Chunju Wang

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.

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.

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.

Effects of tribological behavior of DLC film on micro-deep drawing processes

To decrease the size effects of friction in microforming, three kinds of surface coatings, such as diamond-like carbon (DLC), TiN and MoS2, were deposited on surfaces of dies with plasma based ion implantation and deposition (PBII & D) method and magnetron sputtering technique, respectively. The tribological behavior of surface coatings was analyzed considering plastic deformation of specimen at contact interface. The analyses indicate that there is a lower coefficient of friction (COF) and a high wear resistance under the condition of large strain/stress when using the DLC film. The graphitization of DLC film occurs after 100 times of tests. The mechanism of graphitization was analyzed considering energy induced by friction work. The effects of DLC film properties on qualities of micro-deep drawn parts were investigated by analyzing the reduction of wall thickness, etc. The results indicate that DLC film is very helpful for improving the qualities of the micro-parts.

Size effects on plastic deformation behavior in micro radial compression of pure copper

Abstract Micro radial compression tests were carried out on cylindrical specimens of pure copper polycrystals with different grain sizes. Experimental results indicated that phenomena of decreasing forming force, increasing scatter of forming force and more irregular surface topography occurred with the increase of grain size. A modified surface model based on dislocations pile-up in surface layer grains, and a flow stress scattering formulation based on standard deviation and grain size distribution were proposed to analyze size effects on forming force in micro compression. The inhomogeneous deformation of surface layer grains was discussed by the main deformation manner of rotation. A good agreement with the experimental results was achieved.

Size Effect on Friction of C3602 in Cylinder Compression

Microparts are more difficult to form than macroparts because of the size effect on material properties and the size effect on friction. Friction size effect was studied in this research. Cylinder compression was carried out with using lubrication talc powder, petroleum jelly, and soybean oil and without lubrication. The specimen material was copper alloy C3602 that was thermally treated at 623, 723, 823, and 923 K for 1 h in nitrogen atmosphere, respectively. The specimens were manufactured in an instrument lathe with initial diameters of 6, 4, 2, and 1 mm, respectively, and a height-to-diameter ratio h0/d0 = 1.5. The experiment was carried out at room temperature on a universal testing machine, and the strain rate ϵ = 0.0025 s−1. The results show that the friction size effect was obvious in the case of lubrication with soybean oil. The friction factor increased distinctly with the miniaturization of the specimen size. However, the friction size effect was not found in the case of other lubrication conditions. The reason for this phenomenon is also discussed in this article.