微机械加工技术─LIGA技术

简要介绍了LIGA（利嘎）工艺过程、特点和应用实例。     

Fatigue crack growth of titanium alloy joints by electron beam welding

Electron beam welding（EBW） has been widely used in the manufacture of titanium alloy welded blisk for aircraft engines. Based on fatigue crack growth tests on titanium alloy electron beam welding（EBW） joints, mechanism of fracture was investigated under scanning electron microscope（SEM）. The results show that fatigue crack growth rate increases as the experimental load increases under the same stress ratio and stress intensity factor range. At the beginning of crack growth, the extension mechanism of fatigue crack is the typical mechanism of cleavage fracture. In the steady extention stage, crack extends along the weld seam firstly.Then, crack growth direction changes to extend along the base metal. The extension mechanism of fatigue crack in the weld seam is the main mechanism of cleavage fracture and the extension mechanism of fatigue crack in the base metal is the main extension mechanism of fatigue band. In the instantaneous fracture stage, the extension mechanism of fatigue crack is the typical dimple-type static fracture mechanism.Crack growth was simulated by conventional finite element method and extended finite element method.     

Effect of manufacturing processes on formability and surface topography of proton exchange membrane fuel cell metallic bipolar plates

Metallic bipolar plates in PEM fuel cells offer low-volume, low-mass and low-cost stack fabrication in addition to superior durability when compared to composite bipolar plates, which suffer due to their much higher thickness and less durability. This study aims to address the formability and surface topography issues of metallic bipolar plates fabricated by stamping and hydroforming technologies. Particular emphasis was given to process repeatability, surface topology, and dimensional quality of bipolar plates that would greatly affect the corrosion and contact resistance characteristics. Thin metal sheets of several alloys (i.e., SS304, SS316L, SS430, Ni270, Ti grades 1 and 2) were used in the fabrication experiments. SS304 and SS316L were shown to possess better formability when compared to other alloys that were used in this study, while SS430 and Ti grade 2 demonstrated the worst among all. Channel formability was observed to be greatly affected by the hydroforming pressure, while it does not differ much above certain level of stamping force. The confocal microscopy analyses showed that surface roughness values of the formed samples were altered significantly when compared to the initial flat blanks. In general, increasing hydroforming pressure and stamping force yielded higher surface roughness values at channel peaks. In addition, the surface topography was shown to be influenced mainly by the pressure level rather than the pressure rate in hydroforming process.     

Investigation on shape transferability in ultraprecision glass molding press for microgrooves

Glass molding press (GMP) has been applied to produce the microgrooves on glass plates by using electroless-plated nickel phosphorus (Ni-P) molds. The GMP process for microgrooves was analyzed by finite element method (FEM) simulation. The effects of various pressing conditions, such as the molding temperature, the pressing velocity and the friction in glass deformation, were studied. Then, based on the simulation results, optimal pressing conditions were determined and used in the GMP experiments. By comparing the shape of the molded microgrooves with that of the corresponding microgrooves on the Ni-P mold, the shape transferability was evaluated and the reasons for form error were analyzed. The results show that the GMP process is an effective way to fabricate precision microgrooves on glass.     

微型铜圆柱体镦粗实验研究

为了找出微小尺度下材料流动及流动应力等的新特点及为微成形工艺设计、模具设计提供相关数据,在常温下以不同变形速度和变形量进行了微小尺度下圆柱体镦粗实验。结果表明,材料流动呈现出各向异性,且随压下量增大而明显;屈服应力降低,出现较大波动;流动应力出现明显波动;变形速度对材料流动、屈服应力、流动应力没有明显影响。本研究对于微成形技术的实用化具有一定意义。     

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 sizedependent 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.     

Construction of a composite model of decreasing flow stress scale effect

Based on the composite model of polycrystalline plastic deformation, a new composite model was built, with which the relation of flow stress to grain size, dimension and shape of the billet can be well explained. Through the comparison of this model with experimental data and the surface layer model, its rationality was testified. Based on this model, with the definition of scale effect factor, the decrease of flow stress in micro scale was discussed and predicted, and conclusions were drawn as below: with the downscaling of billet dimension, the decrease of flow stress is gradually increased; with the increase of grain size, the decrease of flow stress is also gradually increased; the shape of billet has a significant influence on the decreasing flow stress scale effect.     

Tribology in microforming

As the tribological conditions between tool and workpiece in metal working are of greatest importance for process feasibility and process quality this is even more true when the process is scaled from conventional down to micro dimensions. As can be shown by scaled friction tests the friction is increased by a factor of 20 and more which will have a distinct impact on all factors characterising the process. This cannot be described by the conventionally used friction laws but can be explained by the model of open and closed lubricant pockets characterising the surface topography. In the present paper, it will be shown that - starting from this model - the size effect in friction can be described by the general friction law developed by Wanheim/Bay.     

Microimprinting of nanocrystalline metals – Influence of microstructure and work hardening

The microimprinting behaviour of a wide variety of materials with grain sizes ranging from 25 nm up to a few mm was studied, using a flat punch indenter with a circular cavity with a diameter of 5 μm and a cavity line width of 350 nm. The flat punch die was fabricated by focussed ion beam machining. The imprinted ring geometry has been evaluated using atomic force microscopy in terms of extrusion height, relative to residual imprinting depth. The experimental work was complemented by Finite Element Analysis (FEA) of the forming process, using experimentally determined stress–strain data of the imprinted materials as input for the finite element simulations. Even so only macroscopic properties of the materials have been used for the modelling, both experiment and simulation yield comparable results. However, the relative extrusion height from FEA are much larger compared to the experimental results. It is found that grain size and the subsequent work hardening behaviour of the material has a strong influence on the local flow behaviour during imprinting. Nanocrystalline materials with nearly ideal-plastic deformation behaviour exhibit the best formability. The influence of the work hardening behaviour on the local flow of the material during microimprinting is moreover confirmed by imprinting of a pre-strained material. A reduction in the work hardenability of the material leads to an increasing microflow during microimprinting.     

Experimental Investigation on Micro Deep Drawing of Stainless Steel Foils with Different Microstructural Characteristics

In the present work, austenitic stainless steel(ASS) 304 foils with a thickness of 50 μm were first annealed at temperatures ranging from 700 to 1100℃ for 1 h to obtain different microstructural characteristics.Then the effects of microstructural characteristics on the formability of ASS 304 foils and the quality of drawn cups using micro deep drawing(MDD) were studied, and the mechanism involved was discussed. The results show that the as-received ASS304 foil has a poor formability and cannot be used to form a cup using MDD. Serious wrinkling problem occurs on the drawn cup, and the height profile distribution on the mouth and the symmetry of the drawn cup is quite nonuniform when the annealing temperature is 700 ℃. At annealing temperatures of 900 and 950 ℃, the drawn cups are both characterized with very few wrinkles, and the distribution of height profile, symmetry and mouth thickness are uniform on the mouths of the drawn cups. The wrinkling becomes increasingly significant with a further increase of annealing temperature from 950 to 1100 ℃. The optimal annealing temperatures obtained in this study are 900 and 950 ℃ for reducing the generation of wrinkling, and therefore improving the quality of drawn cups. With nonoptimized microstructure, the distribution of the compressive stress in the circumferential direction of the drawn foils becomes inhomogeneous, which is thought to be the cause of the occurrence of localized deformation till wrinkling during MDD.