Thermo-mechanically Affected Zone Research Articles

Creep Behavior and Fracture Mechanisms of the Dissimilar Inertia Friction Welded Joints of Deformed and Powder Metallurgy Ni‐Based Superalloys

ABSTRACTIn this research, the microstructure and precipitate characteristics of the dissimilar inertia friction welded (IFW) joints of deformed and powder metallurgy (PM) nickel‐based superalloys were studied using optical, scanning electron, and transmission electron microscopy. In addition, several creep tests were conducted. The high‐temperature mechanical properties of the IFW joints were systematically analyzed. Under the creep testing condition of 680°C, the specimens exhibited creep fracture at the thermomechanically affected zone (TMAZ) of the PM superalloys. Further, the failure lifetime is enhanced with a reduction in the applied creep loading. Owing to the IFW process, various γ′ precipitates and carbide distributions were observed in the various zones of a dissimilar IFW joint. Undissolved powder particle boundary (PPB) defects in the TMAZ of the PM superalloy initiated creep cracks under creep loading. Based on the experimental results and theoretical analysis, the creep fracture mechanisms of the dissimilar IFW joints were revealed. Thus, the findings of this study provide guidance for controlling the microstructures and properties of dissimilar deformed/PM nickel‐based superalloy IFW joints.

Structure-property correlation for friction stir welded joints of 2219Al alloys microalloyed with Cd

Present study aimed at experimental investigation of microstructural evolution, microhardness profile, tensile and impact properties of friction stir welded (FSW) joints of 2219Al alloys microalloyed with varying (up to 0.1 wt%) Cd contents. FSW was performed on the cast and homogenized alloys. Microstructural analysis from weld line up to base metal, identified three sequential heat affected weld zones having separate grain morphologies, namely Weld Nugget Zone (WNZ), Thermo-Mechanical Affected Zone (TMAZ) and Heat Affected Zone (HAZ). Vickers microhardness value increased from weld line towards the WNZ, then decreased towards the TMAZ, and finally increased through HAZ towards the base metal, which exhibited higher hardness compared to all the heat affected weld zones. Microhardness, yield and tensile strengths of the FSW joint of 2219Al alloy increased, due to microalloying with 0.06 wt% of Cd contents, which was attributed to continuous grain refinement. While tensile ductility and toughness, and impact toughness of the welded joint reduced, resulting from trace additions of Cd. Investigated alloys retained significant mechanical strength, ductility and toughness, on the respective joints, following to the FSW operation. Cd was observed to be a potential microalloying element to control the microstructure, refine grain size and improve mechanical strength and hardness of the welded joint of 2219Al alloy. Present experimental results established a structure-property correlation, in order to validate potential application of FSW technique on investigated 2219Al alloys with trace additions of Cd, to attain desirable weld-quality avoiding welding imperfections.

Microstructure, MAO performance, interfacial characteristics and corrosion behavior of FSW joint of Al–Mg-Sc alloy

The microstructure evolution, micro-arc oxidation (MAO) performance, and corrosion behavior of Al–Mg-Sc alloy friction stir welded (FSW) joint were investigated. The microstructure observations indicated that compared with the base metal (BM), the stirring zone (SZ) and thermo-mechanical affected zone (TMAZ) showed notable grain defects and an enormous number of high angle grain boundaries (HAGBs). Additionally, a more conspicuous presence of non-uniform recrystallized grains and HAGBs was noticed in the thickness direction within the SZ. FSW process induced the precipitation of Al3Mg2 (β phase) at grain boundaries in the heat-affected zone (HAZ) of joint. The microstructural changes and precipitation induced by the FSW process influenced the electrical conductivity, resulting in the differences in micro-arc discharge in various zones of the FSW joint in MAO process, which in turn affected the thickness, porosity, and corrosion resistance of MAO ceramic film. HRTEM observation suggested that a transition layer composed of nanocrystalline and amorphous Al2O3 with average thickness of 2–4 nm was found at the film/substrate interface. Electrochemical tests suggested that a heterogeneous structure in various regions of FSW joint resulted in varying susceptibility to localized corrosion. HAZ/TMAZ had the worst anti-corrosion performance. After MAO treatment, the anti-corrosion performance of SZ and HAZ/TMAZ in FSW joint was significantly improved, especially SZ.

Microstructural evolution and mechanical performance of friction stir spot welded ultrafine carbide-free bainitic steel: Role of dwell time

This study focuses on unraveling the role of dwell time on microstructural evolution, mechanical performance, and failure characteristics of friction stir spot welded ultrafine carbide-free bainitic steel. Results clarify that the microstructures of the different weld zones exhibit a receptive behavior to extended dwell time, by which not only coarser martensite is acquired within the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and fine-grain heat affected zone (FGHAZ) but also sub-critical heat affected zone (SCHAZ) experiences a more pronounced tempering. Applying a dwell time of 3 s is sufficient to make an ample bonding width containing refined martensite, being effective in retarding the crack propagation, and in turn, delivering a joint with the highest load-bearing capacity. However, enlarging the bonding width at dwell times beyond 3 s fails to provide higher peak loads owing to the formation of coarse martensite within the SZ. The failure modes of the welds are classified into two distinct modes: interfacial and partial pullout. The reduced upper sheet thickness (∼685 μm) paves the way for crack propagation along the thickness direction, allowing for partial pullout failure to occur.

Comparative study of microstructural evolution and mechanical properties in friction stir processed, reinforced friction stir processed, and heat-treated reinforced friction stir processed Al composites

The development of friction stir welding (FSW) created the basic foundation for friction stir processing (FSP). This process entails refining the local microstructure and achieving the localised plastic deformation due to the stirring effect of the tool, which may involve adding additional filler material as required. The rotating tool involved in the process has a shoulder and pin responsible for heat generation for plastic deformation and localised mixing. The rotating tool is inserted and traversed on the desired surface, tailoring the material properties. The tool’s exposure to the surface causes localised plastic deformation and thermal gradient, and microstructural changes occur locally. The present investigation uses Friction Stir Processing (FSP) to fabricate an aluminium metal matrix composite. Recently, the use of aluminium alloy has been replaced by composites because of their increased mechanical and physical properties (specific properties). The experiment leads to manufacturing an aluminium metal matrix composite using aluminium oxide, boron carbide and commercially pure copper dust. Later, the effect of heat treatment on the respective metal matrix composite is studied. A comparative study among three different types of FSP techniques viz. FSP, Reinforced FSP, and Heat-treated Reinforced FSP have been performed. The tool revolution speed (RPM) and tool traverse speed of 400 RPM and 10 mm/min had been implemented, respectively. The T4 Heat-Treatment cycle was used during the current investigation. This investigative study was performed to analyse the changes in grain sizes and hardness values at the stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and on the base materials (BM) of the samples. The reinforced FSP metal matrix composite (MMC) showed an 81.26% increase in the micro-hardness value (HV) compared to the base metal. Further on analysing the microstructures, the reinforced FSP composite showed a 65.20% reduction in grain size. Moreover, the Heat-Treated sample showed an increase in the micro-hardness value by 99.8%.

Grain Microstructure in Friction-Stir-Welded Dissimilar Al/Mg Joints of Thin Sheets with/without Ultrasonic Vibration.

Electron backscattered diffraction (EBSD) characterization was conducted on the typical regions in friction-stir-welded dissimilar Al/Mg joints of 2 mm thick sheets with/without ultrasonic assistance. The effects of ultrasonic vibration (UV) on the grain size, recrystallization mechanisms, and degree of recrystallization on both sides of the Al-Mg bonding interface and the intermetallic compounds (IMCs) were investigated. It was found that on the Mg side of the weld nugget zone (WNZ), the primary dynamic recrystallization (DRX) mechanisms were discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX), with geometric dynamic recrystallization (GDRX) playing a secondary role. On the Al side of the WNZ, CDRX was identified as the primary mechanism, with GDRX as a secondary contributor. While UV did not significantly alter the DRX mechanisms in either alloy within the WNZ, it promoted the aggregation and rearrangement of dislocations. This led to an increase in high-angle grain boundaries (HAGBs) and an enhanced degree of recrystallization in the welds. The average grain size in both the Al and Mg alloys of the WNZ followed a pattern of initially increasing and then decreasing along the thickness direction, reaching a maximum in the upper-middle part and a minimum at the bottom. The influence of UV on the average grain size in the WNZ was minimal, with only slight grain refinement observed, and the minimum refinement degree was only 0.9%. The Schmid factor (SF) on the WNZ and thermo-mechanically affected zone (TMAZ) boundary regions of the advancing side (AS) indicates that the application of UV increased the likelihood of basal slip and extension twinning in the crystal structure. In addition, UV reduced the thickness of IMCs and improved the strength of the Al-Mg bonding interface. These results suggest a higher probability of fracture along the TMAZ and WNZ boundary on the AS when UV was applied.

Microstructure and mechanical properties of continuous drive friction welded Ti2AlNb alloy under different rotational rates

Ti2AlNb-based alloy was joined in a continuous drive friction welding machine under different rotational rates (500, 1000 and 1500 r/min). The microstructure and mechanical properties of the joints were investigated. It is shown that the weld zone (WZ) is fully composed of recrystallized B2 phase, and the grain size decreases with increasing rotational rate. The thermo-mechanically affected zone (TMAZ) suffers severe deformation during welding, due to which most of original precipitation phase is dissolved and streamlines are present. In the heat affected zone (HAZ), only the fine O phase is dissolved. The as-welded joint produced using 1000 r/min has the best mechanical properties, whose strength and elongation are both close to those of the base metal, while the as-welded joint obtained using 500 r/min exhibits the worst mechanical properties. Post-weld annealing treatment annihilates the deformation microstructure and fine O phase precipitates in the joints, consequently improving the mechanical properties significantly. Decomposed α2 phase is a weakness for the mechanical performance of the joint since microcracks are apt to form in it in the tensile test.

Processing, experimental characterization and thermal analysis for friction stir welded joint of 2219Al alloy

ABSTRACT Present study aimed at thermal modelling and numerical analysis of dynamic heat flow behaviour, for friction stir welded (FSW) joint of 2219Al alloy, further experimentally correlated with microstructural evolution and microhardness profile through different weld zones. FSW was performed on cast 2219Al alloy, with varying process parameters of welding and rotational speed of the tool, and transient temperatures measured at different distances from weld line. From microstructural analysis, three different heat affected weld zones having separate grain morphologies were identified, namely Weld Nugget Zone (WNZ), Thermo-Mechanical Affected Zone (TMAZ) and Heat Affected Zone (HAZ). Vickers microhardness was evaluated at varying distances from weld line, through different weld zones. To investigate transient temperature profiles over the welded plate, a 3-D Finite Element (FE) thermal model was developed, incorporating volumetric heat source as heat generation. Transient temperature profiles computed from FEM analysis, were successfully compared and validated with experimental results, with fairly good accuracy within 4.86 %. Individual influences of welding parameters, on transient temperature profile, weld zones, microstructural and mechanical property were investigated, to optimise and achieve desired weld quality. Present study established structure-property correlation, along with thermal analysis, to validate and optimise the potential application of FSW on 2219Al alloy.

Driving forces of solid-state Cu-to-Cu direct bonding suppressing the work-hardening loss by refill friction stir spot welding

In this study, the solid-state bonding of the Cu-to-Cu parallel connection with no fusion zone and keyhole formation was achieved using the refill friction stir spot welding (RFSSW) process. During the refilling process, significant plastic flow and thermo-compressive behavior promoted geometric and continuous dynamic recrystallization behaviors and highly oriented (111) microstructures, serving as key mechanisms to prevent work-hardening loss and reinforce the joint. Furthermore, the refinement of the grain size without work-hardening loss, increased by 50.8 % which was from 61 Hv in the base material to 92 Hv in the thermo-mechanically affected zone (TMAZ). Even, the hardness of the TMAZ/stir zone interface noticeably increased up to 138 Hv as the reduction in the stacking fault energy (SFE) due to the infiltration of Al element at the trace from the tool. The contribution of microstructural features to the mechanical property was quantitatively investigated by a modified Hall-Petch relationship in terms of the grain size, sub-grain size, and dislocation density. This study systematically investigated the driving forces for the implementation of Cu-to-Cu direct bonding, where work-hardening loss can be suppressed, through the RFSSW process from the perspective of dynamic recrystallization behavior and SFE due to partial alloying of Al elements.

Fracture toughness and microstructural analysis of rotary friction welded S355J2 and SS316L steels for critical applications

Dissimilar metal welding is seeing growing adoption across industries to enhance structural functionality and efficiency. Achieving high-quality, defect-free dissimilar weld joints requires a comprehensive understanding of the interrelationships between the welding-induced microstructural changes and the material's performance characteristics, particularly its fracture-related properties. This study investigates the impact of microstructural changes on the fracture toughness of dissimilar welds between structural low-carbon steel (S355J2) and austenitic stainless steel (SS316L) prepared using the Rotary Friction Welding (RFW) technique. Welding preforms were created from respective pipe pup pieces. The evaluation involves microstructural analysis, tensile testing, hardness testing, and fracture toughness testing using compact tension specimens derived from various zones of the weld joints. Results revealed significant microstructural differences across the weld joint. The weld region exhibited stable hardness with a maximum of 208 HV1 in S355J2′s thermo-mechanically affected zone (TMAZ). High tensile strength (Ultimate Tensile Strength 540 MPa, Yield Strength 367 MPa) with failures mainly on the S355J2 side. The fracture toughness (KQ) matched parent metal values, with the RFW weld centre line (WCL) showing superior crack tip opening displacement (CTOD) of 0.35 mm. Fractography generally indicates ductile failure.

Thermo-mechanical characterization of pinless friction stir welding in AA6060-T4 thin sheets

Friction Stir Welding (FSW) is a solid-state welding method, offering superior weld quality for aluminum alloys. Despite its advantages, the thermo-mechanical characteristics of pinless FSW, especially for AA6060-T4 aluminum thin sheets, remain underexplored. This study aims to fill this gap by examining the effects of different welding parameters on the mechanical properties and thermal distribution in pinless FSW of 2 mm AA6060-T4 sheets. Experimental welding tests were conducted using high alloy steel tools and a modified milling machine, with variations in rotation and feed rates. The results show that higher rotation speeds correlate with lower maximum temperatures, and increased feed rates further reduce temperatures. Optimal welding parameters achieved efficiency coefficients as high as 84%. Microhardness profiles indicated minimum hardness in the Thermo-Mechanically Affected Zone (TMAZ) and a significant increase on the advancing side. These findings enhance the understanding of pinless FSW, emphasizing the interaction between welding parameters and mechanical properties. This research advances the application of AA6060-T4 in various industrial sectors by providing valuable insights into optimizing weld quality and mechanical performance.

Experimental investigation of friction stir welding with special spherical ball shoulder different probe tools and parameters for enhanced material properties of AA2219 aluminium alloy

This research investigates the impact of unique spherical ball shoulder enhancement tool pin configurations on the friction stir welding (FSW) of AA2219 aluminum alloy under various parameters. FSW was performed using three different pin shapes: square, hexagonal, and octagonal. Testing samples were prepared using an L9 orthogonal array layout with varying welding parameters. The material behavior in the three distinct weld zones was evaluated for each pin configuration. Results indicated superior microhardness values in the heat-affected zone (HAZ) and thermo-mechanically affected zone (TMAZ) compared to the weld nugget zone. Elemental composition analysis of the TMAZ revealed significant differences, highlighting the influence of thermal stress and material behavior due to the tool pin and shoulder configuration. The square-headed tool demonstrated improved tensile strength and microhardness. Optimal welding performance was achieved with a spindle speed of 1000 RPM, a travel speed of 25 mm/min, and a tilt angle of 3°. Variations in Cu content in the TMAZ and HAZ underscored localized modifications within the material structure, driven by the spherical ball spinning effects on the shoulder. Tensile tests showed that welds made with the square pin consistently exhibited higher tensile strength compared to those made with hexagonal and octagonal pins.

Improved mechanical behavior of friction stir drilled 6082 aluminum alloy via T6 treatment

6082 Al-alloy underwent a friction stir drilling (FSD) processing. Subsequently, a two-stage post-heat treatment (PHT) procedure, i.e., solution treatment at 550 °C for 2 h, followed by artificial aging at 175 °C for 18 h, was implemented to significantly enhance microstructure and mechanical performance of the alloy. The microstructure was characterized by optical, laser, scanning electron and electron backscattered diffraction microscopes, while the mechanical behavior was assessed by a micro-indentation hardness tester, employing the indentation hardness (HIT) technique. Corrosion behavior of FSD as well as PHT specimens was investigated through electrochemical techniques.The microstructure of the base metal (BM) manifested an α-Al matrix with an average grain size of approximately 50 μm. FSD resulted in the creation of stirring zone (SZ) and thermo-mechanical affected zone (TMAZ), with respective widths of 400 ± 50 and 950 ± 100 μm. The SZ exhibited a very fine structure with an average grain size of 0.5 μm, thereby displaying increased hardness. The PHT microstructure displayed the formation of fine βʹ and βʺ second phase precipitate types, whereas the HIT hardness of the PHT α-aluminum matrix in the SZ, was lower than TMAZ and BM, due to reductions of βʹ in the SZ compared to TMAZ and BM. Moreover, the corrosion rate of the SZ decreased after the heat-treatment cycle, attributed to the recovery of strain energy induced during friction drilling and the increased solubility of solute atoms within the α-Al matrix.

Chloride-Induced Stress Corrosion Cracking of Friction Stir-Welded 304L Stainless Steel: Effect of Microstructure and Temperature

Dry storage canisters of used nuclear fuels are fabricated using SUS 304L stainless steel. Chloride-induced stress corrosion cracking (CISCC) is one of the major failure modes of dry storage canisters. The cracked canisters can be repaired by friction stir welding (FSW), a low-heat input ‘solid-phase’ welding process. It is important to evaluate the ClSCC resistance of the friction stir welded material. Stress corrosion cracking (SCC) studies were carried out on mill-annealed base materials and friction stir welded 304L stainless U-bend specimens in 3.5% NaCl + 5 N H2SO4 solution at room temperature and boiling MgCl2 solution at 155 °C. The engineering stress on the outer fiber of the FSW U-bend specimen was ~60% higher than that of the base metal (BM). In spite of the higher stress level of the FSW, both materials (FSW and BM) showed almost similar SCC failure times in the two different test solutions. The SCC occurred in the thermo-mechanically affected zone (TMAZ) of the FSW specimens in the 3.5% NaCl + 5 N H2SO4 solution at room temperature, while the stirred zone (SZ) was relatively crack-free. The failure occurred at the stirred zone when tested in the boiling MgCl2 solution. Hydrogen reduction was the cathodic reaction in the boiling MgCl2 solution, which promoted hydrogen-assisted cracking of the heavily deformed stirred zone. The emergence of the slip step followed by passive film rupture and dissolution of the slip step could be the SCC events in the 3.5% NaCl + 5 N H2SO4 solution at room temperature. However, the slip step height was not sufficient to cause passivity breakdown in the fine-grained SZ. Therefore, the SCC occurred in the partially recrystallized softer TMAZ. Overall, the friction-stirred 304L showed higher tolerance to ClSCC than the 304L base metal.

Study on the microstructure and properties of spray formed 7055-T76 aluminum alloy joints by rotating magnetic field assisted friction stir welding

This study explores an electrically-magnetically-assisted friction stir welding (EMAFSW) method, conducting butt welding experiments on T76 tempered spray-formed 7055 aluminum alloy. Compared to traditional friction stir welding (FSW) processes, the EMAFSW method maintained basically consistent weld hardness, while tensile strength and elongation increased by 13.1% and 72.3%, respectively. The rotating magnetic field enhanced the mechanical stirring action of the welding tool, reduced the size of precipitates, promoted dislocation pinning, and increased the mobility of dislocations. The stirred zone (SZ) and thermo-mechanically affected zone (TMAZ) of EMAFSW weld joint showed significant grain refinement and increased recrystallization proportion, enhancing the weld joint toughness. The joint's fracture behavior shifted from brittle to ductile fracture.

Optimizing friction stir welding of AA7075 and AA8090 aluminum alloys: a desirability-driven investigation into mechanical and microstructural enhancement

This research explores the multifaceted analysis of a friction-welded joint, employing Central Composite Design of Response Surface Methodology. The study integrates microstructural investigations and fracture analyses to explain the effect of process parameters on mechanical properties. The optimum settings for Friction Stir Welding of AA7075 and AA8090 were determined by assessing desirability indices. These settings comprised a tool rotation speed of 1927.7 rpm, a tool travel speed of 35 mm min−1, and a tool tilt angle of 0.9°. This specific combination yielded a noteworthy combined desirability index of 0.79, considering both Ultimate Tensile Strength (UTS) and Tensile Elongation (TE). Microstructural examinations revealed distinct characteristics in the Heat-Affected Zone (HAZ), Thermo-Mechanically Affected Zone (TMAZ), and Nugget Zone (NZ). Notably, fine grain structure in the NZ was attributed to the stirring effect created by the tool pin. Fracture analyses indicated ductile fractures, with dimple size variation correlating to tensile strength. Lower dimple density in low-strength joints suggested insufficient material mixing during welding. The maximum tensile strength sample exhibited a high dimple density. These findings contribute to a comprehensive understanding of the welding process’s influence on microstructure and fracture characteristics, providing valuable insights for optimizing mechanical properties in friction-welded joints.

Comparative numerical analysis between butt and lap joints of Mg alloy friction stir welding with considering tilted tool effect

Magnesium (Mg) alloy, as an important lightweight structural metal, can be effectively joined by the friction stir welding (FSW) method. Understanding FSW temperature and material flow coupling process in butt and lap welding configurations has important theoretical significance. In this study, the tilted tool induced incomplete contact distance is calculated by process parameters and slip rate, and the deflection of the contact interface is related to the tool torque. The non-uniform boundary conditions are considered. The numerical model is proposed and experimentally validated by using the measured temperatures and thermo-mechanically affected zone (TMAZ) profiles. The comparative numerical analysis between butt and lap welding with three levels of process parameters is further conducted. The peak temperature and material flow velocity along the specified circumferences in FSBW are more than 10 K and 4 mm/s higher than these in FSLW, respectively. When the rotation rate increases from 600 rpm to 800 rpm, the peak temperature and material flow velocity along the specified circumferences increase about 20 K and 9 mm/s, respectively. When the welding speed decreases from 150 mm/min to 30 mm/min, the temperature and material flow velocity fluctuations along the specified circumference decreases about 25 K and 14 mm/s, respectively.

Role of oscillation frequency and amplitude on the microstructure and properties of linear friction welded nickel aluminium bronze joints

This study explores the influence of oscillation frequency and amplitude on the quality of linear friction welded joints using as-cast nickel aluminium bronze. Welding was conducted at 30 Hz, 50 Hz, and 70 Hz oscillated frequencies and amplitudes of 1.5 mm, 2 mm, and 3 mm. The joint’s performance was thoroughly investigated through systematic analysis, including macrostructure examination, peak interfacial temperature measurement, microstructure evaluation, strain assessment, cooling rate determination, microhardness testing, and tensile property characterization. The width of the weld zone varied from 183 μm to 297 μm, and the thermomechanical affected zone (TMAZ) area ranged from 4.48 mm2 to 14.79 mm2 across different process parameters. In the parent material, the volume fraction of the β-phase was as low as 20.2%, contrasting with the dominant α-phase at 79.8%. The average grain size of the lamellar and globular α-phase mixture was 26.4 μm. Notably, the weld zone exhibited extremely refined α-phase grains, with diameters less than 5 μm in all cases. The volume fraction of the β‘-phase increased significantly with higher frequencies, from 15.299% at 30 Hz to 26.98% at 50 Hz, peaking at 40.08% at 70 Hz, leading to varying k phases. This variation in microstructure had a substantial impact on mechanical properties. Tensile strength ranged from 503 MPa to 582 MPa, while ductility varied from 13.5% to 21.7%. Additionally, the hardness of the parent material increased from approximately 155 Hv to 260 Hv. This study demonstrates that controlling the oscillation frequency and amplitude in linear friction welding processes can yield consistent, high-quality welds in nickel aluminium bronze.

Enhanced mechanical properties of thermo-mechanically affected zone of friction-stir welded AZ61 magnesium alloy joint

Friction-stir welded (FSW) magnesium alloy joints frequently fracture at thermo-mechanically affected zone (TMAZ), and the joints exhibit unsatisfactory welding efficiency and ductility. The TMAZ is considered as the weakest region of the entire joint. Enhance the mechanical properties of TMAZ can improve the performance of the welded joint. In this work, subsequent aging treatment and pre-strain of 3% along the weld seam were conducted on the FSW AZ61 magnesium alloy joint. The microstructure of the TMAZ is characterized by abundant nanoscale second phase particles and {10–12} twins. The reduced basal texture intensity decreased the anisotropy of plastic deformation. The welded joint showed a good synergic of welding efficiency (96%) and ductility (20%). The enhanced strength is attributed to the blocking effect of nanoscale particles and grain/twin boundaries to dislocation movement. The increasing of twin boundaries is the main reason for the improvement of work hardening ability and ductility during tensile deformation. This work demonstrated that introducing {10–12} twins and nanoscale particles into TMAZ is an effective way to improve the balance of strength and ductility of the welded joint due to the synergistic effect of multiple strengthening and toughening mechanisms.

Microstructural response characteristics of SAC305/Cu column joint under friction embedded welding conditions

The friction welding method for macro-scale weldments was introduced into the manufacturing of electronic products. Without mold assistance, the positioning and connection of micro copper columns in column grid array packaging (CGA) devices were achieved. The average connection strength of 40.6 N and the intermetallic compound interface layer meant that effective connections were achieved. In the poorly identified stirring-mixing zone (SMZ) in scanning electron microscopy (SEM) images, electron backscatter diffraction (EBSD) results showed that this region was composed of recrystallized grains, twin grains, and Ag3Sn and Cu6Sn5 particles distributed inside or at the grain boundaries. The grain size was about 2 μm, and the Ag3Sn and Cu6Sn5 particle sizes were broken to 250 nm and 120 nm due to stirring. The SMZ at the bottom of the copper column only underwent deformation-induced dynamic recrystallization, while the SMZ on the side was discharged from the bottom of the copper column, so it underwent dynamic recrystallization through the process of cutting crushing-extruding and flow-bonding-deformation. Therefore, the hardness of SMZ was significantly reduced after welding. Unlike traditional friction welding, in the thermo-mechanical affected zone (TMAZ), the solder not only underwent deformation but also formed 47.8% recrystallization microstructure and 14.6% recovery microstructure. Therefore, the effect of deformation strengthening was weakened and recrystallization led to a decrease in hardness. Recovery was the main response behavior in the micro-deformation zone (MDZ). Combined with the numerical simulation results, the temperature and stress driving conditions of the microstructure of each micro-region in instantaneous friction embedded welding (FEW) were also discussed.