Lan Cai

Laser shock forming on coated metal sheets characterized by ultrahigh-strain-rate plastic deformation

We report on a dynamic ultrahigh-strain-rate forming method driven by laser impact. This investigation reveals that laser shock forming is a mechanical, not a thermal process, and the strain rate can go up to 107–109 s−1, two or more orders higher than that of all the existing forming methods. By investigating the hardness and residual stress of the surfaces, we conclude that laser shock forming is a technique with combining laser shock strengthening and metal forming, which introduces strain hardening and compressive residual stresses. We also discover some nonlinear plastic deformation characters in laser shock forming.

Blood Platelet’s Behavior on Nanostructured Superhydrophobic Surface

Regular arrays of micro-pillars and nano-grooves structures on the silicon wafer are fabricated by using soft lithography, and the three dimension morphology of textured surface is observed by using scanning electron microscopy (SEM) and atomic force microscope (AFM). The static water contact angles are measured by using contact angle meter to characterize the wettabilities of these surfaces. To investigate how the presence of topography and the variations of wettability affect the haemocompatibility of textured surface contacted with blood, different patterned surfaces are designed and fabricated, and blood platelet adhesion test is carried out on these surfaces. The adhesion and coagulation of platelets are inspected by scanning electron microscopy (SEM). Experimental data presented in this paper indicate that different surface roughness and wettability are the important factors for blood platelet adhesion. The amount of adsorbed blood platelet is low on textured surfaces, compared with that on the flat surface. Especially, there is no coagulation and activation on the surface with nanometer grooves. That is to say, the superhydrophobic surface is apt to decrease blood platelet adhesion. The study suggests that surface with suitable wettabililty and textured structures exhibits superior blood compatibility.

Self-comparison measurement for slippage on superhydrophobic surfaces based on the wetting transition

This paper describes a self-comparison measuring method for the slip length of fluid flow over superhydrophobic surfaces consisting of a pillar array based on the wetting transition. The wetting transition of the liquid resting on a superhydrophobic surface in the measuring course may lead to the disappearance of the slippage effect, which can be used to measure the slip length before the wetting transition. Based on this feature, the slip length formula for this measuring method is deduced. Then, to verify this method, a rheological experiment for 70 wt% glycerin solution flow over a superhydrophobic surface with carbon nanotube (CNT) forest was carried out on a commercial rheological system with plate-and-plate configuration. Results show that even on the CNT-coated surface that can induce little slip length, the experimental slip length is still on the same scale as the theoretical one. This measuring method can be used to measure the slip length of liquid flow over the superhydrophobic surface consisting of a pillar array with good flexibility.

A Study on the New Method of Laser Scratch Testing Technique to Characterize Interfacial Adhesion Between Thin Film and Substrate

The interfacial adhesion between thin film and substrate are often the predominant factor in determining the performance and reliability of thin film-substrate system. A new method of laser scratch testing technique is presented to characterize the interfacial adhesion between film and substrate, which synthesizes the advantages of traditional scratching technique and laser measuring technique. The laser scratching mechanism is analyzed----thermal-stress spallation caused by the quasi-static heating of long pulse infrared laser and cold denudation of short pulse far-ultraviolet radiation laser, and the laser scratch equipment is introduced as well. Long pulse infrared laser scratch testing technique can be applied to detect thermal barrier film, such as hard tool film, decoration film, DLC film, function film and so on, while far-ultraviolet radiation laser scratch testing technique can be applied to detect macromolecule film and semiconductor material. Introduction The film mentioned is the general designation of surface plate film, surface coating or surface modifying layer. Thin film is one of the important means applied to improve surface performance of material, which gives the parts the property such as high wear-resistance, high corrosion-resistance, high heat-resistance, high fatigue-resistance and so on. As the result, resource and energy can be saved, the performance of substrate and film can be fully performed respectively, and the production cost can be reduced. Thin film has been used for a variety of purposes in myriad applications such as aeronautics and astronautics, mechanical engineering, electronic technology, optical engineering and computer science, etc. Regardless of its different applications, an adequate adhesion to the underlying substrate is one of the most importances. Concomitantly, the necessarity to develop advanced techniques for quantitative and correct assessment of adhesion of thin film and substrate is apparent [1]. Various kinds of advanced films appear with each passing day such as hard film, photoelectricity film, magnetic film and LB film, etc., and great achievements have been constantly made in the fields of various kinds of advanced film manufacturing technology. On the other hand, the technology of film performance measurement and control seems no much progress, especially the experimental technology of quantitative analysis of interfacial adhesion between thin film and substrate. This has affected the further development of thin film science, and the contradiction becomes bigger day by day. The quantitative measurement of interfacial adhesion between thin film and substrate has already become a headache to which scientists of various countries devote themselves [1-3]. There are nearly 355 techniques for measuring interfacial adhesion, such as scratch test, indentation, laser acoustic, laser ablation, laser spallation, etc. [2-3]. Each technology and mechanism of measurement has some limitation and some problem need to be improved, such as the instability, disagreement between different methods, etc. The difficulty is that the interfacial adhesion to be measured is the synthetic result of the elastic and plastic behaviors of film and substrate, friction and other related parameters. The relationship between critical value of film rupture and interfacial adhesion has not yet been clarified so far, and there is no one single technique which can be applied Key Engineering Materials Online: 2004-03-15 ISSN: 1662-9795, Vols. 259-260, pp 615-619 doi:10.4028/www.scientific.net/KEM.259-260.615

Finite Element Modelling and Analysis of Cutting-Direction Burr Formation

To prevent or reduce the formation of burr efficiently in metal cutting, it is necessary to reveal the burr formation mechanism. A finite element model of cutting-direction burr formation in orthogonal machining was presented in this paper. The simulation of the burr formation process was conducted. Undeformed chip thickness, rake angle, rounded cutting edge radius and workpiece material were included in cutting conditions, whose influences on burr formation were investigated, according to the simulation results. By comparing the results of the simulation and the experiment, good consistency is achieved which proves that the finite element model of burr formation in this paper is significant and effective to predict burr formation.

Hemocompatibility research on the micro-structure surface of a bionic heart valve

In order to study how the geometric parameters and shape of the micro-structure surface of a bionic heart valve affects hemocompatibility, mastoid micro-structures with different periodic space were fabricated using a femtosecond laser on a polyurethane (PU) surface. The apparent contact angles of droplets on the micro-structure surfaces were measured to characterize their wettability. Then a series of blood compatibility experiments, including platelet adhesion, dynamic coagulation and hemolysis were completed. The experimental results showed that the micro-structure on the biomaterial surface helped improve its hydrophobicity and hemocompatibility. Also, the periodic space affected not only the hydrophobicity but also the hemocompatibility of the biomaterial. With the increasing of the periodic space, the apparent contact angle increased, the number of platelet adhesion decreased, the dynamic clotting time became longer and the hemolysis ratio reduced. In addition, the shape of the micro-structure also affected the hemocompatibility of the biomaterial.

Three-dimensional photonic crystals containing designed defects achieved with two-photon photopolymerization

Two-photon photopolymerization (TPP) with femtosecond laser is a promising method to fabricate three-dimensional (3D) photonic crystals (PCs). Based on the TPP principle, the micro-fabrication system has been built. The 3D woodpile PCs with rod space of 2000 nm are fabricated easily and different defects are introduced in order to form the cross-waveguide and the micro-laser structure PCs. Simulation results of the optical field intensity distributions using finite-difference time domain (FDTD) method are given, which support the designs and implementation of the PC of two types in theory.

Investigation into the Warm Extrusion for Micropart with Laser-Aided Heating Method

The rapid market need of MEMS (Micro Electromechanical System) and metal microparts are increasing gradually. The plastic microforming has received more interests and applications in MEMS because it ensures high precision and manufacturing efficiency. However, it raises the higher demand on the microforming and plastic processing technology. Here we present one new process to micro-extrude microparts with laser-aided heating method. The new process is to selectively heat the workpiece with the laser radiation method to the suitable temperature that is between room temperature and recrystallization temperature, and to extrude the workpiece. This process is characterized by decreasing the influence of strain strengthening and lowering the resistance to deformation, and increasing the uniformity of material flow. The key elements of the approach concerning the warm micro-extrusion and temperature control, such as process parameters, laser heating system, micro-extrusion die structure, have been advanced. Furthermore, an important aspect of the research is that the developed system and theory of micro warm extrusion based on laser heating can be explored for further investigation and understanding of plastic microforming.

Numerical Simulation of Laser Surface Micro-Texturing

In order to predict the geometry character of laser surface micro-texturing, the finite element analyzing software ANSYS is used to simulate temperature field and crater on the laser ablation. The influence and change regulation of laser intensity, laser pulse number and pulse duration in laser surface texturing are analyzed in detail. The simulation results conclude the best laser intensity in laser-pulse and materials interactions on certain conditions, and the best pulse duration in nanosecond laser micromachining. This research establishes the foundation for laser machining regular non-smooth surface in a rapid and effective way.

Finite Element Analysis of the Effects of Coated Tool on High Speed Orthogonal Machining

Two 3-D finite element models of coated tool and uncoated tool were established using the finite element code DEFORM-2D based on the updated Lagrangian formula. And their machinability on high speed orthogonal machining was simulated and compared. The investigation results indicate that the coated tool has higher surface temperature and lower inside temperature compared with the uncoated tool. Moreover, the cutting forces of the model using coated tool are lower than that using uncoated tool.