激光冲击强化技术原理及研究发展

激光冲击强化技术(LSP)是一种新型的激光应用表面处理技术。与传统表面改性技术相比,激光冲击强化技术能给材料带来更深的残余应力层,使材料表层晶粒细化甚至出现纳米晶,同时大幅提高材料的疲劳寿命。利用高能激光辐照约束层材料(黑漆、黑胶带或铝箔),约束层材料在瞬间熔融气化并产生高温高压的等离子体。等离子体冲击波是一种爆轰波,可以通过C-J模型计算冲击波的峰值压力。等离子体冲击波在约束层(水、光学玻璃)的约束下向材料内部传播,其压力远远超过了材料的弹性屈服极限,材料经历了弹性-塑性变形,最终材料表面形成稳定的残余应力场并发生微弱的塑性变形。本文介绍了激光冲击强化技术的研究发展历程,在此基础上对该技术发展方向进行了展望。

     

The microstructural mechanism for mechanical property of LY2 aluminum alloy after laser shock processing

This paper described nanoindentation techniques for measuring thin films mechanical properties, including elastic modulus and nano-hardness. The effects of laser shock processing (LSP) on elastic modulus and nano-hardness of the sample manufactured by LY2 aluminum alloy were experimentally investigated by nanoindentation techniques. Transmission electron microscope (TEM) observations of the microstructures in different regions after LSP are carried out. Experimental results showed that the values of nano-hardness and elastic modulus in the laser-shocked region were obviously increased by 58.13% and 61.74% compared to those in the non-shocked region, respectively. The influences of LSP on microstructure and grain size of LY2 aluminum alloy were discussed, and the enhancement mechanism of LSP on nano-hardness and elastic modulus was also addressed.     

激光冲击强化技术的应用现状与发展

激光冲击强化是一种利用激光诱导等离子体冲击波来提高材料疲劳寿命的新型表面改性技术,具有强化效果显著、可控性强、适应性好等优点,对提高结构可靠性和部件疲劳强度、延长材料使用寿命具有重要作用。近年来,该技术受到了广泛重视,得到了快速发展。本文简要介绍了激光冲击强化技术的基本原理、特点与应用领域;总结了国内外激光冲击强化技术的发展状况与研究成果;并针对国内外激光冲击强化技术的现状,提出了一些现在需要解决的强化工艺问题;最后对激光冲击强化技术的应用前景进行了展望。

     

激光冲击处理对AISI304钨极氩弧焊接接头残余应力的影响

利用激光冲击波对AISI304不锈钢氩弧焊接接头进行了表面强化处理,用X-350A型X射线应力仪测定了其激光冲击处理后残余应力,分析了残余应力的产生机理.结果表明,激光冲击处理后AISI304焊接接头残余应力为110MPa左右,其强化效果十分明显,显著地降低了焊接接头残余拉应力,并使残余应力分布趋向均匀.     

Effect of laser shock processing on the fatigue crack initiation and propagation of 7050-T7451 aluminum alloy

The aim of this paper was to identify the effect of laser shock peening (LSP) on the fatigue crack initiation and propagation of 7050-T7451 aluminum alloy. The laser shocked specimen in which residual compressive stress is mechanically produced into the surface showed a very high dislocation density within the grains. This was evident throughout the LSP region. The spacing among the fatigue striations in the LSP region was narrow, which indicated that LSP had an obvious inhibitory action to fatigue crack initiation and growth. In contrast, the region without LSP exhibited an extremely low dislocation density. And LSP improved 7050-T7451 alloy specimens’ fatigue intensity.     

铝合金激光表面强化的研究进展

综述了铝合金表面激光强化的工艺方法和特点，阐述了铝合金激光表面强化层的组织特征、硬度变化及其耐磨、耐蚀性能，总结了国内外在此研究领域的最新进展．     

A finite element analysis of thermal relaxation of residual stress in laser shock processing Ni-based alloy GH4169

The residual stress induced by laser shock processing and the thermal relaxation behaviors of residual stress in Ni-based alloy GH4169 were investigated by means of three-dimensional nonlinear finite element analysis. To study the effect of different given exposure time and different temperatures on residual stress in laser shock processing Ni-based alloy GH4169, Johnson–Cook material model was used in order to account for the nonlinear constitutive behavior. The influence of heating temperature and exposure time on the stress thermal relaxation was studied. It was concluded that stress relaxation mainly occured during the initial period of exposure, and the degree of relaxation increased as the temperature risen. The results would provide a theoretical basis for controlling the laser shock processing and guiding subsequent experiments.     

Simulation,analysis and validation of residual stresses on LY2 aluminium alloy by laser shock processing with elliptical spot

Abstract

The effects of laser shock processing on the residual stresses of the LY2 aluminium alloy samples with elliptical spot (long axis length, 12 mm; short axis length, 3 mm) were experimentally investigated, and the effects of the overlapping rate on the residual stresses were simulated using the Abaqus software. The simulated residual stresses were basically in agreement with the measured data, and the relationship between the magnitude and uniformity of residual stress and the overlapping rate was also addressed. Results show that the largest stress magnitudes are located on the top surface of the sample, and the greatest uniformity is achieved by the overlapping of elliptical laser spots. The overlapping rate is critical for the uniformity of the residual stress across the surface. Within a certain impact number range of one to four times, increasing the shocked number can increase the magnitude of residual stress near the surface but not effectively increase the plastically affected depth.     

Laser shock peening on fatigue crack growth behaviour of aluminium alloy

The effect of laser shock peening (LPS) in the fatigue crack growth behaviour of a 2024‐T3 aluminium alloy with various notch geometries was investigated. LPS was performed under a ‘confined ablation mode’ using an Nd: glass laser at a laser power density of 5 GW cm^?2. A black paint coating layer and water layer was used as a sacrificial and plasma confinement layer, respectively. The shock wave propagates into the material, causing the surface layer to deform plastically, and thereby, develop a residual compressive stress at the surface. The residual compressive stress as a function of depth was measured by X‐ray diffraction technique. The fatigue crack initiation life and fatigue crack growth rates of an Al alloy with different preexisting notch configurations were characterized and compared with those of the unpeened material. The results clearly show that LSP is an effective surface treatment technique for suppressing the fatigue crack growth of Al alloys with various preexisting notch configurations.     

选区激光熔化AlSi10Mg合金不同温度原位拉伸变形行为及断裂机理研究

为了提高选区激光熔化AlSi10Mg合金在航空航天领域的应用,基于自主研发的原位SEM高温拉伸台,本文对比分析了原位拉伸非校准样品的选区激光熔化AlSi10Mg合金在室温、200、300 ℃条件下的力学性能与显微组织动态演化,并总结了断裂机理。结果表明,选区激光熔化AlSi10Mg合金的显微结构由α-Al基体、共晶Si和大量的气孔组成,且共晶Si呈连续网格状均匀分布在α-Al基体上。随着温度的升高,选区激光熔化AlSi10Mg合金的强度降低。屈服强度从室温的207 MPa降低到300 ℃时的52 MPa,极限抗拉强度从室温的304 MPa降低到300 ℃时的71 MPa,延伸率则随温度的升高而增大,从室温的7.4%增大到300 ℃时的59.5%。室温拉伸过程中试样并未出现明显的颈缩现象,而是随着温度的升高,试样的颈缩现象逐渐明显,表明试样经历了更加充分的塑性变形, 并且随着温度的升高,试样的断裂位置越来越偏离标距段中心。通过对试样变形行为的研究发现,200 ℃时,变形主要集中在晶内,发生晶内滑移;而300 ℃时滑移主要集中在晶界,导致晶界滑移。由于试样表面及内部存在大量缺陷,因此,室温下选区激光熔化AlSi10Mg合金的断裂机理为熔池边界的组织突变结合孔洞连通造成的准解理断裂。随着温度的升高,由于初始孔洞边缘的应力集中产生新的孔洞形核,新形核的孔洞与相邻孔洞相连通,导致试样的最终断裂。     

Effect of initial surface topography on the surface status of LY2 aluminum alloy treated by laser shock processing

Samples manufactured by LY2 aluminum (Al) alloy with different initial surface topography were treated by laser shock processing (LSP), and then the surface topographies before and after LSP were carefully investigated with a non-contact optical profiler (NCOP). Moreover, the residual stress and microhardness were also examined. Results showed the following three aspects: (a) Initial surface topography will influence the surface roughness of LY2 when treated by LSP. The values of surface roughness of all the tested samples would tend to be stable after one LSP impact, and there was an ultimate value for the surface roughness after multiple LSP impacts, which was about 0.58 μm. (b) With the increase of initial surface roughness, the compressive residual stress decreased when subjected to one LSP impact. The surface residual stress of all the samples tended to be saturated after three LSP impacts, and the saturated value was nearly equal. (c) With the increase of initial surface roughness, the microhardness of all the samples increased when subjected to one LSP impact.     

激光诱导等离子冲击波改善金属材料疲劳性能及强化机理的研究进展

激光冲击是一种利用等离子冲击波效应的表面强化技术,该技术能显著提高金属材料抗疲劳、磨损、腐蚀等性能。简要阐述了激光冲击强化技术原理、特点及激光诱导的等离子体特性。从激光冲击强化后金属的疲劳行为、强化机理及疲劳延寿机制3个方面总结了国内外激光冲击强化在金属零部件抗疲劳性能方面的研究进展。激光冲击强化机理由最初的残余压应力强化机制转变为目前普遍接受的残余压应力和表面纳米化复合强化机制。冲击后的金属零部件表层硬度显著提高,由表层向内部引入较大的残余压应力,表层晶粒碎化至纳米级,而表面粗糙度基本保持不变,尤其适合表面粗糙度要求较高的最终零部件的强化。在总结疲劳性能研究及强化机理的基础上,对目前激光冲击强化研究中存在的问题进行探讨,并指出下一步研究的关键问题。     

Effect of laser shock processing on fatigue crack growth of duplex stainless steel

Duplex stainless steels have wide application in different fields like the ship, petrochemical and chemical industries that is due to their high strength and excellent toughness properties as well as their high corrosion resistance. In this work an investigation is performed to evaluate the effect of laser shock processing on some mechanical properties of 2205 duplex stainless steel. Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field which increases fatigue crack initiation life and reduces fatigue crack growth rate. A convergent lens is used to deliver 2.5 J, 8 ns laser pulses by a Q-switched Nd:YAG laser, operating at 10 Hz with infrared (1064 nm) radiation. The pulses are focused to a diameter of 1.5 mm. Effect of pulse density in the residual stress field is evaluated. Residual stress distribution as a function of depth is determined by the contour method. It is observed that the higher the pulse density the greater the compressive residual stress. Pulse densities of 900, 1600 and 2500 pul/cm² are used. Pre-cracked compact tension specimens were subjected to LSP process and then tested under cyclic loading with R = 0.1. Fatigue crack growth rate is determined and the effect of LSP process parameters is evaluated. In addition fracture toughness is determined in specimens with and without LSP treatment. It is observed that LSP reduces fatigue crack growth and increases fracture toughness if this steel.     

激光冲击处理对金属微结构及其性能的影响

论述了激光冲击处理的基本原理,分析了激光诱发的冲击波在材料中的传播过程.激光冲击处理会使金属材料表层发生塑性变形而产生很高的残余压应力;冲击区的显微组织中会产生高密度位错,有时亦有孪晶与相变产生;金属材料表层的塑性变形、高残余压应力、高密度位错、孪晶以及相变,这些因素的共同作用使得金属材料表面硬度、疲劳寿命获得很大的提高.     

激光冲击处理后Ni基高温合金的性能

对Ni基高温合金进行了激光冲击强化处理,测量了处理后Ni基高温合金的残余应力、表面粗糙度分布规律和疲劳强度．结果表明,激光冲击处理后凹坑内残余压应力值为200—400MPa,其分布服从泊松正态分布规律;承载能力比未冲击试样提高大约150MPa;激光冲击区的粗糙度降低一个等级,凹坑内粗糙度数值分布也服从正态分布规律．激光冲击处理后表面出现了组织细化的硬化层,强化效果明显,残余压应力有极大的提高,有效地提高了抗疲劳寿命．     

激光冲击ZrO2陶瓷的微观分析

为了研究ZrO2陶瓷的冲击性能,分析其失效特征,从而改善ZrO2陶瓷的脆性,使用高功率钕玻璃脉冲激光器对ZrO2陶瓷进行了冲击试验.通过扫描电子显微技术,观察其断口形貌,发现陶瓷的失效主要是由层裂所引起的,层裂表面以典型的脆性断口为主,同时有较多的晶粒拔出现象,层裂面并不是沿单一平面扩展,而是形成大量的台阶,层裂方向和密度分布表明失效是由反射拉伸波造成的.同时材料表面层由于压应力而产生约几十μm的致密压缩层,压缩层内无层裂现象发生.由此机理表明在合适的激光能量下可形成表面强化而基体增韧的ZrO2陶瓷材料.     

Microstructure and microhardness of pearlitic steel after laser shock processing and annealing

The microstructure evolution of the high carbon pearlitic steel after laser shock processing (LSP) with different laser pulse energy and high temperature annealing was investigated. After LSP, the cementite lamella were bent, fractured and broken into granules. Fragmentation and dissolution of the cementite lamella were enhanced by increasing the laser pulse energy. Results show that the ferrite lattice parameter increased due to carbon atom dissolution in the ferrite matrix, and the corresponding ferrite X-ray diffraction peaks shifted significantly towards the smaller diffraction angles. After annealing at 650°C for 30?min, an ultrafine duplex microstructure (ferrite+cementite) was formed on the surface. After LSP with a high energy, equiaxed ferrite grains were refined to 400?nm and the cementite lamella were fully spheroidised with the particle diameter of ～150?nm. The corresponding grain size of ferrite and cementite under low pulse energy was 500 and 300?nm respectively. After annealing, the ferrite peaks significantly shifted towards the higher diffraction angles, and the ferrite lattice parameter decreased. The microhardness initially increases after LSP and then slightly decreases after subsequent annealing but remained higher than without LSP.     

激光冲击处理对Ti6Al4V力学性能的影响

通过对钛合金Ti6Al4V的激光冲击处理,研究了激光冲击处理工艺对钛合金Ti6Al4V力学性能的影响.实验表明:激光冲击处理能有效提升Ti6Al4V的力学性能,在激光功率密度由1.15GW/cm2增加到2.31GW/cm2过程中,其冲击波峰值压力线性增加,表面最大残余压应力也相应增大,最高达-264MPa,表面硬化层的显微硬度高达510Hv,硬化层深度约为0.25mm,经过激光冲击处理后硬度相对于原始钛板提高了64%,随着激光能量的增加,冲击区域的抗拉强度极大增强,塑性降低.     

High temperature mechanical properties and surface fatigue behavior improving of steel alloy via laser shock peening

Laser shock peening was carried out to reveal the effects on ASTM: 410L 00C_r12 microstructures and fatigue resistance in the temperature range 25–600 °C. The new conception of pinning effect was proposed to explain the improvements at the high temperature. Residual stress was measured by X-ray diffraction with sin²ψ method, a high temperature extensometer was utilized to measure the strain and control the strain signal. The grain and precipitated phase evolutionary process were observed by scanning electron microscopy. These results show that a deep layer of compressive residual stress is developed by laser shock peening, and ultimately the isothermal stress-controlled fatigue behavior is enhanced significantly. The formation of high density dislocation structure and the pinning effect at the high temperature, which induces a stronger surface, lower residual stress relaxation and more stable dislocation arrangement. The results have profound guiding significance for fatigue strengthening mechanism of components at the elevated temperature.     

The effect of laser power density on the fatigue life of laser-shock-peened 7050 aluminium alloy

Laser shock peening (LSP) is an innovative surface treatment method that can result in significant improvement in the fatigue life of many metallic components. The process produces very little or no surface profile modification while producing a considerably deeper compressive residual stress layer than traditional shot peening operations. The work discussed here was designed to: (a) quantify the fatigue life improvement achieved by LSP in a typical high strength aircraft aluminium alloy and (b) identify any technological risks associated with its use. It is shown that when LSP conditions are optimal for the material and specimen configuration, a —three to four times increase in fatigue life over the as-machined specimens could be achieved for a representative fighter aircraft loading spectrum when applied at a representative load level. However, if the process parameters are not optimal for the material investigated here, fatigue lives of LSP treated specimens may be reduced instead of increased due to the occurrence of internal cracking. This paper details the effect of laser power density on fatigue life of 7050-T7451 aluminium alloy by experimental and numerical analysis.