Thermal Softening Effect Research Articles

Softening Behavior and Mechanism of Laser Rapid Forming Ti-6Al-4V Alloy Deformed at the High Temperature

Using the gleeble-1500 thermal simulator, the high temperature quasi-static compression experiment on cylindrical samples of Laser Rapid Forming Ti-6Al-4V(LRF TC4) titanium alloy have been conducted. Softening behaviour and mechanism of LRF TC4 alloy deformed at different high temperatures were investigated by means of optical microscope (OM) and transmission electron microscope (TEM). It shows that, the strength of LRF TC4 was observably reduced with the rise of the deforming temperature, showing stronger thermal softening effect and superplasticity ability. Microstructure analysis on the samples deformed at the temperature from 400°C to 900°C showed that the original acicular microstructure of LRF TC4 has two significant changes: Acicular microstructure bents at the temperature from 400°C to 600°C, while acicular microstructure bents drasticly, fragmented, spheroidized at the temperature from 700°C to 900°C. No matter bending, fragmentation, or spheroidizing of acicular microstructure, stress in the deformed sample is released. Therefore, these three factors and their superimposition action were the source of thermal softening of LRF TC4 deformed at the high temperature.

Impact of Stack Orientation on Self-Piercing Riveted and Friction Self-Piercing Riveted Aluminum Alloy and Magnesium Alloy Joints

Self-piercing riveting (SPR) is a mature method to join dissimilar materials in vehicle body assembling. Friction self-piercing riveting (F-SPR) is a newly developed technology for joining low-ductility materials by combining SPR and friction stir spot welding processes. In this paper, the SPR and F-SPR were employed to join AA6061-T6 aluminum alloy and AZ31B magnesium alloy. The two processes were studied in parallel to investigate the effects of stack orientation on riveting force, macro-geometrical features, hardness distributions, and mechanical performance of the joints. The results indicate that both processes exhibit a better overall joint quality by riveting from AZ31B to AA6061-T6. Major cracking in the Mg sheet is produced when riveting from AA6061-T6 to AZ31B in the case of SPR, and the cracking is inhibited with the thermal softening effect by friction heat in the case of F-SPR. The F-SPR process requires approximately one-third of the riveting forces of the SPR process but exhibits a maximum of 45.4% and 59.1% higher tensile–shear strength for the stack orientation with AZ31B on top of AA6061-T6 and the opposite direction, respectively, than those of the SPR joints. The stack orientation of riveting from AZ31B to AA6061-T6 renders better cross-section quality and higher tensile–shear strength and is recommended for both processes.

Thermal/tribological effects of superimposed ultrasonic vibration on viscoelastic responses and mold-filling capacity of optical glass: A comparative study

Ultrasonic-assisted glass molding (UGM) has recently gained a promising start in fast replication of tailored functional structures onto glasses; however, the underlying mechanisms of the unique thermomechanical and micro-filling behaviors of glasses in UGM remain largely unrevealed. This study presents a full demonstration and elucidation of the ultrasonic-induced thermal/tribological effects on viscoelastic responses and filling capacity of the typical optical glass L-BAL42. First, conventional precision glass molding (PGM) and UGM experiments with partial-filling settings are implemented, whereby glass arrays with surface protrusions of varied depths (460–780 μm) are directly formed. Subsequently, the molding force, forming time and filling depth of the glass under varying pressing speeds/loads are comparatively evaluated. Furthermore, experimental quantifications of ultrasonic-induced heat increment and friction reduction are performed to account for the differentiated molding effects in UGM and PGM. The results indicate that compared with PGM, the molding force and forming time in UGM are greatly reduced, while the average filling depth of the UGM-formed glass array is effectively improved. This overall enhancement can be attributed to the ultrasonic-induced thermal softening, friction reduction and stress superposition effects, among which the thermal contribution is dominant. The findings in this study will provide new references for ultrasonic-assisted precision molding of glass-based micro/meso components.

Analysis and modelling of tensile and torsional behaviour at different strain rates of Ti6Al4V alloy additive manufactured by electron beam melting (EBM)

Additive manufactured Ti6Al4V alloy has become a widely used metallic material in many structural applications including aerospace components and medical implants, where its balance of mechanical performance and flexibility in assuming complex shapes is very appreciated. As a consequence, extensive efforts have been directed towards its mechanical characterization and extended data collections are reported in the literature. However, in the basic characterization, some prominent aspects related to “true” stress-strain behaviour and strain rate effects appear to have been substantially neglected. In this work the tensile behaviour of a Ti6Al4V alloy produced by Electron Beam Melting is investigated at static and dynamic rates together with the torsion response at static rates, in order to evaluate the effects of the strain rate and the Lode stress-state parameter on the elastoplastic response and the fracture initiation. The experimental procedure adopted here relied on enhanced true stress-true strain measurements based on optical measurements of the necking cross section of the specimen, continuously acquired by video recordings, as an alternative to the traditional elongation-based approach to the true stress-true strain evaluation. In the case of static torsion tests, the Nádai method has been used, delivering the equivalent stress-strain curves directly from the torque-rotation experimental data. Moreover, the real stress amplification due to high strain rates has been obtained subtracting the thermal softening effect from the flow curves of the dynamic tests. In addition to the mechanical characterization, micrographic analyses allowed to relate the microstructural properties to the additive process conditions and the fracture morphology to the various test conditions imposed to different specimens. Finally the stress-strain results from experiments are compared with literature data for Ti6Al4V alloy, manufactured by the same or different production processes, evidencing how not only manufacturing technique and process conditions, but also the approach used in the characterization affects the results of material behaviour assessment.

Deformation mechanism of fine grained Mg–7Gd–5Y–1.2Nd–0.5Zr alloy under high temperature and high strain rates

Fine grained Mg–7Gd–5Y–1.2Nd–0.5Zr alloy was investigated by dynamic compression tests using a Split Hopkinson Pressure Bar under the strain rates in the range 1000–2000 s − 1 and the temperature range 293–573 K along the normal direction. The microstructure was measured by optical microscopy, electron back-scattering diffraction, transmission electron microscopy and X-ray diffractometry. The results showed that Mg–7Gd–5Y–1.2Nd–0.5Zr alloy had the positive strain rate strengthening effect and thermal softening effect at high temperature. The solid solution of Gd and Y atoms in Mg–7Gd–5Y–1.2Nd–0.5Zr alloy reduced the asymmetry of α-Mg crystals and changed the critical shear stress of various deformation mechanisms. The main deformation mechanisms were prismatic slip and pyramidal 〈a〉 slip, {102} tension twinning, and dynamic recrystallization caused by local deformation such as particle-stimulated nucleation.

Multi-scale crystal plasticity finite element simulations of the microstructural evolution and formation mechanism of adiabatic shear bands in dual-phase Ti20C alloy under complex dynamic loading

A dynamic compression test was performed on α + β dual-phase titanium alloy Ti20C using a split Hopkinson pressure bar. The formation of adiabatic shear bands generated during the compression process was studied by combining the proposed multi-scale crystal plasticity finite element method with experimental measurements. The complex local micro region load was progressively extracted from the simulation results of a macro model and applied to an established three-dimensional multi-grain microstructure model. Subsequently, the evolution histories of the grain shape, size, and orientation inside the adiabatic shear band were quantitatively simulated. The results corresponded closely to the experimental results obtained via transmission electron microscopy and precession electron diffraction. Furthermore, by calculating the grain rotation and temperature rise inside the adiabatic shear band, the microstructural softening and thermal softening effects of typical heavily-deformed α grains were successfully decoupled. The results revealed that the microstructural softening stress was triggered and then stabilized (in general) at a relatively high value. This indicated that the mechanical strength was lowered mainly by the grain orientation evolution or dynamic recrystallization occurring during early plastic deformation. Subsequently, thermal softening increased linearly and became the main softening mechanism. Noticeably, in the final stage, the thermal softening stress accounted for 78.4 % of the total softening stress due to the sharp temperature increase, which inevitably leads to the stress collapse and potential failure of the alloy.

Effect of temperature buildup on milling forces in additive/subtractive hybrid manufacturing of Ti-6Al-4V

An additive/subtractive hybrid manufacturing (ASHM) combines the advantages of both additive and subtractive processes for fabricating complex parts of a better quality. While in an additive manufacturing process, temperature buildup is fast because of heat accumulation from a laser or electron beam, which exerts a great influence on the successive subtractive process. This study firstly measures the temperature variation of a Ti-6Al-4V workpiece in the direct material deposition (DMD) process, and then designs a heating device for a milling experiment at elevated temperatures. The effect of temperature buildup on milling forces is investigated through the experiment and a 2D thermal-mechanical coupling model. For work hardening effect and rapid tool wear, the milling forces are hardly decreased when the preheating temperature is lower than 300 °C. At a preheating temperature of 300 °C or higher, a significant reduction in milling forces is recorded, which may be attributed to the thermal softening effect on the workpiece material. The decreasing trend is more obvious at a larger feed-per-tooth. The thermal softening effect is reflected in the deformation layer in the subsurface of the machined workpiece. The study provides a guide to the determination of ASHM process parameters for Ti-6Al-4V.

Characterization of the Dynamic Response and Constitutive Behavior of PTFE/Al/W Reactive Materials

AbstractIn this study, the static and dynamic mechanical behaviors of four types of PTFE/Al/W reactive materials with different component mass ratios were studied. The mechanical properties of reactive materials at elevated strain rates and temperatures were tested by quasi‐static compression and Split Hopkinson Pressure Bar (SHPB). Parametric study on material properties was carried out, and the Johnson‐Cook constitutive constants were well determined. Based on the good agreement between the predicted and tested constitutive response, a systematic comparison of the static and dynamic behaviors, as well as the comprehensive stress‐strain relationships were conducted, regarding the strain rate and temperature effect. Microscopic fractographic analysis of the tested samples reveals the localized thermal softening effects and fiber network formation phenomenon of the matrix, which dominate the overall mechanical response of the four types of PTFE/Al/W reactive materials, and contribute to the elasto‐plastic property, strain hardening, strain rate strengthening and thermal softening effects.

High temperature tensile properties, fracture behaviors and nanoscale precipitate variation of an Al–Zn–Mg–Cu alloy

In order to assess Al–Zn–Mg–Cu alloys as potential high temperature structural materials, the hardness, tensile properties and fracture behaviors of 7085 Al alloy were investigated at various temperatures from room temperature to 175 °C. High-resolution transmission electron microscopy was used to investigate the evolutions of precipitates at different temperatures, particularly on the relationship between microstructural evolution and tensile strength. The results reveal that both the microstructure and mechanical properties of the alloy are quite sensitive to the environmental temperature. As the temperature increases, the hardness and strength decrease while the elongation and reduction of area increase. As tensile testing temperature rises, the strain hardening exponent (n) decreases due to the thermal softening effect. The fracture mode of the alloy transforms from mixture of intergranular and transgranular fracture to completely transgranular dimples when tensile testing temperature reaches 150 °C. The precipitate sequence during high temperature tests is coincident with that of aging. With the increase of tensile testing temperature, the mean precipitate radius grows larger, and the distribution of grain boundary precipitates transforms from continuous to discontinuous.

Development of a dynamic constitutive model with particle damage and thermal softening for Al/SiCp composites

A dynamic constitutive model of Al/SiCp composites can not only promote the application of composites in frontier fields, but also guarantee the reliability of the simulation modeling and analytical modeling of the composites. However, the influence of particles damage and thermal softening has not been considered in the dynamic constitutive model of Al/SiCp composites established so far. Therefore, a dynamic constitutive model of Al/SiCp composites with particle damage and thermal softening was established based on the Weibull statistical distribution, the Eshelby equivalent inclusion method, and the constitutive relation of the Al matrix. Then, this dynamic constitutive model was verified. The verification results indicated that the dynamic constitutive model with particle damage and thermal softening was able to predict the dynamic compressive behavior of various Al/SiCp composites accurately. In addition, theoretical calculations of the parameters of the dynamic constitutive model were analyzed to reveal the effect of particle damage and thermal softening on the dynamic compressive behavior. The result shows that an increase in the probability of particle damage in Al/SiCp composites reduces the strengthening effect of the particles. An increase in temperature enhances the fluidity of the matrix, and the dynamic compressive behavior of the Al/SiCp composite gradually became closer to those of tough matrix materials.

On dynamic response and fracture-induced initiation characteristics of aluminum particle filled PTFE reactive material using hat-shaped specimens

Dynamic response and fracture-induced initiation characteristics of aluminum particle filled polytetrafluoroethylene (PTFE/Al) reactive material was studied in two experimental configurations: (a) thin disks for compression and (b) hat-shaped specimens with predesigned shear bands of different widths for compression-shear, at elevated strain rates and temperatures. Compression stress-strain curves demonstrate typical elasto-plastic behavior with prominent strain hardening, strain rate strengthening, as well as thermal softening effects of the material. While growing oscillations and marked strength decrease due to shear band widths decrease observable on the shear stress-strain curves illustrate the localized shear induced adiabatic thermal effect and size effect of polymer materials. Post compression-shear to initiation phenomenon recorded by high-speed photography reveals the necessity of fracture process to initiation, apart from the intense shear work and loading rate effect. Employing the constitutive parameters determined at elevated strain rates and temperatures, 3D full size finite element modeling was performed. On the basis of a good agreement of the simulated and tested shear stress-strain curves for specimens of different shear band widths, a more direct insight into the deformation and stress evolution at the vicinity of the predesigned shear zones was analyzed and obtained.

Surface integrity comparison of wire electric discharge machined Inconel 718 surfaces at different machining stabilities

Abstract Current study aims to investigate the effect of machining stability on the surface integrity of the wire electric discharge machined Inconel 718 superalloy. The wire electrode material used for machining is hard zinc coated brass. Experiments were conducted at various levels of machining stabilities, defined with respect to discharge energies and machining gap conditions. The topographical characteristics were analysed by contact surface profilometer and non-contact 3D profilometer. Scanning electron microscope (SEM) images were analyzed to observe and compare the surface defects like microvoids, micro-cracks, micro globules, micro-craters on the samples machined at different stability levels. The least stable machining condition, with highest discharge energy, lowest inter electrode gap and least pulse off time, produced the most uneven topography. Recast layer (RCL) thickness was analyzed by observing the polished cross-sectional view of machined specimens under SEM. The conditions that provided the least RCL thickness were the most stable machining conditions. Furthermore, the surface layer characteristics like elemental contamination and thermal softening effects were analyzed using EDS and micro hardness tester respectively. All these characteristics have close relationship with the degree of machining stability.

Viscous Heating in Compressed Liquid Films

The greatest challenge in high-shear viscometry is the recognition of the difference between constitutive behavior and thermal feedback. The failure to realize this distinction was the cause of a serious mistake which has had a profound effect upon the EHL field for four decades. A traction curve is not a rheological flow curve. The classical approach to EHL has ignored the measurement of viscosity in viscometers at elevated pressures. As a consequence, the field has not benefitted from the understanding gained from directly observing the thermal softening effects of viscous heating. The Eyring sinh-law appears in high-shear viscometry when viscous heating causes thermal softening. The thermal limit of pressurized thin film Couette viscometers with high thermal conductivity cylinders is well-defined by the Nahme–Griffith number. The Spikes and Jie equation overcorrects at low values of the Nahme–Griffith number and undercorrects at high values. The viscous heating in a shear-thinning liquid may cause an apparent transition from power-law to sinh-law above some viscous power. Surprisingly, the sinh-law in this instance extrapolates back to the correct low-shear viscosity. The same response has been observed in one set of NEMD simulations.

Influence of thermal recovery on predictions of the residual mechanical state during melting and solidification

A thermomechanically consistent Eulerian plasticity model with work hardening is adopted for studying the residual mechanical state resulting from loading at elevated temperatures. The isotropic plasticity model includes the standard effect of thermal softening as well as specific modeling of thermal recovery. The model parameters and functions were calibrated to data for an austenitic stainless steel 316L. The model is applied in two numerical examples: a case of uniaxial tension and a circular disk that is exposed to a temperature load. The influence of thermal recovery is examined for each example by comparing the response of the complete model with thermal recovery to that when thermal recovery is omitted. The results of the second example indicate the importance of modeling thermal recovery for accurate prediction of residual stresses for problems dealing with melting and solidification.

Research on machining compacted graphite iron under oil-on-water cooling and lubrication conditions based on modified material model

Oil-on-water (OoW) techniques, including external OoW sprayed to both the rake face and flank face (EOoWrf), as well as cryogenic air mixed with OoW (CAOoW), improve the machinability of compacted graphite iron (CGI). This paper proposes a modified material model for RuT400 CGI cutting, based on flow softening and weak thermal softening effect, as observed during split-Hopkinson pressure bar tests. Employing a modified material model and Cock-Latham damage model, a thermo-mechanical coupled finite element (FE) model, for RuT400 cutting, is presented. The simulations, considering dry cutting, EOoWrf, and CAOoW, are conducted for the purpose of revealing the causes for the RuT400 difficult-to-cut property and for establishing how the mechanism of OoW can improve cutting performance. The results show that the effective stress, required to form chip segments in RuT400 machining, is much lower than in hardened steel, so RuT400 is more likely to form a serrated chip. In dry cutting, the chip bottom surface is subject to high strain and temperature, leading to the work-material phase transformation, followed by aggravating adhesive and abrasive wears. Although EOoWrf has little influence on chip morphology and effective stress, it reduces the temperature and strain on chip bottom surface by reducing friction, thus suppressing the occurrence of phase transformation. Due to low friction and high heat exchange, CAOoW produces the lowest tool-chip interface temperature and minimal adhesive wear. For high-speed cutting of RuT400, compared to dry cutting, EOoWrf and CAOoW can effectively reduce cutting forces and maintain the tool temperature in a lower range, where low friction on tool-chip interface plays a key role. The proposed FE model can be used in the future to improve CGI’s machinability, by changing cooling parameters, tool geometry/material, and other machining variables.

High‐temperature torsion of aluminum bars

AbstractThis paper studies high‐temperature torsion of aluminum bars within the simplified thermodynamic dislocation theory that ignores the excess dislocations. Employing a small set of physics‐based parameters, we are able to fit experimental torque‐twist curves for five different twist rates. The thermal softening effect is shown to be essential at high twist rates.

Modeling of dynamic elasto-plastic growth of a nano-void with surface energy, inertia and thermal softening effects

Surface effect, inertia and thermal softening are important factors affecting the elasto-plastic evolution of voids. In this study, we theoretically examine the dynamic evolution of a nano-void in an elasto-plastic medium, particularly focusing on influences of surface energy, inertia and thermal softening. A two-stage strategy of analyzing surface effect is adopted, i.e. the first stage which is a quasi-static stage from a fictitious stress-free configuration to the actual initial configuration, and the second one which is the dynamic process of deformation under both surface stress and external dynamic loadings. The void will grow dynamically and quickly under high external loading, and the accelerations of material points in the neighborhood of the void are extremely high, resulting in a profound inertia effect. Under impact-like loading conditions, the plastic work done during the elasto-plastic growing process of the void is mainly transformed into heat, leading to thermal softening of material. Furthermore, heat transfer can be well neglected due to the transient characteristic of the problem studied. Tendencies related to all above influence factors are systematically analyzed.

Coupling effect of deformation mode and temperature on tensile properties in TWIP type Ti–Mo alloy

The coupling effect of deformation mode and deformation temperature from 298 K to 673 K on tensile properties was systematically studied in a Ti–15Mo alloy with {332}<113> twinning-induced plasticity (TWIP) effect and a comparison material of Ti–15Mo–1Fe alloy deformed by dislocation slip. The yield strength remained relatively constant in Ti–15Mo alloy, while it monotonously decreased in Ti–15Mo–1Fe alloy as increasing deformation temperature. Due to enhancement of β phase stability from ω phase precipitation, the {332}<113> twinning was suppressed and the contribution of dislocation slip to plastic deformation increased, which compensated the thermal softening effect in Ti–15Mo alloy. High strain hardening rate was induced in Ti–15Mo alloy at 298 K due to the dynamic Hall-Petch effect from amounts of {332}<113> twins and formation of abundant geometrically necessary dislocations (GNDs). As increasing deformation temperature, the suppression of dynamic Hall-Petch effect and GNDs formation as well as the enhancement of dynamic recovery rate of dislocations resulted in the continuous decrease in strain hardening rate.

INFLUENCIA DEL NIVEL DE LUBRICACIÓN EN EL TALADRADO DE APILADOS HÍBRIDOS FIBRA DE CARBONO-TITANIO APLICANDO MQL (MÍNIMA CANTIDAD DE LUBRICANTE): MONITORIZACIÓN DEL PROCESO

The purpose of this paper is to analyze the influence of MQL (minimum quantity lubrication) lubrication level on hybrid stacks (Ti/CFRP/Ti) drilling process with diamond coated carbide tools. Spindle and feed motor power consumptions have been monitored which provides valuable information about the operation in real time which can be used to control and optimize the machining process and allows the implementation of 4.0 industry concepts. Furthermore, tool wear has been characterized and hole quality assessed. The tests were done with two lubrication levels with the same machining centers, tools, materials and cutting conditions as in the drilling processes of aeronautical components. With the fresh tool, the power consumptions were very similar for both MQL levels considered. Along the carbon fiber layers, the increase of cutting power with tool wear was higher for the test done with the lower MQL level. However, during Ti layers drilling, the increase of power was smaller for the lower MQL level, probably due to a higher titanium adhesion which modifies the chip formation process and a higher influence of the thermal softening effect on the workpiece. By using a reduced MQL level it was found a smaller tool wear. However, the quality of the holes was similar for the both lubrication levels tested. Keywords: MQL, hybrid stacks drilling, hole quality, tool wear, monitoring

Thermo-mechanical coupled finite element analysis of rolling contact fatigue and wear properties of a rail steel under different slip ratios

Abstract A multi-step FE modeling strategy has been proposed to predict the temperature evolution, thermo-mechanical response, surface fatigue life and wear behaviors in railway rail for multi wheel passages. Equivalent contact pressure and heat source were coupled and moved simultaneously on rail surface to achieve a balanced advantage of low computational cost and high accuracy. Wheel configuration at locomotive plays a significant role on rail temperature. The highest temperature of 776.05 °C was developed with a slip ratio of 9.43% after 9 wheel passages. Rolling contact fatigue life of rail material decreased quickly with slip ratio due to significant thermal softening effect. Transition from mild to severe wear was found at slip ratio of 2.38% according to the corresponding rail flash temperatures.