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试验中采用10mm厚5083-O铝合金板,尺寸为100mm×50mm×10mm。铝合金平板堆焊的填充材料选用ER5087焊丝,焊丝直径为∅1.2mm。母材及焊丝的化学成分见表 1。
Table 1. Chemical composition (mass fraction) of 5083 aluminum alloy and filler wire
material Si Fe Cu Mn Mg 5083 aluminum alloy 0.004 0.004 0.001 0.004~ 0.01 0.04~ 0.049 ER5087 0.00022 0.0015 0.00005 0.009 0.048 material Cr Zr Zn Ti Al 5083 aluminum alloy 0.0005~ 0.0025 — 0.0025 0.0015 balance ER5087 — 0.00082 — — balance -
本试验中采用德国TRUMPF公司生产的HL4006D型Nd:YAG激光器、松下Panasonic YM-350AG2型MIG焊机及KUKA机器人组成的自动化复合焊接系统对5083铝合金板进行平板堆焊试验。YAG激光波长λ=1064nm,光束质量因子为25mm·mrad,光斑直径D=0.6mm,送丝电机额定电流为8.0A,激光功率P=3.0kW,焊接电流I=200.0A,电弧电压U=20.8V,焊接速率v=1m/min,离焦量Δf=-2mm,焊丝末端在工作表面的接触点和激光束在工作表面的作用点之间的距离DLA=3mm,保护气体为纯度99.9%的氩气其流量为25L/min。除此之外试验过程中所涉及的焊接喷嘴形状、尺寸参量见表 2。
Table 2. Nozzle shapes and size parameters
nozzle shape multiple row circular tube a single round tube a single square tube section inside diameter/mm 4×∅10 ∅15 15×15 焊接之前用钢刷对试样进行打磨以去除表面氧化膜,然后用丙酮擦拭去除表面粉尘及油污。试验中采用高速相机和汉诺威电弧分析仪分别对熔滴过渡形式和实时的电流、电压变化进行观察和监测,从而分析整个焊接过程的稳定性。高速相机像素为480×480,拍摄速率为5000frame/s。为了便于高速相机拍摄,采取焊枪位置不动,工作台行走的方式。焊接采用电弧引导激光的方式,激光束采用垂直入射,MIG焊枪与铝合金板平面成60°,激光束与焊枪夹角为30°。图 1为工件、焊枪及激光束位置关系。
焊接完成后,使用线切割机床将工件沿着横截面切开,并依次标为P1~P3,按照金相试样制作标准对试样进行研磨、抛光,并用Keller试剂(95.0mL水+2.5mL HNO3+1.5mL HCL+1.0mL HF)进行腐蚀。对焊缝熔深d、熔宽w及余高h等主要参量及气孔试样进行测量和标定。焊缝截面形貌参量标定如图 2所示。
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物理守恒定律决定保护气体的空间流动,而基本的守恒定律有质量守恒定律、动量守恒定律、能量守恒定律。模拟保护气体喷嘴外部流场的运动状况除了这些基本守恒定律,湍流模型的选择对于数值精度影响很大。标准k-ε模型在强旋流、弯曲曲面或弯曲流线流动时,会产生一定的失真。在湍流模型的选择上,本次模拟选择重正规化群(re-normalization group, RNG) k-ε双方程模型,这是一种改进的k-ε模型[17-19],其k和ε相对应的输运方程为:
$ \begin{align} &\ \ \ \ \ \ \ \ \ \ \ \ \ \ \frac{\partial (\rho k)}{\partial t}+\frac{\partial (\rho k{{u}_{i}})}{\partial {{x}_{i}}}= \\ &\frac{\partial }{\partial {{x}_{j}}}\left( {{\alpha }_{k}}{{\mu }_{\text{eff}}}\frac{\partial k}{\partial {{x}_{j}}} \right)+{{G}_{k}}+{{G}_{{\rm{b}}}}-\rho \varepsilon-{{Y}_{Ma}}+{{S}_{k}} \\ \end{align} $
(1) $ \begin{align} & \ \ \ \ \ \ \ \ \ \ \ \ \frac{\partial (\rho k)}{\partial t}+\frac{\partial (\rho k{{u}_{i}})}{\partial {{x}_{i}}}= \\ & \frac{\partial }{\partial {{x}_{j}}}\left( {{\alpha }_{\varepsilon }}{{\mu }_{\text{eff}}}\frac{\partial k}{\partial {{x}_{j}}} \right)+{{C}_{1\varepsilon }}\frac{\varepsilon }{k}\left( {{G}_{k}}+{{C}_{3\varepsilon }}{{G}_{\text{b}}} \right)- \\ & \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ {{C}_{2\varepsilon }}\rho \frac{{{\varepsilon }^{2}}}{k}-{{R}_{\varepsilon }}+{{S}_{\varepsilon }} \\ \end{align} $
(2) 式中,xi和xj是迹线在i和j方向的分量;t是运动时间; ui是平均速率在i方向的分量; ρ为液体密度;μi和μj是动力粘度系数;k为湍动能;ε为耗散能; C1ε, C2ε和C3ε为经验常数,取C1ε=1.44,C2ε=1.92,C3ε=0.09;αk和αε是湍动能k和耗散能ε对应的Prandtl数,其中αk=1.39,αε=1.39;Sk和Sε为用户自定义源项,可以根据不同情况设定;Gk是由层流速度梯度而产生的湍流动能;Gb是由浮力而产生的湍流动能;YMa是湍流中脉动扩张产生的波动能;μeff和Rε为修正参量。
在RNG k-ε模型中的μeff, Rε等其它修正参量,使得RNG k-ε模型相比于标准k-ε模型对瞬变流和流线弯曲的影响作出更好的反应。
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本文中的模型进口流量为已知条件,入口速度大小可以由流体的体积流量Q和喷嘴入口面积S决定,如下所示:
$ v=\frac{Q}{S} $
(3) 在确定入口速度后还要设定求解湍流模型的其它各项参量,其计算公式为:
$ R\mathit{e=}\frac{\rho vd}{\mu } $
(4) $ Ma=\frac{v}{c} $
(5) $ L=\frac{A}{\chi } $
(6) 式中, Re是入口雷诺数;v是流体的入口速率(m/s);d是当量直径,当管道为圆管时,当量直径等于圆管直径;当管道为非圆管时,当量直径等于4倍的水力半径(m);μ是动力粘度系数,取为2.13×10-5kg/(m·s);ρ是流体密度,取氩气密度为1.62kg/m3;Ma为马赫数;c为当地声速,取为340m/s;L为水力半径(m);A为过流断面积(m2);χ为湿周长(m)。
依据上述公式得到数值仿真所需要的各项参量,如表 3所示。
Table 3. Parameters data of numerical simulation
nozzle shape serial number gas velocity v/(m·s-1) Reynolds number Re Mach number Ma multiple row circular tube P1 1.3 1013 0.004 a single round tube P2 2.4 2701 0.007 a single square tube P3 1.9 2121 0.006 -
试验中喷嘴出口气体速率均小于5m/s,Ma$\ll $1,故保护气体的密度可以忽略[20],将氩气简化成不可压缩气体,采用压力求解器进行数值求解。
湍流是一种高度复杂的3维非稳态,带旋转的不规则流动,采用湍流模型模拟喷嘴外流场,更符合保护气体真实分布。
采用FLUENT软件计算喷嘴外流场,不考虑气体与激光束、气体与工件之间的热交换。
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用SOLIDWORKS 2014建立保护气体喷嘴3维模型,气体流场的计算区域大小是底面为∅100mm、高为80mm的一个圆柱体。
利用WORKBENCH 17.0对模型进行网格划分和边界条件的设置。由于在喷嘴处应力较大,因此采用更小的网格单元,如图 3所示。
采用流体动力学(computational fluid dynamics, CFD)软件FLUENT 17.0对在相同气体流量下不同形状喷嘴的气体外流场进行模拟计算。
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由图 4可知:保护气体从喷嘴喷出后,在A处速度梯度大,衰减较快,湍流剧烈,在其周围形成多个小漩涡;外侧喷嘴边缘处形成反方向涡流,随后以接近层流的状态,沿轴线方向向两侧流动。B处是焊接作用区,保护气体均布,保护作用的范围大,气流没有较大的偏移和旋转,流动稳定,但是该区气流速率在0.5m/s左右,气流的挺度小,抵抗外界干扰的能力较低,同时气体作用在工件表面的静压力小,保护效果将受到影响。增加保护气体的流量将会提高该处的保护效果。在C处形成一个巨大的涡流区,由于气流挺度进一步减小,无法克服外界干扰,流线变得杂乱无序。
由图 5可知:保护气体从喷嘴喷出后,在A处喷嘴口的边缘处形成“手镯状”的涡流区,湍流剧烈,由于“手镯状”的涡流区范围较大,使得B处的边缘区产生多个小漩涡,影响对工件的保护效果;在B处,气体中间流线由于受到A处大涡流的间接影响产生一个收缩现象,使保护作用区域减小,也会对焊接效果造成影响。在C处,由于距“手镯状”的涡流区较远,大涡流区的影响较小,气流中间流线呈“伞状”发散。
由图 6可知:保护气体从喷嘴流出后,在A处喷嘴口的边缘形成“方形环状”涡流,由于速度梯度较小,湍流平缓,而且涡流区在轴线方向的影响距离小,对焊接区B处的影响小。在B处,由于中间流线的速度梯度小,速度衰减较慢,气流的挺度高,作用在工件表面的静压力较大,焊接保护效果较好。但是与多排喷嘴相比,单个方管的有效保护区域小,从而会影响保护效果。在C处,气流呈“棒状”分布,流线均布,流场稳定,主要是单个方管的速度衰减慢,高挺度的保护气流足以克服外界干扰,从而使流动更稳定。
喷嘴形状对铝合金复合焊接头成形质量的影响
Influence of nozzle shape on the quality of aluminum joints product by laser-arc hybrid welding
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摘要: 为了研究气体喷嘴形状对焊接成形质量的影响,采用了建立不同形状喷嘴的外流场数值模型的方法,进行了数值仿真和实验验证。在相同工艺参量下,采用YAG激光-MIG电弧复合热源,对10mm厚的5083铝合金板进行了平板堆焊试验。结果表明,多排圆管喷嘴的焊接过程最稳定,焊缝熔深和熔宽最大,分别为9.41mm和4.45mm,但其气孔率却最高达7.34%;单个方管的保护效果最好,但其焊接过程稳定性、熔深和熔宽均介于多排圆管与单个圆管喷嘴之间;而单个圆管除了保护效果介于多排圆管与单个方管之间外,其过程稳定性最差,焊缝形貌特征值最小;保护气体喷嘴为方形时,保护效果最佳。该结果验证了数值模型计算所得到的各种喷嘴有效保护范围和气流挺度。Abstract: In order to research the effect of different nozzle shapes on the quality of weld forming joint, numerical simulation of flow field of different shape nozzles were established by FLUENT software. The testing of bead-on-plate welding to 5083 aluminum alloy plate with 10mm thickness was carried out under the same processing parameters by using Nd:YAG-MIG hybrid welding. After experimental verification, the results show that, the welding stability of multi-row round nozzle is the best, and weld penetration and weld width are the largest, 9.41mm and 4.45mm respectively, but the porosity is up to 7.34%. The protective effect of single side square nozzle is the best, but its welding stability, weld penetration and weld width are between multi row round nozzle and single side round nozzle. Protection effect of single side round nozzle is between multi row round nozzle and single side square nozzle, and then, welding stability and characteristic value of welding appearance have the worst results. Protection effect of square nozzle is the best. The research illustrates numerical simulation of protection effective range and airflow stiffness of different nozzle shapes.
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Key words:
- laser technique /
- nozzle shape /
- laser-arc hybrid welding /
- numerical simulation /
- welding appearance
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Table 1. Chemical composition (mass fraction) of 5083 aluminum alloy and filler wire
material Si Fe Cu Mn Mg 5083 aluminum alloy 0.004 0.004 0.001 0.004~ 0.01 0.04~ 0.049 ER5087 0.00022 0.0015 0.00005 0.009 0.048 material Cr Zr Zn Ti Al 5083 aluminum alloy 0.0005~ 0.0025 — 0.0025 0.0015 balance ER5087 — 0.00082 — — balance Table 2. Nozzle shapes and size parameters
nozzle shape multiple row circular tube a single round tube a single square tube section inside diameter/mm 4×∅10 ∅15 15×15 Table 3. Parameters data of numerical simulation
nozzle shape serial number gas velocity v/(m·s-1) Reynolds number Re Mach number Ma multiple row circular tube P1 1.3 1013 0.004 a single round tube P2 2.4 2701 0.007 a single square tube P3 1.9 2121 0.006 -
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