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对高斯热源的研究,主要分为3类,分别是体热源、面热源以及面体组合热源。
在激光束焊接过程中, 高斯面热源是指利用一定区域内的热源对试样进行加热。面热源可表示为:
$ {Q_{\rm{s}}} = \left( {\frac{{{\rm{2}}P{\eta _{\rm{s}}}}}{{{\rm{ \mathsf{ π} }}{R^2}}}} \right){\rm{exp}}\left( {\frac{{ - {\rm{3}}{r^2}}}{{{R^2}}}} \right) $
(1) 式中,R为光斑半径;P为激光输入功率;r为点到热源中心的距离;ηs为高斯面热源在总热源的占比。
高斯圆柱体热源是利用圆柱体内均匀的热源对试样进行加热[19]。圆柱体热源能够反映小孔效应在激光深熔焊过程中材料对能量能量的吸收机制[20],体热源的表达式为:
$ {Q_{\rm{v}}} = \left( {\frac{{{\rm{2}}{\eta _{\rm{v}}}P}}{{{\rm{ \mathsf{ π} }}{r_{\rm{v}}}{h_{\rm{v}}}}}} \right){\rm{exp}}\left[ {\frac{{ - {\rm{2(}}{x^2} + {y^2})}}{{{r_{\rm{v}}}^2}}} \right]u(z) $
(2) 式中,ηv为高斯体热源在总热源的占比, rv为高斯体热源作用的有效半径, hv为体热源作用的有效深度, x2+y2为节点到热源中心的距离, u(z)为单位阶跃函数。
因为在实际的焊接中,激光点是移动的。在移动激光作用下,试样表面和内部所形成的熔池将不再是单一的圆柱或锥形,并且实际焊接过程中存在等离子体云和小孔效应, 所以本文中采用面热源与体热源结合的模型,其热量表达式为:
$ Q = {Q_{\rm{s}}} + {Q_{\rm{v}}} $
(3) 一些学者的研究发现, 通过高斯面体热源模型得到的焊缝形貌的仿真数据,与实际实验中所得到的数据之间的误差小于5%, 说明面热源和体热源组合的形式能够计算出较为准确的结果。根据上述高斯面热源与高斯圆柱体热源及面体热源模型表达式,可以得到高斯面热源与体热源结合的热流密度:
$ Q = \left( {\frac{{{\rm{2}}P{\eta _{\rm{s}}}}}{{{\rm{ \mathsf{ π} }}{R^2}}}} \right){\rm{exp}}\left[ {\frac{{ - {\rm{3}}({x^2} + {y^2})}}{{{R^2}}}} \right]\\ + \left( {\frac{{2{\eta _{\rm{v}}}P}}{{{\rm{ \mathsf{ π} }}{r_{\rm{v}}}{h_{\rm{v}}}}}} \right){\rm{exp}}\left[ {\frac{{ - 2\left( {{x^2} + {y^2})} \right]}}{{{r_{\rm{v}}}^2}}} \right]u(z) $
(4) 由于本文中焊接试样较薄,为1.5mm厚的殷瓦合金,因此热源比例取ηs∶ηv=3∶1。因实际焊接过程中,都是采用保护气体对试样进行焊接保护,在焊接过程中保护气体能把工件表面的等离子体云吹散,所以作者在模型假设中不考虑等离子体与试样的对流和热辐射等。根据传热学原理,建立温度T随点(x,y,z)及时间t的关系(导热微分方程):
$ T = f(x, y, z, t) $
(5) 据能量守恒定律,温度场的微分方程表示为:
$ {\rm{ }}\rho c\frac{{\partial T}}{{\partial t}} = {\bf{ \pmb{\mathsf{ λ}} }}\left( {\frac{{{\partial ^2}T}}{{\partial {x^2}}} + \frac{{{\partial ^2}T}}{{\partial {y^2}}} + \frac{{{\partial ^2}T}}{{\partial {z^2}}}} \right) + \varphi $
(6) 式中, ρ, c, λ, φ分别为焊接材料的密度、比热容、导热系数和在单位时间里单位体积中内热源的生成热。
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焊接过程是一个快速加热和快速冷却的过程,由于在加热阶段,焊缝融合区温度可以达到很高,温度在25℃~3000℃范围内变化,在此范围里,材料的物性参量变化大,不可采用恒定的参量。对此本文中采用的材料参量与参考文献[21]相同。
为了提高仿真的准确性,在焊接过程中除了材料参量会影响分析结果外,焊接时的接触状态和上下板之间的间隙也会对分析结果有影响,因为接触时的接触间隙中主要是空气层,对热的传递阻碍较大,这将直接影响接触面的传热方式从固-固导热到固-气-固导热和辐射,因为辐射强度相较于对流导热而言较弱,在本文计算过程中忽略不计。根据以往的文献报道[22],不锈钢材料在有一定粗糙度的情况下, 单位面积的接触热阻介于2.2×10-4m2·K/W~5.88×10-4m2·K/W之间,由于实验中使用的轧制退火态的殷瓦合金,材料表面光滑无毛刺,且在焊接中用U型夹具对试样施加一定载荷,所以将殷瓦合金单位面积的接触热阻设为4.04×10-4m2·K/W。
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本文中采用的试样几何尺寸为60mm×40mm×1.5mm,同时仿真中涉及到的物理变化相对复杂,如在温度场、流场、微观相变等, 为了提高运算精度及减小运算量,对模型进行简化,在搭接接头部位采用半径为2.5mm的圆弧过渡。建立模型后对开展3维瞬态和稳态分析。在ANSYS分析中,综合考虑计算进度及单元类型,本文中用solid70八节点六面体单元,将网格单元划分为0.75mm×0.75mm×0.75mm,网格划分如图 6所示。
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为了判断出合适的因素取值范围,降低试验成本,本文中通过数值仿真模拟分别对激光功率及焊接速率对焊接温度场的影响进行了分析。由图 7可知,激光功率2.6kW~3.6kW范围内变化时,激光功率越大,下板熔深、搭接处熔宽也越大。经测量可知,当激光功率从3kW增长为3.6kW时,下板熔深从0.57mm增长为0.8mm,熔宽从2.2mm增长到2.4mm; 当激光功率为2.6kW时,试样下板未被熔透,不符合殷瓦合金搭接焊技术要求,因此,激光功率工艺参量范围下限应该比2.6kW大。
在图 8中,焊接速率和下板熔深、搭接处熔宽呈负相关。当焊接速率从0.9m/min增长到1.7m/min时,下板熔深从0.8mm减小到0.26mm,熔宽从2.6mm减小到1.9mm。根据技术要求,选取最优的焊接速率参量为1.1m/min~1.7m/min。
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结合数值仿真模拟获得的激光功率、焊接速率工艺参量范围和气孔缺陷抑制方案,正交实验中将激光功率(kW)作为因素A、焊接速率(m/min)作为因素B、离焦量(mm)作为因素C、激光入射角(°)作为因素D、光斑能量分布(即激光光斑落在上板和下板的分布比例)作为因素变量E。光斑能量分布(1∶1;2∶1;4∶1)如图 9所示。
每个因素取3个水平,主要目标函数有下板焊缝的熔深、下板的熔宽和最小半径等。通过仿真模拟结果确定了正交实验的因素水平,表 1为三水平五因素正交表。
Table 1. Orthogonal experiment factors and level
level factor A factor B factor C factor D factor E laser power/kW welding speed/(m·min-1) defocusing amount/mm incident angle/(°) energy distribution/% 1 3 1.1 16 0 1∶1 2 3.2 1.3 18 5 2∶1 3 3.4 1.5 20 10 4∶1 在实验过程中,每个因素做3组实验,取3组实验结果的平均值作为实验结果,以确保表中实验结果的可靠性和准确性,表 2为正交试验表。
Table 2. Orthogonal experiment table
experiment number factor result 1 result 2 result 3 A/kW B/(m·min-1) C/mm D/(°) E/% weld depth/μm weld width/μm radius/μm 1 3 1.1 16 0 1 855 2101 1234 2 3 1.3 18 5 2 371 1793 1307 3 3 1.5 20 10 4 245 1837 1300 4 3.2 1.1 16 5 2 406 2450 1425 5 3.2 1.3 18 10 4 344 1955 1371 6 3.2 1.5 20 0 1 513 2078 1111 7 3.4 1.1 18 0 4 644 2313 1274 8 3.4 1.3 20 5 1 785 2151 1138 9 3.4 1.5 16 10 2 455 2349 1334 10 3 1.1 20 10 2 970 2157 1200 11 3 1.1 16 0 4 298 1868 1351 12 3 1.3 18 5 1 634 1865 1030 13 3.2 1.5 18 10 1 1009 2160 1123 14 3.2 1.1 20 0 2 472 2310 1254 15 3.2 1.3 16 5 4 300 1907 1353 16 3.4 1.5 16 5 4 556 2300 1360 17 3.4 1.1 18 10 1 826 2140 1174
LNG船用薄板Invar36激光搭接焊工艺参量优化
Process parameters optimization of laser lap welding of thin plate Invar36 for LNG ships
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摘要: 为了解决殷瓦合金焊接中易出现的焊接缺陷问题,研究了液化天然气(LNG)船用薄板殷瓦合金搭接焊(上下板厚度均为1.5mm)工艺参量。通过设计制造气体保护盒,采用数值仿真分析和正交试验的方法,进行了理论分析和实验验证,取得了殷瓦合金搭接焊最优工艺参量。结果表明,气孔缺陷在完全保护气下得到有效抑制,殷瓦合金搭接焊的最优工艺参量为激光功率3.4kW、焊接速率1.3m/min、离焦量+20mm、激光入射角5°、激光光斑能量分布2∶1;在此最优工艺参量下,焊缝表面呈银白色,无气孔等缺陷,焊缝处硬度小于母材但大于热影响区,拉伸强度为417.16MPa,达到母材的94.8%,且仿真结果与试验结果误差较小,证明了本文中所建立的仿真模型的可靠性。这一结果对殷瓦合金激光搭接焊工艺参量数据库搭建是有帮助的。
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关键词:
- 激光技术 /
- 工艺参量优化 /
- 激光自熔搭接焊 /
- Invar36合金材料
Abstract: In order to solve the problem of welding defects in Invar alloy welding, the technological parameters of Invar alloy lap welding (the thickness of the upper and lower plates with 1.5mm) for liquefied natural gas (LNG) ship sheet were studied. Though designing and manufacturing a gas protection box, by means of numerical simulation analysis and orthogonal experiment, the optimal process parameters of Invar alloy lap welding were obtained.The experimental results show that the porosity defects are effectively suppressed under complete shielding gas, and the optimal technological parameters for the lap welding of Invar alloy are obtained as: Laser power is 3.4kW, welding speed is 1.3m/min, defocus is +20mm, laser incident angle is 5°, and laser spot energy distribution is 2∶1, respectively. Under the optimal process parameters, the weld surface is silvery white, and there is no defect such as air holes. The hardness of the weld is smaller than that of the base metal but larger than that of the heat affected zone. The tensile strength is 417.16MPa, which is 94.8% of that of the base metal. The error between the simulation results and the test results is small, which proves the reliability of the simulation model established in this paper. This result is helpful for the construction of the later process parameter database of Invar laser lap welding. -
Table 1. Orthogonal experiment factors and level
level factor A factor B factor C factor D factor E laser power/kW welding speed/(m·min-1) defocusing amount/mm incident angle/(°) energy distribution/% 1 3 1.1 16 0 1∶1 2 3.2 1.3 18 5 2∶1 3 3.4 1.5 20 10 4∶1 Table 2. Orthogonal experiment table
experiment number factor result 1 result 2 result 3 A/kW B/(m·min-1) C/mm D/(°) E/% weld depth/μm weld width/μm radius/μm 1 3 1.1 16 0 1 855 2101 1234 2 3 1.3 18 5 2 371 1793 1307 3 3 1.5 20 10 4 245 1837 1300 4 3.2 1.1 16 5 2 406 2450 1425 5 3.2 1.3 18 10 4 344 1955 1371 6 3.2 1.5 20 0 1 513 2078 1111 7 3.4 1.1 18 0 4 644 2313 1274 8 3.4 1.3 20 5 1 785 2151 1138 9 3.4 1.5 16 10 2 455 2349 1334 10 3 1.1 20 10 2 970 2157 1200 11 3 1.1 16 0 4 298 1868 1351 12 3 1.3 18 5 1 634 1865 1030 13 3.2 1.5 18 10 1 1009 2160 1123 14 3.2 1.1 20 0 2 472 2310 1254 15 3.2 1.3 16 5 4 300 1907 1353 16 3.4 1.5 16 5 4 556 2300 1360 17 3.4 1.1 18 10 1 826 2140 1174
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