Primary Shear Zone Research Articles

A New Cutting Mechanics Model for Improved Shear Angle Prediction in Orthogonal Cutting Process

Abstract In metal cutting processes, accurately determining the shear angle is essential, as it governs chip formation and cutting force generation. Despite extensive research conducted on this topic, the accurate prediction of the shear angle remains a subject of ongoing investigation. This paper presents a new analytical model for predicting the shear angle, taking into account the direction difference between the shear stress at the boundary of the primary shear zone and the maximum shear stress. The constitutive property of the workpiece material with respect to the strain, strain rate, and temperature is considered in predicting the shear angle. The results show that the solution for the shear angle is not unique for a given rake and friction angle, and is highly dependent on the flow stress response of the workpiece material. Orthogonal cutting experiments were conducted on steel and aluminum alloys under various uncut chip thicknesses, cutting speeds, and tool rake angles to characterize the chip thickness and shear angle. Based on a comparison between model predictions, experimental results, and data from the literature for various workpiece materials and cutting conditions, it is shown that the proposed model results in an improved prediction for shear angle by considering the stress transformation within the primary shear zone.

A Visualized Microstructure Evolution Model Integrating an Analytical Cutting Model with a Cellular Automaton Method during NiTi Smart Alloy Machining

In this study, a visualized microstructure evolution model for the primary shear zone during NiTi smart alloy machining was established by integrating an analytical cutting model with a cellular automaton method. Experimental verification was conducted using an invented electromagnet rotation-type quick-stop device. The flow stress curve during the dynamic recrystallization of the NiTi smart alloy, the influence of relevant parameters on the dynamic recrystallization process, and the distribution of dynamic recrystallization in the primary shear zone were studied via the model. The simulation results showed that strain rate and deformation temperature significantly affect the relevant parameters during the dynamic recrystallization process. Three typical shear planes were selected for a comparison between simulation results and experimental results, with a minimum error of 3.76% and a maximum error of 11.26%, demonstrating that the model accurately simulates the microstructure evolution of the NiTi smart alloy during the cutting process. These results contribute theoretical and experimental insights into understanding the cutting mechanism of the NiTi smart alloy.

Evaluation method for microscale mechanical properties of an epoxy resin

This study offers a detailed quantitative evaluation of microscale properties of polymers utilizing the surface and interfacial cutting analysis system (SAICAS) technique. The investigation focuses on the formation of the primary shear zone (PSZ) during cutting, leading to plastic deformation and subsequent chip curling. This research explores various cutting thicknesses, each demonstrating unique chip formation, and applies a lubricant to mitigate friction noise and stabilize chip formation. An in-depth analysis of force geometries is performed to gain insights into chip formation. The shear strength and shear viscosity are calculated using relevant equations, thus enabling the quantitative characterization of the viscoelastic behavior of polymers. Remarkably, the polymers exhibit shear thinning behavior, where their resistance to shear decreases as the shear rate increases. The shear viscosity–shear rate plot follows the Ostwald-de Waele relationship, indicative of plastic flow behavior. The viscoelastic characteristics were validated by comparing the values calculated based on Atkins's theory with the quantified values. Then, the physical meaning of those values were discussed. This study facilitates the direct measurement of mechanical properties in solidified form of the polymer, offering a more precise representation than traditional test piece measurements. In addition, shear viscosity determination of cross-linked materials without pretreatment is achieved, broadening the scope of material analysis through quantitative comparisons.

The relationship between the cutting-edge, tool wear, and chip formation during Inconel 718 dry cutting

This study comprehensively addresses the machining of nickel alloys, focusing its attention on crucial aspects related to chip formation and tool wear. Detailed characterization of the morphology and the chip formation process was performed by analyzing parameters such as chip segmentation ratio and variables such as shear band thickness and strain rate. Additionally, a numerical model was used to quantify stresses and temperatures at the tool/chip interface and to evaluate damage, thus contributing to the understanding of the development of chip formation. A transition in chip shapes as the toothing increases is highlighted, evidenced by segmentation ratio values below 0.5, indicative of the presence of discontinuous chips. The increase in cutting-edge radius is associated with a gradual increase in the compression ratio, indicating a higher plastic energy requirement in chip formation. Numerical simulations support this theory of failure. A significant correlation of 80% was identified between flank wear and the increase in shear force oscillation amplitude, indicating that flank wear contributes to system vibration. It is also noted that the adiabatic shear bands (ASB) are narrow, revealing a marked plastic deformation in the primary shear zone. Consequently, the remarkable incidence of wear with cutting parameters on chip formation is demonstrated, affecting the cutting force amplitude and, hence, the workpiece topography.

Mechanism of chip segmentation transition from shear slip to shear fracture of Zirconium-based bulk metallic glass in mechanical machining

An in-depth understanding of material cutting deformation forms the foundation for manufacturing Zirconium-based bulk metallic glass (Zr-based BMG) parts. This study explores the transition in chip segmentation mode from shear slip to shear fracture in Zr-based BMG during mechanical machining. The investigation covers chip micromorphology, cyclic cutting force, machined surface quality, and the themo-mechanical properties under various chip segmentation modes linked to cutting conditions. A size-dependent transition in chip segmentation has been observed in the cutting of Zr-based BMG. At a specific cutting speed, a smaller uncut chip thickness favors chip formation via shear slip, resulting in a smooth primary shear zone (PSZ). Conversely, a larger uncut chip thickness leads to chip shear fracture, with the PSZ displaying fishbone-like and dimple patterns. The integration of the modified cutting kinematic model with the free volume-dependent shear transformation zone (STZ) model suggests that the transition from shear slip to fracture is due to an increase in uncut chip thickness, which causes a greater STZ volume in the PSZ. When the STZ volume surpasses a threshold, the intrinsic structural recovery of material from shearing cannot fully compensate for the free volume softening, leading to excessive free volume that ultimately triggers shear fracture. During the chip shear fracture process, the evolution from fishbone-like patterns to dimples in the PSZ is attributed to faster crack propagation at greater uncut chip thickness or higher cutting speeds. In practical applications, it is advisable to avoid chip shear fracture during machining to maintain high-quality machined surface quality.

How does the uncut chip thickness affect the deformation states within the primary shear zone during metal cutting?

The deformation states within the primary shear zone (PSZ) significantly affect material removal during machining. Uncut chip thickness (UCT) is an important factor that influences the material deformation states. However, the specific mechanism by which UCT influences the deformation states within PSZ remains unknown. This study aims to investigate the relationship between the deformation states in PSZ and UCTs via in-situ measurement and microscopic characterization techniques. Using the digital image correlation (DIC) technique, strain and strain rate distributions were derived to reveal the discrepant deformation in PSZ with increasing UCT. Furthermore, velocity vector fields and Electron Back-Scattered Diffraction (EBSD) characterizations were employed to examine the heterogeneity of deformation modes. To determine the specific deformation information, a deformation extraction framework based on the deformation gradient tensor theory was developed. Thus, strong and weak shear modes within PSZ were revealed based on the full-field deformation information of compression and extension. As the UCT increased, the transition of deformation states from a strong shear state to a hybrid shear state was determined. This work presents a new understanding of the deformation mechanism within PSZ in a ductile material of pure iron. A critical UCT was proposed to guide the cutting process to avoid inefficient weak shear mode.

Characterization of serrated chip formation based on in-situ imaging analysis in orthogonal cutting

Abstract The in-situ imaging of the cutting process exhibits outstanding advantages in reconstructing the precise and visual thermoplastic deformation fields. The physical and geometric characteristics of deformation fields provide a deeper understanding of the cutting processes. In this paper, a mechanism-image hybrid analysis method is proposed to acquire the characteristics of the serrated chip deformation in the orthogonal cutting of TA15 titanium alloy based on in-situ imaging. The established hybrid analysis method combines the shear-plane theory with the streamline method and image segmentation method, which realizes the identification of pixel coordinates of the main shear plane (MSP) and the primary shear zone (PSZ) and then the extraction of the physical and geometric variables from the digital image correlation (DIC) full-field measurements. Consequently, the variations of equivalent strain rate, strain, temperature, and the geometric characterizations of MSP and PSZ during an individual serration formation of TA15 titanium alloy were quantitatively investigated. It was found that the physical and geometric variables reached stability in the final stage of serration evolution and were averaged as the DIC-based equivalent characterizations to analyze the impact of cutting depth and tool rake angle. Meanwhile, the DIC-based equivalent characterizations were compared with the results obtained by the classical analytical models to illustrate the advantages of the DIC-based analysis. The findings also support that the established hybrid analysis method holds the potential to characterize the serrated chip formation of other materials and improve the models of PSZ.

In situ thermomechanical analysis of the primary shear zone in Inconel 718 orthogonal cutting

In situ thermomechanical analysis of the primary shear zone in Inconel 718 orthogonal cutting

Inverse analysis of machining residual stress based on hybrid model

Machining-induced residual stress significantly impacts the fatigue life and service performance of machined components. Appropriate compressive residual stress can enhance resistance to crack initiation, while tensile residual stress may be detrimental. In this study, an inverse analysis is conducted to optimize the cutting parameters for achieving desired residual stresses. The analysis begins by predicting the machining-induced thermal-mechanical loads in the primary shear zones using analytical methods. Subsequently, an Arbitrary Lagrangian-Eulerian (ALE) based numerical model is proposed to calculate the resulting residual stresses by incorporating the determined thermal-mechanical loads. This analytical-numerical hybrid method facilitates rapid and accurate modeling of residual stresses. Then, the Newton’s method is employed to deduce cutting parameters iteratively, ensuring a proper residual stress. Finally, the results of inverse analysis are compared with experimental measurements to validate its efficacy.

Investigation of the Influence of Built-Up Layer on the Stress State in the Primary Shear Zone Using Particle Image Velocimetry Analysis

In this study, a novel methodology was proposed to investigate the influence of the built-up layer (BUL) formation on the stress state distribution in the primary shear zone (PSZ) using analytical model and particle image velocimetry (PIV) analysis. Orthogonal cutting tests were performed under a range of uncut chip thicknesses and cutting speeds using two uncoated cemented carbide tools with different rake angles. A series of shear strain, shear strain rate, and velocity distributions in PSZ were obtained by PIV analysis. Al7075-T6511 was used as the workpiece. Subsequently, the influences of cutting conditions on the BUL/built-up edge (BUE) formation and the plastic deformation in PSZ were investigated. Using these results, the parameters of the proposed analytical model were identified, and the influences of the BUL/BUE formation on the stress state distribution were investigated. From the experimental results, it was found that in the cutting speed range below 2 m/min, only BUE is formed, and the uncut chip thickness and tool rake angle have a significant influence on its formation. The agreement between the measured and calculated results demonstrated the effectiveness of the proposed methodology. The results confirmed that the BUE formation has little effect on the bell-shaped distribution of shear strain rate, but has a significant influence on the thickness of PSZ, chip sliding velocity near the outlet boundary of PSZ, maximum shear strain rate, stress state, and temperature in PSZ. It was also confirmed that the stress triaxiality plays an important role in the BUE formation. These results provide a deeper understanding of the BUL/BUE formation.

Multi-physics analytical modeling of the primary shear zone and milling force prediction

Multi-physics analytical modeling of the primary shear zone and milling force prediction

Determination of fracture toughness and yield strength of Inconel 718 by milling operation

This paper presents an analytical scheme to determine the fracture toughness and the yield strength of Inconel 718 by the widely used milling operation. It is noted that there exists material fracture at the tool tip and the removed layer of material undergoes plastic deformation along the primary shear plane. The fracture toughness is considered at the tool tip and the yield strength is applied on the primary shear plane. Subsequently, the modified Williams’ model and the modified Williams-minimization model are both proposed on the basis of the geometries and mechanics in the oblique cutting process. The modelling framework indicates that milling can be developed as an alternative method to determine the fracture toughness and the yield strength from the serrated chip thicknesses combined with the corresponding cutting forces. This is because the experimental method by milling is a convenient scheme as it only requires the cutting forces per unit width of cut with respect to a series of uncut chip thicknesses. In addition, the normal shear angle is predicted by applying the principle of energy minimization. Finally, milling experiments are conducted to validate the two proposed methods. Results show that the two analytical methods return similar values of fracture toughness and yield strength, respectively. Besides, it is noted that high values of yield strength are achieved based on Tresca criterion, which would indicate work hardening in the primary shear zone during the serrated chip formation. The proposed methods exhibit advantages of convenience and generality to derive the fracture toughness and the yield strength, since it is still a challenge for the traditional standard mechanical tests for Inconel 718 due to its toughness and ductility.

Insight on turning of stainless steel 431 and modelling by ANN

ABSTRACT An insight study has been addressed to communicate the machining behavior of SS 431 in terms of tool wear rate (TWR) and chip study. Experimental investigation shows that cutting speed (VC) as the most significant parameter to have TWR for SS 431. At low VC 19.5 m/min, chip formed was continuous hairy structure with regular saw fractured morphology, principally promoted due to intensely concentrated shear bands between neighboring segments resulting from shear localized instability in the primary shear zone. 25.9 and 30.6 m/min produces continuous ribbon shape chip and twisted chip, respectively. At high VC 30.6 m/min, owing to high-temperature generation, shear deformation zone was softer and perfectively plastic with least chip contact length encouraged to produce least TWR. The parameters 30.6 m/min, 0.08 mm/rev, and 0.15 mm of VC, feed, and depth-of-cut found optimum to reach least TWR. Further ANN was exercised to predict TWR is good agreement with the experiments.

Modeling energy consumption inside cutting deformation zone to predict the stress distributions, temperature and microstructure by micro irreversible entropic thermodynamics

It is important to analyze the process of chip formation by means of the energy consumption during metal cutting, which is related to the cutting power (or cutting force), the cutting temperature and the machined surface integrity. In this study, the consumed energy field inside the cutting deformation zone is established by considering four micro irreversible processes including the dislocation generation, annihilation, movement, and continuous dynamic recrystallization. A new shear-angle solution is proposed on the basis of the steady conditions of irreversible processes (i.e., the entropy production rate reaches the minimum), whose adequacy is verified by the experimental results of the shear angle and friction coefficient along the tool-chip interface. Based on the predicted consumed energy field inside the cutting deformation zone, the deformation mechanism, microstructure, stress and temperature are predicted and analyzed. The results show that three stages including the loading stage, unloading stage and steady stage are divided during chip formation according to the consumed energy field. The consumed energy and statistically stored dislocation density are both of gradient distribution in the primary shear zone, and continuous dynamic recrystallization is the main mechanism of grain refinement during chip formation, which is dominated in the secondary deformation zone. The stress and temperature inside the cutting deformation zone as well as the hardness of the machined surface are predicted according to the energy balance relationship and consumed energy field, indicating the feasibility of using energy consumption to comprehensively describe the internal changes of workpiece material inside the cutting deformation zone.

Effects of cryogenic cooling on machining of acrylonitrile-butadiene rubber

The basic idea of cryogenic machining of elastomers is to lower the process temperature under glass transition temperature, causing the transformation of the viscoelastic properties of elastomers into brittle with better machinability outcomes. However, because of the heat generated by plastic deformation and chip formation in the primary shear zone and friction between the tool and the workspace, even with cryogenic cooling, the resulting temperature in the cutting area is often higher than the glass transition temperature. As a result, it can cause a partially rubbery state of the workpiece. In this paper, the effect of cryogenic cooling on the milling of acrylonitrile-butadiene rubber was analysed. Three different cooling setups, namely, indirect, direct and flow cooling, were proposed and their influence on temperature distribution in the cutting zone was studied in conjunction with different tool geometries and parameters (depth of cut, rotational speed, and feed rate). Direct cooling provides the best-resulting temperature distribution with the lowest surface roughness (1.27 to 1.47 μm) as it acts as a lubricant between the tool and workpiece and cools the tool and workpiece simultaneously.On the other hand, increasing the depth of cut and rotational speed also increases surface roughness. The best results show samples with grooves obtained with a rotational speed of 5000 rpm, depth of cut 0.25 mm and feed rates between 75 and 300 mm/min with surface roughness between 0.86 and 1.29 μm. Those samples show clean grooves with sharp edges, minimal surface roughness and geometric deviation, with defined ductile chip formation.

Estimation of temperature in the cutting area during orthogonal turning of grade 2 titanium

The authors introduce an experimental-analytical method for determining the average temperature values in the PSZ (primary shear zone) and the SSZ (secondary shear zone) during orthogonal turning of grade 2 titanium with a use of an uncoated carbide tool with a positive rake angle and a flat rake face. The presented method is based on an algorithm in which the values of shear stress in the PSZ and the SSZ are calculated by means of the Johnson–Cook constitutive equation and Oxley’s model of cutting mechanics. Average temperature values in the PSZ and the SSZ are determined by iteratively finding the minimum difference between the calculated stress values. As its inputs, the algorithm uses the values of the feed and the tangential cutting force components, the value of chip flow velocity on the rake face, and the constants of the Johnson–Cook constitutive equation. The model was validated with a use of empirical data collected during the experiments. The test rig consisted of a KNUTH Masterturn 400 precision lathe equipped with a dynamometer, a high-speed camera, and a thermal imaging camera.

Kinematic fields measurement during Ti-6Al-4V chip formation using new high-speed imaging system

Orthogonal cutting on Ti-6Al-4V produces segmented chips which results from localized deformation within the Primary Shear Zone (PSZ). For in situ visualization of the material flow during Ti-6Al-4V chip formation, a new high-speed optical system is proposed. Difficulties arise from the submillimetric size of the cutting zone. Therefore, a dedicated optical system with coaxial illumination is designed allowing for local scale analysis of chip formation. In this paper, the cutting forces were measured highlighting the unsteady nature of Ti-6Al-4V chip segmentation. For kinematic fields measurement, a novel method of Digital Image Correlation (DIC) technique is applied on the recorded images from the cutting zone. It allows for the identification of the localized deformation bands. Then, the level of the cumulative strain fields reached in the PSZ was presented and analyzed. The effect of the rake angle on the strain fields was studied. Using a 0∘ rake angle, the segment chip was found to be more subjected to deformation than in the case using 15∘. Accuracy of the measured strain fields was discussed function of the main source of errors. In addition, the chip morphology and microstructure were investigated with a scanning electron microscopy (SEM). It shows crack opening along the PSZ and high material failure near the tool-chip interface which explains the difficulties of DIC application in the context of kinematic fields measurement during orthogonal cutting. Finally, correlation between the measured strain fields and the mean value of the chip segment width was made.

An Analytical and Experimental Study on Cutting Characteristics and Transient Cutting Force Modeling in Feed Directional Ultrasonic Vibration-Assisted Cutting of High Strength Alloys.

Ultrasonic vibration-assisted cutting (UVC) is progressively being used in machining as it can significantly promote the fabrication process. However, the ultrasonic vibration affecting the cutting process is still controversial. The full-transient cutting process is proposed in this study to analyze the affecting mechanism induced by ultrasonic vibration in the cutting process. This novel model is the first developed based on the fact that ultrasonic vibration would change mechanical behaviors and the cutting process. For example, the reduction of shear flowing stress in the primary shear zone and alteration of the shear angle in the UVC process. Then, considering those coupled effects, a novel model is proposed to determine the average and transient cutting forces. Here, insight and understanding into the physical phenomenon in UVC are provided. The effectiveness of the proposed model is verified by comparison with experimental results and analytical models available in the literature, with cutting parameters varying from macro to micro-scale. The results show that the ultrasonic vibration affects the cutting process in a complicated way, which is determined by transient characteristics, acoustic softening, thermal softening, plowing, and friction. Those effects on cutting performances in the UVC process under various cutting scenarios are investigated and discussed systematically. The average deviation of cutting forces between experiments and values predicted by the proposed model for Ti6Al4V, AISI 1045, and Al6063 is about 7%, 10.2%, and 11%, respectively. The deviation decreases with the increase of cutting speed in the machining of Ti6Al4V, which is different from the machining of other materials. This is contributed by the varied effect of ultrasonic vibration on the cutting process.

High frequency cutting force measurement during saw tooth chip formation of Al-7075-T6 for critical flow stress estimation

Shear banding is an unstable mode of deformation caused by material beginning to soften or weaken with additional strain, resulting in narrow bands of highly deformed material called shear bands. Shear bands occur in metal cutting when deforming material in the primary shear zone is subject to weakening by ductile damage and/or thermal softening by adiabatic heating. Recent work [1] has shown that the force supported by shear bands induced in hat-shaped test specimens subject to high-speed compression can be measured during the evolution of shear bands. This has given rise to the question whether very high band-width force measurement can be used to quantitatively measure the strain softening of material during high-speed machining. The challenge impeding measurement of cyclic changes in cutting and thrust force during shear banding cycles in orthogonal cutting is the rate at which these shear bands typically occur – from a few kHz to 100 kHz. This is well above the lowest natural frequency of even stiff cutting force dynamometer setups. This paper describes the development and validation of a high bandwidth dynamometer. This is used to measure both the cutting and thrust forces during orthogonal cutting of AA 7075-T6 with unprecedented fidelity. The critical flow stress at which flow softening initiates is calculated from the measured forces and chip morphology.

On the Mechanical Response of Aluminum Alloy 6061-T6 under Extreme Strains and Strain Rates, and Rapid Heating

The flow stress of aluminum alloy 6061-T6 produced by conventional thermomechanical methods has been investigated under the application of strains up to 80% at a strain rate of the order of 50001/s, and temperatures ranging from ambient to near melting. The high strain and strain rate deformation was imposed using a Kolsky bar compression apparatus equipped with a fast pulse-current heating system to reach the target temperatures at high heating rates. The flow stress was measured under strains, strain rates, temperatures and heating rates that match thermomechanical conditions developed in the primary shear zone of a specially designed machining with chip pulling test method. Flow stress error is estimated by statistics obtained from experimental replicates. It is expected that these measurements will enable the formulation of realistic machining models for Al6061-T6. Temperature measurements were obtained using non-contact infrared (IR) full field imaging together with embedded micro-thermocouple (TC) point probing. The TC measurements were used to estimate the emissivity of the aluminum specimen via separate, in situ calibration tests, and to track temperature evolution during the compression, also via separate testing, not compromising the flow stress measurements. Type-K TC measurements were made using the separated junction principle by embedding the TC wires into two micromachined holes in the specimen. The temperature measurement technique is discussed in detail, and temperature uncertainties are estimated. The Kolsky bar and machining test with chip pulling will be used going forward to study the effect of heating time on the dynamic thermal softening behavior of Al6061-T6, which has been shown to be time-sensitive under quasi-static loading when temperatures exceed 200°C, due to Mg-Si precipitate growth.