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金属学报, 2023, 59(9): 1173-1189 DOI: 10.11900/0412.1961.2023.00134

综述

镍基高温合金表面冲击强化机制及应用研究进展

王磊,1, 刘梦雅1, 刘杨,1, 宋秀1, 孟凡强2

1东北大学 材料各向异性与织构教育部重点实验室 沈阳 110819

2中山大学 中法核工程与技术学院 珠海 519000

Research Progress on Surface Impact Strengthening Mechanisms and Application of Nickel-Based Superalloys

WANG Lei,1, LIU Mengya1, LIU Yang,1, SONG Xiu1, MENG Fanqiang2

1Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang 110819, China

2Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-Sen University, Zhuhai 519000, China

通讯作者: 王 磊，wanglei@mail.neu.edu.cn，主要从事高温合金微观组织控制与强韧化研究；刘 杨，liuyang@mail.neu.edu.cn，主要从事高温合金强韧化与近服役力学行为研究

责任编辑: 毕淑娟

收稿日期: 2023-03-29   修回日期: 2023-05-05  

基金资助: 国家重点研发计划项目(2022YFB3705102)
国家重点研发计划项目(2022YFB3705101)
国家科技重大专项项目(J2019-VI-0020-0136)
国家自然科学基金项目(U1708253)
国家自然科学基金项目(51571052)

Corresponding authors: WANG Lei, professor, Tel:(024)83681685, E-mail:wanglei@mail.neu.edu.cn;LIU Yang, professor, Tel:(024)83672799, E-mail:liuyang@mail.neu.edu.cn

Received: 2023-03-29   Revised: 2023-05-05  

Fund supported: National Key Research and Development Program of China(2022YFB3705102)
National Key Research and Development Program of China(2022YFB3705101)
National Science and Technology Major Project of China(J2019-VI-0020-0136)
National Natural Science Foundation of China(U1708253)
National Natural Science Foundation of China(51571052)

作者简介 About authors

王 磊，男，1961年生，教授，博士

摘要

为满足不断攀升的两机涡轮动力系统的快速发展，表面冲击强化技术在涡轮转子用高温合金表面强化的应用及相应机制的研究受到了广泛关注。然而，高温合金表面硬化层在高温服役环境下的回复、再结晶行为难以避免，由此引起的表面强韧化、抗疲劳效果的退化，成为制约表面冲击强化技术在先进高温合金关键部件深入应用的瓶颈。本文总结了近年来镍基高温合金表面冲击强化机制及应用研究进展，分析了表面冲击强化对镍基高温合金表面强韧性及抗疲劳的作用规律，探究了高温合金表面冲击硬化层在高温及长期时效过程中的显微组织、微结构演化及其对高温稳定性的作用机理。以期为发展镍基高温合金表面冲击强化、提高两机涡轮转子疲劳抗力提供基础。

关键词： 镍基高温合金; 表面强化处理; 抗疲劳制造; 硬化层; 组织和性能高温稳定性

Abstract

There has been rapid development in the turbine power systems of aeroengines and gas turbines. Consequently, the application of surface impact strengthening technology for the surface strengthening of superalloys used in turbine rotors and its corresponding mechanisms have attracted wide attention. However, it is difficult to prevent the recovery and recrystallization of the surface hardened layer of superalloys serviced at high temperatures. This leads to the degradation of both the surface strengthening/toughening and fatigue resistance. This is the main hurdle restricting the wide application of surface impact strengthening technology for key components of advanced superalloys. In this paper, the progress made in surface impact strengthening mechanisms and the applications of nickel-based superalloys in recent years are summarized. The effect of surface impact strengthening on the surface strength, toughness, and fatigue resistance of nickel-based superalloys is analyzed. The evolution of the microstructure of the hardened surface of the alloys during long-term aging at high temperatures, and its effect on high-temperature stability are explored. The paper aims to provide essential and important information for developing surface impact strengthening mechanisms of nickel-based superalloys and improving the fatigue resistance of turbine rotors of aeroengines and gas turbines.

Keywords： nickel-based superalloy; surface strengthening technology; anti-fatigue manufacturing; hardened layer; high temperature stability of microstructure and property

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本文引用格式

王磊, 刘梦雅, 刘杨, 宋秀, 孟凡强. 镍基高温合金表面冲击强化机制及应用研究进展[J]. 金属学报, 2023, 59(9): 1173-1189 DOI:10.11900/0412.1961.2023.00134

WANG Lei, LIU Mengya, LIU Yang, SONG Xiu, MENG Fanqiang. Research Progress on Surface Impact Strengthening Mechanisms and Application of Nickel-Based Superalloys[J]. Acta Metallurgica Sinica, 2023, 59(9): 1173-1189 DOI:10.11900/0412.1961.2023.00134

高温合金具有优异的高温强韧性、良好的抗氧化和抗热腐蚀性能、显著的抗蠕变与抗疲劳性能以及优良的组织结构高温稳定性，被广泛应用于航空航天、石油化工等领域关键热端部件的制造[1~4]。其中涡轮动力系统关键热端部件(涡轮盘、涡轮叶片等)被誉为涡轮发动机的心脏，需长时间在高温、热腐蚀环境服役。因此，提高涡轮转子关键热端部件用高温合金的高温性能，始终是涡轮动力系统能效跃升、服役安全的核心保障[5,6]。随着高端航空发动机、重型燃气轮机研制技术的飞速发展，高品质涡轮转子用高温合金，更需在长期高温、疲劳环境下面临“高温强韧化”和“性能稳定性”的双重考验[7]。而强韧化设计与微结构热力学往往相互制约[8~10]，成为先进涡轮系统研发的瓶颈之一[11]。

近年来，高能喷丸(shot peening，SP)、表面高速机械挤压(high-speed surface extrusion，HSE)、超声表面滚压(ultrasonic surface rolling process，USRP)和激光冲击(laser shocking processing，LSP)等表面冲击强化技术在两机涡轮转子抗疲劳制造领域的应用研究广泛深入地展开，显现出积极的效果和广阔的前景[12~14]。表面冲击强化通过在金属表层引入可观的塑性变形和残余压应力，诱导高密度位错、孪晶等表面微结构的改变[15~17]，从而获得具有显著综合强化效果的梯度硬化层，有望成为高强金属材料有效的表面抗疲劳制造技术[16~21]。

然而，表面冲击强化在金属表面获得的高残余压应力硬化层[15]，仍以垂直表面向内的冲击形变强化为主[12,15,17]。硬化层在剧烈塑性变形(severe plastic deformation，SPD)时产生的高密度位错、晶格畸变和亚稳界面，使表层获得可观残余压应力、显著强化效果的同时，也在表面引入了缺陷密度梯度分布的微结构亚稳层[18]。而对高温合金而言，600℃甚至以上的高温条件下，硬化层中的亚稳微结构易发生回复和退化，导致残余压应力的松弛甚至消失。这直接影响高温合金表面冲击硬化层高温强化作用的稳定性，也一直是制约表面强化技术在高温合金涡轮转子表面强韧化、抗疲劳制造中深入应用的瓶颈性问题[12,13]，长期受到设计、制造和研究者的广泛密切关注。

迄今为止，关于高温合金表面冲击强化的诸多实际应用和研究报道中，表面硬化层高密度亚结构的回复推迟、残余压应力可保留的机理尚不明确。本文归纳了镍基高温合金表面冲击强化技术及应用的研究进展，在分析不同表面冲击强化技术提升镍基高温合金表面强韧性及抗疲劳作用规律的基础上，结合近年来本课题组的研究结果，探究了高温合金表面硬化层在高温下的显微组织、微结构演化机理。分析高温下维持表面强化作用的关键机制，展望镍基高温合金新型表面冲击强化技术和抗疲劳调控机制的未来发展方向，以期为具有更高承温能力的镍基高温合金关键涡轮转子强韧化与抗疲劳制造提供思路和参考。

1 高温合金的表面喷丸处理及喷丸强化

涡轮动力系统的高温合金关键热端部件，装机前需经多道次复杂机械加工，构件表面完整性易被破坏，表面粗糙度增大。加之机械加工难以避免在表层引起残余拉应力等，从而劣化了高温合金的高温疲劳性能[22]。尤其对于涡轮转子，其在长期服役过程中的高温抗疲劳性能，更是动力系统服役安全的关键[23,24]。因此，诸多金属材料的表面强化技术在高温合金转子构件的抗疲劳制造领域倍受关注[23~25]。表面喷丸处理，作为便于实施、低成本和冲击能量输入可准确调控的表面处理技术，较早被用于高温合金的表面强化[26,27]。喷丸处理采用硬质钢丸、铸铁丸、陶瓷丸等轰击工件表面，在表层引入残余压应力，可有效抵消机械加工的表面残余拉应力，提高材料的抗疲劳能力[28~31]。喷丸处理对高温合金构件的影响主要有：硬化层残余应力分布状态、表面形态和疲劳裂纹萌生阻力等[13,28]。

1.1 高温合金喷丸处理后的表面应力状态及分布

高温合金表面喷丸强化的研究[29~31]表明，喷丸处理可明显改变机械加工的残留表面拉应力，在合金表面引入了较为理想的残余压应力层，这对高温疲劳抗力要求很高的高温合金涡轮转子，无疑具有高的实用价值。钟丽琼等[28]的研究发现，对FGH97合金进行陶瓷弹丸90°喷射4 min处理后，次表层(距表面几十微米)处获得了较高的残余压应力，应力场深度可达280 μm左右。Wang等[30]对FGH96合金进行了车削、陶瓷喷丸和复合喷丸(高强铸铁弹丸和低强陶瓷弹丸)残余压应力场的对比研究，发现喷丸处理明显增加了表层残余压应力，并且应力最高值、深度均随喷丸强度的增加而增大(图1[30])。在铸造高温合金、变形高温合金的相关研究中也获得了相似的效果。K4169铸造高温合金喷丸处理后表面残余压应力由258 MPa增加至1079 MPa，进而显著提高了合金的疲劳寿命[32]。

图1

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图1   喷丸处理对FGH96合金表面硬化层残余压应力分布的影响[30]

Fig.1   Effects of shot peening on the residual compress-ive stress distribution of surface hardened layer of FGH96 alloy[30]

(a) before shot peening (turning)

(b) ceramic bead peening and compound shot peening (SP1—parallel to the tool mark, SP2—perpendicular to the tool mark)


1.2 高温合金喷丸处理后的表面硬化层组织

喷丸处理过程中，高温合金表面产生了理想残余压应力的同时，显微组织亦发生了显著变化。Zhao等[33]对GH4169合金喷丸强化的研究发现，经过喷丸强化后合金表面形成厚度约96 μm的硬化层，而且表面形成平均尺寸约为9 nm的等轴纳米晶(图2[33])。在一定深度范围内还形成高密度形变孪晶，进一步细化了硬化层合金的显微组织，提高了合金的高温疲劳抗力。在IN718合金的研究[34]中发现，超声喷丸处理也可获得厚度约为90 μm、平均晶粒尺寸约为12 nm的表面纳米晶层，有效地提高了合金的高温疲劳抗力。喷丸处理中，高温合金表面产生显著塑性变形，必然引起位错密度增加，并在晶界处形成大量的位错塞积与缠结，这是喷丸处理获得表面硬化层的根本原因。

图2

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图2   GH4169合金喷丸处理获得的表面硬化层截面梯度分布的纳米晶和形变孪晶层[33]

Fig.2   Nanocrystalline and deformation twins layer with gradient distribution in the cross-sectional surface hardened layer obtained by shot peening of GH4169 alloy[33]


同时发现，喷丸处理硬化层的硬化程度呈梯度分布、总深度有限。Dong等[35]对Udimet 720Li合金喷丸硬化层的研究发现，喷丸后合金表面显微硬度由440 HV增至590 HV，但在距离表面50 μm处即开始迅速下降。在铸造高温合金的喷丸处理研究中也发现类似规律，喷丸处理后的K4169[32]、K403[36]铸造合金表面形成深度为100~140 μm的表面硬化层，表层均具有较好的硬化效果。

1.3 高温合金喷丸处理后的表面粗糙度

虽然喷丸处理可在高温合金表面形成具有显著残余压应力的硬化层，但其对构件机械加工表面粗糙度的影响规律并不一致，改善作用有限[30,32,36]。大部分关于高温合金表面喷丸处理的研究[13,36]均表明，表面粗糙度随喷丸强度的升高而增大。图3[13]为GH4169合金经不同强度喷丸处理后的表面形貌。可见，随喷丸强度的增加，材料表面粗糙度明显增大。而Wang等[30]的研究表明，喷丸处理后，FGH96合金表面的车削加工痕迹可完全(或大部分)被弹丸冲击弹坑所覆盖，但喷丸后表面粗糙度却有明显升高之趋势。Shen等[36]对K403铸造高温合金进行超声喷丸处理，发现超声喷丸对合金的表面粗糙度无明显影响。

图3

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图3   喷丸处理前后GH4169合金的表面形貌[13]

Fig.3   Surface morphologies of GH4169 alloy before (a) and after shot peening with intensities of 0.15 mmA (b) and 0.3 mmA (c)[13]


1.4 高温合金表面喷丸强化的抗疲劳作用及影响因素

高温、循环载荷作用下，高温合金喷丸硬化层内的高密度缺陷和亚稳微结构，可阻碍位错运动，提高微观塑性变形抗力，增加裂纹萌生阻力，进而提高喷丸处理合金的疲劳裂纹萌生临界应力值和疲劳强度[33]。Dong等[35]的研究表明，由于喷丸处理形成的硬化层残余压应力和高密度位错缠结，表面裂纹萌生抗力增加，喷丸后合金的疲劳裂纹萌生位置从表层转移到内部(图4[35])。而在其他条件相同的情况下，疲劳裂纹由内部萌生所需的能量远高于由表面萌生的能量，因此可显著提高材料的疲劳寿命[37,38]。

图4

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图4   喷丸处理对Udimet 720Li合金相同载荷幅条件下疲劳裂纹萌生位置的影响[35]

Fig.4   Effects of shot peening on fatigue crack initiation location of Udimet 720Li alloy before (a, c) and after (b, d) shot peening under the same load amplitude[35] (Fig.4c enlarged for red frame in Fig.4a, Fig.4d enlarged for the lake blue frame in Fig.4b)


亦有研究表明，高温合金喷丸处理的抗疲劳强化作用与应力状态、应力幅值均有关。Dong等[35]对Udimet 720Li合金的研究发现，与未喷丸合金相比，喷丸处理后合金的缺口低周疲劳寿命显著提高，其在1390和1070 MPa应力幅下的增幅分别为2.05倍和6.07倍。由此可见，喷丸处理在低应力水平下对提高疲劳寿命的作用更加显著。此外，应力状态还影响喷丸硬化层的残余应力松弛程度。高玉魁等[31]对FGH97合金喷丸前后的不同应力状态疲劳寿命进行对比，发现在630 MPa的低应力状态下，喷丸硬化层的残余压应力可有效延缓表面裂纹萌生，提高合金的疲劳寿命。而在740 MPa高应力状态下硬化层的残余压应力作用减弱，抗疲劳作用并不明显。Luo和Bowen[38]对Udimet 720Li合金的研究也证实了600℃高温条件下，在较低的应力幅范围内，喷丸处理可提高合金的疲劳寿命；而当处于高应力幅时，喷丸处理反而降低了合金的疲劳寿命。

同时应注意到，高温合金喷丸处理的抗疲劳作用亦与喷丸强度有关。过高的喷丸强度反而会降低抗疲劳的功效[28]。主要是由于过高强度的喷丸处理，会在材料内局部引入残余拉应力，明显降低合金的疲劳抗力。Qin等[13]对GH4169合金进行不同强度的喷丸处理，发现局部的残余应力状态波动，增大了裂纹尖端附近的应力场起伏，导致裂纹扩展速率增加，降低了喷丸处理合金的疲劳抗力(图5[13])。

图5

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图5   不同强度的表面喷丸处理对GH4169合金疲劳应力幅-循环周次(S-N)曲线的影响[13]

Fig.5   Effects of surface shot peening with different intensities on S-N curves of GH4169 superalloy[13] (S—stress amplitude, N—number of cycle to failure)


综上，高温合金喷丸处理表面硬化层的残余压应力、强化效果和表面粗糙度，均随喷丸强度的提高而增加。其中残余压应力与表面强化可显著改善合金疲劳性能，而表面粗糙度多不利于高温合金疲劳寿命。Wu等[39]和Klotz等[34]对Inconel 718合金的研究也表明，喷丸处理后，表面残余压应力与表面粗糙度2者对抗疲劳的作用不同。在低周疲劳条件下，表面粗糙度为影响疲劳寿命的主要因素；而高周疲劳条件下，残余压应力的分布状态则为提高疲劳寿命的主要影响因素。

2 高温合金的表面机械挤压强化

高温合金的表面强化还包括复杂结构部位的机械挤压强化[40~42]。机械挤压强化方式根据强化部位形状、工艺要求不同，一般包括孔挤压强化和表面机械滚压强化。由于涡轮转子热端部件的连接孔处应力集中明显，孔周围易产生严重的疲劳损伤，对转动部件孔周围通常进行孔挤压强化。而对于平面或相对平缓曲面，多采取表面机械滚压方式进行强化处理。机械挤压处理可有效改善高温合金的表面状态，机械挤压硬化层内的残余压应力增加、组织细化和均匀化、高密度位错缠结和表面粗糙度的降低，可显著提高合金的高温强度和疲劳强度极限。

2.1 高温合金机械挤压表面粗糙度

王欣等[42]的研究表明，孔挤压处理可显著改善GH4169转动部件孔内铰削刀痕引起的粗糙度突变，挤压后孔壁达到镜面，表面粗糙度Ra由1.083 μm降至0.237 μm。罗学昆等[43]对不同过盈量孔挤压处理的IN718合金研究结果显示，孔挤压显著改善了孔壁表面完整性。低过盈量挤压的孔壁表面粗糙度Ra由原始的0.855 μm降至0.306 μm；高过盈量试样的粗糙度Ra降至0.296 μm。可见，与喷丸处理对表面粗糙度影响不同，挤压处理普遍降低高温合金的孔壁表面粗糙度，有利于降低孔壁的表面微观应力集中效应，从而使高温合金构件获得理想的表面强化效果[43,44]。

2.2 高温合金机械挤压硬化层组织及硬度

Yin等[45]对Inconel 690合金表面的超声滚压处理研究表明，经表面超声滚压后，表层塑性变形引起位错密度增加，产生加工硬化效应，硬化层硬度提高幅度达到44.45%。孙智妍等[46]研究发现，电脉冲的辅助可显著增加GH4169合金表面超声滚压硬化层深度与强化效果。而且，电脉冲辅助超声滚压(电脉冲/超声滚压耦合场强化)能够突破传统超声滚压处理的变形临界极限，获得更细的晶粒、更深的硬化层。

2.3 高温合金机械挤压硬化层的残余压应力

机械挤压强化可在高温合金表面获得比喷丸处理更显著的残余压应力层。Yin等[45]研究了表面超声滚压处理Inconel 690合金硬化层的残余压应力分布，发现经超声滚压处理后表面形成明显的残余压应力层，压应力提高幅度达到33.5%，层深约达400 μm。对孔挤压处理IN718合金的研究[43]也表明，挤压强化后甚至可在孔壁形成超过3000 μm的残余压应力层(图6[43])，这对于合金的表面强化和抗疲劳作用无疑具有积极的作用。

图6

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图6   不同孔挤压处理的IN718合金孔壁硬化层的残余压应力分布[43]

Fig.6   Residual compressive stress distributions of hard-ened layer of IN718 alloys with different hole extrusion treatments[43] (Inset shows the stress and distance direction of hole extrusion treatment)


3 激光冲击处理高温合金表面强化

激光表面冲击强化，是利用高能激光诱导的冲击波在金属表面引入高速塑性变形的强化技术，可广泛应用于高温合金、高强钢、钛合金和铝合金等关键构件的抗疲劳制造[15,16]。与传统表面强化处理方式相比，LSP处理更适合型面复杂结构、大型构件的在线处理[12]。LSP处理能够在金属表层引入可观的残余压应力和塑性变形，诱导高密度位错、孪晶等，显著改变表面微结构[17,47]，获得显著的硬化层，成为目前最有效的高强金属材料表面抗疲劳制造技术。

3.1 高温合金LSP处理的表面粗糙度

Tan等[48]对抛光的FGH96合金LSP处理的研究表明，表面激光冲击波作用后，合金抛光划痕减少，表面粗糙度略有增大，但增加幅度甚微(图7[48])。与传统的喷丸、机械挤压强化不同，由于LSP处理过程中，高能冲击波与合金表面为非直接接触，可显著减缓表面强烈塑性变形引起的粗糙度增大。

图7

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图7   FGH96高温合金经不同表面处理后的表面形貌[48]

Fig.7   Surface morphologies of FGH96 alloys with different surface treatments[48]

(a) mechanical polishing

(b) laser shocking processing (LSP)


3.2 高温合金LSP处理的硬化层组织及硬度

Rozmus-Górnikowska等[49]对Inconel 625合金LSP处理的研究发现，LSP处理后合金表面形成较大塑性变形，产生高密度位错和密集分布的滑移带，表面形成呈梯度分布的冲击硬化层。Cao等[50]关于GH4586合金LSP处理的研究表明，LSP表面硬化层的位错密度显著增加，形成高密度缠结位错、位错壁和位错胞结构，LSP处理后合金的疲劳寿命增加了近1倍。而且，LSP硬化层表面的平均晶粒尺寸降低约43.6%，同时发现基体中形变孪晶数量亦明显增多，使GH4586合金LSP硬化层表面获得了明显的协同强化作用，硬度提高幅度达到27.4%。值得关注的是，经LSP处理的GH4586合金表层γʹ相中也观察到大量的层错以及高密度位错，这在传统机械作用表面处理的高温合金硬化层中未见报道。LSP处理后，GH4586合金硬化层中不同尺度、高密度微结构的形成与细化，有效抑制了疲劳裂纹的萌生和扩展，显著提高了合金的疲劳抗力。Luo等[51]对K417合金进行LSP处理的研究发现，激光冲击引入剧烈塑性变形，在合金表面产生更高密度的位错缠结，硬化层表面形成纳米晶，使表层硬度明显增加。此外，LSP处理后进行900℃、10 h的保温，K417合金硬化层中高密度位错、晶粒细化等微结构特征并未见显著变化，强化效果表现出了较好的高温稳定性，这主要与LSP硬化层的高密度位错与纳米晶复合结构有关。

研究者们[12,13,19]对如何在高温合金表面获得高温稳定的抗疲劳硬化层进行了大量的研究，发现引入温度场的温度辅助耦合激光冲击强化技术(warm laser shocking processing，WLSP)，可获得高温强化增幅更显著的抗疲劳硬化层。本课题组[19,52]对比研究了LSP和WLSP处理IN718合金的表面硬化层微结构特征，发现WLSP作用下能够在高温合金表面获得强化相/高密度位错复合微结构(图8[52])。而且，值得关注的是，IN718合金WLSP硬化层中的γ′′相内部观察到高密度层错及纳米尺寸孪晶(图8b[52]中蓝色箭头所示)。硬化层中有序强化相内部纳米尺度微结构的形成和细化，显著提高了硬化层的强化效果和高温稳定性。

图8

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图8   IN718合金LSP、温度辅助耦合激光冲击强化技术(WLSP)硬化层中的γ″相及位错形态[52]

Fig.8   Morphologies of γ″ phase and dislocation patterns in the hardened layers of IN718 alloy treated with LSP (a) and warm laser shocking processing (WLSP) (Blue arrows show the γ″ phase/high-density dislocation complex structure containing stacking faults and nanosized twins) (b)[52]


3.3 高温合金LSP处理的硬化层残余应力状态

与传统的常规喷丸、机械挤压处理相比，LSP硬化层最大残余压应力影响深度超过了3倍，约达700 μm[48]。因此，LSP处理更深的残余压应力层可更加显著地阻碍疲劳裂纹的萌生和扩展，更有效地提高合金的疲劳强度极限、延长疲劳寿命。Pan等[53]关于带孔的ЭΠ741ΠН合金LSP处理的研究表明，LSP处理可在孔附近圆环区域引入深度约为750 μm、最大残余压应力超过650 MPa的残余压应力场。研究认为，高于传统机械表面强化处理硬化层的残余压应力场，与表层微结构的显著细化协同作用提高了合金LSP硬化层的疲劳极限。

LSP处理GH4586合金表面沿单冲击点直径方向分布的残余应力研究[50]显示，最大残余应力位于距激光光斑中心约1 mm半径的位置(图9[50])，而且沿深度方向呈降低趋势。光斑中心附近的残余压应力相对有所降低，这种现象被称为“残余应力孔”，其主要取决于激光冲击波在合金表面的作用特征。

图9

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图9   单激光脉冲LSP处理后GH4586合金残余压应力的分布[50]

Fig.9   Residual compressive stress distributions of GH4586 alloy treated with LSP of single laser pulse[50]

(a) surface layer along the diameter direction

(b) depth direction from the surface to the inside


本课题组[19,50]对LSP和WLSP处理IN718合金的硬化层残余应力进行了对比研究，发现WLSP硬化层中形成的纳米尺度的强化相/高密度位错复合微结构，在合金表层产生了更高的残余压应力。因此，IN718合金的WLSP硬化层表现出更明显的高温强化和抗疲劳作用。而且，即使在高温长期时效条件下合金表面硬化层残余压应力有所松弛，相比于LSP硬化层，WLSP硬化层的残余压应力仅小幅降低，具有更好的高温稳定性，在高温强化、抗疲劳方面均更突显优势。

4 高温合金的表面冲击强化微观机制

4.1 喷丸及机械冲击表面硬化机理

高温合金表面喷丸、机械挤压处理硬化层的形成，主要是表面机械冲击过程中基体剧烈塑性变形的结果[32~34,54~59]，塑性变形程度及影响层深度与弹丸、机械冲击的能量输入直接相关。孔挤压强化IN718合金孔内壁表面160 μm深度范围内可观察到明显的旋转塑性变形，越靠近内壁表面，变形程度越大[60]。RR1000合金经过喷丸处理后，表面硬化层剧烈塑性变形的晶粒比例明显增加，形成大量亚结构[61]。FGH96合金[30]、FGH97合金[28]经喷丸处理后，硬化层小角晶界比例显著增加。随表面喷丸或机械冲击能量输入的增加，高温合金硬化层表面发生了明显的晶粒细化。对K417合金的喷丸硬化层进行后处理，亦可形成较大深度范围的细晶硬化层，有效抑制表面疲劳裂纹的萌生[62]。

表面机械变形层中变形孪晶的产生与高温合金的层错能有关，层错能越低位错的交滑移受到的阻碍程度越大，变形机制越趋向于孪生[45]。Zhao等[33]研究GH4169合金的超声喷丸硬化层发现，表面剧烈塑性变形产生的位错密度足够高时，位错湮灭、重组现象显著，形成位错胞。随超声喷丸应变量和应变速率进一步增加，形变孪生机制被激发，基体中形成高密度形变孪晶。而且，表面硬化层形成的形变孪晶，与高密度缺陷、纳米晶等微结构特征相比，在高温下稳定性更高，具有提高高温合金表面硬化层高温稳定性之功效。

4.2 LSP和WLSP处理表面强化机理

与传统表面喷丸、机械冲击/挤压强化处理不同，LSP处理是源于高能量密度(109 W/cm2量级)、短脉冲宽度(10~30 ns量级)的强激光诱发等离子体与合金表面的剧烈相互作用。因此，LSP处理可获得比传统表面喷丸、机械冲击/挤压强化更加显著的表层强化程度和层深度[50~53,61~65]。

LSP处理后高温合金表层中结构细化，晶界和孪晶界数量增多。Cao等[50]研究了GH4586合金LSP处理后硬化层微结构发现，LSP处理使合金表层晶粒明显细化、孪晶数量增加，同时LSP硬化层的晶界比例明显提高(图10[50])。在K417合金的LSP硬化层中，观察到在强激光冲击作用下表层产生了高密度缺陷和晶粒尺寸为20~200 nm的纳米晶。同时发现，纳米晶在垂直激光冲击波方向被明显拉长，随着离表面深度的增加，激光冲击压力减弱，不足以形成纳米晶，微结构特征以高密度位错缠结为主。

图10

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图10   LSP处理对GH4586合金表面晶粒尺寸及孪晶数量的影响[50]

Fig.10   Effect of LSP on grain size and number of twins on the surface of GH4586 alloy[50]

(a) untreated (b) LSP


调整LSP处理参数可在高温合金表面硬化层形成不同的组织结构。Yu等[64]对GH4169合金利用高能LSP、低能LSP和飞秒LSP处理调控硬化层微结构，发现飞秒LSP硬化层深度基本小于100 μm，而低能LSP处理后，合金硬化层深度增加，表面可诱导形成高密度亚结构。但由于能量输入较低，低能LSP处理仅在距表面约100 μm范围内形成了致密的亚结构，在100 μm以下的区域，主要以位错的增殖或缠结为主，而未发现致密亚结构层。当提高输入能量时，高能LSP处理的剧烈变形影响层深可达几百微米甚至几毫米，层深1400~1600 μm时，仍可观察到致密亚结构。

LSP处理除诱发高温合金表面塑性变形层中更高的基体位错密度，明显降低晶粒尺寸，进而形成纳米晶外，更重要的是能够诱导合金中共格有序强化相的微结构改变。Geng等[65]研究某镍基单晶高温合金LSP硬化层发现，LSP处理后合金中产生了高密度的复杂位错结构，且分布不均匀，大量位错在γ基体以及γ/γ′界面形成位错缠结和复杂位错网络。同时，γ相中高密度塞积的位错切入γ'相形成滑移带，在γ′相内部还出现了短位错线和位错环(图11[65])，形成硬化层中包含有序强化相的高密度位错复合结构。

图11

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图11   LSP处理对某镍基单晶高温合金表面硬化层基体位错密度及γ′相内部微结构的影响[65]

Fig.11   Effect of LSP on the dislocation density of matrix and microstructure of γ′ phase in the surface hardened layer in nickel-based single crystal superalloy[65]

(a) untreated

(b-d) low (b) and high (c) magnified images of samples treated by LSP, and SAED pattern of Fig.11c (d)


Cao等[50]研究GH4586合金LSP硬化层微结构发现，LSP处理后合金中γ′相周围形成大量的位错缠结，γ′相结构、形态和尺寸并未发生显著变化。而γ′相内部产生了高密度的层错(图10[50])。可见，与常规喷丸、表面机械冲击/挤压处理对比，LSP处理的优势在于，可以在较低的应变量下产生更高密度的位错缠结和强化相内部的特殊亚结构。

近年来，WLSP技术越来越受到高温合金、超高强钢表面强化领域的关注。由于在强激光脉冲冲击过程中，辅助温度场耦合更易于在含有沉淀强化相的高强结构材料表面诱导形成特殊微结构。本课题组[19]对IN718合金LSP和WLSP硬化层微结构的对比研究发现，2种处理后IN718合金均形成了明显的表面硬化层，与未处理状态相比，LSP和WLSP处理后合金表面硬度分别提高了14.562%和22.689%，WLSP处理获得了硬度更高、更深的表面硬化层(图12[19])。

图12

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图12   LSP和WLSP处理对IN718合金表面硬度分布的影响[19]

Fig.12   Effect of LSP and WLSP on the surface microhardness distribution of IN718 alloy[19]


在WLSP处理IN718合金表面硬化层中，观察到γ″相/高密度位错复合微结构。这种特殊微结构尺寸较均匀，以γ″相为内核，弥散状分布在γ基体上(图13[19])。结合此位置γ″相暗场像可发现：在γ″相上存在大量的纳米尺寸微孪晶，微孪晶的孪晶界面方向基本平行，且与γ″相的长轴方向具有较明显的取向关系。微孪晶主要分布在γ/γ″相界面附近，孪晶一侧贯穿γ″相，另一侧终止于γ″相旁的γ基体中的高密度位错缠结区(图13c[19]中蓝色箭头所示)。形成了WLSP处理的IN718合金中“内含微孪晶的γ″相/高密度位错复合体”这一特殊结构。

图13

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图13   WLSP处理后IN718合金表面硬化层γ″相/高密度位错复合体形态及强冲击作用下γ″相中出现的微孪晶[19]

Fig.13   Morphologies of complex structure of γ″ phase/high-density dislocation and micro-twins in γ″ phase induced by strong impact in the surface hardened layer of WLSP-treated IN718 superalloy[19]

(a) morphology of complex structure of γ″ phase/high-density dislocation

(b) dark field morphology of γ″ phase in Fig.13a (Blue arrows show micro-twins)

(c) micro-twins in γ″ phase induced by strong impact (Blue arrows show micro-twins)

(d) HRTEM image of micro-twins at the γ/γ″ interface


深入研究WLSP处理IN718合金硬化层的这种特殊微结构发现，WLSP处理后γ″相周围形成了高密度的原子尺度微区压向相对应变集中，同时γ″相内部形成了微区应变[52] (图14[52])。而且，层错、微孪晶处微区应变明显加剧。在微孪晶贯穿γ″/γ相界面并延伸至γ基体的位置，界面两侧垂直延伸方向均发生拉向/压向微区应变的起伏(图14b和c[52])。同时，在γ″/γ相界面的γ相一侧，微区应变的集中也较明显。γ″相内部可见层错、微孪晶的交错，而且层错、微孪晶中的微区压向应变明显高于其他位置(图14e和f [52])。

图14

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图14   经WLSP处理后IN718合金表面硬化层中γ″/γ界面细节的HRTEM像[52]

Fig.14   HRTEM images of the details of γ″/γ interface in the surface hardened layer of WLSP-treated IN718 superalloy[52] (a, d) HRTEM images of γ″/γ interface (a) and γ″ phase (d) in the surface hardened layer (Insets show fast Fourier transform (FFT) diffraction patterns) (b, e) magnified parts in the red boxes in Fig.14a (b) and Fig.14d (e), showing HRTEM images and corresponding maps of the geometric phase images (GPA) strain component εxx (εxx —x-direction in-plane strain) (c, f) line profiles of strain maps scanned along lines 1, 2, and 3 in Fig.14b (c) and lines 4, 5, and 6 in Fig.14e (f)


WLSP处理IN718合金硬化层中这种特殊结构贯穿γ″/γ相界面，其改变了γ″/γ相界面形态，使得γ″相内部及其周围应变分布的复杂程度提高。这种特殊结构势必阻碍高温服役下位错反应和位错回复。同时，高温下这些高密度位错包裹、内含微孪晶的γ″相，其长大以及γ″→δ相转变的复杂程度均将加剧。

综上，高温合金表面LSP和WLSP硬化层的强化本质，除剧烈塑性变形在基体中引入的高密度缺陷及亚结构和大量形变孪晶外，还包括在γ'、γ″主要相内部诱导形成大量层错、微孪晶等特殊微结构。与传统喷丸处理和表面机械冲击/挤压强化相比，LSP和WLSP处理合金表面硬化增量更显著、硬化层更深，并且形成了稳定强化相/高密度位错复合结构和剧烈塑性变形亚结构，有利于提高高温合金的表面强化和抗疲劳能力。那么，如能利用耦合场在硬化层引入更多微纳尺度“异质结构(heterostructure)”[66~68]、“化学界面工程(chemical boundary engineering)”[69,70]等强化因素，有望在高温合金表面处理过程中收获更显著的强韧化效果。

5 高温合金表面强化的高温稳定性

5.1 硬化层高密度亚结构的演化与位错反应

高温合金表面形成的显著硬化层和残余压应力，均与基体中高密度位错及亚结构的形成和分布密不可分[71~80]。但在高温下，空位、位错等缺陷均会发生回复和强烈的相互作用，服役温度越高缺陷演化越剧烈，导致残余压应力不同程度的松弛甚至消失。

王欣等[42]对比研究了孔挤压GH4169合金在400和600℃条件下抗疲劳强化的高温稳定性。发现，即使孔挤压合金获得了明显的强化；但高温下，位错运动激活能降低、位错运动能力增加，残余压应力易发生应力松弛，硬化层的疲劳抗力降低；当温度提高到600℃时，基体高密度位错的强化作用下降更加明显。

激光冲击高温合金硬化层中基体的高密度位错及亚结构的强化贡献占比亦不容忽视。在本课题组[52]关于IN718合金表面LSP和WLSP硬化层高温稳定性的研究中发现，650℃长期时效后，LSP硬化层中基体位错密度显著降低，缺陷回复明显，导致硬化层表面硬度和硬化层深度均下降较为明显(图15[52])。

图15

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图15   650℃时效200 h对LSP和WLSP处理IN718合金表面硬化层硬度分布的影响[52]

Fig.15   Effects of aging on the microhardness distribution in the surface hardened layer of LSP and WLSP samples of IN718 alloy at 650oC for 200 h[52] (Insets show optical micrographs of the Vickers indentation. LTA—long-term aging)


基体中高密度位错缺陷在高温下的快速演化，在经表面机械冲击/挤压、LSP和WLSP硬化层中普遍发生。机械冲击/挤压处理的高温合金硬化层，由于表面强化的主要微结构为高密度缺陷，高温下这些缺陷的减少，导致硬化层稳定性降低、强化作用退减。

5.2 高温下硬化层细晶组织及形变孪晶的演化

超细晶(纳米晶)层并非高温合金表面硬化层的普遍性微结构特征，多出现在高能机械冲击/挤压叠加硬化层和LSP硬化层表面，其分布深度也仅约100 μm。高温时效中，表面晶粒长大迅速，硬化层稳定性降低。而高温合金表面硬化层的形变孪晶比纳米晶更为常见，且生成深度达200~500 μm，是高能机械冲击/挤压处理高温合金硬化层的典型微结构特征。Zhao等[33]研究GH4169合金超声喷丸硬化层形变孪晶高温稳定性发现，350℃保温后硬化层形变孪晶无明显改变；当温度升高至650℃时，孪晶间距明显加宽，导致硬化层的硬度降低。

5.3 冲击诱导强化相内部微结构的高温稳定性

研究[65,81~93]发现，与机械冲击/挤压硬化层相比，LSP和WLSP处理硬化层在高温长期时效后的抵抗疲劳裂纹萌生、扩展方面更具优势。Buchanan等[82]对比研究了IN100合金的喷丸处理、LSP处理硬化层的高温应力松弛，发现650℃、100 h时效后，喷丸硬化层最大残余压应力下降约800 MPa，而LSP处理的仅下降200 MPa (图16[82])。可见，LSP处理除使表面强化幅度和硬化层深度明显增加外，硬化层高温服役的稳定性也更佳。Lu等[83]研究GH4169合金WLSP硬化层的高温长期时效组织、性能稳定性发现，WLSP硬化层的残余应力降低幅度明显小于LSP硬化层。而且，600℃的时效未观察到表面残余压应力的显著改变。但当时效温度升高至650℃时，LSP硬化层的残余压应力几乎完全释放，而WLSP处理的残余压应力仅降低约8%。当时效温度增加到700℃时，WLSP硬化层的残余应力在时效100 h后仍可保持在原来的50%左右，表现出极高的高温稳定性。

图16

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图16   喷丸处理与LSP处理后IN100合金表面硬化层在650℃、100 h时效后的残余应力分布对比[82]

Fig.16   Comparisons of residual stress distribution of the surface hardened layer of IN100 alloy after aging at 650oC for 100 h[82]

(a) shot peening (b) LSP


本课题组[81]对长期时效后LSP和WLSP处理IN718合金的硬化层硬度进行了系统对比，发现600和650℃时效200 h后，LSP和WLSP硬化层硬度、深度均有所降低。但经过650℃及以下温度长期时效后，WLSP处理的IN718合金表面硬化层硬度、深度的降低幅度均小于LSP处理(图17[81])。尤其是，WLSP处理合金经650℃、200 h长期时效后表面硬度增量的下降幅度仅为40.79%，而相同时效条件下LSP处理合金表面硬度增量的下降幅度达71.69%。680℃、200 h时效后，LSP处理合金的表面硬化现象几乎完全消失，而此时WLSP处理合金表面硬度仍保持一定的增量。研究[81]还发现，表面硬化层的强化增量主要来源于LSP和WLSP处理后合金中的高密度位错增殖和塞积(图18[81])。尤其对于WLSP硬化层而言，更显著的强化作用主要来源于硬化层γ″相/高密度位错复合微结构。同样，WLSP硬化层的强化现象在600℃以上长期时效后仍得以保持，也主要取决于WLSP处理诱发的高密度位错和特殊亚结构。

图17

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图17   不同温度长期时效后IN718合金LSP和WLSP硬化层硬度分布及硬化层深度变化[81]

Fig.17   Micro-hardness distributions and depth changes of the surface hardened layer of LSP and WLSP IN718 alloys after aging at high temperatures (NA—non-aging)[81]

(a) micro-hardness of surface hardened layer after aging at different temperatures treated by LSP and WLSP

(b) comparison of micro-hardness of hardened layer

(c) comparison of hardened layer depth


图18

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图18   长期时效对LSP和WLSP处理的IN718合金表面硬化层强化机制贡献增量的影响[81]

Fig.18   Effects of long-term aging on the contribution increment of strengthening mechanism of the surface hardened layer of LSP and WLSP IN718 alloys[81] (ΔσD—strength contribution from dislocation strengthening, ΔσGB—strength contribution from grain boundary strengthening)


本课题组[54]在研究长期时效后LSP和WLSP处理IN718合金表面硬化层的几何必需位错(geometrically necessary dislocation，GND)演化行为发现，WLSP硬化层的GND密度明显高于LSP处理，升高幅度约为248.71% (图19[52])。硬化层中GND密度的增加，除形成各向同性强化外，更重要的是可产生更加显著、稳定的背应力强化。经长期时效处理后，LSP硬化层中的GND密度明显降低，而WLSP硬化层的GND密度虽有降低，但降幅很小。而且长期时效后，LSP处理合金GND密度统计分布的相对频率峰位，向低位错密度方向发生了明显的移动，同时相对频率峰值也呈现出明显的上升趋势。但WLSP硬化层在长期时效后GND密度统计分布相对频率的峰值、峰位、峰宽无明显变化。这进一步说明，WLSP处理不仅仅使IN718合金获得了更高的GND密度，而且GND密度的集中程度和分布位置变化并不明显，WLSP处理合金均表现出了比LSP处理合金更显著的高温长期服役稳定性。

图19

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图19   650℃长期时效后LSP和WLSP处理IN718合金表面硬化层的几何必需位错(GND)密度分布图及正态分布统计图[52]

Fig.19   Geometrically necessary dislocation (GND) density (ρGND) maps (a, b, d, e) and corresponding normal distribution statistical diagrams of GND density (c, f) of the surface hardened layer of LSP (a-c) and WLSP (d-f) IN718 alloys before (a, d) and after (b, e) long-term aging at 650oC[52] (RD—rolling direction, TD—transverse direction, ND—normal direction)


综上，LSP和WLSP处理高温合金硬化层微结构方面的深入研究[94~96]发现，弥散分布的γ'、γ″相在剧烈塑性变形时形成的特殊微结构，是LSP和WLSP硬化层区别于传统机械冲击/挤压强化，获得优异、稳定强化效果的关键。而WLSP处理更突显的高温强化作用和高温稳定性，不仅与特殊微结构、高的缺陷密度有关，更应与“内含层错、纳米尺寸微孪晶的有序强化相/高密度位错复合体”这种复杂微结构自身的高温稳定性直接相关[52,81]。在高温合金表面硬化层中诱导和调控高密度特殊微结构的形成和分布状态，将是获得合金稳定高温抗疲劳强化的核心。

6 总结与展望

本文基于先进涡轮动力系统用镍基高温合金的典型表面强化技术，以及强化作用高温稳定性的研究和应用分析，就镍基高温合金表面冲击强化总结与展望如下。

(1) 高温合金的表面机械喷丸、挤压/滚压强化处理，均可产生明显残余压应力和硬化层，有效阻碍裂纹在表面萌生，从而提高高温合金的疲劳抗力。但应用中需注意控制喷丸处理造成的表面粗糙度增大、挤压/滚压处理的残余压应力梯度调控以及表面机械强化超过500℃后的强化作用衰减。

(2) LSP和WLSP处理等表面高能冲击强化技术具有不受构件规格、形状限制的独特优势，同时其强化增幅及层深均优于机械表面强化处理，由此经其处理的高温合金显示出更高的疲劳抗力。WLSP处理在高温合金表面硬化层中形成的“有序强化相/高密度位错复合特殊微结构”，进一步提升了表面强化的作用效果。

(3) 硬化层微结构的高温、长时稳定性，是高温合金表面强化技术飞跃发展与深入应用的关键。高温合金表面剧烈塑性变形产生的基体微结构中，高密度缺陷的高温演化与回复显著，不具备高温稳定能力。表面细晶(纳米晶)层较浅，高温下长大倾向明显，稳定性亦不足。而形变孪生可在600℃以下保持一定的高温稳定性。耦合场高能强冲击(LSP和WLSP等)处理在获得上述基体微结构的同时，诱导γ′、γ″相内部形成内含大量层错、微孪晶的“强化相/高密度位错复合特殊微结构”，是使硬化层兼具强化作用、高温稳定性与长时稳定性的关键特殊微结构。

(4) WLSP处理在高温合金表面冲击强化作用高温稳定性方面所凸显的优势，启发人们应关注硬化层不同尺度关键微结构的综合调控。如能利用高能冲击耦合场将诸如“异质结构”、“化学界面工程”等微纳尺度强韧化理念引入高温合金的表面微结构精准设计，在硬化层精准诱导“有序强化相/高密度位错特殊微结构”的梯度分布，构筑具有更高高温强度与高温稳定性的表面硬化层，将有望开辟高温合金表面强化相关研究的新方向。

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The aim of this work was to investigate the influence of laser shock peening on the topography, microstructure, surface roughness and the mechanical properties of the Inconel 625 nickel alloy. Examination of the topography and microstructure of the nickel alloy after laser treatment was carried out by means of scanning electron microscopy as well as atomic force microscopy. The roughness of the surface was measured by WYKO NT9300 equipment. Nanohardness test was carried out using a nanoindenter NHT 50-183 of CSM Instruments equipped with a Berkovich diamond indenter. Additionally, transmission electron microscopy was used to examine the microstructural changes on the surface layer after laser treatment. The investigations showed that the laser process produced an ablation and melting of the surface layer and, hence, increased the surface roughness of the Inconel 625. On the other hand, the presence of the slip bands on the surface and on the cross section of the treated material, a high density of dislocations and a higher hardness of the treated region indicated that the laser shock processing caused severe plastic deformation of the surface layer. Additionally, due to the high plastic deformation, cracking of the carbide precipitates was observed.

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A residual stress depth profile up to 1 mm is determined with the Ortner method in a single crystal of a nickel-based superalloy which has been subjected to shot-peening. An optimization procedure is assessed to minimize uncertainties connected to Bragg angle, mosaic spread and numerical stability. The theoretical background is reviewed to highlight the connections between Bragg angle positions and the stress tensor components in different coordinate systems and also to obtain a mathematically consistent formulation. Transformation matrices required to express the strain components with respect to the initial state are provided for the general case. It is shown that, when a stress gradient occurs beneath the sample surface plane, the value of the σ33component of the stress tensor determined from measurements is twice its true value. For a sample surface oriented along a 〈100〉 crystallographic direction, the data analysis shows that the compressive stresses which develop in the 150 µm-thick surface layer are compensated for by small tensile stresses developing at long scale rather than a specific layer of finite size featuring high tensile stresses. At least 17 Bragg angles are required to have stable solutions with standard deviations close to 30 MPa. Maximum compressive stresses of 1000 or 1400 MPa depending on the assumption used to describe the initial state occur at a 30 µm depth.

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采用透射电子显微术，研究了ＧＨ１６９高温合金孔挤压强化层的微观结构。实验结果表明，强化层内位错呈现平面状滑移和交滑移混合特征。随距孔边距离的增加，滑移带变得不甚明显，位错胞的直径略有增加。强化层内的孪晶数量很少，孪生不是ＧＨ１６９合金的主要变形方式。在合金中的γ'、γ”及δ相周围，存在着高密度的位错缠结结构；ＧＨ１６９合金具有较大范围的孔挤压强化层，约为３ｍｍ的深度

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The purpose of this paper is to analyze the residual stress relaxation in laser shock‐peened and shot‐peened IN100 subject to thermal exposure.

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Laser shock peening (LSP), as an innovative surface treatment technology, can effectively improve fatigue life, surface hardness, corrosion resistance, and residual compressive stress. Compared with laser shock peening, warm laser shock peening (WLSP) is a newer surface treatment technology used to improve materials’ surface performances, which takes advantage of thermal mechanical effects on stress strengthening and microstructure strengthening, resulting in a more stable distribution of residual compressive stress under the heating and cyclic loading process. In this paper, the microstructure of the GH4169 nickel superalloy processed by WLSP technology with different laser parameters was investigated. The proliferation and tangling of dislocations in GH4169 were observed, and the dislocation density increased after WLSP treatment. The influences of different treatments by LSP and WLSP on the microhardness distribution of the surface and along the cross-sectional depth were investigated. The microstructure evolution of the GH4169 alloy being shocked with WLSP was studied by TEM. The effect of temperature on the stability of the high-temperature microstructure and properties of the GH4169 alloy shocked by WLSP was investigated.

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DOI     

The effects of cold work produced by peening processes in residual stress relaxation under elevated temperatures were investigated. Two types of peening processes namely shot peening and laser peening were used to generate different amount of cold work and residual stresses in the study. Characterisation of residual stress induced by cold working were carried out using X-ray diffraction coupled with electrochemical polishing method to measure the variation of residual stress with depth. Residual stresses in shot peened samples were observed to relax more rapidly than that of laser peened ones when subjected to thermal exposure. The data collected from the experimental investigation was then used to build a correlation function to model the effects of cold work on residual stress relaxation. A parameter describing cold work’s contribution to stress relaxation is introduced into the well-known Zener-Wert-Avrami (ZWA) model. The model was validated against experimental measurements and showed good agreement in both shot peened and laser peened samples.

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An austenitic stainless steel 1Cr18Ni9Ti and a solid solution-strengthened Ni-base superalloy GH30 were shock processed using a Q-switched pulsed Nd-glass laser. Microstructure, hardness and residual stress of the laser shock processed surface were investigated as functions of laser processing parameters. Results show that high density of dislocations and fine deformation twins are produced in the laser shock processed surface layers in both the austenitic stainless steel and the nickel-base superalloy. Extensive strain-induced martensite was also observed in the laser shock processed zone of the austenitic steel. The hardness of the laser shock processed surface was significantly enhanced and compressive stress as high as 400 MPa was produced in the laser shock processed surface.

Zhu H Y, Qu X M, Cao J, et al.

Study on stress relaxation characteristics of FGH95 powder superalloy treated by laser shock peening

[J]. Mater. Res. Express, 2022, 9: 106502

DOI     

Aiming at the phenomenon that the residual stress induced by Laser Shock Peening (LSP) will relax and redistribute under various loads, temperature, cyclic load, and the dual treatment of temperature and cyclic load on the residual stress relaxation of FGH95 powder superalloy after LSP treatment were studied, and the analysis model of relevant residual stress relaxation was constructed. The purpose is to understand the strengthening effect and stability of the alloy under temperature and cyclic load after LSP treatment. With the increase of treatment temperature, the relaxation of residual stress became more and more obvious. Most of the residual stress relaxation occurred in the first 30 min of temperature treatment, then slowed down and stabilized after 1 h. The residual stress was initially relaxed in the first 50 cycles, remained roughly unchanged between 50 and 5000 cycles. The intensify of the cyclic load increasing, adding material yield level, further plastic deformation and residual stress relaxation rate increases. With the increase of load intensify and load ratio, residual stress relaxation was also increased. The residual stress relaxation rate after 600 °C and cyclic load treatment was 56.2%, both greater than that after 600 °C or cyclic treatment of 25 °C, but less than the sum of the two conditions. The results of this paper provide a reference for the LSP of the FGH95 powder superalloy turbine disk and other aero engine parts.

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Quantification of room temperature strengthening of laser shock peened Ni-based superalloy using synchrotron microdiffraction

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As a metal surface strengthening technology, oblique laser shock peening uses the force effect of pulsed laser beams to significantly improve the mechanical properties of materials and prolong service life. However, the processing of metal parts still requires a high cost for testing the quality of the finished product. At present, there is no systematic and complete measurement method or evaluation system for the finished products of oblique laser shock peening technology. In this paper, a principal component analysis method and a comprehensive index method are organically combined to construct a method for evaluating the effect of oblique laser shock peening of FGH95 superalloy turbine disks. We measured 10 real data of FGH95 superalloy specimens after oblique laser shock peening by using the orthogonal test method and verified the data. Based on the evaluation results, we put forward a series of suggestions to improve the quality of oblique laser shock peening of FGH95 high-temperature turbine disks, providing a certain reference and guidance for processing FGH95 superalloy turbine disks.

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