Separative and Comprehensive Effects of Grain Coarsening and Grain Refinement of Ni-38Cr-3.8Al Alloy during Thermal Deformation Process.

During thermal deformation, grain coarsening due to grain growth and grain refinement resulting from dynamic recrystallization (DRX) collectively influence the deformed grain size. To investigate the separative and comprehensive effects of the two mechanisms in the Ni-38Cr-3.8Al alloy, grain growth experiments and isothermal compression tests were conducted. Kinetics models for grain growth and DRX behaviors were established based on the experimental data, which were integrated with finite element (FE) techniques to simulate the evolution of grain size throughout the entire thermal compression process. The effects of grain coarsening and grain refinement during this process were separated and quantified based on the simulation data. The results revealed that grain coarsening predominated during the heating and holding stages, with a longer holding time and higher holding temperatures intensifying this effect. However, during the compression stage, grain coarsening and grain refinement co-existed, and their competition was influenced by deformation parameters. Specifically, grain refinement dominated at strain rates exceeding 0.1 s-1, while grain coarsening dominated at lower strain rates (<0.1 s-1) and higher deformation temperatures (>1373 K). The simulated grain sizes closely matched the experimental observations.

Achievement of a Parameter Window for the Selective Laser Melting Formation of a GH3625 Alloy.

In the selective laser melting (SLM) process, adjusting process parameters contributes to achieving the desired molten pool morphology, thereby enhancing the mechanical properties and dimensional accuracy of manufactured components. The parameter window characterizing the relationship between molten pool morphology and process parameters serves as an effective tool to improve SLM's forming quality. This work established a mesoscale model of the SLM process for a GH3625 alloy based on the discrete element method (DEM) and computational fluid dynamics (CFD) to simulate the forming process of a single molten track. Subsequently, the formation mechanism and evolution process of the molten pool were revealed. The effects of laser power and scanning speed on the molten pool size and molten track morphology were analyzed. Finally, a parameter window was established from the simulation results. The results indicated that reducing the scanning speed and increasing the laser power would lead to an increase in molten pool depth and width, resulting in the formation of an uneven width in the molten track. Moreover, accelerating the scanning speed and decreasing the laser power cause a reduction in molten pool depth and width, causing narrow and discontinuous molten tracks. The accuracy of the simulation was validated by comparing experimental and simulated molten pool sizes.

A Review on Controlling Grain Boundary Character Distribution during Twinning-Related Grain Boundary Engineering of Face-Centered Cubic Materials.

Grain boundary engineering (GBE) is considered to be an attractive approach to microstructure control, which significantly enhances the grain-boundary-related properties of face-centered cubic (FCC) metals. During the twinning-related GBE, the microstructures are characterized as abundant special twin boundaries that sufficiently disrupt the connectivity of the random boundary network. However, controlling the grain boundary character distribution (GBCD) is an extremely difficult issue, as it strongly depends on diverse processing parameters. This article provides a comprehensive review of controlling GBCD during the twinning-related GBE of FCC materials. To commence, this review elaborates on the theory of twinning-related GBE, the microscopic mechanisms used in the optimization of GBCD, and the optimization objectives of GBCD. Aiming to achieve control over the GBCD, the influence of the initial microstructure, thermo-mechanical processing (TMP) routes, and thermal deformation parameters on the twinning-related microstructures and associated evolution mechanisms are discussed thoroughly. Especially, the development of twinning-related kinetics models for predicting the evolution of twin density is highlighted. Furthermore, this review addresses the applications of twinning-related GBE in enhancing the mechanical properties and corrosion resistance of FCC materials. Finally, future prospects in terms of controlling the GBCD during twinning-related GBE are proposed. This study will contribute to optimizing the GBCD and designing GBE routes for better grain-boundary-related properties in terms of FCC materials.

Characterization of Flow Behaviors by a PSO-BP Integrated Model for a Medium Carbon Alloy Steel.

In order to characterize the flow behaviors of SAE 5137H steel, isothermal compression tests at the temperatures of 1123 K, 1213 K, 1303 K, 1393 K, and 1483 K, and the strain rates of 0.01 s-1, 0.1 s-1, 1 s-1, and 10 s-1 were performed using a Gleeble 3500 thermo-mechanical simulator. The analysis results of true stress-strain curves show that the flow stress decreases with temperature increasing and strain rate decreasing. In order to accurately and efficiently characterize the complex flow behaviors, the intelligent learning method backpropagation-artificial neural network (BP-ANN) was combined with the particle swarm optimization (PSO), namely, the PSO-BP integrated model. Detailed comparisons of the semi-physical model with improved Arrhenius-Type, BP-ANN, and PSO-BP integrated model for the flow behaviors of SAE 5137H steel in terms of generative ability, predictive ability, and modeling efficiency were presented. The comparison results show that the PSO-BP integrated model has the best comprehensive ability, BP-ANN is the second, and semi-physical model with improved Arrhenius-Type is the lowest. It indicates that the PSO-BP integrated model can accurately describe the flow behaviors of SAE 5137H steel.

A Tailored Preparation Method of Variable Strength for Ultra-High-Strength Steel Sheet and Mapping Mechanism between Process and Property.

The spatiotemporal phase transformation during hot stamping would considerably effect the microstructure and mechanical properties of steels. In order to manufacture hot-stamping components of ultra-high-strength steel with tailored mechanical properties, the effect of the quenching time and the temperature of the die on the phase-transformation characteristics and mechanical property of ultra-high-strength steel was deeply studied. A finite element (FE) model coupled with a thermomechanical phase was employed to perform a succession of simulations for hot stamping corresponding to different quenching times and the temperatures of die, and the corresponding hot stamping experiments were performed. The 3D mapping surfaces of the temperature; quenching time; and three microstructures, namely austenite, bainite, and martensite, were constructed, and the mapping relationships in such surfaces were further explained by microstructural observations. Subsequently, based on the test results of the mechanical properties, the relationship curves of hardness and tensile strength, hardness, and elongation at break were fitted respectively, and then the 3D mapping surfaces were constructed for hardness, tensile strength, and elongation at break, which varied with the temperature die and quenching time. Finally, the quenching parameters of the automobile B-pillar were designed according to the constructed mapping relationship, and the hot-stamping FE simulation of the automobile B-pillar was developed. The result shows that those constructed mapping surfaces are helpful for adjusting the local mechanical property of the steels by designing the parameters.

Influencing Mechanisms of Prior Cold Deformation on Mixed Grain Boundary Network in the Thermal Deformation of Ni80A Superalloy.

Within the grain boundary engineering (GBE) of alloys, a mixed grain boundary network with random grain boundaries interrupted by twin boundaries, contributes to enhancing the overall grain boundary-related properties. The higher density of twin boundaries is pursued herein. Furthermore, a two-stage deformation method, i.e., prior cold deformation followed by thermal deformation, was proposed for improving the mixed grain boundary network in the thermal deformation of Ni80A superalloy. The influence of prior cold deformation on the mixed grain boundary network was investigated through a series of two-stage deformation experiments. The analysis of the stress-strain curves shows that the critical strain for dynamic recrystallization (DRX) and peak strains decrease significantly under the effect of prior cold deformation. In comparison to the necklace-like microstructures that occur after a single thermal deformation, the microstructures apparent after a two-stage deformation are characterized by finer DRX grains with abundant Σ3n twin boundaries, with a significantly improved density of the Σ3n twin boundaries (BLDΣ3n) by a factor of around nine. With increasing prior cold strain, the grain size, after a two-stage deformation, decreases continuously, while the BLDΣ3n increases firstly and then decreases. The mechanisms for improving the mixed grain boundary network via two-stage deformation were uncovered. The sub-grain boundaries formed in prior cold deformation stimulate the nucleation of DRX grains and twins; meanwhile, the driving force for grain boundary migration is enhanced due to prior stored energy. Then, DRX is activated in advance and occurs more completely, thereby promoting the formation of Σ3n twin boundaries.

Clarification of Temperature Field Evolution in Large-Scale Electric Upsetting Process of Ni80A Superalloy through Finite Element Method.

Electric upsetting has been widely employed to manufacture the preformed workpiece of large-scale exhaust valves. The temperature field in the electric upsetting process plays an important role in microstructure evolution and defect formation. In order to uncover the temperature evolution in a larger-scale electric upsetting process, the electric-thermal-mechanical multi-field coupling finite element model was developed to simulate the electric upsetting forming process of Ni80A superalloy. The temperature distribution characteristics and their formation mechanisms under different stages were analyzed systematically. Results indicate that at the preheating stage, the billet temperature increases from 20 °C to 516.7 °C, and the higher temperature region firstly appears at the contact surface between billet and anvil due to the combined effects of contact resistance and volume resistance. With increasing preheating time, the higher temperature region is transferred to the interior of the billet because the contact resistance is reduced with increasing temperature. As for the forming process, the billet is gradually deformed into an onion shape. The highest billet temperature increases to 1150 °C and keeps relatively constant. The high temperature region always appears at the neck of the onion due to the relatively higher current density at this place. It enlarges continuously in the primary stage and intermediate stage, and then decreases at the stable deformation stage. The low temperature regions lie in the contact surface and the outer surface of the onion because a lot of heat is lost to the anvil and surroundings through thermal conduction and radiation. Finally, the established finite element model was verified by an actual electric upsetting experiment. The average relative error between simulated temperatures and experimental ones was estimated as 7.54%. The longitudinal and radial errors between simulated onion shape and the experimental one were calculated as 1.38% and 2.70%, respectively.

Description of Dynamic Recrystallization Behaviors and Grain Evolution Mechanisms during the Hot Forming Process for SAE 5137H Steel.

Describing the dynamic recrystallization (DRX) behaviors and grain evolution mechanisms in the hot forming process contributes to controlling microstructures and enhancing mechanical properties of materials. Here, the isothermal compression experiments for SAE 5137H steel were conducted under temperatures of 1123-1483 K and strain rates of 0.01-10 s-1. The DRX kinetics models, including DRX volume fraction and grain size models, and the meso-scale cellular automaton (CA) models, were established based on the obtained true stress-strain curves and microstructure observation results. In order to dynamically reveal DRX behaviors and grain morphology evolution, a multi-field and multi-scale coupling finite element (FE) model for the hot compression process was developed by embedding the solved DRX kinetics models and CA models. Results show that the DRX volume fraction and grain size increase with temperature increasing and strain rate decreasing. The DRX grains are easier to nucleate at the initial grain boundaries. As strain increases, DRX grains grow up by devouring the matrix grains until DRX occurs completely. The microstructures after compression are composed of equiaxed DRX grains. Finally, the comparisons of grain size between experimental results and simulation results were performed. The mean relative errors between experimental results and predicted results from DRX kinetics models, and between experimental results and predicted results from CA models, were evaluated as 6.5% and 6.0%, respectively. It proves that the developed FE model can well describe the microstructure evolution in the hot deformation process of SAE 5137H steel.

Response Surface of Speed-Loading Path to Grain Refinement during Current-Heating Compression at SAE 5137H Steel.

In thermal deformation of materials, grain refinement induced by dynamic recrystallization (DRX) is often pursued to obtain excellent mechanical properties. Here, the thermal deformation behaviors of SAE 5137H steel were investigated and characterized at temperature and strain rate range of 1123-1483 K and 0.01-10 s-1. Meanwhile, a design approach in speed-loading paths for grain refinement during current-heating compression was proposed, and these paths are linked to a typical three-dimensional (3D) response surface. Depending on the acquired stress-strain curves, the flow behaviors of this steel were analyzed and the typical 3D processing map was constructed to clarify the stable processing parameter domains during the continuous deformation process. Then, by the typical 3D processing map and microstructure observation, the 3D deformation mechanism map was constructed to connect the processing parameters and microstructural mechanisms. Subsequently, the 3D activation energy map was constructed to evaluate these deformation mechanisms, and the enhanced deformation mechanism map was constructed. Eventually, based on the enhanced deformation mechanism map, the speed-loading paths for SAE 5137H steel during current-heating compression were designed and they are mapped in a 3D response surface.

Wear-Resistance Improvement of 65Mn Low-Alloy Steel through Adjusting Grain Refinement by Cyclic Heat Treatment.

Refined microstructures achieved by cyclic heat treatment significantly contribute to improving the wear resistance of steels. To acquire the refined microstructures of 65Mn low-alloy steel, first, the specimens were solid solution-treated; then, they were subjected to cyclic heat treatment at cyclic quenching temperatures of 790-870 °C and quenching times of 1-4 with a fixed holding time of 5 min. The mechanical properties of 65Mn low-alloy steel in terms of hardness, tensile strength, elongation and wear resistance were characterized. Afterwards, the effect of cyclic heat treatment on microstructure evolution and the relationships between grain refinement and mechanical properties' improvement were discussed. The results show that the average grain size firstly decreased and then increased with the increase in the quenching temperature. Hardness increased with grain refinement when the temperature was lower than 830 °C. Once the temperature exceeded 830 °C, hardness increased with the temperature increase owing to the enrichment of carbon content in the martensite. With the increase in cyclic quenching times, hardness continuously increased with grain refinement strengthening. In addition, both tensile strength and elongation could be significantly improved through grain refinement. The relationships among wear loss, hardness and average grain size showed that wear resistance was affected by the synthesis reaction of grain refinement and hardness. Higher hardness and refined grain size contributed to improving the wear resistance of 65Mn low-alloy steel.

An Innovative Approach of Parameter Loading Path Design for Grain Refinement and Its Application in Ni80A Superalloy.

In order to obtain the desired mechanical properties of products, an innovative method of loading parameter designs for acquiring the desired grain refinement is proposed, and it has been applied in the compression process of Ni80A superalloy. The deformation mechanism maps derived from processing maps based on the Dynamic Materials Model (DMM) theory were constructed, since the critical indicator values corresponding to dynamic recrystallization (DRX) and dynamic recovery (DRV) mechanisms were determined. The processing-parameter domains with DRX mechanisms were separated from the deformation mechanism map, while such domains were chaotic and difficult to apply in innovative parameter loading path design. The speed-loading path derived from strain rate-loading path in a compression process was pursued. The grain refinement domains are discretized into a finite series of sub-domains with clear processing parameters, and the optimal strain rate of each sub-domain is determined by step-by-step finite element simulation. A 3D response surface of the innovative optimal loading path of strain rate was fitted by interpolating methods. Finally, the isothermal compression experiments for Ni80A superalloy were conducted, and the microstructure observations indicated that the desired grain refinement was achieved. This innovative method of parameter loading path design contributes to the microstructure adjustment of the alloys with DRX mechanism.

A comparative study on improved Arrhenius-type and artificial neural network models to predict high-temperature flow behaviors in 20MnNiMo alloy.

The stress-strain data of 20MnNiMo alloy were collected from a series of hot compressions on Gleeble-1500 thermal-mechanical simulator in the temperature range of 1173 ∼ 1473 K and strain rate range of 0.01 ∼ 10 s(-1). Based on the experimental data, the improved Arrhenius-type constitutive model and the artificial neural network (ANN) model were established to predict the high temperature flow stress of as-cast 20MnNiMo alloy. The accuracy and reliability of the improved Arrhenius-type model and the trained ANN model were further evaluated in terms of the correlation coefficient (R), the average absolute relative error (AARE), and the relative error (η). For the former, R and AARE were found to be 0.9954 and 5.26%, respectively, while, for the latter, 0.9997 and 1.02%, respectively. The relative errors (η) of the improved Arrhenius-type model and the ANN model were, respectively, in the range of -39.99% ∼ 35.05% and -3.77% ∼ 16.74%. As for the former, only 16.3% of the test data set possesses η-values within ± 1%, while, as for the latter, more than 79% possesses. The results indicate that the ANN model presents a higher predictable ability than the improved Arrhenius-type constitutive model.

Evaluation of varying ductile fracture criteria for 42CrMo steel by compressions at different temperatures and strain rates.

Fracturing by ductile damage occurs quite naturally in metal forming processes, and ductile fracture of strain-softening alloy, here 42CrMo steel, cannot be evaluated through simple procedures such as tension testing. Under these circumstances, it is very significant and economical to find a way to evaluate the ductile fracture criteria (DFC) and identify the relationships between damage evolution and deformation conditions. Under the guidance of the Cockcroft-Latham fracture criteria, an innovative approach involving hot compression tests, numerical simulations, and mathematic computations provides mutual support to evaluate ductile damage cumulating process and DFC diagram along with deformation conditions, which has not been expounded by Cockcroft and Latham. The results show that the maximum damage value appears in the region of upsetting drum, while the minimal value appears in the middle region. Furthermore, DFC of 42CrMo steel at temperature range of 1123~1348 K and strain rate of 0.01~10 s(-1) are not constant but change in a range of 0.160~0.226; thus, they have been defined as varying ductile fracture criteria (VDFC) and characterized by a function of temperature and strain rate. In bulk forming operations, VDFC help technicians to choose suitable process parameters and avoid the occurrence of fracture.