Experimental investigation on laser flattening of sheet metal

The purpose of this research is to establish the technique of laser flattening and to consider the fundamental mechanism. The thermal stress produced by heating with a laser beam is used to make a flat sheet from a sheet metal of protruded distortion. Three kinds of protrusions are chosen as the typical protruded distortion; point protrusion, line protrusion and face protrusion. For point protrusion, laser irradiation along the circular path is effective when the height of protrusion is large, and the laser irradiation along the radial path is effective when it is small. For line protrusion, laser beam is irradiated along the short straight path whose direction is normal to the centerline of the protrusion. For face protrusion, the height decreases from 1-0.1 mm by the laser irradiation along the circular path. The residual stress at the convex surface of a point protrusion on the sheet metal changes from a large compressive stress to a small tensile stress by the laser irradiation.     

板料激光成型技术的实验研究

激光成型是一种利用激光作为热源的热应力无模成型新技术。介绍了板料激光成型技术的工艺过程及影响激光成型的主要因素 ,通过实验研究了激光能量因素、板料的材料性能及几何参数对板料弯曲角度的影响     

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.     

Characteristic of energy input for laser forming sheet metal

Laser forming is a process in which laser-induced thermal deformation is used to form sheet metal without a hard forming tool or external forces. The energy input of laser beam is the key factor for the temperature and stress distribution of sheet metal. The purpose of this work is to investigate the influence of energy input condition on heat input and deformation angle for two-dimension laser forming. Variations in heat input resulting from material deformation was calculated and discussed in this paper at first. Furthermore, in laser forming under the condition of constant laser energy input, the effects of energy input mode on deformation angle and temperature field were investigated.     

3D laser-forming strategies for sheet metal by geometrical information

Forming sheet metal by laser-induced thermal stress (laser forming) is considered to offer great potential for rapid prototyping and flexible manufacturing. Accordingly, many studies have been carried out in different areas of laser forming. However, in order to apply the laser-forming process to real 3D products, a method that encompasses the whole process planning is required, including the laser irradiation patterns, laser power, and travel speed, when the target shape is given. In this work, a new method for 3D laser forming of sheet metal is proposed. This method uses geometrical information rather than a complicated stress–strain analysis. Using this new method the total calculation time is reduced considerably while affording strong potential for enhanced accuracy. Two different target shapes were formed by laser irradiation with the proposed procedure to validate the algorithm.     

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.     

Feedback control for 2D free curve laser forming

Forming sheet metal by laser-induced thermal stress (laser forming) is considered to have a great potential for rapid prototyping and other flexible manufacturing. However, the previous researches have mainly focused on analyzing the phenomena of the forming process. In 2D free curve laser forming, a feedback control scheme for each single bending angle was suggested in this study by incorporating a statistical method and the effect of the remaining errors was discussed. Methods of compensating for the remaining errors were proposed and analyzed by computer simulations. Experiments verified the applicability of the proposed methods.     

Ultrahigh-strain-rate plastic deformation of a stainless-steel sheet with TiN coatings driven by laser shock waves

In this paper, a novel dynamic ultrahigh-strain-rate forming method driven by laser impact is reported. The technique is based on a mechanical, not thermal, effect. It is found that the ultrahigh-strain-rate is the most important feature for laser shock forming. Usually it is about 10⁷–10⁹ s^-1, two or more orders of magnitude higher than that of explosive forming, a method with the largest strain rate previously. Studies on the hardness and residual stress of the surfaces indicate that laser shock forming has some peculiarities other forming methods lack. It introduces strain hardening and compressive residual stresses on both surfaces of the metal sheet, resulting in an obvious improvement in fatigue and corrosion resistance. We also discover some non-linear plastic deformation characteristics in laser shock forming. PACS 42.62.Cf; 81.70.C; 62.20.-x; 81.40.Vw; 62.50.+p; 81.65.-b     

Short pulse laser microforming of thin metal sheets for MEMS manufacturing

Continuous and long-pulse lasers have been used for the forming of metal sheets for macroscopic mechanical applications. However, for the manufacturing of micro-electro-mechanical systems (MEMS), the applicability of such type of lasers is limited by the long-relaxation-time of the thermal fields responsible for the forming phenomena. As a consequence of such slow relaxation, the final sheet deformation state is attained only after a certain time, what makes the generated internal residual stress fields more dependent on ambient conditions and might make difficult the subsequent assembly process for MEMS manufacturing from the point of view of residual stresses due to adjustment.The use of ns laser pulses provides a suitable parameter matching for the laser forming of an important range of sheet components used in MEMS that, preserving the short interaction time scale required for the predominantly mechanic (shock) induction of deformation residual stresses, allows for the successful processing of components in a medium range of miniaturization but particularly important according to its frequent use in such systems.In the present paper, a discussion is presented on the specific features of laser interaction in the timescale and intensity range needed for thin sheet microforming with ns-pulse lasers along with relevant modelling and experimental results and a primary delimitation of the parametric space of the considered class of lasers for the referred processes.     

腔镜变形对平凹稳腔激光振荡模式影响的数值研究

 用Fox-Li迭代法研究了激光谐振腔镜面变形对激光振荡模式的影响, 给出了几种镜面变形条件下的激光谐振腔自再现模,即镜面上光场振幅和位相分布的模拟结果。对平凹稳定腔,为获得严格的基模激光振荡,应将这种镜面变形控制在1/20波长范围内;若镜面变形超过1/10波长,则该谐振腔不存在严格的基模自再现模。同时,高功率激光腔镜的边缘冷却方式将造成较大的镜面形状变化,从而影响激光光束质量。     

Deformation behavior of laser bending of circular sheet metal

The application of a thermal source in non-contact forming of sheet metal has long been used. However, the replacement of this thermal source with a laser beam promises much greater controllability of the process. This yields a process with strong potential for application in aerospace, shipbuilding, automobile, and manufacturing industries, as well as the rapid manufacturing of prototypes and adjustment of misaligned components. Forming is made possible through laser-induced non-uniform thermal stresses. In this letter, we use the geometrical transition from rectangular to circle-shaped specimen and ring-shaped specimen to observe the effect of geometry on deformation in laser forming. We conduct a series of experiments on a wide range of specimen geometries. The reasons for this behavior are also analyzed. Experimental results are compared with simulated values using the software ABAQUS. The utilization of line energy is found to be higher in the case of laser forming along linear irradiation than along curved ones. We also analyze the effect of strain hindrance. The findings of the study may be useful for the inverse problem, which involves acquiring the process parameters for a known target shape of a wide range of complex shape geometries.     

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.     

Micro scale laser shock forming of pure copper and titanium sheet with forming/blanking compound die

A new process fabricating micro parts of thin metal foils by laser shock waves with forming/blanking compound die is reported in this article, in which flexible rubber material was used as the soft punch to act on the thin metal sheet. Systematic studies were carried out experimentally on the process with different laser energies and materials. The formed parts were examined in terms of their morphology, surface roughness, forming depth and mechanical properties (including nanohardness, plasticity and elastic modulus) characterized by nanoindentation test. According to the results, the ablation states of confinement medium and the surface roughness of the different regions change with energies. Additionally, the proper energies are necessary to form complex parts and the forming process can be applied to manufacture parts with good surface quality. What׳s more, the nanoindentation test results showed that the nanohardness, plasticity and elastic modulus of material were increased after impact. The increase in nanohardness and plasticity can attribute to higher stiffness of the parts. The enhanced elastic modulus indicates an increased stiffness of the parts, providing an evidence for the reduced spring back of copper during laser shocking.     

平面光学镜低温温度场和热变形分析方法

在综合分析了温度场和热变形理论分析方法的基础上,提出了对空间光学系统中平面光学反射镜进行有限元分析具体方法;介绍了温度场特性经典理论,并对径向温度分布遵循二次规律的表达式作了简单推导;同时介绍了温度变化引起光学表面的Zern ike多项式法和用热力矩分析平面圆形板面形的方法;最后给出了用ANSYS和I-DEAS软件对联合光学元件热变形的计算流程。     

Nd：YAG板条激光增益介质封装技术研究进展北大核心CSCD

为实现大尺寸、高储能的Nd:YAG板条激光增益介质模块的高可靠性工作,必须找到合适的封装工艺解决大尺寸无空洞、低热阻界面连接问题和界面低应力、低透射波前畸变问题。在充分了解板条激光增益介质和冷却单元的特性后,选择了延展性好的铟作为焊料,实验得到最佳焊料层厚度,通过改进封装工艺的钎焊技术将这两部分可靠地连接在一起。改进的封装工艺实现了钎焊面积大于40cm2,空洞率小于0.5%,最大空洞面积小于1mm2的技术指标,工艺重复性大于90%。通过对焊料层的优化实现了尺寸为150.2mm×30mm×2.5mm板条激光增益介质静态透射波前畸变小于1μm,成品率优于80%,静态透射波前畸变小于1.5μm的模块成品率接近100%的技术指标。采用改进封装工艺焊接的单模块Nd:YAG板条激光器稳定输出功率达到4000 W。     

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.     

A finite element model using multi-layered shell element in laser forming

Due to its enormously high flexibility laser forming has been gaining importance in recent years. This rapidness and flexibility demand very precise controlling strategies especially when simulating the process of large plates and challenging the limited computation power of the current workstation. A simple, robust and accurate modeling method of laser forming has been demonstrated to solve this problem. The simplified model is meshed by multi-layered shell element, simulated with a more real scanning method and fewer parameters. The intelligent meshing strategies have reduced the number of elements dramatically. Thus the simulation efficiency has been improved significantly. By comparing the simulation results under the simplified model with the results under the traditional model for laser forming, the applicability of proposed method has been proven. The method of these simplified models is also suitable to simulate complex finite element models, which take much time to simulate. It would throw some light on the thermal mechanically coupled-field simulation of large sheet.     

Semianalytical analysis of thermal effect in LD double-side-pumped rectangular laser crystal

A semianalytical method to analyze the thermal effect in a LD double-side-pumped rectangular laser crystal is put forward. Through the analysis of working characteristics of the laser crystal, a thermal model that matches actual situations of the laser crystal is established. General expressions of temperature field and thermal distortion field in the laser crystal can be obtained by a novel method to solve the heat conduction equation of orthotropic material. This semianalytical method can be used to calculate the temperature field and thermal distortion field in other LD double-side-pumped laser crystals and is applied to Nd:YVO₄ crystal in detail in this paper, and two methods of effectively reducing thermal distortion in the laser crystal are offered. Results show that a maximum temperature rise of 362.2 °C and a maximum thermal distortion of 5.55 μm are obtained in Nd:YVO₄ crystal when the output power of the two laser diodes are both 30 W. When the off-center distance is 0.6 mm, the maximum thermal distortion is reduced by 37.7%; when the thickness of the crystal is reduced from 2.0 mm to 1.4 mm, the maximum thermal distortion is reduced by 31.7%. Results in this paper can offer theoretical base for better solving thermal problems in laser system.     

应用于水下能分辨粒径10 μm～500 μm尾流气泡的成像系统

针对尾流气泡成像存在的难点,采用片光源切片扫描与高速摄影相结合的技术手段,研制了一套水下激光成像系统。该系统采用激光片光照明,避免了尾流区气泡图像层叠;设计了3组倍率可切换镜头,实现了对大动态范围（10 m～500 m）粒径小气泡的成像;将镜头分成前后组,以平行光中继,通过片光与前镜组的同步移动,可实现尾流区一定体积内气泡图像的采样,同时还能保证扫描过程中始终成像清晰。此系统通过在海域进行试验,放置深度为5 m,气泡由目标船产生,产生气泡粒径范围10 m～2 000 m,结果表明,系统成像效果良好,满足项目技术要求。     

水冷10 Hz Yb:YAG激光放大器热管理

为了控制重频放大器的热致波前畸变,设计并加工了均匀冷却的背面水冷激活镜激光放大器,对放大器的热畸变特性开展了实验研究,实验发现在泵浦功率密度较高即重复频率达到10 Hz,平均功率密度达到200 W/cm2时,放大器的热畸变既影响远场分布又对近场产生显著的调制。近场的调制会给放大器带来较大的损伤风险。为了消除热畸变对近场的调制,首先对泵浦强度分布进行了匀化,然后对介质进行了边缘热平衡控制,消除了热畸变引起的近场调制。通过对上述因素的控制,采用水冷激活镜构型的四程放大器实现了在10 Hz频率下良好运行。在没有进行主动补偿的情况下,实现了远场焦斑优于5倍衍射极限的输出。