Product miniaturization is an emerging trend for facilitating product usage, enabling unique product functions to be implemented in micro-scaled geometries and features, and further reducing product ...weight and volume. Recently, a demand for microparts increased significantly in many industry clusters. Development of the advanced micromanufacturing technologies for fabrication of such microparts has thus become a critical issue. Microforming, which offers attractive characteristics including high productivity, low cost and good quality of the formed parts, provides a promising approach to fabricating metallic microparts. In the last two decades, a lot of effort has been made to the researches on size effect related deformation behaviors in microforming process and the development of the process. Having a panorama of these researches is necessary to support micropart design and development via microforming, and further advance this micromanufacturing process. In this paper, an intensive review on the latest development of microforming technologies is presented. First of all, the paper is focused on the review of the size effect-affected deformation behaviors and the mechanisms of the changes of flow stress, flow behavior, fracture behavior, elastic recovery, tooling–workpiece interfacial friction and the surface finish of the formed parts. The state-of-the-art microforming processes, including micro deep drawing, microembossing, micropunching, microcoining, microextrusion, microheading, and micro progressive forming are then presented. Finally, some research issues from the implementation of mass production perspective are also discussed.
The effect of grain size on micro-void evolution and further macroscopic fracture in an austenite steel 316LN was studied through a series of experiments together with the full-field crystal ...plasticity finite element method (CPFEM) simulations. To probe the grain size effect on void behaviors, macroscopic tensile tests and microscopic fractography characterizations were conducted for samples with different grain sizes. A hierarchy modeling approach based on CPFEM was adopted to quantify the gran size effect accordingly. Authentic boundary conditions were enforced on the high-resolved representative volume elements (RVEs) with realistic grain structures and voids. The simulation results demonstrate that the deformation heterogeneity and the scatter of void growth increase with grain size. Via the quantitative analysis of the void dimension, an extended void growth model involving the effect of grain size was proposed on the basis of the Rice and Tracey model. The extended model adopts the Gaussian distribution to describe the non-uniform void growth induced by the grain-scale deformation heterogeneity and manifests the increase of void size deviation with grain size. The variation of void growth with grain size further leads to a transition of fracture modes. For the fine grain sample, coalescence of densely distributed voids dominates the fracture initiation, and the total void volume fraction thus plays a key role; For the coarse grain sample, however, the growth and coalescence of individual large void are critical for fracture occurrence. With the grain size affected void behavior, the ductility of the material is also shown to be grain size dependent. This study thus advances the comprehensive understanding of the micro-mechanics of ductile fracture and the relationship between microstructure and the macroscopic fracture behavior.
•Identify the grain size effect on void behavior via experiments and full-field CP modeling.•Deformation and void size distribution become more uneven as grain size is increased.•An extended void growth model involving the effect of grain size was proposed.•Explain different ductile fracture mechanisms related to grain size.•Identify the relationship between ductility and grain size.
Nickel-based superalloys were thermomechanically processed at temperatures of 1050–1170 °C and strain rates of 0.001–10 s−1 to study the hot-deformation behavior and dynamic recrystallization (DRX) ...of the columnar structures. Electron backscatter diffraction (EBSD) was employed to characterize the DRX microstructure of the columnar grains during the thermomechanical processing of the alloys. A linear relationship between the critical strain-hardening rate and critical stress was derived and validated. The critical conditions for DRX were directly determined from the corresponding peak values, which could be used to effectively predict the critical conditions for DRX to occur. It was determined that discontinuous-DRX (DDRX) and continuous-DRX (CDRX) both have an important role in the microstructural evolution. During deformation of the columnar grains, the DRX nuclei first developed at the serrated grain boundaries (GBs) via grain boundary bulging. Subsequently, the continuous misorientation accumulation led to subgrain rotation, which accelerated nucleation within the deformed grains. Owing to DDRX, the DRX nuclei tended to develop at the GBs parallel to the loading direction, leading to the heterogeneous distribution of the newly formed, fine DRX grains. The findings of this study provide a basis for the precise control of the microstructures of nickel-based superalloys with columnar grains during the hot-working process, and therefore, could be used to tailor their properties.
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•A relation between critical work-hardening rate and critical stress is derived.•DRX nucleation process was captured by using tailored deformation parameters.•Grain boundary bulging and subgrain rotation take effect in different conditions.•DRX nuclei prefer the grain boundaries parallel to the loading direction.
Size effects (SEs) exist in many domains, and are caused by changes in the values of parameters of materials, structures, or systems individually or collectively known as size effect factors. SEs ...induce a variety of scale-dependent behaviours, phenomena, and performances during the multiscale processing and manufacturing of materials, such as the macro-, meso-, and microscale ones examined by the researches described in this paper. Furthermore, the parts and components fabricated from multiscale processing and manufacturing exhibit the scatters of quality and properties. This paper uses multiscale machining and deformation-based manufacturing as the case studies of material processing and manufacturing to review and analyse SEs and their manifestations. The current status of multiscale manufacturing research and its possible future avenues are articulated and discussed, and key problems that must be solved and unknowns that must be understood are highlighted. This paper thus presents a broad understanding and insight into SEs that influence the material processing and manufacturing for making multiscale parts and components. It also examines the bottleneck problems generated by SEs in such processing and manufacturing arena, and how these can be solved to enable the high efficiencies of multi-scale materials processing and manufacturing to be realised.
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•Size effect (SE) factors and their classification are given.•Manifestations of SEs and the classification into three identified phenomena are articulated.•The SE available mechanisms are summarised and the loopholes are analysed.•The SE-induced behaviour and phenomena are delineated.•SE caused scatters are explicated and the impacts in manufacturing are discussed.
Summary
Recent studies found that hepatitis C virus (HCV) may invade the central nervous system, and both HCV and Parkinson's disease (PD) have in common the overexpression of inflammatory ...biomarkers. We analysed data from a community‐based integrated screening programme based on a total of 62 276 subjects. We used logistic regression models to investigate association between HCV infection and PD. The neurotoxicity of HCV was evaluated in the midbrain neuron–glia coculture system in rats. The cytokine/chemokine array was performed to measure the differences of amounts of cytokines released from midbrain in the presence and absence of HCV. The crude odds ratios (ORs) for having PD were 0.62 95% confidence interval (CI), 0.48–0.81 and 1.91 (95% CI, 1.48–2.47) for hepatitis B virus (HBV) and HCV. After controlling for potential confounders, the association between HCV and PD remained statistically significant (adjusted OR = 1.39; 95% CI, 1.07–1.80), but not significantly different between HBV and PD. The HCV induced 60% dopaminergic neuron death in the midbrain neuron–glia coculture system in rats, similar to that of 1‐methyl‐4‐phenylpyridinium (MPP+) but not caused by HBV. This link was further supported by the finding that HCV infection may release the inflammatory cytokines, which may play a role in the pathogenesis of PD. In conclusion, our study demonstrated a significantly positive epidemiological association between HCV infection and PD and corroborated the dopaminergic toxicity of HCV similar to that of MPP+.
Taking the importance of γ′ phase into consideration, an as-cast nickel-based superalloy was investigated by uniaxial compression experiments performed at sub-solvus and super-solvus temperatures. ...Several different strains were utilized to trace the processes of deformation and microstructural evolution. Electron backscatter diffraction was employed to characterize the microstructures. The results reveal that the volume fraction of DRX grains increased with strain at both sub-solvus and super-solvus temperatures, and also the fraction of high angle grain boundaries. Meanwhile, both the correlated and uncorrelated misorientations gradually approached the random distribution curves during continuous strains. Specially, the fast migration of grain boundaries caused by the unpinning from γ′ particles promoted twin nucleation and generated a number of twins at sub-solvus temperatures. Sub-solvus deformations were found to be more efficient to weaken the initial textures of as-cast superalloys. Finally, it was confirmed that discontinuous dynamic recrystallization (DDRX) dominated the DRX process during the hot deformation carried out at both sub-solvus and super-solvus temperatures. Continuous dynamic recrystallization (CDRX) was the second mechanism, and played a very important role during plastic deformation at sub-solvus temperatures, which is different from the DRX mechanisms reported in the fine-grain structures of wrought billets. The findings improve the understanding of DDRX and CDRX, which does benefit to the accurate control of microstructures of nickel-based superalloys, and also tailoring the properties of final components used in aero-engine.
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•The fractions of DRX grains and HAGBs increased with strain.•The fast migration of GBs caused by unpinning from γ′ phase promoted twin nucleation.•Sub-solvus deformations weakened the initial casting textures more efficiently.•DDRX was the dominant mechanism during sub-solvus and super-solvus deformations.•CDRX played a more important role during sub-solvus deformations.
Numerous criteria have been developed for ductile fracture (DF) prediction in metal plastic deformation. Finding a way to select these DF criteria (DFCs) and identify their applicability and ...reliability, however, is a non-trivial issue that still needs to be addressed in greater depth. In this study, several criteria under the categories of ‘uncoupled damage criterion’ and the ‘coupled damage criterion’, including the continuum damage mechanics (CDM)-based Lemaitre model and the Gurson–Tvergaard–Needleman (GTN) model, are investigated to determine their reliability in ductile failure prediction. To create diverse stress and strain states and fracture modes, different deformation scenarios are generated using tensile and compression tests of Al-alloy 6061 (T6) with different sample geometries and dimensions. The two categories of criteria are coded into finite element (FE) models based on the unconditional stress integration algorithm in the VUMAT/ABAQUS platform. Through physical experiments, computations and three industrial case studies, the entire correlation panorama of the DFCs, deformation modes and DF mechanisms is established and articulated. The experimental and simulation results show the following. (1) The mixed DF mode exists in every deformation of concern in this study, even in the tensile test of the round bar sample with the smallest notch radius. A decrease of stress triaxiality (
η-value) leads to a reduction in the accuracy of DF prediction by the two DFC categories of DFCs, due to the interplay between the principal stress dominant fracture and the shear–stress dominant factor. (2) For deformations with a higher
η-value, both categories of DFCs predict the fracture location reasonably well. For those with a lower or even negative
η-value, the GTN and CDM-based criteria and some of the uncoupled criteria, including the C&L, Ayada and Oyane models, provide relatively better predictions. Only the Tresca and Freudenthal models can properly predict the shear dominant fracture. The reliability sequence of fracture moment prediction is thus the GTN model, followed by the CDM-based model and the uncoupled models. (3) The applicability of the DFCs depends on the use of suitable damage evolution rules (void nucleation/growth/coalescence and shear band) and consideration of several influential factors, including pressure stress, stress triaxiality, the Lode parameter, and the equivalent plastic strain or shear stress. These parameters determine the deformation mode (shear dominant or maximum principal stress dominant deformation) and, further, the DF mechanism (dimple fracture/shear fracture/mixed fracture).
•A DDD model involving dislocation-TB interactions for TWIP steel with low SFE was established.•The complex dislocation reactions for the 60° dislocation at the TB were determined based on the energy ...criterion.•An improved DDB model was established by considering the dislocation type in describing the dislocation-TB interactions.•The mechanical response at different loading directions can be well predicted by this improved DDB model.
Deformation twins contribute to the unique deformation behaviors and characteristics in the plastic deformation of TWIP steels since twin boundary (TB) blocks the movement of dislocations and absorbs them in deformation process. On the other hand, dislocations can traverse TB. However, there is still no consensus on how TB influences the plastic deformation of TWIP steels among prior researches. Therefore, exploring the interaction between dislocation and TB is critical to understand the effects of twins on the macro deformation behaviors and exploit the strengthening potential of the alloys. In this study, a dislocation-TB interactions model for TWIP steel was proposed, developed and implemented in discrete dislocation dynamics (DDD) simulation, the complicated dislocation reactions at the TB were determined by the energy criterion, which serves as a feasible approach to represent the micro deformation characteristics under different tension directions with respect to the twin plane normal of TWIP steel micropillar. Furthermore, the effect of dislocation type and reaction characteristic at the TB in DDD are incorporated into the conventional dislocation density-based (DDB) model, and then the improved DDB model is used to quantitatively describe the macro plastic behavior of TWIP steel micropillar. The DDD simulation results show that the dislocation-TB interactions are related to the dislocation type and the angular relationship between loading direction and twin plane normal. The TB has a significant strengthening effect if the loading direction is perpendicular to the twin plane (0°) due to the increase of the back stress induced by the activated 60° dislocation pileups. For other orientations (45° and 90°), however, the strain hardening becomes negligible. Meanwhile, the stress and dislocation density-strain curves under different directions with respect to the twin plane normal are predicted by the improved DDB model and have a good agreement with the DDD simulation and experimental results. The research thus advances the understanding of dislocation-TB interaction mechanisms in plastic deformation of TWIP steels.
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Electrically-assisted forming has been proven to be an efficient process in terms of reducing forming flow stress, improving formability and supressing the springback for different hard-to-deform ...alloys. However, there is a controversial debate about whether the flow stress reduction with the introduction of electric current into a metallic specimen undergoing plastic deformation is attributed to Joule heat, additional athermal effect, namely the electroplasticity, or their combination thereof. There is a lack of thorough understanding of different mechanisms inducing the flow stress reduction and formability improvement, which would hinder the development and application of innovative electrically-assisted forming processes. Most prior works have examined only one or two materials with similar electroplastic behaviours, making it difficult to resolve this debate. In this study, three typical materials with diverse electroplastic behaviours, i.e. copper, SS304 and Ti6Al5V, were investigated and compared via electrically-assisted tensile tests and thermally-assisted tensile tests. The flow stress of the copper specimens in the electrically-assisted tensile test was lower than that in the thermally-assisted tensile test, which verifies the existence of electroplasticity. However, the flow stresses of the SS304 specimens in the electrically-assisted tensile test and thermally-assisted tensile test were similar at the same temperature, indicating that the athermal effect on the flow stress is not evident. For the Ti6Al4V specimens, the athermal stress reduction was not significant at lower current densities. However, the electrically-assisted and thermally-assisted tensile tests results began to differ when the current density exceeded 37.6 A/mm2. According to the further discussion on the microstructure evolution of different materials, the introduced current improves the dislocation motions and recovery and consequently reduces the flow stress in simple face-centered cubic structure materials such as copper. However, these effects can be suppressed by stronger effects such as the solution hardening in stainless steels. The current also affects the phase transition, which in turn influences the hardening behaviours of Ti6Al4V. By clarifying the cause of conflicting electroplastic effects, a uniform analytical method calculating the short- and long-range stresses that considers the correlation of electrical and thermal effects on dislocation motion, solution hardening and phase transformation was developed for electrically-assisted forming processes. The model was also experimentally verified.
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•The controversial electroplasticity in different FCC and HCP alloys was clarified.•Athermal electrical effect leads to easier dislocation motion for copper.•Athermal stress reduction of SS304 was not obvious due to solution hardening.•The electrical effect on phase transition of Ti6Al4V leads to electroplasticity.•Uniform short-range and long-range stresses models were further developed.
• Interactive effect of grain and specimen sizes on deformation behavior is studied. • The volume fraction of grain boundary changes with the specimen and grain sizes. • The flow stress decreases ...linearly with the ratio of specimen to grain sizes. • Flow stress decreases with the increasing volume fraction of surface grains. • Methodology to identify the surface and internal grain properties is proposed.
In this research, the interactive effect of grain and specimen sizes on the flow stress of sheet metal in microforming is investigated via the tensile test of pure copper and numerical modeling. Models based on different assumptions are proposed to analyze the size effect phenomenon. It is found that the flow stress decreases linearly with the decrease of the ratio of specimen to grain sizes. The grain boundary thickness decreases and its volume fraction increases with the decrease of grain size. The variation of grain boundary thickness is not proportional to the variation of grain size. Furthermore, the fraction of grain boundary increases with the strain and the ratio of specimen to grain sizes. Based on the FE simulation, it is found that the simulated flow stress, which is modeled based on the identified grain boundary thicknesses using the proposed models, has a good agreement with the experimental result. In addition, the size effect on flow stress is also analyzed based on the surface layer model. The methodology to identify the surface and internal grain properties is proposed based on the experimental result. The identified properties are applicable in modeling of the interactive effect of specimen and grain sizes on flow stress. This research thus provides an in-depth understanding of the plastic deformation behavior in microforming process.
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