超声辅助激光修复镍基高温合金V形槽
Ultrasonic assisted laser repair of V-grooves in nickel-based superalloy
通讯作者:
收稿日期: 2020-11-3
基金资助: |
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Received: 2020-11-3
Fund supported: | 国家自然科学基金资助项目(51705460);浙江省重点研发计划资助项目(2019C04004);浙江省属高校基本科研业务费专项资金资助项目(RF-B2020002) |
作者简介 About authors
姚喆赫(1987—),男,副研究员,从事激光加工、超声/振动辅助制造研究.orcid.org/0000-0001-5252-463X.E-mail:
为了提高失效零件的修复质量,以V形槽为修复对象开展研究,在修复过程中引入超声振动. 耦合声场、温度场与流场建立超声辅助激光修复V形槽的非线性瞬态模型,对修复过程熔池形貌及其温度场进行数值模拟,并开展相关实验研究,分析超声对修复区组织及性能的影响机制. 数值模拟结果表明,超声振动能加速熔池流动与散热,提高熔深,细化晶粒并改善V形槽底部难熔区域的修复效果. 金相、扫描电子显微镜和电子背散射衍射分析结果表明,在超声作用下,晶粒尺寸和一次枝晶间距得到细化,Laves相更加弥散,Nb元素的富集受到抑制;常温和高温硬度在超声作用下分别提高了11.1%、10.4%. 研究表明,超声振动能有效改善镍基高温合金V形槽激光修复区的微观形貌,提高力学性能.
关键词:
The V-shaped groove was taken as the repair object, and the ultrasonic vibration was introduced in the repair process, in order to improve the repair quality of failed parts. The acoustic field, temperature field and flow field were coupled to establish a nonlinear transient model for ultrasonic assisted laser repairing of V-groove. The morphology and the temperature field of the molten pool during the repairing process were numerically studied, and the related experimental studies were carried out. The influence mechanisms of ultrasonic vibration on the microstructure and the performance of the repaired region were analyzed. Numerical simulation results showed that the ultrasonic vibration would accelerate the flow and heat dissipation of the molten pool, increase the penetration depth and refine the grains, leading to the improvement of the repair effect of the refractory region at the bottom of the V-shaped groove. Results of metallographic, scanning electron microscope and electron backscatter diffraction analysis show that the crystal grains and primary dendrite spacing were refined with the effect of ultrasonic vibration, resulting in the dispersion of the Laves phase and the suppression of the Nb element enrichment. The room-temperature hardness and the elevated-temperature hardness increased by 11.1% and 10.4%, respectively. Research shows that the ultrasonic vibration effectively improves the microscopic morphology of the laser repair zone of the nickel-based superalloy V-groove, and improves the mechanical properties.
Keywords:
本文引用格式
姚喆赫, 张操棋, 宋其伟, 卢习江, 孔建强, 姚建华.
YAO Zhe-he, ZHANG Cao-qi, SONG Qi-wei, LU Xi-jiang, KONG Jian-qiang, YAO Jian-hua.
为了克服激光缺陷问题,近年来,国内外学者通过将超声振动引入激光熔覆成形过程,在组织细化、缺陷抑制、性能改善等方面取得了一定成果. 例如,Gorunov等[8]研究发现施加超声振动后,熔覆层的拉伸强度提高了30%;Ning等[9]将超声振动引入TiB增强Ti基复合材料激光近净成形过程,结果表明超声振动能够降低孔隙率,细化晶粒组织;张安峰等[10]研究发现超声振动能够有效降低激光熔覆Ti-6Al-4V成形件的表面粗糙度与残余应力;Yao等[11]研究发现在三维高频振动的作用下,激光熔覆SS316L涂层孔隙率明显降低,柱状枝晶尺寸细化,显微硬度得到提高;Todaro等[12]报道了将超声引入Ti-6Al-4V激光粉末沉积过程,实现了柱状晶向等轴晶的转变. 上述文献研究超声振动对于激光熔覆与增材制造的作用效果,而针对受损零部件的超声辅助激光增材修复的研究则较少. 激光增材修复相较于激光熔覆与增材制造更加关注界面结合与过渡、热输入与热影响区等因素,因此,超声辅助激光增材修复仍待进一步深入研究.
1. 实验设备及方案
图 1
图 1 超声辅助激光修复实验平台示意图
Fig.1 Diagram of experimental setup for ultrasonic-assisted laser repairing
表 1 超声辅助激光修复实验研究中的工艺参数
Tab.1
参数名称 | 参数 |
激光功率/W | 500 ~ 1 000 |
光斑直径/mm | 2.2 |
扫描速度/(mm∙s−1) | 5 |
送粉速率/(g∙min−1) | 10 |
超声振动频率/kHz | 20 |
超声振幅/μm | 20 |
2. 数值模拟研究
2.1. 数值模型的构建
基于上述假设建立的数值模型如下.
1) 激光热源模型. 采用平面热源模型[17],热流密度的表达式为
式中:a为激光吸收率,P为激光功率,r为光斑半径.
式中:ρ为密度,c为比热容,T为温度,t为时间,u为熔体流速,
3) 流场控制方程. 除了传热控制方程外,超声辅助激光修复数值模型的控制方程还包括连续方程和动量方程[20],分别为
式中:μ为动力黏度;f为单位体积流体所受到的热浮力源项FB和Darcy源项FD之和.
合金密度会随着温度而变化,且密度发生变化将引起浮力源项的改变,因此采用Boussinesq假设,热浮力源项表达式[16]为
式中:β为热膨胀系数,Tref为相对参考温度,g为重力加速度.
Darcy源项表达式为
式中:m为动量阻尼系数;n为无穷小的辅助参数,避免分母为零;
式中:Ts为固相线温度;Tl为液相线温度;
4) 超声振动的引入方式. 在平衡状态下,激光熔池内部存在稳定压强,即通常定义的静压力p0. 在引入超声能场后,声波在熔池内部传播时将引起压力场的变化,定义为pm. 引入超声振动前、后熔池内压力差表达式为
式中:cu为超声在熔池中的传播速度;v为超声振动作用下的基板瞬时速度,
通过边界应力条件将超声以声压的形式引入模型,进而研究超声对熔池的影响机理[21],控制方程为
式中:ρb为基板材料密度.
2.2. V形槽修复过程熔池数值模拟研究
表 2 超声辅助激光修复数值模拟的参数设置
Tab.2
图 2
图 2 激光修复V形槽熔池形貌及其温度场数值模拟
Fig.2 Numerical simulation of molten pool topography and temperature field distribution in laser repaired V-groove
图 3
图 3 不同条件下的激光修复V形槽熔合线对比
Fig.3 Comparison of fusion line in laser repaired V-groove with various conditions
3. 实验结果与讨论
3.1. V形槽修复区形貌及微观组织分析
图 4
图 4 V形槽激光修复横截面形貌
Fig.4 Cross-sectional morphology of laser remanufactured V-groove
为了定量描述修复效果,定义熔深比为
式中:h为熔合线底部距槽顶深度;h0为V形槽初始深度.
熔深比汇总如图5所示. 可以看出,随着激光功率的增大,熔深比增大;在相同的功率下,施加超声振动后的熔深比相较无超声时有较为明显的提高,显示出超声振动对于尖角修复的促进效果. 该实验结果与2.2节所述数值模拟预测吻合.
图 5
图 5 不同激光功率下激光修复V形槽熔深比
Fig.5 Penetration ratio in laser repaired V-groove with various laser powers
图 6
图 6 V形槽激光修复区枝晶组织形貌
Fig.6 Dendritic morphology in laser repaired zone of V-groove
图 7
图 7 V形槽激光修复区一次枝晶宽度
Fig.7 Primary dendritic spacing in laser repaired zone of V-groove
式中:λ为一次枝晶间距平均值,n为与材料成分相关的系数,ε为冷却速率.
图 8
图 8 V形槽激光修复过渡区横截面形貌
Fig.8 Cross-sectional morphology of transition region in laser remanufactured V-groove
3.2. V形槽修复区析出相及其形貌研究
为了比较研究超声振动对V形槽修复区析出相及其形貌的影响,针对1 000 W激光功率(其他参数见表1)下的修复试样,采用扫描电子显微镜(ZEISS EVO18)及能谱仪(EDS,BRUKERXFlash 6130),对V形槽修复区和结合区的微观组织及析出相进行观测,并在析出相附近进行元素质量分数线扫描分析,记录不同位置(与起点距离为d)的脉冲数pul,结果如图9所示. 在未施加超声时,Laves相主要沿树枝晶间以不连续的链状方式析出,且析出量较多,尺寸相对较大,线扫描结果表明Laves相形成时伴随着Nb、Mo元素的大量富集,其中Nb元素质量分数为19.68%. 在超声作用下,Laves相的析出形貌由链状分布转变为尺寸较小的点状分布,其中Nb元素质量分数显著降低至7.62%,对比未施加超声的Nb元素质量分数降低了61.28%.
图 9
图 9 V形槽激光修复区析出相形貌及元素线扫描结果
Fig.9 Precipitation phase morphology and element line-scanning results of laser repaired zone in V-groove
镍基高温合金析出的Laves相一般被认为是有害相,相关研究[35]表明,当Laves相尺寸较大且以不连续的链状分布于树枝晶间时,将阻碍基体的塑性变形,影响修复区的拉伸性能,而当Laves相尺寸较小且呈点状分布时,其断裂机制为韧性断裂,拉伸强度与延展性得到提高. 因此,采用超声振动将修复区析出相转变为弥散状分布,将有助于提高其综合力学性能.
3.3. V形槽修复区晶体取向及晶粒尺寸研究
图 10
图 10 V形槽激光修复区EBSD分析结果
Fig.10 EBSD analysis results of laser repaired zone in V-groove
3.4. V形槽修复区显微硬度分析
对于1 000 W激光功率(其他参数见表1)下的修复试样,开展常温及600 °C高温条件下的硬度测试,在0.2 kgf试验力下获得维氏硬度(HV0.2). 在常温条件下,修复区显微硬度分布曲线如图11所示. 可以看出,在未施加超声时,修复区内的平均显微硬度Hm≈270 HV0.2,在引入超声振动后修复区平均显微硬度提升至约300 HV0.2,提高了11.1 %. 高温显微硬度测试结果如图12所示,基体部分的显微硬度约为210 HV0.2,相较于常温硬度降低约20 HV0.2. 在未施加超声时,修复区高温显微硬度约为240 HV0.2,施加超声后的高温硬度升至约265 HV0.2,提高了10.4 %.
图 11
图 11 V形槽激光修复区室温硬度
Fig.11 Room-temperature hardness of laser repaired zone in V-groove
图 12
图 12 V形槽激光修复区600 ℃高温硬度
Fig.12 Elevated-temperature hardness of laser repaired zone in V-groove at 600 °C
基体及修复区的高温硬度相较于常温硬度略有下降. 分析认为,在600 °C下,低熔点共晶产物未达到其1170~1180 °C的熔点[36],因此共晶产物中的强化元素Nb、Mo、Ti等元素未固溶于基体γ相中,并且该温度未达到γ′强化相的析出温度,显微组织发生一定的粗化,进而在高温显微硬度下表现为硬度的略微降低.
在超声作用下,基于3.1节和3.3节研究,晶粒和枝晶结构的细化将提升其硬度,此外,超声振动抑制了强化元素Nb和Mo的偏析,Laves相及MC相的体积分数降低,更多的强化元素被固溶于γ奥氏体基体相中,进而提升其显微硬度.
4. 结 论
(1)数值模拟和实验研究均表明,超声振动将促进能量向V形槽底部传输,增大熔深,抑制V形槽底部熔合不良的产生.
(2)在超声振动作用下,V形槽修复区一次枝晶间距减小;Laves相尺寸减小,并由链状分布转为呈点状分布,且析出相中Nb元素质量分数显著降低.
(3)在未施加超声振动时,V形槽修复区主要以取向性较强的柱状晶生长,在施加超声振动后,柱状晶生长取向相对杂乱,且晶粒尺寸减小.
(4)在超声作用下,V形槽修复区常温与高温显微硬度分别提升了11.1 %、10.4 %.
(5)研究表明,超声振动有效改善了镍基高温合金V形槽激光修复区的微观形貌,提高了力学性能.
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