Stir Zone Region Research Articles

EXPERIMENTAL INVESTIGATION OF MECHANICAL AND METALLURGICAL PROPERTIES IN FRICTION STIR SPOT WELDING OF DISSIMILAR AA6061-T6 TO COPPER

In this study, the effects of important parameters of the FSSW process, rotational speed and dwell time, have been investigated on mechanical and metallurgical properties of dissimilar AA6061-T6 to the pure copper welded joint. In addition, the results were compared with the similar Al-Al and Cu-Cu welded joints. The results indicate that an increase in the rotational speed from 1250 to 1600 rpm and dwell time from 5 to 10 seconds improves the tensile and flexural strength of all the welded samples. Furthermore, at rotational speeds lower than 1250 rpm, due to reduced frictional heat generation, and above 1600 rpm, due to excessive heat production, the quality of the joints decreases. In this welding process, the amount of frictional heat generated directly affects the metallurgical structure, hardness of different welding zones, and intermetallic formations formed. The microstructure and the microhardness studies reveal that different welding parameters lead to the formation of fine grains and recrystallization in the stir zone and other regions with an increase in rotational speed and dwell time. This phenomenon is a result of the heat generated by friction and severe material deformation during welding. The refinement of grains contributes to increasing the hardness and strength. Moreover, the hardness analysis of the welded regions shows that the dissolution and growth of precipitates in the weld metal and its vicinity create a weaker region compared to the base metal, resulting in a decrease in hardness relative to the base metal.

Investigation on microstructural evolution and local mechanical performance of friction stir lap welded AA6061-T6/ AA7075-T6 joints

The current study uses the friction stir lap welding (FSLW) technique to successfully fabricate two layered dissimilar AA6061/AA7075 joints. A defect-free joint with superior interfacial properties was obtained at the optimized process parameters (tool rotational speed / transverse speed) of 850 RPM/ 55 mm/min. The detailed investigations of microstructure evolution, crystallographic texture, and strength correlation were performed using the field emission scanning electron microscope (FE-SEM), optical microscope (OM), electron backscattered diffraction (EBSD), Vickers microhardness, nano-indentation, and miniature tensile tests. EBSD analysis reveals the presence of a significant grain refinement at the bottom (3.65 µm), middle (4.58 µm), and top region (6.34 µm) of stir zone (SZ) compared to AA6061 (42.56 µm) and AA7075 (34 µm) base metals. An increasing trend in the fraction of high angle grain boundary (FHAGBs) and recrystallization fraction was noticed from top to bottom of the build within the SZ. Continuous dynamic recrystallization (CDRX) with the gradient in strain, strain rate, and temperature leads to these microstructural variations within the SZ. The microhardness study shows a decrease in microhardness in the SZ due to thermal softening and precipitate dissolution at higher temperatures. Appreciable interfacial material mixing leads to remarkable strength, ductility, and strain-hardening behaviour within the SZ, especially at the interface section. Pole figures (PF) reveal the presence of major shear texture components (B/B̅, and C) with some minor presence of A1*/A2*and A/A̅ that confirms the presence of higher strains within the SZ. The dominant presence of recrystallization textures components (like cube {001} 〈110〉, Goss {011} 〈100〉, and P {011} 〈122〉 ) within the SZ confirms the presence of the CDRX mechanism.

Parametric effect on microstructure evolution, grain size and mechanical behaviour of friction stir butt welding of AA6061-T6 alloy

This study examines how friction stir welding (FSW) changes the microstructure and mechanical properties of an AA6061-T6 alloy joint. Results showed that welding speeds (WSs) affected microstructure evolution, joint impact toughness, tensile strength, and ductility. Frictional heat and deformation rate affect grain size and shape. Additionally, the agitated zone on the advancing side had substantially less precipitates than on the retreating side. Tensile strength and impact toughness increased mostly due to high angle grain boundaries (HABs) and grain refining. X-ray diffraction (XRD) results revealed that intermetallic compounds, such as Mg2Si, MgZn2, and Al2CuMg were formed in various regions of stir zone (SZ), whereas β(Mg2Si) and β″(Mg5Si6) were found in Si-rich zones. Impact test revealed a drop in impact energy at all set of parameters when comparing to ambient temperatures (25 °C), i.e. 23%, 20%, and 22% for sample A1 (rotational speed (RS)/WS-710/40), 15%, 24%, and 30% for sample A2 (RS/WS-710/100), and 12%, 7.5%, and 17% for sample A3 (RS/WS-710/160) at 0 °C, −20 °C, and −40 °C, respectively. The maximum joint efficiency of 74.5% was obtained for sample A3. Hardness value was also found to be higher for sample A3 in SZ due to the increase the precipitates and the dislocation densities walls and substructures.

Friction Stir-Spot Welding of AA5052-H32 Alloy Sheets: Effects of Dwell Time on Mechanical Properties and Microstructural Evolution

Friction stir-spot welding (FSSW) as a solid-state joining process for local welding offers a number of benefits for applications in the automotive, aerospace, and marine industries. In these industries, and from an economic point of view, producing spot welds at a low rotating speed and in a short time is critical for saving energy and enhancing productivity. This investigation helped fill a knowledge gap in the literature about FSSW of 4 mm similar lap joints of AA5052-H32 sheet materials, in which welding takes place over a short time period with a slow tool rotation speed. Consequently, the purpose of this work was to investigate the feasibility of FSSW 2 mm thick AA5052-H32 aluminum alloy sheets to produce 4 mm thick similar spot lap joints at various low dwell times of 1, 2, and 3 s and a constant relatively low tool rotation speed of 500 rpm. The introduced heat input for the friction stir-spot welded (FSSWed) lap joints was calculated based on the applied processing parameters. Joint appearance, cross-section macrostructures, and microstructure features of all the spot welds were evaluated. The mechanical properties (hardness contour maps and maximum tensile shear loads) were also examined. The results show that joining 2 mm sheet thickness AA5052-H32 at a low heat input in defect-free similar lap joints could be successfully achieved. The stir zone (SZ) region became wider as the dwell time increased from 1 to 3 s. The hardness value of the SZ was higher than that attained by the AA5052-H32 base material (BM) for all applied dwell times. Especially at 2 s, the hardness of the SZ was approximately 48% higher than that of the BM. This increase in hardness may be attributed to the high grain refinement of the new dynamically recrystallized grain (4 µm) in the SZ compared to the cold-rolled BM grain size (40 µm). Among the tried FSSW process variables, the dwell time of 2 s at a rotation rate of 500 rpm also produced the maximum tensile shear load of 4330 N. Finally, the locations and features of the fracture surfaces of the FSSWed joints were examined using a scanning electron microscope (SEM) and the obtained results were discussed.

Comparison of microstructure, texture, and mechanical properties of the SQ and thread pin profile FSW joint of AA6061-T6 with Al2O3 particle reinforcement

The paper presents the effect of flat and thread pin profiles on friction stir welding (FSW) of 6061-T6 aluminium alloy with particle reinforcement. The thread geometry on the tool pin enhances the material mixing and improves the material flow in the plastic state. However, in the flat tool pin, the plasticized material was stuck to the side faces of the tool pin surface. The microstructural analysis of the weld specimen revealed that the powder particles were agglomerated in the stir zone (SZ) region in the square tool pin case. However, the powder particles were homogeneously dispersed in the SZ region when the welding was done with the threaded tool pin profile tool. The tensile and hardness tests were also conducted to check the flat and threaded cylindrical pin weld specimen's joint strength and hardness variation. The Electron backscattered diffraction (EBSD) analysis revealed that the average (Av.) grain size of the SZ region of the TCT and SQ pin are 3.74 µm and 5.64 µm, and the fraction of high angle grain boundaries are 0.560 and 0.368, respectively. The texture analysis of the SZ region revealed that the major availability of the shear texture component B/ B ̅ and C in the TCT pin weld specimen.

Effect of rapid cooling on the microstructure and properties of fine-grained 7075 aluminium alloy under friction stir welding

In this study, 7075 aluminium alloy plates with a fine-grained structure were produced by four-pass equal-channel angular pressing and subsequently joined at room temperature (297 K) using carbon dioxide (CO2)-assisted cooling friction stir welding. Electron backscatter diffraction, x-ray diffraction, Thermocouple measuring instrument,optical microscopy and microhardness testing were used to investigate the microstructural and mechanical characteristics of friction stir welded joints. The results indicated that the maximum temperatures of welded joints at room temperature and CO2-assisted were 673 K and 568 K, macroscopic surfaces of the welded joints under CO2-assisted cooling were smoother than those created under room temperature; this was because the rapid cooling of liquid CO2 inhibited the growth of grains and, following dynamic recrystallisation, the grain size was finer (∼2.9 μm). Compared with the fine-grained base material (BM), the proportion of large-angle grain boundaries in the stir zone region of the welded joints increased under both conditions, and the anisotropy was weakened. The precipitation hardening of the joints was obvious; the welded joints appeared to soften at room temperature, while the hardness of the joints was the same as that of the fine-grained BM under the CO2-assisted cooling condition.

Investigation on the friction stir assisted lap joining of pure copper and aluminium 6063 alloy

Among solid-state joining processes, the friction stir welding process stands out for its ability to fabricate specimens with desired thickness and high-quality bonding of the specimens without melting and solidification. In the present work, two dissimilar materials with distinct thermomechanical behavior, such as AA6063 alloy and pure Cu, were used to fabricate the laminated joint through the friction stir welding process (FSW). The FSW was employed to achieve high linear weld density, microstructural, and mechanical analysis of the fabricated samples. The linear weld density results show that the sample was formed with high weld density. A microstructural and EDX study confirms the mixing and purity of both materials in the joining area. Moreover, microscopic analyses of the weld or stir zone were employed to analyze the joint quality in terms of linear weld density and tunneling defects. The Micro-hardness test also showed an improvement in the micro-hardness value at both base materials and stir zone regions. The obtained results show the FSW can fabricate the dissimilar laminate with improved microstructural and mechanical properties.

Microstructure and Mechanical Properties of Friction Stir Welded Austenitic-Ferritic Stainless Steels Using Staggered Joint Configuration

Dissimilar friction stir welding of 304 austenitic stainless steel and 430 ferritic stainless steel was performed by using staggered butt joint configuration. Under proper heat input, welding experiments were carried out to study the effect of tooth dimension on material flow, microstructure evolution and mechanical properties of the joint. The results indicate that the material flow tends to improve with reduced the tooth width. However, when the tooth width is reduced to 1 mm, the mixing degree of base metals in stir zone becomes worse due to the lack of softened metal. It is noted that joint 2 with a tooth width of 2 mm is characterized by river pattern and shows various flow directions in different regions of the stir zone. This pattern is formed by the mixed structure of austenite and ferrite with various grain sizes. Joint 2 presents the maximum tensile strength in all specimens and failed in the ferritic base metal.

Influence of tool offsetting and base metal positioning on the material flow of AA5052-AA6061 dissimilar friction stir welding

This study examines dissimilar friction stir welding of AA5052-AA6061 aluminum alloys with varying tool offsets. The base metals were positioned and fixed at a slight diagonal positioning such that varying tool offset position from the centreline can also be varied along the length of the weld. After the fabrication process, microstructural and mechanical property characterization was subsequently conducted. The results show that, above a certain threshold for tool offset, incomplete consolidation (i.e. kissing bond defects) will occur. Regardless of the base material positioning, a zero tool offset shows optimum intermixing in the stir zone. EDX mapping confirms the presence of a distinct interface between both materials in the stir zone region. However, enhanced material intermixing and better elongation are observed when AA6061 alloy is positioned at the tool advancing side.

Refill friction stir spot welded AA5754-H22/Ti-6Al-4V joints: Microstructural characterization and electrochemical corrosion behavior of aluminum surfaces

The present study assessed the influence of refill friction stir spot welding (RFSSW) process on the microstructure and electrochemical corrosion behavior of the AA5754-H22 alloy surface of an AA5754-H22/Ti-6Al-4 V overlapped joint. The results demonstrate that the RFSSW process promotes substantial microstrutural changes along the welded joint, which, in turn, reflect directly on the mechanical properties and corrosion behavior of distinct welding regions. The potentiodynamic polarization tests and electrochemical impedance spectroscopy measurements indicate that the stir zone region has better corrosion resistance than the other RFSS welded regions [heat affected zone and base metal] due to higher homogeneity and microstructure refinement. The achieved results are evidence that the RFSSW process is an interesting alternative for the welding of overlapping dissimilar joints.

FSW of low carbon steel using tungsten carbide (WC-10wt.%Co) based tool material

Low carbon steels could be simply welded by conventional fusion welding processes. However, fusion joining of these steels results to the problems related to melting of metal and solidification of weld pool. In the present study, friction stir welding (FSW) of low carbon steel plates was undertaken using tungsten alloy tool to determine the effects of welding parameters on joint quality. Welded joints were characterized by microstructural and mechanical properties. Onion rings, banded structure and swirl zone were observed as a result of process temperature, strain rate, plastic deformation, and material transportation. The results indicate that the grain size of the weld zone is different from the base metal (BM) and slightly lesser to the base metal in the middle region of stir zone. Comparing the advancing side (AS) and retreating side, grain size was similar in the heat affected zone and different in the thermo-mechanically affected zone. The microstructure provides the suitable relationship for the properties and micro hardness of the welded region. Tungsten carbides rich areas were found in the stir zone performed at high heat input weld condition. Interestingly, there is noticeable change in grain size and grain distribution of the tungsten tool after welding.

Optimization of Bobbin Friction Stir Welded 1100 Aluminum Alloys using Response Surface Methodology

This paper presents the modelling of the mechanical properties of the bobbin friction stir welded of 6 mm thick AA1100 with control factors of spindle and welding speeds. Face-centered composite design (FCCD) was used to design the experimental work and the results of the responses and the combination of factors were analyzing through analysis of variance (ANOVA). From ANOVA, the result indicates that both spindle and welding speed influence significantly the tensile strength and average hardness at SZ of AA1100. The optimum factors for maximum tensile strength and average hardness of the AA1100 were 950 rpm and welding speed of 130 mm/min. Both models giving a relative small percentage error of 0.8 % and 1.64 % for tensile strength model and average hardness in stir zone (SZ) region, respectively, thus indicate the models were adequate.

Microstructure and mechanical properties of Al-Si-Ni coating on Cu-Cr substrate prepared by multi-permeation and friction stir processing

Abstract The surface of copper-chromium alloy was processed by Al-Si-Ni multi-permeation and friction stir processing, and the microstructure and mechanical properties of the surface layer were tested by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), microhardness tester and friction testing machine. The results show that Al, Si and Ni elements are fully permeated into the surface of copper-chromium alloy after multielement co-infiltration and friction stir processing. In the observation of the microstructures, we found that the reticular structure is fragmented and distributed in the stir zone region. Microstructure becomes finer and grains refinement. The micro hardness of the copper-chromium alloy increased to 129 HV, 44.9% higher than that of the original matrix. The main reasons of microhardness enhancement are solid solution strengthening, fine grains strengthening and dispersion strengthening. The friction test results show that the friction coefficient is basically stable at 0.69 and the wear mass is only 0.0017 g after 10 min of friction test. The improvement of wear resistance was attributed to the increase of microhardness of the alloy surface.

Friction Stir Spot Welding-Brazing of Al and Hot-Dip Aluminized Ti Alloy with Zn Interlayer

Friction stir spot welding (FSSW) of Al to Ti alloys has broad applications in the aerospace and automobile industries, while its narrow joining area limits the improvement of mechanical properties of the joint. In the current study, an Al-coating was prepared on Ti6Al4V alloy by hot-dipping prior to joining, then a Zn interlayer was used during friction stir joining of as-coated Ti alloy to the 2014-Al alloy in a lap configuration to introduce a brazing zone out of the stir zone to increase the joining area. The microstructure of the joint was investigated, and the joint strength was compared with the traditional FSSW joint to confirm the advantages of this new process. Because of the increase of the joining area, the maximum fracture load of such joint is 110% higher than that of the traditional FSSW joint under the same welding parameters. The fracture load of these joints depends on the joining width, including the width of solid-state bonding region in stir zone and brazing region out of stir zone.

Microstructural Characterization and Sliding Wear Behavior of Cu/TiC Copper Matrix Composites Developed Using Friction Stir Processing

The relatively new severe plastic deformation method, friction stir processing (FSP), is a cutting-edge process to synthesize surface and bulk metal matrix composites. The present work is focused to produce Cu/TiC copper matrix composites (CMCs) and investigate the microstructure and sliding wear behavior at room temperature without lubrication. In the beginning of the process, TiC particulates were pressed in a machined groove on the surface of copper plates. The dimensions of the groove were altered to produce four different volume fractions of TiC particulates (0, 6, 12, and 18 vol%). FSP was accomplished by an optimized set of process parameters. The microstructure was observed using optical microscopy, scanning electron microscopy, and electron back-scattered diffraction. The microstructures showed a consistent dispersion of TiC particulates in the copper matrix irrespective of the volume fraction. The dispersion was observed to be uniform across the whole stir zone region. The interfacial bonding with the copper was proper. The reinforcement of TiC particulates enhanced the microhardness and led to a reduction in the wear rate of the composite remarkably. TiC particulates changed the wear mechanism and the geometry of wear debris. Highest hardness and lowest wear rate were observed in Cu/18 vol% TiC CMC.

Resistance and friction stir spot welding of dual-phase (DP 780)\u2014a comparative study

Among the AHSS, the dual-phase (DP) steels have been the subject of particular attention owing to their good combination of high strength and ductility and recent study focus on the weldability of these alloys. Resistance spot welding (RSW) is the primary sheet metal lap joining process in the manufacture of automotive assemblies. However, AHSS in RSW has revealed a number of problems that are a concern to the increased adoption of this range of alloys. Friction stir spot welding (FSSW) has been developed as an extension of friction stir welding (FSW). During FSSW, the rotating tool penetrates the sheets being weld and is then retracted producing a stir zone region that comprises a fine dynamically recrystallized microstructure. Solid-state metallurgical bonding was produced using a range of plunge rate, plunge depth, and tool rotational speed. The objective of this work is to compare the microstructure and mechanical properties on DP780 AHSS (1.6 mm thick) spot welds conducted using both process. The hardness, microstructure, failure mode, and bound area were examined. The results show that RSW has higher strength and bound area: the FSSW with decolorized advantage. Based on these results, we can provide the direction to choose the appropriate weld method.

Electron backscattered diffraction analysis of friction stir processed nanocomposites produced via spark plasma sintering.

In the present study, Spark Plasma Sintered (SPSed) aluminium matrix composites were severely deformed through Friction stir processing (FSP). Pure aluminium powders and bimodal sized Al2 O3 particles (80 nm and 25 μm) were firstly mixed by ball milling and then consolidated by spark plasma sintering. The effect of the heat input as well the bimodal particle size of the alumina on the materials' microstructure and texture development was evaluated by electron back scattered diffraction (EBSD) analysis. The EBSD analysis clearly showed that the SPSed nanocomposites possessed bimodal aluminium matrix grain structure as well as a crystallography characterised by random texture. In addition, microstructural examination revealed that the partial recrystallisation occurred during SPS for all the nanocomposites. Also, it is revealed that the Zener pinning effect of Al2 O3 nanoparticles retarded recrystallised grain growth following recrystallisation during FSP and then leading to grain refinement of the aluminium. The results revealed that the heat generated during FSP has a remarkable effect on the grain distribution as well as on the crystallographic orientation. Also, a mixture of {112} <110> shear elements and an ideal strong B/B¯ component were observed. The microstructural changes, occurred during FSP in the stir zone region for Al-Al2 O3 nanocomposites, were attributed to both the discontinuous along with the continuous recrystallisation (DDRX/CDRX). It should be pointed out that with increasing the heat input, recrystallised grains portion increased.

The Effect of Second Phase Particle Dissolution on the Corrosion of Friction Stir Spot Welded AZ31B

The connection between the corrosion characteristics of friction stir spot welded AZ31B and the microstructural changes induced by the joining operation has been examined. Second phase particle dissolution was investigated through temperature measurements during welding, coupled with microstructural characterization using scanning electron microscopy and nuclear magnetic resonance spectroscopy. The electrochemical properties of the stir zone (SZ) region were characterized by microcapillary polarization. The SZ region was noble to the base metal for all welding parameter settings examined. For a constant dwell time, increasing the tool rotational speed increased the corrosion potential and reduced the corrosion current density of the SZ region. The improvement in the corrosion resistance was attributed to increased dissolution of β-Mg17Al12. Increasing the length of time available for particle dissolution by diffusion had a similar effect for durations up to one second, and longer times had no effect as complete dissolution of β-Mg17Al12 was already achieved. A model capable of predicting the electrochemical properties of the stir zone was developed based on mixed potential theory, changes in stir zone microstructure, and the electrochemical properties of individual phase constituents. The area fraction that supports the anodic dissolution reaction was decreased with increasing extent of second phase dissolution.

The Effect of Microstructural Evolution During Friction Stir Spot Welding on the Corrosion of AZ31B

The connection between corrosion characteristics of friction stir spot welded (FSSW) AZ31B and the microstructural changes induced by the joining operation have been investigated. The effect of second phase particle dissolution was investigated through temperature measurements during welding, coupled with electrochemical characterization of the stir zone (SZ) region by microcapillary polarization. The SZ region was noble to the base metal for all welding conditions tested in this investigation. For a constant dwell time, increasing the rotational speed of the welding tool increased the corrosion potential and reduced the corrosion current density of the SZ. The improvement in the corrosion resistance was attributed to increased dissolution of β-Mg17Al12 particles. Increasing the length of time available for particle dissolution by diffusion had a similar effect for durations up to one second. Further increase in duration produced no effect as complete dissolution of β-Mg17Al12 was already achieved.

Microstructural Evolution during Friction Stir Spot Welding of AZ31 and Its Effect on the Corrosion Resistance of the Joint

In recent years, the corrosion resistance of friction stir spot welds made in magnesium alloys has been the focus of many research projects. Previous reports documented the cathodic behavior of the stir zone (SZ) with respect to the base material, causing increased localized corrosion in the region adjacent to the weld, which is known as the thermo-mechanically affected zone (TMAZ).1,2 While the electrochemical interaction between the SZ and the base metal have been thoroughly studied, the cause for the ennoblement of the weld nugget remains largely unknown. Various studies suggested that the microstructural evolution occurring during the welding process was responsible for the increased corrosion resistance of the stir zone, but the specific contribution from each microstructural feature has not been examined. The present study examines the effect of several microstructural features that are altered during the welding process, ultimately determining their individual contribution to the overall effect on the corrosion characteristics of the joint. Specifically, this investigation examines the effect of (i) residual stress, (ii) grain size and orientation, and (iii) second phase precipitation on the corrosion behavior of FSSW in magnesium alloys. The effect of microstructural variation on the corrosion resistance properties of the stir zone has been evaluated through characterization and electrochemical testing. The corrosion potential and current of the stir zone were determined using microcapillary polarization in 0.1M NaClO4, a localized electrochemical technique developed in previous work.2 Changes in grain size and orientation were monitored through electron back scattered diffraction (EBSD). The effect of residual stress was examined through a series of stress relieving heat treatments. Finally, the effect of second phase particles was investigated by electron microscopy (EM) and nuclear magnetic resonance (NMR). The effect of friction stir spot welding on the microstructure and electrochemical properties of AZ31 is summarized in table I. Each microstructural change was found to have a unique effect on the electrochemical properties of the stir zone region. The relatively small changes in grain size and residual stress induced by the welding operation have been shown to have negligible effects on the corrosion behaviour of the weld nugget.3 Investigation of the grain orientation revealed weaker basal texture in the stir zone than in the base metal. As a result, changes in grain orientation were found to be responsible for the reduction in the corrosion potential and current of the stir zone. Changes in the size and presence of second phase precipitates (Mg-Al and Al-Mn intermetallics) were determined to be the most dominant factor determining the electrochemical properties of the stir zone. Dissolution of these intermetallics eliminated the harmful microgalvanic cells formed between the particles and the surrounding matrix, as well as increased the overall aluminum content in the α-Mg matrix. Ongoing work is focused on producing a model that correlates the welding parameters to the microstructural features present in the SZ region and its subsequent electrochemical properties.