Flattening of sheet metal by laser forming

Laser forming is a thermal process for deformation of sheet metal by thermal stress. In this paper, the technique of laser forming is applied to flatten a protruded distortion on the sheet metal, and the mechanism of flattening is investigated experimentally. The protrusion of some height is intentionally produced by pressing a steel ball on a flat sheet metal. The laser beam was irradiated at the area of distortion, and as a result, the protrusion disappeared and an almost flat sheet metal could be obtained.     

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.     

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.     

Grid-based photogrammetry system for large scale sheet metal strain measurement

This paper presents a new approach to fast strain measurement with high accuracy for large scale sheet metal based on the surface circular grid and digital close range photogrammetry. A multi-block measuring method of discretization is implemented to archive large scale measurement. The sheet metal is separated into several blocks for respective calculating and joined together by common reference points. A surface circular searching method is presented for fast and robust 3D grid generation. A flexible bundle adjustment method is proposed for large amount 3D grid nodes reconstruction, which employs the conception of sampling points and is proved to be efficient. Furthermore, a multi-stage grid registration method is introduced to improve the accuracy of strain field by correcting the true deformation gradient tensor. A novel system is developed and performances well in actual large scale sheet metal strain measurement. Two accuracy tests confirm that the system strain measurement error is less than 0.2%.     

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.     

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     

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.     

Shadowgraphic imaging of laser transfer driven by metal film blistering

A time-resolved shadowgraphic technique was used to investigate local transfer of diamond nanopowder from a thin titanium film on a silica support under irradiation by 450-fs or 50-ps laser pulses. Clean powder ejection driven by blistering of the metal film remaining on the target surface was found possible in a limited fluence range, but the metal film was removed from the target together with the powder when higher laser fluences were applied. The velocity of the powder ejection demonstrates an approximately linear rise in a wide range of the incident fluences, while the slope of the velocity curve decreases for thicker metal layers. The maximum ejection velocity achievable in the blistering regime was evaluated as ∼100 m/s independently of the metal thickness and pulse width.     

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.     

Towards controlled flyer acceleration by a laser-driven mini flyer

A laser driven flyer (LDF) system is designed to blast off a very small, thin flyer plate for impact on a target. When a Nd:YAG laser beam is focused through a transparent substrate onto thin metal, a fraction of the metal is ablated. The blow-off products being contained between the substrate and the flyer make the remaining thin film launch as a separate flyer. Some energy of the laser beam is lost by reflection at the boundary between substrate and metal because of the high reflectivity. By using a proper metal of high absorptance at 1.064 μm wavelength, the laser coupling to the flyer would define the system efficiency of a launch system. An effort is presented here to improve the coupling results in the enhancement of the flyer velocity for a given pulse energy. An optimum energy conversion between laser energy and kinetic energy of the flyer is achieved through a black paint coating technique as opposed to a more conventional means of a multi-layered approach requiring electron beaming or magnetron sputtering that are rather expensive and time consuming. The mini flyer flown under 1.4 km/s showed a controlled flight trajectory without fragmentation, suggesting that performance of this simple system is competitive to if not better than other attempts by the multi-layered LDF systems.     

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     

Detection of cracks in metal sheets using pulsed laser generated ultrasound and EMAT detection

A pulsed Nd:YAG laser with an approximately Gaussian beam shape is directed onto the surface of an aluminium sheet at an energy density below which damage by laser ablation occurs, generating Lamb waves in the sheet. The laser beam is raster scanned across the surface of the sample. The Lamb waves travel radially outwards from the generation point and are detected some distance away by an electromagnetic acoustic transducer with sensitivity to in-plane displacements of the sheet. A number of static EMATs are located around the edges of the sheet, some distance from the generation point. The presence of a crack-like defect on the sheet can be detected by either a sudden change in the ultrasonic waveform or by an enhancement in the frequency content of the waveform when the laser beam illuminates directly onto the crack.     

A high-power millimeter-wave sheet beam free-electron laseramplifier

The results of experiments with a short period (9.6 mm) wiggler sheet electron beam (1.0 mm×2.0 cm) millimeter-wave free electron laser (FEL) amplifier are presented. This FEL amplifier utilized a strong wiggler field for sheet beam confinement in the narrow beam dimension and an offset-pole side-focusing technique for the wide dimension beam confinement. The beam analysis herein includes finite emittance and space-charge effects. High-current beam propagation was achieved as a result of extensive analytical studies and experimental optimization. A design optimization resulted in a low sensitivity to structure errors and beam velocity spread, as well as a low required beam energy. A maximum gain of 24 dB was achieved with a 1-kW injected signal power at 86 GHz, a 450-kV beam voltage, 17-A beam current, 3.8-kG wiggler magnetic field, and a 74-period wiggler length. The maximum gain with a one-watt injected millimeter-wave power was observed to be over 30 dB. The lower gain at higher injection power level indicates that the device has approached saturation. The device was studied over a broad range or experimental parameters. The experimental results have a good agreement with expectations from a one-dimensional simulation code. The successful operation of this device has proven the feasibility of the original concept and demonstrated the advantages of the sheet beam FEL amplifier. The results of the studies will provide guidelines for the future development of sheet beam FELs and/or other kinds of sheet beam devices     

考虑钕玻璃放大器增益特性的光谱色散匀滑系统性能研究

采用数值方法研究了钕玻璃放大器的增益特性对高功率激光系统中光谱色散匀滑单元性能的影响.分析结果表明,入射光中心波长与放大器增益曲线中心波长不一致时,焦斑强度分布会受到一定的影响,且该影响随放大倍数增大趋于明显,而两个波长一致时,强度分布变化较小.靶面焦斑整体的辐照均匀性则主要取决于经过相位调制后的激光束的带宽,放大器的增益特性对其空间功率谱及均匀性无明显的影响.所得结论为光谱色散匀滑单元在激光系统中的实际应用提供了重要的理论参考.     

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.     

Effect of temperature on the anisotropy of AZ31 magnesium alloy rolling sheet under high strain rate deformation

In order to investigate the effect of temperature on the anisotropic behaviour of AZ31 magnesium alloy rolling sheet under high strain rate deformation, the Split Hopkinson Pressure Bar was used to analyse the dynamic mechanical properties of AZ31 magnesium alloy rolling sheet in three directions, rolling direction(RD), transverse direction (TD) and normal direction (ND). The texture of the rolling sheet was characterised by X-ray analysis and the microstructure prior and after high strain rate deformation was observed by optical microscope (OM). The results demonstrated that AZ31magnesium alloy rolling sheet has strong initial {0?0?0?2} texture, which resulted at the obvious anisotropy in high strain rate deformation at 20 °C. The anisotropy reflected in stress–strain curve, yield stress, peak stress and microstructure. The anisotropy became much weaker when the deformation temperature increased up to 250 °C. Continuing to increase the deformation temperature to 350 °C the anisotropy of AZ31 rolling sheet essentially disappeared. The decreasing tendency of anisotropy with increasing temperature was due to the fact that when the deformation temperature increased, the critical resolved shear stress (CRSS) for pyramidal 〈c + a〉 slip, which was the predominant slip mechanism for ND, decreased close to that of twinning, which was the predominant deformation mechanism for RD and TD. The deformation mechanism at different directions and temperatures and the Schmid factor (SF) at different directions were discussed in the present paper.     

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.     

基于激光技术的压痕应变花及测试应变的原理

介绍了一种用于测试面内三个应变分量的应变测量技术,在试件表面预先压制三个压痕,通过激光照射压痕,经衍射后干涉,产生干涉图。通过分析变形前后干涉斑点的光强值的变化来对待测位置进行应变测量。实验表明该方法对待测试件表面损伤小,且压痕应变花尺寸小,是一种新的光学应变测试技术,不仅可以用来进行普通应变测量,尤其对解决小空间应变测试问题有其独特的优点。     

利用侧向阴影照相技术探测靶的飞行速度

 建立起一套侧向阴影照相的光学系统,利用可见光作为探测光,在状态方程实验中对靶的飞行速度进行探测。在天光KrF准分子激光装置上进行激光打靶实验,激光波长为248.4 nm。在激光功率密度为8.3×10¹¹ W/cm²的条件下,测得50 μm厚铝靶的飞行速度为3.28 km/s;在激光功率密度为4.7×1010¹¹ W/cm²的条件下,测得带100 μm厚烧蚀层的13 μm厚铝靶的飞行速度为2.52 km/s。最后进行了误差分析计算,实验中探测激光与靶表面偏离角度最大不会超过2.06°,偏离角对实验精度产生的影响可以被忽略。     

一种新型全光纤速度干涉仪

 基于传统速度干涉仪(VISAR)和光纤速度干涉仪(AFVISAR)的特点，提出了一种由光纤和光纤耦合器组成的工作波长为532 nm的新型全光纤速度干涉仪（NAFVISAR）。该干涉仪采用多模光纤器件构成分离系统,单模光纤器件组成核心部分。由于有两路携带不同信息的光束经不同路径传输到耦合器中，当这两路光束满足干涉条件时，可利用它们的干涉场信息来调解出被测靶的信息，从而区分波面的加减速变化。用该系统进行了Hopkinson森杆一维应力加载下的入射杆端面的速度剖面测试，实测速度最大值为49.36 m/s，与理论速度的最大值50.16 m/s基本符合，实现了全光纤速度干涉仪的实用化。