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1 前言
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核电是一种清洁、环保、低耗的新能源,是当前解决能源短缺和环境恶化双重压力的有效发电方式[1]。自 1951 年 12 月美国首次成功使用核能发电以来,世界核电站发展经历了开发、发展、受阻、复苏四个阶段[2-4]。截至 2019 年核能发电量占全球总发电量约 10%,占低碳发电量 1 / 3[5]。近年来,随着我国的科技发展,秉持可持续发展战略,2019 年全年发电量占全国发电量 4.88%。2022 年,我国在运核电机组 54 台,在建 23 台,处于全球领先地位[5-6]。
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虽然核电能解决能源短缺和环境恶化问题,但核电站核安全仍是发展核能的前提和生命线,1997 年美国三哩岛核事故、1986 年苏联切尔诺贝利核事故及2011年日本福岛核事故向各国敲响了警钟[7]。由于核电领域材料多在极端环境和复杂受力状态下工作,材料的磨损和腐蚀失效严重[8-10],通过表面改性提高金属耐腐耐磨性成为国内外持续关注的热点[11-12]。针对核电领域材料的磨损和腐蚀失效,国内外学者研究发现可通过表面改性提升材料性能。 MAIER 等[13]通过冷喷涂技术在燃料包壳管表面制备 Ti2AlC 涂层,显著提高燃料包壳管的耐腐蚀性和耐磨性。DABNEY 等[14]通过制备 FeCrAl 涂层,发现该涂层相较于燃料包壳管基体耐磨性提高了 3~4 倍。周大勇等[15]采用激光合金化在 Inconel625 合金表面制备出 WC-TiC 合金化层,耐磨性提升 4.1 倍。综上所述,通过引入涂层可以改善核电领域材料耐磨和耐腐蚀性,提高材料使用寿命。但膜基结合性一直是阻碍涂层技术应用的关键,因此采用表面形变强化技术提升核电领域材料使用寿命成为国内外研究热点[16-17]。表面形变强化具有不引入其他材料、膜基结合强度高等优势,是目前被认为最理想的强化手段之一。
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2 表面形变强化技术
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随着科技的发展,表面形变强化技术在实际中逐渐得到应用[18-19]。目前研究、讨论、应用最广的包括喷丸(Shot peening,SP)、滚压(Rolling)、激光冲击强化(Laser shock peening,LSP)等技术,其原理是通过在材料表面引入残余压应力,形成硬化层,细化表面和近表面晶粒,协同作用改善金属各项性能[20-22]。
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喷丸强化是采用球形弹丸高速撞击金属表面,引入残余压应力,使金属表面形成一定厚度的强化层,以达到金属强化的技术[20,23],加工示意图如图1a 所示。滚压强化原理是利用滚轮对金属表面进行滚压,使金属表面发生塑性变形,填充金属表面因加工工艺而存在的凹陷中,从而降低金属表面粗糙度,消除凹陷产生的应力集中,引入残余压应力,进而提升金属的各项性能[24-25],其原理示意图如图1b 所示。在传统喷丸和滚压的基础上,延伸出微喷丸[26]、强力喷丸[27]、超声滚压[28]、超声振动滚压[29] 等多种强化技术。目前,喷丸强化和滚压强化广泛应用在航空航天、核电站及船舶领域。但两种强化技术的劣势也尤为突出。喷丸强化技术噪声大;会对环境造成粉尘污染;受弹丸直径的大小的影响,易造成强化层不均且不能有效撞击精密零件表面。滚压强化技术一般只适合平面和回转体零件,对于结构复杂的精密零件难以实现强化加工。激光冲击强化原理是利用高能脉冲激光束辐照金属表面,金属表面涂覆的吸收层快速吸收激光能量,发生等离子体气化,形成的等离子体团进一步吸收激光能量,形成更大压强的等离子团,在约束层的作用下发生爆炸,生成等离子体冲击波,使金属表面发生塑性变形,引起晶格位错、晶粒细化等,在表面产生残余压应力,从而实现金属的表面强化[30],其工作原理示意图如图1c 所示。与喷丸和滚压技术相比,激光冲击强化技术具有以下几点独特的优势:① 激光光斑大小和作用点精确可控,加工复杂结构零件精度更高;② 激光冲击强化诱导金属表面塑性变形,对金属表面的粗糙度影响较小,对加工薄壁件更有优势;③ 激光技术是一种新型环境友好型技术,在加工过程中对环境无污染;④ 激光冲击强化可以引入更深硬化层及更大幅值的残余压应力[31]。
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图1 几种典型表面形变强化技术示意图
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Fig.1 Schematic diagram of some surface deformation strengthening technology
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3 激光冲击强化技术的特点
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3.1 激光冲击强化技术分类
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图2 为激光冲击强化技术发展过程。自 20 世纪 60 年代发现脉冲激光波以来,激光冲击强化技术经过 70 年左右发展主要分为以下两个阶段。
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第一阶段为 1963—1990 年(兴起阶段)。1963 年美国电气公司 White 首次对脉冲激光在金属材料中冲击波和塑性变形进行探索[32]。直至 1972 年美国巴尔特哥伦布实验室的 FAIRAND 等[33]研究发现激光诱导冲击波可以改善 7075 铝合金屈服强度,随后 FAIRAND 等[34]于 1976 年建立了约束层和吸收层的典型强化模型,基于此模型激光冲击强化技术在不同领域得到应用。第二阶段为 1990 至今(快速发展阶段),在此阶段,针对工艺优化和增强强化效果的研究得到国内外学者的深入讨论。传统的激光冲击强化采用纳秒激光且具有吸收层和约束层,并且吸收层和约束层的种类对金属的强化效果也具有一定的影响[35-37]。
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图2 激光冲击强化技术发展
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Fig.2 Development of laser shock peening technology
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1997 年日本科学家 SANO 等[38]针对水下工作环境材料的表面应力状态进行改善,首次提出去除吸收层的无涂层激光冲击强化技术(Laser shock peening without coating,LSPwC)。无涂层激光冲击强化技术是激光能量较小,且在加工过程中仅保留约束层的激光冲击强化技术。2009 年日本科学家 NAKANO 等[39]首次提出将激光直接引入材料表面的飞秒激光冲击强化技术(Femtosecond laser shock peening,Fs-LSP)。飞秒激光冲击强化是激光脉宽达到飞秒级,且在加工过程中可去除约束层和吸收层的激光冲击强化技术。由于其脉宽极短,可达飞秒级,可以直接照射在金属材料表面,诱导冲击波进入金属内部,并且产生的热对金属影响极小,金属表面所受形变也很小[40]。 2010 年华中科技大学叶畅教授团队于美国[41]首次提出热辅助激光冲击强化技术(Warm laser shock peening,WLSP)。热辅助激光冲击强化技术是在传统技术基础上增加一个温度控制系统,可以实现金属在高温下接受激光冲击强化。在高温作用下,金属产生高密度位错、均匀位错排列和高密度纳米级析出相。高温可以促进金属的动态应变时效,进而提高金属微观结构的稳定性,从而提高循环加载过程中构件表面压残余应力和表面硬度的稳定性[42-43]。次年该团队提出深冷辅助激光冲击强化(Cryogenic laser shock peening,CLSP)。采用液氮对金属进行深冷处理,使金属处于深冷环境,然后进行激光冲击强化[44]。在深冷温度的诱导下,金属的弹性模量提高,在激光能量不变的情况下,塑性应变及塑性变形层深度随着材料弹性模量和动态屈服强度的提高而有所下降,而深冷激光激光冲击所引起的塑性应变饱和值大于传统激光冲击,因此深冷温度下激光喷丸诱导的残余压应力幅值更高[45-46]。2019 年美国科学家 ZHANG 等[47] 提出脉冲电流辅助激光冲击强化技术(Electric pulse-assisted laser shock peening,EP-LSP)。其原理是将金属通电,给金属施加脉冲电流,同时进行激光强化。在电流的作用下,电流焦耳热效应会使金属的流变应力下降,再加上激光冲击强化的作用,金属表面的残余应力得到重新分布,硬化层深度更深,表面硬度更高[48]。图3 为不同类型激光冲击强化技术示意图。随着科技发展,以上技术在航天、核电及船舶领域也开始被逐渐应用[49-50]。
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不同类型的激光冲击强化技术的优缺点也较为明显。YU 等[51]对比了传统 LSP、LSPwC 和 Fs-LSP 对 GH4169 合金表面形貌和影响层深度。图4 为传统 LSP、LSPwC 和 Fs-LSP 处理后的金属表面和和截面硬度云图。由图4 可以看出,无涂层激光冲击强化合金表面烧蚀较为严重,传统激光冲击强化和飞秒激光冲击强化技术金属表面无烧蚀影响。还可以看出,传统激光冲击强化影响层深度最深,飞秒激光冲击强化影响层较浅为 100 μm 以内。金属热效应和影响层浅限制了其应用。
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图3 不同类型的激光冲击强化技术
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Fig.3 Varied types of laser shock peening techniques
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图4 不同类型激光冲击强化后的光镜形貌和截面硬度图[51]
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Fig.4 Optical microscope morphologies and microhardness maps along depth after various LSP treatments[51]
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通过添加辅助手段对强化效果进行增强,图5 显示了不同类型激光冲击强化后的截面硬度分布,由图可以看出,辅助手段虽然各不相同,但增强效果均较为明显,硬度和影响层深度明显得到提升。热辅助激光冲击强化后金属硬度和影响层深度随着温度的升高而升高,主要是由于温度越高使得材料软化,弹性模量降低,因此硬度明显提高。同时,温度升高,材料阻尼降低,激光在材料内部延深度进一步传播,因此硬化层深度进一步提升[52]。虽然通过添加辅助手段可以显著提升强化效果,但是以上技术也存在一定劣势。首先是设备更加复杂,成本更高。然后是约束层材料,传统激光冲击强化可采用水等作为约束层,添加辅助手段的激光强化技术主要采用不同型号的玻璃作为约束层,虽然玻璃相比于水效果更好,但是在加工复杂结构件时适用性更低。最后是吸收层材料,传统激光冲击强化可采用乙烯基胶带等作为吸收层,添加辅助手段的激光强化技术主要采用铝箔材料,加工环境限制了吸收层材质的选择。因此后续针对添加辅助手段在实际工况中的应用需进一步推广,对限制手段进一步改进,推进激光强化技术在极端环境工况下的应用。
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3.2 激光冲击强化机制
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激光冲击强化技术虽然得到广泛的关注和研究,但其强化原理都是借助激光诱导金属表面发生塑性变形,引入残余压应力,进而提升金属性能。金属发生塑性变形的形式由晶粒细化机制主要分为位错滑移和形变孪生,不同的金属晶粒细化机制也不同。有研究表明,晶粒细化机制主要与金属的层错能有关。低层错能金属晶粒细化机制以形变孪晶为主。在激光冲击强化过程中,低层错能金属晶粒细化过程主要分为以下几个部分。图6a 为激光冲击强化低层错能金属晶粒细化形成示意图[54]。首先,激光作用在金属表面,位错带之间发生剧烈的相互作用,产生应力集中区域,促进表面产生形变孪晶;其次,随着金属的进一步塑性变形,位错密度进一步增加,位错线和位错胞开始形成;再次,随着金属塑性变形的进一步加剧,位错密度在达到一定值后,位错开始在形变孪晶内部重新排列,形变孪晶转变为二次孪晶;最后,当累积到一定程度的塑性变形后,孪晶之间开始交错,最终将较大的晶粒分化为细小的均匀晶粒。高层错能金属晶粒细化机制以位错滑移为主。图6b 为激光冲击强化高层错能金属晶粒细化形成示意图[55]。首先,当激光作用在金属表面,金属开始发生塑性变形并产生大量位错。然后,随着塑性变形的进一步增加,位错堆积形成位错壁;其次,当塑性变形到达一定程度时,位错开始湮灭和重新排列,形成亚晶界。最后,形成清晰地多边形亚晶结构。
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图5 不同类型激光冲击强化后的材料截面硬度分布
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Fig.5 Microhardness maps along depth after various LSP treatments
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图6 激光冲击强化诱导金属晶粒细化形成示意图
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Fig.6 Schematic diagram of laser shock peening induced metal grain refinement process
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4 激光冲击强化技术在核电领域的应用
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核电作为清洁高效低碳能源对优化我国能源结构、保障能源安全具有重要意义。随着激光冲击强化技术的不断发展及研究学者的深入研究,该技术不仅在航空航天领域应用广泛,针对极端苛刻环境下的核领域用各种奥氏体不锈钢、镍基合金等材料,也进行了大量强化研究。在提高核领域材料的硬度和残余应力、摩擦磨损性能以及耐腐蚀性能方面国内外学者进行了大量研究。图7 显示了核电装备领域核心部件及其失效形式。传热发生器和蒸汽发生器是核电装备核心部件,其受力和失效形式复杂多样,采用激光冲击强化技术对其进行强化具有重要意义。除传热发生器和蒸汽发生器外,还有蒸汽涡轮叶片及一回路水中合金及其焊缝等核心零部件在工作过程中的失效,也可采用激光冲击强化技术提高其服役寿命,保障核电安全。
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图7 核电装备领域核心部件及其失效形式(以压水堆为例)
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Fig.7 Core components and their failure modes in nuclear power equipment field (pressurized water reactor)
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4.1 力学性能
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由于激光诱导冲击波会导致金属塑性变形,晶粒细化,并引入表面残余压应力,因此金属材料表面和近表面的硬度和残余应力提升较为显著。在核领域中,材料的硬度和残余应力具有重要意义。YAN 等[56]针对核极高温反应度氧化物常用材料 304 奥氏体不锈钢进行了激光冲击强化,并研究了冲击次数对 304 奥氏体不锈钢的硬度和残余应力影响,研究发现,激光冲击强化可以有效提高 OSD 304 钢的残余压应力和硬度,并随着冲击次数的增加表面残余应力由–29 MPa 提升至–369 MPa,表面纳米硬度由 4.1 GPa 提升至 4.6 GPa,提升 12%。同时,ZHOU 等[57]研究发现同样的影响规律,随着冲击次数的增加,硬度和残余应力均有改善,如图8a 和 8b 所示。硬度和残余应力的提升主要归因于金属经过激光冲击,表面发生塑性变形,生成大量位错,随着冲击次数的增加,位错逐渐累积,因而产生机械孪晶,晶粒得到细化,如图8c 所示。
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然后针对激光冲击强化次数影响硬度和残余应力的变化规律,也有研究发现不同的变化规律。 WANG 等[58]通过研究不同激光冲击强化次数增强 ANSI 316L 不锈钢的硬度和残余应力发现,经过激光冲击强化后的 ANSI 316L 不锈钢表面显微硬度和残余应力获得明显改善,随着冲击次数的增加硬度提高了约 28%,但残余应力在一次激光冲击强化后约为–197MPa。两次冲击和三次冲击分别下降了 13.7%和 16.4%。图9 为激光冲击强化后 ANSI 316L 不锈钢截面 EBSD IPF / IQ 图。由图9 可以看出,随着激光冲击强化次数的增加,ANSI 316L 不锈钢表面没有发生明显的晶粒细化,硬度的提高是因为激光冲击诱导表面产生的位错。残余压应力的降低的原因包括以下方面:① 当冲击次数增加时,位错密度由于位错的湮灭和重排而降低,导致晶格畸变减小,在此过程中,一些残余压应力被释放。② 通过增加冲击次数,由于加工硬化,残余应力饱和发生在材料表面。③ 在一次激光冲击强化过程中,侧向变形受周围环境的弹性约束。因此,当激光与材料停止相互作用时,变形体积恢复到原来的尺寸,就会产生一个双轴压缩应力场。当进行多次激光冲击强化时,表面物质有一定的塑性流动。材料的塑性流动会在表面产生一些拉性残余应力。压应力和拉应力的共同作用使之前形成的表面压残余应力减小。 KALAINATHAN 等[59]采用无涂层激光冲击强化技术对 ANSI 316L 进行强化处理后也观察到类似的残余应力演变规律。综上所述,在奥氏体不锈钢上进行多次冲击时,表层硬度和残余应力的演化非常复杂,因此在此后研究过程中需通过有限元仿真对激光冲击强化过程中金属表层硬度和残余应力的演变规律。
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图8 激光冲击强化 304 不锈钢表层残余压应力、硬度和微观组织的演变[57]
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Fig.8 Evolution of residual compressive stress, hardness, and microhardness in surface layer of 304 SS after LSP[57]
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图9 不同强化次数后 316L 不锈钢截面 EBSD IPE / IQ 图[58]
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Fig.9 Cross-sectional EBSD IPE / IQ diagram of ANSI 316L stainless steel after varied laser shock peening cycles[58]
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除以上介绍的不锈钢材料,各国学者针对激光冲击强化核电领域其他材料的硬度和残余应力也有大量研究,如表1 所示。经过强化后材料的力学性能得到明显改善。综上所述,采用激光冲击强化技术对核电领域材料的硬度和残余应力均有不同程度的改善,表明了激光冲击强化技术在核电领域增强材料硬度和残余应力具有重要作用。但通过改变冲击次数对金属材料硬度和残余应力的研究仍需进行深入研究与讨论。
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4.2 耐摩擦磨损性能
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在核电领域中,多种金属部件的功能反复滑动和冲击,导致零件出现磨损现象,由磨损产生的减薄会导致零件失效,严重时会引发核泄漏等安全问题。激光冲击强化后金属发生晶粒细化对金属耐磨性可以有效改善,因此 PRAVEENKUMAR 等[68]采用无涂层激光冲击强化技术针对核电领域常用 304 奥氏体不锈钢进行处理,并对其耐磨性进行研究与分析。研究发现,强化后的不锈钢随着冲击强化次数由 1 次(PP1)的增加至 5 次(PP5),表面残余压应力最大为–380 MPa 和–581 MPa,硬度提高了 31%和 71%。通过对其在不同法向载荷(5、10 和 20 N)磨损性能进行研究发现,随着激光冲击强化次数的增加平均摩擦因数和磨损量均呈现减小趋势,如图10a 所示表明激光冲击强化可显著提高材料耐磨性能,对图10b 分析可以发现,随着激光冲击强化次数的增加,材料晶粒得到细化。在晶粒细化、硬度和残余应力提升的共同作用下,材料耐磨性能得到改善。
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图10 304 不锈钢激光冲击强化层摩擦磨损行为及 IPF[68]
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Fig.10 Tribology behavior and IPF of 304 stainless steel treated by LSP[68]
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在核电领域中常用的Inconel718合金具有良好的强度,在高温下具有优良的抗蠕变性能和优越的抗氧化性,然而其耐磨性较差,因此 KUMAR 等[69] 采用激光冲击强化技术针对其耐磨减摩性的影响进行研究,并研究不同对磨副对合金的磨损影响。图11 为不同法向载荷下对磨副为钢和氧化铝球与合金摩擦 1 万次循环后对应的摩擦力与位移循环曲线以及磨损体积,由图可以看出在随着载荷的增加合金和对磨副(氧化铝和钢)的接触状态由完全滑移向部分滑移转变。与氧化铝球对磨副相比,钢对磨副测试的试样表现出较低的切向力,并且可以看出强化后的合金磨损量明显低于强化前合金。激光冲击强化使合金表面形成纳米晶,晶粒细化,表面硬度增加,残余压应力增加。激光冲击强化后合金由于硬度较高,附着力较小,因此激光冲击强化后合金的切向力系数小于未强化合金。与对磨副为钢球相比,氧化铝球的微动磨损试验切向力系数更高。这是由于 Inconel718 合金与氧化铝基体之间存在较高的赫兹接触应力和摩擦化学反应。由于激光喷丸试样具有较高的表面硬度、较低的 TFC 和较高的残余压应力,因此激光喷丸试的微动磨损性能优于未喷丸试样。
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图11 激光冲击强化 Inconel718 合金微动磨损性能[69]
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Fig.11 LSP of Inconel718 alloy for fretting wear properties[69]
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核电领域材料磨损失效是较为严重的核安全问题,针对核电领域材料的微动磨损,通过激光诱导材料表面残余压应力和硬度的提高,且表面晶粒细化共同作用,可以有效改善其耐磨性。但目前采用激光冲击强化针对核电领域材料的磨损研究相对较少,因此针对核电领域其他材料磨损问题仍需进行大量研究,为激光冲击强化技术在提高核电领域材料耐磨性提供理论基础。
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4.3 耐腐蚀性能
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在核电领域中,因其复杂的外部环境,材料的腐蚀是造成材料失效的又一重要原因[70]。Inconel600 合金作为蒸发器传热管常用材料,其应力腐蚀开裂和电化学腐蚀是其腐蚀失效主要形式[71-72]。学者们针对 Inconel600 合金的应力腐蚀失效进行了大量的机理探讨,发现材料发生应力腐蚀失效的机理主要是晶间腐蚀开裂[73]。为了延长合金在核电站的使用寿命,采用激光冲击对合金进行冲击强化,引入残余压应力,提高 Inconel600 合金的抗应力腐蚀能力受到广泛研究。KARTHIK等[72]针对Inconel600 合金的抗一次水应力腐蚀电化学性进行了研究,图12a 为 600 合金在 0.6M NaCl 溶液中的极化曲线。由图12b 可以看出,采用无涂层激光冲击强化技术对合金进行强化,其工作电位向正电压移动,腐蚀电流密度仅为 4 nA·cm−2,远小于未处理合金的腐蚀电流密度(2.6 mA·cm−2)。除了一次水环境下的电化学腐蚀,TELANG 等[74]研究了激光冲击强化后合金在四硫酸盐环境中的抗应力腐蚀行为。研究发现,采用激光冲击技术在材料表面引入约为 −500 MPa 的残余应力,影响层深度约为 0.4 mm。采用恒定载荷试验对激光冲击强化增强合金应力腐蚀开裂性能进行研究,图12b 为激光冲击强化前后合金在四硫酸盐溶液中应力腐蚀开裂性能(pH3 代表酸化的四硫酸盐溶液),可以看出强化后合金在相同载荷下失效时间显著增加。这是由于激光冲击强化引入了更大的残余压应力,残余压应力会提高合金的屈服应力和极限抗拉强度,从而抑制样品的敏化作用,提高样品的抗应力腐蚀性能。
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除了高温镍基合金外,LU 等[75]针对核电领域用不锈钢抗应力腐蚀性能进行激光冲击强化研究,研究了三种不同类型 U 型材料的抗应力腐蚀性能,第一种样品是不进行激光冲击强化,第二种是先对板材进行激光冲击再进行弯曲,第三种是先将板材弯曲,然后进行激光冲击。研究发现,前两种样品在 142±1℃的 MgCl2 溶液中平均开裂时间分别为 16.06 h、110.43 h,第三种样品在经过 300 h 以上的抗应力腐蚀试验并未发现明显的应力腐蚀裂纹,加工示意图和裂纹形貌如图13a 所示。通过对裂纹形貌分析可以发现,第一种样品表面出现了大量长度达数百微米的裂纹,第二种样品仅有 1 个向侧翼分支的裂纹,第三种样品无明显裂纹,说明激光冲击强化可以有效提升材料的抗应力腐蚀性能。强化机制如图13b 所示,未经强化的样品,材料表面应力呈现拉应力状态,拉应力的纯在导致裂纹萌生快且明显,第二种样品材料表面应力虽然同样呈现拉应力状态,但经过激光冲击强化后材料表层晶粒得到细化,晶粒细化可以提升材料抗应力腐蚀性能,在这种竞争机制的引导下,裂纹萌生但不明显。第三种样品表面应力呈现压应力状态,且表层晶粒得到细化,两种机制共同作用下材料抗应力腐蚀性能得到显著提升,且无明显裂纹产生。针对 304 不锈钢合金的电化学腐蚀,WEI 等[76]采用电化学阻抗谱研究了激光冲击处理对 AISI 304 不锈钢在氯酸溶液中的腐蚀行为的影响。结果表明,在两种氯酸溶液中,单次激光冲击强化处理的样品的电荷转移电阻,始终高于两次 LSP 处理的样品和未处理的样品,这意味着由于形成了致密的钝化膜,缺陷较少,因此单次激光冲击强化具有更高的耐蚀性。
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表2 列示了激光冲击强化技术对不同核材料抗腐蚀性能影响。通过以上总结发现,激光冲击强化可以有效提高核电领域材料的抗应力腐蚀,是由于激光冲击强化抑制了材料的吸氢能力,引入较大的残余压应力,改善了材料抗应力腐蚀能力;激光冲击强化技术也可以有效提高材料耐电化学腐蚀性能,因强化后在材料表面更易形成钝化膜,改善基体和电解液之间的接触方式,进而提升材料的耐电化学腐蚀性能。
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图13 激光冲击强化提高 304 奥氏体不锈钢应力腐蚀性能[75]
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Fig.13 Effects of LSP on stress corrosion cracking (SCC) of ANSI 304 austenitic stainless steel[75]
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为了将激光冲击强化技术应用于核电装备,日本科学家 SANO 等[80]设计了如图14 所示的沸水堆堆芯罩水下激光强化系统示意图,实现了脉冲能量达 100 mJ、脉宽 5 ns 的稳定光纤传输,并且成功在运行的核电站完成了直径 9 mm 管内表面的强化。同时发明设计了图15 所示多功能光学头,可以实现激光超声检测、激光重熔和激光冲击的协同完成。
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图14 沸水堆堆芯罩水下激光强化系统结构示意图[80]
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Fig.14 Configuration of underwater laser peening system for a core shroud in a boiling water reactor[80]
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图15 具有焊接、喷丸、检测功能多功能光学头[80]
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Fig.15 Multi-purpose optical head with welding, peening and inspection functions[80]
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核电装备服役环境异常复杂和苛刻,很多装备备本身或其运行环境具有放射性,同时还兼具水下、高温、高压、强辐照等特点,摩擦磨损、腐蚀、材料可能是同时发生[81-85],激光冲击强化对核电装备领域很多材料的力学性能、耐磨性能和耐腐蚀性明显改善。目前由于国内外激光冲击强化技术多受限制,多处于实验室研究阶段。因此首先针对复杂工况下的研究进行试验研究,在工艺成熟后,在实际应用中进行针对性使用,针对不同的核电装备领域零部件选择合适的激光冲击强化技术进行针对性性能提升。
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5 结论与展望
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系统介绍不同形变强化技术的强化原理,重点阐述了激光冲击强化技术的分类和强化机制,并综述该技术在核电领域材料的研究与应用进展。得到以下结论:
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(1)激光冲击强化技术相较于喷丸和滚压具有加工精度高、对零件表面损伤小、无污染且影响层更深的优势。
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(2)激光冲击强化技术主要发展为工艺简化和增强效果两个方向。强化机制主要和材料的层错能有关。低层错能主要为形变孪晶,高层错能主要为位错滑移。
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(3)激光冲击强化技术在提升核电领域材料性能的主要原因一方面为在材料表面生成较大的残余压应力,另一方面为形成大量位错或产生晶粒细化现象,并且该技术在核电领域关键零部件进行开发和应用。
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激光冲击强化技术可改善核电领域材料各项性能。但目前针对该技术在核电领域的研究和应用仍未完善,需进一步在以下方面深入研究:
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(1)传统激光冲击强化和添加辅助手段激光冲击强化技术受约束层和吸收层的影响,限制以上技术在核电装备领域的实际应用。
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(2)无涂层激光冲击强化技术对材料表面有热影响,材料表面的质量对材料寿命有较大影响。飞秒激光冲击强化技术影响层浅,强化效果较弱。如何改善无涂层和飞秒激光冲击强化技术的劣势需进一步深入研究。
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(3)目前传统激光冲击强化技术在核电领域的应用研究较多,其他激光冲击强化技术主要针对核电领域材料的硬度和残余应力改善研究,需进一步针对其他激光强化技术对核电领域材料耐磨和耐腐蚀性能进行深入研究,并结合有限元模拟进一步验证激光冲击强化过程和性能提升机制。
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摘要
激光冲击强化技术属于改善金属性能的重要表面形变强化技术,因其独特的技术优势在航天及船舶领域获得广泛应用。随着科技的发展研发出多种激光冲击强化技术,并逐渐开始在核电装备领域获得应用,然而针对该技术在核电领域的研究进展缺乏系统的综述。先通过介绍不同表面形变强化技术,叙述激光冲击强化技术的发展,阐述激光冲击强化机制,最后综述激光冲击强化技术在核电领域的应用研究进展。总结发现,激光冲击强化技术可有效改善核电领域材料力学、摩擦磨损及腐蚀性能,但传统和添加辅助手段激光冲击强化技术受约束层和吸收层影响较大,无涂激光冲击强化技术对金属易产生热效应,飞秒激光冲击强化影响层浅且强化效果差,不同工艺技术在核电领域提升摩擦磨损性能研究较少。对不同工艺激光冲击强化机理及在核电领域材料不同性能的提升进行深入研究,为进一步提升激光冲击强化技术在核电领域材料的应用提供理论基础,可为核电领域关键装备进行强化、提升核电装备运行寿命提供参考。
Abstract
Nuclear power is an effective method of generating electricity to address energy shortages and environmental degradation. However, nuclear power safety has been a lifeline for the development of nuclear energy. Materials in nuclear power fields remain in extreme environments and operate under complex stress states. Surface-strengthening technology is currently an important means of enhancing the life of nuclear power equipment materials. Among them, surface deformation strengthening technology is considered one of the most ideal strengthening methods owing to its advantages of not introducing new materials and the high bonding strength of the membrane base. The most widely used surface deformation strengthening technologies with high maturity include shot peening, rolling, and laser shock peening. Laser shock peening has the advantages of a precise and controllable process, a small effect on the metal surface roughness, no pollution, and a deeper impact layer. To reduce the limitations of its applications, laser shock peening has been developed without coating and femtosecond laser shock peening by simplifying the process. In comparison, the impact layer obtained by conventional laser shock peening was deeper, but the surface roughness was the highest. Laser shock peening without coating has a relatively small effect on the surface roughness of the metal but produces thermal effects, forming holes and cracks. Femtosecond laser shock peening has the smallest effect on the surface roughness but has the shallowest impact layer. Warm laser shock peening, cryogenic laser shock, and electric pulse-assisted laser shock peening have been developed to improve this strengthening effect. Through this review, it was deduced that auxiliary means to enhance the strengthening effect are mainly used in traditional laser shock peening. There has been no targeted research on improving the effect of laser shock peening without coating or femtosecond laser shock peening. The mechanism of laser shock peening is mainly related to the stacking fault energy of the material. The low stacking fault energy metal grain refinement mechanism is dominated by deformation twinning. The high stacking fault energy metal grain refinement mechanism was dominated by the dislocation slip. Currently, the application of nuclear power fields is primarily for basic research on nuclear power materials. Two different strengthening mechanisms are used to improve the mechanical properties of materials commonly used in nuclear power generation. One is the enhancement of the material hardness caused by grain refinement. The other is not grain refinement but a large number of dislocations owing to the material hardness enhancement. The improvement in the wear and corrosion resistance of materials commonly used in nuclear power generation is mainly due to grain refinement and the generation of large residual compressive stresses on the material surface. Through this review, it was deduced that the current research mainly focuses on the in-depth theoretical study of materials and less on the application of practical components. The effects of different processes of laser shock peening on the material performance improvement mechanism are not clear enough, and still need to be coupled with experiments and simulations to verify and reveal the strengthening mechanism. This paper mainly reviews research progress in the field of nuclear power and provides an outlook on the future development direction of laser shock peening in the field of nuclear power to provide a solid theoretical foundation for the laser shock peening with the aim of improving the application of laser shock peening in the field of nuclear power.