Dead Metal Zone Research Articles

A slip-line field model for independently characterizing shearing and ploughing effects in metal cutting processes

This study proposes a new slip-line field model to separately characterize the shearing-included cutting process and the pure-plough cutting process. The shear plane and dead metal zone in relation to the shearing effect are modelled by straight slip-lines, while the deformation area in relation to the ploughing effect is treated to have straight boundaries. The boundaries of the dead metal zone and ploughing area are determined by considering tool-workpiece frictional behaviours. The stresses acting on the boundaries of the ploughing area, dead metal zone and shear plane are separately modelled considering the thermal–mechanical coupling effect. Based on the minimum energy principle, the shear angle is originally modelled by following the dynamic requirement of chip flow to balance the forces acting on dead metal zone, shear plane and rake face. By iteratively solving the coupling effect among the temperature, stress and shear angle, the total cutting forces are acquired by integration operation through straight slip-lines. Experiment results and finite element simulations validate the proposed slip-line field model in predicting the shear angle and cutting forces for both micro and regular milling processes.

Prediction of tool-chip contact length based on slip-line modeling of orthogonal cutting with rounded-edge cutting tools

It is known that no cutting tool has a perfectly sharp cutting edge and that there is always a small rounding on the cutting edge of the tool. In this study, a new tool-chip contact length equation is presented for cutting tools with rounded-edge in orthogonal cutting. This equation was obtained based on a newly developed slip-line model. This newly developed slip-line model has rounded-edge cutting and considers the presence of a dead metal zone in front of the cutting edge. To verify the newly developed tool-chip contact length equation, experimental studies were conducted at various cutting speeds and uncut chip thickness values at both 0° and 6° rake angles, using cutting tools with 50 μm–100 μm–150 μm edge radii. Orthogonal cutting experiments were carried out using the shaping-based quick stop device and brass (CuZn30) material was chosen as a workpiece that allows continuous chip formation without built-up edge. Strong correlation was observed between the experiment results and values that were obtained from the newly developed tool-chip contact length equation.

Modelling of grain size effects in progressive microforming using CPFEM

Progressive microforming is widely recognized as one of the most efficient and desirable methods of mass production in micromanufacturing arena. To predict the deformation behaviour and size effects of materials induced in progressive microforming, the finite element method (FEM) employed for modelling the microforming process needs to account for microstructure details and deformation/failure mechanisms of the materials. This led to the development of the novel crystal plasticity finite element method (CPFEM) and cohesive zone model (CZM). Previous research and application of CPFEM have been mainly limited to simple deformations such as uniaxial tension and compression, whereas the new method can provide physical insights into how the grain size affects the interplay between crystallographic slip and mechanical twinning in complex microforming, and further material deformation during sheet blanking. A case study was conducted to manufacture a hexagonal socket part using a three-step progressive microforming system, with the comparison between experiments and CPFEM simulations focusing on microstructure evolution, deformation load, and product quality. The CPFEM was found to be more reliable than the conventional FEM in predicting complex deformation, particularly in microstructure and texture evolution, dimensional accuracy and irregular geometries. Results showed that the total height of the part increases with the decreasing grain size, while the head diameter rises with grain size. Simulations successfully anticipated the distributions of dead metal zones and shear bands and identified hole and rollover geometries and corresponding grain size effects. In conclusion, this research facilitates the understanding of grain size effects on the deformation behaviour in progressive microforming and presents a novel approach and strategy for modelling, prediction, and product quality assurance in complex micro deformation and forming processes.

Charge weld evolution in hollow aluminum extrusion: Experiments and modeling

Charge welds occur in billet-to-billet extrusion processes due to the transition between successive billets. The material in this transition zone is typically characterized by substandard mechanical properties. In industrial practice, a specified length of the extruded profile is cut away and recycled as in-process scrap. Therefore, understanding the extrusion mechanisms affecting the properties of the charge weld is crucial to controlling product quality, reducing non-conformities and improving the extrusion yield. This paper provides new insights into thermo-mechanical mechanisms associated with charge weld of circular aluminum tubes. A large number of carefully-controlled, full-scale extrusion experiments were conducted in an industrial environment to research charge weld evolution mechanisms and influential parameters. In addition, a finite element (FE) model was developed to predict the charge weld evolution across the cross section and along the profile. The experimental and numerical results show good agreement in terms of the location of the charge weld within the extruded profile. Moreover, a comparison with analytical models in the literature reveals that the FE model provides a significantly more accurate prediction of the length of the extruded profile affected by the charge weld (‘scrap length’). Based on the validated FE model, a sensitivity study was done to explore the effect of process parameters on charge weld evolution, particularly focusing on material flow and dead metal zone. The results show that the ram speed influences the charge weld evolution, while changes in billet temperature are insignificant. In conclusion, the findings presented in this study provide new insights that serve as practical guidance to the mechanism of charge weld evolution. They also highlight the applicability and limitations of numerical and analytical methods in assessing industrial extrusion problems.

On unsteady-cutting state material separation and dead metal zone modeling considering chip fracture

An accurate dead metal zone (DMZ) model can well understand the material separation and flow mechanism in the cutting process. Most available DMZ models considered the cutting state steady with complete chip formation and sufficient material piled up. To investigate unsteady-cutting state material separation, this paper proposed an improved DMZ model considering the chip fracture. The uncut chip thickness (UCT), which causes the instantaneous maximum pressure, is chosen as the criterion of the DMZ generation at an unsteady-cutting state. The DMZ model is divided into four types, whose geometry and position were theoretically calculated and verified by finite-element method (FEM) and turning experiments corresponding to Al7075-T6 with a maximum error of 12.5 %. Based on the DMZ modeling, the instantaneous shearing and ploughing effects are quantitatively analyzed. Furthermore, metallographic analyses were carried out for further observing the material flow mechanism. The results showed that different UCTs can result in distinct cutting states. Lower UCT led to the pure ploughing or ploughing dominating the cutting process, resulting in trivial chip generation. With the UCT increasing, the shearing dominates gradually and produces continuous chips. This research can contribute to in deeply investigating the instantaneous material flow and further material removal mechanism in the machining process.

A novel variable rake angle tool design for fabricating ultrafine-grained pure copper strips with improved formability and strain controllability

Extrusion machining (EM) is an efficient and convenient method to prepare ultrafine-grained (UFG) materials. As a vital parameter in the EM process, tool rake angle (α) is critical to control the material's shear strain and grain size. The grains tend to refine with decreasing α, whereas a small α leads to difficult cutting, chip breakage, and poor surface quality. To achieve the controllable strain distribution, grain size adjustment, and improved formability of UFG pure copper strips, this paper proposed a novel design of the variable rake angle (VRA) tool. Combining experiments and finite element simulation, extrusion machining with variable rake angle tools (VRA-EM) was investigated. The results implied that a deviating angle (β) for the material flow existed during VRA-EM. The β increased as the cutting thickness (tc) increased, resulting in decreased plastic deformation, strain, and microhardness. The microstructure was also characterized by electron backscatter diffraction system (EBSD) and transmission electron microscopy (TEM) techniques. A dead metal zone was formed using the VRA tool, in which the material flow behavior was quite weak. The obtained UFG strip was further divided into the low-strain zone (LSZ) and high-strain zone (HSZ) based on strain distribution. And a new theoretical model of shear strain was established and conformed to experimental values. Accordingly, VRA-EM process was able to improve the mechanical properties and strain controllability, showing promising prospects for efficiently fabricating UFG materials.

Optimization of dead metal zone to reduce cutting forces in micro milling of Inconel 718 using RSM

This study focuses on the mechanism of DMZ (dead metal zone) creation, as well as the impact of cutting edge geometries (sharp, chamfered, double chamfered, and blunt edges), cutting speed, and coefficient of friction on DMZ formation while milling Inconel 718 material (FEM). A non-contact type sensor called a laser doppler vibrometer (LDV) is used to monitor the vibration of rotating surfaces. In current research work, the LDV is used to measure the mill cutter vibration in micro-milling of Inconel 718 in terms of acoustic optic emission signals. A FFT (fast fourier transformer) is used for signals processing in to frequency domain. Design of experiments as per Taguchi, experiments were performed on the alloy at three levels of spindle speeds, depth of cuts, feed rates. Experimental results on the amplitude o vibration of tool along X and Y directions, surface roughness were measured and analysed using response surface methodology. Analysis obtained from the variance was used to recognize the significant parameters which effect the vibration of tool and roughness of surface. RSM was implemented and optimized process parameters for the minimum vibration amplitude and surface roughness.

Energy balance model to predict the critical edge radius for adhesion formation with tool wear during micro-milling

Cutting edge of the micro-milling tool rapidly loses its sharpness and experiences early adhesion that degrade the machinability and surface quality. Although rapid edge-rounding and early adhesion were reported in the literature, the condition that promotes the adhesion development was not established. For dry machining of Ti-6Al-4V with TiAlN-coated WC-6Co micro-mills, an energy balance model is proposed in this article that guides the condition for the onset of adhesion with progressive tool wear. Initially, tool wear progression through successive stages of rapid abrasion, steady abrasion, adhesion, and edge-chipping is experimentally characterised. Adhesion at the tool-tip is found to develop when the abrasively worn-out edge radius reaches a critical value. Thereafter, a model is presented considering the dynamic growth of the dead metal zone (DMZ) with tool wear to capture the changes in the tribological contact and stress distribution at the interfaces. The chip-DMZ contact is the crucial factor in governing the critical edge radius for adhesion initiation, while the chip-tool contact has no direct role. Adhesion occurs when worn-out edge radius reaches 3.98–4.26 μm during micro-milling at a low feed of 1.0 μm/flute, whereas 4.0 μm/flute feed permits adhesion-free cutting up to 5.02–5.32 μm edge radius. Higher speed can favourably retard adhesion development. The inherently occurring phenomena like chip accumulation and non-cutting passes in low-feed micro-milling can favourably extend adhesion-free machining. However, coating delamination promotes adherence at an early stage in high-feed machining.

The Effect of the Bridge’s Angle during Porthole Die Extrusion of Aluminum AA6082

During the porthole die extrusion, the separated metal streams are welded together in the welding chamber. The conditions under which this occurs and the integrity of weld seam in the extrudate are impacted by the design of the bridge, including features such as its shape and dimensions. In this research, the commercial finite element method (FEM) software package, DEFORM, was used to run a series of simulation experiments in order to quantitatively understand the relationship between the bridge design and the thermal mechanical history experienced by the material during welding and the impact this has on final weld seam quality. The bridge can be roughly divided into two parts: the lower part, close to the welding chamber, and the upper part, which initially split the billet into metal streams. The results showed that increasing the lower bridge angle led to slightly higher extrusion loads and higher extrudate exit temperatures. As the lower bridge angle increased, creating a streamlined profile to a blunt profile, a dead metal zone formed under the bridge that produced higher strains near the surface of the material. In contrast, changes to the geometry of the upper bridge had little effect on the porthole die extrusion process or the thermal mechanical conditions experienced by the material.

Effect of Elongation of a Rod on Strain Inhomogeneity and Shape Change During the Compression-Type Forming Process

Abstract The influence of elongation on the strain inhomogeneity and shape change in twinning-induced plasticity steel rod is systematically investigated to understand the macroscopic shear band (MSB) formation mechanism and to decrease the strain inhomogeneity during the compression-type forming processes. Specimens fabricated by rod flat rolling with elongation (3D rod) and by plane compression without elongation (2D rod) are compared using both finite element analysis and experiment. Despite the similar final product shape, the 2D rod presents a lower effective strain at the surface region than the 3D rod, leading to a high strain inhomogeneity. The higher effective strain at the surface region of the 3D rod is mainly attributed to the elongation of the 3D rod during the rolling. In contrast, the 2D rod exhibits strong dead metal zones owing to the lack of elongation of the specimen. Therefore, the formation of MSBs or strain inhomogeneity of a specimen can be reduced by increasing the elongation of the specimens during the forming process. Both the contact width and lateral spread of the 3D rod are lower than those of the 2D rod because of the elongation of the 3D rod originating from the slip effect at the rod–roll interface during the rolling process. The small frictional effect at the rod and roll interface increased the elongation of the rod, leading to a decrease in the strain inhomogeneity and lateral spreading in the 3D rod.

Thermal–mechanical model for machining with chamfered insert considering tool flank wear

The deformation of material leads to severe thermal–mechanical effects during metal cutting with chamfered insert. The high cutting temperature on the contact face results in the wear of the insert which is a critical issue. However, there is still a lack of a comprehensive understanding of thermal–mechanical effects, especially with respect to the presence of the tool flank wear. To address this, an analytical thermal–mechanical model is developed in this paper for the prediction of the temperature distribution with a worn insert based on a modified slip-line field approach. Firstly, an analytical slip-line field model is introduced based on material plasticity and plowing theory considering the tool flank wear. The structure of the slip-line field model, especially at the dead metal zone and the flank zone, is modified to reveal the material flow mechanism. Then, heat sources, including primary heat source, secondary heat source, tertiary heat source, and fourth heat source, are clarified to explain the heat generation phenomenon in the cutting process. In addition, a cutting temperature field model is developed to show the effect of tertiary heat source based on the imaginary heat source theory. Finally, the slip-line field geometry and temperature distribution are extracted from two-dimensional finite element simulation to verify the proposed model. And orthogonal experiments are carried out for measuring the maximum cutting temperature. The good agreement of predicted results, simulated results, and experimental results verifies the accuracy of the proposed model.

New slip-line field model based on dead metal zone and material flow ın negative rake orthogonal cutting

New slip-line field model based on dead metal zone and material flow ın negative rake orthogonal cutting

On cutting process damping for small cutters by including the influences of the dead metal zone and elastic recovery

Existing ploughing-based process damping models oriented for the cutting processes did not consider two important factors, i.e. the dead metal zone (DMZ) in front of the rounded cutting edge and the material elastic recovery rate under the clearance face, and thus, they cannot exactly detect the underlying mechanism of process damping in the cutting processes with small cutters. This article systematically investigates the generation mechanism of cutting process damping for small cutters, and proves that the DMZ and the elastic recovery rate have combined contribution to the process damping of small cutters’ cutting. It is theoretically found that in contrast to the full elastic recovery assumed in the macro cutting processes, the material under the cutter’s clearance face is partially recovered in the cutting processes with small cutters, due to the relatively high proportion of plastic deformation. The actual recovery height is numerically calibrated based on the ideal plastic assumption of the stress-strain relationship. Expressions of the statically and dynamically indented areas, which are required for deriving process damping model, are analytically formulated as the functions of the DMZ and the actual recovery height since they greatly change the indentation boundaries between the rounded cutting edge and the clearance face. Above derivations are then utilized to derive the expressions of dynamic cutting forces, and subsequently, the process damping effect is considered to analyze the milling stability. Finally, a series of milling tests with small cutter prove the reasonability of the proposed process damping model.

Study on the Formation Mechanism of Cutting Dead Metal Zone for Turning AISI4340 with Different Chamfering Tools.

Tools with chamfered edges are often used in high speed machining of hard materials because they provide compelling cutting toughness and reduced tool wear. Chamfered tools are also responsible for the dead metal zone (DMZ). Through numerical simulation of orthogonal cutting with AISI 4340 steel, this paper examines the mechanism of the DMZ, the cutting speed, the impacts of the chamfer angle, and the coefficient of friction on the generation of the DMZ. The analysis is based upon the Arbitrary Lagrangian-Eulerian (ALE) finite element method (FEM) for the continuous process of chip formation. The different chamfered angles, cutting speeds, and friction coefficient conditions are utilized in the simulation. The research demonstrates that a zone of trapped material called DMZ has been formed beneath the chamfer and serves as an effective cutting edge of the tool. Additionally, the dead metal zone DMZ becomes smaller while the cutting speed increases or the friction coefficient decreases. The machining forces rise with increasing chamfer angles, rise with increasing friction coefficients, and fall with increasing cutting speed in both the cutting and thrust directions. In this paper, the effect of different chamfering tools on AISI 4340 steel using carbide tools in the simulation environment is studied. It has certain reference significance for studying the formation mechanism of the dead zone of difficult-to-machine materials such as AISI4340 and improving the processing efficiency and workpiece surface quality.

Strain and strain rate hardening effects on the macroscopic shear bands and deformation shape of a caliber-rolled wire

The effect of the strain hardening exponent (n) and strain rate sensitivity (m) on the strain distribution and surface profile of a workpiece during the caliber rolling process has been investigated to fully understand the influence of material properties on shape change and strain distribution of the caliber-rolled wire. Twinning-induced plasticity (TWIP) steel with high n and low m values and low carbon ferritic steel with low n and high m values were mainly compared using both experimental test and finite element analysis. The macroscopic shear bands were clearly observed in the caliber-rolled wire, indicating that the deformation is highly inhomogeneous during the caliber rolling process. The homogeneity of the strain distribution in caliber-rolled wire increased with increasing n and m values due to the fast propagation of the plastic region to other areas from the first hardened area in a material. The effect of the n value was higher than that of the m value. Consequently, TWIP steel had weaker macroscopic shear bands compared to plain carbon steel. The length of the maximum spread of caliber-rolled wire in the free surface increased with decreasing n and m values because the dead metal zone can be easily formed with decreasing n and m values. Therefore, the caliber roll design applied to plain carbon steel cannot be applied to TWIP steel because the roundness of caliber-rolled TWIP steel was poor in such a case. A roll gap thus needs to be modified with the n and m values of a material to fabricate the desired shape of a product.

Effects of welding angle on microstructure and mechanical properties of longitudinal weld in 6063 aluminum alloy extrusion profiles

In order to explore the effects of welding angle on microstructure and mechanical properties of longitudinal weld, a porthole die with different welding angles was designed and manufactured, by using which various extrusion experiments were carried out. The microstructure and mechanical properties of longitudinal weld were characterized, and the extrusion process was simulated. The results show that as welding angle increases, the longitudinal weld gradually transforms from a linear shape to a cross shape. A larger welding angle will not only reduce stress triaxiality and strain rate in the main welding zone, but also increase area of the dead metal zone, thus leading to poor welding quality. The crystal orientations of <100> and <111>//ED can be found in the welding zones of the profiles extruded by using the dies with different welding angles, and the texture components exhibit significant differences. There are a certain number of dislocations, a small amount of Mg2Si precipitates and insoluble iron-containing intermetallic compounds in the welding zones. By comprehensively considering the influence of welding angle on welding quality, extrusion load, effective stress and strain of the porthole die, the welding angle ranges under different welding quality requirements were determined. It is found that the welding quality is higher when the range of welding angle is 38–46°, the welding quality is lower when the range of welding angle is 54–68°. If taking in account both of welding quality and porthole die strength requirement, the preferred welding angle is recommended to be in the range of 46–54°.

Flow behavior and microstructure evolution of Ti-6Al-4V titanium alloy produced by selective laser melting compared to wrought

Nowadays, selective laser melting (SLM) represents an option for manufacturing parts from titanium alloys, especially from Ti-6Al-4V alloy. However, mechanical properties of parts made of SLM-produced Ti-6Al-4V, such as ductility and fatigue resistance, are significantly lower than that of wrought ones. The promising way to improve mechanical properties of SLM parts without expensive hot isostatic pressing can be combination SLM and deformation post-processing. Therefore, the goal of the present study is to investigate the flow stress behavior and microstructure evolution of Ti-6Al-4V fabricated by SLM in a wide temperature range in comparison with conventional manufactured material. In contrast to wrought material, SLM-produced Ti-6Al-4V shows high plastic flow instability at test temperatures of 20–900 °С, which was demonstrated via temperature sensitivity and softening rate. FEM-simulation of hot isothermal compression of samples was conducted in order to compare the deformation inhomogeneity between SLM-produced and wrought materials, namely, local strains e in the severe plastic deformation and dead metal zones. The microstructure in these characteristic zones of both materials deformed in (α + β)-field was examined by means of optical microscopy. The type of microstructure and grain size of the SLM-produced and wrought material were studied. It turned out that the deformation of SLM-produced Ti-6Al-4V at a lower temperature (800 °C) and a higher strain rate (1 s−1) makes it possible to obtain ultrafine-grained microstructure. This, in turn, opens the door to the production of Ti-6Al-4V parts with high mechanical properties via processing route SLM + metal forming.

Finite element analysis of the Non-Equal Channel Angular Pressing (NECAP) with different die geometries

The Non Equal Channel Angular Pressing is one of the promising severe plastic deformation techniques which is used to produce ultra-fine grained and nanostructured materials. In the present investigation, the deformation behavior of Al1100 alloy during non-equal channel angular pressing was studied using two dimensional finite element simulation. Results showed when the ratio of the width of output to input channel is higher than 0.7, the corner gap is formed in the outer side of the intersecting area of two channels and the amount of plastic strain decreases in the lower part of the sample. In contrast, when the amount of this ratio is lower than 0.7, the dead metal zone is formed in the outer region of the intersecting area and the amount of strain is increased in the lower side of sample. It is also observed that the uniformity of the applied plastic strain is decreased with the formation of dead metal zone. In addition, the amount of damage factor is increased with increasing the ratio of the width of output to input channel and the region with higher amounts of damage factor is shifted from the lower side to the middle region of deformed sample. Finally, the results of FEM simulation showed that the amount of pressing force is decreased with increasing the width of output channel.

Experimental and numerical study of the size effect on compound Meso/Microforming behaviors and performances for making bulk parts by directly using sheet metals

Meso/microforming of bulk multi-scaled parts and components by directly using sheet metals is an efficient approach to realizing mass production of meso-/micro-scaled bulk structures with good productivity and low cost. This process is promising with the large-scaled application potentials. In this unique deformation-based meso-/micro-scaled manufacturing, size effect arises due to the size scaling up and down of the extrinsic and intrinsic parameters of materials and forming systems, which further induces different mechanical responses and deformation behaviors in meso/microscale from those in macroscale. In this research, a compound microforming system for a blanking-heading process was developed to produce plug-shaped bulk parts by directly using copper sheets as a case study. Different punch-die clearances and grain sizes of specimen were employed to study the interactive effects of geometry and grain sizes on the microforming process and the micro-formed part. Through numerical simulations and experimental measurements of the final parts, the influences of size effect on microstructural evolution, geometrical precision and surface defects of the meso-/micro-formed parts and the load-stroke relationship were comprehensively investigated. The results reveal that when punch-die clearance equals grain size, the maximum ultimate shear stress of blanking and the highest burr are obtained. The larger grain size and punch-die clearance increase the material loss and reduce the bulge diameter of the produced parts. Three shear bands and three dead metal zones were identified on the cross-section of parts, and various defects including sunken area, pits, crack and surface damage were observed on the surface of the parts. These findings facilitate the production of plug-shaped microparts in the aspects of process monitoring and product qualities control and enrich the understanding of sheet-metal bulk forming in this progressive and compound meso/microforming.

Solid-state welding and microstructural features of an aluminium alloy subjected to a novel two-billet differential velocity sideways extrusion process

A novel process for fabricating cross-sectional shapes of curved profiles in industrial applications, two-billet differential velocity sideways extrusion (DVSE), is proposed in this study. The feasibility of the process is demonstrated by fabricating solid bars of the aluminium alloy AA1070 through the solid-state welding of two billets at elevated temperature. Microstructural examination has shown that the weld formed between the two billets consists of an unsound area in the dead metal zone within the die and a sound bonding area in the welding zone. Along the welding path, the effective and normal strains gradually increase while the shear strain decreases, leading to the transformation of grains from equiaxed to bamboo-like structures and increases in the hardness, average grain size, and fraction of low angle grain boundaries. The shear strength of the welded extrudate is larger than that of the material without welding. The effective strain in the welding zone is larger than that in other zones. Increasing the temperature or speed of extrusion decreases the unsound bonding area length and the shearing angle, thus improving the uniformity of distribution of the shear strain. The grain diameter is refined from ∼500 μm in the initial billet to ∼47 μm in the welding zone of the extrudate formed at 0.05 mm/s and 450 ℃. The hardness of the extrudate formed at 400 °C and 0.1 mm/s is ∼19 % larger than that of the initial billet, and is decreased by decreasing the extrusion speed or increasing the temperature.