Formation Of Adiabatic Shear Bands Research Articles

Mechanism of ultra-high-speed cutting of Ti-6Al-4V alloy considering time-dependent microstructure and mechanical behaviors

High-speed cutting (HSC) and ultra-high-speed cutting (u-HSC) (6.07–32.7 m/s) tests of Ti-6Al-4V alloy were developed based on an improved Split Hopkinson Pressure Bars (SHPB) system. Specific cutting energy was calculated and scaling effect was observed in HSC with different uncut chip thicknesses (20–200 μm). Considering time-dependent (in situ and ex situ) microstructure and mechanical parameters, chip morphology, adiabatic shear band (ASB), chip-free surface, material separation characteristic at chip-workpiece interface, and surface integrity were investigated. The micro-hardness within ASB was 25% higher than that outside of the ASB. Ultra-fine grains (77% grain sizes less than 330 nm) and micro-texture were observed by electron backscattering diffraction (EBSD) test. Grain refinement in ASB was explained by rotation dynamic recrystallization. The maximum shear strain and the range of shear strain rate in ASB at the condition of v = 10 m/s and t1 = 100 μm were approximately 17 and 6.44 × 106 s−1–8.5 × 106 s−1, respectively. Severe plastic deformation was observed by the dimples at ductile fracture surface along ASB. Mechanism of parabolic and ellipsoid dimples caused by shear and tensile stresses was explained for HSC of Ti-6Al-4V alloy. Additionally, the micro-hardness of machined subsurface and the ASB (ex situ parameter) exhibits a relationship with specific energy (in situ parameter), which can provide an effective way to control the formation of ASB and surface integrity by the control of in situ parameters in HSC and u-HSC conditions.

Dynamic shear behaviors and microstructural deformation mechanisms in FeNiAlC dual-phase high strength alloy

Adiabatic shear band (ASB) is a common failure mechanism of metals and alloys under high strain rate dynamic loading. The hat-shaped samples of Fe-24.86Ni-5.8Al-0.38C dual-phase steel with different microstructures were impacted by the Hopkinson pressure bar device to investigate their dynamic shear behaviors and microstructural deformation mechanisms. The coarse grained (CG) structure after solution treatment was subjected to cold rolling (CR) in order to obtain various microstructures. The evolution of microstructure during dynamic shear deformation was extensively studied using transmission electron microscopy (TEM) and scanning electron microscope (SEM). The results revealed that the FeNiAlC dual-phase steel has excellent dynamic shear properties with dynamic shear strength of 1.3 GPa and uniform dynamic shear strain of 1.5. The dual-phase steel was found to be composed of austenite phase (γ) and around 20% martensite phase (α) before deformation. The deformation process was found to be dominated by dislocations slip and twinning. Moreover, martensite transformation was found to be suppressed due to the high strain rates. ASBs were observed to be formed in all samples with various microstructures after impact, and dynamic recrystallization was found to occur with formed ultra-fined grains of about 300 nm and without transformation in ASBs. For the width of ASBs, the theoretical result (about 12.3 μm) was found to be in good agreement with the experimental value (about 14.6 μm) in CR samples. However, the measured width of ASBs was found to be about 15.8 μm, which is far smaller to the calculated value (about 30 μm) in CG samples. This may be attributed to the incompletely adiabatic conditions in CG samples. The adiabatic temperature rise due to the plastic work was found to be about 720 K (for CG sample) and 190 K (for CR sample). Through the analysis of the experimental results and the theory of the thermoplastic model, it can be concluded that the adiabatic temperature rise is not the only factor for ASB formation in the course of impact loading, and the localized deformation induced microstructure evolution in materials should also be considered.

Effect of heat treatment conditions on ballistic behaviour of various zones of friction stir welded magnesium alloy joints

Abstract Ballistic behaviour of different zones of post-weld heat-treated (PWHT) magnesium alloy (AZ31B) target against 7.62 mm × 39 mm armour-piercing (AP) projectile with a striking velocity of (430±20) m/s was determined. Magnesium alloy (AZ31B) welded joints were prepared by using friction stir welding (FSW) process and subjected to different heat treatment conditions. The microhardness values of non-heat-treated and heat-treated FSW joints were investigated. The results indicated that PWHT process (250 °C, 1 h) has improved the microhardness of heat-treated FSW joints. Scanning electron microscope (SEM) microstructure showed that heat treatment has caused the formation of fine α-Mg grains and tiny precipitates and made the dissolution of β-Mg17Al12 phase into the Mg matrix. The ballistic behaviour of PWHT zones was estimated by measuring the depth of penetration (DOP) of the projectile. Lower DOP value was observed for the base metal zone (BMZ) of a heat-treated welded joint. Post ballistic SEM examinations on the cross-section of all three zones of crater region showed the formation of adiabatic shear band (ASB).

Enhanced dynamic mechanical properties and resistance to the formation of adiabatic shear band by Cu-rich nano-precipitates in high-strength steels

Cu-rich nano-precipitate strengthened steel is a kind of promising impact-resistant structural materials due to its perfect combination of strength and ductility. In this study, the effects of the Cu-rich nano-precipitates on the dynamic mechanical properties, deformation resistance and adiabatic shear band formation were systematically investigated. It is found that the Cu-rich nano-precipitates can effectively improve the strength at high strain rate during dynamic deformation. The maximum flow stress reaches ~2565 MPa at the strain rate of 6000 s−1, presenting the outstanding dynamic mechanical properties. The nano-precipitate-strengthened steel has a great positive strain rate sensitivity at a high strain rate, showing the excellent resistance to high-speed deformation and delayed fracture. The nano-precipitates can effectively hinder the formation and development of the adiabatic shear bands and improve significantly the maximum flow stress and microstructure stability during the high-speed impact.

Micro-Mechanisms of Shear Deformation Localization of Ti6Al4V Alloy under Shear-Compressive Loading Conditions.

Titanium Ti6Al4V alloy is a superior material that has extremely high strength, hardness and good anti-corrosion resistance. Dynamic shear-compression experiments were carried out on the alloy to investigate the micro-mechanisms of adiabatic shear banding (ASB) formation. The split Hopkinson pressure bar (SHPB) setup were used for the tests at high strain rates. It was found that the shear deformation localization (SDL) was considerably affected by the complex loading conditions. The micro-mechanisms for the ASB formation relied on different shear compressive proportion of loadings (SCLPs). Scanning electron microscope (SEM) observations showed that the ASB width was related with the SCLP and the fracture failure of alloy was induced by the nucleation and growth of microvoids. In transmission electron microscope (TEM) analysis, the microstructural changes of material within the ASB were characterized by dynamic recrystallization (DRX) and twining grain formation, dislocation migration, and stacking and grain refining processes. The results in this article demonstrates a complex image of microstructural evolution of alloy in the shear localization process.

Numerical simulation of adiabatic shear bands formation processes on two-dimensional eulerian meshes

In this work we suggest a mathematical model of motion of the elasto-plastic materials with nonlinear plasticity constitutive law; we also propose an effective numerical method for numerical simulations of such tasks on two-dimensional eulerian meshes. Based on the method, we research formation of multiple adiabatic shear bands (ASB) at high-speed shear deformations. We test our approach on two-dimensional problem where the initial heterogeneity of temperature leads to formation of adiabatic shear band.

A new method to elucidate fracture mechanism and microstructure evolution in titanium during dissimilar friction stir welding of aluminum and titanium

In the friction stir welding (FSW) of dissimilar materials, the weld nugget exhibits composite properties and is composed of hard particles (high-strength material) distributed in a soft matrix material. The distribution of these particles influences the properties of the weld. Therefore, it is useful to characterize the deformation and fragmentation of the high-strength material from which they originate. In the current study, FSW of aluminum (Al) to titanium (Ti) was performed and a new technique was introduced to remove Al from the post-weld sample to characterize the deformation and fragmentation of Ti in the weld nugget. The post-weld sample showed that Ti particles were inhomogeneously distributed. It was understood that the plastic deformation of the Ti depends on its location of the weld. A detailed investigation revealed a number of fracture zones in titanium. A systematic analysis revealed the evolution of twins in Ti in the thermos-mechanically affected zone (TMAZ), whereas no-twins and recrystallized grains were noticed on the Ti flake that was consolidated in the weld nugget. The X-ray computed tomography (XCT) showed that Ti flakes were subjected to deformation and bending. Furthermore, it was noticed that the width of the flakes was not identical along their length and that the flakes were broken at the middle region into two parts. It can be concluded that Ti, at the interface and as flakes, was subjected to severe deformation at a lower homologous temperature that leads to the formation of adiabatic shear bands (ASBs). The local evolution of temperature within the ASBs was much higher than the general weld temperature, leading to the recrystallization of Ti at the interface. Moreover, ASBs are prone to crack nucleation and propagation; hence, particles with different morphology and sizes were noticed in the nugget zone. The distribution of particles was inhomogeneous due to variation in the particle size and complex mechanical mixing. The distribution of particles varied with location across the weld nugget. A homogeneous mechanical mixing was noticed at the center of the weld nugget due to complex material flow and distribution of finer particles. With this understanding of the deformation and fracture mechanism of titanium during dissimilar FSW of Al to Ti, it is possible to engineer the developed composites and/or weld nuggets of dissimilar materials to achieve the required combination of mechanical properties.

Mechanical Properties and Fracture Mechanism of Laser Solid Formed TC4 Titanium Alloy under Dynamic Compression

The mechanical properties and fracture mechanism of laser solid formed TC4 titanium alloy under different temperatures and strain rates were investigated experimentally. The results show that there is no obvious anisotropy in compression property, and the susceptibility to adiabatic shear deformation is nearly the same. The microscopic analysis shows that the adiabatic shear bands (ASBs) can be formed easily under the dynamic compression loading, and the high experimental temperature may suppress the formation of ASBs. With the Johnson-Cook constitutive mode, the critical strain of ASB was calculated. By means of microscopic observation and mechanical analysis, the formation mechanism of fracture surface was explained, that is, voids and cracks form in shear zone and propagation of cracks along the interface of α/β forms fracture surface.

The Influence of the Microstructure of an HHA Steel on the Formation of Adiabatic Shear Bands, after High Deformation Rates

High hardness armor (HHA) steels, when subjected to high deformation rates, are prone to adiabatic shear bands formation. These heterogeneities, formed in a narrow zone, are the result of an intense plastic deformation in which the rate of heat dissipation is low. The generated shear bands can lead to a decrease in ballistic performance and impose to an armor a catastrophic failures. The appearance of these bands may be related to the microstructure or the deformation rate to which the material is subjected. Therefore, this work aims to analyze the influence of the microstructure of an HHA steel, tempered at 310, 425 and 610° C for 2 h, after high deformation rates, in the appearance of adiabatic shear bands. Specimens were dynamic tested in a split Hopkinson pressure bar. It was shown that tempering temperature at 310 ° C for 2h, which produced bainite and martensite with high hardness, was the most susceptible to the adiabatic shear bands appearance.

Characteristics and its formation reason analysis of adiabatic shear band produced by gear fracture

The formation and characteristics of adiabatic shear band during the fracture of gear assembly were studied. It shows that in the fracture process, the strain required by the adiabatic shear band can be achieved near the failure position of the gear, and thus the adiabatic shear band is generated. The distribution of the adiabatic shear band is discontinuous, and the angle with the gear is about 20 ° to 45 °. With the appearance of adiabatic shear band, a high strain rate is produced, and a lot of heat is produced in some areas. And because of the extremely fast speed, these heat can not be transferred in time, resulting in deformation, which leads to the fracture of gear assembly. At the same time, the critical strain and critical temperature are calculated.

Dynamic forced shear characteristics of Ti-6Al-4V alloy using flat hat-shaped specimen

The relation between stress collapse and nucleation of adiabatic shear band (ASB) remains incompletely understood. To further understand this topic, the evolution of shear deformation of Ti-6Al-4V alloy under forced shear loading is systematically investigated using digital image correlation (DIC) technique. DIC results indicate that ASB nucleates after the maximum stress and drastic stress drop corresponds to the initiation and propagation of crack, and the forced shear process can be divided into homogeneous, inhomogeneous and highly localized deformation stages. Numerical simulation results suggest that thermal softening might not have had a pronounced effect on the onset of adiabatic shear band and dynamic recrystallization formation. “Cohesive fracture” can be identified as the dynamic failure mechanism for Ti-6Al-4V alloy on the basis of the crack propagation features, and the microstructure gets increasingly refined due to the occurrence of dislocations, stacking faults and cell structures.

Dynamic shear instabilities in metallic sheets subjected to shear-compression loading

This paper presents an investigation on the formation of adiabatic shear bands in metallic sheets subjected to dynamic shear-compression loading under plane-stress conditions. For that purpose, we have developed a theoretical model based on perturbation analysis, and have performed finite element calculations. The material behavior has been modeled with von Mises plasticity, and the evolution of the yield stress has been considered dependent on plastic strain, plastic strain rate, and temperature. The theoretical model extends the linear stability analysis for simple-shear formulated by Molinari (1997) to shear-compression loading. The numerical simulations have been performed in ABAQUS/Explicit (2016) using a unit-cell model based on the work of Rodríguez-Martínez et al. (2015) in which the shear band formation is favored introducing geometric and material imperfections. Moreover, we have developed a calibration procedure for the stability analysis that enables to make qualitative and quantitative comparisons with the finite element calculations. Both stability analysis predictions and finite element results display the same overall trends, and show that any small negative triaxiality has a profound stabilizing effect on the material behavior delaying, or even preventing, the formation of a shear band. This key outcome has been substantiated for a wide range of strain rates and for materials with different strain hardening coefficients, strain rate sensitivities, and thermal softening behaviors.

Effects of prior ECAP process on the dynamic impact behaviors of hypereutectic Al-Si alloy

The dynamic mechanical responses of as-cast and ECAPed hypereutectic Al-Si alloy subjected to high strain rate compression by split Hopkinson pressure bar (SHPB) system were investigated. The result shows that the dynamic yield strength of the investigated alloy first decreases with the first ECAP pass, followed by an increase in strength with further increase in ECAP passes. Increasing the number of ECAP passes also results in the refinement of both grains and deleterious crack-nucleating particles and in turn, results in the decrease in susceptibility to thermo-mechanical instability and formation of adiabatic shear bands (ASBs). The activation of dynamic recovery (DRV) and dynamic recrystallization (DRX) by increasing the number of ECAP passes were found to significantly suppress dynamic mechanical damage.

Dynamic mechanical properties and failure characteristics of electron beam melted Ti-6Al-4V under high strain rate impact loadings

This study presents an investigation on the effects of building direction on microstructure, dynamic mechanical properties, and deformation mechanisms of electron beam melted Ti-6Al-4V (EBM-Ti64) cylindrical rods. Microstructural features were characterized using optical microscopy (OM) and scanning electron microscopy (SEM). The initial microstructure in both directions consists of transformed α+β phases and grain boundary-α (αGB)along prior β-grain boundaries. The vertically built cylindrical specimens have a finer grain structure, including lower interlamellar spacing and finer α-laths compared to the horizontally built ones. Dynamic impact tests using Split-Hopkinson Pressure Bar (SHPB) were conducted on both horizontally and vertically built samples at strain rates ranging between 1150 and 2700 s−1. Dynamic mechanical properties are strain-rate sensitive; the maximum flow stress of 1960 MPa (at 2100 s−1) and 2160 MPa (1650 s−1) were obtained for horizontal and vertical specimens, respectively. Horizontal specimens fragmented when deformed at 2700 s−1, whereas the vertical specimens failed at a much lower strain rate (1900 s−1). At a given strain rate, vertical specimens exhibited better dynamic strength and lower strain (total strain) due to their finer microstructure. The temperature rise during deformation primarily governs flow softening at all conditions, which led to the formation of adiabatic shear bands (ASBs). Microstructures of deformed specimens revealed thermal softening features such as voids formation. These voids coalesced and grew, leading to crack initiation and propagation along ASBs. Fractographic examination of the fragmented specimens under impact loading revealed ductile dimples and smoother surfaces, which indicate a combination of both ductile and brittle fracture. The contribution of twinning and pyramidal <c+a> slip systems is the primary deformation mechanism during high strain rate impact loading of EBM-Ti64. The experimentally obtained flow curves are in good agreement with the Chang-Asaro equation-based constitutive modeling results.

Effect of tempering conditions on adiabatic shear banding during dynamic compression and ballistic impact tests of ultra-high-strength armor steel

In this study, roles of strain-hardening rate on susceptibility of adiabatic shear band (ASB) formation and subsequent cracking were investigated in two ultra-high-strength armor steel plates heat-treated differently. The quenched and tempered steel contained ~2% of retained austenite in the tempered martensitic matrix, while the quenched and austempered steel contained ~4% retained austenite in the bainitic matrix partly with the tempered martensite. The actual ballistic impact test results revealed the lower sensitivity of ASB formation in the austempered steel than in the tempered steel, which corresponded well to the higher critical strain for ASB formation in the dynamic compressive test using a laboratory-scale split Hopkinson pressure bar (SHPB). The austempered steel caused the higher internal stress among various constituents, and all the retained austenite transformed into martensite during the deformation, thereby leading to transformation-induced plasticity (TRIP) effect. The higher strain-hardening rate induced by these higher internal stress and TRIP effect increased resistance to ASB formation, which was confirmed by a calculation of ASB susceptibility. Thus, the austempered steel was much less susceptible to the ASB formation during the ultra-high-speed deformation. Consequently, the increased resistance to ASB formation retarded the initiation and propagation of ASBs and cracks.

Multi-scale crystal plasticity finite element simulations of the microstructural evolution and formation mechanism of adiabatic shear bands in dual-phase Ti20C alloy under complex dynamic loading

A dynamic compression test was performed on α + β dual-phase titanium alloy Ti20C using a split Hopkinson pressure bar. The formation of adiabatic shear bands generated during the compression process was studied by combining the proposed multi-scale crystal plasticity finite element method with experimental measurements. The complex local micro region load was progressively extracted from the simulation results of a macro model and applied to an established three-dimensional multi-grain microstructure model. Subsequently, the evolution histories of the grain shape, size, and orientation inside the adiabatic shear band were quantitatively simulated. The results corresponded closely to the experimental results obtained via transmission electron microscopy and precession electron diffraction. Furthermore, by calculating the grain rotation and temperature rise inside the adiabatic shear band, the microstructural softening and thermal softening effects of typical heavily-deformed α grains were successfully decoupled. The results revealed that the microstructural softening stress was triggered and then stabilized (in general) at a relatively high value. This indicated that the mechanical strength was lowered mainly by the grain orientation evolution or dynamic recrystallization occurring during early plastic deformation. Subsequently, thermal softening increased linearly and became the main softening mechanism. Noticeably, in the final stage, the thermal softening stress accounted for 78.4 % of the total softening stress due to the sharp temperature increase, which inevitably leads to the stress collapse and potential failure of the alloy.

Experimental ballistic performance determination of friction stir welded magnesium (AZ31B) targets

Nowadays there is an increasing demand to develop light weight defense vehicles, fighter aircrafts and warships in order to enhance their mobility and fuel economy. It can be achieved by using the light weight structures made up of Aluminum and Magnesium plates which will reduce the weight, increase the payload carrying capacity and efficiency of the vehicles. In the present work, experimental ballistic performance of Magnesium (AZ31B) Base Metal (BM), friction-stir-welded (FSW) and post-weld-heat treated (PWHT) target plates have been determined by using the 7.62 mm Armor Piercing Projectiles (APP). Prior to ballistic testing, Magnesium joints were prepared by the FSW process for various tool rotational and welding speeds. Defect-free welds with better tensile and impact properties, were obtained with tool rotational speed of 1200 rpm and the welding speed of 50 mm/min. PWHT was carried out with the annealing temperature at 250 °C for about 1 hour. Microhardness and microstructure of the FSW and PWHT joints were investigated. Highest hardness (67 HV) was obtained for fine grain structure and precipitated particles were observed for PWHT targets. Depth of Penetration of PWHT target plates was approximately 17.55% and 16.31% lower than the BM and FSW target plates respectively. Scanning Electron Microscopic observation of projectile penetrated channel showed the formation of Adiabatic Shear Bands (ASBs). Due to a larger number of ASB lines, ballistic impacted FSW surface showed more number of macro cracks compared to PWHT surface. The PWHT joints showed the absence of fragmentation failure, lesser cracks and ASB lines which increased the ballistic performance.

Microstructural characteristics and formation mechanism of adiabatic shear bands in Al–Zn–Mg–Cu alloy under dynamic shear loading

The microstructural characteristics and formation mechanism of adiabatic shear bands in ultra-high strength aluminum alloy are studied under dynamic shear loading. It is found that thermal softening alone is insufficient to trigger the initiation of adiabatic shear bands (ASBs). Dynamic recrystallization induced softening is attributed to be the main cause of ASB initiation, with temperature rise playing a secondary role by accelerating the deformation localization process. It is believed that abrupt temperature rise takes place after ASB initiation, and the temperature rise prior to ASB formation is relatively small. Recrystallized ultrafine grains with a strong texture of {112} <110> are formed in ASBs, based on a progressive subgrain misorientation recrystallization mechanism. It is proposed that the translational–rotational motion of the dynamically recrystallized nanograins in the form of vortexes at the mesoscale can account for the microstructural softening, thus causing shear instability, eventually leading to fracture by the combination of ductile and brittle failure mechanisms.

Forecast of Adiabatic Shear Band Formation in Two Commercial Ultra-high-Strength Armor Steels by Split Hopkinson Pressure Bar

Formation possibilities of adiabatic shear bands (ASBs) or cracks in two commercial ultra-high-strength armor steels composed of martensite were evaluated by a split Hopkinson pressure bar so that a plausible forecasting method of ASB formation and cracking during the dynamic compression or ballistic impact might be suggested. The present dynamic compressive test effectively investigated the ASB formation behavior and provided a good idea on how many ASBs or cracks form during the ballistic impact.

Understanding of adiabatic shear band evolution during high-strain-rate deformation in high-strength armor steel

The microstructural evolution and formation mechanism of adiabatic shear band (ASB) in a high-strength armor steel were investigated using a laboratory-scale split Hopkinson pressure bar, and the results were correlated with the actual ballistic impact behavior. The interrupted dynamic compressive test results reveal that a deformed ASB (dASB) starts to form right after the stress collapse and it develops into a transformed ASB (tASB). In the ballistic impact, wide tASBs form mostly at the perforated surface, and narrower tASBs are branched from the tASB. Very fine equiaxed grains of ∼190 nm in the tASB developed during the dynamic compression indicates that the dynamic recrystallization occurs even in 86.5 μs, and then the grains grow up to 260 nm in 9.5 μs. Rotational dynamic recrystallization mechanism and grain-growth rate model were proposed based on the calculation of temperature rise from a thermo-elasto-plastic finite element method, which provide a reasonable explanation for the formation and growth of fine equiaxed grains during both the dynamic compression and ballistic impact. A linkage of equiaxed subgrains and elongated parent subgrains demonstrates that the equiaxed subgrains did not evolve from nucleation and growth processes but from the sub-boundary rotation. Based on the underlying formation mechanisms and kinetics of ASBs, this study would suggest a reliable method to interpret the ASB formation and associated fracture mechanism during the ballistic impact.