Effect Of Strain Path Change Research Articles

Deformation, recrystallization and plastic anisotropy of asymmetrically rolled aluminum sheets

Materials processed under various thermo-mechanical conditions exhibit a different plastic anisotropy. Both conventional and asymmetric rolling processes were applied to a 6016 aluminum alloy for automotive application. A sequence of different rolling schedules bears an effect on the recrystallization and plastic behavior of the material through the annealing texture. The effect of strain path change and annealing conditions on the recrystallization texture is investigated and the experimental data are modeled with the help of crystal plasticity models. The plastic anisotropy is analyzed based on experimentally measured Lankford values and calculated limiting drawing ratios. The calculated limiting drawing ratio of ∼2.17 for asymmetrically rolled sheets represents a considerable improvement as compared to the conventionally produced materials with the highest limiting drawing ratio of ∼2.09.

Research on the drawing process with a large total deformation wires of AZ31 alloy

Magnesium and their alloys have been extensively studied in recent years, not only because of their potential applications as light-weight engineering materials, but also owing to their biodegradability. Due to their hexagonal close-packed crystallographic structure, cold plastic processing of magnesium alloys is difficult. The preliminary researches carried out by the authors have indicated that the application of the KOBO method, based on the effect of cyclic strain path change, for the deformation of magnesium alloys, provides the possibility of obtaining a fine-grained structure material to be used for further cold plastic processing with large total deformation.The main purpose of this work is to present research findings concerning a detailed analysis of mechanical properties and changes occurring in the structure of AZ31 alloy wire during the multistage cold drawing process. The appropriate selection of drawing parameters and the application of multistep heat treatment operations enable the deformation of the AZ31 alloy in the cold drawing process with a total draft of about 90%.

The Material Flow Analysis in the Modified Orbital Forging Technology

The main aim of this work is the computer aided design of the new orbital forging process. The finite element model was developed and used in research on possibility of modification of the classical orbital forging technology based on the Marciniak press to obtain more effective process. Obtained numerical results from simulations of the new orbital process are compared with the experimental analysis, performed on the orbital press with the developed device. However, due to the novelty of the developed approach the investigation on direction of material flow during deformation is of particular interest in this work. Direction of material flow and strain path change effect due to incremental character of deformation is analyzed. Obtained results confirm good predictive capability of the FE model and are the basis for the comparison of the two processes and discussion on the effectiveness of the modified incremental forming process.

Development and Validation of a Numerical Model of Rolling with Cyclic Horizontal Movement of Rolls

AbstractThe development of a numerical model capable of simulating the forming processes characterized by additional reversible movement of tools is the subject of this paper. The major assumption of this process is the introduction of additional shear stresses into the material in order to induce the strain path change effect. The scope of the problem is very broad, so the focus in the present work is on the process of flat rolling with additional oscillatory movement of rolls along their axes. Due to the sophisticated nature of this process particular attention is paid to the reduction in computational time by minimizing the number of required finite elements. The developed model is used to study differences in material behaviour during rolling with different roll velocities. A detailed description of the developed numerical model and examples of obtained results are presented in this paper. A comparison with experimental analysis is presented, as well.

Evaluation of the Work-Hardening of Brass Sheets Following Strain Path Changes

The strain paths followed by metals during sheet forming can be quite complex, especially when successive forming steps are involved. The work hardening of metals associated with these strain paths differs from that caused only by monotonic straining, such as simple tension or compression. It is important to have an adequate description of the work hardening of the material under processing, especially when numerical simulations of the forming are used. The experimental evaluation of the effect of strain path changes on the material work hardening is usually performed through tensile testing following the strain path changes. This technique, however, demands complex machining operations of the formed sheets and the imposed strain is severely limited by impending necking. The present paper utilizes simple shear as a tool for the determination of the work hardening of CuZn34 brass sheets following various strain path changes associated with combinations of different modes of deformation such as rolling, tension, cyclic and forward shears. The results indicate that the cyclic shearing delays the occurrence of plastic instabilities for brass previously tensioned, occurring the opposite for final monotonic shearing. These phenomena were correlated with the probable microstructural evolution of the CuZn34 brass.

Influence of Strain Path on Microstructure Evolution of Low Carbon Steels

Changes in strain path represent one of the most important processing parameters that characterise hot metal forming processes. In the present study, the effect of strain path change on dynamic recrystallisation, strain-induced precipitation processes and phase transformation behaviour in plain carbon and Nb-microalloyed steels was investigated. To assess the effect of strain-path change, forward/forward and forward/reverse torsion tests were conducted. It has been shown that the strain reversal delays the dynamic recrystallisation kinetics whereas its effect on strain-induced precipitation process of Nb(C,N) is rather negligible. Also the onset of austenite-ferrite transformation is delayed; its products however doesn’t change significantly. This can be due to the fact that ferrite nucleation density plays the second order role compared to the geometry of deformation.

A composite dislocation cell model to describe strain path change effects in BCC metals

Sheet metal forming processes are within the core of many modern manufacturing technologies, as applied in, e.g., automotive and packaging industries. Initially flat sheet material is forced to transform plastically into a three-dimensional shape through complex loading modes. Deviation from a proportional strain path is associated with hardening or softening of the material due to the induced plastic anisotropy resulting from the prior deformation. The main cause of these transient anisotropic effects at moderate strains is attributed to the evolving underlying dislocation microstructures. In this paper, a composite dislocation cell model, which explicitly describes the dislocation structure evolution, is combined with a BCC crystal plasticity framework to bridge the microstructure evolution and its macroscopic anisotropic effects. Monotonic and multi-stage loading simulations are conducted for a single crystal and polycrystal BCC metal, and the obtained macroscopic results and dislocation substructure evolution are compared qualitatively with the published experimental observations.

The effect of strain path change on subgrain volume fraction determined from in situ X-ray measurements

The evolution of dislocation structures in individual bulk grains in copper during strain path changes is studied with a new in situ synchrotron technique which combines high angular resolution with fast three-dimensional reciprocal space mapping. Deformed copper contains regions with vanishing dislocation density called subgrains bounded by dislocation rich walls. With the new technique reciprocal space maps, consisting of sharp peaks arising from the subgrains superimposed on a cloud of lower intensity arising from the dislocation walls, are obtained, which allows properties such as subgrain volume fraction to be quantified. The studied strain path changes are tension-tension sequences. Polycrystalline copper sheets are pre-deformed in tension to 5% strain, and tensile samples are cut with varying angles between the first and second loading axis. The second tensile deformation up to additional 5% strain is performed in situ while mapping a selected X-ray reflection from one particular bulk grain with high angular resolution. The reciprocal space maps are analyzed with a recently developed fitting method, and a correlation is found between the evolution of the subgrain volume fraction and the degree of strain path change the sample is subjected to.

Orientation and Path Dependence of Formability in the Stress- and the Extended Stress-Based Forming Limit Curves

This article analyzes the formability data sets for aluminum killed steel (Laukonis, J. V., and Ghosh, A. K., 1978, “Effects of Strain Path Changes on the Formability of Sheet Metals,” Metall. Trans. A., 9, pp. 1849–1856), for Al 2008-T4 (Graf, A., and Hosford, W., 1993, “Effect of Changing Strain Paths on Forming Limit Diagrams of Al 2008-T4,” Metall. Trans. A, 24A, pp. 2503–2512) and for Al 6111-T4 (Graf, A., and Hosford, W., 1994, “The Influence of Strain-Path Changes on Forming Limit Diagrams of Al 6111 T4,” Int. J. Mech. Sci., 36, pp. 897–910). These articles present strain-based forming limit curves (ϵFLCs) for both as-received and prestrained sheets. Using phenomenological yield functions, and assuming isotropic hardening, the ϵFLCs are transformed into principal stress space to obtain stress-based forming limit curves (σFLCs) and the principal stresses are transformed into effective stress and mean stress space to obtain the extended stress-based forming limit curves (XSFLCs). A definition of path dependence for the σFLC and XSFLC is proposed and used to classify the obtained limit curves as path dependent or independent. The path dependence of forming limit stresses is observed for some of the prestrain paths. Based on the results, a novel criterion that, with a knowledge of the forming limit stresses of the as-received material, can be used to predict whether the limit stresses are path dependent or independent for a given prestrain path is proposed. The results also suggest that kinematic hardening and transient hardening effects may explain the path dependence observed in some of the prestrain paths.

Influence of strain path change on the rolling behavior of twin roll cast magnesium alloy

The effect of strain path change on the rolling behavior of twin roll cast (TRC) magnesium alloy AZ31 was investigated by subjecting the TRC material to hot unidirectional and cross rolling, respectively. In general, unidirectional rolling exhibited stronger texture intensities compared to cross rolling. Increasing the thickness reduction per rolling pass, i.e. reducing the total number of rolling passes, gave rise to weaker textures. For cross rolling, the texture intensity was independent of the strain amount per pass.

Strain Path Change Effect on Deformation Behaviour of Materials with Low-to-Moderate Stacking Fault Energy

Stacking fault energy (SFE) plays an important role in face centred cubic (f.c.c.) metals and alloys in determining the prevailing mechanisms of plastic deformation. Low SFE metals and alloys have a tendency to develop mechanical twinning, besides dislocation slip, during plastic deformations. Deformation behaviour and microstructure evolution under simple and complex strain paths were studied in 70/30 brass, with small and intermediate grain sizes, which corresponds to a f.c.c. material with low SFE. Simple (rolling and tension) and complex (tension normal to previous rolling) strain paths were performed. The macroscopic deformation behaviour of materials studied is discussed in terms of equivalent true stress vs. equivalent true strain responses and strain hardening rates normalized by shear modulus (dσ/dε)/G as vs. (σ – σ0)/G (σ0 is the initial yield stress of the material and G is the shear modulus). The mechanical behaviour is discussed with respect to dislocation and twin microstructure evolution developed in both, simple and complex strain paths.

The role of plastic slip anisotropy in the modelling of strain path change effects

Most industrial metal forming processes are characterised by a complex strain path history. A change in strain path may have a significant effect on the mechanical response of metals. This paper concentrates on the role of the plastic slip anisotropy in the strain path dependency of materials containing a dislocation cell structure. The cell structure model, developed in Viatkina et al. [Viatkina, E., Brekelmans, W., Geers, M., 2007. Modelling of the internal stress in dislocation cell structures. Eur. J. Mech. A: Solids 26(6), 982–998], is extended here by employing a crystal plasticity model for describing the local material behaviour. This, more physically based description of the plastic behaviour, allows accounting for plastic anisotropy resulting from the crystal lattice of a metal. The extended cell structure model accounts for four sources of anisotropy: (1) geometrical anisotropy due to the configuration of the cell structure, (2) anisotropy due to the internal stresses associated with the cell structure, (3) slip anisotropy and (4) textural anisotropy. The relative contributions of the four sources of anisotropy are discussed.

Effect of Strain-Path Reversal on Microstructure Evolution and Cavitation during Hot Torsion Testing of Ti-6Al-4V

Hot torsion testing comprising multiple twist reversals was used to establish the effect of strain-path changes on concurrent dynamic globularization and cavitation of Ti-6Al-4V with a colony-alpha starting microstructure. Optical microscopy was used to quantify the cavity area fraction and the effect of globularization on cavitation. The deformation of the hard and soft colonies surrounding the largest cavities and self-consistent-model calculations of strain partitioning were used to estimate the macroscopic and local strains at which colonies with different strengths globularize. It was found that both hard and soft colonies undergo dynamic globularization at the same local strain (i.e., strain within the colony). In addition, cavitation behavior during torsion with multiple strain-path changes was interpreted by taking into account the break up of the colonies into a globular structure. It was found that cavity growth (or shrinkage) persisted as long as there was a flow-stress difference in adjacent regions/colonies surrounding a given cavity. When the microstructure became uniform (as in the case of full globularization), the cavity area fraction did not measurably change with subsequent additional deformation.

Numerical analysis of strain path dependency in FCC metals

Strain path dependency in FCC metals is often associated with the anisotropy induced by the dislocation cell structure in deformed metals. In this paper, the mechanical behaviour of metals under various nonuniform deformation paths is studied with the use of a recently developed dislocation cell structure model. It is shown that this model correctly captures the essential features of strain path change effects for moderate strain path changes, i.e. the anisotropy and the dependency on the amount of prestrain. Next, a numerical analysis is performed to assess the micromechanical origin of the modified reloading yield stress and the transient hardening after strain path changes. The separate influence of anisotropic effects due to the cell structure morphology and residual internal stresses are thereby addressed and illustrated. The transient hardening behaviour after a strain path change is related to the adjustment of the internal stresses to the new loading. Results obtained are consistent with related experimental findings reported in literature.

Static Recovery and the Orthogonal Strain Path Change Effect in IF Steel

Tensile tests have been carried out in the rolling and transverse directions of 'interstitialfree' (IF) steel cold rolled to a strain of εh= -0.18. Tests in the transverse direction showed the characteristic features of the orthogonal strain path change effect, with an initially increased flow stress- compared to tests in the rolling direction- followed by a transient regime of very low strain hardening. Tests were also carried out following recovery annealing of the prestrained sheet at 500°C and 600°C. Static recovery had a marked effect on the strain-induced anisotropy, but this was not eliminated even when the cell structure generated by prestraining haD condensed to one consisting of low-angle boundaries. This supports the view that the length scale, with respect to active slip systems, between boundary obstacles is a significant factor in the orthogonal path change effect.

Modelling the evolution of dislocation structures upon stress reversal

The non-uniform distribution of dislocations in metals causes a material anisotropy that manifests itself through strain path dependency of the mechanical response. This paper focuses on the micromechanical modelling of FCC metals with a dislocation cell structure. The objective is to enhance the continuum cell structure model, developed in Viatkina et al. [Viatkina, E., Brekelmans, W., Geers, M., submitted for publication. Modelling of the internal stress in dislocation cell structures], with an improved description of the dislocation density evolution enabling a correct prediction of strain path change effects under complete or partial stress reversal. Therefore, attention is concentrated on the dislocation mechanisms accompanying a stress reversal. Physically based evolution equations for the local density of the statistically stored dislocations are formulated to describe the formation and dissolution of a dislocation structure under deformation. Incorporation of these equations in the cell structure model results in improved predictions for the effects of large strain path changes. The simulation results show a good agreement with experimental data, including the well-known Bauschinger effect. The contributions of the dislocation mechanisms and the internal stresses to the resulting macroscopic strain path change effects are analysed. The dislocation dissolution is concluded to have a significant influence on the macroscopic behaviour of FCC metals after stress reversals.

Effect of strain path change on the plastic responses during uniaxial loading of materials pre-deformed by equal channel angular extrusion

Strain path change is involved in uniaxial tension or compression tests of materials pre-deformed by equal channel angular extrusion (ECAE). It is shown to vary with the ECAE die angle and the testing direction, and contribute to the anisotropy of plastic responses (e.g. slip activities, yield stress, work-hardening or softening) during uniaxial loading.

Plastic Behaviour of Copper Polycrystal Subjected to Fatigue-Tension Sequential Loading Tests

Sequences of fatigue-tension tests were performed on copper polycrystal sheet, with 32µm mean grain size. The effect of strain path change on subsequent reloading yield stress as well as work hardening rate has been investigated. Dislocation microstructure was observed by transmission electron microscopy after mechanical tests. Under present conditions, it was found that fatigue prestraining caused the increase of reloading yield stress, larger amplitude of strain path change resulted in higher reloading yield stress and lower work hardening rate. Reloading tensile curves are independent of predeformation plastic strain amplitudes in both cases. Many isolated dislocation lines between cell walls have been detected for Φ=0° case when the subsequent tension strain amount is 5%, this can be well understood from the microscopic dislocation slip mechanisms. When the reloading tension tests have been done until rupture, dislocation structures become typical of monotonic tension without preloading. The correlation of mechanical properties and microstructural observations was discussed in this paper.

The anisotropy of aluminum and aluminum alloys

The anisotropy of textured aluminum is approximated by a yield criterion with an exponent of eight. The use of this criterion in metal-forming analyses has improved the understanding of the formability of aluminum and other metals. The effect of anisotropy on the limiting drawing ratio in cupping is less than that expected from the quadratic Hill yield criterion and the effect of texture on forming limit diagrams is negligible. A method of predicting the effect of strain-path changes on forming limit curves of aluminum alloy sheets has proven to agree with experiments.

EBSD investigation of the effect of strain path changes on the microstructure and texture of duplex stainless steel during hot deformation

The effect of strain path changes on the microstructure/crystallographic texture characteristics was studied in a 22Cr-6Ni-3Mo duplex stainless steel subjected to several deformation regimes comprising forward and reverse torsion, performed at a temperature of 1000 °C using a strain rate of 1 s−1. A high-resolution EBSD technique, utilising an FEI Sirion FEG SEM in conjunction with a HKL Technology EBSD attachment, was successfully implemented for precise phase and substructural characterisation of the above steel. The austenite/ferrite ratio as well as the crystallographic texture, subgrain size and misorientation angles corresponding to each phase were determined over large sample areas. The results indicated that the austenite and ferrite phases, present in approximately equal volume fractions, softened via dynamic recovery and extended dynamic recovery, respectively. The phase ratio appeared to remain stable during straining. There seemed to be generally more strain partitioned to the ferrite compared to the austenite. Twin relationships in the austenite appeared partly restored after the full strain reversal. The austenite crystallographic texture displayed a tendency to revert back to the original texture after the strain reversal whereas the ferrite texture evolution appeared more complex. The obtained subgrain characteristics indicated that the reversed torsion might have caused some dissolution (and possible re-formation) of subboundaries in both phases. Nevertheless, the effect of strain path changes on the substructure characteristics generally appeared relatively moderate.