Large Strain Extrusion Machining Research Articles

Grain Refinement and Thermal Stability of AISI1020 Strips Prepared by Large Strain Extrusion Machining

Low carbon steel (AISI1020) strips with grain refinement were successfully produced by large strain extrusion machining (LSEM).A finite element simulation was performed to make comprehensible the deformation behavior of LSEM process. The influence of annealing temperature and annealing time on the microstructure and mechanical properties of strips was investigated by two sets of heat treatments and Vickers hardness test. As a result, strips can maintain high hardness under 400°C but start losing it as the temperature increased to 500°C and above. When annealED at 300°C for 1~9h, hardness of strips can maintain at almost the same level as that before annealing. Obvious hardening was found when annealing at 200~300°C mainly because of the dislocations atresia. Despite of the anneal-hardening behavior, these results indicated that the extruded AISI1020 strip has a good thermal stability at temperature below 400°C.

Direct Single-Stage Processing of Lightweight Alloys Into Sheet by Hybrid Cutting–Extrusion

Widespread application of lightweight magnesium and titanium alloys sheet is limited mainly because of their poor-workability issues, both in primary processing by rolling and secondary sheet forming. This study describes a hybrid cutting–extrusion process, large-strain extrusion machining (LSEM), for producing sheet and foil. By utilizing a constraining edge placed across from the cutting tool edge, the usual cutting process is transformed into continuous shear-deformation process, wherein the thickness of the sheet at its exit from the deformation zone is directly controlled. The confinement of the deformation field in LSEM enables near-adiabatic heating in the deformation zone. Consequently, external workpiece heating, intrinsic to sheet manufacturing by multistage rolling in alloys of poor workability (e.g., hexagonal close packed (hcp) alloys and cast materials), is minimized. Furthermore, the deformation parameters, such as strain, strain rate, and strain path, can be controlled to refine the microstructure and induce shear-type crystallographic textures that enable enhanced sheet mechanical properties (strength and formability). This application of LSEM is demonstrated using magnesium alloy AZ31B as a model system. Since LSEM is a single-stage process for sheet production, it is potentially attractive in terms of production economics and energy. Implications for process scale-up and control of plastic flow localization are briefly discussed.

Suppression of repeated adiabatic shear banding by dynamic large strain extrusion machining

High speed machining (HSM) is an advanced production technology with great future potential. Chip serration or segmentation is a commonly observed phenomenon during high speed machining of metals, which is found to be ascribed to a repeated shear band formation fueled by thermo-plastic instability occurring within the primary shear zone. The occurrence of serrated chips leads to the cutting force fluctuation, decreased tool life, degradation of the surface finish and less accuracy in machine parts during high speed machining. Hence, understanding and controlling serrated chip formation in HSM are extremely important. In this work, a novel dynamic large strain extrusion machining (DLSEM) technique is developed for suppressing formation of serrated chips. The systematic DLSEM experiments of Ti–6Al–4V and Inconel 718 alloy with varying degrees of imposed extrusion constraint were carried out. It is found that there is a prominent chip morphology transition from serrated to continuous state and shear band spacing decreases with the constraint degree increasing. In order to uncover underlying mechanism of the imposed extrusion constraint suppressing repeated adiabatic shear banding in DLSEM, new theoretical models are developed where the effects of extrusion constraint, material convection due to chip flow and momentum diffusion during shear band propagation are included. The analytical expressions for the onset criterion of adiabatic shear band and shear band spacing in DLSEM are obtained. The theoretical predictions are in agreement with the experimental results.

Characterization of the deformation field in large-strain extrusion machining

Large-strain extrusion machining (LSEM) has been emerged as a promising severe plastic deformation methodology for the creation of nano or ultra-fined grained materials. To realize deformation control, the key issue involved is the strain estimation in LSEM. In order to characterize the deformation field in LSEM, the experiments of LSEM oxygen-free high-conductivity copper were conducted by using a specially designed LSEM device. Based upon the deformation field measured by high speed imaging and digital image correlation (DIC), a new strain estimation model considering the extrusion process of constraint is proposed in this paper. The theoretical predicted strain agrees well with the measurements.

Enabling shear textures and fine-grained structures in Magnesium sheet by machining-based deformation processing

The production of Mg alloy AZ31B sheet in a single deformation step by large- strain extrusion machining (LSEM) is detailed. LSEM imposes intense simple shear in a narrow zone by constrained chip formation. The confined deformation and the associated in situ adiabatic heating are found to be the key factors in production of the Mg sheet without need for external (pre-) heating. A range of shear textures with basal planes inclined to the sheet surface are achieved by this processing. The basal plane inclination could be varied by controlling the strain path. Microstructures, both ultrafine-grained (100-500 nm) and conventional fine-grained (2-5 μm), have been obtained by controlling the adiabatic heating and the extent of dynamic recrystallization. The LSEM sheet with shear texture and fine grain size shows superior combinations of formability and strength compared to rolled sheet.

Thermal stability of ultrafine grained aluminium alloy prepared by large strain extrusion machining

In the present study, ultrafine grained (UFG) Al alloy chips with average grain sizes of ∼200 nm were successfully prepared by large strain extrusion machining (LSEM) process using a combined cutting tool with rake angle of 10° and chip compression ratio of 1.0. The tests showed that the Vickers hardness of the UFG Al alloy is significantly improved due to grain size reduction. To understand effect of heat on the microstructure and mechanical properties, the UFG chips were subjected to heat treatment at different temperatures and different annealing time durations. When annealed <100°C, most of fine grains within the UFG chips were found to be replaced by elongated grains whose grain sizes increased with a significant increase in the aspect ratio as the annealing time increased. Despite such increase in grain size, the Vickers hardness was not reduced as expected because of the precipitation of secondary phases. When annealed at temperatures up to 200°C, recrystallisation occurred, along with grain growth, but the Vickers hardness did not deteriorate because of precipitation of secondary phases, as before. However, annealing at temperatures of 300°C and above resulted in significant reduction in hardness of the chips due to dominance of grain growth over secondary precipitation. These results indicated that UFG Al alloy chips have a good thermal stability at temperatures <200°C.

Investigation of the Effect of Rake Angle on Large Strain Extrusion Machining

This paper describes how tool rake angle affect the chip formation during large strain extrusion machining. The simulation studies were corroborated by experiments where pure copper workpieces were extrusion machined using different rake angles and the machined chips were characterized for microstructure and mechanical properties. Both simulation and experimental results showed that rake angle had a significant effect on deformation field parameters and cutting force. Associated to the nanostructured materials formation, grain refinement in primary deformation zone is due to the larger strain imposed. And the increase in hardness in machined chips was a consequence of the reduced grain sizes.

Incipient straining in severe plastic deformation methods

Abstract

Effect of Constraining Tool Corner Radius on Large Strain Extrusion Machining

The severe plastic deformation (SPD) technique called large strain extrusion machining (LSEM) is being increasingly being used to fabricate bulk nanostructured materials. During LSEM, the morphology (microstructure) and physical/mechanical properties of the deformed chip depend on the amount of strain induced during processing. It is therefore critical to understand the mechanism and process of workpiece deformation and the correlation between the deformation and machining parameters has also been studied. The current study attempts to understand the influence of constraining tool corner radius (R) on the deformation behavior in LSEM. Finite element analysis (FEA) has been presented in this article to understand the relationship between strain and strain rate and changes in constraining tool corner radii. FEA results show different effective strains that can be produced during LSEM by changing the constraining tool corner radius, and thus the micro and nanostructure of the resulting bulk material can be controlled. Samples have also been fabricated by LSEM using various tool corner radii and their microstructures have been characterized.

Controlling texture in magnesium alloy sheet by shear-based deformation processing

Constrained chip (sheet) formation by large strain extrusion machining is used to impose effective strains of ∼1 in Mg alloy AZ31B sheet in a single step of deformation. High-speed image analysis shows the deformation underlying sheet formation to be simple shear that is confined to a narrow zone. This confinement of the deformation limits the need for pre-heating of the workpiece to realize continuous sheet forms. Tilted-basal textures, wherein the basal poles are inclined from the sheet surface normal, are achieved by this processing. These textures are quite different from those prevalent in rolled sheet. By controlling the strain path, the basal pole inclination could be varied in the range 32–53°. The primary texture component is the B-fiber, indicating basal slip to be the main deformation mode over the temperature range 165–400°C. An additional C2-fiber component appears above 250°C due to the activation of pyramidal 〈c+a〉 slip. In conjunction with these textures, microstructures ranging from ultrafine-grained (∼200nm) to fine-grained (∼2μm) could be obtained by controlling the deformation temperature. Implications of the results for production of Mg sheet are discussed.

Effect of the Friction Coefficient on Large Strain Extrusion Machining

The present study focused on the influence of the friction coefficient on the deformation behavior in large strain extrusion machining (LSEM). A series of simulation results of effective strain were obtained under different friction coefficients by conducting finite element simulations with a FEM code. The results show that LSEM can produce different effective strains by changing the friction coefficients, thus enabling the fabrication of bulk nanostructured materials. An analysis of the variation of effective strain through the chip demonstrated that the chip deformed much more inhomogeneously when the friction coefficient became larger. The obtained results can offer valuable guidelines for later LSEM studies.

Mechanics of large strain extrusion machining and application to deformation processing of magnesium alloys

An analysis of the mechanics of large strain extrusion machining (LSEM), a constrained chip formation process, is presented for deformation processing of bulk alloys. The deformation field is shown to be narrowly confined and controllable, with attributes ranging from conventional deformation processing to severe plastic deformation. Controllable deformation parameters include strain/strain rate, hydrostatic pressure, temperature and deformation path. These attributes are highlighted in deformation processing of Mg AZ31B, an alloy of commercial significance but noted for its poor workability, into sheet and foil forms. Noteworthy features of the processing are suppression of segmentation, realization of a range of strains and deformation rates, engineering of microstructures ranging from conventional to ultrafine grained, and creation of sheet/foil from the bulk in a single step of deformation without pre-heating. Guidelines for realizing specific sheet attributes, and scalability of LSEM for production are analyzed and discussed.

Analysis of Large‐Strain Extrusion Machining with Different Chip Compression Ratios

Large‐Strain Extrusion Machining (LSEM) is a novel‐introduced process for deforming materials to very high plastic strains to produce ultra‐fine nanostructured materials. Before the technique can be exploited, it is important to understand the deformation behavior of the workpiece and its relationship to the machining parameters and friction conditions. This paper reports finite‐element method (FEM) analysis of the LSEM process to understand the evolution of temperature field, effective strain, and strain rate under different chip compression ratios. The cutting and thrust forces are also analyzed with respect to time. The results show that LSEM can produce very high strains by changing in the value of chip compression ratio, thereby enabling the production of nanostructured materials. The shape of the chip produced by LSEM can also be geometrically well constrained.

Deformation field in large-strain extrusion machining and implications for deformation processing

The deformation field in large-strain extrusion machining, a constrained chip formation process, is characterized using high-speed imaging and particle image velocimetry. The field is shown to be controllable and narrowly confined, with attributes ranging from conventional deformation processing to severe plastic deformation. Implications for deformation processing of sheets, engineering of microstructures in bulk forms and on machined surfaces, and study of large-strain deformation phenomena in metal alloys are highlighted.

Influence of processing parameters on wear resistance of nanostructured OFHC copper manufactured by large strain extrusion machining

The tribological behaviour of three ultra-fine grained oxygen-free high conductivity (OFHC) copper materials manufactured by large strain extrusion machining (LSEM), under variable shear strain, is studied under ball-on-flat reciprocating configuration and compared with that of conventional microstructured copper. The results are discussed as a function of microstructure and sliding direction. The lowest wear volume is obtained when the sliding takes place in the perpendicular direction to that of grain orientation. The highest wear resistance is observed for nanostructured copper material with an elongated grain structure in the extrusion direction. The wear resistance of this anisotropic material depends on the sliding direction. Wear mechanisms are discussed from scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) observations.

Analysis of Severe Plastic Deformation by Large Strain Extrusion Machining

Large strain extrusion machining (LSEM), a constrained chip formation process, is examined as a method of severe plastic deformation (SPD) at small deformation rates for production of ultra-fine-grained (UFG) materials. A finite element procedure is developed for prediction of deformation field parameters such as effective strain, strain rate, and their variation across the thickness of the chip for various cutting (extrusion) ratios. The cutting force (extrusion pressure) and hydrostatic pressures within the deformation zone are also analyzed. A consideration of the deformation occurring in chip formation suggests bounds on the extrusion ratios that can be realized. Implications of the results for production of bulk chip samples of controlled geometry and with an UFG microstructure are discussed.

Micro-scale components from high-strength nanostructured alloys

A general approach for manufacturing of micro-scale components from high-strength, nanostructured materials is presented. The approach utilizes severe plastic deformation by large-strain extrusion machining to create the nanostructured material in a high-strength alloy system, and conventional micro-machining to produce the components. Manufacture of small-scale gears from nickel-based superalloy Inconel 718 is illustrated.

Production analysis of new machining-based deformation processes for nanostructured materials.

Nanostructured materials, composed of sub-micron sized grains (crystals), show novel attributes not commonly found in their microcrystalline counterparts. Furthermore, these attributes can be varied by changing the grain size. Nanostructured solids have high hardness, strength and ductility; and interesting electrical and magnetic properties. Superplasticity has been observed at relatively low temperatures in some of these materials [1]. Many new and exciting applications of these materials have been sought; however, the cost of creating these materials, frequently quoted in excess of three hundred euros per kilogram [2], and an inability to make these on a large-scale with reproducible properties have restricted their broad application. We describe new machining-based manufacturing processes for making nanostructured alloys that are likely to be much less expensive. These processes are structured around chip formation by large-strain machining. It has been found that chips of a variety of metals and alloys produced during normal machining operations are composed entirely of nanocrystalline and/or ultrafine grained (UFG) structures of high hardness and strength [3-5]. Furthermore, by imposing a constraint to the chip at the point of its formation, it is feasible to produce nanostructured metals and alloys directly in bulk forms (e.g., sheet, bar, foil); the constrained chip formation process is called large-strain extrusion machining [6].

Friction and wear of nanostructured metals created by large strain extrusion machining

Tribological behavior of nanocrystalline oxygen-free high conductivity (OFHC) copper and commercially pure titanium produced by large strain extrusion machining (LSEM) is compared with the coarse grained counterparts. Friction and wear of these materials have been determined using a pin-on-disk tribometer, sliding against AISI 52100 steel pins. Although friction coefficients are very similar, microstructure refinement reduces wear factors for these conditions. Wear mechanisms are discussed from optical microscopy and SEM observation of wear tracks, wear debris morphology and transfer tribolayers.

Bulk nanostructured materials by large strain extrusion machining

Large strain extrusion machining (LSEM) is presented as a method of severe plastic deformation for the creation of bulk nanostructured materials. This method combines inherent advantages afforded by large strain deformation in chip formation by machining, with simultaneous dimensional control of extrusion in a single step of deformation. Bulk nanostructured materials in the form of foils, plates, and bars of controlled dimensions are shown to result by appropriately controlling the geometric parameters of the deformation in large strain extrusion machining.