Adiabatic Shear Research Articles

Crystallographic orientation dependence of dynamic deformation behaviours in additively manufactured stainless steel

This study reveals the dynamic compressive deformation anisotropy among 316L stainless steel fabricated via wire and arc additive manufacturing process(WAAM). Cylindrical samples extracted from as-built WAAM 316L plate with 35°, 45°, 55°and 90° from scanning direction, were referred to as <111>, <110>, <112> and <100> crystallographic orientations hereinafter, which had been confirmed through backscattered diffraction. Cylindrical samples were subjected to dynamic tests by a split Hopkinson pressure bar. The corresponding mechanical behaviors and deformed microstructures are associated. There are approximately 10% higher dynamic flow stresses from the <111> samples than other samples during dynamic tests at strain rates less than 3500 s−1, which can be attributed to lower density of thinner deformation twins. The lower tendency towards deformation twinning in <111> samples arises from the increase of effective stacking fault energy. WAAM 316L presents an anisotropy of resistance to adiabatic shearing, of which the order is < 100>, <110>, <112> and <111>. Two adiabatic shear bands(ASB) in <111> sample present distinct orientation characters, where the sub-grains in the first ASB show <111> and <112>-fiber while that in the subsequent one take <110>-fiber. This micro-texture discrepancy from two ASBs could be contributed to the precondition change of macro texture in the matrix which give birth to.

The effect of thermal and strain-induced aging on the mechanical behavior of room temperature ECAP processing of WE43 magnesium alloy

The WE43 magnesium alloy, in case of improved toughness, can be an appealing material for biomedical applications. The effects of equal-channel angular pressing (ECAP) and post-deformation aging (PDA) on the mechanical behavior of the WE43 alloy were investigated in this study. ECAP of WE43 magnesium alloy at room temperature is extremely challenging because of the insufficient number of slip systems. An optimized core–sheath configuration is proposed, which applies fully compressive stress on the WE43 alloy as the core, resulting in ECAP of the WE43 alloy at room temperature. Unprecedented strain-induced precipitation of both Mg41Nd5 and Mg24Y5 was observed owing to stored mechanical energy and temperature raise in the adiabatic shear bands during ECAP at room temperature. Moreover, the formation of oriented needle-like Mg3(Y, Nd) precipitates and partial recrystallization simultaneously increased the strength to 410 MPa and the strain to fracture to 11.4% after PDA. A combination of bimodal grain size distribution and a high volume fraction of precipitates increased the toughness to 38.96 MJ/m3 after ECAP and PDA.

Effects of process parameters and loading direction on the impact strength of additively manufactured 18%Ni-M350 maraging steel under dynamic impact loading

The microstructural and damage evolution of 18%Ni M350 maraging steel fabricated by laser-based directed energy deposition (DED) process under dynamic impact loading are investigated in this study. The influence of additive manufacturing (AM) processing parameters, including laser power, powder feed rate, and energy area density (EAD) are discussed. The mechanical testing of the additively manufactured alloy was done using the split Hopkinson pressure bar (SHPB) system. The dynamic impact test was conducted using strain rates ranging from 100 to 4000 s−1. The test was conducted on cylindrical specimens (7 mm diameter and 8 mm long) with axes parallel to the build direction (Z-axis) and perpendicular (X- and Y-axis) to the build direction to evaluate the effects of loading directions on mechanical responses and microstructural evolution in the alloy under the impact loading conditions. The test results revealed that the materials processed using the lowest EAD parameters exhibit the greatest impact strength under dynamic impact loading for all directions. Specimens impacted along the build direction (Z-axis) exhibit a continuous drop in peak flow stress once the strain rates exceeded 2300 s−1 indicating the greatest susceptibility to thermal softening at high strain rates. Strain localization leading to the formation of adiabatic shear bands (ASB) was observed in some specimens. Cracks developed along the ASBs, while some AM-process-related defects, such as the lack-of-fusion, interacted with adiabatic shear bands, promoting cracking inside the shear bands. Despite possessing the highest impact strength, the materials produced by the lowest EAD parameters are the most susceptible to the development of adiabatic shear bands leading to cracking and fracture.

Evading dynamic strength and ductility trade-off in a high-entropy alloy via local chemical ordering

Strength-ductility trade-off in metals is sever under dynamic loading due to the strain rate effect and adiabatic shear failure. Here, we demonstrate that both strength and ductility progressively increase with increasing strain rate in a body-centered cubic TiZrNbTa refractory high-entropy alloy. We find that a prominent strain rate effect occurs, with a yield strength of 1879 ± 10 MPa at a strain rate of 6500 s−1, which is double that compared to a strain rate of 10−3 s−1. Simultaneously, Zr- and (Nb, Ta)-enriched local chemical ordering stimulates dislocation slip, enhancing homogeneous deformation capacity and adiabatic shear resistance under high strain rates. These findings suggest the importance of local chemical ordering to the dynamic properties of high-entropy alloys, and offer a way to develop metallic materials with improved dynamic mechanical properties.

Square L12 precipitates inducing high susceptibility to adiabatic shear in 3D printed fine-grained tungsten heavy alloy with high-entropy alloy matrix

Tungsten heavy alloys with a L12 precipitation-hardening high-entropy alloy matrix exhibit high susceptibility to adiabatic shear, but the relationship between the size and morphology of L12 precipitates and the adiabatic shear behavior is not well understood. This work systematically studies the effect of different size and morphology L12 precipitates on the mechanical properties and susceptibility to adiabatic shear of the tungsten heavy alloy with an AlCrFeNiV-system high-entropy alloy matrix (W-HEA). The results show that the W-HEA with square L12 precipitates has tensile strength of 1300 MPa and dynamic compression strength of 2100 MPa at a stain rate of 6000 s−1. The higher interfacial energy of square L12 precipitates leads to the faster re-dissolution of square L12 precipitates into the matrix than ultrafine and lath-like L12 precipitates under temperature rise during shearing, which can sharply soften the shearing location and contribute to the narrowest adiabatic shear band (ASB) with width of 15 μm.

Effect of stress state on adiabatic shear banding in Al2024-T351

Cubic uniaxial compression and angled cubic biaxial shear-compression specimens are a preferable choice in the context of developing empirical data for models such as the Generalized Incremental Stress State Dependent Damage Model (GISSMO), with a strain-rate dependent extension. The angled cubic specimens introduce a multiaxial stress state of combined shear and compression. Al2024-T351 specimens have been impacted using a direct impact Hopkinson bar coupled with Digital Image Correlation (2D-DIC). 2D-DIC has been used to track the full-field strain evolution of all specimens and obtain the fracture strains for fractured specimens. The shear strain evolution related to failure along the shear planes can be acquired using 2D-DIC, and a quantitative measurement of fracture initiation is acquired from the DIC data. It is also highlighted that the transverse displacement contours can offer qualitative information about the effects of friction on proportional loading during uniaxial compression. Interrupted tests using stop rings are conducted for angled cubic specimens to identify the critical strains for the initiation of adiabatic shear bands (ASBs), which can be defined as the GISSMO instability strains. ASBs are confirmed using the optical microscope, and it is found that for a constant impact momentum, the specimens with a greater shear to compressive stress state exhibit a higher tendency to form ASBs at lower major axial strains. It is also found that ASBs are observed using the optical microscope with no loss of flow stress, and that angled specimens exhibit a lower flow stress than uniaxial compression specimens at higher strain rates. Lastly, pre-impact and non-fractured post-impact Vickers microhardness tests have been conducted, and it is found that for all specimens, the material is on average about 20% harder after impact in regions away from the ASB, and further that regardless of stress-state, the ASB is consistently about 30% harder than the pre-impact microstructure.

Effect of deep cryogenic treatment on the microstructural, mechanical and ballistic properties of AA7075-T6 aluminum alloy

The study focused on investigating the effect of Deep Cryogenic Treatment (DCT) on the mechanical and ballistic properties of AA7075-T6 aluminum alloy. The microstructure, microhardness, tensile strength, and impact strength of the Base Material (BM) and DCT-treated 7075 samples were analyzed through metallographic analysis and mechanical tests. The microstructure of the DCT-treated 7075 samples revealed fine grains and a distribution of secondary phase particles. The tensile strength, impact strength, and microhardness of DCT-treated samples increased by 7.41%, 4%, and 9.68%, respectively, compared to the BM samples. The fractography analysis of the tensile samples showed cleavage facets, microvoids, and dimples in both the samples. The ballistic behavior of the BM and DCT target plates were studied by impacting hard steel core projectiles at a velocity of 750 ± 10 m/s. The target plates failed due to petaling and ductile hole enlargement, and the depth of penetration (DOP) of the DCT target was less than that of the BM target, indicating a higher ballistic resistance. The post-ballistic microstructure examination of the target plates showed the formation of an Adiabatic Shear Band (ASB) without any cracks. It was concluded that the DCT treatment improved the mechanical and ballistic properties of the aluminum alloy due to grain refinement and high dislocation density.

Dynamic mechanical properties of nanoparticle-enhanced aluminum alloys fabricated by arc-directed energy deposition

The nanoparticle-enhanced aluminum alloys fabricated by arc-directed energy deposition (Arc-DED) have gained much attention due to their improved microstructure and enhanced mechanical properties, and are expected to be applied in aerospace, defense, and marine fields. The dynamic properties of nanoparticle-enhanced aluminum alloys are required in the aforementioned fields. In this work, the dynamic behaviors of AA7075 and TiC/AA7075 samples fabricated by Arc-DED in as-built and heat-treated states are investigated by Split-Hopkinson pressure bars tests with the strain rates of 2000–5000 s−1 at room temperature. The results show that adding TiC nanoparticles improve the dynamic properties of aluminum alloys, especially their performance after heat treatment. The improved performance can be attributed to the refined grain and the generated strengthening phase. The metallographic structure shows that the adiabatic shear band (ASB) is first generated with an increase in the strain rate. Then, cracks appear along ASB, leading to the final shear failure. A modified Johnson-Cook constitutive model is proposed based on the strain softening and strain rate softening effects. The results obtained by the proposed model agree with the experimental results. This work is the first to study the dynamic behavior of aluminum alloys fabricated by Arc-DED, which has potential value for applicating aluminum alloys in various industries.

Microstructure evolution and characterization of the adiabatic shear bands in AZ31 magnesium alloy

The diffuse adiabatic shear bands (ASBs) of the AZ31 magnesium alloy under high-strain-rate compression are systematically studied. Dynamic compression experiments at a strain rate of 1200 s−1 were conducted using a split Hopkinson pressure bar, and the microstructure of the specimen was characterized via optical microscopy, electron backscatter diffraction, and transmission electron microscopy. The results show that the microstructure after high-strain-rate deformation can be divided into three regions, namely the matrix region, transition region, and ASB region, and the corresponding grain crystal orientations are random, textured, and random, respectively. The microstructure exhibits high-density twins after high-strain-rate deformation. The dynamic recrystallization (DRX) of the ASB region is controlled by the combination of the twinning-induced DRX, continuous DRX, and discontinuous DRX.

A reformulated non-ordinary state-based peridynamic method for dynamic failure of ductile materials

A non-local correspondence peridynamic model is reformulated to describe the dynamic failure of ductile materials, under the framework of non-ordinary state-based peridynamics. To eliminate the zero-energy mode as well as to improve the numerical stability, the deformation gradient is updated on the bond level, which owns information including both the non-local deformation and the bond deviation. Peridynamic thermo-viscoplastic constitutive formulations are presented for the failure analyses of ductile materials. Moreover, a mixed strain-stress criterion describing the bond breaking and considering the temperature effect is presented on the bond level, in which the bonds can keep intact in calculations. Several comprehensive cases, including a notched tension test and Kalthoff-Winkler impact tests with different velocities, are investigated to verify the applicability of the proposed model for ductile fracture and impact failure. Numerical results, in terms of failure patterns, cracking switching and propagation, as well as adiabatic shear band propagation, show a reasonable agreement with corresponding experimental observations.

Mechanical behavior of nanorubber reinforced epoxy over a wide strain rate loading

Nanorubber/epoxy composites containing 0, 2, 6 and 10 ​wt% nanorubber are subjected to uniaxial compression over a wide range of strain rate from 8 ​× ​10−4 s−1 to ∼2 ​× ​104 s−1. Unexpectedly, their strain rate sensitivity and strain hardening index increase with increasing nanorubber content. Potential mechanisms are proposed based on numerical simulations using a unit cell model. An increase in the strain rate sensitivity with increasing nanorubber content results from the fact that the nanorubber becomes less incompressible at high strain, generating a higher hydro-static pressure. Adiabatic shear localization starts to occur in the epoxy under a strain rate of 22,000 s−1 when the strain exceeds 0.35. The presence of nanorubber in the epoxy reduces adiabatic shear localization by preventing it from propagating.

Adiabatic shearing in railway wheel steel of high-speed train

Adiabatic shearing behavior in the web and the rim of high-speed train railway wheel steel was investigated using a split Hopkinson pressure bar (SHPB) with hat-shaped punch shear technique. Different initial microstructure characteristics were observed in the web and the rim specimens experiencing different heat treatment. For the first time we show that the pre-eutectoid ferrite deforms at first and pearlite interlamellar spacing has a significant influence on structural softening sequence during shear band evolution in the railway wheel steel, and shear bands are more prone to emerge in the rim.

Effect of Aging on the Ballistic Performance of AA 7017 Alloy

&#x0D; This study presents the effect of aging on microstructure, texture, mechanical and ballistic properties of AA-7017 aluminium alloy. AA-7017 alloy is subjected to three different aging namely under-aging, peak-aging and over-aging. Significant difference is observed in the mechanical properties after aging. The alloy exhibits maximum strength and hardness in the peak-aged condition. From texture measurements, it is noticed that the overall intensity of orientation distribution function increases from under-aged to peak-aged condition. The AA-7017 aged plates are impacted with 7.62 mm steel core projectiles at 820±10 m/s to evaluate the ballistic performance. It is noticed that AA-7017 alloy displays best ballistic resistance in peak-aged condition. Distorted material flow lines and adiabatic shear band induced cracking is detected in post ballistic microstructure. The ballistic performance of AA-7017 alloy is correlated with the variations in the microstructure and mechanical properties with aging&#x0D;

Primary hot working characteristics and microstructural evolution of as-cast and homogenized Ti-4Al-2.5V-1.5Fe alloy

Hot deformation characteristics of Ti-4Al-2.5V-1.5Fe-0.25O alloy in as-cast and homogenized condition was studied using isothermal hot compression tests in both β and α + β fields at strain rates ranging from 3 × 10−4 s−1 to 1 s−1. To understand the hot deformation behaviour and to identify the optimum regime for the primary breakdown of titanium alloy ingot, processing map based on strain rate sensitivity (m), kinetic analysis, and microstructural characterization were used. Micro-mechanisms of deformation in the β-field were analyzed with the help of reconstruction of parent β grain orientations using electron backscatter diffraction data. Kinetic analysis and microstructural characterization of samples deformed at low strain rates (ε̇<10−2 s−1) in β-field confirmed dynamic recovery assisted by dislocation glide/climb. In comparison, the samples deformed at higher strain rates (ε̇>10‐2s−1) in β-field exhibited deformation bands and recrystallized grains formed through static recrystallization. In the α + β field, kinetic parameters at low strain rates (ε̇<10−2 s−1) indicated dynamic globurization, which were corroborated by microstructural analysis. Samples deformed at high strain rates (ε̇>10−2 s−1) in the α + β field exhibited adiabatic shear bands, strain-induced porosity and lamellar α-kinking. The optimum regime for thermomechanical processing (900°C to 1150°C and 3ⅹ10−4 s−1 to 3ⅹ10−2 s−1) is arrived at by combining the strain rate sensitivity map and microstructural analysis. Further, constitutive equations have been developed to predict the steady-state flow stress values in β and α + β fields.

Atomistic characterization of impact bonding in cold spray deposition of copper

We investigate the cold-spray additive manufacturing process of nanometric copper particles impacting a copper substrate using molecular dynamics (MD) and focus on the interplay between particle size (2–32 nm), particle velocity (100–2800 m/s), ensuing particle penetration depth and the impact bonding energy (IBE) binding the impinging particle with the substrate. It is observed, based on trends in the IBE, that even at low particle velocities (∼100 m/s) smaller sized particles (<8 nm) bind to the substrate, while there is a size-dependent critical velocity (≥200 m/s) for binding of larger particles (≥8 nm). At velocities near and above 500 m/s, material jetting and significant increase in particle penetration into the substrate are observed for larger particles. Rapid temperature increase, accompanied by adiabatic shear banding and instability (ASI), is observed, especially at the particle-substrate interface, resulting in particle-cratering at higher velocities in the range studied. Further, while jetting and ASI are observed together at high impact velocities, simulations show that ASI is not essential for jetting, consistent with recent investigations. In addition, both ASI and jetting are not prerequisites for particle binding to the substrate. A significant observation involves self-similar variations in particle penetration depth and IBE with particle kinetic energy across all particle sizes, which is captured by a quantitative constitutive relationship governing the particle's kinetic energy, penetration depth, and IBE.

Superior dynamic shear properties by structures with dual gradients in medium entropy alloys

Structures with single gradient and dual gradients have been designed and fabricated in an Al0.5Cr0.9FeNi2.5V0.2 medium entropy alloy. Structures with dual gradients (with increasing grain size and a decreasing volume fraction of nanoprecipitates from the surface to the center) were observed to show much better dynamic shear properties compared to both structures with single grain-size gradient and coarse-grained structures with homogeneously distributed nanoprecipitates. Thus, the dual gradients have a synergetic strengthening/toughening effect as compared to the sole effect of a single gradient and the sole precipitation effect. Initiation of the adiabatic shear band (ASB) is delayed and propagation of ASB is slowed down in structures with dual gradients compared to structures with single gradients, resulting in better dynamic shear properties. A higher magnitude of strain gradient and higher density of geometrically necessary dislocations are induced in the structures with dual gradients, resulting in extra strain hardening. Higher density dislocations, stacking faults, and Lomer-Cottrell locks can be accumulated by the interactions between these defects and B2/L12 precipitates, due to the higher volume fraction of nanoprecipitates in the surface layer of the structures with dual gradients, which could retard the early strain localization in the surface layer for better dynamic shear properties.

Influence of cutting parameters on cutting specific energy of Inconel718 based on strain gradient

In this paper, the Johnson-Cook constitutive model is modified by considering the influence of hard point such as TiC and NbC in the matrix of Inconel718 on the deformation stress. The theoretical model of cutting specific energy in the main deformation zone of Inconel718 is modified based on the new constitutive model by combining the strain gradient theory. The effect of different cutting parameters on cutting specific energy is studied, and the effect of cutting specific energy on cutting deformation and the resulting dimensional effect are also analyzed. The research results show that cutting specific energy increases with the increase of cutting speed. With the increase of cutting thickness, the cutting specific energy reduced, and the trend is non-linear. The change of undeformed chip thickness will cause size effect. The cutting specific energy increases with the reduction of the thickness of undeformed chip, and the impact of the thickness of undeformed chip on the cutting specific energy becomes smaller and smaller as the speed increases. The existence of hard points makes the main deformation zone generate a large amount of heat energy and deformation energy, which leads to dimensional effects and makes the material be more prone to adiabatic shear instability, then leads to the increase of cutting specific energy. With the increase of cutting specific energy, the width of adiabatic shear band is narrowed, the degree of serration is aggravated, and the chip morphology is closer to the experimental results.

Specificity of the stress-strain and thermodynamic state simulation of the titanium alloy workpiece in the cutting zone

Titanium alloys have specific mechanical and physical-chemical properties that significantly reduce their machinability. Providing high productivity and accuracy has a great sense at the stage of technological preparation of machine-building production, taking into account the high cost of products from titanium alloys, as well as the need to provide increased operational properties. Simulation of machining processes of titanium alloys has many differences from the simulation of cutting of medium-carbon steels. This article focuses on recommendations for selecting an effective cutting model, describing the specific rheological properties of titanium alloys, optimizing solver selection, and cutting fracture criteria. The stages of adiabatic shear are described. The use of these recommendations contributes to the development of a more adequate model of machining titanium alloys and the choice of optimal cutting parameters to ensure a given quality of the machined surface layer of the workpiece.

Special features of the dynamic cutting process analysis in the machining of titanium alloys

The achievement of high accuracy parameters and the surface layer quality of the machined surface greatly depends on the dynamic stability of the system “Machine-Fixture-Tool-Workpiece”. It is especially important for machining titanium alloys because intensive tool vibrations are generated due to the adiabatic shear phenomenon. The causes and consequences of such effects are described in the article. The paper also describes a specific method for studying the dynamic state of the cutting system of titanium alloys, based on the system integration of the classical method of the analytical description of the vibration processes arising when machining and the method of rheological simulation of the cutting process. Two important problems are proposed to be solved for the effective analysis of the system modeling results: first, it is the filtering of the noise signal with preservation of the limiting values of modeling results and maximum sensitivity using the Savitsky-Golay mathematical apparatus; second, the problem of approximation and interpolation of the filtered function of cutting parameters by Fourier transform in time.

An investigation on the hot workability and microstructural evolution of a novel dual-phase Mg–Li alloy by using 3D processing maps

Hot compression tests of a novel dual-phase Mg–Li alloy were performed in temperature range of 200–400 °C and strain rate range of 0.01–10s−1. 2D and 3D processing maps were established using Prasad's and Murty's instability criteria (methods) based on the dynamic materials model. It was found that one instability region obtained based on Prasad's instability criterion exist in the domain of power dissipation efficiency at a strain of 0.7. Prasad's and Murty's instability criteria are difficult to accurately characterize the instability characteristics of the alloy. The accuracy processing map rectified was obtained by means of Murty's and Prasad's instability criteria and microstructural analysis. On the basis of the processing map rectified, the optimum hot working parameters were obtained to be the temperature range of 320–400 °C and strain rate range of 0.01–0.4s−1 with the peak power dissipation efficiency. In this domain, the α phase undergone dynamic recrystallization (DRX) and dynamic recovery (DRV). The β-phase occurred DRX during the hot deformation. The instability regions lie in the temperature range of 200–250 °C with strain rate range of 0.01–1s−1 and the temperature range of 200–312 °C with strain rate range of 1–10s−1. Microstructural observations indicate that characteristics of the flow instabilities were flow localization, deformation twinning, the adiabatic shear bands and macro/micro cracking. The rectified processing map was successfully verified by the hot rolling test. The rectified processing map could accurately characterize workability characteristics of a novel dual-phase Mg–Li alloy and could be used for the optimization of hot working process parameters.