Search Results (294)

Taking the repair of prefabricated hole defects in 2024 aluminum alloy thin-walled components
by friction stir spot welding (FSSW) as the research object, the evolution laws
of microstructure and mechanical properties of FSSW-repaired joints of thin-walled components
were clarified through process experiments and numerical simulations. The
collaborative effect of the temperature field and material flow field during the FSSW repair
process and their regulation laws on the microstructure and properties were revealed. The
results show that as the repair speed increases, the macroscopic surface quality of the FSSW
joint improves. When the repair speed reaches 2000 r/min, a high-quality repaired joint
with a smooth and flat surface and no porosity defects can be obtained. Meanwhile, within
the repair speed range of 800 to 2000 r/min, the grains undergo dynamic recrystallization
(DRX) due to the combined effect of heat and mechanical forces, eventually forming a
uniform equiaxed grain structure in the weld core area. ABAQUS 2023 simulation verifies
the temperature distribution during the FSSW repair process. When the repair speed is
2000 r/min, the maximum temperature obtained from the simulation is 431.1 ◦C, which
agrees with the measured value from the experiment. The simulation results further reveal
that when the repair speed increases from 1200 r/min to 2000 r/min, the material fluidity
significantly enhances, and the flow velocity on the advancing side is always higher than
that in other areas. At the rotational speed of 2000 r/min, the plastic material flows continuously
from the periphery and eventually fills the defect area completely. The fracture
mode of the FSSW-repaired joint is mainly ductile fracture. With the increase in the repair
speed, the number of dimples at the fracture surface increases significantly. When the
rotational speed reaches 2000 r/min, the joint achieves the best mechanical properties, and
the FSSW-repaired joint reaches the maximum tensile strength of 169 MPa. Full article

During hot deformation of GH4706 alloy forgings, to achieve effective control over the uniformity of its microstructure, it is first necessary to establish a quantitative relationship between the microstructural characteristics of an entire hot-formed turbine disk forging and process parameters using reliable methods. Based on hot compression test results of GH4706 alloy at temperatures ranging from 950 °C to 1150 °C and strain rates from 0.001 s⁻¹ to 1 s⁻¹, this study developed a microstructure evolution model. Multi-scale high-precision numerical simulations were performed to predict the parameter field distribution and microstructure distribution of the turbine disk. The results reveal that the inhomogeneity of strain distribution is the primary cause of mixed grain formation. Statistical comparisons between simulation predictions at six validation points and industrial experimental data revealed the following relative deviations: 5.21% for average grain size (AVG), 9.65% for the standard deviation of grain size distribution (SD), and 5.31% for DRX fraction. Additionally, the standard deviations of prediction error for these three parameters are 3.26% for AVG, 4.06% for SD, and 3.12% for DRX fraction. These results demonstrate that the multi-scale dynamic recrystallization model developed in this study exhibits satisfactory prediction accuracy and stability. The modeling approach presented in this paper is of great significance for precisely controlling the uniformity of microstructural distribution during the hot deformation of GH4706 alloy. Full article

Thermomechanical processing (TMP) is of critical importance for tailoring microstructures and properties of high-strength titanium alloys and their composites. In this study, mechanical response and microstructural evolution mechanisms of 2 vol.% TiB/Ti-5Al-5Mo-5V-3Cr-1Zr (Ti-55531) matrix composites during isothermal compression at varied deformation temperatures (785–925 °C) and strain rates (0.001–1 s⁻¹) are comprehensively investigated by kinetic calculation and microstructural characterization. Strain-compensated constitutive equations in α + β and β phase regions were established. Results show that deformation temperature and strain rate influence flow behavior and microstructure through dynamic recovery (DRV) and dynamic recrystallization (DRX) of the β phase as well as dynamic spheroidization of the α phase. Crucially, three DRX mechanisms of β phase were identified, wherein TiB-induced β-DRX dominates, with α-assisted β-DRX and continuous dynamic recrystallization (CDRX) as secondary mechanisms. Dynamic spheroidization mechanisms of the α phase, including β-wedge penetration as well as α interaction and kinking, were elucidated. A comprehensive microstructural evolution mechanism map was constructed, and an optimized hot-processing window was proposed. Notably, the introduction of TiB significantly promotes β-DRX, which tends to randomize crystallographic orientations of the β phase, and enhances microstructural stability. This study provides theoretical complement and practical guidance for hot processing and microstructure control of metastable β titanium matrix composites. Full article

To reveal the high-temperature hot deformation mechanism and optimize the thermal processing window of Zr-Nb alloy, hot compression tests were implemented via a thermal simulation apparatus over temperatures ranging from 500 to 900 °C and strain rates of 0.01~10 s⁻¹. The material’s flow performance, dynamic recrystallization (DRX) and phase transformation were systematically explored in this work. The results indicate that flow stress presents a negative correlation with deformation temperature and a positive correlation with strain rate. On the basis of measured stress–strain curves, the Arrhenius constitutive equation and processing map were established to quantitatively describe the alloy’s hot deformation behavior. Flow instability areas are mainly distributed in the low-temperature domain and the high strain rate region of medium-high temperature zones. The evolution of DRX under different strain rates and microstructural variations during phase transformation was analyzed in detail. Elevated strain rate leads to gradual grain refinement. Continuous dynamic recrystallization (CDRX) acts as the predominant DRX mode for Zr-Nb alloy, with merely minor discontinuous dynamic recrystallization (DDRX) features observed in the microstructure. Full article

Single-pass differential speed rolling (DSR) is an effective route for strengthening magnesium alloys through grain refinement induced by dynamic recrystallization (DRX); however, the accompanying strong basal texture often limits ductility and formability. In previous work, an Mg–2Al–0.5Ca–0.4Mn alloy (namely AXM20504) in the T4 condition was subjected to single-pass DSR with thickness reductions of 20% and 40% with a preheat temperature of 400 °C and a roll temperature of 300 °C, followed by post-annealing at 350–450 °C for durations of 20–60 min to systematically investigate static recrystallization (SRX), texture evolution, and mechanical response. Electron backscatter diffraction (EBSD) revealed that post-annealing promoted progressive SRX, with nearly complete recrystallization achieved at 450 °C for 40 min. This transition was accompanied by substantial basal texture weakening, reduced kernel average misorientation (KAM), and significantly lower grain orientation spread (GOS), indicating effective stress relief and formation of strain-free grains. As a result, tensile ductility increased from ~5% in the 40% as-rolled condition to ~12% after optimized post-annealing, while ultimate tensile strengths were retained above 200 MPa, much higher than the initial T4 strength. While these findings demonstrate that post-annealing is a critical step in restoring ductility and enhancing the formability of DSR-processed Mg alloys, certain types of alloying can also assist in a favorable balance between strength and formability for sheet forming applications. Alloying with Zn has been shown to improve ductility to a higher than 20% elongation at break as compared to 5% for the T4 AXM base material, showing that processing techniques and alloying have a high impact on the formability of Mg-based alloys. The zinc-based alloys in this study include Mg-2Al-0.5Ca-0.4Mn-0.5Zn (namely AXMZ2050405) and Mg-2Al-0.5Ca-0.4Mn-1Zn (namely AXMZ205041), both of which demonstrated improvements in mechanical properties as compared to the base alloy, AXM20504. Full article

The development of unconventional deep oil and gas resources requires 205 ksi high-Nb Ni-based corrosion-resistant alloys; however, the lack of hot-forming databases and processing maps for this grade has frequently resulted in hot cracking during ingot forging. To address this gap, this study investigated the hot-deformation behavior of this alloy and optimized its forming parameters. Isothermal-compression tests were performed using a Gleeble 3800-GTC simulator (Dynamic Systems Inc., Poestenkill, NY, USA) at 1173–1323 K and strain rates of 0.01–10 s⁻¹, followed by quantitative electron backscatter diffraction (EBSD) characterization of grain size, kernel average misorientation (KAM), and grain-boundary misorientation. A strain-compensated Arrhenius constitutive model was developed, giving a deformation activation energy of 540.33 kJ/mol and an average absolute relative error of only 4.77%. Dynamic materials model (DMM)-based power-dissipation and instability maps were constructed for this alloy for the first time, and an optimal hot-working window of 1260–1300 K and 0.01–0.1 s⁻¹ was identified. Under the representative condition of 1273 K and 0.1 s⁻¹, a uniform fine-grained microstructure with well-developed dynamic recrystallization was obtained without cracking, with an average grain size of 10.91 μm, a dynamic recrystallization (DRX) fraction of 48.4%, and an average KAM value of 0.476°. This work establishes the first complete hot-processing map system for the 205 ksi high-Nb Ni-based alloy, clarifies the coupled effects of dislocation evolution and dynamic recrystallization, and provides theoretical guidance for industrial hot cogging and microstructural control. Full article

A recently developed high-strength nickel-based corrosion-resistant alloy has attracted increasing interest for drilling and production operations in unconventional oil and gas fields. Owing to its high resistance to media containing H₂S, CO₂ and chloride ions, together with its ultra-high strength and favorable strength–toughness balance, this alloy is suitable for demanding service conditions. During hot working, dynamic recrystallization (DRX) governs deformation softening, grain refinement and the subsequent microstructural state, and thus has a direct influence on final properties. In this work, isothermal compression experiments were conducted on this ultra-high-strength nickel-based corrosion-resistant alloy using a Gleeble thermal simulator at 1000–1150 °C and strain rates of 0.01–10 s⁻¹. Electron backscatter diffraction (EBSD) was used to quantify grain size, grain-boundary misorientation, kernel average misorientation (KAM) and the DRX volume fraction. The results indicate that higher deformation temperature generally accelerates DRX, lowers the KAM value and increases the recrystallized-grain fraction. Under a constant deformation temperature, the DRX volume fraction changes non-monotonically with strain rate, showing an initial increase followed by a decrease. Based on the EBSD-derived DRX fractions, linear and quadratic single-parameter models using the Zener–Hollomon parameter were examined first, but neither provided satisfactory fitting accuracy. A two-variable empirical model was therefore formulated for a fixed true strain of ε = 0.92 by considering deformation temperature and strain rate separately. The predicted values agree well with the experimental data, giving R² = 0.91278 and an average relative error of 8.53%. The proposed model captures the main variation tendency of the DRX volume fraction within the studied processing window and provides a useful basis for microstructure control and hot-working parameter design for ultra-high-strength nickel-based corrosion-resistant alloys. Full article

The constitutive analysis model and hot processing map of the ZM6 alloy across various deformation conditions were investigated during hot compression experiments. True stress-strain curves within 300–450 °C and 0.0001–0.1 s⁻¹ were obtained from compression tests on a Gleeble-1500 platform. The results showed that higher strain rates (e.g., 0.1 s⁻¹) induced pronounced work hardening, whereas high temperatures (300–400 °C) combined with low strain rates (10⁻⁴ s⁻¹) promoted conditions conducive to dynamic recrystallization (DRX), leading to a softening tendency of steady-state flow stress. Additionally, a modified strain-compensated constitutive model was built for flow stress prediction. Material constants were plotted as fifth-order polynomial functions of strain (0.025–0.80) for precise stress predictions. The derived activation energy (Q = 182.38 kJ/mol) falls within the typical range for Mg-RE alloys. Leave-one-temperature-out cross-validation showed average AARE values of 7.2–9.8%, demonstrating the model’s interpolation capability and its sensitivity to extrapolation. Cross-validation within the training dataset showed reasonable consistency between experimental and predicted stresses (R > 0.997, AARE < 4.35%). Using the dynamic materials model, hot processing maps identified safe deformation zones and instability zones of the ZM6 alloy. Flow instability was observed at strain rates >0.01 s⁻¹, particularly at low temperatures (300–350 °C). Optimal processing windows appeared in high-energy dissipation (η > 30%) regions, e.g., 400–450 °C/10⁻⁴–10⁻³ s⁻¹. Optical microscopy confirmed that at high temperatures (≥400 °C) and low strain rates (≤0.001 s⁻¹), a uniform, fine-grained, fully recrystallized structure can be obtained, whereas low temperatures (350 °C) and high strain rates (0.1 s⁻¹) produce coarse elongated grains with limited DRX, consistent with the instability regime predicted by the processing maps. Under intermediate conditions (e.g., 400 °C, 0.01 s⁻¹), a bimodal grain distribution indicates incomplete recrystallization. Although EBSD analysis was not performed in this study, the optical microstructures directly validate the predicted safe and unstable windows. Together, all these findings provide preliminary model-based guidance for optimizing hot working parameters to balance microstructural stability and processing efficiency. Full article

Wire arc additive manufacturing of aluminum generates coarse, anisotropic solidification microstructures that limit mechanical performance, and interlayer friction stir processing (FSP) is increasingly applied to refine them. This study reports the layer-resolved grain morphology and the recrystallized crystal evolution in MIG + FSP-fabricated aluminum alloy 4043 walls, pairing the FSP spindle torque recorded from the CNC controller with multi-descriptor grain morphology in a coupling that, to the authors’ knowledge, has not been previously reported in the WAAM + FSP literature. Methodologically, two four-bead, three-layer walls were co-fabricated under identical deposition conditions on a HAAS VF-3 CNC platform, one by MIG deposition alone and one by the complete MIG + FSP route; the FSP spindle torque was measured at three positions per layer (118 ± 6 N·m at 600 RPM for L1, and 19.1 ± 1.0 and 26.6 ± 1.3 N·m at 1200 RPM for L2 and L3), and quantitative image analysis of 10,091 grains provided the layer-resolved mean grain area, equivalent diameter, aspect ratio, perimeter-to-area ratio, and circularity. The results show that the mean grain area increased from 8.55 μm² (L1) to 12.96 μm² (L3) while the aspect ratio decreased monotonically (1.389 to 1.323), indicating progressive grain equiaxiality with build height; the P/A ratio followed a non-monotonic layer dependence (2.54 to 2.11 to 2.50 μm⁻¹), with the L2 minimum consistent with reduced boundary line density under the combined thermal influence of two adjacent FSP events. The MIG + FSP route produced grain areas 29–48× smaller per layer than the MIG wall and a 45.8% higher hardness (75.8 ± 7.7 versus 52.0 ± 1.3 HV; n = 6; p = 0.0027). In conclusion, the L3 torque exceeds the L2 torque at equal 1200 RPM, qualitatively consistent with the d^−p term in the grain-size-explicit creep framework $\stackrel{.}{\gamma }$ = C·(τⁿ/d^p)·exp(−Q/RT), although temperature, strain rate, and grain size cannot be fully decoupled from the present three-layer dataset. The morphology and the distributional evidence are consistent with dynamic recrystallization (DRX); discrimination between continuous and discontinuous DRX requires EBSD. Full article

To elucidate the evolution of dynamic recrystallization (DRX) mechanisms in cold-worked Cr4Mo4Ni4V martensitic steel, tensile tests were conducted on a 50% cold-deformed material at 600–850 °C at a fixed strain rate of 0.001 s⁻¹, combined with systematic microstructural characterization. Under this specific strain rate, the results reveal a temperature-dependent transition from continuous dynamic recrystallization (CDRX) to discontinuous dynamic recrystallization (DDRX). At 600 °C, CDRX dominates, producing recrystallized grains with orientations close to the parent matrix and relatively strong texture. At 750 °C, CDRX and DDRX coexist, while DDRX is significantly enhanced, characterized by grain boundary nucleation and random orientations, leading to a marked reduction in texture intensity; simultaneously, the fraction of recrystallized grains and high-angle grain boundaries reaches a maximum. At 850 °C, DDRX becomes dominant. This transition in DRX mechanism governs the high-temperature plasticity, with optimal superplasticity achieved at 800 °C, corresponding to an elongation of 748%. Cavities are primarily initiated at carbide/matrix interfaces, and their growth and coalescence dominate the fracture process. These findings clarify the temperature-dependent DRX evolution and its role in regulating superplasticity, providing guidance for microstructure design and superplastic forming of martensitic steels. Full article

Dynamic recrystallization processes are known to significantly affect both the mechanical properties and the microstructure of materials. In this paper, we investigate the influence of discontinuous dynamic recrystallization (dDRX) during deformation at high strain rates (from 10⁴ to 10⁵ s⁻¹) and elevated temperatures in pure aluminum and copper (in the range of 700–800 K for aluminum and 800–1100 K for copper). For this purpose, we propose a theoretical model in which the material is described within the framework of continuum mechanics, plastic deformations are modeled using a dislocation plasticity approach, the equation of state is represented by a neural network, and the microstructure evolution is simulated using the cellular automata method. The model is applied to uniaxial compression and tension of copper and aluminum polycrystals with an initial average grain size of 14 μm. It is shown that grain refinement occurs in all systems. The average grain size decreases from 14 μm to 4–5 μm. The distribution of plastic and total strains in the polycrystals is presented. In all considered systems, deformation localization is observed, and the localization pattern changes due to the nucleation of new grains and grain boundary surfaces during dynamic recrystallization. Full article

Ductility dip cracking (DDC) remains a persistent challenge in multipass welds of high-chromium nickel-based alloys used in the nuclear power generation industry. While dynamic recrystallization (DRX) has been observed to arrest DDC crack growth and has been associated with weld regions that experience less DDC, there exists no quantitative relationship between the extent of recrystallization in a microstructure and DDC susceptibility. This research examines the influence of intragranular carbides on DRX behavior and establishes an experimental relationship between DDC susceptibility and extent of recrystallization in high-chromium nickel-based weld metals, novel contributions for this alloy system. In this work, the DRX behavior of the weld metal of high-chromium nickel-based filler metals (FM-52, FM-52M, FM-52i, and FM-52xl) was investigated under controlled thermo-mechanical conditions, and its effect on DDC susceptibility was established. Weld metal specimens were subjected to uniaxial deformation at 1100 °C to a true strain of 2% at strain rates of 10⁻³/s and 10⁻⁴/s using a Gleeble 3800^TM. Recrystallization was quantified using electron backscatter diffraction (EBSD) via grain orientation spread (GOS) analysis and dislocation–precipitate interactions were examined using transmission electron microscopy (TEM). Strain-to-fracture (STF) testing at 950 °C was employed to assess DDC susceptibility as a function of the extent of recrystallization and grain surface area. All tested weld metals exhibited increased recrystallization and grain refinement, as the strain rate decreased from 10⁻³/s to 10⁻⁴ s. The FM-52i weld metal specimens exhibited the highest grain refinement under high temperature deformation, followed by the FM-52xl, FM-52, and FM-52M weld metals with a percent reduction in average grain surface area of 51.22%, 41.66%, 35.48%, and 24.40%, respectively. The FM-52i weld metal specimens also exhibited the highest recrystallization response, followed by FM-52M, FM-52xl, and FM-52 weld metals at 75%, 40%, 39% and 21% recrystallized, respectively. Weld metals containing strong carbide formers experienced higher recrystallization responses than those without due to precipitate–carbide interactions. All tested weld metals experienced drastic reductions in DDC response with increasing extent of recrystallization and decreasing average grain surface areas. DRX in STF specimens was observed to facilitate uniform plastic strain accumulation, lowering overall DDC susceptibility compared to non-recrystallized specimens. Full article

Friction stir welding (FSW) is a key technology for manufacturing T-shaped thin-walled structures and avoiding fusion welding defects. However, the quantitative relationship between its process parameters and the microstructure properties of the joint remains unclear. To address this, this study established regression models via response surface methodology (RSM) relating rotational speed (w), welding speed (v), and plunge depth (h) to the mechanical properties of T-joints. The optimal process parameters (400 rpm, 60 mm/min, 0.21 mm) were determined, under which the ultimate tensile strength (UTS) and weld nugget hardness (WNH) of the joint reached 74.1% (377 MPa) and 94.4% (153 Hv) of the base materials (BM) respectively, with v showing the most significant influence on joint mechanical properties. Microstructural observations revealed that from the BM to the stirring zone (SZ), the grains underwent a continuous evolution from coarsening, partial recrystallization to complete dynamic recrystallization (DRX). In the SZ, due to severe plastic deformation and high heat input, the continuous dynamic recrystallization (CDRX) was the dominant mechanism, and the grain was significantly refined. The heat input in the thermomechanical affected zone (TMAZ) is relatively low, mainly geometric dynamic recrystallization (GDRX). DRX-driven grain refinement was the primary strengthening factor in the joint, with hardness closely related to grain size. However, thermal cycling induced softening in the heat-affected zone (HAZ) and promoted the precipitation of brittle compounds such as Al₃Mg₂ and MgZn₂, which caused crack initiation exhibiting intergranular brittle fracture. Subsequently, under stress drive, it extends to SZ, mainly characterized by ductile fracture. Full article

To give more insight into the microstructural evolution and deformation mechanisms governing the long-term service performance of additively manufactured TiAl-based composites at elevated temperatures, this study investigated the high-temperature compressive creep behavior of a laser powder bed-fused LaB₆ reinforced high-Nb TiAl-based composite after hot isostatically pressing (HIP), with emphasis on the creep response and dynamic recrystallization (DRX) mechanisms under different applied stress levels. The results showed that, as the applied stress increased from 200 MPa to 450 MPa, the steady-state creep rate rose from 2.88 × 10⁻⁸ s⁻¹ to 3.85 × 10⁻⁷ s⁻¹. Stress exponent analysis indicated that creep deformation was predominantly controlled by dislocation climb, and no tertiary creep stage was observed within the investigated stress range. At 200 MPa and 300 MPa, a certain fraction of recrystallized grains formed during prolonged creep exposure. When the stress increased to 400 MPa, the recrystallization process was restricted due to the limited creep duration. In contrast, at 450 MPa, the accelerated accumulation of strain energy significantly promoted recrystallization. Both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) were identified, jointly governing the microstructural evolution. Superior creep resistance can be attributed to multiple synergistic strengthening mechanisms, including the refined α₂/γ lamellar structure induced by HIP treatment, the strong pinning effect of dispersed La₂O₃ nanoparticles on dislocation motion, and the suppression of diffusion-controlled dislocation climb by Nb addition. These combined effects enhance the high-temperature creep performance of the TiAl composite and provide important insights for the application of LPBF-fabricated TiAl-based composites under elevated-temperature service conditions. Full article

To study the influence of freckle defects on the hot deformation behavior and the inheritance of dynamic recrystallization (DRX) structure in GH4706 alloy, the microstructures of specimens with and without freckles and the evolution laws of hot-processing parameters were compared. Hot compression experiments were conducted on a thermal simulation testing machine at 950–1150 °C, strain rates of 0.001–1 s⁻¹, and 55% deformation. Freckle-containing specimens were tested under DRX critical conditions. The flow stresses of both specimens increase with strain rate or with decreasing temperature. The power dissipation coefficient (η) and instability value (ξ) follow complex laws. Electron back-scattering diffraction (EBSD) was used to analyze DRX microstructures and nucleation mechanisms. The DRX degree of freckle-containing specimens is lower, with a larger average grain size. The DRX mechanism initiates preferentially in freckle-containing specimens, and its volume fraction changes in a complex manner. Grain coarsening occurs in freckle-containing specimens at high temperatures and low strain rates. Freckle defects lead to significant differences in the DRX mechanism of GH4706 alloy. Freckle-containing specimens exhibit both discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX), whereas freckle-free specimens primarily display DDRX and second-phase particle-stimulated nucleation (PSN). The presence of MC carbides and Laves phases within freckle defects provides nucleation sites, further supporting a typical second-phase particle-stimulated nucleation mechanism. Full article