Large Strain Extrusion Machining Research Articles

Surface integrity of pure titanium strips produced by large strain extrusion machining at varying speeds

Surface integrity of pure titanium strips produced by large strain extrusion machining at varying speeds

Crystallographic texture evolutions of Ti-6Al-4V chip foils in relation to strain path and high strain rate arising from large strain extrusion machining process

Large strain extrusion machining (LSEM) is a promising process to create chip foils with ultrafine grains and a certain type of crystallographic texture. Disclosing the interconnection between the deformation histories and the crystallographic texture variations resulting from the LSEM process is vital for the texture generation and control of Ti-6Al-4V chip foils. Electron-backscattered diffraction and viscoplastic self-consistent (VPSC) simulation were combined based on numerical simulation to study evolutions of crystallographic textures for chip foils during LSEM of Ti-6Al-4V. The deformation histories during LSEM process were also analyzed by coupling theoretical analysis with digital image correlation technique. The role of individual type of slip system on the texture variation of Ti-6Al-4V chip foils was identified by modifying the Voce hardening parameters in VPSC model. It is seen that strain path controlled by the chip thickness ratio changes the inclination of generated textures in chip foils. Basal and 1-st order pyramidal <c+a> slip systems are the main deformation modes in high strain rate deformation condition of LSEM process. Similar textures are formed under large strain and high strain rate deformation in LSEM comparing with that in free machining process. The conclusions are useful to control the crystallographic texture generation in LSEM of Ti-6Al-4V by optimizing cutting speeds and chip thickness ratios.

Experimental And Numerical Investigation Of Large Strain Extrusion Machining Of AZ31 Magnesium Alloy For Biomedical Applications

Magnesium alloys represent promising bioresorbable orthopedic materials thanks to their biocompatibility, non-toxicity, and mechanical properties that are pretty close to the ones of the human bone. Nevertheless, the major drawback that impairs their widespread adoption in the biomedical field is represented by their unsatisfactory corrosion resistance once placed in the human body environment. Several research studies have demonstrated that an effective method to control the degradation rate of magnesium alloys is represented by a proper conditioning of the surface characteristics of the alloy. With this in mind, the present work proves the feasibility of using Large-Strain Extrusion Machining (LSEM), namely a hybrid cutting-extrusion process, as a strategy to sensibly affect the surface features of the AZ31 magnesium alloy, and, as a consequence, its corrosion performances. The microstructure and micro-hardness close to the machined surface was investigated after LSEM. The experimental results were exploited to develop, calibrate and validate numerical simulations able to describe the microstructural and mechanical phenomena that occur under LSEM. The proposed approach shows that the simulation model can represent a useful tool to predict the magnesium alloy machining performances, thus reducing the need for numerous and time-consuming experimental tests to a great extent.

Research on the influence of tool geometric parameters on the LSEM mechanism of 7A04 aluminum alloy

In order to explore the influence of tool geometric parameters on the large-strain extrusion machining (LSEM) mechanism of 7A04 aluminum alloy, based on the Abaqus finite element (FE) analysis software, the LSEM process of 7A04 aluminum alloy is simulated, with focus on how equivalent stress, equivalent strain, cutting temperature and cutting force change with tool geometric parameters. The results show that as cutting tool rake angle increases, equivalent stress first decreases then increases; equivalent strain first increases then decreases and then increases again; cutting temperature first decreases then increases; cutting force first decreases then increases. As constraining tool corner radius increases, equivalent stress first increases then decreases; equivalent strain, cutting temperature and cutting force are all positively correlated with constraining tool corner radius. Adding micro-textures can effectively increase the equivalent strain generated by LSEM and reduce the cutting force generated by LSEM. If a smaller cutting force were considered an indicator of a better tool geometric parameter, then α = 15°, R = 0, and adding rectangular-sectioned strip texture only on the constraining tool should be the best option.

A Finite Element Study of Large Strain Extrusion Machining Using Modified Zerilli–Armstrong Constitutive Relation

Abstract The objective of this work is to study the performance of modified Zerilli–Armstrong constitutive relation proposed in our previous study for the finite element modeling of a severe plastic deformation technique called large strain extrusion machining. The modified Zerilli–Armstrong constitutive relation is implemented in a finite element model of large strain extrusion machining of Inconel 718 to analyze the influence of process parameters, i.e., the chip compression ratio and tool–chip friction, on deformation, effective strain distribution, and hydrostatic pressure distribution along the extruded chip. The predicted strain values for different chip compression ratios were validated by comparison with those obtained through an analytical model. The finite element predictions also served as a guideline in designing the large strain extrusion-machining setup on which experiments were conducted to generate Inconel 718 foils with superior mechanical properties. The predicted limits of chip compression ratio were in close agreement with experimentally realizable values. Furthermore, the predicted strain distribution through the thickness of the chip was validated with the results of hardness measurement tests. Microstructural characterization of the Inconel 718 foils was carried out by using both optical and transmission-electron microscopic studies in order to reveal the presence of fine-grain structures. The validations showed the effectiveness of the modified Zerilli–Armstrong constitutive relation in modeling large strain extrusion machining—a variant of the conventional machining process.

Sliding wear behaviour of CP titanium laminates produced by large strain extrusion machining

Titanium sheets find their wide application in aerospace industry. Rolling process is commonly used for making these sheets. Inducing large amount of strain in high strength materials with HCP structure such as Magnesium and Titanium alloys makes rolling a multistage process. Moreover, controlling crystallographic texture in multistage deformation processes is also very critical. To overcome these limitations, in present research attempt large strain extrusion machining (LSEM) process is used to produce laminates from CP titanium sheets with controlled microstructure in a single continuous operation. These CP Ti laminates are produced at different strain rate. Microstructural analysis and hardness measurement of fabricated laminates are carried out using scanning electron microscopy and Vickers microhardness tester respectively. Roughness tester is used for roughness measurement. X-Ray diffraction is used for deformation analysis of the laminates. Sliding wear tests on produced laminates are performed using tribometer. Hardness measurement revealed a ~25–52% increase in hardness value in fabricated laminates as compared to bulk material. This increase in hardness may be attributed to grain refinement resulted from high deformation induced by LSEM process. The surface roughness of fabricated laminates decreased with increase in strain rate. Wear rate analysis of the samples have shown improved wear performance of laminates fabricated by the process in comparison to bulk material. Wear rate is observed to be lower in direction perpendicular to laminate formation direction.

Towards understanding the microstructure and temperature rule in large strain extrusion machining

Large strain extrusion machining (LSEM) is a typical process for preparing ultrafine or nanocrystalline strips. It is based on large plastic deformation. The processing parameters of LSEM in this study were optimized by experiments and simulations. Using the orthogonal array, signal-to-noise ratio, and analysis of variance, the influence and contribution of processing parameters on response variables were analyzed. Because of the difference in processing parameters between optimizing the average grain size and the maximum temperature, the response variables analyzed must be correctly selected. Furthermore, the optimal processing parameters for obtaining the minimum average grain size and the lowest maximum temperature are analyzed. The results show that the tool rake angle is the most important factor. However, the level of this factor required to achieve the minimum average grain size is different from that required to obtain the lowest maximum temperature. The validity of the method is verified through experiments and simulations.

Single-Step Shear-Based Deformation Processing of Electrical Conductor Wires

Abstract Commercial electrical conductor wires are currently produced from aluminum alloys by multi-step deformation processing involving rolling and drawing. These processes typically require 10 to 20 steps of deformation, since the plastic strain or reduction that can be imposed in a single step is limited by material workability and process mechanics. Here, we demonstrate a fundamentally different, single-step approach to produce flat wire aluminum products using machining-based deformation that also ensures adequate material workability in the formed product. Two process routes are proposed: (1) chip formation by free-machining (FM), with a post-machining, light drawing reduction (&amp;lt;20%) to achieve desired finish and (2) constrained chip formation by large strain extrusion machining (LSEM). Using commercially pure aluminum conductor alloys (Al 1100 and EC1350) as representative material systems, we demonstrate key features of the machining-based processing, including (a) single-step processing to achieve flat wire geometries, (b) surface finish (Ra = 0.2 to 1.0 μm) comparable to that of commercial wire products made by drawing/rolling, (c) deformation control independent of wire size, and (d) hardness increases of 50–150% over that of annealed wires, while retaining high electrical conductivity (&amp;gt;56% IACS). The wire microstructure, which can also be varied via the large-strain deformation parameters, is correlated with mechanical and electrical properties. Implications for commercial manufacture of flat wire products are discussed.

Microstructure evolution of ultrafine grained aluminum alloy prepared by large strain extrusion machining during annealing

As a new type of severe plastic deformation (SPD) method for efficiently preparing ultrafine grained (UFG) material, large strain extrusion machining (LSEM) has attracted the attention of researchers. At the same time, the UFG material prepared by the process has a small size, and the influence of external heat is involved in the subsequent utilization. Therefore, in this paper, the microstructure evolution of UFG 6061 Al alloy chips prepared by LSEM during annealing has been investigated. The UFG chips were subjected to isochronous and isothermal annealing treatments. The microstructure of UFG chips was characterized by scanning electron microscope (SEM) with electron backscattered diffraction (EBSD) system. The results showed that with the increase of annealing temperature, the UFG aluminum alloy chips underwent three processes of recovery, recrystallization and grain growth. When the annealing temperature was 160 °C, with the extension of annealing time, the UFG aluminum alloy chips remained at the recovery stage, the point defects and dislocations were reduced, the dislocation density was reduced, and the grain size was not changed significantly.

Investigation of large strain extrusion machining (LSEM) of pure magnesium (Mg)

Large strain extrusion machining (LSEM) is used to produce pure magnesium strips directly from as-cast billets. LSEM reduces manufacturing steps and the special shear direction inclines the unfavorable strong basal texture of Mg. Finding suitable parameter ranges to control geometry and properties of the produced strip is a great challenge. In the present research, a finite element model is proposed and verified with experiments and previous findings. Next, effects of process parameters (temperature and pressure) on geometrical and microstructural properties of the produced strips are investigated. Studying the involved failure micro-mechanisms shows that the process temperature and hydrostatic pressure have adverse effects on the production of a flawless strip. To reduce the temperature and hydrostatic pressure throughout the LSEM process many different conditions are tested by the FEM model. Among all the examined conditions, 0.85 mmRev feed and 530 Revmin rotation speed yields the most promising results it terms of low temperature, low hydrostatic pressure, and stable machining force. The proposed conditions are applied in the experimental setup and a uniform and flawless strip is obtained.

Quantitative analysis of microstructure refinement in ultrafine-grained strips of Al6063 fabricated using large strain extrusion machining

In present research work, ultrafine-grained strips of Al-6063 alloy were fabricated using hybrid extrusion machining technique known as “large strain extrusion machining (LSEM).” Fabrication of strips was done using the customized HSS tools of different rake angles varying from 0° to 10° under different machining conditions. Microstructural and mechanical characterizations of these strips were done to ascertain the effect of different parameters on their properties. From the results of hardness measurement of strips, it was concluded that hardness of the strips increased by 34–97% of the base material as of the refinement of grain size occurred. Surface lay was improved by 30% with higher cutting velocity and rake angle. Crystallite size was found to decrease with increase in the rate of strain. The shear strain was increased as chip compression ratio increased and rake angle decreased. Fabrication ability of strips increased due to increase in strain hardening exponent and it may result in the large scope of their applications. Nano-hardness of the strips was found to be more than bulk alloy. These above said results showed that ultrafine strips fabricated using LSEM process can become a good choice for future material fabrication.

Study on Surface Hardness and Microstructure of Pure Copper Chip Strips Prepared by LSEM

Large strain extrusion machining (LSEM) is one of the severe plastic deformation (SPD) methods that can improve the mechanical properties of materials. The purpose of this experiment is to study the surface hardness and microstructure of the pure copper chip strips. It was found that most of the grains of the chip strips had been refined to the ultrafine grain grade. Finite element analysis (FEA) simulations were conducted to predict the von Mises equivalent strains. Based on the analysis of variance (ANOVA), further study indicated that the surface hardness of the chip strips was decided by several key parameters including the chip thickness compression ratio, rake angle, and uncut chip thickness during LSEM. Through this analysis, a set of parameters which have the greatest impact on the properties of the material can be found. This set of parameters helps us to achieve the strip with the best performance.

Ultrafine grained Al 7075 alloy fabricated by cryogenic temperature large strain extrusion machining combined with aging treatment

In this study, cryogenic temperature large strain extrusion machining (CT-LSEM) as a severe plastic deformation (SPD) method is applied to fabricate chips with ultra-fine grained (UFG) structure. The chip morphology, microstructure evolution, aging behavior, as well as micro-hardness characteristics of a solution treated (ST) Al 7075 alloy subjected to CT-LSEM and room temperature (RT) LSEM are investigated. It is observed that the processing temperature has a great influence on the chip morphology, i.e., the chips produced by CT-LSEM have better surface quality than that by RT-LSEM. The SPD process of LSEM, especially under CT, can accelerate the aging kinetics of the alloy. Much more deformation energy stored in the CT-LSEM sample could promote the dynamic recrystallization. A higher dislocation density exists in the CT-LSEM samples due to effective suppression of dynamic recovery. The optimized hardness for all samples is obtained at an aging temperature of 120 °C. The hardness of CT and RT LSEM samples after peak aging (PA) have increased from 102 Hv to 201 Hv and 192 Hv respectively. A large number of fine and closely spaced ηʹ and η precipitates are present in the LSEM samples after PA treatment, accounting for the further enhancement of hardness. The ST Al 7075 samples subjected to LSEM are severely deformed. The mean grain/sub-grain size of PA samples has hardly changed as compared with the initial LSEM samples. It is feasible to realize simultaneous hardening including grain refinement, dislocation and precipitation in the Al 7075 alloy via application of CT-LSEM and subsequent aging.

Effects of Machining Velocity on Ultra-Fine Grained Al 7075 Alloy Produced by Cryogenic Temperature Large Strain Extrusion Machining.

In this study, cryogenic temperature large strain extrusion machining (CT-LSEM) as a novel severe plastic deformation (SPD) method for producing ultra-fine grained (UFG) microstructure is investigated. Solution treated Al 7075 alloy was subjected to CT-LSEM, room temperature (RT) LSEM, as well as CT free machining (CT-FM) with different machining velocities to study their comparative effects. The microstructure evolution and mechanical properties were characterized by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Vickers hardness measurements. It is observed that the hardness of the sample has increased from 105 HV to 169 HV and the chip can be fully extruded under CT-LSEM at the velocity of 5.4 m/min. The chip thickness and hardness decrease with velocity except for RT-LSEM at the machining velocity of 21.6 m/min, under which the precipitation hardening exceeds the softening effect. The constraining tool and processing temperature play a significant role in chip morphology. DSC analysis suggests that the LSEM process can accelerate the aging kinetics of the alloy. A higher dislocation density, which is due to the suppression of dynamic recovery, contributes to the CT-LSEM samples, resulting in greater hardness than the RT-LSEM samples.

Development of ultrafine grained Al 7075 by cryogenic temperature large strain extrusion machining

Abstract

Thermal Stability of Ultrafine Grained Pure Copper Prepared by Large Strain Extrusion Machining

Ultrafine grained (UFG) pure copper chips with improved material strength have been successfully prepared by large strain extrusion machining (LSEM). However, the thermal stability of the UFG chips has been a key characteristic that has restricted their use in practical applications. To understand the influence of annealing temperature and annealing time on their microstructures and mechanical properties, the UFG chips were subjected to isochronous and isothermal annealing treatments as well as Vickers hardness tests in the present study. From the results, we found that the UFG chips maintain high hardness when annealing at temperatures up to 160 °C but begin to exhibit a reduction in their hardness while the annealing temperature reached above 200 °C. When annealed at 280 °C for 10–240 min, the grain size increased slightly and reached a stable value of 2 µm with an increase in annealing time and with a decrease in the hardness of the chips. These results indicated that UFG pure copper chips have good thermal stability at temperatures below 160 °C.

Enhancing workability in sheet production of high silicon content electrical steel through large shear deformation

Enhanced workability, as characterized by the magnitude and heterogeneity of accommodated plastic strains during sheet processing, is demonstrated in high Si content Fe-Si alloys containing 4 and 6.5 wt% Si using two single-step, simple-shear deformation techniques – peeling and large strain extrusion machining (LSEM). The model Fe-Si material system was selected for its intrinsically poor material workability, and well-known applications potential in next-generation electric machines. In a comparative study of the deformation characteristics of the shear processes with conventional rolling, two distinct manifestations of workability are observed. For rolling, the relatively diffuse and unconfined deformation zone geometry leads to cracking at low strains, with sheet structures characterized by extensive deformation twinning and banding. Workpiece pre-heating is required to improve the workability in rolling. In contrast, peeling and LSEM produce continuous sheet at large plastic strains without cracking, the result of more confined deformation geometries that enhances the workability. Peeling, however, results in heterogeneous, shear-banded microstructures, pointing to a second type of workability issue – flow localization – that limits sheet processing. This shear banding is to a large extent facilitated by unrestricted flow at the sheet surface, unavoidable in peeling. With additional confinement of this free surface deformation and appropriately designed deformation zone geometry, LSEM is shown to suppress shear banding, resulting in continuous sheet with homogeneous microstructure. Thus LSEM is shown to produce the greatest enhancement in process workability for producing sheet. These workability findings are explained and discussed based on differences in process mechanics and deformation zone geometry.

Large strain extrusion machining of magnesium alloys for biomedical applications

Recently, magnesium alloys are attracting more and more attention as degradable materials for manufacturing temporary biomedical devices, although their rapid degradation in physiological environment limits their clinical applications to a great extent. Different Severe Plastic Deformation (SPD) processes have been recently applied to magnesium alloys in order to improve the surface integrity, which is directly correlated to their corrosion resistance. The current study investigates the possibility of exploiting Large Strain Extrusion Machining (LSEM) as a processing route to increase corrosion resistance of magnesium alloys for biomedical applications. Different cooling conditions and cutting speeds were adopted during LSEM and their effects on the surface integrity and corrosion resistance on both the machined workpiece and obtained chips were studied. For the first time, liquid nitrogen was used as cooling medium in LSEM and its effect was properly investigated. Results showed that LSEM, regardless of the adopted cutting parameters, is an effective method to obtain a workpiece with improved functional performances. Similar results pertain to the chips, but a careful choice of process parameters is even more mandatory than in the case of the workpiece.

Large Strain Extrusion Machining under Cryogenic Cooling to Enhance Corrosion Resistance of Magnesium Alloys for Biomedical Applications

Magnesium alloys are attracting more and more attention for producing temporary prosthetic devices thanks to their bioresorbable characteristics in human environment. However, they present a reduced corrosion resistance to body fluids, which still limits their applications to a great extent. In this work, severe plastic deformation through large strain extrusion machining is explored, as a route for the surface and sub-surface modification of the AZ61 magnesium alloy with the aim of improving its corrosion resistance. Different process parameters, namely cooling condition and cutting speed, were adopted and their effects on the machined surface integrity and corrosion resistance were investigated. The obtained results show that cryogenic cooling applied to large strain extrusion machining can represent an efficient strategy to enhance the magnesium alloys poor corrosion behavior, regardless the adopted cutting speed.

Creation of ultrafine-grained surfaces by large strain extrusion machining (LSEM)

ABSTRACTLarge strain extrusion machining (LSEM) is examined as a route for achieving controlled microstructure refinement at freshly generated surfaces in a single pass of the machining tool. It is shown that the extrusion ratio λ of LSEM, which is the ratio of the thickness of the chip to that of the preset depth of cut, controls the extent of the ultrafine-grained (UFG) zone. Microstructure analysis was performed using orientation imaging microscopy (OIM) and mechanical testing using nanoindentation was used to characterize the UFG microstructure beneath the freshly generated surfaces. The mechanics of deformation in LSEM were examined using infrared thermography and modeled. The present research demonstrates LSEM as a novel platform for tailoring surficial microstructures and controlling their spatial extents in fabricated components.