Material Strengthening Effect Research Articles

Classification criteria for austenitic stainless steel H-section under cyclic loading about major-axis

Stainless steel exhibits strain hardening characteristics under cyclic loading, hence presenting significant potential for application in seismic design. However, the current stainless steel structural design code fails to account for this beneficial effect in the cross-section classification. Given the scarcity of research about the behavior of structural stainless steel members under cyclic stress, this paper establishes the preliminary finite element model for the austenitic stainless steel (EN 1.4301) H-section. Firstly, finite element model validation of the existing cyclic tests on stainless steel members from literature was performed, followed by numerical analyses of a total of 370 beam and beam-columns members with different axial compression ratios and plate width-thickness ratios, and subsequently evaluated the cross-section classifications specified in prEN 1993-1-4, AISC 370 and CECS 410 through extensive parametric studies. The results indicate that the current design codes are conservative in the provisions for cross-section classification because they neglect the influence of plate interactions and material strengthening effects during cyclic loading. In this paper, limit and generalized expressions for austenitic stainless steel H-section with different axial compression ratios and cross-section classes under cyclic loading about major-axis are proposed. Additionally, the effect of material strengthening on cross-section classification under cyclic loading is examined.

Crystal plasticity modeling for the strengthening effect of multilayered copper–graphene nanocomposites

The paper explores plastic deformation mechanisms in metal–graphene nanocomposites, illustrating their material strengthening effects through a crystal plasticity finite element (CPFE) model. This model is compared with published experimental results, which have previously shown that the two-dimensional shape of graphene effectively controls dislocation motion and significantly enhances the strength of metals. Simulated nanopillar compression tests, using a physics-based CP model that incorporates surface nucleation and single-arm source dislocation mechanisms, help us understand dislocation motions at submicron length scales. The crystal plasticity models are applied to nanolayered composites with copper grain layers and monolayer graphene, featuring repeat layer spacings of 200 nm, 125 nm, and 70 nm, respectively. This study quantifies the accumulation of dislocations at the graphene interfaces, contributing to the ultra-high strength of copper–graphene composites. Moreover, a Hall–Petch-like correlation is established between yield strength and the number of embedded graphene layers.

Thermal gradient and its contribution to size effect of specific cutting energy

The size effect of specific cutting energy is one of the most fundamental and challenging problems in metal cutting. The size effect phenomenon has been attributed to the material strengthening effect due to strain, strain rate, and strain gradient. However, the contribution of the material strengthening caused by temperature effect in the primary shear zone is rarely studied. Here, orthogonal cutting experiments under different initial temperature were conducted. It shows the significant dependence of the size effect phenomenon on the temperature effect. The finite element methods were further conducted to reveal the underlying reason of the size effect. It is found that there exists high-temperature gradient in the primary shear zone along the shear direction, which causes the average temperature in the primary shear zone to drop with decreased uncut chip thickness, giving rise to the size effect phenomenon. Furthermore, a new approach is developed to predict the highest temperature in the primary shear zone by numerical fitting.

Development and experimental validation of a mechanistic model of cutting forces in micro- ball end milling of full slots

ABSTRACTIn the present work, a mechanistic model of cutting forces is developed with a novel approach to arrive at the cutting edge geometry as well as the cutting mechanics. The geometry of cutting elements derived and verified using a virtual tool generated in CAD environment is considered. The cutting and edge force coefficients at every discrete point on the cutting edge of micro-ball end mill are established in a novel way from the basic metal cutting principles and fundamental properties of materials, considering edge radius and material strengthening effects. Further, measured edge radius is used in the model. Full slot micro-ball end milling experiments are conducted on a high-precision high-speed machining center using a 0.4 mm diameter tungsten carbide tool and cutting forces are measured using a high-sensitive piezo-electric dynamometer. It is established that the predicted as well as experimental cutting forces are higher at very low uncut chip thickness in comparison with the cutting edge radius in micro-ball end milling also. Amplitudes of cutting forces and instantaneous values with incremental rotation of the tool are compared with predicted values over two revolutions for validation of proposed model.

Micro-flank milling forces considering stiffness of thin-walled parts

A novel micro-flank milling force prediction model is developed in this study by considering the deflection of tool and workpiece, tool run-out, and material strengthening effects during the flank milling of thin-walled parts. The model of the cutting tool applied in this study is closer to the actual structure and its deflection model is established based on Euler Bernoulli cantilever beam theory under mesoscale. The values of the workpiece deflection are obtained with an online Keyence LK-H020 laser sensor. The Johnson−Cook constitutive model is adopted to estimate the flow stress σ JC, which takes in consideration the effects that strain-hardening, strain-rate, and thermal softening have on the flow stress. The mechanistic model is validated by a series of micro-thin-wall experiments with a two-flute KENNA micro-milling cutter as tool and Ti-6Al-4V titanium alloy as workpiece material. Experimental results illustrate that the proposed model performs well for the micro-flank milling forces in the x-direction, with an average error of 4.153%, while the error in the y-direction is slightly larger at 4.458%.

Finite Element Simulations of Micro Turning of Ti-6Al-4V using PCD and Coated Carbide tools

The demand for manufacturing axi-symmetric Ti-6Al-4V implants is increasing in biomedical applications and it involves micro turning process. To understand the micro turning process, in this work, a 3D finite element model has been developed for predicting the tool chip interface temperature, cutting, thrust and axial forces. Strain gradient effect has been included in the Johnson–Cook material model to represent the flow stress of the work material. To verify the simulation results, experiments have been conducted at four different feed rates and at three different cutting speeds. Since titanium alloy has low Young’s modulus, spring back effect is predominant for higher edge radius coated carbide tool which leads to the increase in the forces. Whereas, polycrystalline diamond (PCD) tool has smaller edge radius that leads to lesser forces and decrease in tool chip interface temperature due to high thermal conductivity. Tool chip interface temperature increases by increasing the cutting speed, however the increase is less for PCD tool as compared to the coated carbide tool. When uncut chip thickness decreases, there is an increase in specific cutting energy due to material strengthening effects. Surface roughness is higher for coated carbide tool due to ploughing effect when compared with PCD tool. The average prediction error of finite element model for cutting and thrust forces are 11.45 and 14.87 % respectively.

Analytical modeling and experimental validation of residual stress in micro-end-milling

Micro-milling is widely used in aerospace and precision optical part manufacturing. Residual stress is an important index of surface integrity, which signally affects the performance of the micro-parts. This paper presents an analytical model to predict micro-milling residual stresses considering tool edge radius, material strengthening effects, and initial stress. A micro-milling cutting force prediction model is proposed, in which tool edge radius and material strengthening effects are taken into account. The imaginary heat source is utilized to estimate the temperature distribution in the workpiece. This model considers the prediction results of cutting force and temperature as thermomechanical loads experienced by the workpiece. Also, the effect of initial stress is taken into account during the estimation of residual stresses. After loading, unloading, and stresses release, the results of residual stresses in micro-milling are finally obtained. Both the micro-milling cutting force and residual stresses prediction results are validated by NAK80 steel on a three-axis ultra-precision machine. The predicted results capture the experiment results well in terms of distribution and value. Finally, the model is analyzed and discussed. The influences of tool edge radius, rake angle, feed per tooth, and spindle speed on residual stresses are preliminarily explored.

Analytical modeling and experimental validation of micro end-milling cutting forces considering edge radius and material strengthening effects

This paper presents a novel micro end-milling cutting forces prediction methodology including the edge radius, material strengthening, varying sliding friction coefficient and run-out together. A new iterative algorithm is proposed to evaluate the effective rake angle, shear angle and friction angle, which takes into account the effects of edge radius as well as varying sliding friction coefficient. A modified Johnson–Cook constitutive model is introduced to estimate the shear flow stress. This model considers not only the strain-hardening, strain-rate and temperature but also the material strengthening. Furthermore, a generalized algorithm is presented to calculate uncut chip thickness considering run-out. The cutting forces model is calibrated and validated by NAK80 steel, and the relevant micro slot end-milling experiments are carried out on a 3-axis ultra-precision micro-milling machine. The comparison of the predicted and measured cutting forces shows that the proposed model can provide very accurate predicted results. Finally, the effects of material strengthening, edge radius and cutting speed on the cutting forces are investigated by the proposed model and some conclusions are given as follows: (1) the material strengthening behavior has significant effect on micro end-milling process at the micron level. (2) Cutting forces predicted increase with the increase of edge radius. (3) Considering varying sliding friction coefficient can enhance the sensitivity of the predicted cutting forces to cutting speed.

Investigations into Cutting Forces and Surface Roughness in Micro Turning of Titanium Alloy Using Coated Carbide Tool

Micro turning is one of the tool based micromachining process used for manufacturing axi-symmetric miniaturized parts. This paper presents the development of micro turning setup and investigations on cutting forces and surface roughness during micro turning process. Titanium alloy (Ti6Al4V) and coated carbide tool (TiN/AlTiN) is considered as work piece and tool material respectively. Experiments have been conducted by varying the cutting speed, feed and depth of cut. Piezoelectric dynamometer is used to measure the cutting forces during the process. Cutting forces decreases by increasing the cutting speed due to thermal softening where as at low depth of cut, cutting forces increases with increase of cutting speed due to material strengthening effect. Surface roughness increases when uncut chip thickness is less than the edge radius, due to rubbing and ploughing action. Improved roughness is observed when uncut chip thickness and depth of cut is greater than edge radius.

Mechanistic model for prediction of cutting forces in micro end-milling and experimental comparison

Micro end milling is an important process in the manufacture of micro and meso scale products and has an advantage of creating more complex geometry in a wider variety of materials in comparison with other micro-machining methods. In this paper, a new methodology for predicting the cutting coefficients considering the edge radius and material strengthening effects is presented. Further a mechanistic model is developed to predict the cutting forces in micro end milling operation taking into account overlapping tooth engagements. The mechanistic model, derived from basics considering material property and principles of metal cutting, is valid for a wider range of cutting parameters. The model is validated with the results from micro slot end-milling of mild steel carried out on the basis of full factorial design. On comparing the amplitudes of cutting forces, it is seen that mechanistic model predicts the transverse force with an average absolute error of 12.29%, while a higher prediction error of 19.49% is obtained for feed force. Additionally the mechanistic model is able to predict the variations in the cutting forces with rotation of the cutter and average absolute deviations of 13% and 11% are obtained for feed and transverse forces, respectively.

Material Strengthening Mechanisms and Their Contribution to Size Effect in Micro-Cutting

The specific cutting energy in machining is known to increase nonlinearly with decrease in uncut chip thickness. It has been reported in the literature that this phenomenon is dependent on several factors such as material strengthening, ploughing due to finite edge radius, and material separation effects. This paper examines the material strengthening effect where the material strength increases nonlinearly as the uncut chip thickness is reduced to a few microns. This increase in strength has been attributed in the past to various factors such as strain rate, strain gradient, and temperature effects. Given that the increase in material strength can occur due to many factors, it is important to understand the contributions of each factor to the increase in specific cutting energy and the conditions under which they are dominant. This paper analyzes two material strengthening factors, (i) the contribution of the decrease in the secondary deformation zone cutting temperature and (ii) strain gradient strengthening, and their relative contributions to the increase in specific cutting energy as the uncut chip thickness is reduced. Finite element (FE)-based orthogonal cutting simulations are performed with Aluminum 5083-H116, a work material with a small strain rate hardening exponent, thus minimizing strain rate effects. Suitable cutting conditions are identified under which the temperature and strain gradient effects are dominant. Orthogonal cutting experiments are used to validate the model in terms of cutting forces. The simulation results are then analyzed to identify the contributions of the material strengthening factors to the size effect in specific cutting energy.