侧吹辅助气流对激光深熔焊接光致等离子体的影响_张林杰.pdf

收稿日期:2005-08-15基金项目:国家自然科学基金(50475093);国家教育部博士点基金资助项目(20030698018)侧吹辅助气流对激光深熔焊接光致等离子体的影响张林杰1,张建勋1,段爱琴2(11 西安交通大学 金属材料强度国家重点实验室,西安 710049;21 北京航空制造工程研究所,北京 100024)摘 要:为研究侧吹辅助气流对激光焊接光致等离子体的影响及其机理,利用CCD高速摄影研究了不同侧吹条件下激光深熔焊接过程中光致等离子体形态的变化,建立了模拟激光深熔焊接过程气体流场的二维可压缩模型,计算了侧吹喷嘴高度和侧吹流速的变化对激光焊接光致等离子体的影响。结果表明,侧吹喷嘴高度和侧吹流速的变化对光致等离子体行为和焊缝形貌有显著的影响;侧吹气体的冷却作用及其对向上喷射的炽热等离子体云团的“压制”作用是侧吹气体能够有效抑制光致等离子体的重要原因。关键词:等离子体云团;侧吹辅助气体;数值模拟;CCD高速摄影中图分类号:TG45617 文献标识码:A 文章编号:0253-360X(2006)10-037-04张林杰0 序 言激光深熔焊接过程中,会产生光致等离子体。光致等离子体形成后由于逆韧致辐射吸收和折射对入射激光束产生屏蔽效应,影响焊缝熔深、焊缝成分以及焊缝中气孔的生成,使激光深熔焊接可焊的最大工件厚度远低于同属高能束焊接方法的电子束焊1。实践证明在CO2激光焊接过程中,侧向吹辅助气体能有效地抑制光致等离子体的屏蔽作用。国内外研究人员对CO2激光焊接过程中辅助气流的影响已经进行了许多研究。唐霞辉等人指出对不同种类气体只要保护气压力大于等离子体压力都可获得最佳焊缝2。肖荣诗等人研究了不同辅助气体对等离子体屏蔽临界功率密度的影响3。王振家等人研究了侧吹角度、侧吹流速等因素对熔深的影响4。Hamadou等人研究了喷嘴形状以及侧吹气流速对焊缝熔深、熔宽的影响5。Beck等人发现当氦和氩按照3比1的比例混合抑制等离子体的效果最好6。Ancona等人采用一种双喷嘴侧吹装置使得激光焊热传递效率提高,焊缝深宽比增大7。激光深熔焊接时,小孔内的温度通常为10 000K左右小孔孔口处等离子体温度也高达数千度,激光焊接的熔池尺寸很小并有强光的干扰,很难通过试验手段来得知侧吹气体对小孔内外等离子体云团中的温度、压力和速度分布会产生什么样的影响。数值模拟和计算在这方面具有独特的优势,为激光深熔焊接机理的研究提供了有力的手段。陈熙和王海兴采用三维不可压缩性模型模拟研究了激光焊接等离子体各种特性参数的空间分布8。Amara等采用二维可压缩模型对不同环境压力下激光深熔焊接过程中小孔内部的压力、温度和速度分布进行了数值研究,但该模型中没有考虑侧吹辅助气体的影响9。为了研究侧吹条件的变化对激光深熔焊接光致等离子体和焊缝形貌的影响及其机理,建立了分析激光深熔焊接过程气体流场的二维可压缩模型,并计算了不同侧吹工艺参数下等离子体云团内部的温度和速度分布。1 高速摄影试验方法焊接试板厚度2 mm,试板尺寸是100 mm200 mm。激光焊接设备是PRC23000型快速轴流CO2激光器,光斑直径0.4 mm。侧吹喷嘴内半径2.5 mm。试验中侧吹辅助气为氩气。如图1所示,试验中热源和摄像机固定,“vS”是工件运动速度,“h”为侧吹喷嘴高度,是侧吹喷嘴下端和工件上表面之间的距离;“”是侧吹喷嘴轴线和水平方向之间的夹角;“H”表示顶吹喷嘴高度,是顶吹喷嘴下沿和工件第27卷 第10期2 0 0 6年1 0月焊 接 学 报TRANSACTIONS OF THE CHINA WELDING INSTITUTIONVol.27No.10October2 0 0 6上表面之间的距离。在激光焊接过程中用Phan2tomV4.0高速摄影机沿垂直于焊缝对称面的方向拍摄不同条件下光致等离子体的形态。拍摄频率为1 000幅/s。图1 试验装置示意图Fig.1Schematic diagram of experiment set2up2 激光焊接过程气体流场的数值模拟211 基本假设计算中作如下假设:金属蒸发速度为常数;模型为二维稳态;可压缩、湍流流动;考虑对流换热和粘性耗散,忽略辐射换热;激光焊接小孔为圆柱状盲孔。212 控制方程采用FLUENT软件来模拟研究辅助气流的影响。FLUENT软件采用的质量、动量和能量守恒控制方程,在该软件帮助文档中有详细的介绍。213 边界条件整个计算区域如图2所示,网格划分通过FLU2ENT软件包的前处理软件G AMBIT来完成,最小单元尺寸0101mm0105mm。小孔半径为0.25 mm,小孔深2 mm。根据侧吹喷嘴高度的变化对网格分别进行重新划分,当喷嘴高度为5.5 mm侧吹角度为20 时,模型包括68 490个单元。小孔内壁设置为无滑的壁面边界,并认为壁面温度等于工件材料的蒸发温度。工件上表面、侧吹喷管表面和顶吹喷嘴表面都设置为壁面边界,并且温度等于环境温度。侧吹喷嘴和顶吹喷嘴的进气口设置为质量入口,质量流速根据单位时间的体积流量来确定。其余边界设置为压力出口边界,并设置出口压力等于环境压力,回流温度等于环境温度。金属蒸气以恒定的初始温度和质量速率在小孔底部进入计算区域。小孔内金属蒸气的质量蒸发速(a)计算区域的实物边界(b)计算区域边界类型图2 计算区域和边界类型示意图Fig.2Schematic diagram of computational domain andboundary type率(T)由下式来确定,即(T)=AILf+Lv+C(T-T0),(1)式中:C是比热;T0是环境温度;Lf是熔化潜热;Lv是蒸发潜热;T是金属蒸气温度;A是能量净吸收系数。假设金属蒸气以等于金属蒸发温度的初温从小孔底部进入计算区域,进入计算区域后,由于孔内的金属蒸气中存在大量的自由电子,对激光产生有效的逆韧致吸收,蒸气温度进一步升高。小孔内的逆韧致吸收的能量在能量方程中用源项Sh表示,即Sh=I,(2)=(3.310-39ne2)/Te3/2,(3)式中:是逆韧致辐射吸收系数;I是激光功率密度;ne是电子密度;Te是电子温度。利用FLUENT的用户自定义函数可以通过编制用户子程序并编译链接的方法把质量源项和能量源项加入到控制方程中。214 求解使用基于有限容积法的FLUENT软件,结合上述边界条件对控制方程组进行求解。采用耦合隐式求解方法,湍流模型选用Spalart-Allmaras模型。3 结果及讨论311 侧吹喷嘴高度的影响图3和图4分别是不同侧吹喷嘴高度下的计算结果和试验结果。图3中vg是侧吹气体流速,图438焊 接 学 报第27卷中的焊缝熔透率r表示一条焊缝中焊缝熔透部分的累计长度和焊缝总长度的比值。图3 不同侧吹喷嘴高度下激光束轴线上的温度和速度计算结果Fig.3Computed temperature and velocity distribution onlaser beam axis at different h 由于影响入射激光束的主要是激光束路径上的温度分布,所以图3a中给出不同侧吹喷嘴高度下激光束路径上温度分布的计算结果。描述激光焊接光致等离子体中电子、离子与原子数密度的Saha方程可表示为1ne=An1/2Te3/4exp(-/Te)1/2,(4)式中:n是气体密度;A和是与气体种类有关的常数。从式(4)可知等离子体温度越低则其电子密度越小。图3中的预测结果显示随着侧吹喷嘴高度的增大,光束路径上的温度将减小。根据式(4)可以推断光束路径上的电子密度会随着侧吹喷嘴高度的增大而减小,从而对入射激光束的屏蔽会逐渐减弱,输入工件的有效能量将会随着侧吹喷嘴高度的增大呈增加趋势。图4中的试验结果显示随着侧吹喷嘴高度的增加,等离子体平均面积逐渐减小。等离子体面积大小取决于等离子体高亮度区域(即高温区)的大小,进而取决于等离子体高电子密度区域的大小。因此,从图4所示试验结果可知,等离子体中的电子密度随着侧吹喷嘴高度的增加而减小,屏蔽效应随着图4 不同侧吹喷嘴高度下的试验结果Fig.4Experiment results at different h侧吹喷嘴高度的增加而减小。所以随着侧吹喷嘴高度的增加,焊缝熔透率和焊缝背面熔宽会增大。312 侧吹气体流速的影响图5给出的是不同侧吹流速下激光束路径上温度和速度分布的计算结果。随着侧吹流速的增大,等离子体受到更强的抑制作用,激光束传播路径上的速度和温度都呈下降趋势。图6是不同侧吹流速下的试验结果。试验中随着侧吹流速的增大,等离子体平均面积减小,焊缝熔透率增加,焊缝背面熔宽增大,说明输入工件的有效热量在增大。试验结果和计算结果预测的趋势一致。313 侧吹参数影响激光深熔焊接过程的机理侧吹气体对等离子体的产生抑制作用的一个重要原因是侧吹气体可以促进对流冷却作用。从图3,图5中的数值模拟结果可看到,在各种情况下光束传播路径上温度的下降总是伴随着速度的下降。说明侧吹气体能够有效地抑制激光焊接过程中的光致等离子体,还有一个重要的原因是侧吹气体对向上喷射的炽热等离子体云团起到了“压制”的作用。侧吹气体对等离子体云团的“压制”作用可以使其高度减小,而通过逆韧致辐射效应被等离子体吸收的激光能量是随着光束在等离子体中穿行距离的第10期张林杰,等:侧吹辅助气流对激光深熔焊接光致等离子体的影响39图5 不同侧吹流速下激光束轴线上温度和速度分布的计算结果Fig.5Computed temperature and velocity distribution on laserbeam axis at different vg图6 不同侧吹流速下的试验结果Fig.6Experiment results at different vg减小而减小的。折射对入射光束产生屏蔽作用主要是由于折射使得工件表面的束斑直径变大,功率密度降低。折射引起的光束边缘发散角可用下式表示8=1n9n9xy,(5)式中:n是折射系数;9n/9x是折射系数在垂直光束路径方向上的导数;是入射光束边缘在光束轴线方向长度为y的距离上产生的发散角。由上式可知折射引起的光束边缘发散角也随着等离子体云团高度的减小而减小。发散角减小了,束斑直径也就减小了。4 结 论(1)激光焊接光致等离子体对侧吹条件很敏感,采用优化的侧吹喷嘴高度和侧吹流速可以有效地抑制光致等离子体,使焊接过程热传递效率提高。(2)对流冷却作用是侧吹气体能够抑制等离子体的重要原因。此外,侧吹气体的冷却作用是光束路径上温度的减小总伴随着光束路径上气流速度的减小,说明侧吹气体对向上喷射的炽热等离子体云团的“压制”作用也是其能够有效抑制光致等离子体的重要原因。参考文献:1Duley WW.Laser weldingM.New Y ork:Jhon W iley&S ons Inc,1998.2 唐霞辉,周 毅,何艳艳,等.Influence of assistant gas flux on con2trolling laser induced plasma J.激光杂志,2002,23(6):62-63.3 肖荣诗,梅汉华,左铁钏.辅助气体对激光焊接光致等离子体屏蔽的影响J ,中国激光,1998,23(11):1045-1050.4 王振家,苏 严,陈武柱.激光焊接侧吹工艺研究J.热加工工艺,2004,6(4):49-50.5Hamadou M,Fabbro R,Caillibotte G,et al.Study of assist gas flowbehavior during laser weldingC.Proceedings of ICALEO 2002.6Beck M,Berger P,Hugel H.The effect of plasma formation on beamfocusing in deep penetration welding with CO2 lasersJ ,J.Phys.D:Appl.Phys,1995,28:2430-2442.7Ancona A,Sibillano T,Tricarico L,et al.Comparison of two differentnozzles for laser beam welding of AA5083 aluminium alloyJ.Journalof Materials Processing Technology,2005,164-165,971-977.8Wang Haixing,Chen Xi.Three dimensional modeling of the laser in2duced plasma plume characteristica in laser weldingJ ,J.Phys.D:Appl.Phys,2003,36:628-639.9Amara E H,Fabbro R,Bendib A.Modeling of the compressi2ble va2por flow induced in a keyhole during laser welding J ,Journal of Ap2plied Physics,2003,51(2):1805-1814.作者简介:张林杰,男,1973年5月出生,博士研究生,从事激光深熔焊接机理及激光焊接应力与变形的研究,发表论文5篇。Email:zljspace 40焊 接 学 报第27卷welding of aluminum alloy.The experiment results demonstrate thatthe addition of the metal powder enhances the energy coupling effi2ciency and reduces the power density threshold value during laserdeep welding.Such as the reducing rate is about 50%for A2219 al2loy.The experiment results also indicate that the powder can enterthe weld pool availablyonly the powder was delivened to the suitablearea after laser beam at the surface of a part.The proper quantity ofthe alloy powder must match with the right power density and energyinput,which can assure to obtain good weld appearance.Meanwhile,the addition of the powder can reduce the fluctuation of the laser2in2duced plasma and maintain its stability effectively in the laser weld2ing.So a stable welding process and good weld appearance can beobtained in the experiments.Key words:aluminum alloy;metal powder;CO2laser weld2ing;laser2induced plasma;power density threshold valueEffect of side gas on laser induced plasma during laser deeppenetration weldingZHANGLin2jie1,ZHANGJian2xun1,DU2AN Ai2qin2(1.State KeyLaboratoryof Mechanical Behavior for Ma2terials,XianJiaotong University,Xian 710049,China;2.BeijingAeronautical Manufacturing Technology Research Institute,Beijing100024,China).p37-40Abstract:A two dimensional compressible gas flow model oflaser penetretion welding process was founded for a better under2standing of the influence of side gas flow conditions on the behaviorof laser induced plasma.The distributionsof temperature and veloci2ty within plasma plume under different side gas flow conditions werecalculated.The shape and size of plasma in laser welding processwas studied with high2speed CCD carnera.The result shows thatlaser induced plasma and weld geometry are sensitive to side nozzleheight,its inclination angle and side gasflow rate;that cooling effectand assist2gas/plasma2plume momentum ration play an important rolein controlling the plasma plume.Key words:laser deep penatration welding;side assist gas;numerical simulation;high2speed CCD imagingMechanical properties and microstructures of QFP micro2jointssoldered with diode2laser soldering systemHAN Z ong2jie,XUE Song2bai,WANG Jian2xin,CHEN Xu(College of MaterialsScience and Technology,Nanjing University of Aeronautics and As2tronautics,Nanjing 210016,China).p41-44Abstract:Soldering experimentsof two kindsof devices QFP32and QFP48 were carried out using diode2laser soldering system andIR reflow soldering method,and the distribution regulations of thetensile strength of QFP micro2joints with Sn2Pb solder and Sn2Ag2Culead2free solder were studied by STR21000 micro2joints tester,andthe characteristics of fracture microstructures of micro2joints were al2so analyzed by SEM.The results indicate that tensile strengthof QFPmicro2joints soldered with laser soldering system is larger than thatwith IR reflow soldering method,and tensile strength of QFP48 mi2cro2joints is larger than that of QFP32 micro2joints.Fracture mecha2nism of micro2joints soldered with laser soldering system is toughnessfracture,while fracture mechanism of micro2joints soldered with IRreflow soldering method includes brittle fracture and toughness frac2ture.Key words:diode2laser soldering;QFP devices;mechanicalproperties of micro2joints;microstructuresActive vision sensing method for weld location by“circle2depthrelation”algorithmXU Pei2quan,TANG Xin2hua,LU Jian2bo,Yao Shun(School of Material Science and Engineering,Shang2hai Jiaotong University,Shanghai 200030,China).p45-48Abstract:Three vital angles were put forward,so the weldcould be described by three2dimensional information.On this basis,mathematical model of depth recovery according to”circle2depth re2lation”algorithm and projection rules was constructed.The relationship between depth values and off2axis angleunder gas tungstenarc welding experiment condition was addressed by real experimentand the characteristic pointsof weld was described and located.Meansquare error was used to analyze the recovery precision.The resultsshowed:(1)Depth recovery for butt joint and ramp welded seamsrealized;(2)Weld can be located very well by using proposed vi2sion sensor based on circular laser trajectory;and proposed visionsensor manifested a promising result with high precision and high ef2ficiency.Key words:laser cone projection;circular laser trajectory;depth recovery;seam locationInstantaneous energy waveform of short circuiting transition inCO2arc weldingXUE Jia2xiang,Y ANG Guo2hua,WANGZhen2min,LIU Xiao,HONG Sen(College of Mechanical Engineer2ing,South China University of Technology,Guangzhou 510640,China).p49-52,84Abstract:The welding process of three arc welding machineshad been tested and analyzed with the welding dynamic arc waveletanalyzer developed by the anthers1The experimental results indicatethat the instantaneous energy can synthetically reflect the influence ofcurrent and voltage on dynamic arc characteristics1T o make are ig2nite successfully,the instantaneous energy should increase to enoughhigh quickly along with the current1In a cycle of short circutingtransition during the steady welding process,the instantaneous ener2gy during arc burning should be kept at the constant base,and thenit falls to smaller value rapidly,finally,it rises rapidly and com2pletes the short circuit transition.The instantaneous energy falls a2gain rapidly to base after arc re2striking1While arc extinguishing,the instantaneous energy falls slowly to zero and avoids serious im2pulsion1The experiment results can provide for the foundation to de2sign and control accurately the waveform of the welding current andvoltage1Key words:instantaneous energy;wavelet analysis;repeata2bility of waveformEffects of temperature and coatings on wettability of Sn2Cu2Nilead2free solder WANG Jian2xin1,XUE Song2bai1,HANZ ong2jie1,WANGning1,Y U Sheng2lin2(1.College of Materials Sci2ence and Technology,Nanjing University of Aeronautics and Astro22006,Vol.27,No.10TRANSACTIONS OF THE CHINA WELDING INSTITUTION