Temperature gradient mechanism in laser forming of thin plates

To obtain further insight into the deformation of a plate in the laser forming process, the temperature gradient mechanism (TGM) is studied. Through the investigation, it can be found that, under the processing conditions of TGM, the plate not only bends about the x-axis but also about the y-axis. An analytical model estimate of the bending angle about the y-axis is constructed based on the theories of heat transfer and the mechanics of elastoplasticity. Numerical simulations are carried out to investigate the deformation of the plate about the y-axis by choosing the different process parameters. The analytically based estimate is used to suggest suitable starting values for the simulation process of calculated results. The study of the bending about the y-axis may describe more fully the deformation of a plate, which is helpful in high-precision forming.     

Development of irradiation strategies for free curve laser forming

Although forming sheet metal by laser-induced thermal stresses (laser forming) has been extensively studied, the research has mainly focused on a single angle forming process. The task of free curve laser forming of sheet metal is to determine a set of process parameters such as laser scanning paths, laser power and scanning speed that will make a given shape. Two methods were used for generating the laser scanning paths and the bending angles of each path. Each method was analyzed by computer simulation and the two methods were compared. Experiments verified the applicability of the proposed methods.     

An experimental investigation of underwater pulsed laser forming

Laser forming is a new forming technology, which deforms a metal sheet using laser-induced thermal stresses. This paper presents an experimental investigation of pulsed laser forming of stainless steel in water and air. The effects of cooling conditions on bending angle and morphology of the heat affected zone (HAZ) are studied. It is shown that the case of the top surface in air and the bottom surface immersed in water has the greatest bending angle based on the forming mechanism of TGM. The water layer above the sample decreases the coupling energy, leading to a small bending angle. For a thin water thickness (1 mm), the water effects on the HAZ are limited. As water layer thickness increases (5 mm), the concave shape of the HAZ is more remarkable and irregular because the shock waves by high laser energy heating water are fully developed. However, the area and the depth of the HAZ become less significant when water thickness is 10 mm due to the long pathway that laser undergoes.     

切向气流对激光加热金属板非熔化穿孔效应的影响

针对切向气流加载导致激光加热金属板在熔化前的穿孔效应,利用金属薄板的弹性弯曲理论,推导出了两种典型光束（方形和圆形）照射下的弯曲挠度表达式,利用Mises理论给出了非熔化穿孔的破坏判据。研究结果表明：激光加热下材料强度降低是出现非熔化穿孔破坏的主要机理;薄板在光斑区的最大变形与气流速度、光斑直径、板厚与弹性模量（U2a4/Eh3）相关,穿孔破坏温度与气流速度、光斑直径及板厚（Ua/h）2相关;与方形光斑辐照相比,圆形光斑辐照的破坏阈值稍高一些。数值计算结果表明：0.8 Ma切向气流作用下,铝合金壳体的激光破坏能量阈值大大降低（可达40%～50%）,典型不锈钢壳体的破坏阈值降低相对较小（20%左右）,气流作用导致金属板破坏阈值的下降是需特别关注的问题。     

Surface characteristic of stainless steel sheet after pulsed laser forming

Laser forming is a non-contact and die-less forming technique of producing bending, spatial forming, modifying and adjusting the curvature of the metallic sheet by using the controlled laser beam energy. One of the problems in laser forming is controlling the characteristic of laser scanned surface. The aim of the investigation is to explore the relation between the surface behaviors of heat affected zone (HAZ) scanned by pulse laser and the pulse parameters of the laser. This paper illustrated the fundamental theory of pulsed laser affected material, and pays attention to the microstructure, micro-hardness and the anticorrosion in the HAZ generated by the laser scanning. Metallographic microscope, scanning electron microscope (SEM), micro-hardness testing system are used to examine the surface characteristics. The work presented in this paper is beneficial to understand the mechanism of pulse laser affect to materials and improve controlling the surface behaviors scanned by pulsed laser.     

Experimental study on negative laser bending process of steel foils

In this paper, laser bending experiments were carried out on the stainless foil for producing negative bending angles. BM-dominated laser parameters, such as larger ratio of beam diameter to specimen thickness and lower scanning speed, help to produce negative bending angles. However, the bending direction in BM-dominated process will be uncertain due to the influence of the specimen's initial stress state and surface conditions. For this reason, experiments on stainless foil loaded with controlled pre-stresses were carried out for producing certain negative bending angles. The effect of the pre-stress and laser parameters are investigated experimentally. The experimental results show that negative bending angles could be produced conveniently when the pre-stresses were induced by elastic pre-bending which direction away from the laser beam, and the angles increase remarkably with the pre-stresses increasing. A set of reasonable laser parameters (laser power, scanning speed and beam diameter) for good quality negative bending angles forming were gotten by analyzing the effect of each parameter respectively in the experiments.     

Laser forming of a bowl shaped surface with a stationary laser beam

Despite a lot of research done in the field of laser forming, generation of a symmetric bowl shaped surface by this process is still a challenge mainly because only a portion of the sheet is momentarily deformed in this process, unlike conventional sheet metal forming like deep drawing where the entire blank undergoes forming simultaneously reducing asymmetry to a minimum. The motion of laser beam also makes the process asymmetric. To counter these limitations this work proposes a new approach for laser forming of a bowl shaped surface by irradiating the centre of a flat circular blank with a stationary laser beam. With high power lasers, power density sufficient for laser forming, can be availed at reasonably large spot sizes. This advantage is exploited in this technique. Effects of duration of laser irradiation and beam spot diameter on the amount of bending and asymmetry in the formed surface were investigated. Laser power was kept constant while varying irradiation time. While varying laser spot diameter laser power was chosen so as to keep the surface temperature nearly constant at just below melting. Experimental conditions promoted almost uniform heating through sheet thickness. The amount of bending increased with irradiation time and spot diameter. It was interesting to observe that blanks bent towards the laser beam for smaller laser beam diameters and the reverse happened for larger spot diameters (~10 times of the sheet thickness). Effect of spot diameter variation has been explained with the help of coupled thermal-structural finite element simulations.     

A modular scanning tunneling microscope with an interchangeable elastic closed cell and external actuators

We introduce a novel modular cell based scanning tunneling microscope with external piezoelectric actuators. A tip and a sample are contained in a closed interchangeable cell, consisting of a stiff top plate and a bottom part, fastened together by an elastic material. The bottom part, containing a scanning tip, is fastened to a base unit while the top plate, containing a sample, is capable of scanning motion by external piezoelectric actuators mounted in the same base unit. The actuators are pre-loaded by the deformation of the elastic material of the cell, giving an increased stability. This design is expected to simplify the scanning tunneling microscope (STM) operation in difficult environments greatly by enclosing only the tip and sample in a small cell-module, which is pluggable to a scanning mechanism and other supportive functionalities. A frequency characterization and an image scan showing atomic resolution of highly oriented graphite in air, at room temperature, is presented.     

A study of the effect of laser beam geometries on laser bending of sheet metal by buckling mechanism

Laser beam forming has emerged as a new and very promising technique to form sheet metal by thermal residual stresses. The objective of this work is to investigate numerically the effect of rectangular beam geometries, with different transverse width to length aspect ratio, on laser bending process of thin metal sheets, which is dominated by buckling mechanism. In this paper, a comprehensive thermal and structural finite element (FE) analysis is conducted to investigate the effect that these laser beam geometries have on the process and on the final product characteristics. To achieve this, temperature distributions, deformations, plastic strains, stresses, and residual stresses produced by different beam geometries are compared. The results suggest that beam geometries play an important role in the resulting temperature distributions on the workpiece. Longer beam dimensions in the scanning direction (in relation to its lateral dimension) produce higher temperatures due to longer beam–material interaction time. This affects the bending direction and the magnitude of the bending angles. Higher temperatures produce more plastic strains and hence higher deformation. This shows that the temperature-dependent yield stress plays a more dominant role in the deformation of the plate than the spread of the beam in the transverse direction. Also, longer beams have a tendency for the scanning line to curve away from its original position to form a concave shape. This is caused by buckling which develops tensile plastic strains along both ends of the scanning path. The buckling effect produces the opposite curve profile; convex along the tranverse direction and concave along the scanning path.     

Finite element simulation of laser tube bending: Effect of scanning schemes on bending angle, distortions and stress distribution

Laser forming has received considerable attention in recent years. Within laser forming, tube bending is an important industrial activity, with applications in critical engineering systems like micro-machines, heat exchangers, hydraulic systems, boilers, etc. Laser tube bending utilizes the thermal stresses generated during laser scanning to achieve the desired bends. The parameters to control the process are usually laser power, beam diameter, scanning velocity and number of scans. Recently axial scanning has been used for tube bending instead of commonly used circumferential scans. However the comparison between the scanning schemes has involved dissimilar laser beam geometries with circular beam used for circumferential scanning and a rectangular beam for the axial scan. Thermal stresses generated during laser scanning are strongly dependent upon laser beam geometry and scanning direction and hence it is difficult to isolate the contribution made by these two variables. It has recently been established at the Corrosion and Protection Centre, University of Manchester, that corrosion properties of material during laser forming are affected by the number of laser passes. Depending on the material, the corrosion behaviour is either adversely or favourably affected by number of passes. Thus it is of great importance to know how different scanning schemes would affect laser tube bending. Moreover, any scanning scheme which results in greater bending angle would eliminate the need for higher number of passes, making the process faster. However, it is not only the bending angle which is critical, distortions in other planes are also extremely important. Depending on the use of the final product, unwanted distortions may be the final selection criteria. This paper investigates the effect of scanning direction on laser tube bending. Finite-element modelling has been used for the study of the process with some results also validated by experiments.     

Experiment study of powder flow feed behavior of laser solid forming

A photographic system for the powder feed process of laser solid forming (LSF) was developed using a high speed camera, and the powder feed behaviors (the particle speed and the powder flow concentration) were described based on the powder flow images. The influences of the powder feed parameters and the distance below the nozzle exit plane on the powder feed behaviors were discussed, and the influences of the powder feed behaviors on the deposited layer quality were also investigated. It can be seen that the smooth finish of the deposited layer surface was improved remarkably by increasing the particle speed, and the deposited layer height decreases with the increase of the particle speed. It can also be found that the variation of the deposited layer height with the increase of the distance between the deposited surface and the nozzle exit plane is similar to that of the powder mass concentrations on the vertical symmetry axis.     

A two-step nanosecond laser surface texturing process with smooth surface finish

Surface texturing (for example, producing micro dimples on the surface) of mechanical parts has a great potential to improve the surface tribological properties. Surface texturing through nanosecond laser ablation has many associated advantages and hence has drawn lots of attentions. However, the produced micro dimple bottom (if through laser spot scanning) is often very rough, which may harm the surface tribological properties. In this paper, a two-step laser surface texturing process is proposed and studied, where a relatively high-fluence laser ablation step (which is to create dimples) is followed by a low-fluence laser-induced melting, melted material flow and re-solidification step (which is to smooth the ablated dimple bottom surface). The study shows that the two-step laser surface texturing process can produce dimples with very smooth bottom surfaces. The effects of laser pulse duration and scan speed in Step 2 on the dimple bottom surface morphology and roughness have also been investigated, and some very interesting physical phenomena have been found, which have been rarely reported before in literature. Some hypothesized explanations are given for the observed effects, which require future work to completely understand their underlying mechanisms.     

CO2 laser heating of surfaces: Melt pool formation at surface

The melt pool formation during the heating of titanium and steel surfaces by a moving CO₂ laser beam is examined. The repetitive pulses are introduced in the simulations and the Marangoni effect in the melt pool is incorporated in the model study. The influence of laser scanning speed and the laser intensity parameter on the melt pool size is also considered. The enthalpy–porosity method is adopted to account for the phase change in the irradiated spot. It is found that the influence of laser scanning speed on the melt pool size is considerable, which is more pronounced for laser beam parameter β=1. The melt pool size is smaller for stainless steel as compared to that corresponding to titanium.     

Transient deformation of thin metal sheets during pulsed laser forming

The transient deformation of thin grade 304 stainless steel metal sheets heated by a single pulse of a CO₂ laser beam is simulated in this paper. The laser beam is assumed to be line-shaped and the problem is treated as three-dimensional thermo-elastoplastic. The temperature field, deformation pattern, stress–strain states and the residual stress distribution of the specimens have been calculated numerically and the transient response of the bending angle has been validated by experiments. Good agreement has been obtained between the numerical simulation and the experiments under various operating conditions. The numerical study reveals that a high temperature gradient exists for a positive bending angle and a low one for a negative angle. It transpires that the mechanisms of pulsed laser forming are dependent mainly upon the laser power, the heating time, the clamping arrangement, as well as the geometry, the thermal properties and the original stress states of the specimen.     

Study on mechanical properties after laser forming

Laser forming is a means of processing materials in a novel manner. The mechanical properties of specimens after laser forming are investigated. By tension tests, the tension properties are analyzed to establish Ramberg–Osgood constitutive equations under different laser processing parameters. Experimental data show that the yield strength and tensile strength are improved after laser forming, while the elongation percentage is reduced. Based on the distribution of residual stresses as well as residual strains after the laser forming process, the fatigue life under different laser processing parameters is studied using low-cycle fatigue tests. The residual compressive plastic strain is the most important reason for improving the fatigue life of low carbon steel after laser forming. The fatigue fracture mechanism is shown through the analysis of macro-fracture and micro-fracture using the scanning electronic microscope.     

基于透镜畸变效应的仿人眼扫描系统设计

仿人眼扫描具有旋转与尺度不变性、背景信息压缩及变分辨率信息采集等优点,但传统的仿人眼扫描方法存在系统结构复杂、扫描速度慢等缺点,提出一种基于透镜畸变实现扫描环增长,采用阿贝棱镜旋转实现对目标视场扫描的仿人眼扫描方法。利用透镜畸变对不同视场下入射光斑进行放大,进而实现仿人眼光斑结构。通过物像关系求解透镜初始参数,将仿人眼扫描光斑的环间增长系数作为透镜畸变优化目标函数,对透镜参数进行非球面优化,获得透镜结构参数。设计的仿人眼扫描系统焦距为14.24 mm,工作距离为25 mm。仿真结果表明:入射高度为20 mm的1×16线阵激光能够对直径为27.66 mm的像面实现16环仿人眼扫描,扫描光斑的最大环增长系数为1.08。相比于传统仿人眼扫描方式,此文提出的方法具有成本低、结构简单、扫描速度快等优点。     

Longwave Marangoni convection in an inhomogeneously heated liquid

The supercritical Marangoni convection has been studied in a plane-parallel liquid layer, bounded by a free deformable gas-liquid interface from above and by a low-heat-conductivity wall from below, occurring under conditions of inhomogeneous heating in the horizontal plane. In a longwave approximation with a small inhomogeneity of heat flux, the process is described by a system of two-dimensional nonlinear equations for the temperature perturbations, vorticity, and free surface deformation. The concept of quasiequilibrium, implying stability of long-range flows, is introduced, which allows the inhomogeneous heat flux to be modeled by a step function. The linear stability is analyzed in the cases of planar and axisymmetric heat fluxes. The boundaries of stability of the convection regimes are determined on the plane of parameters characterizing the degree of supercriticity inside a heated spot and the depth of damping outside the spot. For an axisymmetric spot, the domains of stability with respect to perturbations for various azimuthal numbers are established.     

Porous microstructures induced by picosecond laser scanning irradiation on stainless steel surface

A study of porous surfaces having micropores significantly smaller than laser spot on the stainless steel 304L sample surface induced by a picosecond regenerative amplified laser, operating at 1064 nm, is presented. Variations in the interaction regime of picosecond laser pulses with stainless steel surfaces at peak irradiation fluences(F_pk=0.378–4.496 J/cm²) with scanning speeds(v=125–1000 μm/s) and scan line spacings(s=0–50 μm) have been observed and thoroughly investigated. It is observed that interactions within these parameters allows for the generation of well-defined structured surfaces. To investigate the formation mechanism of sub-focus micropores, the influence of key processing parameters has been analyzed using a pre-designed laser pulse scanning layout. Appearances of sub-focus ripples and micropores with the variation of laser peak fluence, scanning speed and scan line spacing have been observed. The dependencies of surface structures on these interaction parameters have been preliminarily verified. With the help of the experimental results obtained, interaction parameters for fabrication of large area homogeneous porous structures with the feature sizes in the range of 3–15 μm are determined.     

A three-dimensional model of multiple reflections for high-speed deep penetration laser welding based on an actual keyhole

Keyhole shapes are observed experimentally by two high-speed cameras from two perpendicular directions in high-speed laser welding of glass. From the obtained keyhole pictures, it can be seen that in high-speed deep penetration laser welding, the keyhole is not only seriously bent in the direction opposite to that of welding speed, but also elongated along the direction of the welding speed. Based on the so-obtained keyhole photograph, the keyhole profiles in both the symmetric plane and its perpendicular plane (i.e., the cross-section plane) are determined by the method of polynomial fitting. Then, under the assumption of elliptical cross-section of the keyhole at each keyhole depth, a 3D bending keyhole is reconstructed, the behavior of focused Gaussian laser beam in the keyhole is analyzed by tracing a ray of light using geometrical optics theory. Fresnel absorption and multiple reflections in the keyhole are systematically studied, and the laser intensities absorbed on the keyhole walls are calculated. In determining the distribution of laser intensity on the keyhole wall, the bending of the keyhole plays the dominant role, elongation of the keyhole plays just a minor role. Because of the bending of the keyhole, not all the keyhole wall can be irradiated directly by laser beam. The absorbed laser intensity cannot be uniformly distributed on the keyhole wall even after multiple reflections. The keyhole wall absorbs laser intensity mainly on the small area near the front keyhole wall. Recoil pressure plays a dominant role in forming a keyhole and keeping it open.     

Active suppression of panel flutter with piezoelectric actuators using eigenvector orientation method

This paper examines the use of eigenvector orientation method to detect the onset of subsonic and supersonic flutter of panels modeled by finite elements. The accuracy of the eigenvector orientation method for prediction of the flutter boundary (indicated by a gradual loss of orthogonality between two eigenvectors) is demonstrated by using the examples of a swept-back cantilever plate model at subsonic speed and a simply supported plate model at supersonic speed. Piezoelectric layers are assumed to be bonded to the top and bottom surfaces of the simply supported plate in order to provide bending moments to control motions of each finite element. An approach of optimal control design is presented to actively suppress the possible flutter based on linear quadratic regulator theory and the nonlinear modal equations of motions. To illustrate the applicability and effectiveness of using the piezoelectric layers as controllers, several cases are studied and presented. The effects of varying locations of control moments are studied so as to fulfill the objective of adjusting the flutter speed to be within a desirable range. The results illustrate that the control moment manipulation can offset the flutter occurrence and additionally generate a lead time for possibly executing flutter control.