The Effect of Concurrent Straining on Phase Transformations in NiAl Bronze During the Friction Stir Processing Thermomechanical Cycle

Equivalent strains up to a value of ≈2.7 were determined by evaluation of the shape changes of the phases in a duplex α(fcc)/β(bcc) microstructure formed ahead of the pin tool extraction site during the friction stir processing (FSP) thermomechanical cycle in a cast NiAl bronze alloy. Correlation of the local strains with volume fractions of the various microstructure constituents in this alloy shows that the concurrent straining of FSP results in acceleration of the α + β → β reaction in the thermomechanically affected zone (TMAZ) ahead of the pin extraction site. The resulting volume fraction of β (as determined by the volume fraction of its transformation products formed during post-FSP cooling) corresponds closely to the volume fraction expected for the peak stir zone temperature measured separately by means of thermocouples embedded within the tool pin profile along the tool path. The stir zone (SZ) in this material exhibits near-equilibrium microstructures despite brief dwells near the peak temperature (T _peak ≈ 0.95T _melt), reflecting large local strains and strain rates associated with this process.     

Some Observations on Microstructural Changes in a Mg-Based AE42 Alloy Subjected to Friction Stir Processing

The main aim of the current study is the analysis of friction stir processing (FSP) of Mg-based alloys as a possible tool for nanocomposites production. The study reports microstructural changes taking place in a Mg-based alloy (AE42) subjected to FSP under different cooling conditions. The FSP process was carried out with single as well as multipass options. The friction stir processed samples were characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), focused ion beam (FIB)-scanning ion microscopy (SIM), and X-ray diffraction (XRD). It was observed that FSP tends to fragment the elongated precipitates and produces near homogeneous distribution of fine particles. The smallest particle size was observed to be produced by double-pass FSP supplemented by rapid cooling, thereby generating in situ nanocomposites. Vickers microhardness testing was done along the thickness (transverse direction) of the specimen to study and understand the variation of hardness with thickness. Nearly a two-times increase in the microhardness of AE42 was observed in the case of double-pass, FSP AE42 with cooling at temperature of approximately 253 K (–20 °C). To confirm these observations, another magnesium alloy AM50 was also friction stir processed under similar conditions. The fine submicron grain structure produced in AE42 alloy contributed immensely toward grain boundary strengthening and Orowan strengthening had only marginal influence. Subgrain boundary pinning by in situ nanoparticles contributed significantly in the strengthening process.     

Formation of Liquid and Intermetallics in Al-to-Mg Friction Stir Welding

In dissimilar-metal friction stir welding (FSW), intermetallic compounds can form in the stir zone and significantly reduce the joint strength. The formation of intermetallic compounds in Al-to-Mg FSW was investigated in lap and butt FSW of the widely used 6061 Al and AZ31B Mg and discussed using the binary Al-Mg phase diagram as an approximation. Temperature measurements during lap FSW indicated a 703 K (430 °C) peak temperature, slightly below the eutectic reaction (Mg) + Al₁₂Mg₁₇ → L at 710 K (437 °C), because the thermocouples were pushed downward during welding. The intermetallic compounds in the stir zone were revealed by color etching and identified by X-ray diffraction (XRD), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM) as Al₃Mg₂ and Al₁₂Mg₁₇. Additional FSW was conducted near the edge of the upper sheet, and the liquid droplets squeezed out during welding solidified along the edge. Optical microscopy of the solidified droplets and EPMA revealed dendrites of Al₃Mg₂ and Al₁₂Mg₁₇ and interdendritic eutectics, thus indicating eutectic reactions (Mg) + Al₁₂Mg₁₇ → L (710 K (437 °C)) and (Al) + Al₃Mg₂ → L (723 K (450 °C)). Differential scanning calorimetry (DSC) confirmed that the solidified droplets melted at 709 K (436 °C) and 722 K (449 °C), nearly identical to the eutectic temperatures. Formation of intermetallic compounds on the order of 1 mm in size suggests they form upon solidification of the liquated material instead of solid-state diffusion.     

The influence of friction stir processing parameters on microstructure of as-cast NiAl bronze

The influence of friction stir processing (FSP) parameters on the evolution of microstructure in an equilibrium-cooled, as-cast NiAl bronze (NAB) material was evaluated by optical microscopy (OM) and transmission electron microscopy (TEM) methods. A threaded pin tool was employed and tool rotation and traversing rates were varied in order to examine the spatial variation of stir zone microstructures in relation to FSP parameters. For processing at low rotation and traversing rates, the microstructure throughout the stir zone consists of elongated and banded grains of the primary α and transformation products of the β phase. Such microstructures reflect severe deformation at temperatures up to ～900 °C in the α+β two-phase region for this NAB material. Increasing rotation and traversing rates, coarse Widmanstätten α near the surface in contact with the tool became apparent. The appearance of this constituent reflects nearly complete transformation to β during FSP with peak temperatures of ～1000 °C. Also, complex stir zone flow patterns, often referred to as onion ring structures, become distinct in the mid regions of the stir zones as rotation and traversing rates increase. Schematic representations illustrating the effect of FSP parameters on thermal cycles at various locations in stir zones were prepared based on microstructure observations. Thus, processing at higher rotation and traversing rates results in higher peak temperatures near the surface in contact with the tool but also in steeper temperature gradients when compared to lower rotation and traversing rates.     

Influence of Friction Stir Processing Parameters on the Microstructure of Aluminum Foams

In the present study, friction stir processing (FSP) has been used to fabricate aluminium foams. The effects of the number of FSP passes, FSP tool rotational speed, foaming time and temperature on the porosity have been investigated. Aluminium foam with porosity up to 40% was successfully fabricated. In the samples foamed at 923 K (650 °C), a few irregular pores were produced as a result of high aluminium matrix stiffness in this temperature. In general with increase in foaming temperature the porosity increased. However, in the samples foamed for 30 or 60 min, lower porosity was detected at higher foaming temperature. Also, in the samples which were produced with more FSP passes, the foaming time decreased and more uniform pore structure was obtained.     

Effect of Multiple-Pass Friction Stir Processing Overlapping on Microstructure and Mechanical Properties of As-Cast NiAl Bronze

As-cast Cu-9Al-4.5Ni-4Fe NiAl bronze alloy (NAB) was subjected to multiple-pass friction stir processing (FSP) with a 50 pct overlap. After FSP, the coarse microstructure of the base metal (BM) was transformed to defect-free material with fine microstructure. While the torchlike patterns in the stir zone (SZ) and the uplifted grains in the transitional zones (TZs) between two passes were observed in the multiple-pass FSP region, no grain coarsening was found in the remnant zone of the previous SZ after subsequent FSP pass. The hardness value of the FSP materials was higher than that of the BM and was homogeneously distributed throughout the entire multiple-pass FSP region. The FSP materials showed greatly improved tensile properties compared to the BM, and the TZs showed similar tensile strength and ductility to the single-pass FSP materials. The BM broke in a mixture mode of brittle cleavage and microvoid coalescence fracture, whereas the FSP and TZ samples failed in the latter fracture mode. The results showed that the multiple-pass overlapping (MPO) FSP was feasible to modify the microstructure of large-sized plate of the NAB.     

Effect of Friction Stir Processing on Microstructure and Mechanical Properties of a Cast-Magnesium–Rare Earth Alloy

Single-pass friction stir processing (FSP) was used to increase the mechanical properties of a cast Mg-Zn-Zr-rare earth (RE) alloy, Elektron 21. A fine grain size was achieved through intense plastic deformation and the control of heat input during processing. The effects of processing and heat treatment on the mechanical and microstructural properties were evaluated. An aging treatment of 16 hours at 200 °C resulted in a 0.2 pct proof stress of 275 MPa in the FSP material, a 61 pct improvement over the cast + T6 condition.     

Stir zone microstructure and strain rate during Al 7075-T6 friction stir spot welding

The factors determining the temperature, heating rate, microstructure, and strain rate in Al 7075-T6 friction stir spot welds are investigated. Stir zone microstructure was examined using a combination of transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) microscopy, while the strain rate during spot welding was calculated by incorporating measured temperatures and the average subgrain dimensions in the Zener-Hollomon relation. The highest temperature during friction stir spot welding (527 °C) was observed in spot welds made using a tool rotational speed of 3000 rpm. The stir zone regions comprised fine-grained, equiaxed, fully recrystallized microstructures. The calculated strain rate in Al 7075-T6 spot welds decreased from 650 to about 20 s⁻¹ when the tool rotational speed increased from 1000 to 3000 rpm. It is suggested that the decrease in strain rate results when tool slippage occurs when the welding parameter settings facilitate transient local melting during the spot welding operation. Transient local melting and tool slippage are produced when the welding parameters produce sufficiently high heating rates and temperatures during spot welding. However, transient local melting and tool slippage is not produced in Al 7075-T6 spot welds made using a rotational speed of 1000 rpm since the peak temperature is always less than 475 °C.     

Influence of Tool Rotational Speed and Post-Weld Heat Treatments on Friction Stir Welded Reduced Activation Ferritic-Martensitic Steel

The effects of tool rotational speed (200 and 700 rpm) on evolving microstructure during friction stir welding (FSW) of a reduced activation ferritic-martensitic steel (RAFMS) in the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ) have been explored in detail. The influence of post-weld direct tempering (PWDT: 1033 K (760 °C)/ 90 minutes + air cooling) and post-weld normalizing and tempering (PWNT: 1253 K (980 °C)/30 minutes + air cooling + tempering 1033 K (760 °C)/90 minutes + air cooling) treatments on microstructure and mechanical properties has also been assessed. The base metal (BM) microstructure was tempered martensite comprising Cr-rich M₂₃C₆ on prior austenite grain and lath boundaries with intra-lath precipitation of V- and Ta-rich MC precipitates. The tool rotational speed exerted profound influence on evolving microstructure in SZ, TMAZ, and HAZ in the as-welded and post-weld heat-treated states. Very high proportion of prior austenitic grains and martensite lath boundaries in SZ and TMAZ in the as-welded state showed lack of strengthening precipitates, though very high hardness was recorded in SZ irrespective of the tool speed. Very fine-needle-like Fe₃C precipitates were found at both the rotational speeds in SZ. The Fe₃C was dissolved and fresh precipitation of strengthening precipitates occurred on both prior austenite grain and sub-grain boundaries in SZ during PWNT and PWDT. The post-weld direct tempering caused coarsening and coalescence of strengthening precipitates, in both matrix and grain boundary regions of TMAZ and HAZ, which led to inhomogeneous distribution of hardness across the weld joint. The PWNT heat treatment has shown fresh precipitation of M₂₃C₆ on lath and grain boundaries and very fine V-rich MC precipitates in the intragranular regions, which is very much similar to that prevailed in BM prior to FSW. Both the PWDT and PWNT treatments caused considerable reduction in the hardness of SZ. In the as-welded state, the 200 rpm joints have shown room temperature impact toughness close to that of BM, whereas 700 rpm joints exhibited very poor impact toughness. The best combination of microstructure and mechanical properties could be obtained by employing low rotational speed of 200 rpm followed by PWNT cycle. The type and size of various precipitates, grain size, and evolving dislocation substructure have been presented and comprehensively discussed.     

Transient Heat and Material Flow Modeling of Friction Stir Processing of Magnesium Alloy using Threaded Tool

A three-dimensional transient computational fluid dynamics (CFD) model was developed to investigate the material flow and heat transfer during friction stir processing (FSP) in an AZ31B magnesium alloy. The material was assumed to be a non-Newtonian viscoplastic fluid, and the Zener-Hollomon parameter was used to describe the dependence of material viscosity on temperature and strain rate. The material constants used in the constitutive equation were determined experimentally from compression tests of the AZ31B Mg alloy under a wide range of strain rates and temperatures. A dynamic mesh method, combining both Lagrangian and Eulerian formulations, was used to capture the material flow induced by the movement of the threaded tool pin. Massless inert particles were embedded in the simulation domain to track the detailed history of material flow. The actual FSP was also carried out on a wrought Mg plate where temperature profiles were recorded by embedding thermocouples. The predicted transient temperature history was found to be consistent with that measured during FSP. Finally, the influence of the thread on the simulated results of thermal history and material flow was studied by comparing two models: one with threaded pin and the other with smooth pin surface.     

Microstructure and Texture Evolution during Single- and Multiple-Pass Friction Stir Processing of Heat-Treatable Aluminum Alloy 2024

Microstructure and crystallographic texture evolution during single- and multiple-pass friction stir processing (FSP) of an age-hardenable aluminum alloy 2024 (Al-Cu-Mg) was investigated. Multiple-pass experiments were carried out using two different processing strategies, multi-pass FSP, and multi-track FSP. Effect of a post-FSP heat treatment above and below the solutionizing temperature of the alloy was also studied. FSP experiments were carried out using an optimal set of parameters. Characterization tools used in the study include scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), electron probe micro-analyser (EPMA), and X-ray diffraction (XRD). Microstructural features indicate the occurrence of particle stimulated nucleation (PSN) assisted dynamic recrystallization (DRX) as the dominant microstructural evolution mechanism in the nugget zone. Geometrical coalescence occurred, leading to the formation of some larger grains in the nugget zone. Heterogenous micro-texture distribution was observed in the nugget zone with the bulk textures consisting of FCC shear texture components dominated by A ₁*/A ₂* and C. Microstructure and texture in the nugget zone remained stable after both routes of multiple-pass processing, demonstrating the possibility of FSP to produce bulk volume of fine-grained materials. Post-FSP heat treatment indicated the stability of microstructure and texture up to 723 K (450 °C) owing to relatively lower strain energies retained after FSP.

     

Microstructure Evolution during Friction Stir Welding of Mill-Annealed Ti-6Al-4V

In this study, mill-annealed Ti-6Al-4V plates were successfully friction stir welded over a wide range of processing parameters using a tungsten-1 pct La₂O₃ tool. Two K-type thermocouples embedded in the tool indicated that approximately 25 pct of the heat generated during welding was transferred out of the workpiece and into the tool. The thermocouple data, combined with observations of the microstructure, indicated that the stir zone of all welds exceeded the β transus. The microstructure and texture of two representative welds made just above and high above the β transus were investigated with scanning electron microscopy and electron backscatter diffraction (EBSD). The β phase orientations were reconstructed with a fully automated technique from the as-collected α phase data through knowledge of the Burgers orientation relationship. The results suggest that the fine β grains in the stir zone are formed from the base material ahead of the advancing tool by dissolution of secondary and primary α phase, and there is no further recrystallization. These grains subsequently deform by slip and rotate toward the orientations that are most stable with respect to the shear deformation induced by the tool. In the highest temperature weld, diffusion tool wear in the form of periodically spaced bands provided an internal marker of the tool/workpiece interface during welding. The flow patterns evident within the tungsten-enriched bands suggest that flow is considerably more chaotic on the advancing side than in the central stir zone.     

Effect of Backing Plate Thermal Property on Friction Stir Welding of 25-mm-Thick AA6061

By using backing plates made out of materials with widely varying thermal diffusivity this work seeks to elucidate the effects of the root side thermal boundary condition on weld process variables and resulting joint properties. Welds were made in 25.4-mm-thick AA6061 using ceramic, titanium, steel, and aluminum as backing plate (BP) material. Welds were also made using a “composite backing plate” consisting of longitudinal narrow strip of low diffusivity material at the center and two side plates of high diffusivity aluminum. Stir zone temperature during the welding was measured using two thermocouples spot welded at the core of the probe: one at the midplane height and another near the tip of the probe corresponding to the root of the weld. Steady state midplane probe temperatures for all the BPs used were found to be very similar. Near root peak temperature, however, varied significantly among weld made with different BPs all other things being equal. Whereas the near root and midplane temperature were the same in the case of ceramic backing plate, the root peak temperature was 318 K (45 °C) less than the midplane temperature in the case of aluminum BP. The trends of nugget hardness and grain size in through thickness direction were in agreement with the measured probe temperatures. Hardness and tensile test results show that the use of composite BP results in stronger joint compared to monolithic steel BP.     

A microtexture investigation of recrystallization during friction stir processing of as-cast NiAl bronze

As-cast NiAl bronze (NAB) was subjected to friction stir processing (FSP). Orientation imaging microscopy (OIM) methods were used to obtain microtexture data in the stir zone (SZ) and along its periphery. At selected SZ locations, orientation data were obtained by convergent beam electron diffraction (CBED) methods in transmission electron microscopy (TEM). Random α phase textures were apparent in the SZ. The α grains tended to be equiaxed, exhibited annealing twins, and were refined to 1 to 2 μm at the edge of the SZ. The population of subgrain boundaries in α phase grains was highest near the plate surface in contact with the tool and decreased with depth in the SZ, reflecting deformation by the tool shoulder after the passage of the tool pin. Distinct shear texture components were apparent in the thermomechanically affected zone (TMAZ) outside of and along the periphery of the SZ. A texture gradient from the TMAZ into the SZ was apparent and was steeper on the advancing side and under the SZ center than on the retreating side. The apparent shear plane tended to align with the local interface between the SZ and TMAZ, while the shear direction tended to align with the FSP traversing direction. In this material, the SZ-TMAZ interface is a distinct boundary between recrystallized and deformed regions and the α-phase grain refinement reflects dynamic recrystallization and, in locations near the SZ-TMAZ interface, particle-stimulated nucleation (PSN) at undissolved Fe₃Al particles.     

Modelling Corrosion Behavior of Friction Stir Processed Aluminium Alloy 5083 Using Polynomial: Radial Basis Function

Aluminium alloy 5083, widely used in marine applications, undergoes accelerated corrosion in sea water due to the aggressive reaction of chloride ions with the secondary phase particles and other intermetallics present in the alloy matrix. The corrosion rate of the alloy is also influenced by the temperature difference between the alloy and its environment. Friction stir processing (FSP) is a recent solid state processing technique for improving the surface properties of metals and alloys. In this study, an attempt has been made to explore the possibility of improving the corrosion resistance of AA5083 by FSP. FSP trials were performed by varying the tool rotation speed, tool traverse speed and shoulder diameter of the tool, as per face centered central composite design. The corrosion potential and the corrosion rate of friction stir processed AA5083 was studied using potentiodynamic polarization studies, at three different temperatures. Mathematical models based on polynomial—radial basis function were developed and used to study the effect of process parameters on the corrosion potential and the corrosion rate of friction stir processed AA5083. FSP resulted in refinement of the grain structure, dispersion and partial dissolution of secondary phase particles in the matrix, which increased the corrosion resistance of the alloy.     

Microstructure and Fatigue Properties of a Friction Stir Lap Welded Magnesium Alloy

Friction stir welding (FSW), being an enabling solid-state joining technology, can be suitably applied for the assembly of lightweight magnesium (Mg) alloys. In this investigation, friction stir lap welded (FSLWed) joints of AZ31B-H24 Mg alloy were characterized in terms of the welding defects, microstructure, hardness, and fatigue properties at various combinations of tool rotational rates and welding speeds. It was observed that the hardness decreased from the base metal (BM) to the stir zone (SZ) across the heat-affected zone (HAZ) and thermomechanically affected zone (TMAZ). The lowest value of hardness appeared in the SZ. With increasing tool rotational rate or decreasing welding speed, the average hardness in the SZ decreased owing to increasing grain size, and a Hall–Petch-type relationship was established. Fatigue fracture of the lap welds always occurred at the interface between the SZ and TMAZ on the advancing side where a larger hooking defect was present (in comparison with the retreating side). The welding parameters had a significant influence on the hook height and the subsequent fatigue life. A relatively “cold” weld, conducted at a rotational rate of 1000 rpm and welding speed of 20 mm/s, gave rise to almost complete elimination of the hooking defect, thus considerably (over two orders of magnitude) improving the fatigue life. Fatigue crack propagation was basically characterized by the formation of fatigue striations concomitantly with secondary cracks.     

Microstructure and Texture Evolution during Friction Stir Processing of Fully Lamellar Ti-6Al-4V

Friction stir processing (FSP) was used to locally refine a thin surface layer of the coarse, fully lamellar microstructure of investment-cast Ti-6Al-4V. Depending on the peak temperature reached in the stir zone during processing relative to the β transus, three distinct classes of microstructures were observed. Using accepted wrought product terminology, they are equiaxed, bimodal, and lamellar, except for the case of FSP, the length scale of each was smaller by at least an order of magnitude compared to typical wrought material. The evolution of an initially strain-free fully lamellar microstructure to each of these three refined conditions was characterized with scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The fundamental mechanisms underlying grain refinement during FSP, including both the morphological changes and the formation of high-angle grain boundaries, were discussed.     

Physical Simulation of Deformation and Microstructure Evolution During Friction Stir Processing of Ti-6Al-4V Alloy

The feasibility of using high-strain rate (1.475 to 3.942 s^?1) hot-torsion testing with a Gleeble^® thermomechanical simulator was demonstrated for simulating microstructures consistent with friction stir processing (FSP) of Ti-6Al-4V. The tests were performed on α/β-processed base material at temperatures both above and below the β-transus. Various phenomena including the refinement of α- and β-grains, deformation-induced heating, and deformation instabilities were observed. These tests reproduced the range of microstructures that are observed under FSP processing conditions. The testing methodology can be used for generating constitutive material property equations relevant to computational FSP/friction stir welding models.