In the present investigation, effects of length and geometry of die land/bearing on curved profil... more In the present investigation, effects of length and geometry of die land/bearing on curved profiles/sections produced by a novel process, differential velocity sideways extrusion (DVSE), were studied through physical experiments using plasticine as a model material and finite element modelling. Profile curvature decreases as die land length increases due to its negative influence on exit velocity gradient, and a straight profile is extruded when the ratio of die land length to die orifice diameter exceeds a critical value l 0 which increases as extrusion ratio increases and extrusion velocity ratio v v / 2 1 decreases. Generally, effective strain level of the extrudate slightly increases as the die land length increases. Larger die land length increases the frictional areas between extrudate surface layers and die land (and mandrel for tube extrusion), generating zone of shear along the profile edge and thus increases surface layer effective strain. As a result, the strain homogeneity over the cross-section or wall thickness (for tube extrusion) is decreased. Compared with a sharp die land/container transition corner, a chamfered or radiused die land transition corner leads to an increased curvature due to the decreased effective land length, while it decreases overall effective strain level in the cross-section and strain homogeneity as a result of lower effective strain rate across the deforming region. A sharp die land transition corner is recommended for achieving a relatively large and homogenous effective strain in the cross-section.
Metal-ceramic functionally graded materials (FGMs) have been extensively used in aerospace engine... more Metal-ceramic functionally graded materials (FGMs) have been extensively used in aerospace engineering where high strength and excellent heat insulation materials are desired. In this paper, load distribution in threads of the Thermal Protection System used bolted joint made up of porous ZrO 2 /(ZrO 2 + Ni) FGMs is investigated by ABAQUS codes. The bolted joint is subjected to reentry heating corresponding to the Access to Space Vehicle. Effects of bolt-nut parameters including thread tooth profile, thread pitch, and modulus ratio of bolt to nut on load distribution in threads are analyzed in detail. It is found that uneven load distribution in threads occurs at elevated temperature, which mainly focuses on the first two threads closest to the nut bearing surface, with the first thread carrying 74% of the total load. Bolt-nut parameters have great effects on load distribution in threads, with trapezoidal thread, extra fine thread and greater modulus ratio of bolt to nut leading to more evenly distributed load. Further studies show that nut shape has significant effects on load distribution in threads, the optimized nut is designed to make the maximum load bearing ratio of the thread decrease to 30.21%, and thus the service reliability of the bolted joint is greatly improved.
Poor cyclic performance of lithium-ion batteries is calling for efforts to study its capacity att... more Poor cyclic performance of lithium-ion batteries is calling for efforts to study its capacity attenuation mechanism. The internal stress field produced in the lithium-ion battery during its charging and discharging process is a major factor for its capacity attenuation, research on it appears especially important. We established an electrochemical-mechanical coupling model with the consideration of the influence of elastic stiffening on diffusion for graphite anode materials. The results show that the inner stress field strongly depends on the lithium-ion concentration field, greater concentration gradients lead to greater stresses. The evolution of the stress field is similar to that of the concentration gradient but lags behind it, which shows hysteresis phenomenon. Elastic stiffening can lower the concentration gradient and increase elastic modulus, which are two major factors influencing the inner stress field. We conclude that the latter is more dominant compared to the former, and elastic stiffening acts to increasing the internal stress.
Extruded profiles/sections are increasingly used in the transport industry for lightweight struct... more Extruded profiles/sections are increasingly used in the transport industry for lightweight structures. In this paper, a wide thin-ribbed aluminium profile with asymmetric Z-shape, was manufactured by a novel sideways extrusion process proposed by the authors. A comparative study was conducted by utilising the direct/forward extrusion process at the same extrusion temperature and speed, in which the different process mechanics, resulting microstructures and mechanical properties of profiles have been investigated by experiments and finite element modelling. It was revealed that, compared with sideways extrusion, although the design of a die pocket in forward extrusion induces preform and avoids the use of the large-diameter billet and extrusion container/press needed for extruding wide profiles, it requires a greater extrusion force due to work-piece upsetting necessary to fill the die pocket and leads to a lower effective strain in the profile rib. EBSD characterisation of the regions with an equal effective strain indicated that an increased shear strain is more efficient for obtaining fine grains with a higher average misorientation angle. In the same region of the profile rib made from the two different processes, sideways extrusion results in greater grain refinement due to greater effective strains, and a slightly greater texture intensity was found due to the intensive shear deformation. Tensile tests on formed profiles revealed that sideways extrusion leads to a higher yield strength (YS) and ultimate tensile strength (UTS) but a relatively lower elongation to failure, due to the combined effects of grain refinement, GND and texture intensity enhancement. Compared with the billet, the profile formed by forward and sideways extrusion has a YS increased by about 60% and 79% respectively, and an UTS increased by about 74% and 80% respectively in the extrusion direction, demonstrating an advantage of the sideways extrusion process in improving material strength under the same extrusion condition.
The work described in this paper concerns a novel method for directly forming curved profiles/sec... more The work described in this paper concerns a novel method for directly forming curved profiles/sections from billets in one extrusion operation using two opposing punches. Its mechanics are based on internal differential material flow, and it has been given the acronym, differential velocity sideways extrusion (DVSE). A tool set enabling sideways extrusion to be performed using opposing punches moving with different velocities was used for a series of experiments in which punch velocity ratio and extrusion ratio were process parameters. Plasticine was used as a model work-piece material and a series of compression tests were undertaken, to determine its constitutive properties and gain an estimate of work-piece die friction for use in process simulation. Curvature of extrudate can be controlled and varied using a difference between the velocities of the two punches, defined by velocity ratio. Greater curvature is achieved with lower velocity ratio. Curvature is also dependent on extrusion ratio, an increase in which increases curvature, although curvature is less sensitive to it than to velocity ratio. The extent of work-piece flow velocity gradient across the die exit orifice, which causes curvature, has been identified. Severe plastic deformation of the extrudate occurs in a way similar to channel angular extrusion (CAE), thus a greatly promoted effective strain level is achieved, though it is not always uniform across a section. The inner bending region of an extrudate experiences maximum localised effective strain, which decreases with decrease in curvature. To the authors' knowledge this is the first publication in which extrudate curvature is deliberately induced using opposing punches with differential velocities. Although only fixed velocity ratio values have been used in the work described in this paper the ability to change during operation exists and the process has the potential for the production of a profile with different curvature along its length.
For structural and aerodynamic reasons, curved profiles are widely used in the transport industry... more For structural and aerodynamic reasons, curved profiles are widely used in the transport industry for manufacturing lightweight structures. In the present work, a curved AA1050 bar with fine grains and high strength was manufactured by a novel forming technique, differential velocity sideways extrusion (DVSE). The evolution of grain structure and micro-texture during DVSE and the mechanical properties of the formed bar were studied, and the grain refinement mechanism was revealed. Due to the severe strains arising in the process, (greater than that for conventional one pass equal channel angular extrusion), significant grain refinement in the curved bar (grain size ∼3 μm) was achieved from the original billet (grain size ∼357 μm) in one extrusion operation. Coarse band-like structures containing subgrains with low angle boundaries in the shearing zone gradually transformed into fine shear band-like structures containing equiaxed (sub)grains with a mixture of low and high angle boundaries. The fine shear band-like structures inclined approximately along the shear intersection planes. Severe plastic deformation induced a high dislocation density that initiated subgrain walls with low angle boundaries, which gradually transformed into grain boundaries with high misorientation, indicating that refinement of AA1050 grains in the DVSE process is due mainly to continuous dynamic recrystallization (cDRX). Due to the appearance of greater effective strain on the inner bend of the extruded bar, the grain refinement degree and high angle boundary fraction of the material on the inner bend are slightly greater than those of the material on the outside. DVSE resulted in a weak C-type shear-texture component which can be determined by a proper rotation of the negative simple shear texture. Compared with the billet, significant increase of the hardness, yield strength and ultimate tensile strength by 134.8%, 354.0% and 116.8% respectively was achieved in the formed curved bar, although the elongation to fracture was decreased by 60.0%.
Ceramicemetal functionally graded materials (FGMs) have been extensively used in aerospace engine... more Ceramicemetal functionally graded materials (FGMs) have been extensively used in aerospace engineering where high strength and excellent heat insulation materials are desired. In this paper, the thermodynamic behavior of the Thermal Protection System (TPS) used bolted joints made up of porous ZrO 2 /(ZrO 2 þ Ni) FGMs is investigated by finite-element (FE) modeling. The bolted joint is subjected to reentry heating corresponding to the Access to Space Vehicle. Thermodynamic simulations are carried out to yield the transient response of the porous ZrO 2 /(ZrO 2 þ Ni) functionally graded bolted joint (FGBJ). The effects of the preload on the thermomechanical behavior and service reliability of the bolted joint are numerically analyzed in detail by ABAQUS codes. It is found that the preload relaxation of the bolted joint occurs at elevated temperature, and the preload has significant influence on service reliability of the bolted joint under transient thermomechanical circumstances. With the increase of the preload, stress concentration which occurs at the root of the first thread of the bolt increases rapidly and predominates in service reliability. Proper preload is thus defined to balance the service reliability and tightness of the bolted joint. Further studies show that the shape of the nut has a great effect on the stress concentration of the thread, the optimized nut is designed to reduce the stress concentration of the thread, and thus the reliability of the bolted joint is also improved.
Ceramic-metal functionally graded materials (FGMs) have been extensively used in aerospace engine... more Ceramic-metal functionally graded materials (FGMs) have been extensively used in aerospace engineering where high strength and excellent heat insulation materials are desired. However, their performance highly depends on the internal residual stress, which is generated inevitably due to the thermal mismatch of ceramic and metal. In this paper, based on the nanoindentation test, the field distribution of the residual stress of porous ZrO 2 /(ZrO 2 þNi) ceramic-metal FGMs is measured. The residual stress field measured by nanoindentation agrees qualitatively with the finite element simulation results. Then a constitutive relation is established to investigate the effects of residual stress on the macroscopic deformation behavior of porous ZrO 2 /(ZrO 2 þ Ni) FGMs, which agrees well with the bending and compression experiments. It is found that residual stress can improve both the flexural strength and stiffness of the porous ZrO 2 /(ZrO 2 þ Ni) FGMs, by densifying and compensating the tensile stress of the porous middle layer (ZrO 2) during the bending process, respectively. However, it has no obvious influence on the ultimate compressive strength of the porous ZrO 2 /(ZrO 2 þ Ni) FGMs, but mainly influences its initial stage of elastic deformation in the compressive behavior.
The joining of ceramics with metals is widely used in aerospace engineering where high strength a... more The joining of ceramics with metals is widely used in aerospace engineering where high strength and excellent heat insulation materials are desired. In this paper, a new multifunctional bolted joint with load bearing-heat insulation integration is prepared with porous ZrO 2 /(ZrO 2 þNi) sandwich ceramics. Double-shear behavior of the bolted joint connecting C/SiC plates is analyzed by ABAQUS codes. It is found that shearing damage occurs at shearing faces of the bolt, the shearing failure faces are layered rather than smooth. To improve its shearing strength, we introduce the shear band (ZrO 2 þ V%Ni of thickness h) to its shearing faces. Results show that the shear band can improve the shearing strength and slow down the attenuation of load bearing capacity after reaching the shearing strength, without obviously increasing the thermal conductivity. An optimal structural design is performed and proper shear band is defined to balance the shearing strength and heat insulation performance of the ceramic joint.
In analysing metal forming processes the deformation mechanism map (elastic-plastic, elastic-visc... more In analysing metal forming processes the deformation mechanism map (elastic-plastic, elastic-viscoplastic, or creep type behaviour) for a particular process is commonly built solely in relation to temperature; which can be acceptable for a defined modest strain rate range. However, for a given temperature, if strain rate variation is large, the deformation mechanism could vary significantly. In this paper, a deformation mechanism map is proposed to clarify the interacting effect of deformation conditions (temperature and strain rate) on workpiece behaviour in metal forming processes. Rate type deformation equations which can be used to comprehensively model the effect of temperature and strain rate on deformation mechanism characteristics are elucidated and as examples, determined for Ti-6Al-4V and Al-Mg alloy.
Li-ion batteries are ineluctably subjected to external mechanical loading or stress gradient. Suc... more Li-ion batteries are ineluctably subjected to external mechanical loading or stress gradient. Such stress can be induced in battery electrode during fabrication and under normal operation. In this paper, we develop a model for stresses generated during lithiation in the thin plate electrode considering the effects of external mechanical loading. It is found that diffusion-induced stresses are asymmetrically distributed through the thickness of plate due to the coupling effects of asymmetrically distributed external mechanical stress. At the very early stage during Li-ions insertion, the effects of the external mechanical loading is quite limited and unobvious. With the diffusion time increasing, the external mechanical loading exerts a significant influence on the evolution of stresses generated in the electrode. External compressed electrode is inclined to increase the value of stresses generated during lithiation, while external tensed electrode tends to decrease the value of stresses, and as the diffusion time increases, the effects of the external mechanical loading on the stresses generated during lithiation become more obvious.
The joining of ceramics with metals have been extensively used in applications requiring high str... more The joining of ceramics with metals have been extensively used in applications requiring high strength and excellent heat insulation. However, evaluating the residual stress generated inevitably due to the mismatch in coefficients of thermal expansion of ceramic and metal is challenging, which is very important for fabrication and characterization of layered inhomogeneous material. A simplified analytical model considering the overall deformation compatibility is established to compute the interlaminar residual stresses of the ZrO 2 /(ZrO 2 þNi) sandwich ceramics, which agrees well with the results obtained by the commercial finite element package. The effects of the thickness ratio of the transitional layer to the middle layer, and the number of transitional layers on the properties of the ZrO 2 /(ZrO 2 þNi) sandwich ceramics are researched to obtain the optimal structure.
An analytical upper-bound-based model for predicting curvature of bent metal alloy profiles obtai... more An analytical upper-bound-based model for predicting curvature of bent metal alloy profiles obtained through a novel extrusion process, differential velocity sideways extrusion (DVSE), previously proposed by the authors, has been first-time developed. Finite element modelling and simulation and model material experiments, which were validated by extrusion of AA1050, have been performed to determine the geometry of the deformation zone and assess the accuracy of the analytical model. The extrusion force, curvature, and effective strain predicted by the analytical method agreed well with results from experiments and FE simulation. It was shown that the punch with a lower velocity experiences a lower extrusion force, which increases both with increase of its velocity and the extrusion ratio. The extrusion force on the faster punch with a constant velocity v 1 changes quite slightly with the increase of the velocity v 2 of the slower punch. Various values of curvature, which decrease with the increase of the punch velocity ratio v 2 / v 1 and the decrease of the extrusion ratio, can be achieved through the DVSE process. DVSE is a novel process which leads to larger effective strain per pass than that in the equal channel angular extrusion (ECAE).
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Conventional curved aluminium alloy profile forming methods which depend on apparatus, additional... more Conventional curved aluminium alloy profile forming methods which depend on apparatus, additional to extrusion tooling, to bend extruded, straight sections inevitably decrease manufacturing efficiency and have to deal with springback and crosssectional distortion. In this article, a novel energy-efficient forming method based on internal differential material flow, differential velocity sideways extrusion (DVSE), is proposed, which can directly form billets into curved profiles/sections by extrusion alone, using two opposing punches. A detailed study has been conducted and cold extrusion/bending of round bars and tubes was examined. It was found that curved profiles/sections with no distortion are formed, and adjustable profile curvature can be achieved by changing the velocity ratio of a slower lower punch to a faster upper punch and also extrusion ratio. Lower velocity ratio and larger extrusion ratio tend to increase the material flow velocity gradient at the die exit and lead to greater curvature. Effect of velocity ratio is greater than that of extrusion ratio, especially when velocity ratio is less than 1/3. Effect of the extrusion ratio is small when velocity ratio is greater than 0.5, after which the effect of extrusion ratio becomes more significant as velocity ratio decreases.
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