Conventional Friction Stir Welding Research Articles

Thermal Effect of Bobbin Tool Friction Stir Welding on the Mechanical Behavior of High Density Polyethylene Sheets: Experimental Study

Bobbin friction stir welding (BT-FSW) is a variant of the conventional friction stir welding (C-FSW). This method has been applied of welding high density polyethylene (HDPE) plates; where a rotating symmetrical tool causes a fully penetrated bond, it can weld the upper and lower surface of the work-piece in the same pass. BT-FSW process involves complex heat generation and HDPE flow, which directly affects on the weld area and on mechanical properties of welded joint. Heat generation and material flow during BT-FSW are significantly affected by the tool design features, process parameters and mechanical behavior of work piece materials. Studying the temperature of polyethylene sheets welded by BT-FSW can help in analyzing the mechanism of weld formation and also can provide theoretical guidance for the tool design, process parameter selection and even new process development. In the unique work described in this paper, the 11.4 mm-thick HDPE plates were welded successfully by bobbin-tool friction stir welding. Measurements of the material temperatures were performed by thermocouples which are placed near and at the weld seam. The weld quality was determined in terms of no defects in the stir zone and the tensile strength of the joint. It was found that considerable melting occurred between the rotating shoulders and on the trailing side of the rotating pin. Movement of the molten material by the rotating tool created a very black band in the stir zone. Thermocouples measurements indicated that the temperatures were higher on the advancing side (AS) compared to those on the retreating side (RS). Tensile tests and hardness measurements were performed on welded and seamless sheet samples. The results were analyzed to compare the mechanical properties. To demonstrate the variation in micromechanical properties between welded and seamless sheet samples, micro hardness (HV) testing was used to explain the difference. The HV of the HDPE plates weld by BT-FSW, were relatively symmetrical with respect to the parting line. The maximum hardness levels were reached in the weld bead at around 66 HV in the welded nugget; there was a rise in the level of hardness, in particular at retreating side (RS) and at advancing side (AS) where the value reached 68.60 HV.

Dissimilar welding of AA5083/AA7039 by self-reacting friction stir welding

To evaluate the potential of similar and dissimilar welding, 3 mm thick plates of 5083 and 7039 aluminum alloys were joined by using the self-reactive friction stir welding technique. The bobbin tool was manufactured from H13 tool steel. The effect of several process parameters such as shoulder pinching gap, transverse speed, and tool rotation speed was investigated in order to design a defect-free joint. Tensile testing indicated that the joint efficiency for connections 5083–5083, 7039–7039, and 7039–5083, respectively, is 94.5, 80.4, and 97.4%. Which are much superior to the joint efficiency of fusion welding procedures and at least as good as, if not slightly better than, conventional friction stir welding. Microscopic investigation of the fracture surface of welded specimens indicated that ductile fracture is the primary failure mechanism for similar joints, whereas minor areas of brittle fracture can be seen for dissimilar joints. The hardness profile of the welded components’ cross-sections near the upper and lower shoulders was investigated, and it was revealed that the higher hardness at the lower shoulder is due to the lower temperature of the workpiece in this area.

Acoustic effect on the joint quality and process of friction stir lap welding of aluminum to steel

To gain a high strength-to-weight ratio and economic viability for multiple industrial applications, the Al and steel need to be joined successfully. In the present work, we used conventional friction stir welding (FSW) and ultrasonic vibration enhanced friction stir welding (UVeFSW) techniques to obtain high-strength industrial-grade dissimilar joints of Al and steel. The effects of acoustic energy on the joint quality and welding process were investigated. Macrographs of the joints revealed that ultrasonic vibrations could effectively promote the material flow and increase the mixing degree and width of the Al/steel interface. Steel strips' fragmentation and intermixing with the Al substrate were enhanced under ultrasonic effects. The tool torque, axial force, and spindle power were significantly decreased with ultrasonic vibration. Acoustic assistance was found beneficial to increase the joints’ failure load and change the fracture mechanism. The microhardness of acoustically treated regions was found to be higher compared to its counterparts. The thinning of the brittle intermetallic compounds (IMCs) layer at the interface of the UVeFSW joint was caused by decreased peak temperature during welding. In UVeFSW, when the welding speed was 50 mm/min, the maximum failure load of the joint was obtained under the joint action of the macro interlocks (interface zone), the thinner IMCs layer, and the micro interlocks (laminated structure). When the welding speed was 100 mm/min, the failure load of the joint was improved by 25.8% compared with conventional FSW.

Friction Stir Welding of Aluminum in the Aerospace Industry: The Current Progress and State-of-the-Art Review.

The use of the friction stir welding (FSW) process as a relatively new solid-state welding technology in the aerospace industry has pushed forward several developments in different related aspects of this strategic industry. In terms of the FSW process itself, due to the geometric limitations involved in the conventional FSW process, many variants have been required over time to suit the different types of geometries and structures, which has resulted in the development of numerous variants such as refill friction stir spot welding (RFSSW), stationary shoulder friction stir welding (SSFSW), and bobbin tool friction stir welding (BTFSW). In terms of FSW machines, significant development has occurred in the new design and adaptation of the existing machining equipment through the use of their structures or the new and specially designed FSW heads. In terms of the most used materials in the aerospace industry, there has been development of new high strength-to-weight ratios such as the 3rd generation aluminum-lithium alloys that have become successfully weldable by FSW with fewer welding defects and a significant improvement in the weld quality and geometric accuracy. The purpose of this article is to summarize the state of knowledge regarding the application of the FSW process to join materials used in the aerospace industry and to identify gaps in the state of the art. This work describes the fundamental techniques and tools necessary to make soundly welded joints. Typical applications of FSW processes are surveyed, including friction stir spot welding, RFSSW, SSFSW, BTFSW, and underwater FSW. Conclusions and suggestions for future development are proposed.

Effect of induction preheating on microstructure and mechanical properties of friction stir welded dissimilar material joints of Inconel 718 and SS316L

Using an external heat source during friction stir welding (FSW) of high melting point materials is a promising approach to enhance the material flow and reduce the axial load, improving the tool life. In this study, conventional and induction heating-assisted friction stir welding (I-FSW) in joining dissimilar Inconel 718, and stainless steel (SS316L) alloy using the WC-10%Co tool was experimentally investigated. The welding was performed at a constant rotational speed of 300 rpm with welding speeds of 70 and 140 mm/min and 300 °C preheating temperature in the assisted FSW. The results show that the application of friction stir welding resulted in highly refined grains with deformed carbide particles in the stir zone of both materials, which increased mechanical properties. An increase in temperature during the I-FSW process led to a slight increase in the size of the particles and high atomic diffusion of Cr and Mo elements at the interface of the SS316L and Inconel 718 joint, which showed enhancement of the weld strength. In addition, highly dragged SS316L material into the advancing side of the Inconel 718 was observed, which was directly influenced by the preheating temperature and welding speed. The mechanical properties of the joint with preheating significantly improved at a high welding speed (i.e., at 140 mm/min). The tensile strength of the I-FSW joint at 140 mm/min was 683 MPa which was 99.8% of the base SS316L. Dynamic recrystallization resulted in grain refinement, which raised the hardness value. The results also show that preheating reduces downward axial force, enhancing the tool life.

Evolution of microstructures and intermetallic compounds at bonding interface in friction stir welding of dissimilar Al/Mg alloys with/without ultrasonic assistance

Complete understanding of the evolution behaviors of the microstructures and intermetallic compounds (IMCs) along the interface materials flow path in friction stir welding (FSW) of dissimilar Al to Mg alloys is of great significance. In this study, conventional FSW and ultrasonic vibration enhanced FSW (UVeFSW) experiments of Al/Mg alloys were performed, and the instantaneous evolution features of the interface materials around the tool were "frozen" by using the "sudden stop" and simultaneous cooling techniques. The microstructures and IMCs formation at different locations around the exit hole were observed and characterized by scanning electron microscope, energy dispersive spectrometer and transmission electron microscope. It was found that before the materials started to deposit near the back of the tool, “IMC+Mg+IMC+Al” multilayer microstructure and simple IMC layer with (β+γ) sequentially emerged on the Al/Mg interface. With the application of ultrasonic vibration, the multi-layered interface structure only appeared at the middle stage of materials flow around the pin, and ultrasonic vibration just began to play a suppression role on the growth of two sub-layers IMC at a position where the materials deposit. With assistance of ultrasonic vibration in UVeFSW, the tool drove a larger volume of Mg alloy to move toward the retreating side, and the final IMCs thickness was thinner than that in FSW.

Enhanced strength and ductility of rapid cooling friction stir welded ultralight Mg–14Li–1Al alloy joint

Ultralight Mg–14Li–1Al alloy sheets were subjected to conventional friction stir welding (FSW) and rapid cooling FSW (RCFSW) for comparative study. Compared with decreasing the rotation rate, the use of coolant significantly improved the thermal cycle and inhibited the grain coarsening. The RCFSW600 joint had an ultrafine grain structure with its average grain size of 2.1 μm. The yield strength was mainly determined by the synergic influences of grain boundaries, dislocations, and nanoscale MgLiAl2 precipitations. The ultimate tensile strength was 181 MPa and the welding efficiency reached 94.3%. A relatively good fracture elongation of 9.6% was also obtained. Finally, the improved strength-ductility synergy was regarded as the interaction among dislocations, deformation twins, and stacking faults.

Microstructure and enhanced mechanical performances of dissimilar Mg alloys fabricated by pulse current-assisted friction stir welding (PCaFSW)

The dissimilar AZ31B/AZ61 Mg alloy joints were fabricated by pulse current-assisted friction stir welding (PCaFSW). The maximum tensile strength and elongation were 314.28 MPa and 13.66%, respectively. A micro interlocking structure was formed and the grain refinement effect was enhanced by the PCaFSW process, which was improved relative to that in the conventional FSW. The enhanced strength and plasticity with increasing pulse current were attributable to the grain refinement, weakening of texture strength, formation of nano-scale precipitates and increase of short dislocation lines. The fracture mode was transformed from a brittle fracture mode to a combined fracture mode of brittle and ductile fractures.

Production of novel recycled hybrid metal matrix composites using optimized stir squeeze casting technique

In this work, optimized squeeze stir casting was used to produce recycled Aluminum Metal Matrix Composites (AMC's). Scrap Aluminum Alloy Wheel (SAAW), sourced from the local markets was used as a matrix. Alumina (5 %), SiC (3 & 6 %) and graphite (1, 3 & 4 %) were used as hybrid reinforcements to create the sustainable MMC's. The produced recycled hybrid AMCs were subjected to mechanical tests such as porosity, tension, compression, hardness and wear analysis. The highest ultimate tensile strength of 250 MPa was recorded for AMC with 4 % graphite inclusion, followed by AMC with 3 % SiC. The highest compressive strength was noted at 508 MPa for 4 % graphite, followed by composite with 3 % graphite and 3 % SiC. Similarly highest wear resistance was exhibited by AMC with 4 % graphite and alumina. The results showed that the addition of 4 % graphite and alumina to SAAW increased the strength and decreased the wear rate and frictional forces. The microstructure analysis conducted on these samples confirmed that graphite and alumina exhibited better bonding to the SAAW matrix than SiC. Conventional Friction Stir Welding (FSW) technique was employed to check the weldability of the developed AMCs. Tensile test of the welded samples confirm that recycled AMCs with 4 % graphite are very strong and can be introduced to the industry for various applications.

Effect of Process Parameters on Weld Quality in Vortex- Friction Stir Welding of 6061-T6 Aluminum Alloy.

Vortex- friction stir welding (VFSW) utilizes a stir bar made of an identical material to the workpiece to rub the workpiece's top surface, which avoids the keyhole defect in conventional friction stir welding. It presents great potential in the repair field of aluminum alloys. In this study, the effect of stir bar diameter, rotation speed, and welding speed on the weld formation was investigated in the VFSW of 6061-T6 aluminum alloy. The weld macrostructure, penetration, and mechanical properties were characterized. The results show that a large diameter of the stir bar can enhance the vortex material flow, increase the heat input, and eliminate the incomplete-penetration defect. The increase in rotation speed within limits can enhance the weld penetration and the mechanical properties of the weld nugget zone (WNZ). However, too high a rotation speed reduces the weld penetration and weakens the mechanical properties of the WNZ. The increase in welding speed reduces the weld penetration but enhances the mechanical properties of the heat affected zone. The incomplete-penetration defect significantly weakens the ductility of the VFSW joint. It can be eliminated by enlarging the stir bar diameter and choosing a moderate rotation speed and a lower welding speed.

Evaluation of Pure Titanium Welded Joints Produced by Underwater Friction Stir Welding

Underwater friction stir welding (UWFSW) is an advancement of conventional friction stir welding, which is performed in air. The need for welding underwater calls for more information to be explored in the welding of different materials. The aim of this research is to investigate the UWFSW of 2 mm titanium sheet joints for their tensile and microstructural properties and compare them with the results of friction stir welding (FSW) performed in air. Also, nondestructive tests, such as radiography and dye penetrant tests, were carried out to identify the occurrence of any weld defects. The rotational speed of the FSW tool and its travelling speed were considered process parameters, while other parameters like the tilt angle of the tool, shape of the tool pin, etc. were kept constant. The results show that the welded joints with higher strengths were achieved at a lower travelling speed of 140 mm/min at a rotational speed of 400 and 500 rpm in FSW and UWFSW, respectively. The mechanical properties such as yield strength, tensile strength, and hardness are improved in UWFSW when compared with conventional FSW due to the proper mixing of the metal and effective cooling due to the water environment. In underwater FSW, the improved YS and TS values are observed, which are 95.9% and 75.1% of the base metal values, respectively, when the tool rotational speed is 400 rpm and the travelling speed is 140 mm/min. However, the elongation % is much lower in both FSW and UWFSW when compared with the base metal. These results are supported by microstructure and nondestructive test outcomes.

Effect of tool rotation speed on microstructure and mechanical properties of underwater friction stir welding 6061 aluminium alloy

This work presents the experimental investigation of the effect of tool rotation speed (TRS) on the mechanical properties of the 6061-T6 aluminium alloy weld joint in the underwater friction stir welding (UFSW) process. The experiments are conducted with four different TRS, 710, 900, 1120, and 1400 rpm, and a constant welding speed (WS) of 20 mm/min, respectively. The welding is performed under the water in the butt joint configuration on the vertical milling machine (Model HMT FN2H). The experimental result shows that the average grain size decreases and tensile strength and hardness increases on increasing the TRS up to mid-range of the TRS and further increasing the TRS the trend of grain size and tensile strength and hardness are increases and decreases, respectively. The maximum tensile strength value is 190 MPa, and the minimum grain size is 5 µm at 1120 rpm. Additionally, the optimum condition of tensile strength and weld zone shape is compared with the conventional friction stir welding process, and it is found that tensile strength is increased by 20% and smaller weld zone shape are observed in underwater friction stir welding process. The brittle fracture dominance is observed at lower TRS and ductile dominance fracture in 900, 1120, and 1400 rpm of the weld joint.

Eliminating hole defects and improving microstructure and mechanical properties of friction stir welded joint of 2519 aluminum alloy via TIG arc

Two different joints of 2.5 mm thick 2519-T87 aluminum alloy sheets were obtained by conventional friction stir welding (FSW) and tungsten inert gas (TIG) arc assisted FSW. The mechanical properties of the joints were evaluated by hardness and tensile tests at room temperature. The results show that the introduction of TIG arc results in apparently higher tensile strength and elongation of the joint with the increment rates of 16.9% and 209.1%, respectively. The mechanism has been discussed based on microstructural examination by using optical microscope, scanning electron microscope, electron backscatter diffraction analyses, electron probe microanalyzer and scanning transmission electron microscope. The TIG arc assisted FSW eliminated the hole defects, reduced onion rings area and helped to form a higher density of finer θ′ hardening precipitates, which contributed to the higher mechanical properties.

Development of novel double-side friction stir Z shape butt-lap welding process for dissimilar joining of 12 mm medium-thick Al/Cu plates

Medium-thick Al and Cu plates are commonly used in the power industry. However, it is difficult to achieve high-quality medium-thick Al/Cu dissimilar joints due to the formation of intermetallic compounds (IMCs) layer, which are typical Al2Cu, AlCu and Al4Cu9 phases. Although friction stir welding (FSW) is a suitable welding technique for joining dissimilar metals, the higher heat generation from a large tool shoulder when FSW of medium-thick plates significantly reduces the joint properties. Therefore, a novel double-side friction stir Z shape butt-lap welding (DS-FSZW) is proposed to join medium-thick Al/Cu dissimilar plates. The results indicated that the DS-FSZW reduces the thickness of the IMCs layer compared with conventional double-side FSW (DS-FSW). The maximum tensile strength and elongation of the DS-FSZW joint are 174.17 MPa and 7.32 %, respectively. While the maximum tensile strength and elongation of DS-FSW are 140.98 MPa and 6.2 %, respectively. It indicates the novel DS-FSZW can effectively increase the mechanical properties of the medium-thick Al/Cu dissimilar joints compared to DS-FSW.

The material flow and texture-weakening mechanism in double-sided friction stir welded Mg alloy

In conventional friction stir welding of magnesium alloys, a strong (0001) texture at the welded zone is inevitable, which deteriorates the joint mechanical properties. A novel method called ‘double-sided friction stir welding’ solved this problem by weakening the texture. However, the texture-weakening mechanism remains unclear. In this study, the mechanism of the texture weakening in double-sided friction stir welded Mg alloy was systematically analysed. The welding temperature and the material flow mode were revealed to control the (0001) texture randomisation simultaneously. The unique material flow at the mid-thickness layer caused by the double-sided friction stir welding was revealed to match the ‘beat phenomenon’ for the first time, which was revealed as the dominant mechanism for the (0001) texture randomisation.

Effect of splat cooling on microstructures and mechanical properties of friction stir welded 2195 Al–Li alloy

To achieve high-strength joints of 2195 Al–Li alloy, a novel splat cooling assisted friction stir welding (SCaFSW) was proposed. The microstructures and mechanical properties of conventional friction stir welded and splat cooling assisted friction stir welded 2195 Al–Li alloy were comparatively studied. The work revealed that using splat cooling in friction stir welding could reduce the welding temperature and fluidity of the plastic materials which lead to a rough weld surface in the SCaFSW, and even lead to the formation of void defects at the rotation speed of 400 rpm. However, finer grain size and higher dislocation density were observed in the weld nugget zone of the SCaFSW joint compared to conventional FSW. Furthermore, the application of splat cooling in FSW suppressed the dissolution of precipitates into the matrix during welding, resulting in some coarse residual precipitates in the weld nugget zone of SCaFSW joints. In addition, splat cooling in FSW could significantly increase the tensile strength of defect-free 2195 Al–Li alloy joints but decrease the elongation of the joint. The maximum tensile strength of the SCaFSW joint reached 502.7 MPa, which is 93% of the base material.

Simultaneously enhanced strength-ductility synergy and corrosion resistance in submerged friction stir welded super duplex stainless steel joint via creating ultrafine microstructure

In this paper, submerged friction stir welding (SFSW), capable of significantly weakening the welding thermal cycle compared to conventional FSW, was employed on SAF2507 super duplex stainless steel (SDSS). An ultrafine grained (UFG) dual-phase microstructure, characterized by more uniformly distributed and balanced alpha (α) and gamma (γ) phases with average grain sizes of 0.96 and 0.77 µm, respectively, was achieved in the stir zone (SZ). Such unique microstructure leads to a synergistic enhancement of tensile strength and ductility in the longitudinal SZ at room temperature, which increases from 840 MPa to 18% under AFSW condition to 915 MPa - 22%. As well, this UFG dual-phase microstructure plays a crucial role in determining the resultant higher corrosion resistance. Combining with the reduced width of entire joint, a transverse SFSW joint with a remarkable increased total elongation was successfully achieved. In addition, the relatively lower residual stress across the whole joint actually contributed to the increment of ductility in the SFSW joint, as compared to the traditional FSW joint. The aforementioned results demonstrate an effective strategy in achieving a completely balanced UFG microstructure across the whole SZ during SFSW and a concurrent improvement of strength, elongation and corrosion resistance.

Achievement of defect-free and high-ductility TA5 alloy joint by dynamic supporting friction stir welding

Dynamic supporting friction stir welding (DSFSW) is proposed by developing a novel welding platform. The conventional backing plate is replaced by a small-size supporting column to reduce heat dissipation from weld back in the FSW of titanium alloy. TA5 alloy joints were produced via varied FSW techniques and examinations on macrostructure, microstructure and mechanical properties were carried out. Results reveal that DSFSW enhances material flow by greatly improving the effective heat input into weld, thereby eliminating the flow-related cavity defect. Specifically, DSFSW enables defect-free joint at 250 r/min with a significantly increased welding speed of 55 mm/min (83.3% higher than the conventional FSW). Furthermore, the ductility of joint can be improved more than 20%. Consequently, DSFSW is feasible to obtain high-quality joint of titanium alloy under rather high welding speed.

Microstructures and mechanical properties of non-thinning and penetrating friction stir welded 2219-T6 aluminum alloy

The aim of this paper is to offer a novel friction stir welding technique to achieve non-thinning and lack of kissing bond welds with excellent mechanical properties. In the conventional friction stir welding (CFSW) process, a certain size spindle tilting angle and a shoulder plunge depth were applied in order to obtain high-quality welds. However, what was the reason for the thinning phenomenon of weld and thus the weakening of weld bearing capacity? The reduction of spindle tilting angle and shoulder plunge depth was likely to cause the welding tool not being able to reach the weld root, and thus, it was engendered for the kissing bond or the absence of complete penetration. In the present study, a novel welding tool with large size shoulder and Archimedes spiral groove was designed in order to achieve non-thinning and lack of kissing bond welds. The microstructures and mechanical properties of the joints obtained by the non-thinning and penetrating friction stir welding (NTPFSW) and the penetrating friction stir welding (PFSW) had been analyzed in detail. Compared with PFSW, NTPFSW had a narrow shoulder-affected zone. The thickness of NTPFSW joint is 0.1 mm thicker than that of BM and 0.22 mm thicker than that of PFSW joint. The stir zones (SZ) obtained by the PFSW and NTPFSW were both composed of fine equiaxed grains. The grains in the upper part of NTPFSW joint were larger in size, and the number of precipitates (θ’ phase) was less. NTPFSW completely eliminated the weld thinning phenomenon, and the tensile properties were 365 Mpa, which is 2% higher than that of PFSW.

Microstructure and Mechanical Properties Analysis of Al/Cu Dissimilar Alloys Joining by Using Conventional and Bobbin Tool Friction Stir Welding

The feasibility of producing welding joints between 6061-T6 aluminum and pure copper sheets of 6 mm thickness by conventional friction stir welding (CFSW) and bobbin tool friction stir welding (BTFSW) by using a slot-groove configuration at the joining surface was investigated. The microstructure of the welded samples was examined by using an optical microscope and X-ray diffraction. Furthermore, the mechanical properties of the weld samples are compared based on the results of the tensile test, hardness measurement, and fractography test. The slot-groove configuration resulted in the presence of a bulk-sized Al block on the Cu side. The microscopic observations revealed the dispersion of fine Cu particles in the stir zone. The presence of intermetallic compounds (IMCs) CuAl2, which are hard and brittle, lowered the strength of the weld joints. The strength of the weld joints produced with BTFSW was superior to that of the C-FSW. The maximum hardness values of 214 HV and 211 HV are reported at the stir zone for BTFSW and CFSW, respectively. The fracture location of all the joints was at the intersection of the stir zone and the thermomechanically affected zone was on the Cu side.