Width Of Adiabatic Shear Bands Research Articles

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.

Adiabatic shearing mechanism on Al-4.2%Cu alloy bars subjected to electromagnetic loading

This paper proposed an electromagnetic loading process with the high-speed impact. Al-4.2% Cu alloy bars were used to employ electromagnetic impact (EI) experiments. Deformation mechanism and microstructure evolution of EI samples were revealed by theoretical model and microstructure characterizations. The EI process had impact force (peak value 40 kN) and impact velocity (peak value 6.7 m/s) during a short time period (1.25 ms). Adiabatic shearing mechanism dominated the whole deformation process, causing that significant microstructure characteristic was adiabatic shear bands (ASBs). The theoretical analysis implied that the formation of ASBs was accounted for the radial velocity gradient. Most plastic deformations concentrated in ASBs, and approximately pure shear deformations resulted in adiabatic temperature rise of 0.33–0.42 Tm inside ASBs. The width of ASBs was about 135 μm, in which original equiaxial grains were elongated into laminated sub-structures. TEM observations showed multi-slip systems were simultaneously actuated due to severe shear deformations. High dislocation density and dislocation tangles distributed with the ASBs. Adiabatic temperature rise and distorted energies drove sub-grains rotate into recrystallization grains (70–280 nm) with large angle grain boundaries. The needed maximum time (45 μs) for rotational dynamic recrystallization was far less than that of plastic deformation, indicating that rotational dynamic recrystallization mechanism contributed to the formation of recrystallization grains.

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.

Dynamic Shear Deformation of a Precipitation Hardened Al0.7CoCrFeNi Eutectic High-Entropy Alloy Using Hat-Shaped Specimen Geometry.

Lamellar eutectic structure in Al0.7CoCrFeNi high-entropy alloy (HEA) is emerging as a promising candidate for structural applications because of its high strength-ductility combination. The alloy consists of a fine-scale lamellar fcc + B2 microstructure with high flow stresses > 1300 MPa under quasi-static tensile deformation and >10% ductility. The response to shear loading was not investigated so far. This is the first report on the shear deformation of a eutectic structured HEA and effect of precipitation on shear deformation. A split-Hopkinson pressure bar (SHPB) was used to compress the hat-shaped specimens to study the local dynamic shear response of the alloy. The change in the width of shear bands with respect to precipitation and deformation rates was studied. The precipitation of L12 phase did not delay the formation of adiabatic shear bands (ASB) or affect the ASB width significantly, however, the deformed region around ASB, consisting of high density of twins in fcc phase, was reduced from 80 µm to 20 µm in the stronger precipitation strengthened condition. We observe dynamic recrystallization of grains within ASBs and local mechanical response of individual eutectic lamellae before and after shear deformation and within the shear bands was examined using nano-indentation.

2A10铝合金铆钉电磁铆接微观组织演化研究

本文以2A10铝合金铆钉为材料，研究高速冲击电磁铆接后铆钉镦头微观组织演化，采用金相和透射电镜手段进行分析，结果表明高速冲击作用导致塑性变形主要集中于铆钉镦头的绝热剪切带区域，绝热剪切带宽度约为100 μm。TEM组织表明绝热剪切带内部产生宽度0.5 μm层片状亚晶，并且亚晶内部存在大量缠结的位错。剧烈塑性变形的绝热剪切带显微硬度明显高于其他区域，并且铆钉镦头硬度分布与变形组织分布一致。 In this paper, 2A10 aluminum alloy rivets were used for electromagnetic riveting experiments. The microstructure evolution in rivet tail was investigated after electromagnetic riveting. The mi-crostructure evolutions were analyzed by the optical microscope and transmission electron mi-croscope. Results showed that plastic deformations mainly concentrated in adiabatic shear band of rivet tail due to the high speed impact. The width of adiabatic shear band was about 100 μm. TEM observations demonstrated that lamellar sub-grains with the width of 0.5 μm generated in adiabatic shear band, and many tangled dislocations existed within them. The hardness values in adiabatic shear band with severe plastic deformations were significantly higher than that of other zone, and the hardness distribution law accorded with deformation microstructure distributions.

A unified model for coupling constitutive behavior and micro-defects evolution of aluminum alloys under high-strain-rate deformation

High strain rate forming (HSRF) is promising to break through the conventional forming limit of materials and thus to form hard-to-deform components. During HSRF process, long-lasting micro-defects before fracture, i.e. adiabatic shear bands (ASBs) in compression and voids in tension, are significant characteristics existing in a large strain range. These defects bring about the non-destructive softening of stress and continuous exertion of ductility, in return, the stress responses affect the evolution of micro-defects. However, the interaction between stress responses and defects evolution has not yet been reflected in the existing models, resulting in a limited prediction accuracy. Aiming at this issue, the competition between hardening caused by thermal activated dislocation movements and the flow softening brought by micro-defects evolution was well modeled in this work. During the modeling, the relation between the normal strain and effect zone of ASBs was established via the modification of Bai–Dodd model by considering the geometric features of ASBs. Moreover, by introducing a rate-dependent ASB trace angle, the half width of ASBs was expressed as a function of maximum shear strain and critical instability strain. The effect of strain and strain rate to the evolution of voids under tensile conditions was taken into account by combining Hollomon hardening law with Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. Then, the interactions between micro-defects and structure-related athermal stress were characterized by connecting volume fraction of voids and effective bearing area in tension and the intensity of flow localization with ASBs width in compression. As a consequence, a unified model of constitutive behaviors coupled with micro-defects evolution was established with considering rate-dependent hardening and softening. Applied to aluminum alloys, this model predicts the stress responses, evolution of ASBs and voids, and low or negative strain rate sensitivity with high precision in large ranges of strain (0–0.6) and strain rate (0.001–5000 s−1). The proposed model is thus believed to be with a successful application in precise prediction and optimization of HSRF processes.

Diffusive transformation at high strain rate: On instantaneous dissolution of precipitates in aluminum alloy during adiabatic shear deformation

Abstract

Investigation of serrated chip morphology change regular in the burning of titanium alloys

Titanium alloys are regarded as difficult-machining materials due to their high strength at elevated temperature, relatively low thermal conductivity, small elastic modulus, and high chemical reactivity. It is prone to produce cutting burn defect in end milling. In order to investigate the variation regular of serrated chip morphologies and the relationship between chip macro morphologies and tool flank wear in the burning of milling titanium alloys TC4, a series of cutting experiments were conducted in this study. The chip morphology and cutting vibration characteristic in different tool wear stages were examined using an optical microscope, SEM, and vibration test system. In addition, the variation characteristics of serrated chips in the burning of milling titanium alloy TC4 were analyzed. The results revealed that the occurrence of tool flank wear can make an influence on serrated chips morphologies and surface color in the cutting burning process. Besides, the chip segment degree, shear angle, and width of adiabatic shear band increased rapidly as the cutting burning occurring. The present research could be potentially used to evaluate tool wear and surface quality in the burning of end milling titanium alloy process.

Experimental study of macro–micro dynamic behaviors of 5A0X aluminum alloys in high velocity deformation

Investigation of macro–micro dynamic mechanical behavior of aluminum alloys is crucial to understand and model high velocity forming processes, e.g. the electromagnetic forming. Therefore, this work presents the experimental investigation of the macro–micro dynamic deformation behavior and related mechanisms of 5A0X alloys. With increasing strain rate, notable hardening and serrated flow curves are observed for both 5A06 and 5A02 alloys; the flow softening ratio increases from 10.59% to 15.56% for 5A06 alloy, while increasing up to 34.55% at a strain rate of 3000s−1 for 5A02 alloy. High velocity deformation enhances the ductility of both the alloys via slowing down the neck development rather than suspending the onset of necking. Comparative study in dynamic tensile/compressive behavior of 5A06 alloy indicates a similar hardening rule in these two loading conditions, however, more obvious softening is found in tension than that in compression. The difference can be attributed to the different microstructure-related characteristics, namely, the diffuse necking related to void evolution in tension and the adiabatic shear bands (ASB) in compression. In addition, a mathematical model is used in this work to corroborate the occurrence of ASBs and to predict the half width of ASBs, which is proved reliable by experiments.

Influence of Cutting Conditions on the Width of Adiabatic Shear Bands: An Investigation Using Finite Element Simulation

In this paper, a novel model combining the microstructure prediction model and a modified constitutive model of the Johnson-Cook (JC) model was developed and embedded into FEM software via the user subroutine. The chip formation and microstructure evolution in high speed cutting of Ti-6Al-4V alloy were simulated. The results indicated that dynamic recrystallization mainly happened in adiabatic shear bands (ASBs), where the grain size had a big decline. Then FEM simulations were carried out to investigate the effect of cutting velocity, uncut chip thickness, and the rake angle on the ASBs width of the serrated chips. It can be concluded that the width of ASB increases with the increasing of cutting depth and cutting velocity, and decreases with the increasing of rake angle of the tool.

Experimental Study of Serrated Chip in High Speed Cutting

In this paper, some high speed cutting experiments of hardened 45 steel are carried out. The effects of cutting parameters on the width and space of adiabatic shear band, the frequency and degree of sawtooth are investigated by metallographic observation and theoretical calculation. The results show that the space of adiabatic shear band decreases with the increase of cutting speed and rake angle, but increases with the increase of cutting depth. The width of adiabatic shear band decreases with the increase of cutting speed and cutting depth, however, increases with the increase of rake angle. The frequency of sawtooth increases with the increases of cutting speed, decreases with the increase of rake angle, and has no obvious relationship with cutting depth. The degree of sawtooth increases with the increase of cutting speed and cutting depth, but decreases with the increase of rake angle.

Phenomenon of transformed adiabatic shear band surrounded by deformed adiabatic shear band of ductile metal

The coexistent phenomenon of deformed and transformed adiabatic shear bands(ASBs) of ductile metal was analyzed using the JOHNSON-COOK model and gradient-dependent plasticity(GDP). The effects of melting point, density, heat capacity and work to heat conversion factor were investigated. Higher work to heat conversion factor, lower density, lower heat capacity and higher melting point lead to wider transformed ASB and higher local plastic shear deformation between deformed and transformed ASBs. Higher work to heat conversion factor, lower density, lower heat capacity and lower melting point cause higher local plastic shear deformation in the deformed ASB. Three reasons for the scatter in experimental data on the ASB width were pointed out and the advantages of the work were discussed. If the transformed ASB width is used to back-calculate the internal length parameter in the GDP, undoubtedly, the parameter will be extremely underestimated.

Width of adiabatic shear bands formed under combined stresses

An equation for the half width of an adiabatic shear band formed under combined stresses is derived. The importance of strain rate, stress, temperature rise, and thermal conductivity is described. Confirmation is found for the proposition that shear band width is independent of stress state and this is confirmed by comparing the theory with the experimental results of other workers.MST/933

Width of adiabatic shear bands formed under combined stresses

Width of adiabatic shear bands formed under combined stresses

Width of adiabatic shear bands

An equation for the half-width of an adiabatic shear band is derived. With reference to the equation, the importance of thermal conductivity, temperature, and viscosity in determining the bandwidth are illustrated. It is shown that calculations of shear band width are fairly close approximations to those obtained experimentally. The apparent insensitivity of shear band width to the loading conditions is in agreement with experimental findings of other researchers.MST/156

Width of adiabatic shear bands

An equation for the half-width of an adiabatic shear band is derived. With reference to the equation, the importance of thermal conductivity, temperature, and viscosity in determining the bandwidth are illustrated. It is shown that calculations of shear band width are fairly close approximations to those obtained experimentally. The apparent insensitivity of shear band width to the loading conditions is in agreement with experimental findings of other researchers.MST/156