Retreating Side Research Articles

Microstructure and texture of friction stir welds in low carbon steel using prior austenite reconstruction method

Friction stir welding (FSW) was performed on thick plates of high-strength low-alloy (HSLA) steel with polycrystalline cubic boron nitride (PCBN) tools. To investigate the mechanism of microstructure formation in the stir zone (SZ), the prior austenite structure undergone through a thermo-mechanical history during the FSW process was investigated. The prior austenite structure was reconstructed by the local crystal orientations of prior austenite calculated from the bainite orientations by using the Kurdjumov–Sachs orientation relationship between the prior austenite and bainite. The reconstructed austenite revealed a heterogeneity in the SZ. The austenite grain size in the advancing side (AS) of SZ was larger than that of the retreating side (RS). Thus, a reheated temperature of AS was considered to be higher than that of RS. The textures of reconstructed austenite were also different in SZ. The rolling texture was observed in RS, but the shear texture was observed in the center and AS. The heterogeneous microstructure and texture in SZ was considered to be caused by the material flow behavior in FSW process.

Microstructure evolution mechanism of Al/Mg dissimilar joint during friction stir welding

A butt friction stir welding (FSW) process was performed on 6061 Al and AZ31 Mg plates. The microstructure evolutions of the three main regions in the nugget zone (NZ) retained in the FSW joint were systematically investigated to clarify the joint formation mechanism during FSW. The differential etching of these microstructural features was found to produce very vivid flow features. During FSW, the material in the shoulder affected zone (SAZ) was mainly driven by the shoulder, and only a small amount of it was driven by the pin. A strip of Al transferred by the pin from the retreating side (RS) to the advancing side (AS) contacted and reacted with Mg, thus forming intermetallic compounds (IMCs) (e.g., Mg17Al12 and Al3Mg2). Due to the stirring action and tilted angle of the threaded pin, a banded structure (BS) feature tilted at approximately 45° was produced by the alternating lamellae of IMCs. The appearance of an onion ring structure occurred in the severely deformed zone (SDZ), which could be attributed to the reflection effect of the imaginary die wall. Finally, the overall flow pattern of the joint was obtained.

Mechanical and metallurgical properties of friction stir welded dissimilar joints of AZ91 magnesium alloy and AA 6082-T6 aluminium alloy

Abstract In the present research work, AZ91 magnesium alloy and AA6082-T6 aluminium alloy were joined by friction stir welding process. The comparison of microstructure and mechanical properties between different joints by varying the different materials on advance and retreating sides were mainly studied. Four different welds have been prepared to find the material mixing between the similar and dissimilar joints. The joint interfaces of the welds have been investigated by employing an optical microscope and scanning electron microscope. When Mg was placed on advancing side (AS), more aluminium content was soluble in nugget zone than the case where Mg was placed on the retreating side (RS). Thin intermetallic layer in the joint interface of Mg/Al and thick intermetallic layer with poor adhesion of the aluminium and magnesium have been observed in the dissimilar joints varying the sides. The highest UTS of 172.3 MPa was found for Mg-Al when Mg was placed on AS and lower UTS of 156.25 MPa was obtained when Mg was placed on RS. Hardness of 86 Hv and 89 Hv were observed in the Stir zone for the dissimilar AZ91 Mg alloy and AA6082-T6 Al alloy when AZ91Mg alloy was placed on the AS and on the RS respectively. Fractography was also carried out to find the mode of failure.

Interfacial bonding features of friction stir additive manufactured build for 2195-T8 aluminum-lithium alloy

Friction stir additive manufacturing (FSAM) was performed successfully using 2 mm thick sheets of 2195-T8 aluminum-lithium alloy. The influence of the tool pin shape and process parameters on the interfacial bonding features among the additive manufactured layers was discussed, and the effects of interfacial defects on the performances of the additive build were analyzed based on microstructures, hardness profiles, and mechanical property evaluations. It is shown that the shape of the tool pin is one of the key factors in influencing the bonding interface between two manufactured layers. The cylindrical pin and the conical pin with three flats are not suitable for the FSAM process since very poor material mixing features are produced along the bonding interface. Although the material mixing degree of bonding interface is obviously improved at the advancing side (AS) interface of the nugget zone (NZ) by using the convex featured pin or the pin with three concave arc grooves, the material mixing degree at the retreating side (RS) interface of the NZ is always insufficient. Meanwhile, the weak-bonding defects along the bonding interfaces could be formed, which are originated from the hooking defects on the RS. The weak-bonding defects are related to the oxides and impurities existing at the original bonding interfaces as well as the insufficient stirring action of the tool pin. The back and forth double passes welding is one of the effective methods to improve the material mixing in the whole NZ and eliminate the hooking defects extending into the NZ. The welding rotation speeds of 800, 900 and 1000 rpm for giving welding speed of 100 mm/min were used in the additive manufacturing processes of 2195-T8 aluminum-lithium alloy, in which the optimum microstructure is obtained with the rotation speed of 800 rpm. The soften degree for the multilayered build is obvious, and the hardness profiles across the different bonding interfaces are always uneven. Meanwhile, compared with the AS interface, the fluctuation of the hardness value at the RS interface is greater. The mechanical properties of the multilayered build are inhomogeneous, and the maximum tensile strength of the multilayered build is only reached the 56.6% of the base metal. The mechanical properties are closely associated with the soften tendency of the material and the degree of the amelioration of weak-bonding defect along the bonding interface.

Temperature Distribution During Friction Stir Welding of AA2014 Aluminum Alloy: Experimental and Statistical Analysis

FSW has shown great potential to weld AA2014 heat-treatable aluminum alloy. But the effectiveness of joint in FSW depends on the temperature distribution across the length and width of weld zone. The present study aims to investigate the temperature distribution during FSW of AA2014 alloy. The temperatures were recorded at eight different positions by inserting K-type thermocouples on pre-drilled AA2014 alloy under different process parameters like tool rotation (N), welding speed (v) and tilt angle. These temperatures were recorded using two thermocouple layouts: First layout was “equal distance and same depth” on advancing side (AS) whereas second layout was “equal distance and varying depth” on retreating side (RS). Experimental results showed that by increasing the N/v ratio, the peak temperature increased whereas the peak temperature decreased by decreasing the N/v ratio. On the other hand, effect of tool tilt angle on peak temperature was also found to be significant and confirmed by experimental as well as statistical analysis. Furthermore, it was concluded that temperature was higher on AS compared to the RS. The conclusion drawn from statistical analysis was in good agreement with experimental results.

Effect of tool/material interface temperature on flow stress and the Zener-Holloman parameter in Al 6082 friction stir butt welds

This paper outlines, the temperature around the tool/material interface during friction stir welding (FSW) of Al 6082-T6 alloy. The first section gives a brief overview on temperature measurement around the tool periphery, i.e. trailing edge (TE), leading edge (LE), retreating side (RS), advancing side (AS), and their corners associated with it. The second section was focused on the plasticity behavior of welded material as a function of temperature, governed by flow stress and Zener Holloman (Z)-parameter. Around the tool periphery in a clockwise direction, the critical observations were made from the obtained data. The peak temperature surges with increasing shoulder diameter and it was higher for the weld done with 24mm shoulder diameter tool. It was noted that during every cycle there was asymmetric temperature distribution at tool/material interface, the rate of heat generation was higher in TE to RS-LE zone and lower in RS-LE to TE zone. The calculated flow stress and log Z values were higher on RS and lower in AS to RS-LE zone of the tool shoulder periphery. Flow stress was almost uniform for all the shoulder diameters except on RS of 18mm shoulder tool. The optimum tool shoulder diameter was found to be 21mm, below which the flow stress was high and above which the plasticity at tool shoulder periphery was negligible.

On the microstructure and mechanical properties of similar and dissimilar AA7075 and AA2024 friction stir welding joints: Effect of rotational speed

The similar and dissimilar aluminum alloys (5 mm thick AA7075-T651 and AA2024-T351) joints were performed by friction stir welding to assess the influence of rotational speed (600, 950, 1300 and 1650 rpm) on the microstructure features, materials flow and mechanical properties of the joints. The results indicated that the width of the thermos-mechanically affected zone (TMAZ) on the retreating side (RS) is greater than that of the advancing side (AS). Increasing the rotational speed results in widening the TMAZ on the AS and RS. The mixing degree in the joints is remarkably affected by the rotational speed. Low rotational speed leads to a limited materials mixing, while the typical onion ring of mixing pattern can be acquired at the high rotational speed. Compared to the base materials, significant grain refinement (average grain size: 1.7 μm) occurs in all the joints at a rotational speed of 600 rpm. Increasing the rotational speed is prone to result in grain coarsening. The crystallographic texture in the nugget zone of all the joints is dominated by a simple shear texture and varies with the rotational speed. The hardness values of all the joints almost increase and then decrease from the top to the bottom along the welding center thickness direction. The tensile fracture locations coincide with that of minimum hardness values in all the joints at various rotational speeds.

Exfoliation corrosion of friction stir welded dissimilar 2024-to-7075 aluminum alloys

The objective of this study was to identify the effect of base metal (BM) locations on the corrosion behavior of friction stir welded dissimilar 2024-T351 to 7075-T651 aluminum alloys joints in an EXCO solution. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were conducted to characterize the microstructure evolution of the BMs and stir zones (SZs). Scanning electron microscopy (SEM) was employed to observe the appearance of these corroded zones. Results reveal that the corrosion resistance of SZs is similar to that of the BM positioning in the retreating side (RS), and intergranular corrosion (IGC) is prevailing. The fine recrystallized grains in SZs exhibit more drastic IGC than that of BMs. The discontinuously distributed grain boundary precipitates and precipitate-free zones in the SZ of 2024 reduce the IGC damage of SZ.

Material flow and void defect formation in friction stir welding of aluminium alloys

ABSTRACTAn ingenious experimental programme by combining artificially thickened oxide layer as marker material and ‘stop-action’ welding were used to study the material flow and defect formation in friction stir welding of aluminium alloys. The results showed that material flow around the pin on the advancing side (AS) was severer than that on the retreating side (RS) and the fastest velocity of material flow in the middle stir zone (SZ) was 43.9 mm s−1. Moreover, the material under the RS shoulder included extruded metal only and the material under the AS shoulder included extruded and rotated metal. Lastly, instantaneous void occurrence and insufficient inflow material were reasons for the preferential formation of void defects in the top SZ on the AS.

Prediction and Experimental Validation of Peak Temperature in Friction Stir Welded AA2024 with AA5083 by Response Surface Methodology

Objective: The work aims at prediction of the peak temperature using Response Surface Methodology. Methods/ Statistical Analysis: Dissimilar Aluminum Alloys AA2024 and AA5083 O were friction stir welded. A fifteen run 3 factorial 3 level Box Behnken design was conducted and temperature generated were measured at both sides of the alloys. Thermocouples were employed for the same. The temperature was recorded and studies conducted. Response surface plots were used to obtain the peak temperature and effect of various parameters. Findings: The relationship between the process parameters and the temperature were established using ANNOVA. Mathematical models were development which can apply for future predictions. The traverse speed, rotational speed and the tool profile contributed to the temperature generation. The peak temperature was obtained at 1200 rpm, 25 m/min and threaded cylindrical tool pin profile. Application/Improvements: The temperature at the re-treating side was recorded to be higher compared to the advancing side. Keywords: Anova, Heat Input, Mathematical Modeling. Peak Temperature, RSM

Experimental and numerical investigations of bonding interface behavior in stationary shoulder friction stir lap welding

Stationary shoulder friction stir lap welding (SSFSLW) was employed to weld 2024 aluminum alloy. A coupled Eulerian-Lagrangian (CEL) model was developed to investigate the lap interface behavior during SSFSLW. Numerical results of material movement and equivalent plastic strain were in good agreement with the experimental work. With increasing welding speed, the distances from the hook tip to the top surface of the upper workpiece on the retreating side (RS) and the advancing side (AS) increase, while the distance between two wave-shaped alclads decreases. A symmetric interface bending is observed on the AS and the RS during plunging, while the interface bending on the AS is bigger than that on the RS during welding. The peak temperature of the interface on the AS is higher than that on the RS. The equivalent plastic strain gradually increases as the distance to the weld center decreases, and its peak value is obtained near the bottom of the weld.

Influence of tool shoulder end features on friction stir weld characteristics of Al-Mg-Si alloy

In the present study, the tool shoulder end features such as concentric circle shoulder tool (TCC), ridge shoulder tool (TR), knurling shoulder tool (TK), and scroll shoulder tool (TS) were designed, and their effect on weld size, vertical force, temperature distribution, and material flow was investigated during friction stir welding (FSW) of Al-Mg-Si alloy. The vertical force, tool torque, and temperature during the welds were measured and compared for all the tools. The deformed marker material was used to analyze the material flow caused by the shoulder and pin. The experimental results show that the welds with shoulder end featured tools of smaller shoulder diameter (ϕ18mm)were comparable with the welds with plane shoulder tool (TP) of larger shoulder diameter (ϕ21mm) in the aspects of temperature and weld strength with reduced weld size. At initial stage, the plasticized material, beneath the tool shoulder, moves from advancing side (AS) to retreating side (RS) whereas the material around the pin moves from RS to AS. Overall, the welds produced with the proposed tool shoulder end features achieved lower vertical force, higher temperature, and minimum/no flash compared to TP of equal/higher shoulder diameter. The welds produced with TR resulted in better mechanical properties with lower vertical force, i.e., approximately 36% than that of (TP)21 tool and the hardness of weld zone is at a value of 65–78 HV0.1.

Effect of lateral offset on microstructure and strength of friction stir welded 2A14-T6 aluminum alloy

In this paper, the effects of different lateral offset directions on the microstructure and mechanical properties of the welded joints of 2A14-T6 aluminum alloy during friction stir welding (FSW) were studied. The results indicate that the center of the weld nugget (WN) always coincides with the center of the welding tool. There is some influence on the shape of the WN when the offset condition is present. The movement of the location of the WN with the offset of the welding tool changes the location of the initial contact surface (ICS), resulting in the change of the severe level of stirring at the ICS and the pressure at the joint root. There are varying degrees of lack of penetration when the welding tool shifts to the retreating side (RS) and the advancing side (AS) respectively. The presence of the defects has an important influence on the mechanical property and fracture, especially when the welding tool shifts to the AS. The joint performance has obvious reduction. In summary, the quality of the joint is more sensitive to the condition of AS offset.

Microstructural features of friction stir welded dissimilar Aluminium alloys AA2219-AA7475

High strength, good corrosion resistance, light weight make aluminium alloys a material of choice in many industrial sectors like aerospace, marine etc. Problems associated with welding of these alloys by fusion welding processes restricted their use in various industries. Friction stir welding (FSW), a clean solid-state joining process, easily overcomes various difficulties encountered during conventional fusion welding processes. In the present work, the effect of rotational speed (710 rpm, 900 rpm and 1120 rpm) on micro-hardness distribution and microstructure of FSWed dissimilar aluminium alloy joints were analyzed. Plates of AA7475-T761 and AA2219-O having thickness of 2.5 mm were welded by fixing AA7475 on retreating side (RS) and AA2219 on advancing side (AS). Welded joints were characterized by Vickers micro-hardness testing, scanning electron microscopy (SEM) and optical microscopy (OM). Results revealed that rotational speed significantly affects the micro-hardness due to increase in grain size, coarsening and dissolution of strengthening precipitates and re-precipitation. Higher micro-hardness values were observed in stir zone due to grain refinement and re-precipitation. Minimum micro-hardness value was observed at the TMAZ/HAZ of advancing side due to thermal softening.

Effect of Processing Parameters on Plastic Flow and Defect Formation in Friction-Stir-Welded Aluminum Alloy

The effect of processing parameters on material flow and defect formation during friction stir welding (FSW) was investigated on 6.0-mm-thick 2014Al-T6 rolled plates with an artificially thickened oxide layer on the butt surface as the marker material. It was found that the “S” line in the stir zone (SZ) rotated with the pin and stayed on the retreating side (RS) and advancing side (AS) at low and high heat inputs, respectively. When the tool rotation rate was extremely low, the oxide layer under the pin moved to the RS first and then to the AS perpendicular to the welding direction, rather than rotating with the pin. The material flow was driven by the shear stresses produced by the forces at the pin–workpiece interface. With increases of the rotation rate, the depth of the shoulder-affected zone (SAZ) first decreased and then increased due to the decreasing shoulder friction force and increasing heat input. Insufficient material flow appeared in the whole of the SZ at low rotation rates and in the bottom of the SZ at high rotation rates, resulting in the formation of the “S” line. The extremely inadequate material flow is the reason for the lack of penetration and the kissing bonds in the bottom of the SZ at extremely low and low rotation rates, respectively.

Influence of ridges shoulder with polygonal pins on material flow and friction stir weld characteristics of 6082 aluminum alloy

In the present paper, the flow of stirred material under ridges shoulder combined with various polygonal pin profiles during friction stir welding (FSW) of 6082 aluminum alloy was investigated through marker insert technique. The marker inserts (copper) were placed in ‘parallel to/along the weld’ and ‘perpendicular to weld configurations’. The axial force and tool torque during the welding were measured and compared for all the tools. Final weldments were subjected to radiographic and sectioned macro and microscopic observation to understand the detailed flow of marker material. The results show that the material driven by pin moves from bottom to top which improves the flow in the top layer of material i.e., shoulder region. The individual effect of pin profiles on material flow was clarified. The welds with square pin tool, (TR)S resulted in superior mechanical properties and uniform weld zone hardness with complete mixing and distribution of material on both advancing side (AS) and retreating side (RS).

Analysis of cooling media effects on microstructure and mechanical properties during FSW/UFSW of AA 6082-T6

The aim of the present study is to investigate the effect of cooling media on the temperature distribution, microstructure and mechanical properties of the joint produced during Underwater Friction Stir Welding (UFSW) in normal water, cold water (water with crushed ice (CFSW)) and air (FSW), for aluminum alloy (AA) 6082-T6. The results showed that peak temperature during UFSW and CFSW were significantly lower than the FSW. The temperature at the advancing side (AS) of the joint was higher than the retreating side (RS). Substantial reduction in TMAZ/HAZ width was observed during UFSW and CFSW as compared to FSW. Al-Mn-Fe-Si intermetallic phases were seen in all the joints along with the BM. The main strengthening precipitates found in UFSW and CFSW was β″ (Mg5Si6) which changed to β (Mg2Si) precipitates during FSW due to increased temperature. The tensile strength of the joints was best during UFSW followed by FSW and CFSW. The controlled temperature distribution resulted in improved tensile strength whereas both undercooling and overcooling resulted in decreased tensile strength, however, increased cooling rate does not improve the elongation. A typical ‘W’ shape hardness profile was observed in all the joints irrespective of the cooling media used. Maximum hardness was obtained in the UFSW joint due to refined grain structure, high-density dislocations and presence of β″ phases.

Visualization of Contact-Pressure Distribution between Material and Backing Plate in Friction Stir Processing

In this study, the authors evaluated pressure distribution on a backing plate in friction-stir processing (FSP) utilizing an embedded pressure pin connected to a load sensor. They conducted FSP on aluminum alloy plates repeatedly offsetting the path-lines from the center of the pin and recorded change of forming pressure with tool position, which was compiled from the bearing load of the pin. The authors mapped the results to visualize the two-dimensional contact pressure distribution on a backing plate during FSP. They then compared the height distribution of the wall fabricated by friction-stir forming (FSF) utilizing a die having a groove with the observed distribution of pressure. Consequently, maximum pressure was observed beneath the rim of the tool probe at the retreating side (RS), and the highest points of the wall were observed at the RS.

Effect of Exchanging Advancing and Retreating Side Materials on Mechanical Properties and Electrochemical Corrosion Resistance of Dissimilar 6013-T4 and 7003 Aluminum Alloys FSW Joints

Friction stir welding (FSW) was used to weld dissimilar joints between Al 6013-T4 and Al7003 alloys in this work. The effect of exchanging advancing (AS) and retreating (RS) side material on microstructure, mechanical behaviors and electrochemical corrosion resistance was discussed. Results showed that different joint cross sections were obtained when exchanging AS and RS materials. The material on the AS would be more deformed during the welding process. When the Al6013 placed on the AS, the plastic flow of weld is more sufficient. Whether on the AS or RS, the Al6013-T4 side is the weak region for both tensile specimens and hardness samples. The fracture position corresponds to the minimum hardness position. Also, more strengthening phase can be retained in the joint, and the joint of A6R7 has better corrosion resistance.

Residual Stresses in Dissimilar Friction Stir Welding of AA2024 and AZ31: Experimental and Numerical Study

Thermal history and residual stresses in dissimilar friction stir welding (FSW) of AA2024 and AZ31 were studied under different tool offsets using a coupled Eulerian–Lagrangian (CEL) finite element model and a mechanical model. Welding experiments and residual stresses' measurements were conducted to validate the models. Comparisons between the experimental and numerical results indicated good agreement. The maximum temperature in the welded zone was predicted to be slightly lower than 400 °C, regardless of offset, and that its location shifted with tool offset from the advancing side (AS) to the retreating side (RS). Longitudinal residual stresses changed from tensile under the tool shoulder to compressive beyond this region and it appeared to be the dominant stress component. The transverse stresses were tensile and of lower magnitude. Both the longitudinal and transverse residual stresses have their maximum values within the weld zone near the end of the weld length. For both peak temperatures and residual stresses, higher values were obtained at the AS with no tool offset and 1 mm offset to the AS, and at the RS with 1 mm offset to the RS. Lower residual stresses and better weld quality were obtained with tool offset to the aluminum side.