Thermo-mechanically Affected Zone Research Articles

Numerical and experimental investigation on friction stir welding of Ti- and Nb-modified 12 % Cr ferritic stainless steel

Abstract A friction stir butt-welded Ti- and Nb-modified 12 % Cr ferritic stainless steel joint without any defects was successfully produced under low welding heat input. The welding heat cycles, temperature profiles, and degree of plastic deformation during welding were quantitatively evaluated based on the results of a rigid-viscoplastic thermo-mechanically coupled model. Microstructure development in the stir zone (SZ) and thermo-mechanically affected zone (TMAZ) was studied and comparatively analyzed. A fine duplex microstructure containing nearly 62.9 % martensite and 37.1 % ferrite was obtained in the SZ. Undissolved nanometer-sized precipitates during welding played a crucial role in grain refinement in the weld. Retardation of dynamic recrystallization (DRX) occurred in the SZ, and the recrystallized grains were no more than 8% of the materials. The dynamic recovery and continuous DRX experienced by the ferrite grains in the TMAZ of the advancing side were more intense than those on the retreating side, leading to a higher amount of high-angle boundaries and a 20.8 % reduction in dislocation density. The hardness and strength in the SZ apparently increased owing to refined grains and phase transformation. Both the heat-affected zone with a fully ferritic structure and the SZ exhibited superior low-temperature toughness.

Characterization of the evolution of 2219-T87 aluminum as a function of the friction stir welding process

The aluminum-copper alloy AA2219 is commonly joined by friction stir welding (FSW). The strengthening Al2Cu phases are affected by the temperatures produced as a function of the FSW process. This work characterizes the effect of FSW on the fatigue crack growth and fracture response of the of AA2219-T87. Specifically, the virgin material’s microhardness, strength, and θ-phase evolution are studied as a function of time and temperature. This data is compared to the microhardness resulting in the stir zone, thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ), and base material. Furthermore, the residual stresses produced by the FSW process are quantified.

Linear friction welding of a beta titanium alloy: experimental investigations on microstructure evolution and mechanical properties

ABSTRACT Linear friction welding was used to join Ti–3Al–8V–6Cr–4Mo–4Zr alloy under different pressures, and the microstructure, texture evolution and mechanical properties of the joint were investigated. Results show that the incomplete continuous dynamic recrystallisation (CDRX) occurs during welding, which refines the grains and produces a large number of subgrains in the thermo-mechanically affected zone (TMAZ) and weld centre zone (WCZ). At a higher friction pressure, the width of the WCZ becomes narrower, with the decrease of grain size and the increase of recrystallisation degree. In addition, the texture in the WCZ has changed from {112}<111> to {110}<111>, while the texture in the TMAZ remains at {110}<112>. The tensile properties of the two joints are similar to the base metal (BM), and the microhardness in WCZ and TMAZ are also close to that in BM due to the incomplete CDRX and the decrease of alpha phase.

Effect of friction stir welding (FSW) on the electrochemical behavior and galvanic coupling of AA2024-T3 and AA7475-T651

In this study, the local electrochemical activity of AA2024-T3 and AA7475-T761 joined by friction stir welding (FSW) was investigated as a function of time by electrochemical tests in 0.01 mol L−1 NaCl solution using a mini cell. The welding procedure resulted in increased electrochemical activity of the weld affected zones of both alloys, which electrochemical activities increased with immersion time, as demonstrated by the electrochemical behavior. In the heat affected zone (HAZ) and the thermomechanically affected zone (TMAZ) of AA7475-T651 the increased activity was due to precipitation of η phase at the grain boundaries stimulated by thermomechanical effects. On the other hand, the enhanced activity in the TMAZ/HAZ of the AA2024-T3 was associated to a large concentration of copper rich cathodic particles broken by tool rotation that were spread along this zone. However, the zone of highest electrochemical activity was the stir zone (SZ) and this was ascribed to galvanic coupling between the two alloys, AA2024-T3 and the AA7475-T651, where the former acted as cathodic and the latter as anodic area.

Effects of welding parameters on micro-junction structure and fracture behavior of refill friction stir spot welded joints for 2060 aluminum alloys

Due to different filling modes of the material in sleeve-affected zone (SAZ), probe-affected zone (PAZ), and thermo-mechanically affected zone (TMAZ), the micro-junction structure is formed at the bottom of the SAZ of the refill FSSW joint for 3.2-mm-thick 2060 aluminum alloys. The plasticized degree of the filling material and refilling time greatly affect the morphologies of micro-junction structure. Thus, the welding defects such as void with different sizes, kissing bond, and crack are easily formed in the micro-junction structure. Besides, the tensile-shear properties are related to the micro-junction structure and bonding strength of the lap interface. The increase of the plunge depth or rotation speed within a certain range enhances the flow of material in the stir zone and increases the atomic diffusion capacity of the lap interface, which is beneficial to increasing the tensile-shear load. The maximum tensile-shear load was obtained when the rotation speed of 2500 rpm and the plunge depth of 4 mm are used, and the tensile-shear specimen fractures along the bottom of the SAZ. Other fracture paths are also obtained under different welding parameters combinations, first along the lap interface, and second, not only along the lap interface but along the SAZ bottom.

On the effect of a thermal treatment on the tensile and fatigue properties of weak zones of similar Ti17 linear friction welded joints and parent material

The tensile and fatigue properties and the damage mechanisms of as welded (LFW–AW) and post-weld heat treated (PWHT) similar Ti17 linear friction welded (LFW) joints as well as Ti17 parent material (PM) have been studied together with their underlying microstructures. The LFW–AW joints exhibited a depletion of the α phases in the weld centre zone (WCZ) and in the thermo-mechanically affected zone (TMAZ), which was previously qualified as a weak zone. Stereo Digital Image Correlation (SDIC) analysis of the strain field for monotonic loading of the PM showed strain heterogeneity that was linked by EBSD and Schmid's factor analysis to the large β grains containing α precipitates. Compared to the PM tensile specimens, the LFW–AW joint highlighted lower ultimate tensile strength (UTS) and a strongly reduced and scattered macroscopic ductility. Maps and spatio-temporal graphs of normalized strain rate fields obtained by SDIC clearly highlighted localization at the weak zone. For the target fatigue life of 105 cycles, the fatigue strength of the AW joint was about half that of the PM with fatigue crack initiation at the β grain boundary in the weak zones. Remarkably, the PWHT made the AW joints comparable to the PM in terms of microstructure and mechanical properties. The former weak zones recrystallized into β grains with α, the UTS and fatigue strength were almost recovered, macroscopic and local ductility were enhanced, failure during monotonic and cyclic loadings occurred far from the former weak zones.

Joining of Dissimilar Alloys Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo-0.1Si Using Linear Friction Welding.

Dissimilar joints between Ti-6Al-4V (Ti-64) and Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) were manufactured using linear friction welding. The weld quality, in terms of the microstructure and mechanical properties, was investigated after stress relief annealing (SRA) at 750 °C for 2 h and compared with the as-welded (AWed) results. The central weld zone (CWZ) microstructure in the AWed condition consisted of recrystallized prior-β grains with α’ martensite, which transformed into an acicular α+β structure after SRA. The hardness in the AWed condition was highest in the CWZ and decreased sharply through the thermomechanically affected zones (TMAZ) to the parent materials (PMs). After SRA, the hardness of the CWZ decreased, mainly due to tempering of the α’ martensite microstructure. Static tensile testing of the dissimilar welds in both the AWed and stress relief annealed (SRAed) conditions resulted in ductile fracture occurring exclusively in the Ti-6Al-4V side of the joint. The promising results on joining of Ti-64 to Ti-6242 provide valuable insight for tailoring performance of next-generation aero-engine products.

Microstructural and mechanical behavior of Al 6061/SiC-Al2O3 composites processed through friction stir processing

Metal Matrix Composite (MMC) reinforced in friction stir processing (FSP) has increased insights that can affectively attain the desired mechanical properties for the manufactured samples. The favorable conditions of carbides are considered for reinforcing the SiC particles into the Aluminum 6061. The methodology of fabricating Aluminum 6061 comprises of three materials, Al 6061-SiC-Al2O3. The experimental evaluation of the composite Aluminum 6061-SiC-Al2O3 includes the influence of process parameters on microhardness, tensile strength, and microstructure. As a result of the reinforcement of nanoparticles processed in FSP, the properties of composite material increased satisfactorily. The sample S3 observed to be having a maximum tensile strength of 185 MPa. The larger, the better condition is adopted to analyze the tensile strength of the fabricated samples. The optimum condition for maximum tensile strength was found at 900 RPM, 15 mm/min, and composition 3. The hardness profiles at different zones of friction stir processing (FSP), viz., Heat Affected Zone (HAZ), Thermo Mechanical Affected Zone (TMAZ), Nugget Zone (NZ) were examined. The characterization techniques deployed were optical microscope (OM), and scanning electron microscope (SEM) studies for microstructural behavior. The result shows that the reinforcements were tightly embedded into the base material surface. The spherical grains are formed in the reinforcement region.

Strength, fatigue strength and toughness of dissimilar Ti17–Ti64 linear friction welded joints: Effect of soft surface contamination and depletion of α precipitates

The effect of microstructure of three dissimilar Ti17–Ti64 linear friction welded joints on the strength, fatigue strength and fracture toughness was studied. A special attention was paid to role of soft contaminants and α precipitate depletion. Three joints were produced: one in the as machined state after electrical discharge machining, one ground prior to welding, and one welded in the as machined state and post-weld heat treated. The microstructure of the weld centre zone (WCZ) exhibited an acicular entangled α' martensite with a Widmanstätten morphology on the Ti64 side and fine equiaxed distorted β grains on the Ti17 side. A depletion of the strengthening α precipitates was observed in the WCZ and the thermo-mechanically affected zone (TMAZ) on the Ti17 side. For the studied welding parameters, the LFW self-cleaning mechanism failed to extrude the contaminants in the weld interface of the joint welded in the as machined state. These contaminants were related to prior to welding machining and were denominated soft contaminants due to their relatively low melting point. A defect layer was formed that led to a pseudo-brittle fracture in at the WCZ during tensile testing, cyclic loading and fracture toughness testing. During tensile testing of the joint that was ground prior to welding, failure occurred after plastic deformation sometimes at the Ti64 parent material (PM) and sometimes at the Ti17 TMAZ. Fatigue cracks initiated at the latter weak zone with a reduced fatigue strength. Compared to the PM, fracture toughness of samples even without fatigue pre-crack was also reduced. Failure at the weak zone highlighted unexpected colonies of elongated dimples that nucleated on transgranular deformation lines.

Microstructure and Mechanical Properties of Low-Carbon Q235 Steel Welded Using Friction Stir Welding

This study focuses on the microstructure and mechanical properties of the joints of Q235 mild steel, which was formed by the friction stir welding (FSW). The results indicated that, after the FSW, the heat-affected zone (HAZ) of the retreating side (HAZRS) and the HAZ of the advancing side (HAZAS) recovered under the influence of the heating cycle. The transformation of the phases in the thermo-mechanically affected zone (TMAZ) of the retreating side (TMAZRS), the stir zone (SZ) and the TMAZ of the advancing side (TMAZAS) generated the pearlite and acicular ferrite. The continuous dynamic recrystallization occurred in all the three zones, whereas the grains were refined. The SZ mainly consisted of D1, D2 and F shear textures, while the TMAZAS was made up of only the F shear texture. The fine-grained structure, pearlite and the acicular ferrite improved the hardness and tensile strength of the joint. Its ultimate tensile strength was 479 MPa, which was 1.3% higher than that of the base metal. However, the uniform elongation was 16%, which showed a decrease of 33%. The fracture was a ductile fracture with the appearance of dimples. Besides, the joints of the FSW showed an excellent bending performance.

Effect of Friction Stir Processing at High Rotational Speed on Aging of a HPDC Mg9Al1Zn

Friction stir processing (FSP) has confirmed its valuable contribution to refining microstructures and the improvement of mechanical properties for Mg-Al alloys. Reference papers illustrate that post-processing aging treatments enhance hardness, but with the application of a solution heat treatment prior to FSP. In this work, aging was performed at two different temperatures (170 and 300 °C) directly on friction stir processed samples of AZ91 produced using high pressure die casting (HPDC). High rotational speeds of the tools (2500 and 3000 rpm) increases the heat input and the temperature of the plates during the process up to 270 °C. Vickers microhardness (HV) increased by 15–20% in the nugget, compared to the as FSPed condition; moreover, a greater homogeneity of hardness values was found at the higher aging temperature used. The β-Mg17Al12 precipitates are randomly distributed inside grains of the stirred area, while in the thermomechanical affected zone (TMAZ) they have grown in a network similar to the old eutectic. A preliminary investigation of FSPed samples deformed at 300 °C found that an equivalent hardness increase is achievable using hot deformation due to dynamic precipitation; to find local homogeneity in hardness, it is worth performing a final aging.

Microstructural heredity and its effect on mechanical properties of linear friction welded Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy joints

The Ti-6.5Al-3.5Mo-1.5Zr-0.3Si blocks with three different original microstructures obtained by heat treatment, were joined successfully with linear friction welding (LFW). The microstructure evolution and mechanical properties were investigated in detail to clarify the microstructural heredity during welding process for the first time. Results show that all joints contain a weld center zone (WCZ), thermomechanically-affected zone (TMAZ) and base metal (BM) of different sizes. The microstructure heredity during LFW is partial. All three WCZs contain fine β grains with acicular α laths of similar texture distribution indicating no microstructural heredity in this area. Similar dynamic recrystallization process happens during LFW in three joints, and the β grain size in BM has no effect on the final grain size in WCZ. While the material in the TMAZ shows partial phase transformation and deformation compared with the original microstructural characteristic of BM. The similar microstructures across the WCZ of the three joints produce similar hardness as that of the water-quenched base metal. The different microstructures in the TMAZ and the BM produce different hardness distributions and different fracture modes during tensile strength testing.

Friction stir welding of 2060–T8 Al[sbnd]Li alloy. Part I: Microstructure evolution mechanism and mechanical properties

2 mm thick rolled 2060–T8 AlLi alloy plates were subjected to friction stir welding (FSW) under rotation rates of 400–1200 rpm and welding speeds of 50–200 mm/min. In the nugget zone (NZ), FSW resulted in the dissolution of the precipitates and the formation of δ′ (Al3Zr) precipitates, as well as the formation of Guinier–Preston (GP) and Guinier–Preston–Bagaryatsky (GPB) zones except for at 400 rpm–200 mm/min. In the heat affected zone (HAZ), FSW led to the dissolution and coarsening of T1 (Al2CuLi) and S′ (Al2CuMg), and the subsequent formation of new θ′ (Al2Cu) and σ (Al5Cu6Mg2) precipitates. The lowest hardness zones (LHZs) of the FSW joints were located at either the NZ or the HAZ under varied welding parameters. Different from the FSW joints of conventional precipitation–hardened aluminum alloys, the ultimate tensile strength of FSW 2060–T8 joints increased as the rotation rate increased from 400 to 800 rpm, but was unchanged with further increasing the rotation rate from 800 to 1200 rpm under a constant welding speed of 200 mm/min. On the other hand, the joint strength increased as the welding speed increased from 50 to 200 mm/min under a constant rotation rate of 1200 rpm. While the FSW joints fractured at the LHZs under 400 rpm–200 mm/min and 1200 rpm–50 mm/min, abnormal fracture at the thermo–mechanically affected zone (TMAZ) was observed under 800 rpm–200 mm/min and 1200 rpm–200 mm/min.

Friction stir welding of 2060–T8 Al-Li alloy. Part II: Tensile fracture behavior

For friction stir welded (FSW) joints of conventional precipitation-hardened aluminum alloys, the lowest hardness zone (LHZ) was located at heat affected zone (HAZ) and determined the tensile fracture behavior with the fracture occurring at the LHZ. For FSW 2060–T8 joints, however, the LHZs were located at the nugget zone (NZ) under 400 rpm–200 mm/min, but at the HAZ under 800 rpm–200 mm/min, 1200 rpm–200 mm/min and 1200 rpm–50 mm/min. While the FSW 2060–T8 joints normally fractured along the LHZ under 400 rpm–200 mm/min and 1200 rpm–50 mm/min, abnormal fracture occurred at the thermo–mechanically affected zone (TMAZ) under 800 rpm–200 mm/min and 1200 rpm–200 mm/min. The FSW 2060–T8 joints with 800 rpm–200 mm/min and 1200 rpm–200 mm/min presented obvious diversity of crystal orientation and grain morphology, and subsequent deformation in the LHZs, the lower TMAZ and upper TMAZ. The large Schmid factor, coarse grains and δ′/β′ precipitates caused the preferential deformation and planar slip at the lower TMAZ, weakened the mechanical properties of the TMAZ, and therefore resulted in the abnormal failure at the TMAZ of FSW 2060–T8 joints under 800 rpm–200 mm/min and 1200 rpm–200 mm/min.

Thermal and mechanical induced texture evolution of inertia friction welding in α + β titanium alloy

Shear and compression textures of inertia friction welding in the welding region of the α + β titanium alloy were measured using electron backscatter diffraction. The texture of substantial remnant β in the thermo-mechanical affected zone (TMAZ) was 〈100〉 fiber texture, while, it was the 〈111〉 fiber texture in the dynamic recrystallization zone (DRZ). In the DRZ, continuous dynamic recrystallization occurred which retained the deformation texture. To summarize, we concluded that the predominant deformation in the TMAZ was compression, while that in DRZ was shear.

Effects of Friction Stir Welding Parameters of Dissimilar Aluminum Alloys on Residual Stress and Microhardness

The aim of this research is to use a full factorial design to determine the significant factors of friction stir welding (FSW) process of dissimilar AA6061-T6 and AA7075-T651 aluminum alloys, on the surface residual stress and microhardness. There are a total of three factors studied: rotation speed, welding speed and workpiece layout. The results showed that the factors that have a statistically significant effect on residual stress occurring are welding speed, workpiece layout and interaction between rotation speed and welding speed. The appropriate parameters for FSW process of dissimilar aluminum alloys are rotation speed of 1400 rpm, welding speed of 50 mm/min and the layout of the workpiece by advancing side (AS) uses AA6061-T6 sheet, while retreating side (RS) uses AA7075-T651 sheet, which will result in the surface residual stress of −34.33 MPa. This compressive residual stress will be beneficial to the welded joint for retard the occurrence of cracks caused by fatigue. The hardness of all workpieces have similar tendency, thermo-mechanically affected zone (TMAZ) area in both AS and RS gave the least hardness, which found that the AA6061-T6 and AA7075-T651 sheets had a hardness of approximately 55HV and 110HV respectively.

Fatigue behavior and fractography in friction stir welding zones of dissimilar aluminum alloys (AA5086-H32 with AA6061-T6)

Dissimilar aluminum alloys of AA5086-H32 with AA6061-T6 were welded by friction stir welding method using three different values of linear and rotating speed; V = 50, 75 and 90 mm/min., N = 680, 920 and 1500RPM. Tensile test was used to determine the efficiency of welded samples. The microstructure, hardness, XRD analysis and fatigue tests were studied for the welded sample with the highest tensile strength. The fatigue test was studied in the welding and the nearby region at a distance of x=5, 10 and 15 mm from the welding line. The fractured fatigue surface was analyzed using SEM. The welding speed used has an effect on the efficiency of the welded samples where the maximum welding efficiency (82%) was obtained at N = 680 and V = 75 mm/min. The microstructural analysis indicated that there is a significant elongation and bending in the grains of the thermo-mechanical affected zone (TMAZ) in the advance side (AA5086-H32) of the weld compared to the retreating side (AA6061-T6). The results of XRD and EDS indicated that there is no transition of phases between the two alloys. The maximum value of hardness was at the weld line and started to decrease away from it, hence the values of hardness at the advance side was higher compared to the retreating side. Fatigue strength of the welded samples was less than the wrought alloys.

Microstructural Analysis of Friction Stir Butt Welded Al-Mg-Sc-Zr Alloy Heavy Gauge Sheets

Friction stir welding (FSW) on a heavy gauge sheet of a hereditary fine-grained Al-Mg-Sc-Zr alloy was carried out to study the specifics of plasticized metal flow and microstructural evolution in different sections and zones of the joint. It was found that the stir zone (SZ) macrostructure may contain either a single or many nugget zones depending on the metal sheet thickness and the seam length. The effect of grain kinking in a thermomechanically affected zone (TMAZ) under pressure from the stir zone metal was discovered. The stir zone metal was fine-grained but had a microhardness lower than that of the base metal, which may be explained by the overaging effect of FSW on the Al3Sc precipitates. The tensile strength of the joint was almost equal to that of the base metal (BM). The grain size distributions were obtained in different sections below the sheet surface and away from the exit hole, which allowed us to suggest the specific adhesion-assisted layer-by layer metal transfer mechanism in FSW.

Microstructure evolution of thermo-mechanically affected zone in dissimilar AA2024/7075 joint produced by friction stir welding

In the present research, the microstructure characterization of the thermo-mechanically affected zone (TMAZ) in dissimilar FSW AA2024/7075 joint was systematically investigated. The results showed that many Cu-rich and Zn-rich constituent particles are generated in the TMAZ on the advancing side (AS-TMAZ) and the retreating side (RS-TMAZ), respectively. This mainly depends on the corresponding base materials (BMs) at the AS and RS. The precipitated particles are coarsened in the AS-TMAZ and RS-TMAZ compared to the corresponding BMs owing to the thermal cycle produced during FSW. Grain bending occurs in the TMAZ, and the degree of grain curvature in the RS-TMAZ are more pronounced than that in the AS-TMAZ. Many dislocations and sub-grains including a large number of LAGBs can be found in the AS-TMAZ and RS-TMAZ. The entire texture intensity is stronger in the AS-TMAZ and weaker in the RS-TMAZ as compared to the corresponding BMs. The original texture components are changed and some new shear components can be produced as a result of the severe shear deformation during FSW. The hardness value of the AS-TMAZ and RS-TMAZ is lower than that of the corresponding BMs due to the precipitated particles coarsening during FSW.

Microstructure and Mechanical Properties of Inertia‐Friction‐Welded Fe–Cr–Ni–Mo High‐Strength Steel

Inertia friction welding (IFW) is a solid‐state welding technology that avoids defects associated with molten weld processes; however, this process has rarely been used to weld ultrahigh‐strength steel (UHSS). Herein, 32CrMnSi2Ni6MoV UHSS is joined successfully via IFW. The mechanical properties after welding at different rotational speeds and the microstructure at 2800 rpm are studied in detail. The temperature distributions in the peripheral and central areas are modeled according to the temperature at the weld zone. The microstructural transformation is analyzed for different temperature intervals, and the microstructural characteristics of each interval correspond to the actual microstructure of the welded joint according to the temperature distribution model. High‐strength martensite forms at the weld zone, and the thermomechanical‐affected zone (TMAZ) in the peripheral region is divided into phase transformation zone, partial recrystallization zone, and plastic deformation zone, whereas the TMAZ in the central region includes only the partial recrystallization and pure plastic deformation zones. This difference is due to the higher frictional heat caused by the higher peripheral linear velocity during rotation. A tensile test shows that fracture occurs in the base metal (BM) region, and the yield strength, tensile strength, and elongation are 805 MPa, 1064 MPa, and 13.88%, respectively.