In this work, the hot deformation behavior of Alloy 925 with intermediate strain rates of 0.01s−1-10s−1 ranging from 900 °C to 1150 °C has been investigated through the isothermal hot compression tests. Some features of the flow curves corresponding to negative strain rate sensitivity have been observed and discussed associated with the microstructural evolution, showing the nucleation mechanism of dynamic recrystallization and the occurrence of dynamic strain ageing. Besides, we have employed two typical constitutive models, namely, Arrhenius model and backpropagation artificial neural network (BP ANN) model to describe the flow behavior, and also developed a modified BP ANN model based on genetic algorithm (GA-BP ANN). The results show that the GA-BP ANN model has the highest accuracy and stability for predicting the flow stress. The correlation coefficient between the predicted and experimental values is 99.99 %, and the average absolute relative error is only 0.54 %. The comparative investigation on the predicted values of different ANN models reflects that GA can reduce the randomness of initial weights and thresholds of BP ANN and also can further increase the accuracy and stability of ANN model. Moreover, the increasing number of input training data can improve the prediction performance of neural network. For a single-layer neural network, 12 hidden layers can effectively ensure the reliability of the constitutive model.

Dynamic behavior and modified artificial neural network model for predicting flow stress during hot deformation of Alloy 925

The mechanism of dynamic strain aging for type A serrations in tensile flow curves of Fe-18Mn-0.55C (wt.%) twinning-induced plasticity steel

This review article provides an in-depth discussion on the hot deformability, processing map analysis, and microstructural development in superalloys, as well as the associated constitutive equations employed in flow stress prediction. It describes how hot working can improve the grain structure of superalloys by dynamic recrystallization (DRX), reduce defects, and enhance their mechanical characteristics. The formation of necklace structures, work-hardening analysis to identify the existence and commencement of dynamically recrystallized grains and the influence of processing conditions on DRX grain size are all addressed in this article. The influence of deformation variables, described by the Zener-Hollomon parameter - the occurrence of phases, dynamic precipitation, and alloying elements on the thermomechanical response, and the restoration processes of DRV and DRX are discussed in detail. The utilisation of processing maps as a means to determine the most favourable processing conditions and identify the instability regime, encompassing flow instability, defects and cracking, that may arise during the hot-working of superalloys are discussed. Specifically concerning the constitutive modelling of flow stress for characterizing material flow (at various deformation strain rates, temperatures, and strain), the application threshold stress (as a result of phase transformation during hot deformation), temperature-dependent Young's modulus, and comparing the experimentally observed activation energy and deformation stress exponent to the values predicted by creep theories, are discussed. Also, an analysis of the various modelling techniques and equations for predicting flow curves during hot-working processing is evaluated. Finally, some recommendations are made regarding the potential future research directions. 

Hot deformation behaviour, constitutive model description, and processing map analysis of Superalloys: An overview of nascent developments

High-temperature tribology, which is often involved during hot metal forming, is controlled via oxidation on a rubbing surface. However, for high chromium stainless steel (ST), where oxidation is strongly inhibited, the effect of counterface materials on tribological behavior is yet to be elucidated. In this study, the effects of counterfaces on the tribological behavior of 253MA ST and mild steel (MS) are investigated via a ball-on-disc test at 900 °C using a 20 N load. The results reveal that high-speed steel (HSS) experiences severe abrasive wear with MS and causes severe sticking problems with ST. Si3N4 and SiC present substantially stronger abrasive wear resistance than HSS with MS, and the friction coefficients are dependent on the type of ceramic. Both ceramics can facilitate the establishment of a thick tribo-oxide layer (> 3 µm) on ST to prevent sticking; however, this is accompanied by severe pull-out and fracture wear. The effects of the counterface on the mechanical properties of the tribo-oxide layer, near-surface transformation, and the responses of the tribo-oxide layer to friction and wear are discussed. This study contributes to the understanding of interfacial tribological behaviors when different types of tools are used on MS and ST.

/pdf/tribological-behavior-comparisons-of-high-chromium-stainless-3508ka2iqm.pdf

Tribological behavior comparisons of high chromium stainless and mild steels against high-speed steel and ceramics at high temperatures

This study compares the tensile response of laser powder bed fusion (LPBF) fabricated Hastelloy X (HX) coupons with those of conventionally manufactured HX over the temperature range of 22-800°C and strain rate of 10−4-10−2 s−1. Comparable ultimate tensile strength and higher yield strengths were observed for LPBF HX. The stress-strain response of LPBF HX showed pronounced evidence of dynamic strain aging (DSA) during tensile testing at temperatures from 300°C to 600°C and also limited evidence of DSA at 200° and 700°C. The transition from type-A/B to type-C DSA for LPBF HX happens at lower temperatures than that for conventionally manufactured HX. Furthermore, the activation energy for DSA is 66 kJ/mol for LPBF HX, which is lower than the reported 80-130 kJ/mol for conventionally manufactured HX. The observed differences in the tensile and DSA responses of the two variants of HX were attributed to the differences in their dislocation microstructure, observed through scanning transmission electron microscopy. 

Effects of temperature and strain rate on tensile properties and dynamic strain aging behaviour of LPBF Hastelloy X

In this research, we performed an experimental investigation into the influence of dynamic strain aging (DSA) and precipitation behavior on the plastic deformation and microstructure evolution in alloy 253MA considering its service temperature. The serrated flow behavior, stress anomaly behavior and negative strain rate sensitivity are considered as the manifestation of DSA within a certain temperature range (TDSA), which can be classified as low (550–700 °C), intermediate (600–750 °C) and high temperatures (700–790 °C) at 0.001 s−1, 0.01 s−1 and 0.1 s−1, respectively. The activation energy of DSA was determined to be 242 kJ/mol, indicating that the diffusion of substitutional solutes, such as Cr and Ni, is mainly responsible for the DSA phenomenon. The characteristic of dislocation substructures is prone to change from linear structure to cellularization structure with a rising deformation temperature in the DSA regime. The stress anomaly behavior can be attributed to the rapid dissolution of σ phase. The initiation of dynamic recrystallization should contribute to the disappearance of DSA by suppressing the dynamical solute atom–dislocation interaction.

Effect of dynamic strain aging and precipitation on the hot deformation behavior of 253MA heat-resistant alloy

The invention belongs to the technical field of electrolytic polishing, and discloses an electrolytic polishing method of an Al/Ti cold rolled composite plate. Through the novel electrolytic polishingmethod, a sample is clamped by using a clamping tool to be placed into a Stael Lectropol-5 electrolytic polishing instrument, the control speed flow is 12, the voltage is 10-15 V, the temperature is2-5 DEG C, and the time is 100-120 s, and the sample is washed with the water after polishing, moisturized with the alcohol, and dried by cold wind blowing; and analysis and calibration of the electronic backscatter diffraction (EBSD) are conducted. The surface of the Al/Ti cold rolled composite plate after electrolytic polishing is bright as a mirror and has no obvious corrosion spots and other defects, when the EBSD is conducted, the EBSD calibration can be conducted on the Al layer and Ti layer on the two sides of the interface simultaneously, and the calibration rate is very high and can reach more than 99%.

Electrolytic polishing method of Al/Ti cold rolled composite plate

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Effect of dynamic strain aging and precipitation on the hot deformation behavior of 253MA heat-resistant alloy

Electrolytic polishing method of Al/Ti cold rolled composite plate