Pre-deformed Material Research Articles

Effect of Residual Stresses on the Fatigue Stress Range of a Pre-Deformed Stainless Steel AISI 316L Exposed to Combined Loading

AISI 316L austenitic stainless steel is utilized in various processing industries, due to its abrasion resistance, corrosion resistance, and excellent properties over a wide temperature range. The physical and mechanical properties of a material change during the manufacturing process and plastic deformation, e.g., bending. During the combined tensile and bending loading of a structural component, the stress state changes due to the residual stresses and the loading range. To characterize the component’s stress state, the billet was bent to induce residual stress, but a phase transformation to martensite also occurred. The bent billet was subjected to combined tensile–bending and fatigue loading. The experimentally measured the load vs. displacement of the bent billet was compared with the numerical simulations. The results showed that during fatigue loading of the bent billet, both the initial stress state at the critical point and the stress state during the dynamic loading itself must be considered. Analysis was demonstrated only for one single critical point on the surface of the bent billet. The residual stresses due to the phase transformation of austenite to martensite affected the range and ratio of stress. The model for the stress–strain behaviour of the material was established by comparing the experimentally and numerically obtained load vs. displacement curves. Based on the description of the stress–strain behaviour of the pre-deformed material, guidelines have been provided for reducing residual tensile stresses in pre-deformed structural components.

Dynamic Equations for the Propagation of Acoustic Waves in Pre-Deformed Materials

Dynamic Equations for the Propagation of Acoustic Waves in Pre-Deformed Materials

LCF behavior of 2024AA under uni- and biaxial loading taking into account creep pre-deformation

This study presents the results of experimental low-cycle fatigue (LCF) tests of aluminum 2024 alloy T3511 temper in uni- and biaxial loading states. Tests were carried out on both the as-received material (hardened extruded rods) and material with different pre-deformation histories. These deformations were carried out in the creep process at 200 °C and 300 °C for two different levels of at each temperature. The pre-deformed material’s basic fatigue characteristics were determined and compared with the appropriate characteristics of the as-received material. In-depth macro- and microscopic analysis (SEM) of fracture surfaces was done. The effect of preliminary creep on LCF behavior of investigated alloy was characterized for both uni- and biaxial loading. An increase of fatigue life occurs for large plastic strains in the case of cyclic tension/compression. For in-phase biaxial loading, improvement of life is observed only for material with pre-deformation at 300 °C. Crack initiates in the plane of maximum shear strains for both biaxial loading and pure torsion. For tension/compression – in the plane of maximum principal stress (strain).

The effect of preliminary creep-strain on material behavior under LCF tension/compression and pure torsion regimes

This paper presents the results of experimental tests of monotonic tension and torsion, creep-rupture and preliminary creep at 200 °C and 300 °C, as well as low-cycle fatigue of 2024T3511 aluminum alloy. The fatigue process was conducted at room temperature for uniaxial tension/compression and torsion. The as-received material and pre-deformed material were also investigated during creep at elevated temperature. Basic monotonic, creep and fatigue characteristics of the material were determined. Detailed micro- and macroscopic analyses of fracture surfaces (scanning electron microscopy, SEM) were carried out. The influence of different creep pre-deformation histories at elevated temperature on the behavior of the studied alloy were characterized with respect to monotonic and cyclic loads at room temperature.

Fatigue Crack Propagation on Uniaxial Loading of Biaxially Predeformed Pearlitic Rail Steel

Rolling contact fatigue loading causes the surface material in the top of the rail to severely deform. The microstructure aligns along the shear direction and the mechanical behavior in terms of plastic flow becomes anisotropic. Cracks are initiated in the severely deformed surface region and propagates along the direction of microstructure flow lines. However, the effect of large shear deformation on fatigue crack growth is not yet well understood. In this study, uniaxial fatigue crack propagation of undeformed and biaxially predeformed pearlitic rail steel R260 has been investigated. The material was predeformed to different deformation states using combined axial compression and shear deformation, corresponding to material found at different depths in the severely deformed surface layer of rails. Fatigue crack propagation rate was dependent on the material state and influenced by both work hardening and microstructural anisotropy. It was found that predeformed material exhibited lower crack growth rates than undeformed material under this loading condition. The influence of predeformation on the crack growth direction was limited.

Effect of pre-stress on surface integrity in micro milling: Modelling and experimentation

Recent years have witnessed rapid growth in microscale manufacturing, particularly in the biomedical, electronics and aerospace industries. Therefore, it is required to improve the capabilities of micro-cutting processes such as micro-milling to generate intricate features with high surface quality. The surface integrity of the machined materials is influenced by the cutting conditions, tool geometry and material properties. The novelty of the proposed work is to improve the surface finish and compressive residual stress of the milled slot by tailoring the pre-compressive stress condition. Ultrasonic-assisted abrasive peening is used to tailor the residual stress and obtained grain refinement on the surface. The micro-milling is performed on the peened surface of Ti-6Al-4V, Al-6061, and OFHC-Cu to investigate the surface roughness and residual stress. A hybrid model is developed to estimate the surface roughness considering the effect of tool geometry, process parameters, size effect and workpiece microstructure. Induced residual stress at the machined surface is simulated through the finite element (FE) method. Moreover, the FE model simulated the minimal chip thickness and contact pressure at the flank face and is deployed as an input to the analytical model for estimation of surface roughness. The pre-deformed material by peening reduced the surface roughness by 35 to 42 % on different materials and improved compressive residual stress due to grain refinement. The predicted average surface roughness and residual stress are validated with experimental results.

Decoupling electroplasticity by temporal coordination design of pulse current loading and straining

Multi-mechanism coupled electroplasticity (EP) has not been fully elucidated yet due to the difficulty in eliminating thermal effect, although isothermal, air-cooled, and cryogenic methods are usually used, there are different heating histories from electrically-assisted deformation. The 0–7% pre-strained Ti–6Al–4V alloy is taken as the case material. Based on the two different characteristics of the time-accumulation of thermal effect and the instantaneous nature of the athermal effect, a comparative study is conducted by designing two temporal coordination methods of pulse current and straining, viz., asynchronous loading with high-energy current (ALHC) and synchronous loading with low-energy current (SLLC), where the thermal and athermal effects are highlighted by different loading times, loading and unloading moments of pulse current of different magnitudes. Under ALHC condition, the decreasing yield strength and increasing elongation of the 7% pre-deformed material are observed with increasing current density and loading times, and stress reduction up to 20.7% for yield strength and ductility recovery of maximal 99.1% for elongation are obtained for 14.0 A/mm2 and loading three times. Moreover, stage B of the work hardening rate related to dislocation de-pinning disappears, and the TEM shows an annealing effect that the tangled dislocations are annihilated and rearranged by climbing. Thus, the Joule heat-based thermal effect dominates this condition. Under SLLC condition, the loading and unloading of pulse current of 2.81 A/mm2 in the plastic stage result in an instantaneous drop and rebound in flow stress and work hardening rate (stage B exists), respectively, and are accompanied by the negligible instantaneous temperature rise, while loading at the elastic stage, the overall flow stress and work hardening rate is decreased. It is concluded that the athermal effect can lower the dislocation phonon drag effect and the deformation activation energy, and change the athermal atomic state and dislocation slip mode, which dominates the varied flow stress and elongation. A dislocation dynamics-based physical model is developed by taking into account phonon drag, diffusion activation energy, and deformation activation energy to account for the thermal and athermal effects on dislocations, providing a self-consistent explanation of the above behaviors and mechanisms of the flow stress and ductility under ALHC and SLLC conditions.

Yield surface identification of CP-Ti and its evolution reflecting pre-deformation under complex loading

Uniaxial testing methods to characterize materials provide only limited data that is insufficient to fully understand all aspects of their behaviour, such as initial texture or anisotropy. Therefore, this research aims to conduct complex stress loading experiments to understand the physical mechanism accountable for plastic deformation caused by monotonic tension and tension assisted by proportional cyclic torsion in the CP-Ti (Commercially Pure Titanium). The yield surface approach was applied to assess the variation of mechanical properties in the as-received and pre-deformed material. It was found, that such monotonic tension associated with cyclic torsion caused a significant decrease of the tensile stress. The initial yield surface obtained for the as-received material exhibits anisotropic behaviour, whereas, the sizes of subsequent yield surface reflecting pre-deformation were reduced in all directions with exception of the tension direction.

Axisymmetric Drawing of Parts from Brass Sheets with Regulated Thinning

This paper describes the results of a study that looked at the effect of the dimensions and mechanical properties of L63 brass alloy sheets on the drawing forces and the dimentions of axisymmetrically drawn tubs. Specifically for tubs made of L63 brass, the paper presents a number of different options in terms of the actual process and the pre-hardening parameters for sheet metal. Computer modelling was performed based on the results of pilot passes conducted on a two-high mill 350×450 operated by Kolchugino Non-Ferrous Metal Working Plant. For this, 380×380 mm square 1.5–1.7 mm thick sheets with accumulated strain were driven through the mill, and the workpieces were turned around before the second pass. Due to varying properties of annealed sheets, the yield strength doubled rising from 145 to 280 MPa, whereas the plasticity dropped. The simulation study helped determine that as pre-deformed material becomes 10 % harder, it tends to form more evenly in the Tub stamping operation with L63 brass material used. At the same time, the thinning effect in critical areas decreases. Due to this process solution, the workpiece thickness could be decreased from 1.7 to 1.6 mm. The reduced thinning of the round part is combined with a slight thickening of the flange edge. The paper also decribes a drawing option which could help save half of the required operations due to a 5 mm bigger diameter of the workpiece (leading to an only 3% increase in the surface area of the workpiece). Due to the use of pre-hardened sheet metal, one can avoid big cross-sectional thickness variations in axisymmetrically drawn parts. To implement this process, one is advised to use a single-action press of the P2228A type designed for 630 kN.

VHCF life of AM1 Ni-based single crystal superalloy after pre-deformation

VHCF properties of the Ni-based single crystal superalloy AM1 with pre-deformation have been investigated at 1000 °C. Pre-deformed material did not present life debit under fully reversed conditions (R = −1). Pre-deformation decreased VHCF lifetime under high mean stress conditions (R = 0.5) with a shift from internal to mode I surface crack initiation. The pre-deformed material has microstructure coarsened bands lying on the former {111} slip planes, facilitating microstructure shearing during VHCF at R = 0.5. Slip traces increased surface roughness, causing a surface recrystallization and initiation of the fatal crack from the surface. Rejuvenation heat treatment after pre-deformation can restore the microstructure and the fatigue life.

Effects of Plastic Strain on Mechanical and Electrical Conductivity Response of (100) Monocrystalline Copper

The physical characteristics of material would be influenced by its stress states. In this paper, nanoindentation experiments with a maximum load of 100 mN and strain-sensitive resistance tests were conducted on (100) monocrystalline copper with tension-induced plastic strain to investigate the influences of plastic strain on mechanical and electrical characteristics. By theoretical and experimental analysis, indentation depth, elastic modulus, recovery rate of total work, contact area, pile-up and apparent hardness of the deformed and virgin material were obtained and the corresponding investigation was compared and analyzed. The experimental results revealed that under the same conditions, tensile pre-deformed material would like to generate more penetration depth to achieve the same load during indentation. As the plastic strain increased in the uniform plastic region, both the total and elastic indentation work increased, while the recovery rate of total work, contact area, apparent hardness, and elastic modulus decreased. Meanwhile, the contribution of plastic strain to electrical resistivity was also investigated. The quantitative relation between electrical resistivity and plastic strains was extracted from experiments. The understanding of the role of plastic strain in mechanical and electrical properties of monocrystalline copper would be helpful to the application in damage detection systems and strain sensing sensor.

Quantitative Representation of Mechanical Behavior of the Surface Hardening Layer in Shot-Peened Nickel-Based Superalloy.

Surface hardening treatment can usually introduce severe grain distortion with a large gradient in the surface layer. It results in mechanical properties being difficult to accurately determine through macroscopic tests due to the non-uniformity of the shot-peened material. In this study, the mechanical behavior of uniformly pre-deformed nickel-based superalloy IN718 was investigated with monotonic tensile tests and instrumented indentation tests. For the shot-peened material, the hardness distribution of the surface hardening layer after shot peening was identified through the instrumented indentation method. According to the stress–strain results of pre-deformed materials, Ramberg–Osgood model parameters could be presented with plastic deformation. Assuming the power-law relationship between hardness and plastic deformation, the plastic deformation distribution along the depth of the surface hardening layer was clarified. Based on the results, a method to identify the stress–strain relationships of hardened material at different depths was established. Finally, the finite-element simulations of the instrumented indentation test considered residual stress and strain hardening were built to verify the method presented herein. The results show that the solution to evaluate the mechanical properties of hardening layer materials in the microscopic zone is feasible, which can provide the foundation for the failure analysis of shot-peened materials.

The effect of heavy deformation on the precipitation in an Al-1.3Cu-1.0Mg-0.4Si wt.% alloy

This work has investigated the effect of the combination of natural ageing and 80% pre-deformation after solution heat treatment on precipitation during subsequent artificial ageing in an Al-1.3Cu-1.0Mg-0.4Si wt.% alloy. It was found that a combined use of atomic resolution aberration corrected high-angle annular dark-field scanning transmission electron microscopy and scanning precession electron diffraction enabled a detailed characterisation of precipitates in heavy pre-deformed materials. The dominant phase in the undeformed condition was the L phase. L was also found to be nucleated in the undistorted regions of the Al matrix in the pre-deformed conditions. Two phases with high aspect ratios were nucleated on dislocations: The C phase and a previously reported phase, which we named here the ‘E phase’. The crystal structure of E was solved experimentally as monoclinic with core composition Mg6Al2Si2Cu4, supported by density functional theory calculations. It was determined that the order of pre-deformation and natural ageing had an influence on the relative fractions of the aforementioned phases formed during artificial ageing. An increased fraction of C+E relative to L was found in the condition where the pre-deformation was applied after natural ageing as compared to the condition where the pre-deformation was applied before natural ageing.

Improvement of strength and ductility of an EZ magnesium alloy by applying two different ECAP concepts to processable initial states

The commercial magnesium alloy EZ33A was processed by Equal Channel Angular Pressing (ECAP). Two different tools were used: a conventional one with two channels and one intersection (90°), and a so-called double ECAP (D-ECAP) tool with two intersections (90° and 120°). Four passes were conducted at temperatures subsequently lowered from 350 °C down to 240 °C using route Bc.The as-cast, pre-deformed material and both ECAP conditions were investigated. The microstructure was revealed by means of optical microscopy, scanning and transmission electron microscopy (SEM, TEM), and X-ray diffraction. Mechanical properties were studied by Vickers hardness measurements as well as by compressive and tensile tests.The microstructure investigations show a significant grain refinement as well as fragmentation and redistribution of the hard intermetallic (Mg,Zn)12RE phase by ECAP. This effect is even more pronounced in the material processed by D-ECAP where this phase is two times finer compared to the size after conventional ECAP. Additionally, small Zn-Zr particles of a few nanometers size within grains or subgrains were observed.Microstructure changes evoked by the ECAP processing significantly affect the mechanical properties. This is especially the case after 4 passes in the D-ECAP tool where hardness (90 HV2) is almost 70% higher than in the as-cast condition (54 HV2) and the tensile yield strength is enhanced by nearly 300% (82 MPa up to 327 MPa). Hence, the D-ECAP tool is more efficient than the conventional one in improving mechanical properties.

The Effect of Dynamic Recrystallization on Monotonic and Cyclic Behaviour of Al-Cu-Mg Alloy.

The paper presents an investigation that was conducted to determine the possibility of the occurrence of the process of dynamic recrystallization in 2024 alloy during monotonic tensile and creep tests at the elevated temperatures of 100 °C, 200 °C, and 300 °C. As-extruded material was subjected to creep process with constant force at elevated temperatures, until two varying degrees of deformation were reached. After cooling at ambient temperature, the pre-deformed material was subjected to monotonic and fatigue tests as well as metallographic analysis. The process of dynamic recrystallization was determined in monotonic tests to occur at low strain rate (0.0015/s) only at the temperature of 300 °C. However, in the creep tests, this process occurred with varying efficiency, both during creep at 200 °C and 300 °C. Dynamic recrystallization was indicated to have a significant influence on the monotonic and cyclic properties of the material.

Determination of the Plastic Strain by Spherical Indentation of Uniaxially Deformed Sheet Metals

This work consists of determining the plastic strain value undergone by a material during a forming process using the instrumented indentation technique (IIT). A deep drawing steel DC01 is characterized using tensile, shear and indentation tests. The plastic strain value undergone by this steel during uniaxial tensile tests is determined by indentation. The results show that, the identification from IIT doesn’t lead to an accurate value of the plastic strain if the assumption that the hardening law follows Hollomon law is used. By using a F.E. method, it is shown that using a Voce hardening law improves significantly the identification of the hardening law of a pre-deformed material. Using this type of hardening law coupled to a methodology based on the IIT leads to an accurate determination of the hardening law of a pre-deformed material. Consequently, this will allow determining the plastic strain value and the springback elastic strain value of a material after a mechanical forming operation.

Influence of annealing temperature on the baking response and corrosion properties of an Al–4.6 wt% Mg alloy with 0.54 wt% Cu

The effect of different thermo-mechanical processing parameters on the baking response, and intergranular corrosion (IGC) resistance of an Al–4.6wt% Mg–0.54wt% Cu alloy was investigated. It has been shown that the high temperature anneal at 530°C resulted in superior baking response of pre-deformed material after short annealing at 140–180°C, compared to the annealing at 280°C. Specimens pre-deformed 2% had positive baking response for both initial annealing temperatures, however, pre-deformation of 5% induced softening during the baking treatment of the specimens annealed at 280°C. Prior to sensitization of 7days at 100°C, specimens at all stages of thermo-mechanical treatment were IGC resistant. After the sensitization specimens that underwent annealing at 530°C remained IGC resistant, but specimens that underwent annealing at 280°C became IGC susceptible. Better corrosion properties and baking response are related to the higher Cu content in solid solution after annealing at 530°C than at 280°C.

A new Methodology to Predict Yield Strength Under Biaxial pre-Deformation on Sheet Metals

The use of stamped aluminium alloy components in the automotive industry is commonplace nowadays. To design these components correctly, it is necessary to know how the mechanical properties of sheet metals change when the material is deformed, generally under biaxial strain. In this paper, the influence of the pre-deformation on yield strength in an aluminium alloy 5000 series is studied from a new methodology that combines the Marciniak stretching test (levels of biaxial strain) and the Small Punch Test. New expressions to estimate yield strength using the Small Punch Test in biaxial pre-deformed material have been developed.

Texture development and formability prediction for pre-textured cold rolled body-centred cubic steel

In this study, a polycrystalline material model is introduced. It consists of an artificial grain structure, where the individual crystal geometries are generated by Voronoi cells. In the grains slip planes and sliding directions, corresponding to the slip systems of bcc crystals, are incorporated. The crystallographic behaviour is captured in a consistent continuum mechanics based crystal plasticity framework based on the elastoplastic multiplicative split of the deformation gradient and a viscoplastic regularisation due to a constitutive relation of slip rates and the critical shear stress in the slip systems. A suitable flow rule fulfils the requirement of plastic incompressibility.The specific material considered is the ferritic steel DC04 (material number 1.0338). Based on the results of experimental studies, the initial texture is transferred via Euler angles into the modelled polycrystalline structure. For the determination of initial data as well as the validation of the material behaviour EBSD investigations are carried out. The comparison of the results is done by measuring the strain evolution of grains, the texture and the Schmid factor under uniaxial tensile load. In addition to EBSD measurements of pre-deformed material, in situ EBSD tensile tests are carried out in the SEM. The results of the in situ studies coincide with the results of the pre-deformed material. It is shown that the simulation results agree very well with the experimentally obtained data. The plastic material behaviour is reproduced very well by the simulation, whereas the texture and Schmid factor development shows a good correlation between model and experiment.

The effect of simultaneous deformation and annealing on the precipitation behaviour and mechanical properties of an Al–Mg–Si alloy

The influence of simultaneous deformation and annealing on the precipitation behaviour and the mechanical properties of an AA6060 Al–Mg–Si alloy were investigated by Transmission Electron Microscopy (TEM) and tensile testing. The results were compared to similar heat-treatments of the un-deformed material and of a material which was pre-deformed 10% before a subsequent annealing. The deformations applied in the present work were simultaneous by 0.6% within 10min of heat treatment at 190°C, and by 10% within 100min and 300min at 190°C. The results show that the precipitation kinetics, microstructure and the tensile strength are greatly dependent on the specific deformation mode applied. When deformation and annealing of the material are simultaneous, precipitation takes place at dislocations which are moving through the material. These precipitates are of post-β′′-type. Compared to the pre-deformed material, the strength addition by the precipitates is larger in the simultaneous deformation mode due to a finer precipitate distribution, while the strength addition by dislocations appears to be lower. The total yield strength of all peak hardness conditions is similar, due to a compensation of different strength contributions with the highest contribution by precipitates in the un-deformed material.