141 China M achine Press, Beijing (2001) p 608-643. [10] Savolanen K, M ononen J, Saukkone H H, Koivula J, Friction [3] Sinclair P C, Longhurst W R, Cox C D, Lammlein D H, Strauss A M , Cook G E, Heated Friction Stir Welding, An Experimental and Theoretical Investigation into How Stir Weldability of copper Alloys, In: proceedings of the 5th International Conference on Friction Stir Welding, September M etz, France (2004). Preheating Influences Process Forces. Mater. and Manuf. 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