激光焊接技术简介

1、机械制造根底读书报告激光焊接技术简介 付维亮学号：××××××院系：××××××激光焊接技术简介激光是本世纪最大的也是最实用的创造，是与热核技术、半导体、电子计算机和航天技术相媲美的一个举世瞩目的重大科技成就。自从激光被创造并应用开始，人们对激光的特性进行了研究，发现了激光许多人们梦寐以求的特点，从而激光被用在军事、医学、通讯、电子、工业生产、物理化学研究等各个领域，对现在人们的生产生活产生了极大的影响。在机械制造方面，激光更是发挥出巨大的作用，激光切割、激光焊接、激光淬火、

2、激光抛光、激光微细加工、激光熔覆与激光合金化、激光板料成形等技术都有着不俗的表现，而今天我们就深入了解一下激光焊接技术。一、激光焊接技术的开展激光焊接是利用高能量密度的激光束作为热源的一种高效精密焊接方法。激光焊接技术历经由脉冲波形向连续波形的开展，由小功率薄板焊接向大功率厚件焊接开展，由单工作台单工件加工向多工作台多工件同时焊接开展，以及由简单焊缝形状向可控制的复杂焊缝形状开展，受激物质也包含了多种气体和固体晶体。激光焊接技术的开展重点仍然集中于设备的开发研制，包括如何提高最大输出功率和如何提高光束的质量及其聚焦性能。随着激光焊接技术的广泛应用与技术进步，激光焊接越来越接近人们心目中最理想的

3、焊接方式，也越来越多地被人们所采用，并在理论与实践的双重推动下，开展得越来越快，而且激光焊接技术也越来越成熟。二、激光焊接的原理及过程激光蕴含极高能量，当激光照射到工件时，高能量使工件被照射局部熔化，这是激光焊接技术的根底，当然激光也属于光，因此被处理的工件外表不能太光滑，以减少激光反射。而激光只照射在工件一点，使工件局部到达极高温度而熔化，当激光作用终止，局部熔化的金属又会迅速凝固。按激光束的输出方式不同，可以把激光焊分为脉冲激光焊和连续激光焊。假设根据激光焊时焊缝形成特点，又可以把激光焊分为热导焊和深熔焊。前者使用激光功率低，熔池形成时间长，且熔深浅，多用于小型零件的焊接；后者使用的激光功

4、率密度高，激光辐射区金属熔化速度快，在金属熔化的同时伴随着强烈的汽化，能获得熔深较大的焊缝，焊缝的深宽比较大，可达12：1。 激光焊接时，激光通过光斑向材料“注入热量，材料的升温速度很快，外表以下较深处的材料能在极短的时间内到达很高的温度。焊件的穿透深度可以通过激光的功率密度来控制。由于激光束可利用反射镜任意改变方向，因而能焊接用一般焊接方法难以接近工件的部位。激光焊接系统通常采用l4kW的CO2激光器。由激光器产生的激光束经导光系统传递到抛物面镜反射聚焦，在焦点附近达500020000K的高温光束从喷嘴射出照射在金属板接缝处，瞬间将接缝处的金属材料熔化。激光束向前移动，熔化金属凝固，形成焊缝

5、，将两块别离的金属板连成一整体。由于气体激光器可以连续焊接，适用于焊接01212mm厚的低合金钢、不锈钢、镍、钛、铝等金属及其合金。采用固体激光器焊接时，由于输出能量小(150J)，脉冲激光持续时间短(<10ms)，其焊点可小到几十微米。故可用于05mm以下厚度的金属箔片的点焊和连续点焊以及直径06mm以下金属丝的对焊；也适用于微型、精密、排列密集以及受热敏感的贵重仪器部件的焊接。激光焊接的接头型式与普通焊接相同。三、激光焊接工艺特点激光具有单色性、方向性、相干性、高亮度、高能量等良好特性。在激光焊接过程中，当激光束触及金属材料时，其热量通过热传导传输到工件外表及外表以下更深处。在激光热

6、源的作用下，材料熔化、蒸发，并穿透工件的厚度方向形成狭长空洞，随着激光焊接的进行，小孔沿两工件间的接缝移动，进而形成焊缝。激光焊接LBW的显著特征是大熔深、窄焊道、小热影响区，以及高功率密度。激光焊接代表着一种在微小区域内加热与冷却之间的精细平衡。激光焊接的目的是通过辐射吸收产生液态熔池，并使之长到理想尺寸，然后沿固体界面移动，消除被焊构件间的初始缝隙，形成高质量焊缝。熔池过大、过小，或者蒸发严重，都将导致焊接失败。此外，焊缝的最终质量还可能因其它因素的改变而恶化，如合金成分的蒸发，过大的热梯度导致热裂纹，以及焊接熔池体积与几何形状的不稳定导致气孔和空穴等。激光焊以高能量密度的激光作文光源，对

7、金属进行熔化形成焊接接头。与一般焊接方法相比，激光焊具有以下特点：1可将入热量降到最低的需要量，热影响区金相变化范围小，且因热传导所导致的变形亦最低。 2不需使用电极，没有电极污染或受损的顾虑。且因不属于接触式焊接制程，机具的耗损及变形接可降至最低。 3激光束易于聚焦、对准及受光学仪器所导引，可放置在离工件适当之距离，且可在工件周围的机具或障碍间再导引，其他焊接法那么因受到上述的空间限制而无法发挥。 4工件可放置在封闭的空间经抽真空或内部气体环境在控制下。 5激光束可聚焦在很小的区域，可焊接小型且间隔相近的部件。 6可焊材质种类范围大，亦可相互接合各种异质材料。 7易于以自动化进行高速焊接，亦

8、可以数位或电脑控制。 8焊接薄材或细径线材时，不会像电弧焊接般易有回熔的困扰。9不受磁场所影响电弧焊接及电子束焊接那么容易，能精确的对准焊件。 10可焊接不同物性如不同电阻的两种金属 11不需真空，亦不需做X射线防护。 12假设以穿孔式焊接，焊道深一宽比可达10:1 四、激光焊接的分类根据激光对工件的作用方式或激光束输出方式的不同，可以把激光焊接分为脉冲激光焊和连续激光焊。前者形成一个个圆形焊点，后者形成一条连续的焊缝。脉冲焊接时，输入到工件上的能量是断续的、脉冲的。根据实际作用在工件上的功率密度或激光焊接时焊缝激光焊接时焊缝的形成特点，又可以把激光焊接分为热传导焊接和深熔焊接小孔激光焊接两类

9、。热传导激光焊，激光功率密度较低，一般小于105W/cm2；而深熔焊接功率密度较大，一般大于或等于105W/cm2 。除此之外，还可以根据焊接对象的不同分为金属材料的激光焊接和非金属材料的焊接，具体还可分出异种金属材料焊接、塑料焊接、陶瓷焊接等当下广泛应用的新型焊接方式。异种金属材料焊接是解决构件同时满足多方面性能要求的有效途径。焊接方法有多种，比方氩弧焊(TIG)、电阻焊、摩擦焊、电子束焊以及激光焊等。与其他焊接方法相比，激光焊具有热源密度集中、焊缝深宽比大、热影响区小、可控性好等特点，而且相对电子柬焊，激光焊接气氛要求低，通常不需要真空环境。异种金属激光焊接始于20世纪70年代，目前成为航

10、空航天、船舶制造、汽车制造诸领域重要的先进制造技术之一。异种金属激光焊接过程包含多种物理效应。具体表现为：金属材料对激光的吸收；激光材料相互作用引起的材料相变；能量与动量的传递与转换；光致等离子体对激光的散射与吸收；熔池形成及演化；小孔效应以及熔池凝固等。从复杂物理现象中提取科学问题，并对这些科学问题开展研究工作具有重大意义。五、激光焊接技术的开展前景激光焊接技术的开展前景与面临的挑战目前，在激光焊接技术研究与应用方面处于世界领先水平的国家有德国、日本、瑞士和美国等国。欧美及日本主要的大型船厂已大量采用激光加工技术。采用先进的LEGO原理，船体分段制造，拼接总装而成。另外，I-CORE技术，例

11、如美国最新建造的航空母舰(CVN·79)上广泛使用高强度低合金钢的T型构件。采用激光焊接技术，使该航母的重量降低大约200吨。船体平板为了适应海洋流体力学的需要，都被设计成具有复杂的三维曲率形状，激光辅助平板成型技术可以代替费力，费时，具有一定危险性的机械加热成型工艺，应用前景良好。日本的Kawasaki重工等造船企业已经安装了高功率激光平板切割系统，德国的Meyer Werlt也安装了四台12Kw的C02激光器，用来焊接不同长度的船体加强杆。目前美国，欧洲等地区正在进行大功率光纤激光工业加工设备的开发，正在开发的有2Kw，6Kw输出的工业级光纤激光器的加工设备的二次开发。横流连续C

12、O2激光加工设备的输出功率可达20kW，脉冲Nd1YAG 激光器的最大平均输出功率也已到达4kW，并且实现了纳秒级的脉冲宽度。激光焊接能够实现的材料厚度最大已达80mm，最小为，大局部材料的激光焊接质量均超过传统焊接工艺。激光焊接技术正朝着低本钱、高质量的方向开展，具有很大的开展潜力和开展前景。可以预料，激光焊接工艺将逐步占据焊接领域的主要位置，并取代一些传统落后的焊接方法。激光焊接技术在迅猛开展的同时，也面临着一些新的课题，其中包括：高功率低模式激光器的开发及在焊接中的应用；纳秒级短脉冲顶峰值功率激光焊接过程中激光与材料的作用机制；超薄板材激光焊接工艺的优化与接头性能的检测；激光焊接时声、光

13、、电信号的反响控制；激光焊接过程中等离子体的产生对焊接质量的影响等等。激光焊接技术面临的这些新的挑战，有待于从事激光焊接的研究人员进行深入的探讨，同时，这些新问题的提出也预示着激光焊接技术正向着更加深化的方向开展。参考文献3 虞钢、赵树森、张永杰、何秀丽、庞铭. 异种金属4 王又良，上海市激光技术研究所. 激光加工的最新应用领域，应用激光，2005.10 第5期第25卷6 G. Labeas, S. Tsirkas, J. Diamantakos and A. Kermanidis,Effect of residual stresses due to laser welding on the

14、Stress Intensity Factors of adjacent crack附：外文文献Effect of residual stresses due to laser welding on the Stress Intensity Factors of adjacent crack由于激光焊接应力强度因素相邻的裂缝剩余应力的影响关键词：laser welding 激光焊接，crack 裂缝，stress field 应力场文摘：In the present work the effect of residual stresses due to laser welding on the

15、 Stress Intensity Factors (SIFs) of cracks developing nearby the welded area is studied. The simulation of the welding process and the calculation of SIFs on the cracked structure are performed using an explicit and an implicit Finite Element code, respectively. The developed residual stresses due t

16、o the welding of two flat plates by laser are calculated first, using a thermo-mechanical transient analysis. Consequently, a linear elastic analysis is applied on the calculation of SIFs at the crack tips. The calculated results of the welding simulation are verified by comparing the computed angul

17、ar distortions to the corresponding experimental values. As SIF values can not be experimentally determined due to the existence of the residual stress field, the verification of the fracture mechanics analysis is performed through comparisons between computed and experimental crack opening displace

18、ment (COD) values. Concerning both analysis types, a good agreement between experimental and theoretical results is observed.翻译:现在的工作中剩余应力的影响由于激光焊接应力强度因子(SIFs)附近的裂缝开展研究的焊接区。焊接过程的模拟和计算SIFs裂纹结构上采用一个显式和隐式有限元代码,分别。兴旺国家由于焊接剩余应力的两个平板计算激光第一,使用一个热机瞬态分析。因此,应用线弹性分析计算SIFs裂缝的技巧。计算结果的比较,验证了焊接仿真计算角变形值相应的实验。作为SIF值

19、不能实验性地确定由于存在的剩余应力场、验证的断裂力学分析的执行是通过计算和比照实验裂缝翻开位移(COD)值。两方面的分析类型,一个好的之间的协议的实验和理论结果是观察。关键词：laser welding 激光焊接，crack 裂缝，stress field 应力场Effect of residual stresses due to laser welding on the Stress Intensity Factors of adjacent crackG. Labeas, S. Tsirkas, J. Diamantakos and A. Kermanidis LTSM, Laborato

20、ry of Technology and Strength of Materials Department of Mechanical Engineering & Aeronautics University of Patras, Patras 26500, GREECE 2ISTRAM, Institute of Structures and Advanced Materials Patron-Athinon 57, Patras 26441, GREECE1. IntroductionThe welding processes usually cause development o

21、f flaws or cracks in engineering structures which are subjected to complex stress fields. Many researchers have indicated by experimental proof that the weld residual stresses can significantly affect the fatigue crack growth rates in welded joints. The prediction of crack growth rate in such condit

22、ions is a real engineering problem. In previous studies, a number of analytical and numerical models have been developed to calculate the SIF values of cracked welded structures, however, the residual stress field imposed is rather hypothetical and not predicted by precise numerical simulation of th

23、e laser welding process. The studies comprise analytical 1-3, twodimensional 4 and three-dimensional 5 numerical models predicting the effect of an hypothetical residual stresses field on the SIF values. Unfortunately, the above models do not have the capability of predicting the real welding produc

24、ed residual stresses and take into account the real residual stress field into the SIF calculations.In the present study, a numerical algorithm employing two different FE codes is developed for the prediction of the effect of the laser welding residual stresses on the SIF values of a cracked welded

25、plate. Simulation of the residual welding stresses is performed initially and the simulation results are introduced in a fracture mechanics analysis for the calculation of the SIFs of cracks adjacent to the weld area. For the application of the proposed algorithm, the basic parameters required are t

26、he plate dimensions, the laser power, the laser travel speed, the laser beam diameter, the material absorption coefficient, the temperature dependent material mechanical and thermal properties and the material phase transformation data. The computed results include: (i) the time-dependent temperatur

27、e distribution; (ii) the time-dependent stresses, strains and distortions; (iii) the residual stresses and strains; and (iv) the final shape and (v) the Stress Intensity Factors of the cracked plate.2. Numerical analysisFor the fracture mechanics design of welded plates, the computation of SIFs of c

28、racks developing nearby the welded area is required, by taking into account the residual stress field created during the laser welding process. Therefore, two types of numerical analyses are required. In the present work the explicit FE code SYSWELD 6 is used for the simulation of the laser welding

29、process and the determination of the residual stress field around the welded area, while the implicit FE code ANSYS 7 is applied afterwards, for the calculation of SIFs, as well as, stresses and deformations at the cracked structure.The FE model is initially developed using the preprocessor of ANSYS

30、 code. The entities of the FE model are transferred to SYSWELD code by means of a properly transformed ASCII file, which contains the topology of the model (nodes and elements). Upon simulation of the laser heating and cooling process in SYSWELD, the computed residual stresses are exported from SYSW

31、ELD and are imported as initial stresses in ANSYS, through an in-house developed interface. The methodology followed utilizes the same FE mesh for both numerical analyses, therefore, it ensures excellent cooperation between the two models. However, both the welding simulation and the fracture mechan

32、ics problem are solved with the same FE mesh, although it would be beneficial to refine each mesh separately, according to the specific requirements of each analysis type.For the entire FE model the SOLID45 element type of ANSYS code is used. SOLID45 is a brick element and is defined by eight nodes,

33、 having three degrees of freedom at each node, i.e. translations in the nodal x, y, and z directions. During the creation of the model special consideration is taken into account in order to achieve a fine mesh density at the areas of the laser beam welding, as well as, at the crack tips, which is n

34、ecessary for the successful laser welding simulation and SIFs calculations, respectively.The developed FE model is presented in figure 1. In the same figure two details are shown, one at the laser beam track around the welded plate edge and one at the crack region. The model comprises 35380 nodes an

35、d 27888 elements.2.1 Laser welding simulationDuring the laser welding process, many mechanisms are taking place in the material of the weldments. A very narrow zone under the laser beam is suddenly heated, vaporized and locally fused. After welding and cooling of the melt material, the assembly of t

36、he welded pieces is achieved. On the exposed area a keyhole is shaped. The elevated temperature gradients developed in this area during both heating and cooling phase, along with the sharp decrease of mechanical properties during heating, yield to non-homogeneous permanent strains and residual stres

37、ses after the process.A three-dimensional (3D) finite element model is developed to simulate the laser welding process for the butt-joint of two square plates 150 mm length and 4mm thickness, made of shipbuilding steel AH36 8, using the commercial code SYSWELD. For the laser welding 2000 Watt power

38、and 300 mm/min laser travel speed, was used. The geometry of the weld structure is modeled using the pre-processor of ANSYS code. Three-dimensional volume elements having eight nodes and three degrees of freedom per node are utilized. A dense mesh is used in the area along the weld line and at the c

39、rack tip regions, as shown in figure 1, while a coarser mesh for the rest of the structure.A thermo-elasto-plastic analysis associated with metallurgical transformations is performed using the SYSWELD finite element code enhanced with user subroutines. The solution is generated in two basic steps. F

40、irst a transient heat transfer analysis is performed and the resulting temperature field is used as input for the second step, which is the mechanical analysis. An appropriate time stepping scheme is applied for each analysis in order to achieve fast convergence of the solution and reasonable accura

41、cy.The thermal analysis is conducted using temperature dependent thermal material properties, provided by 8. A metallurgical analysis, based on the material phase transformations laws is applied, in order to simulate the phase transformations taking place during the material heating and cooling due

42、to welding process. During the welding, heat is supplied to the weld pool by the laser beam, which is transferred to the metal by conduction and convection. A part of this heat energy is lost by free convection and radiation. The heat input to the weld is generally calculated from the energy supplie

43、d. The heat input distribution determines the size and shape of the weld pool. In order to simulate the heat distribution and flow in the welding direction, the laser beam is modelled as a threedimensional moving heat source. The model of the heat source assumes a Gaussian heat flux distribution on

44、the weld pool and the shaped keyhole is simulated by a cone 8, 9. The heat flux is implemented into the FE code by developing a FORTRAN subroutine.The heat loss by free convection follows Newtons law, where the coefficient of convective heat transfer is assumed to vary with both temperature and orie

45、ntation of the boundary 8, 9.Heat loss due to thermal radiation between the weldments and the environment are important where the temperature difference is high. This radiation is modelled by the standard Stefan-Boltzman relation 8, 9. The heat loss is implemented into the FE code by developing a FO

46、RTRAN subroutine. Prior to welding the material is assumed to be at room temperature.The results of the thermal analysis, i.e. the temperature distributions, are inputted to the mechanical analysis. Proper kinematic boundary conditions to represent the clamping of the plates are introduced in the mo

47、del. The mechanical analysis is performed using, an elastoplastic material formulation and the Von-Mises yield criterion coupled to a kinematic hardening rule. The temperature dependent mechanical material properties are introduced in the FE code in a suitable table form. The residual stresses distr

48、ibution computed as computed from the simulation of the laser welding process is shown in figure 2.2.2 SIFs calculationAfter the two flat plates are welded together, a central notch is created by means of saw cut, which is then pre-fatigued so that cracks start developing at the notch edges. When th

49、e total crack length reaches 20mm, Compact Tension specimens are cut from the welded structure and the Crack Opening Displacement (COD) is measured by extensometers. To verify the measured COD and consequently the numerical fracture mechanics analysis, a FE model of the Compact Tension specimens is

50、used for the computation of the COD, SIF and strains stresses, under the mechanical loading. A comparison between experimentally measured and numerically computed COD values, has shown a good agreement, thus verifying the success of both the residual stress field computation, as well as, the success of the fracture mechanics analysis, as the COD value is dependent on both. Therefore, reliable computation of Stress Intensity Factors may be performed, for which the displacement extrapolation method is applied, using a fit of the nodal displacements in the vicinity of the c