Stir Pin Research Articles

Effect of dynamic recrystallization on microstructure and properties of multilayer Al foils friction stir welding

In this study, friction stir welding (FSW) was used to weld multi-layer Al foil and Al plate, and the defect-free welded joint was successfully obtained. In the stirring zone (SZ), due to the extrusion of the shoulder and the rotation of the stirring pin, the original layered structure of SZ is destroyed, and the original grains are destroyed and dynamically recrystalized to form fine equiaxed grains. With the decrease of welding speed, the heat input increases, the dynamic recrystallization degree (DRX) of SZ increases, the proportion of low angle grain boundaries (LAGBs) increases from 20.5% to 45.0%, and the grain size of SZ decreases from 1.53 to 0.93 μm, which affects the performance of the joint. The hardness of each welding parameter SZ is higher than that of the base material. With the increasing degree of DRX, the hardness of SZ continues to rise, and the electrical conductivity decreases from 57.5% international annealing copper standard (IACS) to 52.5%-56.2% IACS. The tensile strength of the joint reaches its maximum first and then decreases with the increase of DRX.

Numerical simulation of bidirectional fluid-solid coupling in friction stir welding of aluminium and magnesium dissimilar alloys

Abstract During friction stir welding, the flow degree of metal materials and the welding temperature determine the microstructure and mechanical properties of the joint. This paper establishes a bidirectional fluid-solid coupling model for dissimilar friction stir welding of aluminum and magnesium alloys. Combining simulation results and experimental research, it quantitatively analyzes the distribution laws of the flow field, temperature field, and stress field during the welding process. The results show that in terms of temperature field distribution, the simulated temperature matches the thermal cycle curve of the measured temperature, with the welding temperature peak located in the rear area of the shoulder’s bottom. In addition, the temperature field on both sides of the weld is asymmetrically distributed, with the temperature on the aluminum side being higher than that on the magnesium side. In terms of flow field distribution, the material flowability decreases as the distance from the stirring head increases. Therefore, the material flow patterns around the stirring head and in areas farther from the stirring head are different. It was also found that the best material flow was at the bottom of the shoulder and the material flow rate was higher on the magnesium side than on the aluminium side. It is noteworthy that under the combined action of the stirring pin and the shoulder, the material mixing zone and the magnesium alloy shift towards the aluminum side. In terms of stress field distribution, the equivalent stress of the stirring head is distributed differently on the aluminum side and the magnesium side.

Improved material properties of wire arc additively manufactured Al-Zn-mg-cu alloy through severe deformation interlayer friction stir processing and post-deposition heat treatment

In this study, Al-Zn-Mg-Cu alloys were prepared by wire arc additive manufacturing (WAAM) combined with interlayer friction stir processing (IFSP). To enhance the FSP region, an improved stirring pin was designed to broaden the stir zone effectively. Moreover, the effects of typical T6 and T73 heat treatments on the microstructure and mechanical properties of as-deposited (AS) specimen were meticulously investigated. The results indicated that heat treatments had minimal impact on grain size, dislocation density, and texture strength, but significantly altered the type, size, and distribution of precipitates. The differences in strength were primarily attributed to precipitation strengthening rather than grain boundary or dislocation strengthening. Following T6 heat treatment, the precipitates were predominantly η’, which were smaller in size and exhibited a significantly increased number density compared to AS specimen. Therefore, the average yield strength (YS) and ultimate tensile strength (UTS) increased by 39.51 % (500.21 MPa) and 15.84 % (584.26 MPa), respectively. In contrast, T73 treatment caused a substantial number of fine η’ to transform into coarser η, leading to a significant decrease in precipitate number density. Consequently, compared to T6 specimen, the average YS and UTS decreased by 47.19 % and 13.13 %, respectively.

Solid-state friction rolling repair technology for various forms of defects through multi-layer multi-pass deposition

The current Friction Stir Welding (FSW) based solid-state defect repairing method is an ideal repair technology for high-strength aluminum alloy. However, due to the limitation of pin length and the existence of shoulder, the stirring pin cannot penetrate deep into the root of defects and is difficult to extend in the width direction, which poses great challenges for components with large defects, such as one-dimensional, two-dimensional and volume defects. To overcome these limitations, this study successfully achieved effective repair of various forms of defects using the solid-state Friction Rolling Repair (FRR) technology through multi-layer multi-bead deposition. The microstructure and mechanical properties of the repaired ZL114A cast aluminum alloy joints were investigated. The results demonstrate well-bonded repaired boundaries, significant grain refinement at the junction between the repaired area and boundary, and ultimate tensile strength exceeding 80 % of base material strength. The average grain size of the repaired area can reach a minimum of 2.82 μm. The grain refinement degree of the side joint of the surface defect repair sample is higher than that of the bottom joint, and the corner joint of the bottom surface shows the highest proportion of recrystallized grains. Fracture locations were consistently within the interior of the repair zone in tensile specimens, indicating superior properties at side joints compared to those within the interior. The research proves that FRR has the ability to repair a variety of typical defects, including one-dimensional, two-dimensional and volume defects, in the process of processing and service, which lays a foundation for the industrial application of FRR.

Inhomogeneity of microstructure evolution in stir zone of 2195Al thick-plate FSW joints

An appropriate friction stir welding (FSW) tool has been developed to perform high-quality joining of 12 mm thick Al-Li alloy plates. Threads and triple facets were machined on the stirring pin to improve the material flow. The differences in microstructure characteristics, recrystallization mechanisms, and precipitate evolution along the stir zone (SZ) thickness were systematically clarified. The results indicated that microstructures in SZ were refined by dynamic recrystallization (DRX) and showed an apparent top-to-bottom gradient distribution in grain size. The geometric topology of the stirring pin results in periodic ultrafine-grained bands throughout SZ. At the top and middle regions of SZ, high-density vacancies and dislocations were formed due to elevated temperature and strain rate, facilitating the precipitation of T1 (Al2CuLi) phases through a dislocation-induced heterogeneous nucleation mechanism. In comparison, icosahedral quasi-crystalline T2 (Al6Cu(Li, Mg)3) phases, which can be attributed to the segregation of atoms and high cooling rate, were detected along grain boundaries at SZ-bottom. Furthermore, the ultimate strength values of tensile specimens sectioned from the top, middle, and bottom parts of SZ are 452, 457, and 406 MPa, respectively. Studying the inhomogeneous microstructure and properties of welded joints provides a theoretical basis for manufacturing high-load components.

Effects of Friction Stir Processing on the Microstructure and Mechanical Properties of an Ultralight Mg-Li Alloy

Magnesium–lithium alloys are arguably the lightest metal structural materials but have low strength. In order to increase strength, friction stir processing (FSP) is applied to a hot-rolled Mg-10Li-3Al-3Zn (LA103Z) sheet to study the effects on the microstructure and mechanical properties. In this study, the strengthening mechanisms of various FSP regions of an Mg-Li alloy were clarified by a combination of numerical simulation and experimental method. Based on ANSYS APDL, a finite element model with a moving heat source is established. Rotational speeds of 800, 1000, and 1200 rpm and traverse speeds of 100, 110, and 120 mm/min were used in this research. The simulation results confirm that the influence of the rotation speed on the alloy temperature field is greater than that of the travel speed. The temperature of the processing area increases with an increase in rotation speed and decreases with an increase in travel speed. Then, hot-rolled LA103Z alloy plates are processed by FSP. The correspondence between the numerical simulation and experiment was verified by infrared thermography. The results indicate that FSP decreases the grain size significantly for the dynamic recrystallization and dramatic mechanical crushing of the stirring pin. The α-Mg and AlLi are solid soluted in the β-Li matrix. The tensile strength of the processing zone is 260.67 MPa (1000 rpm, 110 mm/min) versus the 170.47 MPa of the base metal. The SZ has the highest microhardness of 77.8 HV (800 rpm, 120 mm/min) and decreases gradually to the BM. The severe deformation, recrystallization, and solid solution of the α-Mg are important factors contributing to the improved mechanical properties.

搅拌针偏移量影响铝镁异质金属搅拌摩擦焊材料流动的数值模拟

搅拌针偏移量影响铝镁异质金属搅拌摩擦焊材料流动的数值模拟

Thermomechanical behavior and microstructural characteristics of 7055 aluminum alloy during friction stirring welding

This study employed a viscoplastic finite element model and re-meshing technique to investigate the thermomechanical response of 7055 aluminum alloy during friction stir welding (FSW). The stirring pin rotates at 800–1200 rpm and moves at 80–120 mm/min (i.e., welding speed) during the FSW process. The temperature field and viscoplastic flow were mathematically modeled based on a computational solid mechanics method and predicted by a three-dimensional coupled thermomechanical numerical simulation. To validate the simulation results, real-time temperature measurements at the welded joint were acquired via thermocouples embedded in the plate prior to the welding process. Additionally, optical microscopy and electron backscatter diffraction techniques were used to examine the metallographic structure and grain orientation of the welds, respectively. The results revealed a temperature difference of 5–10 ℃ between the advancing and retreating sides of the plate. The material on the advancing side was extruded upward in a circular motion and eventually reached the surface. Meanwhile, the material on the retreating side moved half a revolution around the pin and remained at the original depth of the plate. Complete dynamic recrystallization occurred in the weld nugget, resulting in the formation of fine equiaxed grains with random orientation. In the thermomechanically affected zone, the proximity to the weld nugget was associated with an increased proportion of high-angle grain boundaries and recrystallized structures. Simultaneously, the intensity of the deformation texture decreased, while the recrystallized texture showed an initial strengthening followed by a subsequent weakening. This comprehensive investigation contributes to a deeper understanding of the thermomechanical behavior and metallographic characterization of 7055 aluminum alloy during the FSW process.

Effect of pin offset on interface evolution and fracture behavior of aluminum/copper dissimilar friction stir welded

In this study, triangular prismatic threaded structural pins at different offsets were used to study the forming and performance of Al/Cu dissimilar joints. In the experiments, the Al alloy was placed on the advancing side, and the stirring pin was offset 0.5 mm, 0.8 mm, 1.0 mm, and 1.2 mm towards the Al side, respectively. The results show that sound joints can be obtained under each offset. The number and size of Cu particles in the stirred zone of the joint with a pin offset of 0.5 mm was large. The Cu particles in the stirred zone gradually refine as the stirring pin is offset towards the Al side. According to the EDS analysis, the stirring zone and interfacial intermetallic compounds (IMCs) mainly consist of Al2Cu and Al4Cu9, and a lamellar structure with IMC layers was observed on the Cu side near the Al/Cu interface. The size and number of Cu particles as well as the thickness of the IMCs layer had a significant effect on the tensile strength of the joints, with a maximum tensile strength of 203.4 MPa (equivalent to 71.3% of the Cu base metal) obtained at a pin offset of 1 mm, which was fractured in the heat-affected zone on the Al side, and the fracture surfaces demonstrated ductile fracture.

Effect of Mechanical Stirring on High-Speed GMAW Hump Bead.

High-speed GMAW tends to be accompanied by periodic humping defects, thereby reducing the weld bead quality. A new method was proposed to actively control the weld pool flow for eliminating humping defects. A high-melting point solid pin was designed and inserted into the weld pool to stir the liquid metal during the welding process. The characteristics of the backward molten metal flow were extracted and compared by a high-speed camera. Combined with particle tracing technology, the momentum of the backward metal flow was calculated and analyzed, and the mechanism of hump suppression in high speed GMAW was further revealed. The stirring pin interacted with the liquid molten pool, resulting in a vortex zone behind the stirring pin, which significantly reduced the momentum of the backward molten metal flow, and thus it inhibited the formation of humping beads.

Investigation on material flow and microstructural evolution mechanism in non-thinning and penetrating friction stir welded Al–Cu aluminum alloy

Non-thinning and penetrating friction stir welding (NTPFSW) was proposed through an innovative welding system. The traditional backing plate was replaced by a small-size stationary shoulder with a blind hole to accommodate the stirring pin. AA2219-T6 aluminum alloy joints were produced via NTPFSW and the examinations on material flow, dynamic recrystallization (DRX), precipitate evolution, and microhardness were carried out. The results revealed that the material flow around the root of the NTPFSW joint was unique owing to a stationary shoulder. Due to the bending moment M between the two sides of the NTPFSW tool being enormous, the inclination angle of the tool was large, and the long pin made this phenomenon even more pronounced. Analyses of DRX behavior indicated that discontinuous dynamic recrystallization (DDRX) was the primary microstructures evolution mechanism in the stir zone (SZ), resulting in the refined grains. Additionally, the dissolution of primary precipitates (θ′ phase) entirely occurred. By comparison, continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX) were dominant in thermo-mechanically affected zone (TMAZ), and more θ′ phases were transformed into θ phases. Owing to the different microstructure evolution mechanisms and precipitates evolution behavior, the TMAZ was characterized by lower microhardness than SZ. These findings provided a basis for tailoring the microstructures and mechanical properties of NTPFSW joints.

The influence of convex shoulder on tribological and mechanical behavior of FSLW of aircraft assembly technology

Regarding the challenges of welding for aluminum alloy, this research illustrates the influence of convex shoulder on mechanical and tribological behavior of friction stir lap welding of aircraft assembly technology. This research employs a spiral whirling needle convex shoulder, and the friction diffusion-welding test is carried out with 2024-T4 aluminum alloy sheets. Then the influence of the numbers of vortex-like needle convex shoulders on the cross-section morphology, material flow behavior, and mechanical properties are compared and analyzed. The experimental results show that there is no obvious bending phenomenon at the lapping interface under the action of a vortex-shaped stirring pin, and increasing the number of vortex-shaped structures is beneficial to increase the cross-sectional width of the nugget area. When the four vortex-shaped mixing needles are adopted, the effective lap width and fracture load of the lap joint with tensile fracture mode are greater than those with six vortex structures, respectively; the values are 11.74 mm and 17531.83 N.

The influence of process parameters on the preparation of closed-cell aluminum foam by friction stir processing

In order to obtain high-quality aluminum foam components, a new preparation method for a 7075 closed-cell aluminum foam based on friction stir processing (FSP) was proposed. The influence of process parameters on microstructures of closed-cell Al foam precursor was investigated by optical metallographic microscope and scanning electronic microscope (OM/SEM). The temperature field and flow field of the Al foam precursor during FSP were simulated by Fluent CFD software. The formability of the Al foam precursor was analyzed by the cupping test. It is found that the rotating speed has great influence on the uniformity and density of the precursor prepared by FSP, but the welding speed has little influence. Numerical simulation shows that the maximum flow velocity of material in the tool shoulder increases by 1.5 times when the rotating speed increases from 800 to 2000 r/min, and the peak temperature at the stirring pin reaches 491 °C, which is consistent with the experimental results. The powder ring is continuous and uniform due to sufficient plastic deformation and flow at travel speed of 50 mm/min and the rotating speed of 2000 r/min. The formability of the precursor increases with the increase of deformation temperature. Especially the cupping test value of precursor is close to that of base metal at 450 °C. In the foaming process, the foam pores first appear at the bottom and then expand upwards with the extension of the foaming time. The expansion rate of the whole foam increases rapidly, and the pores with highly spherical are homogeneously distributed when the foaming time is 110 s at 680 °C.

Enhancements in the Bonding Properties of a Friction Stir Lap-Welded Interstitial Free Steel and Al Alloy by Introducing a Ni Interlayer

A pure Ni interlayer with a thickness of 0.1 mm was introduced between high-strength interstitial free steel and Al–Mg–Si alloy, which were friction stir lap welded, producing an excellent welded joint. The interface layer consisted of a γ-Ni solid solution, and the mixed stirring zone contained alternate lamellae of γ-Ni and α-Fe solid solutions. The addition of a Ni interlayer strongly suppressed the reaction between Al and Fe because of the atomic arrangement of Ni. Furthermore, the insertion depth of the stirring pin has a significant influence on the Al/steel interfacial reaction. Under shallow insertion depth, the intermetallic compounds of both FeAl and Fe2Al5 were observed at the interface layer. A maximum tensile-shear fracture load of 4.3 kN was achieved, with fractures being present in the steel substrate far away from the Al/steel weld.

Effects of pin morphology on the interface defects of the FSWed lap joints of 2A12 aluminum alloy

The current work aims to study the effects of stirring pin morphology on the FSWed lap joints defects and mechanical properties. The 2A12 aluminum alloy friction stir welding lap joints were obtained using two shapes and three thread lengths of stirring pins by experiments. Besides the interface defects forming mechanism were studied by joints plastic metal flow numerical simulation. The results show that the conical pin can lead to uniform and continuous plastic metal velocity field which forms a Hook defect with sharp end, smaller deflection angle (minimum 8°) and larger size (maximum 0.18mm). It also will lead to a larger failure load (maximum is 9.5kN), and the fracture position easily occurs at the Advancing side (AS) of lower plate. The three section pin has a periodic shearing effect causing velocity difference between the arc edge and cutting section of the pin which forms a Hook defect with thicker end, larger deflection angle (maximum 42°) and smaller size (minimum 0.06mm). It also causes discontinuous wavy debris forms in the AS. The three section pin leads to a lower failure load (maximum is just 7.6kN), and the fracture position turns to occur at the lap interface. The increase of the thread length can promote the exchange of plastic metal in the thickness direction and interface materials flow velocity. So the angle (from 20°to 150°) and size (from 0.1mm to 0.6mm) of Hook increases but the height of Cold Lap reduces (increasing the effective sheet thickness of retreating side from 0.69mm to 0.95mm). In addition, the increase of the thread length can make the periodic shear effect of three section pin gradually weak. It is also found that the fracture position is closely related to the shape and the thread length of pins. When using conical pin, the fracture occurs at AS of lower plate or Retreating Side of Nugget Zone (NZ-RS) of upper plate; When using three section pin, the fracture occurs at the bonding interface. And with the increase of the thread length, the fracture position changes from the bonding interface to the NZ-RS of upper plate, and finally to the AS of lower plate.

Parametric optimization for friction stir processing in Al-Zn-Mg-Cu alloy

ABSTRACT Friction stir processing (FSP) is an effective technique in microstructural refinement. It is expected to make a great change in the manufacturing industries. However, the weak region of thermo-mechanically affected zone (TMAZ) limits the practical application of FSP. Hence, double-sided FSP (DFSP) was proposed to eliminate such weak zone. In this work, 7050-T7451 alloy plates were processed by the single-sided FSP (SFSP) and DFSP (two parameters: DFSP-0O-50D and DFSP-30O-35D). The DFSP-30O-35D parameters can ensure the overlapping of the former and later stir pin, which prepared the refined and homogeneous microstructure without TMAZ, and result in a significant enhancement of mechanical properties. In the former SZ, the main precipitates are Al2CuMg and η (MgZn2) phase; in the later SZ, the main precipitates are η′ phase with smaller size and higher quantity. It caused a higher microhardness of the later SZ than the former ones.

Effect of Stirring Pin Rotation Speed on Microstructure and Mechanical Properties of 2A14-T4 Alloy T-Joints Produced by Stationary Shoulder Friction Stir Welding.

In this study, 2A14-T4 Al-alloy T-joints were prepared via stationary shoulder friction stir welding (SSFSW) technology where the stirring pin’s rotation speed was set as different values. In combination with the numerical simulation results, the macro-forming, microstructure, and mechanical properties of the joints under different welding conditions were analyzed. The results show that the thermal cycle curves in the SSFSW process are featured by a steep climb and slow decreasing variation trends. As the stirring pin’s rotation speed increased, the grooves on the weld surface became more obvious. The base and rib plates exhibit W- or N-shaped hardness distribution patterns. The hardness of the weld nugget zone (WNZ) was high but was lower than that of the base material. The second weld’s annealing effect contributed to the precipitation and coarsening of the precipitated phase in the first weld nugget zone (WNZ1). The hardness of the heat affect zone (HAZ) in the vicinity of the thermo-mechanically affected zone (TMAZ) dropped to the minimum. As the stirring pin’s rotation speed increased, the tensile strengths of the base and rib plates first increased and then dropped. The base and rib plates exhibited ductile and brittle/ductile fracture patterns, respectively.

Friction stir butt welding of magnesium alloy to steel by truncated cone-shaped stirring pin with threads

Abstract A truncated cone-shaped stirring pin with threads was designed to enhance friction stir butt welding (FSBW) quality of AZ31B magnesium alloy and Q235 low carbon steel. The effects of welding speed and rotation speed on the microstructure and properties of joints were studied. Microstructure characteristics and element distribution along the Mg/steel interface were analyzed. The results showed that the truncated cone-shaped stirring pin with threads effectively improves the metal flow behavior, especially the fluidity of steel. A steel-strip was formed at the bottom of the Mg side weld nugget during the welding process. The stirring pin effectively improved the interfacial reaction at the Mg/steel dissimilar metal. A thin accumulation layer of the Al was observed along with the interface. Discontinuous oxides of Mg appeared in the interface and were mainly distributed in the upper and bottom interfaces of the joint. As the heat input increased, the metallurgical bonding of the Mg/steel interface became stronger, and the joint strength increased. But when the heat input increased to a certain level, the oxidation of the Mg/steel interface became serious. The aggregation of MgO in the Mg/steel interface led to cracks, which decreased the joint strength. When the welding speed was 75 mm/min and the rotation speed was 850 rpm, the highest tensile strength of the joint was 189.4 MPa, which is 75.8 % of magnesium alloy. The joint fractured in the Mg side near the Mg/steel interface.

Influence of laser shock peening on the coefficient of thermal expansion of Al (7075)-based hybrid composites

Integration of laser shock peening (LSP) and friction stir welding (FSW) was conducted for TiC strengthening on 7075 aluminum (Al) alloys. During FSW, TiC nanoparticles were mixed by using a stir pin to obtain TiC coating with FSWed joints. Nanocrystallines on the surface of Al-based hybrid composites with high-density dislocation were generated through LSP. Microstructural variables, including dislocations and precipitates of the different treatment specimens, were characterized through transmission electron microscopy and X-ray diffractometry. Thermal expansion of cylindrical treated samples was measured from room temperature to 300 °C. The coefficient of thermal expansion (CTE) value of the LSPed samples with TiC decreased by approximately 30% compared with that of the as-FSW sample. Differential scanning calorimetry (DSC) provides insights into the thermal expansion and strengthening mechanisms of LSP. These conditions were mainly caused by the acceleration of precipitated phase transformation from metastable η′ to stable η, resulting in the negative growth of the lattice constant of Al matrix and a decrease in CTE. In addition, the low CTE and dislocation pinning are important in providing a significant synergistic contribution to retard the thermal expansion of Al (7075)-based hybrid composites.

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.