Thermomechanical Processing Conditions Research Articles

A study of dynamic recovery and recrystallisation mechanisms in aluminium alloy AA7050 at different thermomechanical processing conditions

The effects of hot deformation strain rates and temperatures on aluminium alloys' dynamic recovery and recrystallisation mechanisms have been widely discussed in the literature. However, their influence remains controversial due to a need for comprehensive and detailed thermomechanical testing and characterisation data across a wide range of hot deformation conditions. In this study, hot compression tests of AA7050 were conducted at strain rates of 0.0005–5 s−1 and temperatures of 380–460 °C. The geometrically necessary dislocation (GND), low-angle grain boundary (LAGB) and high-angle grain boundary (HAGB) were acquired by the latest high angular resolution, high-speed EBSD detector (OI Symmetry 2), and their underlying mechanisms were analysed in relation to Zener-Hollomon parameter (Z). It was found that within the tested range of the hot compression condition (ln (Z) between 20.23 and 29.45), continuous dynamic recrystallisation (CDRX) predominates at lower ln (Z) values with its activity being initially increased and then decreased as ln (Z) rises, while discontinuous dynamic recrystallisation (DDRX) becomes dominant at higher ln (Z) levels. This transition of the dominant mechanism is intriguingly reflected in the evolution of HAGB length, which increases, then decreases and finally increases again. The LAGB length shows a relatively linear relationship with ln (Z), and the average GND density exhibits a near-linear correlation until reaching a plateau. Based on the obtained results, the transformation of the dominant microstructural evolution mechanisms with increasing ln (Z) was clarified: from grain growth, to recovery, to CDRX, and then to DDRX, along with the quantitative trend of HAGB length per unit area. This transformation mechanism is believed to encompass the full range of material processing window, providing a basis for justifying the previous controversial proposition in literature and a guide for defining and fabricating microstructures in manufacturing applications.

Mechanical and corrosion performances of Al–Mg-Mn 5083 rolled alloys microalloyed with Sc and Zr in different thermomechanical processing conditions

Mechanical and corrosion performances of Al–Mg-Mn 5083 rolled alloys microalloyed with Sc and Zr in different thermomechanical processing conditions

Real-time evolution of texture and temperature during friction stir processing of a magnesium alloy: An operando neutron diffraction study

The real-time development of texture and the evolution of temperature during friction stir processing (FSP) of a Mg alloy were investigated using an operando neutron diffraction measurement. A novel approach was applied in this study where the real-time, quasi-steady state measurements performed as a function of position during FSP are converted to a Lagrangian dataset that reveals the transient behavior as a function of time. The in-situ FSP was carried out under two different thermo-mechanical processing conditions represented by the Zener-Hollomon parameter, Z. The results show that: (i) a shear texture develops from the initial strong basal texture with the peak temperature reaching about 774 K during a low-Z processing and (ii) a strong off-normal texture develops with a peak temperature of 616 K during a high-Z processing. Moreover, time-temperature-texture diagrams were established to reveal the real-time development of the texture during the processing for both the low Z and high Z processing conditions. The changes in texture will be discussed in terms of plastic deformation mechanisms active during various stages of the FSP under the two different processing conditions.

Predictive Modeling of Hardness Values and Phase Fraction Percentages in Micro-Alloyed Steel during Heat Treatment Using AI

In this work, we have proposed an AI-based model that can simultaneously predict the hardness and phase fraction percentages of micro-alloyed steel with a predefined chemical composition and thermomechanical processing conditions. Specifically, the model uses a feed-forward neural network enhanced by the ensemble method. The model has been trained on experimental data derived from continuous cooling transformation (CCT) diagrams of 39 different steels. The inputs to the model include a cooling profile defined by a set of time-temperature values and the chemical composition of the steel. Sensitivity analysis was performed on the validated model to understand the impact of key input variables, including individual alloys and the thermomechanical processing conditions. This analysis, which measures the variability in output in response to changes in a specific input variable, showed excellent agreement with experimental data and the trends in the literature. Thus, our model not only predicts steel properties under varied cooling conditions but also aligns with existing theoretical knowledge and experimental data.

Microstructure Based Flow Stress Modelling of Superalloy 718

In order to get the insights about microstructural changes that occurs under the thermo-mechanical processing conditions, the physics based modelling approach is very useful. Therefore, the flow curves of alloy 718 are theoretical simulated using a dislocation density dependent constitutive model for different conditions. Presented model considers the microstructural ingredients that are immobile dislocation density, effective grain size and dislocation cell size as the variables to address the creep. The simulated flow curves show a good agreement with the experimental flow curves. The magnitude of immobile dislocation density and dislocation cell size in between 3.87× 1014 - 3.87× 1014­ m-2 and 8.29-8.45 μm, respectively, at the completion of the simulation. Furthermore, this approach also provides the possibility to quantify and depict the variation in each strengthening contributions.

Modelling the spatial evolution of excess vacancies and its influence on age hardening behaviors in multicomponent aluminium alloys

Excess vacancies play crucial roles in the precipitation of age-hardening precipitates in aluminium alloys, but their spatial evolution across grains during heat treatments is less known. In this work, a numerical model is developed to predict the spatial evolution of non-equilibrium excess vacancies during cooling from solution treatment and during ageing heat treatments of multicomponent aluminium alloys. A finite volume scheme is applied to derive the spatial distribution of vacancy site fraction across grains by solving the diffusion equations of vacancies under the influence of solute elements. Binding energies between solute atoms and vacancies predicted by first-principles calculations has been used to handle the trapping of excess vacancies by solute atoms and atom clusters. The annihilation rates of excess vacancies at grain boundaries and at dislocation jogs have been derived based on a rigorous description of the annihilation mechanisms of vacancies. The evolution of the density of dislocation jogs due to vacancy annihilation has been taken into account. The model is successfully applied to interpret the age hardening behaviors of experimental alloys subjected to different thermomechanical processing conditions. This model will help to reach a deeper understanding of the roles of excess vacancies in precipitation kinetics and therefore is important to further optimize thermomechanical processing parameters and alloy composition to improve the macroscopic mechanical properties of age hardening aluminium alloys.

In-situ SEM/EBSD investigation of tensile deformation-driven microstructural changes in low-carbon HSLA steels with ferrite-bainite microstructures

In this study, we aimed to investigate how microstructural constituents affect the plastic deformation behavior of low-carbon HSLA steels during a uniaxial tensile test using the in-situ SEM/EBSD technique. We fabricated three types of low-carbon HSLA steels, each with distinct ferrite-bainite microstructures, by altering the thermomechanical control processing conditions. All the steels were composed primarily of polygonal ferrite (PF) and bainitic microstructures. As the finish rolling temperature and the cooling rate decreased, the volume fraction of PF increased. The in-situ SEM and EBSD analysis during deformation revealed a greater concentration of plastic strain and an increased mobile dislocation density in the PF than in the bainitic microstructures. Most notably, as the volume fraction of the bainitic microstructure increased, so did the degree of strain gradient between the soft PF and the hard bainitic microstructure. This suggests that at the initial yield stage the soft PF plays a more significant role in mitigating the yield point phenomenon than the hard bainitic microstructure. Our findings offer a deeper understanding of the microstructural factors that influence plastic deformation, and may serve as a guide for designing optimal microstructures to ensure the mechanical properties of low-carbon HSLA steels for a range of applications.

Prediction of Grain Size in a High Cobalt Nickel-Based Superalloy.

With the advancement in computational approaches and experimental, simulation, and modeling tools in recent decades, a trial-and-validation method is attracting more attention in the materials community. The development of powder metallurgy Ni-based superalloys is a vivid example that relies on simulation and experiments to produce desired microstructure and properties in a tightly controlled manner. In this research, we show an integrated approach to predicting the grain size of industrial forgings starting from lab-scale cylindrical compression by employing modeling and experimental validation. (a) Cylindrical compression tests to obtain accurate flow stress data and the hot working processing window; (b) double-cone tests of laboratory scale validation; (c) sub-scale forgings for further validation under production conditions; and (d) application and validation on full-scale industrial forgings. The procedure uses modeling and simulation to predict metal flow, strain, strain rate, temperature, and the resulting grain size as a function of thermo-mechanical processing conditions. The models are calibrated with experimental data until the accuracy of the modeling predictions is at an acceptable level, which is defined as the accuracy at which the results can be used to design and evaluate industrial forgings.

Effect of Thermo-Mechanical Processing on Initiation and Propagation of Stress Corrosion Cracking in 304L Austenitic Stainless Steel

Despite the high corrosion resistance of austenitic stainless steels (SSs), a significant reduction of stress corrosion cracking (SCC) resistance has been reported in cases of high residual stress and metastable microstructural features. In this study, the effect of thermo-mechanical processing (TMP) on the initiation and propagation of SCC in 304L SS was studied. To better understand the SCC mechanisms, three TMPs conditions—welded, solution annealed at 1050 °C for tens of seconds, and straightened—were used. The research focused on analyzing the initial microstructure, residual stress, and hardness along the depth direction to assess SCC resistance and establish correlations with the observed SCC modes. Experimental results demonstrated that transgranular SCC was observed in regions exhibiting elevated residual stress induced by welding and straightening processes. Furthermore, the presence of strain-induced martensite transformation and slip bands formed during plastic deformation were identified as additional factors contributing to the susceptibility of SCC. The study findings highlighted that the magnitude and distribution of residual stresses, in conjunction with microstructural evolution, could be varied depending on the specific TMP condition, leading to different SCC susceptibilities, cracking modes, and directions.

Machine learning approach for predicting yield strength of nitrogen-doped CoCrFeMnNi high entropy alloys at selective thermomechanical processing conditions

The yield strength property is important to consider while designing high entropy alloys (HEAs). In order to obtain the desired yield strength of HEAs through the experimental method it is a difficult, expensive, and time-consuming process due to the broad composition space available. The room temperature yield strength property of nitrogen-doped (CoCrFeMnNi)100-x-Nx HEAs at preferred thermomechanical conditions was predicted using the machine learning (ML) technique based on the linear regression model in the present investigation. The yield strength prediction result of 2% nitrogen-doped CoCrFeMnNi HEA subsequently cold-rolled 91 (%) and annealed at 850 °C temperature consisting of 563.6 MPa is consistent with the experimental value of 556 MPa. It implies that the yield strength predictions of (CoCrFeMnNi)100-x-Nx HEAs are accurate. As a result, selecting suitable models and material parameters to design a wide range of materials with superior properties attributed to various compositions of HEAs through ML technology could be a potential approach.

Decision Science Driven Selection of High-Temperature Conventional Ti Alloys for Aeroengines

Near-α Ti alloys find themselves in advanced aeroengines for applications of up to 600 °C, mainly as compressor components owing to their superior combination of ambient- and elevated-temperature mechanical properties and oxidation resistance. We evaluated, ranked, and selected near-α Ti alloys in the current literature for high-temperature applications in aeroengines driven by decision science by integrating multiple attribute decision making (MADM) and principal component analysis (PCA). A combination of 12 MADM methods ranked a list of 105 alloy variants based on the thermomechanical processing (TMP) conditions of 19 distinct near-α Ti alloys. PCA consolidated the ranks from various MADMs and identified top-ranked alloys for the intended applications as: Ti-6.7Al-1.9Sn-3.9Zr-4.6Mo-0.96W-0.23Si, Ti-4.8Al-2.2Sn-4.1Zr-2Mo-1.1Ge, Ti-6.6Al-1.75Sn-4.12Zr-1.91Mo-0.32W-0.1Si, Ti-4.9Al-2.3Sn-4.1Zr-2Mo-0.1Si-0.8Ge, Ti-4.8Al-2.3Sn-4.2Zr-2Mo, Ti-6.5Al-3Sn-4Hf-0.2Nb-0.4Mo-0.4Si-0.1B, Ti-5.8Al-4Sn-3.5Zr-0.7Mo-0.35Si-0.7Nb-0.06C, and Ti-6Al-3.5Sn-4.5Zr-2.0Ta-0.7Nb-0.5Mo-0.4Si. The alloys have the following metallurgical characteristics: bimodal matrix, aluminum equivalent preferably ~8, and nanocrystalline precipitates of Ti3Al, germanides, or silicides. The analyses, driven by decision science, make metallurgical sense and provide guidelines for developing next-generation commercial near-α Ti alloys. The investigation not only suggests potential replacement or substitute for existing alloys but also provides directions for improvement and development of titanium alloys over the current ones to push out some of the heavier alloys and thus help reduce the engine’s weight to gain advantage.

A pseudo thermo-mechanical model linking process parameters to microstructural evolution in multilayer additive friction stir deposition of magnesium alloy

Additive friction stir deposition has been proposed as a disruptive manufacturing process; involving complex thermo-mechanical mechanisms during multilayer material deposition. The current efforts have attempted to develop a FEM based pseudo-mechanical thermal model accounting for heat generation due to friction and plastic dissipation during multilayer additive friction stir deposition. The primary motivation for development of the model was to seek an understanding of thermo-mechanical mechanisms and their impact on microstructural evolution during additive friction stir deposition. The predicted temperature–time profiles agreed well with the experimentally derived ones. The computational predictions indicate rise of the peak temperatures up to 0.8 times the melting temperature of Mg-alloy. In addition, the Zener-Holloman parameter, Ze evaluated using the computational model was correlated with the microstructural evolution during the deposition process. The unique thermo-mechanical processing conditions during additive friction stir deposition led to dynamic recrystallization followed by grain coarsening. A significant extent of texture strengthening was observed in the AFSD processed samples. The already established phenomenological relationship between Ze and grain size was used to predict the grain size in the present work. The computational predictions indicate that the recrystallized grain size ranged from 4 to 6 µm, and the post deformation grain coarsening varied in the range of 4–24 µm, thereby providing reasonable agreement with the experimental observations.

Structure formation during sheet steel production in a unit of continuous casting and deformation

The paper considers the main problem of improving quality of steel sheets for welded pipes obtained at plate rolling mills and the main reasons that reduce it. There is a possibility of improving quality of steel sheets using technological capabilities of the unit of combined continuous casting and deformation. Problem of determining stress state of metal in cyclic deformation zone is posed and solved. The results of obtaining 09G2S sheet steel in pilot unit of continuous casting and deformation are presented. Magnitude of stresses in the center of cyclic deformation during production of steel sheets was determined for welded pipes at the unit. The authors present results of experimental study of the process of steel sheets production at pilot unit of continuous casting and deformation. The article considers the results of microstructure analysis, microhardness determination and microanalysis of chemical composition of the studied steel in various zones of the strip cross section. Structural inhomogeneity development in ferritic-pearlitic steel is interpreted along the thickness of sheet under conditions of combined thermomechanical processing of metal solidified in the mold. Predominantly ferritic microstructure is generated in the central light interlayer of the strip compared to base metal, characterized by a sharp decrease in carbon concentration in it. There is a possibility of such structure genesis at the first stages of crystallization during forging of low-carbon steel, when the excess phase of high-temperature ferrite that solidifies first is located mainly in the strip center. The size of pearlite grains in the light interlayer covariates with the grain structure of pearlite in the strip base metal and does not form precipitates of lath morphology, which increase tendency of steel to brittle fracture.

Hot deformation behavior and microstructure evolution of non-equimolar Ti2ZrHfV0.5Ta0.2 refractory high-entropy alloy

This work aims to elucidate the hot deformation behavior and microstructure evolution of a non-equimolar TiZrHf-rich refractory high-entropy alloy (RHEA) with a composition of Ti2ZrHfV0.5Ta0.2 (at. %). The as-cast Ti2ZrHfV0.5Ta0.2 alloy composed solely of a body-centered cubic phase showed superior tensile mechanical properties with a yield strength of 865 MPa and a total elongation of 22.5%. The uniaxial compression tests were performed at temperatures ranging from 900 to 1100 °C and strain rates varying from 10−3 to 10−1 s−1. The relation among the flow stress, deformation temperature, and strain rate was represented by the Arrhenius-type constitutive equations. The calculations show a comparatively low apparent activation energy of 144–119 kJ/mol over the entire range of strains, revealing a noticeable contradiction between the concentration of group IV elements (Ti, Zr, Hf) and the high-temperature strength. The excessive addition of Group IV elements tends to deteriorate the high-temperature strength due to the relatively low melting points and self-diffusion activation energies of Group IV elements. The misorientation analysis based on electron back-scattered diffraction indicated that the dynamic recrystallization mechanism depends strongly on the thermo-mechanical processing conditions. A low deformation temperature and high strain rate results in an abnormal discontinuous dynamic recrystallization mechanism. Additionally, the dominant role played by the edge dislocation in RHEAs during deformation is supposed to be the reason for the abnormal discontinuous dynamic recrystallization mechanism. A series of stress drops were observed under low deformation temperatures and high strain rates. Based on an interrupted hot-compression experiment and a strain aging experiment, the stress drop can be attributed to the unlocking of dislocations pinned by the Cottrell atmosphere or short-range ordering.

Correlation between subsurface properties, the thermo-mechanical process conditions and machining parameters using the CEL simulation method

In the present work, the applicability of the coupled Eulerian-Langragian method (CEL) was proved to model a drilling process. The CEL method enables complex three dimensional simulations with large deformations very time efficient with a sufficient accuracy. In this study, different process parameters were applied and the resulting feed force, maximum temperature and residual stress in the subsurface region were evaluated and validated by comparison with experimental data. With the resulting simulation data regression models were developed, which can be used in a process framework to control the thermo-mechanical state during single lip deep hole drilling.

Effect of Strain Rate on Mechanical Behavior and Microstructure Evolution of Ti-Based T110 Alloy

Recently developed thermally hardenable medium-level alloyed titanium alloy T110 was studied from the viewpoint of microstructure influence on the mechanical behavior under quasi-static and high-strain rate deformation. The globular microstructures of two types differing in aspect ratio of α-globules were formed under thermomechanical processing conditions with different reductions (εtotal = 2, and 3) and final annealing at 850 °C, 3 h. The thermally hardened state was formed by conventional solid-solution treatment at a temperature of two-phase α+β field (880 °C, 45 min), water quenching, and final aging (550 °C, 5 h). Stress–strain dependencies were estimated on tension with strain rates varied from 8·10−4 to 4·10−2 s−1, as well as compression with a strain rate of 10−3 s−1 (quasi-static) and under high-strain rates varied from 870 s−1 to 3520 s−1; the deformation with high-strain rates has been achieved using split Hopkinson pressure bar (SHPB) technique. The tested material was also assessed using strain energy (SE) parameter. A parallel study of the microstructure and crystallographic texture formed during testing allowed to propose a reliable deformation and fracture mechanisms, depending on the stress state (tension, compression) and strain rate. A special role of the initial microstructure is noted, which determines the plasticity of the tested alloy, the sites of pore nucleation, features of crack growth, as well as the localization of deformation, which is determined by the mode and rate of loading. It is established and explained why the best balance of strength and ductility, and thus, the highest SE values in the all tests carried out were ensured by the alloy in the state with a more uniform microstructure of globular morphology (after the rolling with reduction εtotal =3), which, in turn, provided general superiority over the widely used Ti-6Al-4V alloy.

Numerical Modelling of the Powder Metallurgical Manufacturing Chain of High Strength Sintered Gears

This paper presents a digital model for the powder metallurgical (PM) production chain of high-performance sintered gears based on an integrated computational materials engineering (ICME) platform. Discrete and finite element methods (DEM and FEM) were combined to describe the macroscopic material response to the thermomechanical loads and process conditions during the entire production process. The microstructural evolution during the sintering process was predicted on the meso-scale using a Monte-Carlo Model. The effective elastic properties were determined by a homogenization method based on modelling a representative volume element (RVE). The results were subsequently used for the FE modelling of the heat treatment process. Through the development of multi-scale models, it was possible obtain characteristics of the microstructural features. The predicted hardness and residual stress distributions allowed the calculation of the tooth root load bearing capacity of the heat-treated sintered gears.

Thermomechanically processed powder metallurgy Ti-5Fe alloy: Effect of microstructure, texture, Fe partitioning and residual porosity on tensile and fatigue behaviour

The combination of low-cost alloying elements like Fe and powder metallurgy, a route with potential to lower the cost of processing titanium alloys, is an interesting option to process cost-effective titanium alloys. Using thermomechanical processing can help improve their behaviour, but limited work has been devoted to it. Additionally, there is a lack of knowledge on their fatigue performance. In this work, different extrusion and heat treatment conditions were applied to develop different microstructures and textures, and their effect on the tensile and fatigue behaviour was thus studied. Small differences in the extrusion and annealing temperatures, when in the α+β phase, result in strong Fe partitioning between α and prior β grains, developing an ultrafine transformed β microstructure instead of lamellar colonies. This results in a remarkable strengthening of the alloy plus an increase in ductility. Despite reaching practically full density after extrusion, residual pores influence the fatigue behaviour (but not the tensile behaviour), and the extent of their influence depends on the intrinsic strength of the alloy. By carefully selecting the thermomechanical processing conditions, the cost-effective powder metallurgy Ti-5Fe alloy can match the fatigue performance of the workhorse Ti-6Al-4V alloy.

Rheological Basics for Modeling of Extrusion Process of Wood Polymer Composites.

Wood polymer composites are materials with pseudoplastic and viscoelastic properties. They have yield stress and exhibit slip during flow. Studies on extrusion and rheology, as well as on process modeling of these highly filled materials are limited. Extensive rheological and extrusion modeling studies on the wood polymer composite based on the polypropylene matrix were performed. Viscous and slip flow properties were determined (with Rabinowitsch, Bagley, and Mooney corrections) at broad (extrusion) range of shear rate and temperature, using a high-pressure capillary rheometer. Rheological models of Klein and power-law were used for flow modeling, and Navier model was applied for slip modeling. A novel global computer model of WPC extrusion with slip effects has been developed, and process simulations were performed to compute the extrusion parameters (throughput, power consumption, pressure, temperature, etc.), and to study the effect of the material rheological characteristics on the process flow. Simulations were validated experimentally, and were discussed with respect to both rheological and process modeling aspects. It was concluded that the location of the operating point of extrusion process, which defines the thermo-mechanical process conditions, is fundamentally dependent on the rheological materials characteristics, including slip effects.

Microstructure Evolution during Strain-Induced Transformation of Austenite in an Austempered Ductile Iron (ADI)

Microstructural evolution during the strain-induced phase transformation of austenite in an Austempered ductile iron (ADI) under various thermomechanical processing conditions is studied in the present study. An alloyed ductile iron is taken as the base material, and thermomechanical treatment is carried out on a Gleeble 3800 thermomechanical simulator coupled with dilatometry. The effect of deformation on the austempering process has been studied by microstructure characterization using optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) techniques. The variations in retained austenite volume fraction and its carbon content with respect to different austempering times are analyzed to study the effect of strain-induced transformation of austenite. It has been observed that the thermomechanical treatment significantly influences the phase transformation kinetics during the austempering process. The thermomechanical treatment produced a martensite free ausferritic microstructure for all austempering times with a high volume fraction of carbon enriched retained austenite as compared to the conventional heat treatment.