Papers by Author: Yan Ling Tian

Abstract: A 3-dimensional (3D) thermal distribution computational methodology for laser cladding is presented. Based on the developed model, which considers the latent heat and change of conductivity in different phase, the transient temperature fields for the preplaced laser cladding processing are performed. The effect of processing parameters including laser power, scanning speed and laser spot diameter on melt pool temperature filed was extensively discussed and some conclusions were drawn.

Abstract: Laser cladding on the titanium alloy is widely applied with the development of laser technology in the industry. However, the cracks, produced during the process, greatly confine the development of laser cladding technology. Cracks are mainly resulted from the over high thermal stress in the coatings, so it is extreme important to confine the thermal stress of cladding coatings. To investigate the influence of process parameters on the cladding coating's stress, the model of laser cladding based on Ti-6Al-4V is built in the way of finite element method (FEM) and three different scanning paths are chosen to calculate the stress of cladding layer. The results show that the spire-in scanning path is the best way to minimize the cladding coatings’ thermal stress. The numerical results provide the theoretical guidance for optimization of the laser cladding process on titanium alloys.

Abstract: Molecular dynamics (MD) simulations were used to investigate the elastic properties of carbon nanotubes (CNTs). Displacements were loaded to CNTs on the tension deformation simulations. In order to better understand the relationship between Young’s modulus and the structure of the CNTs, different chiralities and diameters were involved. It is found that the Young’s modulus will be no more sensitive as in the single-walled carbon nanotubes (SWCNTs) with increasing walls. The tension deformation results also indicate that SWCNTs have better elastic property compared to multi-walled carbon nanotubes (MWCNTs).

Abstract: Titanium alloys which exhibit excellent material properties like a high strength to weight ratio and good corrosion resistance have become the important structural materials in the applications of the aerospace. However, it’s quite difficult to repair the damages of the titanium alloy parts such as fatigue crack and erosion resulted from poor working environment using the traditional manufacturing technology while the problem can be easily solved with the help of laser cladding technology. For the excellent quality of the fixed parts, it is extraordinarily significant to obtain the rule of the temperature and thermal stress distribution in the cladding process. To investigate the influencing rule of cladding coating's temperature and stress on laser cladding process parameters, the model of laser cladding based on TC4 titanium alloy is built by the way of finite element method (FEM). This model encompasses the effects of the temperature-dependent thermal conduction and radiation as well as the latent heat of fusion. Different laser processing parameters are chosen to calculate the temperature and stress of cladding layer.The result shows that the temperature of the clad coating is positive correlation with the raise of laser power and the depth of the powder layer, and negative correlation with the raise of scanning speed and the laser spot diameter. In addition, the transient stress of clad coating is augmented with the increase of laser scanning velocity, laser spot diameter and the depth of the clad coating while it’s negative correlation with the raise of laser power. The numerical results provide the theoretical guidance for optimization of the laser cladding parameters on TC4 titanium alloy.

Abstract: Understanding of the prosperities of the carbon nanotubes (CNTs) probes is crucial when measuring surface using atomic force microscopy (AFM). In this paper, we investigate the deformation of CNTs by adding lateral forces based on molecular dynamics (MD) simulation. In the simulation, Tersoff many-body potential function is used to describe the interaction between atoms. The movement of CNTs is periodic vibration, which is different from traditional material. We analyzed the vibration of different CNTs including single-walled carbon nanotubes (SWCNTs) and sharpened CNTs. Similarities and differences between different CNTs during the deformation are illustrated. It is shown that sharpened CNTs have better stiffness without declining the resolution of AFM. By analyzing the results obtained from the MD simulation, it is found that the sharpened CNTs may be more suitable as AFM probes.

Abstract: This paper focuses on the effects of heat source profiles during thermal analysis of grinding. Three different models of heat source, namely triangular model, parabolic model and elliptic model, have been suggested and their numeric formulas are provided. These models take into account of the variation of heat flux along the contact zone, so as to improve the accuracy of numeric results. Finite Element Analysis (FEA) is utilized to investigate the temperature distributions under different thermal models and the effects of two profile parameters (η and ξ). The result is a) peak temperature decreases as η increases and the location of peak value moves backwards simultaneously; b) peak temperature decreases as ξ increases and the location of peak value moves forwards simultaneously.

Abstract: In order to improve the machining accuracy of the precision surface grinding machine, a 3-DOF micropositioning table is used as an auxiliary table to form the dual infeed system with nanometer level positioning accuracy. This paper mainly deals with the static characteristics of the micropositioning table. The direct and inverse kinematic models are obtained under different orientation descriptions, and the inherent relationship between different orientation descriptions is investigated. By use of Eular angle description, the reachable orientation space of the micro-positioning table is obtained. The theoretical static stiffness on the top surface of the table is also given, and the experimental tests are carried out to verify the established models.

Abstract: In order to realize active grinding control, a nanometer micropositioning table is designed. The table has a circular working area with diameter of 150 mm. Three piezoelectric actuators are utilized in parallel to drive the moving part with flexure guide mechanism. Through cooperation of the three piezoelectric actuators, the moving part can implement 3-DOF nanometer positioning. The flexure hinge mechanism can also provide preload for the actuators. The preload can keep the moving part from separation with the piezoelectric actuators during moving process. The dynamic model of the micropositioning table is developed with consideration of the driving circuit. To improve the dynamic performance of the micropositioning table, a decoupling PID controller is designed by use of frequency domain approach. The experimental tests have been carried out to verify the performance of the micropositioning table and the established decoupling controller.

Abstract: In order to compensate the force induced deformation of the precision grinding, a piezoelectrically driven micropositioning table is developed to actuate the workpiece for error compensation. To better understand the performance of the micropositioning table, the deformation patterns and stiffness distribution of the micropositioning table under normal grinding force are investigated by computational finite element analysis method. It is noted that the moving part of the micropositioning table can be considered as rigid body and the maximum static stiffness is located at the center of the top surface. The experimental tests are carried out to verify the analysis.

Abstract: In order to implement dynamic control during ultrapre cision turning operation, a piezo-electrically driven fast tool servo is developed. The parallel flexure hinge is utilized to guide the moving platform and to provide preload. A high resolution capacitive sensor is used to measure the displacement of the fast tool servo for closed loop control. With consideration of the driving circuit, the dynamic model of the fast tool servo has been established. The effect of the driving circuit on the dynamic response of the fast tool servo is investigated. The experimental tests have been carried out to verify the established model and the performance of the fast tool servo.