Thermal-mechanical Affected Zone Research Articles

Influence of post-weld rolling and artificial aging on microstructure and mechanical properties of friction stir welded 2195-T4 Al-Li alloy joints

Fiction stir welding (FSW) of a naturally aged (T4) 2195 Al-Li alloy was conducted to study the effect of post-weld artificial aging (AA) and rolling+artificial aging (R+AA) on microstructure and mechanical properties of the FSW joints. Grain features were analyzed by electron back scattered diffraction (EBSD) technology, and precipitates were characterized using transmission electron microscopy (TEM) along [011]Al and [001]Al zone axes. Mechanical properties of the joints were evaluated through micro-hardness and tensile testing. The results indicate that grain development, including grain orientation, grain boundary components and dislocation density, varies within different regions of the As-welded joints due to complicated thermal-mechanical history. The alloying elements mostly exist as Guinier-Preston zones in the base material (BM) with a few formations of δ'/β', and slightly precipitate into T1 in the heat affected zone (HAZ), form σ in the thermal-mechanical affected zone (TMAZ), as well as develop T1 and S' in the nugget zone (NZ) after FSW. The lowest hardness zone (LHZ) exhibits elevated precipitation levels, generating coarse T1 and grain boundary precipitates with evident σ and precipitate free zone. The overall precipitation level of the As-welded joints remains minor, resulting in a slight decrease in hardness at the LHZ. Post-weld AA promotes many T1 formations, enhancing the joint strength and hardness. As a result, the ultimate tensile strength (UTS) of the joints increases from 440 MPa to 506.5 MPa, while the elongation decreases from 13.5% to 3.4%. Furthermore, hardness profiles transform from a low-amplitude wave curve to a high-amplitude W-shaped curve, generating a significant LHZ due to the minimal precipitation of T1. Pre-rolling deformation can enhance dislocation density and low angle grain boundaries (LAGBs), promoting the nucleation of high-density, fine T1 during artificial aging, and especially improves precipitation ability of the LHZ, leading to an elevated hardness increment in the region after AA. Compared to the Joint-AA, both strength and elongation of the Joint-R+AA increase, obtaining 530.5 MPa and 3.9%, respectively.

Friction weldability of ultrafine-grained titanium grade 2

The paper presents the results of an experimental study of the friction weldability of Titanium Grade 2 with ultrafine-grained (UFG) microstructure obtained by hydrostatic extrusion and rotary swaging (HE+RS). High-speed friction welding with different rotational speed (n) values was used as a joining method. The best properties were obtained for n=7000 rpm. Metallographic observations confirmed the formation of a continuous friction joint with a changed microstructure of the base material in the thermal-mechanical affected zone (TMAZ) and a friction weld (FW). The results of the mechanical properties showed the greatest decrease in hardness in the TMAZ. The friction welded joint had an 18 % lower tensile strength value than initial state UFG Titanium Grade 2 after the HE+RS process (with UTS=1050 MPa).

Investigation on submerged friction stir welding of AZ31B magnesium alloy under the influence of rotation speed

In this study, submerged friction stir welding (SFSW) was performed to weld 6 mm thick AZ31B Mg alloy plates, aiming to explore the impact of rotation speed on microstructure and tensile behavior. The water medium was used to submerge the samples. The SFSW was conducted at three different speeds (815, 960, and 1200 rpm), to assess the impact of rotation speed on SFSWed joint performance. As the rotation speed increased from 815 to 960 rpm, tensile strength increased plateauing over a range of the rotation speed. However, a significant drop in tensile strength occurred at 1200 rpm due to the formation of void defects. The SZ exhibits a size and width increment in the lower part with increasing rotation speed. The hardness of the stir zone (SZ) gradually rose with increasing rotation speed from 815 to 960 rpm. Fracture locations were observed in the thermal-mechanically affected zone (TMAZ) adjacent to the SZ at a rotation speed of 815 rpm, and in the heat-affected zone (HAZ) adjacent to the TMAZ at a rotation speed of 960 rpm. The joint welded at 1200 rpm fractured within the SZ. This study offers valuable insights into the welding and joining field, particularly regarding their mechanical characteristics.

Effect of base material temper condition on precipitate evolution and mechanical properties of 2195 Al[sbnd]Li alloy friction stir welding joints

In this study, 2195 AlLi alloy plates with different temper conditions (O, T4 and T8) were subjected to friction stir welding (FSW). Precipitate evolution of the three FSW joints (2195-O/T4/T8 joint) was investigated by transmission electron microscopy (TEM). Mechanical properties of these joints were analyzed using micro-hardness and tensile test. Efforts were made to explore the correlation between precipitates, mechanical properties and fracture behavior of the joints. It can be found that the precipitates from the base material are unevenly dissolved in the nugget zone (NZ) of the 2195-O joint, while the original precipitates mostly disappear in NZ of the 2195-T8 joint, and change a little in the NZ of the 2195-T4 joint. In the thermal-mechanically affected zone (TMAZ) of the 2195-O and 2195-T4 joint, precipitates are almost unchanged, while in the TMAZ of the 2195-T8 joint, most of the precipitates are dissolved and the residual is significantly coarsened. Little change of precipitates occurs in the heat affected zone (HAZ) of the 2195-O joint. T1 newly forms in the HAZ of the 2195-T4 joint (T4-HAZ), while this precipitate is mainly dissolved and coarsened in the HAZ of the 2195-T8 joint (T8-HAZ). Moreover, both grain boundary phases (GBPs) and precipitate-free zone (PFZ) are generated in the T4-HAZ and T8-HAZ. Hardness profile is represented as a “n” shape in the 2195-O joint due to significant harden of the NZ, a “wave line” shape in the 2195-T4 joint attributed to the slight change of precipitation degree, and a “w” shape in the 2195-T8 joint mainly owing to the dissolution and coarsening of T1. The three joints are all broken in their weak regions during tensile test and have a similar fracture behavior, indicating that the crack initiation and development are related to the GBPs and PFZ.

Insight on ultrasonic effect of Al[sbnd]Cu aluminum alloy joints fabricated by ultrasonic assisted non-thinning and penetrating friction stir welding

AlCu alloy joints of ultrasonic assisted non-thinning and penetrating friction stir welding (U-NTPFSW) and NTPFSW were prepared. The microstructure and mechanical properties of the joints were studied comparatively to elucidate the effect of ultrasonic. This study provided an in-depth understanding of the microstructure formation mechanism and strengthening mechanism of joints under the ultrasonic effect. Ultrasonics broadened the width of the stir zone (SZ) and stationary shoulder affected zone (SSAZ) by promoting the material plastic flow ability and changing the material flow at the weld root. The ultrasonic addition enhanced plastic heat production and promoted grain growth in SZ. Compared with NTPFSW, the SZ of both joints was dominated by continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) mechanisms, and geometric dynamic recrystallization (GDRX) grains were also present in U-NTPFSW. The mechanical properties of the joint were improved from 339 MPa to 348 MPa. Ultrasonics introduced numerous vacancies into the weld, which promoted the precipitation of the Al2Cu phase and increased the precipitated phase density of the thermal-mechanical affected zone (TMAZ). The interaction mechanism between the precipitated phase and dislocations was analyzed. The geometrically necessary dislocation (GND) density and residual stress of TMAZ were reduced due to ultrasonic vibration.

Microstructure, mechanical and fracture properties of friction stir welded 2195 Al-Li alloy joints

The 8 mm-thick 2195 Al-Li alloy joints were achieved by Friction Stir Welding (FSW). The microstructural evolution, temperature-dependent mechanical properties, and fracture properties were studied. The T1, δ′/β′ and θ′ precipitates were observed in the Base Metal (BM) and the Heat Affected Zone (HAZ). Most of the precipitates, except for re-precipitated δ′/β′ phases, were dissolved in the Nugget Zone (NZ). The tensile specimens that failed at cryogenic temperatures (–196 °C) had the maximum Ultimate Tensile Strength (UTS), and the fracture surface showed the inter-granular fracture characteristics. Compared to those at room temperature (25 °C), the decreasing tensile properties at high temperature (180 °C) were related to microstructure and strain hardening effects. The NZ presented the optimal fracture toughness, and the Crack Tip Opening Displacement (CTOD) was mutually dominated by microhardness and grain size. Analysis on Fatigue Crack Growth (FCG) rates indicates that the Thermal-Mechanically Affected Zone (TMAZ) exhibited the most superior fatigue resistance performance at stress intensity range below 17 MPa·m1/2 due to compressive residual stress and the crack closure effect. The fatigue fracture surfaces reveal that the crack propagation zone was characterized by the striations and secondary cracks. Also, inter-granular fracture behavior was responsible for the fastest FCG rates in the NZ.

Fatigue fracture behavior of a Ti17 joint under various heat treatment specifications prepared by linear friction welding

A comprehensive study to investigate and characterize the fatigue fracture behavior of linear friction welded Ti17 joint and Ti17 joint under various post welding heat treatment (PWHT) specifications is critical to providing a foundation for the life design of a ‘blisk’ manufactured by linear friction welding. This study addressed this issue. High cycle fatigue tests were carried out. EBSD was employed to examine the microstructures in welding center zone (WCZ), thermal mechanical affected zone (TMAZ), heat affected zone (HAZ) and base metal (BM) across the Ti17 friction welded interface. The results show that the as welded Ti17 joint tends to fracture at WCZ &TMAZ during the fatigue service with a life of 45201, where a heterogeneous structure is formed as ‘refined equiaxed β grain - coarse β grain - basket-weave β grain with lamellar α partially dissolved - basket-weave β grain’ corresponding to ‘WCZ - TMAZ - HAZ - BM’. After aging at 550 °C, the Ti17 joint tends to fracture along the boundary between TMAZ and HAZ + BM with a life of 42338, where a heterogeneous structure is formed as ‘α′ colony with coarse ‘α laths’ - basket-weave β grain’ corresponding to ‘WCZ &TMAZ - HAZ & BM’. After aging at 630 °C, Ti17 joint tends to fracture at BM with a life of 75121, where a heterogeneous structure as ‘refined α′ laths with basket-weave characteristics - coarse basket-weave of α′ laths in β grains' corresponding to ‘WCZ &TMAZ -HAZ & BM’ region is formed.

Simultaneous Joining and Forming of Dissimilar Steels by Electrically Assisted Pressure Joining

In the present study, simultaneous joining and forming of dissimilar steels Q235B (ball) and SCM435 (stud) by electrically assisted pressure joining (EAPJ) are experimentally investigated. The target product is intended with a dust‐cover ring in the stud head, then the electrodes and the stud head are designed accordingly so that the deformed stud head can fill the electrode cavity. In EAPJ, compressive deformation and pulsed electric current are simultaneously applied to the workpieces to create diffusion bonding. As a result, the sound solid‐state joints are successfully fabricated by EAPJ, while a target geometry is also successfully formed. In the bending test, the fabricated ball stud is broken through the ball, and the maximum bending load is higher than the target value for commercial applications. The microstructure analysis and hardness test show that the progress suggested in the present study can cause grain refinement, induce tempered martensite, and increase the portion of polygonal ferrite in the deformed zone (DZ), the joining zone (JZ), and the thermal‐mechanically affected zone (TMAZ), respectively. The results of the present study confirm that the concept of EAPJ is applicable to the simultaneous joining and forming of dissimilar steels.

Investigation of microstructure, texture, and mechanical properties of FeCoNiCrMn high entropy alloy during drive friction welding

FeCoNiCrMn high-entropy alloy (HEA) with FCC structure was welded by continuous drive friction welding. The microstructure and mechanical properties of welding joints were investigated using OM, XRD, EBSD, TEM, microhardness, and tensile tests. The microstructure of welding joints samples consist of three zones: the weld zone (WZ), the thermal-mechanically affected zone (TMAZ) and the base metal zone (BM). The microstructure of WZ is consistent with that of BM, which is a single FCC solid solution. Discontinuous dynamic recrystallization (DDRX) and twin-induced dynamic recrystallization (TDRX) mainly occur in WZ and TMAZ. The hardness of TMAZ is significantly increased and higher than that in WZ and BM; the hardness of WZ is slightly higher than that of BM. The increase of the hardness is related to grain refinement. The tensile strength and uniform elongation of the joint samples are increased by 4.2% and 31% respectively compared with the results of the base metal. The results exhibited ductile fracture.

Microstructure and mechanical properties of underwater friction stir welding of CNT/Al-Cu-Mg composites

The friction stir welding (FSW) joints of CNT/Al-Cu-Mg composites have severe flash defects and softening phenomena due to the excessive heat input. In this paper, underwater friction stir welding (UFSW) was used to transfer the heat input and increase the cooling rate of the joint, which was beneficial to suppress the coarsening of the strengthening precipitates in the thermal mechanically affected zone (TMAZ) and narrow the range of TMAZ. The DIC results demonstrate that the joints obtained by UFSW alleviate the strain concentration in the TMAZ. The nugget zone (NZ) with outstanding performance could synergistically engage in plastic deformation. Thus, the mechanical properties of the UFSW joint increase to 523 MPa, reaching 94.7% of the CNT/Al-Cu-Mg composites, providing further improvement of the FSW joints. Additionally, the flash defects have been improved due to the descent of the thermal cycle, which alleviates the difficulty of post-weld processing and promotes welding efficiency.

Investigation of microstructure and mechanical characteristic of underwater friction stir welding for Aluminum 6061 alloy – Silicon carbide (SiC) metal matrix composite

Demand for metal matrix composites (MMCs) is expected to increase in these applications, such as ‎in the aerospace and automotive sectors. Adequate joining techniques, which are important for ‎structural materials, have not yet been developed for Metal Matrix Composite (MMCs), however. ‎This work aimed to demonstrate the feasibility of ‎friction stir welding (FSW) and ‎underwater friction stir welding (UFSW) for joining Al 6061/5, Al 6061/10, and Al ‎‎6061/18 wt. %SiC composites have been produced by utilizing reinforce stir casting technique. Two ‎rotational ‎speeds,1000and 1800 rpm, and traverse speed 10mm \ min were examined. Specimen ‎composite plates 10 mm thick have been successfully welded by FSW. For FSW and UFSW, a tool ‎made of high-speed steel (HSS) with a conical pin shape was used. The result revealed that the ‎ultimate tensile strength of the welded joint by FSW and UFSW at rotation speed 1800 rpm for (Al ‎‎6061/18 wt. ‎‎% SiC composites) was 195 MPa and 230 MPa respectively. The ultimate ‎tensile ‎strength of the welded joint by FSW and UFSW (Al 6061/18 wt.% SiCe composites) was 165 MPa ‎and 180 MPa at rotation speed ‎‎1000 rpm respectively. The microstructural assessment showed that due ‎to larger grain sizes at FSW and UFSW, most of the fractures are located in the thermal ‎mechanically affected zone (TMAZ) adjacent to the weld nugget zone (WNZ). It is observed that in ‎failure, most of the joints show ductile features. As the volume fraction of SiC (18 wt.%) increases, ‎the friction stir welded and underwater friction stir welded efficiency decreases.

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.

Microstructural response at the interface and its effect on the fatigue fracture behavior of rotary friction welded dissimilar titanium alloys: Ti–5Al–2Sn–2Zr–4Mo–4Cr (Ti17) and Ti–6Al–2Sn–4Zr–2Mo (Ti6242)

Ti17 and Ti6242 tubes were subjected to rotary friction welding to fully understand the microstructural response and its effect on the fatigue fracture behavior of the dissimilar joint. Experiment was conducted using friction linear speed 1 m s−1, friction pressure 80 MPa, burn-off length 5 mm and forging pressure 120 MPa as welding parameters. Electron backscattered diffraction was utilized to systematically examine the microstructures and microtextures across the Ti17/Ti6242 friction welded interface. Thereafter, two fatigue tests were conducted to characterize the fatigue performance of Ti17/Ti6242 joint, one of which is used to identify the fatigue fracture location of the joint and another one to analyze the fatigue fracture behavior. Results clinched that the formation of joint exhibited 600 μm thick weld interface comprises of thermal-mechanical affected zone (TMAZ), heat affected zone (HAZ) and welding zone (WZ) with varying thicknesses (i.e. 100 μm to 200 μm). It was also observed that Ti17 WZ is occupied by equiaxed β grains (mean size of 10 μm) with only one texture which was closely similar to {110} < 111 >. Whereas, Ti6242 WZ contained acicular α’ laths (1 μm) with two types of α texture. Ti17 HAZ + TMAZ were consisted of acicular α’ laths within refined equiaxed β grains and three types of texture of α and β, were respectively found in Ti17 HAZ + TMAZ zone. Whereas, Ti6242 HAZ + TMAZ zone was developed by elongated equiaxed α, acicular α’ laths and few β phases and only two types of texture of α and one type texture of β were found in this zone. The microstructural response across the joint concentrates plastic deformation in Ti6242 HAZ + TMAZ during fatigue cycle, which causes the fatigue fracture occurs at a radial distance of 150μm from the boundary between the Ti6242 TMAZ + HAZ and Ti6242 base metal.

Linear friction welding of dissimilar Ni-based superalloys: microstructure evolution and thermo-mechanical interaction

Linear friction welding (LFW) technology, as an advanced manufacturing approach to fabricate the bimetallic turbine blisks, is receiving increasing attention from aerospace industries. Current work experimentally investigated the microstructural evolution and microhardness in LFW of FGH4096 and GH4169 superalloys. A complete dynamic recrystallization (DRX) was obtained in friction interface zone (FIZ) with no obvious secondary phases including δ and carbide phases on GH4169 side, and primary and secondary γ′ phases on FGH4096 side. The thermal-mechanically affected zone (TMAZ) was characterized by incomplete DRX where a significantly lower amount of low angle grain boundaries existed together with partially dissolved secondary phases depending upon the distance from weld line. The above-mentioned distinct microstructural behavior, manifesting different thermo-mechanical fields at different areas in the joint, necessitated the further analysis of numerical simulations of LFW of two dissimilar alloys. A thermo-mechanical coupling model was developed to predict the temperature, plastic strain and stress distributions. Satisfactory comparisons between numerical and experimental results were observed. The findings in the work are believed to be one of few currently available to focus on the study of metallurgical and thermo-mechanical coupling behavior of the dissimilar nickel-based superalloy joint made by the LFW process.

Evaluation of Optimization Parameters of Semi-Solid Metal 6063 Aluminum Alloy from Friction Stir Welding Process Using Factorial Design Analysis

The semi-solid-metal 6063 aluminum alloy was developed for the automotive industry. The objective of this research was to optimize parameters in friction stir welding process that can provide the highest tensile strength. The ANOVA factorial design was used to analyze rotation speed, welding speed, and tool geometry at different factor levels of experimentation. The results showed that the optimized tensile strength was 120.7 MPa from the cylindrical tool, rotation speed was from 1300 to 2100 rpm, and welding speed less than 75 mm/min in the coefficient of determination R2 was 95.09%, as can be considered from the regression equation. The examination of the stir-zone and thermal mechanical affected zone using SEM and EDX showed that the new recrystallization of the microstructure causes fine grain in the stir-zone, coarse grain in advancing-side thermal mechanical affected zone, and equiaxed grain in the retracting-side thermal-mechanical affect zone. The intermetallic compounds of β-Al5FeSi phase transformation phase were formed to three types, i.e., β″-Al5Fe, Mg2Si, and Al8Fe2Si phase were observed. Moreover, in the stir-zone and thermal-mechanical-affected zone, defects were found such as flash defects, void or cavity defects, crack defects, lack of penetration defects, tunnel defects, kissing bond defects, and dendrite formation defects affecting weldability.

Study on the Precipitation Behavior of Precipitates of 7075 Aluminum Alloy Friction Stir Welding Joint

In this work, the microhardness of 7075 aluminum alloy friction stir welding (FSW) joint was measured by a micro vickers hardness tester, the microstructure of the joints was characterised by microscope, the precipitated phases among the welding nugget zone (WNZ), thermal mechanical affected zone (TMAZ), heat affected zone (HAZ) were affirmed by X-ray diffractometer (XRD) and the lattice fringe of transmission electron microscopy (TEM) high resolution image. Based on this, the precipition behavior of precipitated phases was studied. The results show that the microhardness distribution of the 7075 aluminium alloy FSW joints is heterogeneous in comparison with the base metal (BM). The precipitates in the joint mainly include MgZn rod shape and AlCuMg in elliptical shape. In the WNZ, the main precipitate is AlCuMg, and the fine grain strengthening effect is better, so the microhardness in this zone is relatively high. In the TMAZ, the quantity of AlCuMg decreased while the MgZn2 increased relatively in comparison with the WNZ. At the same time, the effect of the fine grain strengthening was weakened, though the strain hardening increased. Therefore, the microhardness in the TMAZ still decreased. In the HAZ, the quantity of MgZn2 increased furtherly, and there is no strain hardening and fine grain strengthening, so the microhardness of the HAZ was the lowest among the FSW joints. Besides, through comparative tests, the optimal process parameters of friction stir welding of 7075 aluminum alloy were obtained.

Effect of process parameters on mechanical properties of AA5052 joints using underwater friction stir welding

Underwater Friction Stir Welding (UFSW) is a solid-state joining technique which uses a non-consumable tool to weld metals. The objective of this investigation is to evaluate the mechanical properties of the AA5052 Aluminium alloy joints prepared by UFSW. The effect of different type of welding tools and welding parameters on the weld joint properties are studied. Square, tapered cylindrical and taper threaded cylindrical type of welding tools have been used to produce the joints with the tool rotational speed varying from 500 rpm to 2000 rpm while the welding speed varying from 50 mm/min to 150 mm/min. Tensile strength, micro-hardness distribution, fracture features, micro-and macrostructure of the fabricated weld joints have been evaluated. The effect of welding process parameters that influences the mechanical properties and fracture characterization of the joints are explained in detail. A maximum Ultimate Tensile Strength (UTS) value of 222.07 MPa is attained with a gauge elongation of 14.78%. Microstructural evaluation revealed that most of the fracture are found on the thermal mechanically affected zone (TMAZ)adjacent to the weld nugget zone (WNZ) due to bigger grain sizes. It is found that most of the joints exhibit ductile characteristics in failure. Fractography analysis has been used to find the behavior of weld joints in failure.

Metallurgical reaction and joining phenomena in friction welded Al/Fe joints

The continuous drive friction welding of aluminum alloy to stainless steel was investigated in this paper. Microstructural characterization, intermetallic compound (IMC), and mechanical properties of the friction welded Al/Fe joint were studied. Results showed that thermal mechanically affected zone (TMAZ) at the Al side was subjected to both thermal cycling and mechanical force, and grain size was 1–3 μm, and streamlines were more dramatically than at the steel side. In fully dynamic recrystallized 127 zone (FDRZ), microstructure became the fine equiaxed grains, and its width at the steel side was about 5 μm. The local Si enrichment IMC layer was formed at the interface area of Al/Fe joint, and its average thickness was about 300 nm. IMC layer was relatively flat at the steel side, while uneven at the Al side. The hardness reached to the maximum value (395.8 HV) at the steel side in FDRZ, and the average hardness was only 76 HV in heat-affected zone (HAZ) at the Al side. When forge pressure was below 220 MPa, the tensile strength of joint was approximately linearly related to the forge pressure.

Study of grain distribution during friction stir welding of Al-Zn-Mg alloys using numerical simulation

During friction stir welding (FSW) of Al-Zn-Mg alloys, the grain size of the welded material significantly affects its mechanical properties. In this study, the FSW process was simulated using the finite element method (FEM). Subsequently, the Zener-Hollomon parameters and grain size were calculated using the results of the stimulation of the temperature and strain rate fields. Also, the accuracy of numerical simulation was proved by experiments. It was found that the Zener-Hollomon relationship correctly predicted the grain size for the weld stirring zone (SZ) and thermal-mechanical affected zone (TMAZ), and we concluded that low rotating and high welding speeds resulted in smaller grain sizes for the SZ. Moreover, the high welding speed could effectively improve the grain condition at the edge of the TMAZ and the specific relationship between Zener-Hollomon parameters and the average grain diameter of SZ can be expressed as: Z = 9.09 × 1013d−4.74.

Microstructure and mechanical properties of dissimilar friction stir welded type 304 austenitic stainless steel to Q235 low carbon steel

In this work, friction stir welding (FSW) was used to weld dissimilar metals (Type 304 stainless steel (SS304) and Q235 low carbon steel). The microstructure, interfacial characteristics, residual stress distribution and mechanical properties of the joint were investigated. The results show that FSW results in grain refinement in the stir zone (SZ) and thermal-mechanical affected zone (TMAZ) in SS304 side. Discontinuous dynamic recrystallization and twinning-induced dynamic recrystallization are the main grain refinement mechanisms. Dynamic recovery occurs in the heat-affected zone (HAZ). On the Q235 steel side, the acicular ferrite and pearlite are generated in the SZ, which is due to its peak temperature of up to Ac3. Additionally, the amount of acicular ferrite decreases in the TMAZ. Continuous and discontinuous dynamic recrystallization is the main recrystallization mechanism in the two regions. The interfacial bonding mechanism of FSW joint consists of both mechanical and metallurgical bonding. In addition, the difference in expansion coefficient and microstructure between the two steels results in the formation of residual compressive stress in the SZ. Grain refinement and acicular ferrite are responsible for the microhardness distribution in the whole joint. The tensile strength of FSW joint is 493 MPa, which is more than that of Q235 steel by around 4%. However, the elongation is 17%, which shows a decrease of around 50%. Furthermore, the fracture surface shows ductile fracture with dimples.