Chuanjie Wang

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.

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.

Effect of Laser Power on the Quality of Drilled Micro Hole Using Cu50Zr50 Amorphous Alloys Foils

The rapid growth of the micro-electro-mechanical systems (MEMS) is being driven by the rapid development of the micro manufacturing processes. Laser beam machining is one of the micro manufacturing processes which can shape almost all ranges of engineering materials. In this study, the effect of laser power on the quality of drilled micro holes using Cu50Zr50 amorphous alloys foils is experimentally investigated. It indicates that both entrance and exit circularities diameters increase with laser power. The circularities of the holes at the entry and the exit are in the range of 0.893 to 0.997. The taper of drilled holes increases quickly to a stable value with the increase of laser power from 60 to 110 W, then decreases quickly when the laser power becomes larger than 170 W. The micro holes with a diameter of 400 to 1200 μm are manufactured successfully by laser drilling processes. The formation mechanism of the quality of the laser drilled hole is analyzed based on the laser beam and the properties of amorphous alloys foils.

Manufacturing High Aspect Ratio Microturbine by Isothermal Microforging Process

An isothermal microforging process is proposed to manufacture a microturbine with high aspect ratio microblade. Two schemes with circular and circular ring preforms were performed by the proposed isothermal micro forging process. A microturbine with a higher microblade is manufactured when using the circular ring preform compared to that using the circular one. Then, the mechanical property of microturbine is evaluated via the shear strength of the micro blade. The mechanical property is improved by solid solution and aging treatment.

Tensile deformation behaviors of pure nickel fine wire with a few grains across diameter

Abstract Size effects on plastic deformation behaviors in uniaxial micro tension of pure nickel fine wires were investigated experimentally, including flow stress and inhomogeneous deformation behaviors. It is found that with the increase of grain size or the decrease of number of grains across the diameter, the flow stress decreases and inhomogeneous deformation degree increases. When there are less than 9.3 grains across the diameter, the flow stress decreases quickly with the increase of grain size. Then, the flow stress size effect in micro tension of fine wires is revealed by a proposed model by introducing the grain boundary size factor. These results also indicate that both the fracture strain and stress decrease with the increase of grain size. When there are less than 14.7 grains across the diameter, both the fracture strain and stress decrease quickly. This indicates that the inhomogeneous deformation degree in micro tension increases with the decrease of the number of grains across the diameter. The fracture topography tends to be more and more irregular with the decrease of the number of grains across the diameter. Then, the formation mechanism of irregular fracture topography was analyzed considering the inhomogeneous distribution of microstructure when there are a few grains across the diameter.

Constitutive model based on dislocation density and ductile fracture of Monel 400 thin sheet under tension

In micro-scaled plastic deformation, material strength and ductile fracture behaviors of thin sheet in tension are quite different from those in macro-scale. In this study, uniaxial tensile tests of Monel 400 thin sheets with different microstructures were carried out to investigate the plastic deformation size effect in micro-scale. The experimental results indicate that the flow stress and fracture strain departure from the traditional empirical formula when there are only fewer grains across the thickness. And the number of dimples on the fracture surface is getting smaller with the decreasing ratio of specimen thickness to grain size. Then, a constitutive model based on dislocation density considering the free surface effect in micro-scale is proposed to reveal the mechanism of the flow stress size effect. In addition, a model is proposed considering the surface roughening inducing the thickness nonuniform and the decrease of micro-voids resulting from the reduction of grain boundary density with the decreasing ratio of specimen thickness to grain size. The interactive effects of the surface roughening and the decrease of micro-voids result in the earlier fracture in micro tension of the specimen with fewer grains across the thickness.

Effect of Heat Treatments on Microstructures and Tensile Properties of Cu–3 wt%Ag–0.5 wt%Zr Alloy

The microstructures and tensile properties of Cu–3 wt%Ag–0.5 wt%Zr alloy sheets under different aging treatments are investigated in this research. As one kind of precipitate, Ag nanoparticles with coherent orientation relationship with matrix precipitate. However, after the peak-age point, most of Ag nanoparticles grow into short rod shape with the interface translating to semi-coherent, which leads to the lower strength of over-aging sample. The yield strength is estimated by considering solid solute, grain boundary and precipitation strengthening mechanisms. The result shows that the Ag precipitates provide the main strengthening role. Then a constitutive equation representing the evolution of dislocation density with plastic strain is built by considering work-hardening behavior coming from shearable and non-shearable precipitates which is mainly the particles containing Zr. The flow stress contributed by shearable particle hardening is higher than that of non-shearable one. Due to the coarsening of grain boundary precipitates and low rate of damage accumulation of these non-shearable particles, the micro-cracks nucleate easily at grain boundary which leads to intergranular fracture.

Influence of laser parameters on micro-hole drilling of Cu50Zr50 amorphous alloys foil

ABSTRACT In this study, the effect of the laser process parameters on the quality of the drilled holes using Cu50Zr50 amorphous alloys foils is investigated experimentally. The results indicate that the diameter of the laser drilled micro-hole increases with the increase of the laser power or laser pulse width. The increasing speed decreases when the laser power or laser pulse width reaching thresholds of 120 W or 1.1 ms, respectively. The variation of diameter with the laser defocusing amount is not monotonic changing and dependent on both the laser defocusing amount and the laser power. The diameter of the laser drilled hole reaches a minimum value under the laser defocusing amount of −0.5 mm for all the laser powers. The variation law of diameter with the laser defocusing amount is changing with the laser power. The formation mechanism of the laser drilled hole is analyzed based on the laser beam and the amorphous alloys foils properties.