Debin Shan

Flow stress and tribology size effects in scaled down cylinder compression

Microforming is an effective method to manufacture small metal parts. However, macro forming can not be transferred to microforming directly because of size effects. Flow stress and tribology size effects were studied. Scaled down copper T2 cylinder compression was carried out with the lubrication of castor oil and without lubrication. The results show that the flow stress decreases with decreasing the initial specimen diameter in both lubrication conditions, and the flow stress decreases by 30 MPa with the initial specimen diameter decreasing from 8 mm to 1 mm. The friction factor increases obviously with decreasing the initial specimen diameter in the case of lubricating with castor oil, and the friction factor increases by 0.11 with the initial specimen diameter decreasing from 8 mm to 1 mm. However, the tribology size effect is not found in the case without lubrication. The reasons of the flow stress and tribology size effects were also discussed.

Hot deformation behavior and microstructure evolution of TC4 titanium alloy

Abstract The hot deformation behavior of Ti-6Al-4V (TC4) titanium alloy was investigated in the temperature range from 650 °C to 950 °C with the strain rate ranging from 7.7×10−4 s−1 to 7.7×10−2 s−1. The hot tension test results indicate that the flow stress decreases with increasing the deformation temperature and increases with increasing the strain rate. XRD analysis result reveals that only deformation temperature affects the phase constitution. The microstructure evolution under different deformation conditions was characterized by TEM observation. For the deformation of TC4 alloy, the work-hardening is dominant at low temperature, while the dynamic recovery and dynamic re-crystallization assisted softening is dominant at high temperature.

Effect of thickness and grain size on material behavior in micro-bending

Abstract Micro-bending tests were performed to investigate effects of thickness and grain size on material behavior in sheet metal forming. The rolling brass C2680 foil was selected as the experimental material, and it was annealed to eliminate the work-hardening and get different grain sizes. A device was specially designed for three-point bending with two load sensors. The results show that the bending force increases with increasing the punch displacement. With the foil of same thickness, a smaller punch radius leads to a larger bending force. When the grain size increases, the bending force becomes smaller. Size effects are observed obviously. These results have been analyzed by work-hardening, Hall-Petch equation and free surface effect.

Microstructure evolution of TA15 titanium alloy during hot power spinning

The microstructure evolution of TA15 titanium alloy during hot power spinning was studied. Effects of wall reduction on microstructure evolution of TA15 titanium alloy were researched. Slip deformation is the main deformation mechanism accompanying with twinning to coordinate the deformation. The microstructure gradually transforms into fine fibrous microstructure and the aspect ratio of primary α grain increases with increasing wall reduction. The recrystallization fraction and microhardness increase with increasing reduction.

Micro bulging of thin T2 copper sheet by electromagnetic forming

Electromagnetic micro bulging experiments on T2 copper were achieved in order to find the effects that different voltages and depths of mold work on deformation characters. Laser scanning confocal microscope and contourgraph were used to study the effects of electromagnetic forming parameters such as voltage and die depth on material. Results show that width and depth of micro channel increase with the increases of voltage with a certain die depth, moreover, forming depth reaches the maximum at 7 500 V. And then rebound emerges and forming depth decreases. Forming depth and width of channel increase with the decease of die depth at a certain voltage; and forming depth reaches maximum at 0.5 mm of die depth. Therefore, rebound is weakened and the traces caused by bad exhaust disappears gradually, and surface roughness decreases simultaneously in electromagnetic bulging experiments.

Mechanism of size effects in microcylindrical compression of pure copper considering grain orientation distribution

In microscale deformation, the magnitudes of specimen and grain sizes are usually identical, and size-dependent phenomena of deformation behavior occur, namely, size effects. In this study, size effects in microcylindrical compression were investigated experimentally. It was found that, with the increase of grain size and decrease of specimen size, flow stress decreases and inhomogeneous material flow increases. These size effects tend to be more distinct with miniaturization. Thereafter, a modified model considering orientation distribution of surface grains and continuity between surface grains and inner grains is developed to model size effects in microforming. Through finite element simulation, the effects of specimen size, grain size, and orientation of surface grains on the flow stress and inhomogeneous deformation were analyzed. There is a good agreement between experimental and simulation results.Graphical abstractThe deformation behavior in micro compression of pure copper has been investigated by a modified surface model considering orientation distribution of surface grains and continuity between surface grains and inner grains.

Effects of specimen size on flow stress of micro rod specimen

Abstract With the miniaturization of parts, size effects occur in the micro-forming processes. To investigate the effects of the specimen size on the flow stress, a series of upsetting deformation experiments were carried out at room temperature for specimens with different diameters. And the grain size of billets was changed by anneal processes to analyze the grain size effects on the size dependence of flow stress. The deviation of stress was observed. The results show that the flow stress decreases with decreasing billet dimensions. As the dislocation accumulation in free surface layer is slight, the reduction degree of flow stress becomes larger when the plastic deformation goes on. The flow stress is enlarged by grain size, which can be analyzed by the grain boundary length per area. The deviation increases with decreasing specimen size. This can be explained by the effects of grain orientation stochastic distribution according to the Schmid law. As a result, the micro-forming process must be considered from the viewpoint of polycrystalline structure, and the single grains of micro-billet dominate the deformation.

Surface finish of micro punch with ion beam irradiation

Abstract Ion beam irradiation was adopted for surface treatment of the micro punch manufactured by precision machining. Ar plasma was used for the ion irradiation process, which was generated by the electron cyclotron resonance(ECR) equipment. The surface finish processes of micro punch were carried out at irradiation angles of 45° and 10°, respectively. The surface roughness and topography were measured to estimate the quality of surface finish. The results show that the ion irradiation is very effective to reduce the surface roughness, which can be improved more significantly at irradiation angle of 10° than at 45°. The technology of surface finish with ion beam irradiation is suitable for the surface treatment of micro die.

Interactive effect of microstructure and cavity dimension on filling behavior in micro coining of pure nickel.

In this study, interactive effects of microstructure and cavity dimension on the filling behaviors in micro coining were investigated. The results indicate that the filling ability is dependent on both the cavity width t and the ratio of cavity width to grain size t/d strongly. The critical ratio t/d for the worst filling ability increases with cavity width t and tends to disappear when the cavity width t increases to 300 μm. A polycrystalline filling model considering the friction size effect, effect of constrained grains by the tools, grain size, cavity width and ratio of cavity width to grain size is proposed to reveal the filling size effect in micro coining. A quasi in-situ Electron Backscatter Diffraction (EBSD) method is proposed to investigate filling mechanism in micro coining. When several grains across the cavity width, each grain deforms heterogeneously to ordinate the deformation compatibility. When there is only one grain across the cavity width, the grain is fragmented into several smaller grains with certain prolongation along the extrusion direction to coordinate the deformation in the cavity. This is different from the understandings before. Then the filling deformation mechanism is revealed by a proposed model considering the plastic flow in micro coining.

Microstructural Evolution at Micro/Meso-Scale in an Ultrafine-Grained Pure Aluminum Processed by Equal-Channel Angular Pressing with Subsequent Annealing Treatment

Micro-forming with ultrafine-grained (UFG) materials is a promising direction for the fabrication of micro-electro-mechanical systems (MEMS) components due to the improved formability, good surface quality, and excellent mechanical properties it provides. In this paper, micro-compression tests were performed using UFG pure aluminum processed by equal-channel angular pressing (ECAP) with subsequent annealing treatment. Microstructural evolution was investigated by electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). The results show that microstructural evolutions during compression tests at the micro/meso-scale in UFG pure Al are absolutely different from the coarse-grained (CG) materials. A lot of low-angle grain boundaries (LAGBs) and recrystallized fine grains are formed inside of the original large grains in CG pure aluminum after micro-compression. By contrast, ultrafine grains are kept with few sub-grain boundaries inside the grains in UFG pure aluminum, which are similar to the original microstructure before micro-compression. The surface roughness and coordinated deformation ability can be significantly improved with UFG pure aluminum, which demonstrates that the UFG materials have a strong potential application in micro/meso-forming.