Local melting and tool slippage during friction stir spot welding of Al-alloys

Local melting and tool slippage during friction stir spot welding of different Al-alloy base materials is examined using a combination of detailed microscopy and temperature measurement. The stir zone peak temperature during welding is limited by either the solidus of the alloy in question or by spontaneous melting of intermetallic particles contained in the as-received base material. When spontaneous melting occurs this facilitates tool slippage at the contact interface. Accurate stir zone temperature and grain size measurements are essential elements when estimating the strain rate using the Zener–Hollomon relation. In Al 2024 and Al 7075 spot welds spontaneous melting of second-phase particles produces a drastic reduction in strain rate values. In Al 5754 and Al 6061 spot welds there is a strong correlation between tool rotational speed and estimated strain values. Local melted films dissolve rapidly in the high temperature stir zone and when the spot weld cools to room temperature following welding. Evidence of local melting is observed in Al 7075 friction stir spot welded joints made using a combination of rapid quenching, high plunge rates, and extremely short dwell time settings.     

Formation and retention of local melted films in AZ91 friction stir spot welds

The formation of local melted films during friction stir spot welding of as-cast AZ91D and thixomolded AZ91 material is investigated. The average temperatures close to the tip of the rotating pin vary from 438 to 454 °C during the dwell period in friction stir spot welding. These measured temperature values are higher than the melting temperature of α-Mg + Mg₁₇Al₁₂ eutectic (437 °C). It is suggested that the temperature in the stir zone during the dwell period is determined by the relative proportions of α-Mg and (α-Mg + Mg₁₇Al₁₂) eutectic material, which are incorporated during friction stir spot welding. Based on the stir zone temperature measurements and a detailed examination of material located at the root of the pin thread it is suggested that material is moved downwards via the pin thread and into the stir zone during the dwell period in friction stir spot welding. Evidence of local melted film formation is observed in the stir zone of AZ91 spot welds. It is suggested that melted films are retained since their dissolution rate is much slower in the high temperature stir zone than it is when melted films is formed in the stir zone during Al 7075-T6 friction stir spot welding. The spontaneous melting temperature, solute diffusion rate and the thermodynamic driving force for droplet dissolution are much higher during Al 7075-T6 friction stir spot welding.     

Microstructures and mechanical properties in dissimilar AZ91/AZ31 spot welds

The stir zone microstructures and mechanical properties of dissimilar AZ91/AZ31 friction stir spot welds made using different tool designs and tool rotational speed settings are investigated. Intermingled AZ91 and AZ31 lamellae are formed in the stir zones of dissimilar spot welds made using threaded, three-flat/0.7 mm/threaded and three-flat/no-thread tools and tool rotational speeds ranging from 1500 to 3000 rpm. The intermingled lamellae have chemical compositions, which are similar to those of the upper and lower sheets in the dissimilar sandwich. The flats on the rotating tool facilitate the downward transfer of upper and lower sheet materials in the location close to the pin periphery and therefore intermingled AZ91 and AZ31 lamellae are formed in the stir zones of dissimilar spot welds produced using a three-flat tool without a thread.The distance (Y) from the tip of the hook region to the keyhole periphery has a dominant influence on the mechanical properties of dissimilar AZ91/AZ31 spot welds, since the hook regions are curved inwards towards the axis of the rotating tool. The highest failure load properties and largest Y-values are found in dissimilar spot welds made using threaded and three-flat/0.7 mm/threaded tools and tool rotational speeds from 1500 to 3000 rpm. Dissimilar spot welds made using a rotational speed of 1000 rpm have the smallest Y-values and the lowest failure load properties.     

Influence of tool geometry and process parameters on macrostructure and static strength in friction stir spot welded polyethylene sheets

The effect of important welding parameters and tool properties that are effective on static strength in friction stir spot welds of polyethylene sheets were studied. Six different tool pin profiles (straight cylindrical, tapered cylindrical, threaded cylindrical, triangular, square and hexagonal) with different shoulder geometries, different pin length, pin angle and concavity angle were used to fabricate the joints. The tool rotational speed, tool plunge depth and dwell time were determined welding parameters. All the welding operations were done at the room temperature. Welding force and welding zone material temperature measurements were also done. Lap-shear tests were carried out to find the weld static strength. Weld cross section appearance observations were also done. From the experiments, the effect of pin profile, pin length, pin angle, dwell time and tool rotational speed on friction stir spot welding formation and weld strength was determined.     

Cracking in the stir zones of Mg-alloy friction stir spot welds

Liquid penetration induced (LPI) cracking is investigated during friction stir spot weld of AZ91, AZ31 and AM60 magnesium alloys. A combination of stir zone temperature measurement and detailed metallography has revealed differences in the cracking tendencies of different magnesium alloys when the dwell time during spot welding is varied. LPI cracking in AZ91 spot welds involves the following sequence of events: the formation of eutectic films in the thermo-mechanically affected zone (TMAZ) region immediately adjacent to the stir zone extremity, engulfment of melted eutectic films as the stir zone width increases during the dwell period, penetration of α−Mg grain boundaries and crack propagation when torque is applied by the rotating tool. Cracking occurs early in the dwell period during AZ91 spot welding and almost the entire stir zone is removed when the rotating tool is withdrawn. However, crack-free AZ31 and AM60 spot welds are produced when a dwell time of 4 s is used since the stir zone temperatures are much higher than the α-Mg + Mg₁₇Al₁₂ eutectic temperature (437 °C) and melted eutectic films dissolve rapidly following their engulfment by the growing stir zone. In contrast, the temperature during the dwell period in AZ91 spot welding is close to 437 °C and melted eutectic films are not completely dissolved so that spot welds produced using a dwell time of 4 s exhibit LPI cracking.     

Material flow during friction stir spot welding of dissimilar Al2024/Al materials

In the present paper, the material flow and intermixing during friction stir spot welding of dissimilar Al2024/Al materials were investigated. The dissimilar materials had quite different strength. The microstructural evolutions taking place during a series of lap and butt welds were observed. The effect of penetration depths, dwell time, rotational speed and tool geometry were systematically investigated. The material flow and formation of the intermixed region were explained by a modified model.     

Analysis of dissimilar friction stir welding process for tensile properties of EN AW 2024 and EN AW 5083

The objective of the present study is to analyze the effect of dissimilar friction stir welding process parameters associated with the tool pin profile. Nine different welding experiments were conducted on EN AW 2024 and EN AW 5083 plates. Each of the welded joints was exposed to metallurgical and mechanical tests to determine the effect of the parameters on the welded joint's strength and characteristics. The welding responses or characteristics were analyzed using the statistical tools, grey relational analysis and analysis of variance. Thus, the contribution of each parameter to the process response (ultimate tensile strength and percentage of elongation) was analyzed, and an optimal welding condition was determined. The results of metallurgical analyses showed that the defective joints were mostly in the welded joints fabricated with a conical threaded pin, and the shape of the stir zone was affected by the pin profile and tool rotational speed. The metallurgical results were consistent with the tensile test results. Statistical analyses showed that the most effective parameter on the welded joint strength and elongation is tool rotational speed with 70.3 %. While the tool pin profile affects the stir zone shape, the strength and elongation are not affected. The effect of welding speed (5.6 %) is not significant on strength and elongation.     

Impact of tool profile on mechanical behavior and material flow in friction stir welding of dissimilar aluminum alloys

The tool pin geometry used in friction stir welding of any material affects the transportation and mixing of the materials at the joint interface during the welding process. This further affects the mechanical properties of the joint. Tapered threaded and unthreaded tool pin profiles were investigated in this research work. The relationship between the material mixing characteristics and mechanical properties of each pin profile were evaluated. The results indicate that more materials mixing occurred in the nugget zone of the welds at lower rotational speed with the threaded tool pin than the unthreaded tool pin. However, at medium rotational speed, more volume of materials was swept into each other better in the unthreaded tool pin than the threaded pin. The tensile strengths of welds with the threaded tool pin were higher than the unthreaded tool pin. Although the two tool pins exhibit similarities in hardness variations across the weld zones however, higher average values of hardness were obtained at the nugget zone for welds performed with the tapered threaded tool pin. These could be as a result of better material mixing and higher opposition to grain dislocations across the dividing lines in the welds from the threaded tool pin.     

Challenges in dissimilar friction stir welding of aluminum 5052 and 304 stainless steel alloys

In this study, dissimilar friction stir welding of aluminum 5052 and stainless steel 304 has been carried out with different process parameters. This investigation provides a better insight regarding the defect formation of the weld joints with tilt angles ranging from 0 ° to 2.5 °. The experiments were conducted according to Taguchi L9 orthogonal array by changing the tool rotational speed, and welding speed. The tool pin was kept 70 % towards the aluminum with the tool rotational speed ranging from 800 min⁻¹ to 1200 min⁻¹ with a varying traverse speed of 5 mm/min to 15 mm/min. The bottom part of the stir zone was perfectly welded without any defects. Tunnel defect was detected just above the bottom welded surface. Microstructural analysis reveals that the weld between both materials is formed on the retreating side, whereas on the advancing side, the weld was formed with void defects. Mostly, the stir zone is filled with irregular shaped aluminum and steel parts which were detached from the base material. Several other defects such as voids, cracks, and fragmental defects were observed in the stir zone irrespective of the process parameters. It was observed from the experimental investigations that the tunnel defect can be reduced by increasing the tilt angle.     

Microstructural and Mechanical Observations of Galvanized TRIP Steel after Friction Stir Spot Welding

Friction stir spot welding was done in transformation-induced plasticity steel sheets coated with zinc. The influence of tool rotational speed and dwell time on the microstructure and mechanical properties of lap-joints were investigated. After processing, different zones were formed in the joints. Microstructures in each zone depended on the welding conditions employed. Higher dwell time coupled with higher rotational speed promoted the deposition of a large amount of allotriomorphic ferrite beside the keyhole left by the pin. Coalesced bainite formation was stimulated by the deformation. Mechanical and chemical stabilization of the austenite occurred in different welding zones. Some zinc from the coating remained in the joint, in the stirring zone, representing a partial bonding between the steel sheets. The strength of the welds depended on a complex interaction between geometrical features, such as bonding ligament length and distance between the zinc and the keyhole left by the pin and the resultant microstructure in the stirring zone. The highest joint strength was observed for the “lowest tool rotational speed–highest dwell time” combination of welding parameters.     

Effect of tool rotational speed and pin profile on microstructure and tensile strength of dissimilar friction stir welded AA5083-H111 and AA6351-T6 aluminum alloys

The relatively new welding process friction stir welding (FSW) was applied in this research work to join 6 mm thick dissimilar aluminum alloys AA5083-H111 and AA6351-T6. The effect of tool rotational speed and pin profile on the microstructure and tensile strength of the joints were studied. Dissimilar joints were made using three different tool rotational speeds of 600 rpm, 950 rpm and 1300 rpm and five different tool pin profiles of straight square (SS), straight hexagon (SH), straight octagon (SO), tapered square (TS), and tapered octagon (TO). Three different regions namely unmixed region, mechanically mixed region and mixed flow region were observed in the weld zone. The tool rotational speed and pin profile considerably influenced the microstructure and tensile strength of the joints. The joint which was fabricated using tool rotational speed of 950 rpm and straight square pin profile yielded highest tensile strength of 273 MPa. The two process parameters affected the joint strength due to variations in material flow behavior, loss of cold work in the HAZ of AA5083 side, dissolution and over aging of precipitates of AA6351 side and formation of macroscopic defects in the weld zone.     

Effect of friction stir welding parameters on morphology and strength of acrylonitrile butadiene styrene plate welds

The aim of this study is to examine the effect of main friction stir welding (FSW) parameters on the quality of acrylonitrile butadiene styrene (ABS) plate welds. Welds were carried out in a FSW machine, using a tool with a stationary shoulder and no external heating system. The welding parameters studied were the tool rotational speed which varied between 1000 and 1500 (rpm); the traverse speed which varied between 50 and 200 (mm/min), and the axial force ranging from 0.75 to 4 (kN). The major novelty is to study the influence of the parameter axial force on FSW of polymers. Produced welds have always a tensile strength below the base material, reaching the maximum efficiencies of above 60 (%) for welds made with higher rotational speed and axial force. Good quality welds are achieved without using external heating, when the tool rotational speed and axial force are above a certain threshold. Above that threshold the formation of cavities and porosity in the retreating side of the stir zone is avoided and the weld region is very uniform and smooth. For low rotational speed and axial force welds have poor material mixing at the retreating side and voids at the nugget. For this reason the strain at break of these welded plates is low when compared with that of base material.     

Mechanical properties of friction stir butt‐welded Al‐5086 H32 plate

Al‐5086 H32 plates with a thickness of 3 mm were friction stir butt‐welded using different welding speeds at a tool rotational speed of 1600 rpm. The effect of welding speed on the weld performance of the joints was investigated by conducting optical microscopy, microhardness measurements and mechanical tests (i.e. tensile and bend tests). The effect of heat input during friction stir welding on the microstructure, and thus mechanical properties, of cold‐rolled Al‐ 5086 plates was also determined. The experimental results indicated that the maximum tensile strength of the joints, which is about 75 % that of the base plate, was obtained with a traverse speed of 200 mm/min at the tool rotational speed used, e.g. 1600 rpm, and the maximum bending angle of the joints can reach 180^o. The maximum ductility performance of the joints was, on the other hand, relatively low, e.g. about 20 %. These results are not unexpected due to the loss of the cold‐work strengthening in the weld region as a result of the heat input during welding, and thus the confined plasticity within the stirred zone owing to strength undermatching. Higher joint performances can also be achieved by increasing the penetration depth of the stirring probe in butt‐friction stir welding of Al‐5086 H32 plates.     

Identification of the pin offset effect on the friction stir welding (FSW) via Taguchi – Grey relational analysis: A Case study for AA 7075 – AA 6013 alloys

The aim of this work is to present a case study relating to the dissimilar friction stir welding (FSW) ability of AA 7075‐T651 and AA 6013‐T6 by applying pin offset technique. An orthogonal array L18 was conducted to perform the overlapped weld seams using three different values of pin offset, welding speed and tool rotational speed along with two different pin profiles determine the impact of welding parameters on the tensile properties of friction stir welded joints. The nugget zone for each of overlapped weld seams exhibited a complex structure and also, the pin offset and profile also were found to have a great impact on the microstructural evolution of the nugget zone. The ultimate tensile strength, elongation at the rapture and bending strength of welded joints were measured in the ranges of 194–215 MPa, 1.79–3.34 % and 203–352 MPa. From the Taguchi based Grey relational analysis, the optimum welding condition was determined for the welded joint performed using a single fluted pin profile with the zero pin offset, tool rotational speed of 630 min^?1 and welding speed of 63 mm/min. Microstructural and macro‐structural observations revealed that welded joints exhibiting lower tensile strength are consistent of various types of defects (e. g. cracks, tunnels and cavities). The fracture location of welded joints was found to be on the heat affected zone and between the heat affected zone and AA 6013‐base metal. The tool and pin wear was not observed during the welding applications     

Effects of Tilt Angle on the Properties of Dissimilar Friction Stir Welding Copper to Aluminum

In the present investigation, dissimilar materials such as electrolytic tough pitch copper, and aluminum 6061-T651 were welded by friction stir welding technology. Effects of tool tilt angle on the mechanical and metallurgical properties were studied experimentally for dissimilar material systems. In the present study, the tool tilt angle was varied from 0° to 4° with an interval of 1°, while the other parameters such as rotational speed, welding speed, tool pin offset, and workpiece material position were kept constant. Macrostructure analysis, tensile test, macro hardness measurement, scanning electron microscopy, and energy dispersive x-ray spectrographic tests were performed to evaluate the weld properties of dissimilar copper–aluminum joints. The results revealed that a defect free dissimilar copper–aluminum friction stir welding was achieved by tilt angles 2°, 3°, and 4°. The maximum tensile strength was reported to be 117 MPa and the macro hardness was reported to be 181 VH (in the nugget zone) at a tilt angle of 4°. The macro hardness was increased as the tilt angle increases from 0° to 4°. In addition to this, the thermo-mechanically affected zone (at the copper side) was found to be the weakest zone for a dissimilar copper–aluminum friction stir welding system.     

Structure, properties and fracture of friction stir welds in a high-temperature Al-8.5Fe-1.3V–1.7Si alloy (AA-8009)

Friction stir welds produced in a rapidly-solidified, powder metallurgy Al-8.5Fe-1.3 V-1.7 Si (wt.%) alloy were characterized in order to investigate the effects of deformation during welding on the weld zone microstructure, hardness, tensile properties, and fracture behavior. A weld produced using a tool rotational speed of 1200 rpm and a traversing rate of 4.3 mm/s exhibited a repetitive pattern of dispersoid-depleted bands that were attributed to the intense deformation that occurred in the vicinity of the tool. The significant softening associated with these regions, and the presence of occasional, irregularly-shaped voids near the boundary between the base metal and the weld zone on the advancing side of the weld, promoted a weld tensile strength of 60–70% of the base metal. The application of a lower tool rotational speed of 428 rpm and a lower traversing rate of 1.9 mm/s promoted fewer bands and a more uniform dispersoid distribution throughout the weld zone, and an absence of defects along the weld zone/base metal interface. Tensile strength of these welds approached 90% of the base metal. Fracture of the transverse-weld oriented tensile specimens for both weld types consistently occurred near the boundary between the weld zone and the base metal on the advancing side of the weld zone, with tensile specimen ductilities appreciably lower than that of the base metal.     

Material stirring during FSW of Al–Cu: Effect of pin profile

Friction stir welding (FSW) joins the material in solid state, and it gets evolved as a new and effective technique to join dissimilar materials such as aluminum and copper. FSW tool design and configuration critically affect the joint quality. This study has evaluated the effect of different pin profiles used during FSW of AA5754 Al alloy and commercially pure copper in a butt configuration on the microstructure, material movement, and microhardness for the different joints. The joining is performed through the different pin profiles of cylindrical, taper, cylindrical cam, taper cam, and square shape at the rotational and welding speed of 900?rpm and 40?mm/min respectively. Among all joints, the square pin profile provides good joining and microhardness. Square tool pin profile facilitates good amount of mixing at nugget zone, which consequently increases the hardness. The material movement in square tool pin profile joint is also studied on the longitudinal plane to understand the effect of pulsating and stirring action on the material mixing pattern in dissimilar FSW. It is evident that the softer material in the stir zone gets more stirring, and the flow lines are clearly visible for the stirred material.     

Formation and mechanical properties of stationary shoulder friction stir welded 6005A-T6 aluminum alloy

6005A-T6 aluminum alloy is welded by stationary shoulder friction stir welding (SSFSW). At a constant rotational velocity of 2000 rpm, the effect of welding speed on mechanical properties of SSFSW joint are investigated in detail. Defect-free joint with gloss surface and small flash is attained and no cracks appear at the bending angle of 180°. Compared with traditional friction stir welding (FSW), width of rotational shoulder affected zone is relatively small because of the smaller diameter of rotational shoulder. Increasing welding speed is benefit for reducing the width of softening region and the softening degree. The fracture position of welding joint locates in thermo-mechanically affected zone and the fracture surface morphology presents the typical ductile fracture. The maximum tensile strength of joint at the welding speed of 400 mm/min reaches 82% of base metal (BM).     

Influence of Tool Geometry in Friction Stir Welding

In this article we highlight the results of a recent study undertaken to understand the influence of tool geometry on friction stir welding (FSW) of an aluminum alloy with specific reference to microstructural development, defect formation, and mechanical response. The welding trials were made on 4.4 mm thick sheets using tools made of die steel and having different diameters of the shoulder and the pin, and the profile of the pin. Throughout the welding operation, the rotational speed, traverse speed, and tool axial tilt were held constant at 1400 rpm, 80 mm/minute, and 0 degrees, respectively. For a shoulder diameter of 20 mm and a pin diameter of 6 mm, the severity of defects in the weld was found to be the least and the resultant tensile strength of the weld was high. For the welds that were made using a tool having a shoulder diameter of 10 mm and a pin diameter of 3 mm the tensile strength of the weld was the least since the degree of defects observed were higher.     

Influence of tool geometry and rotational speed on mechanical properties and defect formation in friction stir lap welded 5456 aluminum alloy sheets

Friction stir welding of AA5456 aluminum alloy in lap joint configuration is with two different tempers, T321 and O, and different thicknesses, 5 mm and 2.5 mm was investigated. The influences of tool geometry and various rotational speeds on macrostructure, microstructure and joint strength are presented. Specifically, four different tool pin profiles (a conical thread pin, a cylindrical–conical thread pin, a stepped conical thread pin and Flared Triflute pin tool) and two rotational speeds, 600 and 800 rpm, were used. The results indicated that, tool geometry influences significantly material flow in the nugget zone and accordingly control the weld mechanical properties. Of particular interest is the stepped conical threaded pin, which is introduced for the first time in the present investigation. Scanning electron microscopy investigation of the fracture location of samples was carried out and the findings correlated with tool geometry features and their influences on material flow and tension test results. The optimum microstructure and mechanical properties were obtained for the joints produced with the stepped conical thread pin profile and rotational speed of 600 rpm. The characteristics of the nugget zone microstructure, hooking height, and fracture location of the weld joints were used as criteria to quantify the influence of processing conditions on joint performance and integrity. The results are interpreted in the framework of physical metallurgy properties and compared with published literature.