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Investigation on friction stir welding parameter design for lap joining of pure titanium
Investigation on friction stir welding parameter design for lap joining of pure titanium F C Liu1, H Liu1,3, K Nakata1, N Yamamoto2, J Liao2 1 Joining and Welding Research Institute, Osaka University, Japan 2 Technology Development Headquarters, Kurimoto Ltd, Japan 3 State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, China
Abstract Friction stir welding (FSW) of pure titanium lap joint using three sized tools was investigated over a wide range of welding parameters. The ranges of the welding parameters were narrow, and out of the ranges led to the emergence of welding defects, such as overheating rough surface, groove-like surface and inner cavitations. The sound joints have sufficient bonding strength in the lap zone, which insured that the fracture occurred in the base metal after a large elongation. Increasing the probe length was beneficial to widen the connection width of the lap joints. Surface polishing prior FSW reduced the inner cavitations. Keywords: Friction stir welding, Titanium, Lap joint, Welding defect, Alloy
1. Introduction Ti and its alloys have been extensively used in the aerospace and chemical industry because of their high specific strength, excellent heat resistant, low density and natural corrosion resistance. Commercially pure (CP) Ti is used primarily for its corrosion resistance. It is also useful in applications requiring high ductility for fabrication but relatively low strength in service. With increasing the use of Ti and its alloys, the bonding of these alloys has become more and more important [1]. Generally, Ti and its alloys can be jointed through fusion welding methods like arc welding, laser welding, electron beam welding, etc. However, fluxed welding of Ti are not recommend because weld embitterment caused by picking up of nitrogen, oxygen, hydrogen from atmosphere, high residual stresses and distortion, particular in welding of thin plates, due to the low conductivity are usually encountered [2]. Friction stir welding (FSW), developed and patented in the UK in early 1990s by The Welding Institute, is an innovative solid state welding method [3,4]. It has received much attention due to the avoidance of solidification problems associated with the fusion welding techniques [4,5,6]. FSW was initially developed for joining low melting metals, such as Al, Mg and Cu alloys. During the past decade, increasingly more attention have been focused on the FSW of high melting metals, such as Ti alloys and steels, with the continued development of FSW. To date, there have only been a limited number of reports on FSW of CP Ti. For example, defect free welds were achieved when the 5.6 mm CP Ti plates were butt welded at a rotation speed of 1100 rpm and a welding speed of 500 mm/min [1]. CP Ti plates with 2 mm in thickness were successfully friction stir butt welded at
a rotation speed of 200 rpm combining with a welding speed range from 50 to 300 mm/min [2]. Very recently, CP Ti lap joints have been produced using FSW by Liu et al. [7,8]. These primary studies demonstrated that the optimum welding condition for the lap joining of CP Ti plates is quite narrow. Although early results have indicated that FSW was a potential welding method for obtaining defect-free joints in CP Ti, further works are still needed to understand the effect of welding conditions on the microstructural evolution and mechanical properties of FSW CP Ti. The lack of investigation in this field can be attributed to the difficulties in FSW of CP Ti. Firstly, CP Ti is difficult to be plasticised sufficiently during FSW because its hexagonal crystal structure has only a limited number of independent slip systems. Secondly, the shortcoming in the design and production of tools which withstand to the tool wear at typical FSW temperature has not yet been fully overcome. The high reactivity of Ti with many potential tool materials at high temperatures further limits the choice of tool materials. Thirdly, it is necessary to shield the joint area during FSW as the CP Ti is very reactive when the plates reach temperatures higher than 500oC. Though there exist many challenges in FSW of Ti and its alloys, the growing demand for joining of Ti and its alloys and the potential excellent mechanical properties of Ti joint using FSW drive us to go into the field FSW of Ti and its alloys further. In this investigation, the FSW of CP Ti lap joint was carried out using tools with various designs. The influences of welding parameters on FSW were investigated extensively and the microstructural evolution and mechanical properties of the joints were studied in detail.
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Fig. 1. Schematic illustration of the FSW tools with probes of: (a) 1.8 mm, (b) 2.0 mm and (c) 2.2 mm in length. 2. Experimental The 2-mm thick CP Ti sheets with a composition (in wt.%) of 0.01 C, 0.03 Fe, 0.01 N, 0.1 O, 0.001 H and Ti balance were used in this study. The FSW of lap joint was carried out using a sintered WC-Co tool (tilted at 3o from the vertical) consisting of a shoulder of 15 mm in diameter, a probe of 6 mm in diameter and 1.8, 2.0 and 2.2 mm in length, respectively, as shown in Fig. 1. During FSW, a load control system was employed and the plunging force was set to be 14.7 kN. The rotation speeds were varied from 200 to 400 rpm, and the welding speeds were at a range of 60 to 200 mm/min. The water cooling and argon shielding systems were utilized to cool the welding tool and minimize surface oxidation. After FSW, the lap joint was cross-sectioned by a wire electrical discharge cutting machine (HSC-300; Brother Ind. Ltd.). The cross-section was mechanically polished using water abrasive paper and finally with 1 mm diamond paste. The polished cross-section was
etched in a solution comprising of hydrofluoric acid, nitric acid and distilled water at a volume ratio of 1 : 1 : 8, and examined with an optical microscope (OM; VH-Z100R; Keyence Corp.). The lap shear strength test was evaluated by means of a tensile test machine (Instron-5500R; Instron Corp., Norwood, MA, USA) at room temperature with a crosshead speed of 1 mm/min. The relative position of pure titanium lap joint for lap shear strength test is shown in Fig. 2. The AS and RS are the advancing side and retreating side, respectively.
Fig. 2. Relative position of CP Ti lap joint for lap shear tensile test.
Fig. 3. Relationship between welding parameters and welding defects for the lap joints welded using probes with lengths of (a) 1.8 mm, (b) 2.0 mm, and (c) 2.2 mm.
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3. Results and discussion The influence of the FSW parameters on welding defects under the load control for different sized tools are summarized in Fig. 3. The typical overheating rough surface together with tool penetration defect was shown in Fig 4a. It was observed that the overheating rough surface and tool penetration defects appeared when the FSW was conducted at low welding speed or high rotation rate (Fig. 3). The appearance of this defect is probably because that the high thermal input sharply accelerated the phase transformation of α to β during FSW. A great deal of soft β phase formed in the stir zone at high temperature, the insert depth of the tool increased rapidly under the load control. Therefore, the penetration defects were observed when the thermal input was too high. The typical groove-like defect was shown in Fig 4c. Such a defect was detected when the welding speed was high or the rotation rate was low (Fig. 3). It indicated that the groove-like defect associated with the insufficient thermal input and material flow. The sound surface appearance (which is typically shown in Fig. 4b) was observed when moderate welding speed and rotation rate were selected (Fig. 3). The cross-sectional examination revealed that although the groove-like defect disappeared and sound surface appearance was achieved after increasing the rotation rate or decreasing the welding speed, the inner cavity defects were observed when the joints were welded at the relatively low thermal input parameters (Figs. 3 and 5). It indicated that the inner cavity defects should associated with the low thermal input and the insufficient material flow during FSW. Under the appropriate heat input, the joints without defect were obtained at the parameter combinations of 300 rpm - 100 mm/min, 250 rpm - 100 mm/min, and 250 rpm - 125 mm/min for the lap joints welded using the tool with a probe length of 1.8 mm and at parameter combinations of 250 rpm - 75 mm/min for the lap joints welded using the tool with a probe length of 2.0 mm. When using the tool with a probe length of 2.2 mm, the sound joint without defect was not achieved. These results showed that the ranges of the welding parameters for lap joining of CP Ti were narrow, and out of the ranges led to the emergence of
welding defects, such as overheating rough surface, groove-like surface and inner cavitations. An increase in the probe length further narrowed the FSW parameter range for the lap joining of CP Ti.
Fig. 4. Typical surface appearances of the pure titanium lap joint processed using 2.0 mm probe at (a) 350 rpm, 100 mm/min, (b) 300 rpm, 100 mm/min, and (c) 350 rpm, 150 mm/min.
Fig. 5. Typical cross section of CP Ti lap joint processed using 2.0 mm probe at (a) 250 rpm, 75 mm/min, and (b) 300 rpm, 85 mm/min.
Fig. 6. Cross section of CP Ti lap joint processed at 350 rpm, 100 mm/min using probe with lengths of (a) 1.8 mm, (b) 2.0 mm and (c) 2.2 mm.
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Fig. 7. CP Ti lap joint processed at 300 rpm, 80 mm/min using probe with lengths of 2.0 mm: (a) as received CP Ti and (b) surface was polished by abrasive paper. Previous investigation [8] showed that the lap width strongly influenced the failure load and the failure load was possible improved by increasing the lap width. This investigation showed that the lap width increased from about 2.6 to 4.0 mm with increasing the probe length from 1.8 to 2.2 mm when the lap join was processed a rotation rate of 300 rpm and a welding speed of 100 mm/min (Fig. 6), demonstrating that the lap zone can be widened through increasing the probe length. However, increasing the probe length raised the likelihood of welding defect. The temperature measurement during FSW indicated that the tool shoulder dominated the heat generation [10]. This is attributed to the fact that the contact area and vertical pressure between the tool shoulder and workpiece are much larger than those between the pin and workpiece, and the tool shoulder has a higher linear velocity than the pin with smaller radius [10]. Because of the poor thermal conductivity of the CP Ti [9], there is a great temperature gradient between the titanium in the probe root and tip parts. The varying deformation ability of the CP Ti at different temperatures resulted in inhomogeneous flow of the titanium during FSW. An increase in the length of the probe would increase the temperature gradients, and thereby increase the inhomogeneous flow of the CP Ti. This is one of the reasons why increasing the probe length raised the possibility of welding defect. Furthermore, increasing the probe length also widened the lap width. Therefore, more oxide surface film was broke and stirred into the stir zone. These broken oxides may increase the possibility of inner cavitation formation. In order to reduce the quantity of oxides in the stir zone, the CP Ti surfaces were mechanically polished using abrasive paper before FSW. Fig. 7 shows that the inner cavitations was detected when the as-received CP Ti was friction stir welded directly at a rotation rate of 300 rpm and a welding speed of 80 mm/min, while the defect was disappeared when the CP Ti sheet was mechanically polished before subjecting to FSW at the same parameters. Thus, surface polishing before FSW is helpful to eliminate the inner cavitations. The tensile curves of the lap joint which was friction stir welded directly using the probe with a length of 2.0 mm at a rotation rate of 250 rpm and a welding speed of 75 is shown in Fig. 8, and the picture of
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fractured sample is inserted in the bottom part. The tensile curves approximately experienced to be horizontal in the middle (10 mm ≤ elongation ≤ 30 mm) and then sharply descended at the end (elongation > 30 mm). The lap join supplied sufficient bonding strength in the lap zone, which led to the fracture happening in the base metal after a large elongation. This result demonstrated that high strength CP Ti lap joint can be obtained if proper weld parameters were used even without any surface pretreatment.
Fig. 8. Tensile curves of CP Ti lap joint processed at 250 rpm, 75 mm/min using probe with lengths of 2.0 mm. A position control system was employed for the frictions stir lap welding of CP Ti in previous work [7]. Electron backscattering diffraction (EBSD) examination showed that the microstructure in the stir zone is different from that in the base metal (BM). The BM can be characterized by coarse-equiaxed grains, while the microstructure at the stir zone can be depicted as fine-equiaxed grains. The fine-equxied grain structure is supposed to result from the dynamic recrystallization, because the stir zone has experienced heavy plastic deformation at relatively high temperature. Because of the grain refinement in the stir zone, the hardness of the stir zone was significantly increased. The reason the fracture location of shear tensile specimen was at the BM is probably attributed to the two aspects, i.e. the hardness increase in the stir zone, which means that the strength in the stir zone is higher than that in the BM, and then enough width of lap zone which provides the sufficient bonding strength and thus prevents the fracture at the lap zone.
4. Conclusion (1) The optimum welding parameter range for the lap joining of CP Ti plates is quite narrow. Out of the ranges led to the emergence of welding defects, such as overheating rough surface, groove-like surface and inner cavitations. (2) Increasing the probe length increased the width of lap zone, but narrowed the range of the optimum welding parameters. (3) The oxide surface film of the CP Ti has a negative effect on the formation of sound joint during FSW. Surface polishing before FSW is helpful to eliminate the inner cavitation defect. (4) The sound joints achieved using the probe with a length of 2.0 mm at a rotation rate of 250 rpm and a welding speed of 75 mm/min exhibited high bonding strength, and the joints fractured in the base material during tensile test. References [1] W.B. Lee, C.Y. Lee, W.S. Chang, Y.M. Yeon, S.B. Jung, Microstructural investigation of friction stir welded pure titanium. Materials Letters 59 (2005) 3315-3318. [2] H. Fujii, Y. Sun, H. Kato, K. Nakata, Investigation of welding parameter dependent microstructure and mechanical properties in friction stir welded pure Ti joints. Mater. Sci. Eng. A 527 (2010) 3386-3391.
[3] W. M. Thomas, E. D. Nicholas, J. C. Needham, M. G. Murch, P. Templesmith, C. J. Dawes, International Patent Application No. PCT/GB92/02203, 1991. [4] R. S. Mishra, Z. Y. Ma, Friction stir welding and processing. Mater. Sci. Eng. R, 50 (2005) 1-78. [5] C. G. Rhodes, M. W. Mahoney, W. H. Bingel, R. A. Spurling, C. C.Bampton, Effects of friction stir welding on microstructure of 7075 aluminum. Scripta. Mater. 36 (1997) 69-75. [6] M.A. Sutton, B. Yang, A.P. Reynolds, R. Taylor, Microstructural studies of friction stir welds in 2024-T3 aluminum. Mater. Sci. Eng. A 323 (2002) 160-166. [7] H. Liu, K Nakata, N. Yamamoto, J. Liao, Mater. Grain Orientation and Texture Evolution in Pure Titanium Lap Joint Produced by Friction Stir WeldingTrans. JIM 51 (2010) 2063-2068. [8] H. Liu, K Nakata, N. Yamamoto, J. Liao, Friction stir welding of pure titanium lap joint. Sci. Tech. Weld. Join. 15 (2010) 428-432. [9] E.O. Ezugwu, Z.M. Wang, Titanium alloys and their machinability - a review. J. Mater. Process. Tech. 68 (1997) 262-274. [10] W. Tang, X. Guo, J.C. McClure, L.E. Murr, Heat input and temperature distribution in friction stir welding. J. Mater. Process. Manuf. Sci. 7 (1998) 163-172.
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Abstract Friction stir welding (FSW) of pure titanium lap joint using three sized tools was investigated over a wide range of welding parameters. The ranges of the welding parameters were narrow, and out of the ranges led to the emergence of welding defects, such as overheating rough surface, groove-like surface and inner cavitations. The sound joints have sufficient bonding strength in the lap zone, which insured that the fracture occurred in the base metal after a large elongation. Increasing the probe length was beneficial to widen the connection width of the lap joints. Surface polishing prior FSW reduced the inner cavitations. Keywords: Friction stir welding, Titanium, Lap joint, Welding defect, Alloy
1. Introduction Ti and its alloys have been extensively used in the aerospace and chemical industry because of their high specific strength, excellent heat resistant, low density and natural corrosion resistance. Commercially pure (CP) Ti is used primarily for its corrosion resistance. It is also useful in applications requiring high ductility for fabrication but relatively low strength in service. With increasing the use of Ti and its alloys, the bonding of these alloys has become more and more important [1]. Generally, Ti and its alloys can be jointed through fusion welding methods like arc welding, laser welding, electron beam welding, etc. However, fluxed welding of Ti are not recommend because weld embitterment caused by picking up of nitrogen, oxygen, hydrogen from atmosphere, high residual stresses and distortion, particular in welding of thin plates, due to the low conductivity are usually encountered [2]. Friction stir welding (FSW), developed and patented in the UK in early 1990s by The Welding Institute, is an innovative solid state welding method [3,4]. It has received much attention due to the avoidance of solidification problems associated with the fusion welding techniques [4,5,6]. FSW was initially developed for joining low melting metals, such as Al, Mg and Cu alloys. During the past decade, increasingly more attention have been focused on the FSW of high melting metals, such as Ti alloys and steels, with the continued development of FSW. To date, there have only been a limited number of reports on FSW of CP Ti. For example, defect free welds were achieved when the 5.6 mm CP Ti plates were butt welded at a rotation speed of 1100 rpm and a welding speed of 500 mm/min [1]. CP Ti plates with 2 mm in thickness were successfully friction stir butt welded at
a rotation speed of 200 rpm combining with a welding speed range from 50 to 300 mm/min [2]. Very recently, CP Ti lap joints have been produced using FSW by Liu et al. [7,8]. These primary studies demonstrated that the optimum welding condition for the lap joining of CP Ti plates is quite narrow. Although early results have indicated that FSW was a potential welding method for obtaining defect-free joints in CP Ti, further works are still needed to understand the effect of welding conditions on the microstructural evolution and mechanical properties of FSW CP Ti. The lack of investigation in this field can be attributed to the difficulties in FSW of CP Ti. Firstly, CP Ti is difficult to be plasticised sufficiently during FSW because its hexagonal crystal structure has only a limited number of independent slip systems. Secondly, the shortcoming in the design and production of tools which withstand to the tool wear at typical FSW temperature has not yet been fully overcome. The high reactivity of Ti with many potential tool materials at high temperatures further limits the choice of tool materials. Thirdly, it is necessary to shield the joint area during FSW as the CP Ti is very reactive when the plates reach temperatures higher than 500oC. Though there exist many challenges in FSW of Ti and its alloys, the growing demand for joining of Ti and its alloys and the potential excellent mechanical properties of Ti joint using FSW drive us to go into the field FSW of Ti and its alloys further. In this investigation, the FSW of CP Ti lap joint was carried out using tools with various designs. The influences of welding parameters on FSW were investigated extensively and the microstructural evolution and mechanical properties of the joints were studied in detail.
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Fig. 1. Schematic illustration of the FSW tools with probes of: (a) 1.8 mm, (b) 2.0 mm and (c) 2.2 mm in length. 2. Experimental The 2-mm thick CP Ti sheets with a composition (in wt.%) of 0.01 C, 0.03 Fe, 0.01 N, 0.1 O, 0.001 H and Ti balance were used in this study. The FSW of lap joint was carried out using a sintered WC-Co tool (tilted at 3o from the vertical) consisting of a shoulder of 15 mm in diameter, a probe of 6 mm in diameter and 1.8, 2.0 and 2.2 mm in length, respectively, as shown in Fig. 1. During FSW, a load control system was employed and the plunging force was set to be 14.7 kN. The rotation speeds were varied from 200 to 400 rpm, and the welding speeds were at a range of 60 to 200 mm/min. The water cooling and argon shielding systems were utilized to cool the welding tool and minimize surface oxidation. After FSW, the lap joint was cross-sectioned by a wire electrical discharge cutting machine (HSC-300; Brother Ind. Ltd.). The cross-section was mechanically polished using water abrasive paper and finally with 1 mm diamond paste. The polished cross-section was
etched in a solution comprising of hydrofluoric acid, nitric acid and distilled water at a volume ratio of 1 : 1 : 8, and examined with an optical microscope (OM; VH-Z100R; Keyence Corp.). The lap shear strength test was evaluated by means of a tensile test machine (Instron-5500R; Instron Corp., Norwood, MA, USA) at room temperature with a crosshead speed of 1 mm/min. The relative position of pure titanium lap joint for lap shear strength test is shown in Fig. 2. The AS and RS are the advancing side and retreating side, respectively.
Fig. 2. Relative position of CP Ti lap joint for lap shear tensile test.
Fig. 3. Relationship between welding parameters and welding defects for the lap joints welded using probes with lengths of (a) 1.8 mm, (b) 2.0 mm, and (c) 2.2 mm.
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3. Results and discussion The influence of the FSW parameters on welding defects under the load control for different sized tools are summarized in Fig. 3. The typical overheating rough surface together with tool penetration defect was shown in Fig 4a. It was observed that the overheating rough surface and tool penetration defects appeared when the FSW was conducted at low welding speed or high rotation rate (Fig. 3). The appearance of this defect is probably because that the high thermal input sharply accelerated the phase transformation of α to β during FSW. A great deal of soft β phase formed in the stir zone at high temperature, the insert depth of the tool increased rapidly under the load control. Therefore, the penetration defects were observed when the thermal input was too high. The typical groove-like defect was shown in Fig 4c. Such a defect was detected when the welding speed was high or the rotation rate was low (Fig. 3). It indicated that the groove-like defect associated with the insufficient thermal input and material flow. The sound surface appearance (which is typically shown in Fig. 4b) was observed when moderate welding speed and rotation rate were selected (Fig. 3). The cross-sectional examination revealed that although the groove-like defect disappeared and sound surface appearance was achieved after increasing the rotation rate or decreasing the welding speed, the inner cavity defects were observed when the joints were welded at the relatively low thermal input parameters (Figs. 3 and 5). It indicated that the inner cavity defects should associated with the low thermal input and the insufficient material flow during FSW. Under the appropriate heat input, the joints without defect were obtained at the parameter combinations of 300 rpm - 100 mm/min, 250 rpm - 100 mm/min, and 250 rpm - 125 mm/min for the lap joints welded using the tool with a probe length of 1.8 mm and at parameter combinations of 250 rpm - 75 mm/min for the lap joints welded using the tool with a probe length of 2.0 mm. When using the tool with a probe length of 2.2 mm, the sound joint without defect was not achieved. These results showed that the ranges of the welding parameters for lap joining of CP Ti were narrow, and out of the ranges led to the emergence of
welding defects, such as overheating rough surface, groove-like surface and inner cavitations. An increase in the probe length further narrowed the FSW parameter range for the lap joining of CP Ti.
Fig. 4. Typical surface appearances of the pure titanium lap joint processed using 2.0 mm probe at (a) 350 rpm, 100 mm/min, (b) 300 rpm, 100 mm/min, and (c) 350 rpm, 150 mm/min.
Fig. 5. Typical cross section of CP Ti lap joint processed using 2.0 mm probe at (a) 250 rpm, 75 mm/min, and (b) 300 rpm, 85 mm/min.
Fig. 6. Cross section of CP Ti lap joint processed at 350 rpm, 100 mm/min using probe with lengths of (a) 1.8 mm, (b) 2.0 mm and (c) 2.2 mm.
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Fig. 7. CP Ti lap joint processed at 300 rpm, 80 mm/min using probe with lengths of 2.0 mm: (a) as received CP Ti and (b) surface was polished by abrasive paper. Previous investigation [8] showed that the lap width strongly influenced the failure load and the failure load was possible improved by increasing the lap width. This investigation showed that the lap width increased from about 2.6 to 4.0 mm with increasing the probe length from 1.8 to 2.2 mm when the lap join was processed a rotation rate of 300 rpm and a welding speed of 100 mm/min (Fig. 6), demonstrating that the lap zone can be widened through increasing the probe length. However, increasing the probe length raised the likelihood of welding defect. The temperature measurement during FSW indicated that the tool shoulder dominated the heat generation [10]. This is attributed to the fact that the contact area and vertical pressure between the tool shoulder and workpiece are much larger than those between the pin and workpiece, and the tool shoulder has a higher linear velocity than the pin with smaller radius [10]. Because of the poor thermal conductivity of the CP Ti [9], there is a great temperature gradient between the titanium in the probe root and tip parts. The varying deformation ability of the CP Ti at different temperatures resulted in inhomogeneous flow of the titanium during FSW. An increase in the length of the probe would increase the temperature gradients, and thereby increase the inhomogeneous flow of the CP Ti. This is one of the reasons why increasing the probe length raised the possibility of welding defect. Furthermore, increasing the probe length also widened the lap width. Therefore, more oxide surface film was broke and stirred into the stir zone. These broken oxides may increase the possibility of inner cavitation formation. In order to reduce the quantity of oxides in the stir zone, the CP Ti surfaces were mechanically polished using abrasive paper before FSW. Fig. 7 shows that the inner cavitations was detected when the as-received CP Ti was friction stir welded directly at a rotation rate of 300 rpm and a welding speed of 80 mm/min, while the defect was disappeared when the CP Ti sheet was mechanically polished before subjecting to FSW at the same parameters. Thus, surface polishing before FSW is helpful to eliminate the inner cavitations. The tensile curves of the lap joint which was friction stir welded directly using the probe with a length of 2.0 mm at a rotation rate of 250 rpm and a welding speed of 75 is shown in Fig. 8, and the picture of
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fractured sample is inserted in the bottom part. The tensile curves approximately experienced to be horizontal in the middle (10 mm ≤ elongation ≤ 30 mm) and then sharply descended at the end (elongation > 30 mm). The lap join supplied sufficient bonding strength in the lap zone, which led to the fracture happening in the base metal after a large elongation. This result demonstrated that high strength CP Ti lap joint can be obtained if proper weld parameters were used even without any surface pretreatment.
Fig. 8. Tensile curves of CP Ti lap joint processed at 250 rpm, 75 mm/min using probe with lengths of 2.0 mm. A position control system was employed for the frictions stir lap welding of CP Ti in previous work [7]. Electron backscattering diffraction (EBSD) examination showed that the microstructure in the stir zone is different from that in the base metal (BM). The BM can be characterized by coarse-equiaxed grains, while the microstructure at the stir zone can be depicted as fine-equiaxed grains. The fine-equxied grain structure is supposed to result from the dynamic recrystallization, because the stir zone has experienced heavy plastic deformation at relatively high temperature. Because of the grain refinement in the stir zone, the hardness of the stir zone was significantly increased. The reason the fracture location of shear tensile specimen was at the BM is probably attributed to the two aspects, i.e. the hardness increase in the stir zone, which means that the strength in the stir zone is higher than that in the BM, and then enough width of lap zone which provides the sufficient bonding strength and thus prevents the fracture at the lap zone.
4. Conclusion (1) The optimum welding parameter range for the lap joining of CP Ti plates is quite narrow. Out of the ranges led to the emergence of welding defects, such as overheating rough surface, groove-like surface and inner cavitations. (2) Increasing the probe length increased the width of lap zone, but narrowed the range of the optimum welding parameters. (3) The oxide surface film of the CP Ti has a negative effect on the formation of sound joint during FSW. Surface polishing before FSW is helpful to eliminate the inner cavitation defect. (4) The sound joints achieved using the probe with a length of 2.0 mm at a rotation rate of 250 rpm and a welding speed of 75 mm/min exhibited high bonding strength, and the joints fractured in the base material during tensile test. References [1] W.B. Lee, C.Y. Lee, W.S. Chang, Y.M. Yeon, S.B. Jung, Microstructural investigation of friction stir welded pure titanium. Materials Letters 59 (2005) 3315-3318. [2] H. Fujii, Y. Sun, H. Kato, K. Nakata, Investigation of welding parameter dependent microstructure and mechanical properties in friction stir welded pure Ti joints. Mater. Sci. Eng. A 527 (2010) 3386-3391.
[3] W. M. Thomas, E. D. Nicholas, J. C. Needham, M. G. Murch, P. Templesmith, C. J. Dawes, International Patent Application No. PCT/GB92/02203, 1991. [4] R. S. Mishra, Z. Y. Ma, Friction stir welding and processing. Mater. Sci. Eng. R, 50 (2005) 1-78. [5] C. G. Rhodes, M. W. Mahoney, W. H. Bingel, R. A. Spurling, C. C.Bampton, Effects of friction stir welding on microstructure of 7075 aluminum. Scripta. Mater. 36 (1997) 69-75. [6] M.A. Sutton, B. Yang, A.P. Reynolds, R. Taylor, Microstructural studies of friction stir welds in 2024-T3 aluminum. Mater. Sci. Eng. A 323 (2002) 160-166. [7] H. Liu, K Nakata, N. Yamamoto, J. Liao, Mater. Grain Orientation and Texture Evolution in Pure Titanium Lap Joint Produced by Friction Stir WeldingTrans. JIM 51 (2010) 2063-2068. [8] H. Liu, K Nakata, N. Yamamoto, J. Liao, Friction stir welding of pure titanium lap joint. Sci. Tech. Weld. Join. 15 (2010) 428-432. [9] E.O. Ezugwu, Z.M. Wang, Titanium alloys and their machinability - a review. J. Mater. Process. Tech. 68 (1997) 262-274. [10] W. Tang, X. Guo, J.C. McClure, L.E. Murr, Heat input and temperature distribution in friction stir welding. J. Mater. Process. Manuf. Sci. 7 (1998) 163-172.
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