Tool Friction Stir Research Articles

Defect formation, microstructure evolution, and mechanical properties of bobbin tool friction–stir welded 2219-T8 alloy

Bobbin tool friction stir welding (BT–FSW) is a novel and effective welding method for aluminum alloys, but microvoid defects are usually formed in the nugget zones (NZ). Therefore, it is important to understand the formation mechanism of microvoid defects and their effect on the fracture behavior of joints. In the present study, 6 mm thick 2219–T8 aluminum alloy plates were subjected to BT–FSW under a constant rotation rate of 300 rpm and welding speeds of 150–500 mm min–1 with the objective of analyzing the evolution of microvoid defects and their effect. The results showed that, lower welding speeds resulted in microvoid defects in the band pattern structure on the retreating side (RS) of the NZ, and the size of the defects decreased with increasing the welding speed, while sound joints were obtained under higher welding speeds. In contrast to conventional FSW precipitation–hardened aluminum alloys with the lowest hardness zone (LHZ) at the heat affected zone (HAZ), the LHZs of the BT–FSW 2219–T8 joints were located at the thermal–mechanically affected zone (TMAZ). Digital image correlation (DIC) of large tensile samples showed that the microvoid defects exhibited an obvious effect on the tensile deformation and fracture behavior of the joints due to the decrease in the maximum strain and lack of necking. Mini-samples in different layers further confirmed that cracks initiated from the microvoid defects in the NZ. However, the sound BT–FSW 2219–T8 joints fractured along the LHZs on the RS, with a maximum joint strength coefficient of 78.1% being achieved.

Effect of Tool Pin Geometry on the Microhardness and Surface Roughness of Friction Stir Processed Recycled AA 6063

Friction stir processing was experimented on recycled aluminum alloy 6063 to investigate the effects of varying friction stir tool pin geometry and friction stir processing parameters on the microhardness and surface roughness. Different tool pin geometry has great influence on the outcome as it alters the ability to provide localized heating and better material flow. This study was performed using two different types of tool pin geometry, namely, the cylindrical threaded and the taper threaded pins, across varying rotational speeds and feed rates. The mechanical properties of the processed workpiece were inspected and analyzed in terms of microhardness, microstructure, and surface roughness. The results show that the taper threaded tool offers the highest improvement in microhardness up to 63% at the lowest rotational speed and highest feed rate at 1150 rpm and 30 mm/min, respectively, and this is supported by microscopy images showing finer grains with the compact and homogenous distribution. The taper threaded tool also provided a better surface roughness than the cylindrical threaded tool. However, the surface produced by cylindrical threaded at 30 mm/min feed rates is as smooth and consistent as that of taper threaded tool.

Influence of Welding Speed on Microstructure and Mechanical Properties of 5251 Aluminum Alloy Joints Fabricated by Self-Reacting Friction Stir Welding.

In the present study, 8 mm-thick 5251 aluminum alloy was self-reacting friction stir welded (SRFSW) employing an optimized friction stir tool to analyze the effect of welding speed from 150 to 450 mm/min on the microstructure and mechanical properties at a constant rotation speed of 400 rpm. The results indicated that high-quality surface finish and defect-free joints were successfully obtained under suitable process parameters. The microhardness distribution profiles on the transverse section of joint exhibited a typical “W” pattern. The lowest hardness values located at the heat-affected zone (HAZ) and the width of the softened region decreased with increasing welding speed. The tensile strength significantly decreased due to the void defect, which showed mixed fracture characteristics induced by the decreasing welding speed. The average tensile strength and elongation achieved by the SRFSW process were 242.61 MPa and 8.3% with optimal welding conditions, and the fracture surface exhibited a typical toughness fracture mode.

In-depth understanding of material flow behavior and refinement mechanism during bobbin tool friction stir welding

Bobbin tool friction stir welding (BT-FSW) is a promising solid-state welding process for fabricating closed or hollow profiles due to its self-supporting nature, which is achieved using a combined tool consisting of two shoulders and a penetrating pin, releasing the backing plate. The joint reliability is mainly affected by its macro-/micro-features including geometric defects and non-uniform grains. However, the underlying thermo-physical process has not been fully understood to clarify how the defects form and grains evolve during welding and how to control them. In this paper, a 3D thermo-mechanically coupled Eulerian-Lagrangian model was developed to analyze the material flow behavior and help understand the defect forming mechanism and recrystallization behavior during BT-FSW of aluminum alloy, wherein tracing particles were specially embedded. The calculated results revealed a complex material migration in the domain driven by the rotating tool. The flow behavior in horizontal/vertical directions at local regions was asynchronous, and eventually converged on the advancing side (AS) to normally form a sound joint, where the combined effects of shearing and squeezing of the material flow throughout thickness affected the morphology of S-line defect. In addition, the non-uniform thermo-mechanical cycling caused an abrupt change in grain structure near the TMAZ/SZ-AS transition region due to the combined continuous and discontinuous dynamic recrystallization, and geometrical effect of strain. The final joint failure was the result of competition between the two softening regions, S-line defect and TMAZ/SZ-AS. These can be applied for further fundamental investigation of parameter optimization or welding structure design, and promote the exploration of post-processing methods to improve the joint performance.

Bobbin Tool Friction Stir Welding of Aluminum Thick Lap Joints: Effect of Process Parameters on Temperature Distribution and Joints' Properties.

Bobbin tool friction stir welding (BT-FSW) is characterized by a fully penetrated pin and double-sided shoulder that promote symmetrical solid-state joints. However, control of the processing parameters to obtain defect-free thick lap joints is still difficult and needs more effort. In this study, the BT-FSW process was used to produce 10 mm AA1050-H14 similar lap joints. A newly designed bobbin tool (BT) with three different pin geometries (cylindrical, square, and triangular) and concave shoulders profile was designed, manufactured, and applied to produce the Al alloy lap joints. The experiments were carried out at a constant tool rotation speed of 600 rpm and a wide range of various welding travel speeds of 200, 400, 600, 800, and 1000 mm/min. The generated temperature during the BT-FSW process was recorded and analyzed at the joints’ center line, and at both advancing and retreating sides. Visual inspection, macrostructures, hardness, and tensile properties were investigated. The fracture surfaces after tensile testing were also examined. The results showed that the pin geometry and travel speed are considered the most important controlling parameters in BT-FSW thick lap joints. The square (Sq) pin geometry gives the highest BT-FSW stir zone temperature compared to the other two pins, cylindrical (Cy) and triangular (Tr), whereas the Tr pin gives the lowest stir zone temperature at all applied travel speeds from 200 to 1000 mm/min. Furthermore, the temperature along the lap joints decreased with increasing the welding speed, and the maximum temperature of 380 °C was obtained at the lowest travel speed of 200 mm/min with applying Sq pin geometry. The temperature at the advancing side (AS) was higher than that at the retreating side (RS) by around 20 °C. Defect-free welds were produced using a bobbin tool with Cy and Sq pin geometries at all the travel welding speeds investigated. BT-FSW at a travel speed of 200 mm/min leads to the highest tensile shear properties, in the case of using the Sq pin. The hardness profiles showed a significant effect for both the tool pin geometry and the welding speed, whereas the width of the softened region is reduced dramatically with increasing the welding speed and using the triangular pin.

Friction Stir Processing Regularities of Cast Aluminum Alloy AlSi12

The paper studies the structure and morphology of the treating zone surface of cast aluminum alloy AlSi12 after 1-4 passes with a friction stir tool. The studies show that after all four passes, the surface of the tool exit zone has traces of material tearing out along the edge of the pin, but the overall appearance of the machining surface improves. The structure after four passes with the tool is dominated by smaller and more rounded particles, while after one pass there is a large number of elongated particles. Friction stir processing leads to improvement of material plasticity by up to 2.5 times and ultimate tensile strength value by 9-10%.

Dual sided composite formation in Al 6061/B4C using novel bobbin tool friction stir processing

Abstract A novel material processing approach using the bobbin tool technique called bobbin tool friction stir processing (BTFSP) was developed to simultaneously fabricate two-side composites. The feasibility of simultaneous fabrication of composites on the top and bottom sides of the workpiece using B4C reinforcement particles into Al 6061 alloy was successfully studied in the present paper. Excellent dispersion of the B4C particles obtained at the bottom side of the workpiece by this technique. The metallurgical investigation of Al 6061/B4C double side composites was carried out using optical and scanning electron microscopy. The mechanical properties include sliding wear behavior, and microhardness of the fabricated composites was studied in detail. The results indicated a more uniform distribution of B4C particles with 3 passes. The grain refinement and homogeneous distribution of the B4C significantly improved microhardness and wear properties of the fabricated composites compared to as-received Al.

Influence of Tool Runout on Force Measurement During Internal Void Monitoring for Friction Stir Welding of 6061-T6 Aluminum

Abstract The goal of this research was to examine how altering the amount of friction stir tool eccentricity while controlling the amount of slant in the tool shoulder (drivers of oscillatory process forces) effects the generation of process force transients during sub-surface void interaction. The knowledge gained will help improve the accuracy of force-based void monitoring methods that have the potential to reduce the need for post-weld inspection. Process force transients during sub-surface void formation were examined for multiple tools with varying magnitudes of kinematic runout. The eccentric motion of the tool produced oscillations in the process forces at the tools rotational frequency that became distorted when features (flats) on the tool probe interacted with voided volumes, generating an amplitude in the force signals at three times the tool rotational frequency (for three-flat tools). A larger tool eccentricity generates a larger amplitude in the force signals at the tool’s rotational frequency that holds a larger potential to create a distortion during void interaction. It was determined that once void becomes large enough to produce an interaction that generates an amplitude at the third harmonic larger than 30% of the amplitude at the rotational frequency in a weld with no interaction (amplitude solely at rotational frequency), the trailing edge of the tool shoulder cannot fully consolidate the void, i.e., it will remain in the final weld. Additionally, once the void exceeds a certain size, the amplitudes of the third harmonics saturate at 70% of the amplitude at the rotational frequency during full consolidation. The interaction between the eccentric probe and sub-surface void was isolated by ensuring any geometric imperfection in the shoulder (slant) with respect to the rotational axis was removed. The results suggest that geometric imperfections (eccentricity and slant) with respect to the tool’s rotational axis must be known when developing a void monitoring method from force transients of this nature.

Особенности процессов структурообразования в соединениях сплава Д16, полученных сваркой трением с перемешиванием с инструментом типа «bobbin tool»

Introduction. One of friction stir welding types is the bobbin friction stir welding (BFSW) process, which allows to obtain welded joints in various configurations without using a substrate and axial embedding force, as well as to reduce heat loss and temperature gradient across the welded material thickness. This makes the BFSW process effective for welding aluminum alloys, which properties are determined by their structural-phase state. According to research data, the temperature and strain rate of the welded material have some value intervals in which strong defect-free joints are formed. At the same time, much less attention has been paid to the mechanisms of structure formation in the BFSW process. Therefore, to solve the problem of obtaining defect-free and strong welded joints by BFSW, an extended understanding of the basic mechanisms of structure formation in the welding process is required. The aim of this work is to research the mechanisms of structure formation in welded joint of AA2024 alloy obtained by bobbin tool friction stir welding with variation of the welding speed. Results and discussion. Weld formation conditions during BFSW process are determined by heat input into a welded material, its fragmentation and plastic flow around the welding tool, which depend on the ratio of tool rotation speed and tool travel speed. Mechanisms of joint formation are based on a combination of equally important processes of adhesive interaction in “tool-material” system and extrusion of metal into the region behind the welding tool. Combined with heat dissipation conditions and the configuration of the “tool-material” system, this leads to material extrusion from a welded joint and its decompaction. This results in formation of extended defects. Increasing in tool travel speed reduce the specific heat input, but in case of extended joints welding an amount of heat released in joint increases because of specific heat removal conditions. As a result, the conditions of adhesion interaction and extrusion processes change, which leads either to the growth of existing defects or to the formation of new ones. Taking into account the complexity of mechanisms of structure formation in joint obtained by BFSW, an obtaining of defect-free joints implies a necessary usage of various nondestructive testing methods in combination with an adaptive control of technological parameters directly in course of a welding process.

Bobbin Tool Friction Stir Welding of Aluminum Using Different Tool Pin Geometries: Mathematical Models for the Heat Generation

In this work, three mathematical models for the heat generation during bobbin tool friction stir welding (BT-FSW) of aluminum using three tool pin geometries have been proposed. The models have utilized and updated the available models for the heat generation during the conventional tool friction stir welding (CT-FSW). For the validation of the models, BT-FSW experiments have been carried out for aluminum alloy AA1050 using three different pin geometries (cylindrical, square, and triangular), at different welding speeds of 200, 400, 600, 800, and 1000 mm/min and a constant tool rotation speed of 600 rpm. The welding temperatures during BT-FSW have been measured to be compared with that calculated from the models at the same parameters. It has been found that the calculated welding temperatures from the models and that measured during BT-FSW are in good agreement at all the investigated welding speeds especially in case of the square and cylindrical pins, proving the validity of the developed models for the predication of the heat generation as well as the welding temperatures. This will allow proper designing of the BT-FSW parameters and avoiding the conditions that can deteriorate the joint quality and properties.

Effect of shoulder diameter on bobbin tool friction stir welding of AA 6061-T6 alloy

The present study investigated the effect of different shoulder diameter on the 6 mm thick AA 6061-T6 aluminium alloy using bobbin tool friction stir welding (BTFSW). Three shoulder diameters as 20, 22, and 24 mm were considered. The maximum temperature of 345 ℃ was observed for 24 mm shoulder diameter sample. Void defect was observed in the weld nugget zone of sample welded with 20 mm shoulder diameter, and defect-free joint was observed with increasing shoulder diameter. All the samples exhibited a fine, equiaxed grain structure in the weld nugget zone. The maximum tensile strength of 166.32 ± 3.6 MPa and & maximum % elongation of 15.8 ± 1.35 was observed for sample welded with 24 mm shoulder diameter.

Effect of different tool pin diameter on mechanical properties of friction stirred welded AISI 304 stainless steel plate

The investigations was carried out to investigate the effect of different friction stir tool pin diameter on mechanical, microstructural properties and welding efficiency of the Stainless Steel AISI 304 friction stir welded specimen. The experiment were carried out at constant tool rotating speed of 1050 rpm, welding speed of 24 mm/min and tools with diameter ranging from 2.5 mm to 4.9 mm at 0.3 mm increasing value. The constant axial force of 10 KN has been applied to perform the friction stir joining. The welded specimens were inspected microscopically and analyzed to obtained mechanical properties such as hardness and UTS, as per the ASTM standard. The maximum tensile strength has been observed as 504 MPa at 2.8 mm tool pin diameter. The maximum welding efficiency of 99% has been observed. The lower tensile strength of the other joints found were due to lower and non-uniform heat distribution at the surface. The microstructure of welded specimen has been analyzed and characterized at different zones by using an optical microscope.

Influence of travel speed on the microstructural evaluation and mechanical characteristics of bobbin tool friction stir-welded thick AA5456-H112 plates

5456-H112 aluminum alloy plates with a thickness of 20 mm were joined using floating-bobbin tool friction stir welding at different travel speeds from 28 to 160 mm/min and a fixed rotational speed of 90 rpm. The results of microstructural analysis indicated that the grain size is decreased at a constant travel speed (at each of the speeds of 28, 40, 56, 80, 112, and 160 mm/min), from the retreating side to the advancing side of the joints whereas the density and the size of the intermetallic compounds are increased. Increasing the travel speed also reduces heat input and recrystallized grain size of the stirred region leading to increased hardness of the joints. The tensile test results showed that the ultimate tensile strength, yield strength, and elongation of the welded samples initially increase by increasing the travel speed and then start to decrease due to the formation of defects in the stir zone. The fracture of all transverse tensile test specimens occurs on the advancing side of the joints. The fracture location of the best specimen is located at the adjacent of the base metal with the heat-affected zone. The maximum mechanical properties are achieved at a travel speed of 40 mm/min, and the welding efficiency in that condition is approximately 97%.

Assessing the Bonding Interface Characteristics and Mechanical Properties of Bobbin Tool Friction Stir Welded Dissimilar Aluminum Alloy Joints

This study focuses on the bonding interface characteristics and mechanical properties of the bobbin tool friction stir welded dissimilar AA6056 and AA2219 aluminum alloy joints using different welding speeds. Voids arise solely in the stir zone at the AA2219 side. A distinct boundary with limited material mixing develops at the middle section of the bonding interface, while excellent material mixing with an irregularly jagged pattern forms at the top and bottom sections of the bonding interface. Increasing the welding speed, the material mixing is rarely changed at the middle section in comparison with the bottom section. Furthermore, a small difference between Guinier–Preston dissolution and Q phase precipitation leads to rare change of hardness in the heat affected zone (HAZ) at the AA6056 side. The increased hardness of the HAZ at the AA2219 side is attributed to avoidance of the dissolution of θ″ phase precipitates. A maximum tensile strength of 181 MPa is obtained at 300 mm min−1. Fractures occur at the AA6056 side near the top and bottom surfaces and at the bonding interface in the middle section of the joints. The regions close to the top and bottom surfaces of the joints show a better ductility.

Understanding the effect of tool geometrical aspects on intensity of mixing and void formation in friction stir process

The reduction of agglomeration of particulate reinforcement in friction stir processing and elimination of joint line remnant in friction stir welding depends on stirring ability of friction stir tool. The magnitude of stirring is indicated by the intensity of material inter-mixing in tool interacted region. Present investigations analyse the influence of geometrical aspects of classical friction stir tool on the intensity of material intermixing, location of high stir regions and void formation during the steady-state phase of friction stir welding. The classical tool was chosen as its geometrical features undergo minimal wear in the harsher processing environment. Investigating material mixing using metallic inserts have their shortcomings due to different flow properties of insert/marker and parent material. Therefore, bi-colour plasticine configuration possessing primary colours was adopted to understand the level of intermixing. The hue attribute of the generated secondary colour was utilised to identify and quantify material mixing. Experimental results revealed that the pin diameter positively influenced mixing and negatively affected void formation. Shoulder size and pin taper angle aided in the closure of void with inferior mixing. The intensity of mixing enhances on selection of tools possessing a shoulder to pin diameter ratio of 2.5. Finite element simulation studies were conducted to understand the reasons for mixing under certain tooling conditions. Since material subjected to stirring is concentrated primarily on a portion of advancing side, components of shear strain, shear strain rates, shear stress and velocity were tracked for this portion of processed material during its interaction with tool pin for one complete rotation.

Effects of traverse speed on weld formation, microstructure and mechanical properties of ZK60 Mg alloy joint by bobbin tool friction stir welding

ZK60B (Mg-6% Zn-0.6% Zr) alloy joints fabricated by bobbin tool friction stir welding (BTFSW) with various traverse speeds were investigated. The sound joint fabricated by the BTFSW was possible under the appropriate welding parameters. The severe plastic deformation during BTFSW resulted in dispersion and segregation of the Zr-rich particles within the stirred zone (SZ) followed by evolution of a bimodal grain structure with distributed bands of 0.8–1.7 μm ultrafine grains and 4.1–7.1 μm equiaxed grains. Micro-hardness of SZ is substantially reduced in contrast to that of parent metal (PM) in spite of the finer grain size owing to dissolution of Mg-Zn based precipitates having hardening effects on alpha-Mg matrix. With the decrease in traverse speed, randomization degree of the plasticized metal flow increases, which is evidenced by the randomized arc line pattern at the low traverse speed. Among all defect-free joints, the 200 mm/min joint exhibits the weakest isotropy of texture within SZ and the best tensile properties, which has reduced ultimate tensile strength and yield strength by 5.4% and by 22.2%, respectively, as compared to the PM. The randomized texture hinders the joint fracturing within SZ at low elongation. Therefore, a relatively high elongation of 10.8% was achieved, which corresponded to 72% of the PM value.

Macrostructure, microstructure and mechanical properties of bobbin tool friction stir welded ZK60 Mg alloy joints

Friction stir welding based on bobbin tool was studied for the joining of ZK60 Mg alloy. Sound joints with the ultimate tensile strength reaching 91.0–94.9% of that of the parent metal can be obtained under the appropriate welding parameters. The welding causes dynamic segregation of Zr-rich precipitates in the stirred zone followed by development of a bimodal recrystallized grain structure with distributed bands of 0.6–1.5 μm ultra-fine grains and 4.4–9.5 μm coarser equiaxed grains. Extensive softening occurred in the stirred zone owing to dissolution of the Mg-Zn based precipitate particles. With the increase in tool rotational speed, the average grain size in the stirred zone increases while the tensile properties are not declined as the enhanced complicated type of plasticized metal flow at the high tool rotational speed produces relatively randomized {0001} textures. The joint produced at the 600 rpm tool rotational speed exhibits the weakest isotropy of {0001} texture and the best combination of strength and ductility. The randomized {0001} textures alleviated the inhomogeneity of plastic deformation during the tensile test, which hindered the joint fracturing with low elongation.

Improvement of Forming Limit in Height with Alternating Tool Path in Penetrating Tool Friction Stir Incremental Forming

Improvement of Forming Limit in Height with Alternating Tool Path in Penetrating Tool Friction Stir Incremental Forming

Quality improvement of bobbin tool friction stir welds in Mg-Zn-Zr alloy by adjusting tool geometry

The bobbin tool friction stir welding (BTFSW) technique was applied to a Mg -Zn- Zr alloy, ZK60-T5. The effects of shoulder diameter and stir pin geometry on the macrostructure, microstructure and mechanical properties of the BTFSW joints were studied. Weld quality was sensitive to the interaction of bobbin tool geometries and processing parameters. The reduction in the shoulder diameter led to less frictional contact and resistance and hence lowered the mechanical load imposed on the tool. The employment of the tapered stir pin reduced the traversing force, which displaced substantially less plasticized metal during the BTFSW than the cylindrical stir pin. The welding speed for the BTFSW was increased benefiting from these mechanical load alleviations. The visual aspects of the BTFSW joints, characterized by limited tunneling and voiding phenomena occurring were improved with the increase in welding speed. The BTFSW resulted in extensive softening in the stir zones of the joints due to dissolution of the β 2 ´precipitates. The tensile properties of the joints were inferior to those of the base metal due to the crystallographic isotropy in the stir zone. The improvements in tensile properties of the BTFSW joint were achieved through texture tailoring by adding threads on the stir pin. The complicated plasticized metal flow introduced by the threaded stir pin provided a randomized crystallographic texture in the stir zone which contributed to the preferable tensile properties.

Microstructure and mechanical properties of bobbin tool friction stir welded ZK60 magnesium alloy

Bobbin tool friction stir welding (BTFSW) technique was applied to a Mg–Zn–Zr alloy ZK60. The results indicated that the plates were joined successfully with no welding defect forming at the investigated processing parameters. BTFSW resulted in significant grain refinement and dissolution of the Mg4Zn7 precipitates in the stir zone (SZ). Extensive softening was observed in the SZ because the strengthening resulted from the grain refinement could not compensate the strength loss caused by the precipitates dissolution. The ultimate tensile strength of the joint reached 80.3–84.4% of that of the ZK60 parent metal. All the joints consistently fractured in the SZ adjacent to the thermo-mechanically affected zone (TMAZ) on the advancing side (AS). The extensive softening in the SZ makes it more susceptible to plastic deformation during transverse uniaxial tension, which is the precondition for the failing in the SZ-side. Within the SZ, the profuse activation of {10–12} extension twinning in the SZ-side on the AS (with <0001> crystal direction parallel to transverse direction) and basal slip in the adjacent region (with <0001> crystal direction at ~45° to transverse direction) caused deformation incompatibility. Additionally, the abrupt change of microstructure from the TMAZ to the SZ aggravated the deformation incompatibility, leading the fracture to initiating in the SZ-side on the AS.