Showing papers on "Strain rate published in 2022"

Additive manufacturing – A review of hot deformation behavior and constitutive modeling of flow stress

Abstract: Hot working, as an important group of post-processing routes for additive manufacturing technology (3D printing), is used to reduce the solidification/processing defects and anisotropy of properties, grain refinement, improvement of mechanical properties, processing of pre-formed parts, and increasing the applicability domain. Accordingly, the present state of the art of the elevated temperature deformation behavior and constitutive description of flow stress during thermomechanical processing of additively manufactured parts is summarized in this monograph. Besides the effects of temperature and strain rate (represented by the Zener-Hollomon parameter), the significance of initial phases and the type of additive manufacturing process on the hot deformed microstructure, restoration processes of dynamic recovery (DRV) and dynamic recrystallization (DRX), flow stress, workability, and hot deformation activation energy is critically discussed. In this regard, the α'-martensite in Ti-6Al-4V titanium alloy produced by selective laser melting (SLM), the precipitates in aluminum alloys (such as 2219 Al alloy) produced by wire and arc additive manufacturing (WAAM), and the Laves phase in Inconel 718 superalloy produced by laser metal deposition (LMD) are remarkable examples. The utilization of innovative methods with in situ hot working effects such as additive friction stir deposition (AFSD) is also enlightened. Regarding the constitutive equations for modeling and prediction of hot flow stress, the reports on the strain-compensated Arrhenius model, artificial neural network (ANN) approach, DRX/DRV kinetics models, Johnson-Cook equation, and Fields-Backofen formula are presented, and the potentials of the modified, simplified, and physically-based approaches are discussed. Finally, the future prospects in this research field such as the hybridization of additive manufacturing with hot forming processes, work-hardening analysis for obtaining the onset of DRX, unraveling the effects of as-built microstructure, developing processing maps, proposing some physical-based unified constitutive models, and investigation of novel and/or widely-used alloys such as austenitic stainless steels, high-entropy alloys, and aluminum alloys (e.g. AlSi10Mg alloy) are proposed.

Experimental Investigation on Fracture Properties of HTPB Propellant with Circumferentially Notched Cylinder Sample

Abstract: To comprehensively understand the fracture properties of solid propellants, the mode I tensile fracture toughness tests at different temperatures (253.15–343.15 K) and loading rates (2–500 mm/min) were conducted on the newly designed circumferentially notched cylinder sample with different initial crack sizes (4.5 and 6.5 mm) for hydroxyl-terminated polybutadiene (HTPB) propellant. Test results reveal that the shape of the tensile fracture stress-strain curves was not significantly influenced by temperature, loading rate and the initial crack size. Higher loading rate and lower temperature can lead to a rise in the tensile fracture toughness of HTPB propellant described by the stress intensity factor, however, continuously increasing loading rate cannot dramatically improve this fracture toughness beyond the rate of 250 mm/min. In addition, the fracture toughness is more sensitive to temperature. Furthermore, the variation of the initial crack size causes obvious changes in the fracture toughness with the coupled effects of low temperature and higher loading rates. At these test conditions, a higher tensile fracture toughness can be obtained with the shorter initial crack. For all the test conditions, there is a linear rise in the strain corresponding to the fracture toughness as temperature increases. Meanwhile, this strain increases with the shorter initial crack. Whereas, the effect of loading rate on this strain is complex. Based on the time-temperature superposition principle (TTSP), the master curves with a log-curvilinear form were constructed to predict the mode I tensile fracture toughness of the propellant at different initial crack sizes in a wide range of loading conditions.

Effect of grain size on strength and strain rate sensitivity in metals

Abstract: The effect of the grain size on the mechanical properties of metallic materials has been a topic of significant interest for researchers and industry. For many decades, a relationship defining the mechanical strength proportional to the inverse of the square root of the grain size has been widely accepted despite some reports of deviations from this behavior. Nevertheless, the initial explanations for this relationship, based mainly on the activation of slip systems by dislocation pileups at grain boundaries, have provided essentially no predictive capability. Here, we show that a physically based model for grain boundary sliding predicts, in excellent agreement with experimental data, the flow stress for plastic deformation for a broad range of materials using the fundamental properties of each material over a wide range of grain sizes and testing conditions. This mechanism also successfully predicts the reported enhanced strain rate sensitivity in ultrafine and nanocrystalline materials at different temperatures.

Modelling of the Steel High-Temperature Deformation Behaviour Using Artificial Neural Network

Abstract: Hot forming is an essential part of the manufacturing of most steel products. The hot deformation behaviour is determined by temperature, strain rate, strain and chemical composition of the steel. To date, constitutive models are constructed for many steels; however, their specific chemical composition limits their application. In this paper, a novel artificial neural network (ANN) model was built to determine the steel flow stress with high accuracy in the wide range of the concentration of the elements in high-alloyed, corrosion-resistant steels. The additional compression tests for stainless Cr12Ni3Cu steel were carried out at the strain rates of 0.1–10 s−1 and the temperatures of 900–1200 °C using thermomechanical simulator Gleeble 3800. The ANN-based model showed high accuracy for both training (the error was 6.6%) and approvement (11.5%) datasets. The values of the effective activation energy for experimental (410 ± 16 kJ/mol) and predicted peak stress values (380 ± 29 kJ/mol) are in good agreement. The implementation of the constructed ANN-based model showed a significant influence of the Cr12Ni3Cu chemical composition variation within the grade on the flow stress at a steady state of the hot deformation.

Effect of solid-solution strengthening on deformation mechanisms and strain hardening in medium-entropy V1-Cr CoNi alloys

Abstract: High- and medium-entropy alloys (HEAs and MEAs) possess high solid-solution strength. Numerous investigations have been conducted on its impact on yield strength, however, there are limited reports regarding the relation between solid-solution strengthening and strain-hardening rate. In addition, no attempt has been made to account for the dislocation-mediated plasticity; most works focused on twinning- or transformation-induced plasticity (TWIP or TRIP). In this work we reveal the role of solid-solution strengthening on the strain-hardening rate via systematically investigating evolutions of deformation structures by controlling the Cr/V ratio in prototypical V1-xCrxCoNi alloys. Comparing the TWIP of CrCoNi with the dislocation slip of V0.4Cr0.6CoNi, the hardening rate of CrCoNi was superior to slip-band refinements of V0.4Cr0.6CoNi due to the dynamic Hall-Petch effect. However, as V content increased further to V0.7Cr0.3CoNi and VCoNi, their rate of slip-band refinement in V0.7Cr0.3CoNi and VCoNi with high solid-solution strength surpassed that of CrCoNi. Although it is generally accepted in conventional alloys that deformation twinning results in a higher strain-hardening rate than dislocation-mediated plasticity, we observed that the latter can be predominant in the former under an activated huge solid-solution strengthening effect. The high solid-solution strength lowered the cross-slip activation and consequently retarded the dislocation rearrangement rate, i.e., the dynamic recovery. This delay in the hardening rate decrease, therefore, increased the strain-hardening rate, results in an overall higher strain-hardening rate of V-rich alloys.

Effects of fibres on ultra-lightweight high strength concrete: Dynamic behaviour and microstructures

Abstract: The use of ultra-lightweight high strength concrete (ULHSC) for prefabricated structures has been recognized as a promising method of construction. Adding fibres commonly enhances the mechanical properties of ULHSC but increases its density. In order to balance the strength and self-weight of ULHSC with different fibres, an extensive investigation was carried out to understand the effects of fibre incorporation on ULHSC made with fly ash cenospheres. This study focuses on the dynamic compressive response and failure mechanism of ULHSC with varied fibre contents. Impact tests were conducted by using a Φ100-mm splitting Hopkinson pressure bar apparatus with different strain rates ranging from approximately 20 s−1 to 120 s−1. The results showed that the compressive properties of ULHSC exhibited a strong strain rate dependency, and the addition of fibres increased the strain rate sensitivity of ULHSC, especially at high strain rates. It was interesting to find that the dynamic compressive strength increased with increasing fibre content, and the dynamic increase factor (DIF) showed the same tendency. These findings indicated that the tested ULHSC with 1 vol% end-hooked steel fibre maintained an excellent balance between the dynamic strength and density. Finally, a recalibrated model considering the effect of fibres was proposed to predict the DIFs of ULHSCs under impact loads.

Dynamic compressive properties and underlying failure mechanisms of selective laser melted Ti-6Al-4V alloy under high temperature and strain rate conditions

Abstract: In this study, high speed impacting tests are systematically conducted on a split Hopkinson pressure bar device to investigate the strain rate and temperature dependence of dynamic compressive properties of Ti-6Al-4 V (TC4) fabricated by selective laser melting (SLM), the ranges of strain rate and temperature are 2000–6000/s and 25–650 ℃, respectively. The results reveal that the yield strength and ultimate compressive strength of the SLM-TC4 alloy increase with the increasing strain rate and the decreasing temperature, showing obvious strain rate and temperature sensitivities. The high speed impacting load intensifies the texture of the SLM-TC4 alloy significantly. Adiabatic shear band (ASB) is more likely to evolve at higher temperatures and strain rates, submicron equiaxed grains formed in the ASB and the surrounding area are mainly ascribed to the combination of dynamic recrystallization, deformation-induced twinning and transverse α-lath splitting. Within the ASB, grains with {0001} pole orientation are rotated by approximately 45° with respect to the shear direction, indicating that the recrystallized grains are able to reorient themselves to accommodate to the shear deformation. The findings in this work provide a theoretical basis to understand the deformation behavior and mechanism of SLM-TC4 alloy under impacting loads, thus is helpful to widen the application of SLM technique and products.

Effect of Ce addition on hot deformation behavior and microstructure evolution of AZ80 magnesium alloy

Abstract: Hot compression tests of AZ80 and AZ80-Ce alloys were conducted on a Gleeble-3800 simulator at strain rates of 0.01–10 s−1 and deformation temperatures of 340–420 °C. The effects of Ce addition on hot deformation behavior and microstructure evolution of AZ80 magnesium alloys were investigated. The rare earth phase formed with added Ce hindered movement of the dislocations and grain boundaries, causing higher peak stress of AZ80-Ce alloy than that of AZ80 alloy. Existing large-size Al4Ce particles as second phase in AZ80-Ce alloy increased the driving force of recrystallization, promoting dynamic recrystallization and reducing dislocation density. After hot compression, AZ80 alloy showed a fiber texture with the c-axis parallel to the compression direction, and addition of Ce caused texture-weakening. The constitutive equations and processing maps of AZ80 and AZ80-Ce alloys were constructed and analyzed. Adding Ce reduced deformation activation energy and Zener-Hollomon parameter, and enlarged non-instability zone in processing maps of AZ80 alloy.

Modeling Dynamic Recrystallization Behavior in a Novel HIPed P/M Superalloy during High-Temperature Deformation

Abstract: The dynamic recrystallization (DRX) features and the evolution of the microstructure of a new hot isostatic pressed (HIPed) powder metallurgy (P/M) superalloy are investigated by hot-compression tests. The sensitivity of grain dimension and DRX behavior to deformation parameters is analyzed. The results reveal that the DRX features and grain-growth behavior are significantly affected by deformation conditions. The DRX process is promoted with a raised temperature/true strain or a reduced strain rate. However, the grains grow up rapidly at relatively high temperatures. At strain rates of o.1 s−1 and 1 s−1, a uniform microstructure and small grains are obtained. Due to the obvious differences in the DRX rate at various temperatures, the piecewise DRX kinetics equations are proposed to predict the DRX behavior. At the same time, a mathematical model for predicting the grain dimension and the grain growth behavior is established. To further analyze the DRX behavior and the changes in grain dimension, the hot deformation process is simulated. The developed grain-growth equation as well as the piecewise DRX kinetics equations are integrated into DEFORM software. The simulated DRX features are consistent with the test results, indicating that the proposed DRX kinetics equations and the established grain-growth model can be well used for describing the microstructure evolution. So, they are very useful for the practical hot forming of P/M superalloy parts.

Dynamic recovery and recrystallization mechanisms in secondary B2 phase and austenite matrix during hot deformation of Fe–Mn–Al–C-(Ni) based austenitic low-density steels

Abstract: In present study two different austenitic based low-density steel (LDS) grades with compositions of Fe–28Mn–9Al-0.9C and Fe–15Mn–9Al-0.9C–5Ni (in weight percent) were produced by vacuum induction melting. The former alloy was a single-phase austenitic grade, whereas the latter alloy contained coarse B2 ordered Fe(Ni)Al secondary phase along with fine κ-carbides in austenite matrix. Hot deformation behaviour of these two LDS grades was studied in as-cast plus homogenized condition under compression over a range of temperature (1173–1423 K) and strain rate (0.005–5 s−1). The stress-strain curves were analysed to evaluate the role of secondary B2 phase on the stress exponent and activation energy of deformation. The evolution of microstructures in these steels as a function of test temperature, strain rate and strain was examined in-detail by electron backscattered diffraction. Both the low-density steels exhibited nearly similar stress exponent value of ∼4; however, the presence of B2 phase led to an increased activation energy of deformation. The effect of B2 phase on flow behaviour and microstructure evolution during hot deformation was found to be more pronounced at low test temperatures. The B2 containing steel showed slower recrystallization kinetics than that of B2-free alloy and is presumably due to the Ni addition, the presence of undissolved κ-carbides and the strain rate-temperature dependent strain partitioning between γ and B2 phases in the former alloy. The examination of microstructural evolution with progress of deformation revealed that the austenite (γ) matrix grains undergo discontinuous dynamic recrystallization, while B2 grains display continuous dynamic recrystallization along with fragmentation.