Thermo-mechanically Affected Zone Research Articles

Characterization of residual stress as a function of friction stir welding parameters in oxide dispersion strengthened (ODS) steel MA956

Friction stir welding (FSW) can generate large residual stresses during solid state joining of oxide dispersion strengthened steels. In this work, a plate of MA956 steel was friction stir welded at three conditions: 500rpm/25mm per minute (mmpm), 400rpm/50mmpm and 400rpm/100mmpm. The residual stresses across these welds were measured using both x-ray and neutron diffraction techniques. The distribution and magnitude of the residual stresses agreed well between the two techniques. Longitudinal residual stresses up to eighty percent of the yield strength were observed for the 400rpm/100mmpm condition. The surface residual stresses were somewhat larger on the root side of the weld than on the crown side. Increases in the relative heat input during FSW decreased the measured residual stresses in the stir zone and the thermomechanically affected zone (TMAZ). Increasing the traverse rate while holding the rotational speed fixed increased the residual stress levels. The fatigue strength of the material is predicted to decrease by at least twenty percent with cracking most likely in the TMAZ.

EBSD study on microstructure and texture of friction stir welded AA5052-O and AA6061-T6 dissimilar joint

Abstract Friction stir welded AA5052-O and AA6061-T6 dissimilar joint has a more obvious impact on microstructure and texture evolution compared to single material welding due to differences in physical and chemical parameters between two aluminum alloys. Microstructure, texture evolution and grain structure of AA5052-O and AA6061-T6 dissimilar joint were investigated by means of OM,EBSD and TEM measurements. Experimental results showed that FS weld was generalized in four regions–nugget zone (NZ),thermomechanically affected zone (TMAZ),heat affected zone (HAZ) and base metals (BM), using standard nomenclatures. NZ exhibited the complex structure of the two materials with flowing shape and mainly composed of the advancing side material Subgrain boundaries in weld nugget zone gradually transformed into high angle grain boundaries by absorbing dislocation and accumulating misorientations. Grain refinement of weld nugget zone was achieved by dynamic recrystallization. In the friction stir welding process, the presence of the shear deformation in weld made {001} C cube texture, {123} S texture in BM gradually transformed into {111} A1∗ shear texture. HABs distribution were most significant in nugget followed by RS and then by AS. In TMAZ and NZ, numerous precipitates and lots of dislocations were observed.

Effect of post-weld heat treatment on friction welded joint of carbon steel to stainless steel

1045 carbon steel was joined to 304 stainless steel by friction welding, and the joint was heat treated at different post-weld heat treatment (PWHT) temperature. The weld interface of as-welded joint was straight and obvious, whereas, it became indistinct after PWHT characterized by severe element diffusion. Chromium carbides such as (Cr, Fe)7C3 and Cr7C3 formed at the weld interface, the amount of which increased with the PWHT temperature increasing. In thermo-mechanically affected zone (TMAZ), the microstructure at carbon steel side of as-welded joint was quite heterogeneous, while it became homogeneous after PWHT at 400°C. Analyses of fracture surfaces showed the amount of chromium carbide and the heterogeneous microstructure in TMAZ at carbon steel side were crucial factors in influencing fracture mechanism and properties of joints. Tensile strength and elongation of the joint were improved substantially after PWHT at 400°C, which could reach up to the equivalent strength of stainless steel and elongation of carbon steel, because the microstructure became homogeneous to some extent and the amount of chromium carbides barely increased. The fracture mode also altered from quasi-cleavage fracture in as-welded joint to the combination of dimple fracture and quasi-cleavage fracture after PWHT.

EBSD Characterization of Microstructural Variations in Solid-State Welds as a Function of Distance from the Weld Interface in Ti-17

Recent advances in welding technology have enabled the aerospace industry to reduce further the weight of aircraft, by welding titanium alloys reliably. Solid-state welding, in which two metal pieces are welded without either one melting, has recently been proven to produce reliable and consistent welds, and is rapidly becoming the preferred method for joining titanium. Rapid heating rates, short hold-times, plastic deformation, thermal gradients, and fast cooling rates all contribute to the non-equilibrium nature of the process. All of these factors vary with respect to their proximity to the weld interface, resulting in various regions of the welded piece. Thus, the resulting microstructure varies with distance from the weld plane, and can typically be divided into four zones: the dynamically recrystallized zone (DRX), the thermo-mechanically affected zone (TMAZ), the heat-affected zone (HAZ) and the base material (BM) [1]. Commercial titanium alloys have two equilibrium phases: the low-temperature hexagonal closepacked (hcp) phase, α, and the high-temperature body-centered cubic (bcc) phase, β. Ti-17 (Ti-5Al-4Cr4Mo-2Sn-2Zr) is a metastable- β alloy, meaning that it can retain essentially 100% β phase, upon fast cooling [2]. Since the mechanical properties of titanium alloys are particularly dependent upon microstructure, it is important to understand the phase transformations occurring and the microstructures produced during the solid-state welding process. Once the various regions of the weld can be characterized, the data can help better inform integrated computational materials engineering (ICME) models. In this study, samples of Ti-17-to-Ti-17 solid-state welds were sectioned via electric-discharge machining (EDM), normal to the weld interface. The cut face was then polished according to metallographic polishing procedures for titanium alloys outlined in [3]. Once polished, the surface was indented with a Buhler microhardness indenter, using 500 g load force, and a 10 s dwell time. The first indent was placed at the weld interface and subsequent intents were placed every 100 µm, extending to 10 mm from the weld interface, on each side. To ensure accuracy of the values, each sample received multiple, parallel, identical rows of indentations. The Vicker’s Microhardness values were recorded and plotted in Figure 1. These microhardness indents could then be used as fiducial markers for recording of EBSD data, enabling precise correlation between microhardness and microstructure. Inverse pole figure maps were used to determine the amount of recrystallization in the beta phase, as well as the degree of dissolution of the alpha phase and whether any re-precipitation occurred. The microhardness profiles displayed symmetry with respect to the weld interface. The lowest hardness levels occurred at the weld interface, in the DRX zone, and generally increased as distance increased. Both sides of the interface showed local peaks and local minima in hardness at distances of approximately 1100 µm, and 1600 µm from the interface, respectively. The hardness returned to the baseline level, at a distance of approximately 2500 µm from the weld interface, which is in agreement with microstructural observations. EBSD data were collected at ~100 µm intervals for the first 2 mm,

Microstructure evolution in a single crystal nickel-based superalloy joint by linear friction welding

Abstract Linear friction welding (LFW) was used with a single crystal nickel-based superalloy to produce sound welds. Microstructural examination shows that the joint has a distinct weld zone (WZ) and thermomechanically affected zone (TMAZ). In the WZ, the microstructure has recrystallized polycrystals instead of a single crystal. In the TMAZ, the amount of γ′ phase increased from the TMAZ/WZ interface to the parent material (PM) and decreased from the periphery to the centre along the weld creating a U-shaped microhardness profile in the TMAZ. The microhardness is however lower than that in the WZ. The average tensile strength was found to be 837.5 ± 50 MPa, comparable to the PM (880 MPa [26]).

Fatigue crack initiation behaviors throughout friction stir welded joints in AA7075-T6 in ultrasonic fatigue

This paper presents the results of experimental investigation on fatigue behaviors of friction stir welded joints in AA7075-T6 with ultrasonic fatigue test system (20kHz). Two kinds of particles, Fe-rich intermetallic compounds and Mg2Si-based particles, governed the fatigue crack initiation. The plastic deformation and recrystallization during welding process led to the changes in particle size and micro crack occurrence between thermo-mechanically affected zone (TMAZ) and nugget zone (NZ). Therefore, the fatigue crack initiation sites leaned to be located at the TMAZ in short fatigue life, or at the NZ in very high cycle fatigue regime.

Study of the Metallurgy of a Dissimilar Ti-6Al-4V – Stainless Steel Linear Fiction Welded Joints

This paper deals with the investigation of the metallurgy of a dissimilar Ti-6Al-4V-stainless steel joint linear friction welded. In particular two different stainless steel were considered: AISI 304 and AISI 316. These two alloys differs in the Molybdemun content. Metallographic observations, EDS analysis and Vickers Microhardness measurements were carried out, particular attention was focused on the study of the intermetallic compounds and on the microstructures of the different zones produced by the process. As usual for solid state welding processes, three different zones can be identified: the parent material, the heat affected zone (HAZ) and the thermo-mechanical affected zone (TMAZ), furthermore a very thin joining line, rich of intermetallic compounds, was also observed. In this zone diffusive phenomena also occurred resulting in a variation of the alpha phase content on the titanium side.In the TMAZ, the bimodal microstructure of the parent material was deformed and the presence of elongated alpha grains with broken beta-phase particles was established. Moreover it was observed that in the weld region, exposure to supertransus temperatures (995°C) combined with hot-deformation working and rapid cooling after joining induced the recrystallization of a martensitic beta grain structure. Concerning the joint between Ti-6Al-4V and AISI 316 some cracks were observed within the weld line, this due to the presence of brittle intermetallics compounds in this zone. The formation of these intermetallics was promoted by the presence of Molybdenum.

Influence of Test Temperature on the Tensile Properties along the Thickness in a Friction Stir Welded Aluminum Alloy

The aim of this study was to evaluate microstructures and the influence of test temperature on the tensile properties, strain hardening behavior and fracture characteristics of friction stir welded (FSWed) 2219-T62 aluminum alloy thick plate joints. A fine and equiaxed recrystallized grain structure had no significant change in grains at the top of weld nugget zone (WNZ) at a rotational rate of 500 r/min compared with 300 r/min, but the grains and second-phase particles at the middle of WNZ exhibited obvious coarsening. The yield strength, ultimate tensile strength and joint efficiency were observed to decrease with increasing test temperatures. However, the elongation presented a contrast trend. Compared with the middle and bottom slices, the top slice (216 and 342 MPa) had a higher strength and a lower elongation (8.5%) at different test temperatures. Hardening capacity and strain hardening exponent of bottom slices were higher than those of the top and middle slices. Both of them at room temperature (RT) were bigger than those at higher temperature (HT) and lower temperature (LT). The FSWed joints basically failed in the border area between the thermo-mechanical affected zone (TMAZ) and heat-affected zone (HAZ) of the top slice, and in the HAZ of the middle or bottom slices, while the fracture surfaces exhibited dimple fracture characteristics at different test temperatures.

Friction stir lap welding of 5456 aluminum alloy with different sheet thickness: process optimization and microstructure evolution

The influences of friction stir welding process parameters on microstructure evolution and mechanical properties of lap-welded 5456 aluminum alloy plates with different thickness and temper conditions were investigated. The upper plate was 5 mm thick, cold rolled aluminum alloy 5456-T321 and the lower plate was 2.5-mm annealed sheet (5456-O). Four different pin geometry types (conical thread pin, cylindrical–conical thread pin, stepped conical thread pin, and flared-triflute pin tool) and two rotational speeds (600 and 800 rpm) were used to produce the joints. Microstructures and microhardness values in the weld nugget (WN), thermomechanically affected zone (TMAZ), and the heat-affected zone (HAZ) were examined and correlated with selected processing conditions. Specifically, the influence of tool geometry on the flow of the plasticized material in the nugget zone, extent of hooking defect, and mechanical properties (microhardness) of the FSW joints were documented and quantified. It was found that weld joints made by using the stepped conical thread pin tool produced a homogeneous microstructure with finer grain size (5.4 μm) and higher microhardness levels than the other tools. The optimum processing conditions resulting in sound and defect-free joints with highest mechanical properties were obtained with the stepped conical thread pin and 600-rpm rotational speed. The evolution of the microhardness in each region is characterized and related to processing conditions.

Characterization of magnetically impelled arc butt welded T11 tubes for high pressure applications

Abstract Magnetically impelled arc butt (MIAB) welding is a pressure welding process used for joining of pipes and tubes with an external magnetic field affecting arc rotation along the tube circumference. In this work, MIAB welding of low alloy steel (T11) tubes were carried out to study the microstructural changes occurring in thermo-mechanically affected zone (TMAZ). To qualify the process for the welding applications where pressure could be up to 300 bar, the MIAB welds are studied with variations of arc current and arc rotation time. It is found that TMAZ shows higher hardness than that in base metal and displays higher weld tensile strength and ductility due to bainitic transformation. The effect of arc current on the weld interface is also detailed and is found to be defect free at higher values of arc currents. The results reveal that MIAB welded samples exhibits good structural property correlation for high pressure applications with an added benefit of enhanced productivity at lower cost. The study will enable the use of MIAB welding for high pressure applications in power and defence sectors.

Correlation of microstructure and mechanical properties in friction stir welded 2198-T8 Al–Li alloy

In this study, the 1.8mm thick cold-rolled sheets of 2198-T8 Al–Li alloy were manufactured by friction stir welding (FSW) at a rotation rate of 800rpm and a travel speed of 300mm/min. The microstructure and mechanical properties of different regions of the produced joint were evaluated by means of optical microscopy (OM), transmission electron microscopy (TEM), hardness testing and tensile testing. Results show that the original “pancake” grains became coarser in the heat affected zone (HAZ), transformed into equiaxed grains in the stir shoulder zone (SsZ) and stir pin zone (SpZ), and formed mixed grains with both “pancake” and equiaxed shapes in the thermo-mechanical affected zone (TMAZ). The hardness distribution in the cross-section of the FSW joint exhibited a “basin” shape. When approaching the weld centre, the hardness gradually decreased compared to the base metal (BM). The BM exhibited the highest strength due to the presence of fine T1 phase. In the HAZ, the strength decreased as T1 phase was partially dissolved. In the SsZ and SpZ, in spite of strength contribution from grain refinement, the strength further decreased as T1 phase was fully dissolved. The minimum strength in the TMAZ was related to the reduced amount of T1 phase and the presence of transition layer with sharp gradient of grain size.

Thermal stability of aluminum after friction stir processing with SiC nanoparticles

Abstract In this research, 1050 aluminum sheets in the conditions of annealed and severely deformed by 2 passes of constrained groove pressing (CGP) process were used. Two passes of friction stir processing (FSP) were applied on the specimens. Also, another CGPed specimen was processed by 2 passes of FSP using SiC nanoparticles. The samples were then heat treated to study thermal stability of base metal, heat affected zone (HAZ), thermo-mechanically affected zone (TMAZ), and stir zone (SZ). Microstructure examinations revealed that the annealed base metal was relatively stable upon heating even up to 400 °C and a little grain growth was evident. On the other hand, the CGPed base metal was fairly stable just up to 200 °C. It was also observed that HAZ and TMAZ of annealed specimen exhibited some grain growth while for CGPed sample, these regions experienced recrystallization and subsequent grain growth during heat treatment. It was found that the SZ of annealed and CGPed specimens remained fine after heat treatment even up to 400 °C and slight grain growth occurred. Microstructural assessments revealed that using SiC nanoparticles during FSP had a potential to form finer grain structure at SZ compared with that in specimen FSPed without any particle. Also, it was found that SiC nanoparticles could hinder grain growth during heat treatment provided that a suitable distribution nanoparticles was formed.

Microstructure and hardness investigation of different welding passes in weld zones

Friction stir welding is a solid state joining process that uses frictional heat generated from non‐consumable rotating tool to weld two metals together. This present work aims at the investigation of the effect of multiple pass welding on microstructure and hardness in weld zones. Aluminium alloy 6061‐T6 sheets with a dimension of 100 mm × 100 mm × 10 mm are butt‐welded in three different passes; single pass, double pass on the same side and double pass on different sides. The microstructure of the welded samples and its hardness are analysed and the effect of passes are investigated. Visual inspection on the weld joint shows that double passes experienced smaller wormhole defect as compared to single pass. Microstructure investigation shows slight difference in the grain size among all passes. The quantity of precipitates is highest at the thermo‐mechanically affected zone (TMAZ) compared with other regions. The double passes on same the side exhibits the highest hardness value with 61.1 HV in the stir zone compared with other welding passes studied. However, all regions show lower hardness values as compared to the parent material in all combinations of passes.

Analysis of integrity and microstructure of linear friction welded Waspaloy

Nickel-base superalloy, Waspaloy, was linear friction welded (LFWed) under different axial shortening conditions of 2.0, 3.4, and 4.6mm. The tensile properties and microhardness of the weldments were investigated in the as-LFWed condition and compared with those in the post-weld heat treated (PWHTed) condition. Mechanical properties were related to microstructures following examination by optical microscopy, high resolution scanning electron microscopy, and electron backscatter diffraction (EBSD). Analyses of the EBSD results in terms of the misorientation angle distribution, which represents the stored energy, were performed. In the as-LFWed condition, the yield strength (YS) and ultimate tensile strength (UTS) increased with axial shortening due to greater expulsion of the softened interfacial material toward the periphery as flash. In contrast, with increasing axial shortening the total elongation initially remained constant and then decreased. This was also related to the expulsion of the softened interfacial material into the bifurcated flash. Extensive dissolution of the strengthening phase (γ′) in the weld area during linear friction welding (LFW) contributed to the lower YS and UTS in the as-welded condition compared to the PWHTed condition where the γ′ particles were recovered. After performing post-weld heat treatment (PWHT), the total elongation improved due to the relaxation of stored energy and grain growth in the thermomechanically affected zone (TMAZ).

Analysis of heat transfer and material flow in reverse dual-rotation friction stir welding

Reverse dual-rotation friction stir welding (RDR-FSW) is a novel variant of conventional FSW process. The key feature is that the tool pin and the assisted shoulder rotate reversely and independently during the process; thus, it has great potential to improve the weld quality and lower the welding loads through adjusting the rotating speeds of the tool pin and the assisted shoulder independently. In this study, a 3D model of RDR-FSW process is developed to conduct the numerical simulation of heat generation, material flow, and temperature profile during the process. Heat generated due to plastic deformation and friction at the tool-workpiece contact interfaces are both considered. Streamlines show that there are two material flows with reverse direction, which is beneficial to the uniformity of both the temperature and the microstructure at the advancing side and retreating side. The simulation results show that rotating speeds of the assisted shoulder and the tool pin have great effects on the shape and size of the thermo-mechanically affected zone (TMAZ). The calculated peak temperatures at typical locations match with the experimentally measured ones. It lays solid foundation to optimize the process parameters in RDR-FSW.

Characterisation of fatigue fracture surfaces of friction stir channelling specimens tested at different temperatures

The fatigue fracture surfaces of friction stir channelling specimens tested at room temperature, 120°C and 200°C were observed in a scanning electron microscope (SEM) in order to analyse their morphology and the crack propagation mechanisms. Three different friction stir channelling conditions were tested and analysed. For all specimens tested the developing fatigue-crack has always initiated at the advancing side, namely on the boundary between the nugget and the thermo mechanically affected zone (TMAZ) into the interior of the specimen. The crack has propagated through the channel nugget with a path tangential to the advancing side. After the crack has reached the processed surface, a second crack initiated at the channel bottom. The fracture surfaces have shown a semi-elliptical shape crack front. This second crack has propagated uniformly through the base material. Fatigue crack propagation on the TMAZ was mainly characterised by fatigue striations. It was found, on most of the surfaces observed, a clear coexistence of the intergranular fracture mode and the transgranular fracture mode. A relationship between the fatigue testing temperature and the roughness of the fracture surfaces was found. The fracture surfaces roughness was considerably lower at a testing temperature of 200°C for the three friction stir channelling conditions analysed.

Evaluation of Residual Stresses in 12%-Cr Steel Friction Stir Welds by the Eigenstrain Reconstruction Method

In the present paper we report the results of a study into Friction Stir Welds (FSWs) made in 13 mm-thick 12%-Cr steel plates. Based on residual strains obtained by diffraction techniques, eigenstrain analysis was performed using the Eigenstrain Reconstruction Method (ERM), which is a novel methodology for the reconstruction of full-field residual strain and stress distributions within engineering components. Significant eigenstrain distributions were found at around Thermo-Mechanically Affected Zone (TMAZ) where the most severe plastic deformation was occurred. Microstructure analysis was used to elucidate this phenomenon showing that the grain structure in TMAZ was bent and not successfully recrystallised, resulting in severe deformation behaviour. The reconstructed residual strain distributions by the ERM agree well with the experimental results. It was found that the approach based on theory of eigenstrain is a powerful basis for reconstructing the full-field residual strain/stress distributions in engineering components and structures.

Analysis of AA 6082-T6 welds strength mismatch: stress versus hardness relationships

In current work, linear stress-hardness relationships are established, relating the hardness in different AA 6082-T6 weld zones with local tensile properties. The welds were performed by friction stir welding (FSW), using varying welding parameters, in order to obtain a diversified range of weld mismatch conditions. Local tensile properties for the thermo-mechanical-affected zone (TMAZ) and heat-affected zone (HAZ) were assessed from tensile testing of transverse samples using digital image correlation (DIC). The proposed stress-hardness relationships, which relate hardness with yield strength and flow stress, at different plastic deformation values, are suitable to be used for assessing plastic properties, from hardness data, for AA 6082 alloys, in the naturally aged and overaged conditions, characteristic of different weld zones of any type of welds in this aluminium alloy. The evolution of friction stir welds’ local and global tensile properties, with varying welding parameters, is also analysed. It is shown that in spite of the large scatter in local mechanical properties, the global strength of the welds does not vary significantly. This behaviour is explained based on local plastic strain distribution analysis.

Microstructure and pitting corrosion resistance of AA2219 Al–Cu alloy friction stir welds – Effect of tool profile

Abstract AA2219 Al–Cu alloy is widely used in defence and aerospace applications due to required combination of high strength-to-weight ratio and toughness. Fabrication of components used for defence always involves welding. Even though the mechanical properties of the base metal are better, but the alloy suffers from poor mechanical and corrosion properties during fusion welding. To overcome the problems of fusion welding, friction stir welding (FSW) is recognized as an alternative solid state joining method aimed to improve the mechanical and corrosion properties. Tool profile is one of the important variables which affect the performance of the friction stir weld. In the present work the effect of tool profile on the microstructure and pitting corrosion of AA2219 aluminium–copper alloy was studied. Electron backscattered diffraction results established that the grain size and orientation of weld nugget of triangle profile is finer than that of conical profile. Differential scanning calorimetric results show the evidence of precipitate dissolution during FSW. It was found that the microstructure changes, such as grain size and its orientation precipitate dissolution during FSW influence the hardness and corrosion behaviour. Pitting corrosion resistance of friction stir welds of AA2219 was found to be better for triangle profile tool compared to conical profile which is attributed to material flow and strengthening precipitate morphology in various zones. Higher amount of heat generation during FSW made using triangle profile tool may be the reason for greater dissolution of strengthening precipitates in nugget zone and coarsening in thermo mechanically affected zone (TMAZ) and heat affected zone (HAZ).

Effects of the Reversely Rotating Assisted Shoulder on Microstructures During the Reverse Dual-rotation Friction Stir Welding

The reverse dual-rotation friction stir welding (RDR-FSW) has the capability to adjust the heat generation because of the separately designed tool shoulder and tool pin. The welding torque exerted on the workpiece by the reversely rotating shoulder is opposite to that exerted by the rotating tool pin, so the total welding torque is reduced, which is beneficial to reducing the clamping requirement of workpieces. In the present paper, a RDR-FSW joint was welded in a condition similar to the optimal welding condition of conventional FSW, and microstructures in various zones were investigated by comparison, aiming to highlight effects of the reversely rotating assisted shoulder. Due to the heat conduction of the middle cylinder and the bottom end cover on which the assisted shoulder was machined, the thermal effect of RDR-FSW was smaller than that of the conventional FSW. Moreover, the effect of assisted shoulder on the plastic flow or deformation of material or was constrained in a thin layer near the weld top surface, and thus the flow of material especially along the thickness direction was clearly decreased in the RDR-FSW. In the heat-affected zone (HAZ), the precipitate coarsening was the main evolution and was completed through the dissolution of small precipitates and the continuous growth of large precipitates. By contrast, the dissolution degree of precipitates increased significantly in the thermo-mechanically affected zone (TMAZ), and a small amount of original meta-stable precipitates transformed to block-shaped stable precipitates. Precipitate evolutions in the shoulder affected zone (SAZ) and the weld nugget zone were similar, i.e. the majority of original meta-stable precipitates dissolved into the matrix and the remainder transformed to stable precipitates, though the dissolution degree was greater in the SAZ. Compared with the conventional FSW joint, the coarsening degrees of precipitates in the HAZ and TMAZ of RDR-FSW joint were much smaller, as well as the dissolution degrees of precipitates in all four specified zones.