Ultrafine Grains Research Articles

Microstructure evolution of ATI718 plus alloy during high-speed machining: Experiments and a combined FE-CA approach

Excellent surface integrity is an eternal pursuit in high performance manufacturing, with microstructure being a crucial component of the surface integrity dataset and a key factor controlling surface properties such as fatigue and creep. The multi-physical fields generated by thermo-mechanical loads during high-speed machining act on the processed surface layer, influencing the evolution of microstructures. To investigate the microstructural evolution mechanisms of ATI718 plus during high-speed machining, cutting experiments and techniques such as Electron back scatter diffraction (EBSD), Transmission Kikuchi diffraction (TKD), and Precession electron diffraction (PED) is conducted to quantitatively analyze the microstructures in the chip shear zone and the machined surface. Subsequently, a combined finite element (FE) and cellular automata (CA) model is developed to simulate the microstructure evolution during the cutting process. The discontinuous dynamic recrystallization (DDRX) mechanism is employed to demonstrate the nucleation and growth of grains under the influence of multiple physical fields. The simulation and experimental results show similar dynamic recrystallization (DRX) grain sizes, indicating acceptable accuracy of the CA model in terms of DRX grain size. The comparison between experimental and simulation results confirms the occurrence of both continuous dynamic recrystallization (CDRX) and DDRX during the cutting process. The synergistic competition between CDRX induced grain lamellar refinement and DDRX induced grain growth emerge as the primary mechanism driving microstructural evolution. A layer of ultrafine grains, with a thickness within 20 μm, is formed on the machined surface. Results under different parameters demonstrate that the temperature has a more significant impact on the thickness of the ultrafine grain layer and the diameter of grains within the layer compared to the strain rate.

Friction welding of UFG copper using the W2Mi prototype machine

When welding ultra-fine-grained metals using conventional methods, the microstructure in the joint area is degraded (mainly due to recrystallization) in the heat-affected zone and a significant deterioration of mechanical properties, including joint strength. The aim of the research presented in this article was to identify the possibility of obtaining joints with strength close to initial material using friction welding of metal materials with ultra-fine grain. For this purpose, UFG (ultra-fine-grained) material was produced from technically pure M1Ez4 copper using a hybrid SPD (severe plastic deformation) process. The welding process was carried out on a machine with a prototype design that allows minimizing the welding time, while generating high force. The process parameters used on the prototype machine resulted in an increase in the hardness of the material by 4% in the joint area. The strength of the joint compared to the base material decreased slightly by 2%. The tests carried out proved that, using appropriate process parameters, it is possible to obtain a UFG metal joint without a decrease in its mechanical strength.

The configuration design of cross-linked CNT networks to realize heterostructure in Cu matrix composite towards prominent mechanical-electrical property synergy

Developing reticular reinforcement and heterogeneous microstructure remains persistent challenges in one-dimensional carbon nanotube (CNT)-Cu system. We herein fabricated dual-heterostructured Cu composite by diazotizing CNT into cross-linked (CL) networks and achieving bimodal grain distribution. The CL-CNT networks uniformly dispersed in Cu matrix, forming strong C–N/O–Cu covalent bonds at the interfaces and effectively retarding grain growth to create ultra-fine grain (UFG) regions around them. The CL-CNT networks/UFG regions, along with the surrounding coarse grain (CG) regions, resulted in a remarkable synergy between mechanical and electrical properties within the composite. Systematic analysis indicates that CL-CNT network acts as a whole to bear loading, remarkably enhances the load-transfer strengthening and promotes the dislocation accumulation around the interface. Meanwhile, hetero-deformation between “hard” CL-CNT networks/UFG regions and “soft” CG regions lead to extra-strengthening and hardening associated with strain partitioning, generation of geometrically necessary dislocation (GND) and strain delocalization. In-situ tensile test further reveals that CL-CNT/Cu with dual-heterostructure introduces the extra-toughening via crack bridging and deflection mechanisms. Besides, CL-CNT network with the robust interface is considered to reduce the interfacial inelastic scattering and provide additional paths for electron transport, accounting for the high electrical conductivity. This work highlights the importance of configuration design of reinforcement in tailoring heterostructured metal matrix composites (MMCs) with outstanding comprehensive performance.

Relationship of evolution of slip-surface structure in Indian sandstone to changes in friction coefficient for a wide range of slip rates

Although many experimental studies have investigated the frictional properties of sandstone, few of them have considered the evolution of slip-surface structures with changes of friction coefficient over a wide range of slip rates. Here we report the results of rock-on-rock rotary shear experiments on specimens of Indian sandstone at slip rates from 2 × 10−4 to 1 m s−1. The resultant slip behaviors and microscale surface structures indicate two types of dependency on slip rate. At high slip rates (≥5 × 10−1 m s−1), the friction between the sandstone blocks fluctuated markedly and the surfaces of the blocks were intensively worn. At low slip rates (≤1 × 10−1 m s−1), the friction decreased gradually with increasing slip distance and the slip surfaces became reflective. The high slip-rate experiments produced abundant rock fragments (∼10–100 μm diameter), whereas the slip surfaces after low slip-rate experiments were highly polished fault mirrors accompanied by abundant rounded ultrafine grains (∼100 nm diameter). Such slip-rate-dependent evolution of slip-surface structures in Indian sandstone may arise from the relatively low cohesiveness of that rock. This insight may further our understanding of faulting and sliding mechanisms in sandstone near the ground surface.

Good strength-ductility synergy achieved in a Fe62Ni16Co9Mn9Ti3Si1 Fe-high-entropy alloy with heterogeneous structures

Developing metal structural materials with a good combination of strength-ductility is of great significance for the rapid development of the aerospace industry and the energy industry. High-entropy alloys (HEAs) have attracted widespread attention due to their excellent mechanical properties, especially the excellent ductility and toughness of face-centered cubic (FCC) high-entropy alloys. However, similar to traditional alloys, there is a trade-off between strength and ductility in HEAs. In this paper, we report the successful preparation of Fe-HEA (Fe62Ni16Co9Mn9Ti3Si1 (atomic percentage, %)) with high strength-ductility synergy at room temperature by cryo-rolling and annealing at 600 °C for 10 min (CR600), which increases the yield strength by 59% compared to room temperature rolling (RR600), while the uniform elongation decreases by only 20%. The ultra-high yield strength and the not significant reduction in uniform elongation are mainly attributed to the formation of high-density dislocations, dual-phase bimodal ultra-fine grain heterostructures, (NiMn)3-xTix and (Fe, Ni)2SiTi composite nanoprecipitates, precipitation-induced chemical composition heterogeneity, and BCC→HCP phase transformation during tensile deformation during CR and subsequent annealing at a reduced temperature (600 °C). In addition, by increasing the annealing temperature of the alloy after CR to 800 °C, its uniform elongation can be increased to 53%, which is mainly due to the excellent strain hardening ability induced by changes in heterogeneous structures, nanoprecipitates, and martensitic phase transformation. Compared with other similar alloys, our CR alloy exhibits higher strength-ductility synergy. The results of this study provide a new perspective for developing HEAs with higher strength-ductility synergy.

Comparison of conventional and severe shot peening effects on the microstructure, texture, roughness, hardness, and electrochemical behavior of austenitic stainless steel

In the present research, microstructure, texture, roughness, hardness, and electrochemical behavior of AISI 316L austenitic stainless steel before and after shot peening were studied to elucidate the effect of conventional and severe shot peening (CSP and SSP) processes. After the shot peening, the fraction of strain-induced martensite (SIM) and mechanical twins (MTs) in the sub-surface layer was increased. The fraction of SIM and MTs in the SSP sample was higher than in the CSP sample. The XRD patterns indicated that the SSP sample had a higher peak broadening compared to the CSP sample. In the CSP and SSP samples, a gradient microstructure was formed along the depth direction. The microstructure of the topmost layer of the CSP and SSP samples exhibited numerous ultrafine grains. The grain refining during severe shot peening was faster because of the accumulation of more strain. The CSP and SSP samples revealed a gradient distribution of elements. After the SSP, the intensity of ⟨110⟩‖ED fiber texture decreased from 12.7 to 11.6 × R and the average intensity of ⟨100⟩‖ED fiber texture increased from 1.7 to 2.0 × R, respectively, compared to the CSP sample. The surface roughness of the SSP sample (Rq = 73.6 nm and Ra = 45.2 nm) was lower than that of the CSP sample which represented the roughness decreased with surface coverage increasing from 100 % to 1500 %. Also, the wettability increased after the conventional and severe shot peening processes. In addition, the microhardness of the CSP and SSP samples showed a gradient distribution. The CSP sample had the lowest corrosion current density (0.13 μA/cm2) whereas the NP (non-peened) sample exhibited the highest current density (0.65 μA/cm2). The presence of ⟨100⟩-oriented grains in both CSP and SSP samples led to the higher corrosion resistance of shot-peened steels compared to the NP sample. The presence of favorable texture with higher intensity in the CSP sample was responsible for the higher corrosion resistance of the CSP sample compared to the SSP sample. Finally, the gradient distribution of elements along the depth direction in the CSP and SSP steels improved the corrosion resistance of the surface.

Microstructure and constitutive modeling of an ultrafine-grained refractory high-entropy alloy fabricated by powder metallurgy

In the present study, a novel AlCr0.3FeMoNbTiV2 alloy was prepared by means of mechanical alloying and subsequent spark plasma sintering, resulting in an ultrafine-grained (UFG) microstructure consisting of a bcc matrix phase with an average grain size of 0.32 μm accompanied by Ti carbide, Laves phases, and minor contents of Al2O3. Excellent thermal stability was exhibited by the alloy, since no considerable phase transformations were observed after a heat treatment at 1350 °C for 16 h, beyond microstructural coarsening, as can be appreciated from the increase of the average bcc grain size up to 1.50 μm. Compression tests were performed at elevated temperatures (950–1100 °C) at strain rates ranging 0.0005–0.01 s−1. According to the constitutive analysis of the peak stress using the hyperbolic sine law, an experimental creep exponent of 2.17 was obtained, which indicates that deformation is mainly controlled by grain boundary sliding, as typically observed in UFG materials, with a large apparent activation energy of 527.7 kJ mol−1, superior to that of other RHEAs reported in literature.

The Effect of Ultrafine-Grained (UFG) Structure Formed by Equal-Channel Angular Pressing in AA7075 on Wear and Friction in Sliding against Steel and Ceramic Counterbodies

The Effect of Ultrafine-Grained (UFG) Structure Formed by Equal-Channel Angular Pressing in AA7075 on Wear and Friction in Sliding against Steel and Ceramic Counterbodies

Digital Image Correlation Analysis of Uniform Deformation and Necking in Solid‐State Welded Nanocrystalline Aluminum via High‐Pressure Torsion

Solid‐state welding of Al 1043 sheets was achieved via high‐pressure torsion (HPT) processing to produce bulk nanostructured Al disks. A homogeneous nanostructure without segregation was observed, with grain sizes of ∽430‐470 nm. Miniature tensile testing, coupled with the digital image correlation (DIC) technique, was employed to determine the room‐temperature tensile deformation behavior, particularly the non‐uniform behavior with necking, of the HPT‐bonded ultrafine‐grained (UFG) aluminum, comparing it with annealed coarse‐grained counterpart. The HPT‐bonded UFG Al exhibited a large fraction of post‐necking strain, which is supported by the estimated high strain rate sensitivity value of m = 0.085, suggesting the delay of local necking leading to tensile fracture. Detailed DIC analysis revealed prolonged diffuse necking, thus delaying local necking, in the HPT‐bonded UFG Al, while the annealed samples show high fractions of local necking during the non‐uniform deformation. Moreover, the DIC data illustrated that local necking predominantly occurred at a limited neck zone, maintaining a plateau strain distribution at the out‐of‐neck zone throughout necking deformation towards tensile failure for both annealed and UFG aluminum. The DIC method offers an alternative means to demonstrate the transition in necking behaviors of materials by estimating the plastic lateral contraction exponent.This article is protected by copyright. All rights reserved.

Crystal Plasticity Finite Element Modeling of the Influences of Ultrafine-Grained Austenite on the Mechanical Response of a Medium-Mn Steel

Medium manganese (medium-Mn) steel, one of the third-generation advanced high-strength steels (AHSS), delivers impressive mechanical properties such as high yield strength, ultimate tensile strength, and uniform elongation. One notable feature of medium-Mn steels is the presence of ultrafine-grained (UFG) austenite, achieved through phase transformation from the parent martensite phase during intercritical annealing. While, in general, UFG is considered a strengthening mechanism, the impact of UFG austenites in medium-Mn steel has not been fully studied. In this manuscript, we advance our previous work on crystal plasticity simulation based on the Taylor model to consider fully resolved high-fidelity microstructures and systematically study the influence of the UFG austenites. The original microstructure with UFG is reconstructed from a set of serial electron backscatter diffraction (EBSD) scans, where the exact grain morphology, orientation, and phase composition are preserved. This microstructure was further analyzed to identify the UFG austenites and recover them to their parent martensite before the intercritical annealing. These two high-fidelity microstructures are used for a comparative study using dislocation density-based crystal plasticity finite modeling to understand the impact of UFG austenites on both the local and overall mechanical responses.

Achieving high strength-ductility synergy in TiBw/near α-Ti composites by ultrafine grains and nanosilicides via low-temperature severe plastic deformation

The common problem of low strength-ductility matching prevails in near-α high-temperature titanium matrix composites (TMCs). In this work, the design strategy of ultrafine grains and dispersed (Ti, Zr)6Si3 nanoprecipitates in the microstructure of TiBw/near α-Ti composites via low-temperature isothermal multidirectional forging (IMDF), is expected to break the trade-off dilemma between strength and ductility. The results show that with the decrease in the temperature of IMDF, the grain scale decreased from 0.98 to 0.59 μm, and the location of silicide precipitation shifted from phase boundaries and grain boundaries to α-grain boundaries and intracrystalline regions. The experiments confirm that the local segregation of Si and the temperature of thermomechanical deformation are the key factors affecting the precipitation behavior of silicides. With the decrease of the deformation temperature, the precipitation mechanism of silicides changes from a single diffusion-controlled precipitation to the coupling of two mechanisms, namely, elemental diffusion and dislocation-assisted nucleation, which facilitates the successive precipitation of nanometer-sized silicides at the grain boundaries and in the inner regions. The ultimate tensile strength (UTS) and elongation of the composites were substantially increased after IMDF at 950 and 800 °C, especially the excellent performance at 800 °C, where the strength reached 1320.3 MPa and the elongation was 5.8 %. The room and high-temperature strengthening and failure mechanisms of the composites are analyzed and discussed, and the yield strength (YS) increments provided by various strengthening mechanisms at room temperature are quantified, aiming to provide a potential preparation strategy for the synergistic strengthening of near-α TMCs with ultrafine grains and nanoparticles.

Achieving high strength and uniform ductility in high-entropy alloys via dynamic-precipitation accelerated transformation-induced plasticity

This study presents a novel approach to fabricating advanced transformation-induced plasticity (TRIP) high-entropy alloys (HEAs) that exhibit an exceptional combination of high strength and uniform ductility, far surpassing the mechanical performance of previously developed TRIP HEAs and non-TRIP multiphase HEAs. The ultrafine-grained Al5Cr20Fe35Co35Ni5 TRIP HEA sample exhibits a yield strength of gigapascal-level, ranging from 1100 to 1260 MPa, with large uniform elongations that range from 29 % to 39 %. Simultaneously, the alloy displays outstanding toughness, reaching 46,000±5390 MPa·%. The achievement of this significant breakthrough rests upon two key factors: (1) achieving an ultrafine-grained FCC matrix in a fully recrystallized microstructure with the aid of thermally induced B2 phase, and (2) dynamic precipitation of nanometer-sized B2 phase particles on high-density stacking faults and slip bands created within the interiors of ultrafine grains. The dynamic precipitation of the B2 phase, facilitated by the formation of an amorphous phase that serves as a precursor to the B2 crystalline phase, is a rare occurrence in HEAs at room temperature. This dynamic precipitation process greatly enhances TRIP-assisted strain hardening (from FCC to HCP phase) of the current alloy at ultrafine grains, enabling the overcoming of the inherent tensile ductility constraints typically associated with ultrafine-grained metals. The current development of TRIP HEAs with an outstanding combination of high strength and high toughness is believed to mark a remarkable milestone in the application field of TRIP HEAs.

Surface nanocrystallization mechanism of additive manufactured 316L stainless steel via high-energy shot peening

High-energy shot peening (HESP) is an effective way to achieve surface nanocrystallization modification of additive manufactured metal parts, significantly improving their mechanical properties. Yet, there is still a lack of deep understanding of the surface nanocrystallization evolution mechanism of additive manufactured metals. For that, this study focuses on the surface nanocrystallization mechanism of selective laser melted 316L stainless steel treated by HESP treatment. The detailed microstructural characteristics are investigated via optical microscope (OM) and transmission electron microscopy (TEM). The experimental results show that the matrix layer, the low-strain matrix layer, the elongated ultrafine grain layer, the short rod-like ultrafine grain layer, and the equiaxed nanograin layer occurred in sequence from the matrix to the topmost surface. HESP treatment enables the gradient grain refinement from the matrix with a micrometer structure of about 120 mm to the topmost surface with an average grain size of about 25 nm. The evolution of surface nanocrystallization has resulted from the interaction of high-density dislocation, dislocation tangles (DTS), and twins. This study could provide a deep understanding of the surface nanocrystallization evolution mechanism of additive manufactured metal parts treated by HESP.

Superior strength-ductility synergy in a near-α titanium alloy fabricated by severe plastic deformation at lower temperatures

This study demonstrates the crucial trade-off between strength and ductility in titanium alloys. High strength-ductility synergy is achieved through severe plastic deformation (SPD) at a relatively lower temperature, resulting in ultrafine grains and well-dispersed (Ti, Zr)6Si3 nano-precipitates. The samples SPD-processed at 800 °C demonstrate excellent room-temperature properties with a tensile strength of 1558.9 MPa and 7.2 % ductility. The findings highlight the impact of SPD temperature and alloying elements on silicide precipitation, revealing two different precipitation mechanisms depending on the SPD processing temperature: single diffusion-controlled nucleation at a higher SPD temperature while a coupled diffusion and dislocation-assisted nucleation at a lower SPD temperature. Lower-temperature SPD is critical to achieving superior strength-ductility synergy.

Enhancing Recovery of Ultra-Fine Magnetite from Low-Iron-Grade Cyanidation Tailings by Optimizing Flow Field Parameters of Low-Intensity Magnetic Separation (LIMS)

The characteristics of iron minerals in cyanidation tailings with a low iron grade were determined via chemical composition analysis, iron phase analysis, and mineral liberation analysis (MLA). The results showed that the cyanidation tailings contained 15.68% iron, mainly occurring in the form of magnetite (19.66%) and limonite (79.91%), in which 16.52% magnetite and 65.90% limonite particles were fully liberated. Most ultra-fine magnetite grains were adjacent and wrapped with limonite to form complex intergrowths, which resulted in low-efficiency magnetite recovery in low-intensity magnetic separation (LIMS) and adversely affected the downstream high-gradient magnetic separation (HGMS) process. Thus, in this work, the optimization of the flow field was proposed to enhance the separation of ultra-fine magnetite from the cyanidation tailings using pilot-scale LIMS separation, and the controllable parameters (including feed flow, separation gap, drum rotating speed, and solid weight) affecting ultra-fine magnetite capture were investigated. Under optimized conditions, a high-grade magnetite concentrate assaying 63.31% Fe with 86.46% magnetite recovery was produced, which, respectively, increased by 0.76% and 15.22%, compared with those obtained from industrial production. In addition, from the flow dynamics simulation, it was found that the magnetite particles in the −6 µm ultra-fine fraction were lost much more easily than those of coarser fractions due to the relatively enhanced hydrodynamic drag force acting on the particles compared with the magnetic force. However, this loss would be effectively reduced with the regulation and control of the flow field. The iron recoveries in the −16~+6 µm and −6 µm fractions of magnetite concentrate increased by 3.66% and 4.42%, respectively, under optimized hydrodynamic conditions. This research outcome provides a valuable reference for the economic and effective utilization of iron resources from such solid wastes.

Effect of Cold Isostatic Pressing and VC Grain Growth Inhibitor Addition on WC Grain Size and Mechanical Properties of WC-8Co Cemented Carbide

In this study, sub-micron WC-Co cemented carbides were fabricated by cold isostatic pressing using recycled WC powder, Co powder, and VC grain growth inhibitor as the initial composite powders. The raw WC-8Co and WC-8Co-0.3VC compositions were prepared according to conventional technology. After that, part of the green compacts was CIP-ed under pressure of 200 MPa. Sintering was carried out in an industrial vacuum sintering furnace at a temperature of 1420 ºC simultaneously for all samples. The microstructures and properties, including density, coercivity, transverse rupture strength, A-type Rockwell hardness and fracture toughness were investigated, characterized and compared systematically. The experimental results showed that the action of cold isostatic pressing in combination with grain growth inhibitor addition promoted ultrafine grains with the finest WC grain size of 0.68 μm and an increase in the fracture toughness up to 15.4 MPa·m1/2 ± 0.2 compared to rest cemented carbides, showing the techniquehas potential to fabricate cermets with both small average WC grain size and high fracture toughness, which is key in providing excellent performance.

Ultrahigh-strengthened intragranular κ-Al2O3 nanoparticles dispersed Mo composite prepared via SPS sintering of core-shell Mo nanocomposite powder

Oxide dispersion strengthening (ODS) is an effective method to improve the mechanical properties of Mo-based materials. However, the mechanical properties of traditional ODS-Mo composites are always limited by the coarsening and intergranular distribution of second-phase particles. In this work, an effective nano-reinforcement dispersion strategy was developed to fabricate an ODS-Mo composite with ultrafine grain and intragranular distribution of second-phase particles. Core-shell structural Mo nanocomposite powders, with internally distributed sub-10 nm Al2O3 dispersoids, were prepared by nano atomization doping (AD) followed by a chemical vapor transport growth strategy. Then, ODS-Mo composites with ultra-fine Mo grain (below 700 nm) and high-density intragranular κ-Al2O3 (below 20 nm) nanoparticles were prepared via spark plasma sintering (SPS), in which a coherent interface between κ-Al2O3 and Mo matrix was formed. The composites present remarkably improved hardness (above 500 HV), bend strength, and compressive yield strength (above 1664 MPa) at room temperature, with a suitable strain to fracture of 27.1 %. The calculation of strengthening mechanisms indicates that the enhancement was mainly attributed to the intragranular κ-Al2O3 nanoparticles. This nano-sized reinforcement distributed within the grain can more effectively pin dislocations and achieve dispersion strengthening in ODS-Mo composites. Therefore, this strategy can efficiently construct intragranular second-phase nanoparticles and open up new avenues to fabricate high-performance ODS-Mo composites.

Extending the mechanical property regime of laser powder bed fusion Sc- and Zr-modified Al6061 alloy by manipulating process parameters and heat treatment

Highly dense compositionally modified Al6061 alloys with Sc and Zr (Al6061MOD) were fabricated using the laser powder bed fusion. The as-built AL6061MOD alloy exhibits a bimodal grain size distribution consisting of coarse columnar grains and ultrafine grains, which is tunable by either varying the printing parameters or post-heat treatment. A high laser energy density along with a lower cooling rate, leaving more time for nucleating primary Al3Sc particles, triggers a higher area fraction of ultrafine grains. The heat treatment-induced evolution of microstructure and mechanical properties is jointly influenced by the size of grains and (Al,Si)3(Sc,Zr) particles. The conventional T6 heat treatment triggers coarsening of grains (∼5.3 μm) and (Al,Si)3(Sc,Zr) particles (∼42 nm), lowering the yield strength (∼280 MPa) compared with that of as-built counterparts. On the contrary, direct aging enhances the yield strength (∼383 MPa) due to the precipitation of nano-sized secondary Al3(Sc,Zr) particles (∼5 nm) as well as retaining the grain size (∼690 nm) of ultrafine grains. Concurrent with the tunable strength, the plasticity behaviors (Lüders band and Portevin-Le Chatelier effect) are manipulated correspondingly with varying microstructure. The broad tailoring of microstructure and mechanical property regime can shed light on designing Al alloys targeting the desired applications by manipulating the printing parameters and heat treatment.

Cr-assisted low-temperature densification of (Ti,Zr,Nb,Ta,Mo)C high-entropy carbides with ultrafine grain and enhanced hardness

Cr-assisted low-temperature densification of (Ti,Zr,Nb,Ta,Mo)C high-entropy carbides with ultrafine grain and enhanced hardness

Underwater friction stir welding of ultrafine-grained laminated composites produced through accumulative roll bonding process

In this work, ultrafine-grained (UFG) plates of AA2024/AA5083 laminated composite with alternating layers of heat-treatable aluminum AA2024 and strain-hardening aluminum AA5083 were fabricated through accumulative roll bonding (ARB) technique and were subsequently joined using underwater friction stir welding (UFSW). Friction stir welding (FSW) is a solid-state welding method which is suitable for the joining of similar and dissimilar UFG alloys of AA2024 and AA5083, which are mainly used in aerospace and aviation industries. Welding variables, such as pin profile, rotational speed, welding speed, and tilt angle were optimized using the Taguchi optimization technique. Different zones of UFSWed joints including stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat affected zone (HAZ) were investigated through OM, TEM, and SEM. The results showed that the tapered cylindrical pin, 1000 rpm rotational speed, 110 mm.min−1 welding speed, and 2.5° tilt angle were the optimal welding variables in terms of tensile strength. The microstructural results showed the formation of nano-size, recrystallized equiaxed grains with a size of 500 nm (the same size as that in the UFG base metal) in the optimal sample. The hardness in the weldment of the optimal UFSW sample decreased slightly, so that the efficiency of 85% was presented and the minimum hardness location (MHL) disappeared. The ultimate tensile strength (UTS) of 425 MPa and 64% efficiency were observed in the optimal conditions. Due to the presence of very fine grains and large dislocations, and the uniform distribution of precipitates in the underwater state, high strength and elongation were obtained.