2024铝合金50％预疲劳后激光冲击强化对微观结构及性能的影响

为了提高2024铝合金疲劳后的性能,先对其进行50%预疲劳再作激光冲击强化处理。采用显微硬度、残余应力测试,EBSD和疲劳试验等技术,研究了2024铝合金50%预疲劳和激光冲击强化后的组织结构和性能;讨论了激光冲击强化提高其预疲劳后性能的机理。结果表明:激光冲击强化可以明显改善2024铝合金50%预疲劳后的疲劳性能,200 MPa疲劳载荷下循环寿命比未强化前提高了1倍多。     

激光冲击强化制备发动机用FGH96镍基高温合金性能分析北大核心CSCD

采用时效工艺处理发动机用FGH96镍基高温合金,利用激光冲击强化方法对其表面进行修复,实验测试分析其组织,残余应力及疲劳寿命。研究结果表明:基体中形成了大量的γ相奥氏体;时效96h后显微组织中产生了大量碳化物,存在沿晶析出。激光冲击强化处理后位于表面附近的晶粒形成更小的尺寸,合金表面晶粒发生明显细化,链状碳化物在晶粒内呈现弥散分布状态,实现FGH96合金的沉淀强化作用,对位错运动产生明显抑制效果。经激光冲击强化处理后试样并未产生新的衍射峰。激光冲击强化可以使试样表面获得更高的残余压应力,使时效试样达到更高的疲劳寿命。激光冲击强化还可以将残余应力引入到基体中,使疲劳裂纹源受到明显抑制,显著降低疲劳裂纹的扩展速度。     

激光冲击强化制备发动机用FGH96镍基高温合金性能分析

采用时效工艺处理发动机用FGH96镍基高温合金,利用激光冲击强化方法对其表面进行修复,实验测试分析其组织,残余应力及疲劳寿命。研究结果表明:基体中形成了大量的γ相奥氏体;时效96h后显微组织中产生了大量碳化物,存在沿晶析出。激光冲击强化处理后位于表面附近的晶粒形成更小的尺寸,合金表面晶粒发生明显细化,链状碳化物在晶粒内呈现弥散分布状态,实现FGH96合金的沉淀强化作用,对位错运动产生明显抑制效果。经激光冲击强化处理后试样并未产生新的衍射峰。激光冲击强化可以使试样表面获得更高的残余压应力,使时效试样达到更高的疲劳寿命。激光冲击强化还可以将残余应力引入到基体中,使疲劳裂纹源受到明显抑制,显著降低疲劳裂纹的扩展速度。     

TC4钛合金选区激光熔化与层间激光冲击强化复合工艺

目的 改善激光选区熔化(Selective Laser Melting,SLM)工艺成形的TC4合金的内部缺陷,提高疲劳寿命。方法 选用TC4钛合金为研究对象,提出了SLM结合层间激光冲击(3D-Laser Shock Peening,3D-LSP)与热处理的强化工艺,对复合制造工艺下的微观组织、内部缺陷和力学性能演变进行了研究,并建立了复合强化工艺制造样品的疲劳寿命模型。结果 在激光冲击影响区域内形成了0.2 mm深度的高幅值残余压应力,并在1 mm深度范围内改善了应力场,且显微硬度得到了提升,内部缺陷数量减少了36%,疲劳寿命提升了40%以上。结论 实现了SLM增材制造TC4钛合金的缺陷在线闭合、微观组织改性和疲劳寿命的提升,揭示了层间激光冲击对内部缺陷的闭合机理,为金属SLM复合增材制造的研究与应用奠定了理论基础。     

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.     

5CrNiMo钢的强度水平和表面强化对冲击疲劳性能的影响

本文探讨了用冲击疫劳试验测定的疲劳寿命和疲劳裂纹扩展速率衡量热锤锻模材料疲劳抗力的合理性。研究了5CrNiMo钢在热锻模要求的硬度范围内,材料强度水平和表面强化对冲击疲劳性能的影响。试验结果表明,在相当小型锻模受载的低冲击能量作用下,适当提高材料强度可延长疲劳寿命;在相当大型锻模受载的高冲击能量作用下,提高材料塑性、韧性可提高疲劳寿命。软氮化并磨去脆性白层可显著提高低冲击能量下的冲击疲劳寿命。     

激光喷丸强化对半圆孔件疲劳寿命的影响

为研究激光喷丸强化对7075-T6铝合金半圆孔件疲劳寿命的影响,对激光喷丸与未喷丸的试样进行了对比试验,利用X射线应力仪测定其表面残余应力,并对试样进行疲劳拉伸试验.用扫描电镜观察了两类试样疲劳断口的形貌,并采用数理统计方法对其疲劳寿命进行分析.研究表明:经激光喷丸处理区域,表面存在较大的残余压应力,幅值为310 MPa;未喷丸试样疲劳裂纹条带的宽度为0.7~0.8μm,而喷丸试样疲劳裂纹条带的宽度为0.3~0.4μm,说明喷丸试样裂纹扩展的速度比未喷丸试样慢很多;激光喷丸后半圆孔件的疲劳寿命比未喷丸的疲劳寿命提高了2.8~7.2倍.     

新型贝氏体钢的组织和冲击疲劳性能研究

通过显微组织观察和冲击疲劳实验,研究了不同热处理新型贝氏体钢的组织和冲击疲劳性能.结果表明:新型贝氏体钢正火低温回火的组织由贝氏体铁素体和奥氏体组成,淬火低温回火组织为回火马氏体和残余奥氏体,正火低温回火热处理的冲击疲劳寿命高于淬火低温回火热处理的冲击疲劳寿命.分析了多冲疲劳裂纹扩展的行为,讨论了正火低温回火提高冲击疲劳的原因.     

高强度合金抗疲劳应用技术研究与发展

评述了超高强度钢、高强度Al合金和Ti合金表面完整性抗疲劳应用技术的研究和发展。高强度合金疲劳性能对应力集中敏感，不适当的加工工艺和切削热等造成的表面损伤和高拉应力使其疲劳和应力腐蚀性能损失殆尽。先进的表面完整性加工尤其是表面改性可显著提高疲劳性能，如激光冲击使7475-T761拉-拉疲劳寿命提高约89％，7075-T6裂纹扩展速率降低到原来的1／1500；超声喷丸使超高强度钢低周疲劳强度提高约50％，Ti7Al4Mo合金高周疲劳强度提高约15％；表面超硬化可使Vasco X-2M齿轮钢接触疲劳寿命提高30～35倍等。     

超声冲击处理MB8镁合金十字接头的表层组织及疲劳性能

采用超声冲击方法对MB8镁合金十字接头焊趾处进行超声冲击处理,对比测试焊态及冲击处理态接头的表层组织和疲劳性能。结果表明:超声冲击处理可以在MB8镁合金焊接接头表面获得纳米晶组织。在循环寿命为2×106条件下,焊态试样的条件疲劳强度为32.07MPa,冲击处理态试样的条件疲劳强度为41.88MPa,提高了30.59%。未冲击接头疲劳断裂大多发生在焊缝缺陷处,冲击处理后的接头则发生在热影响区。超声冲击处理不仅可以大幅提高MB8镁合金十字接头的疲劳寿命,还可以改变接头疲劳断裂位置。此外,热影响区也是MB8镁合金十字接头疲劳断裂的薄弱区域,这与热影响区晶粒粗大有很大的关系。     

Influence of pulse sequence and edge material effect on fatigue life of Al2024-T351 specimens treated by laser shock processing

Laser shock processing (LSP) is being considered as a competitive alternative technology to classical treatments for improving fatigue, corrosion cracking and wear resistance of metallic materials. The purpose of this paper is to present a fully 3D finite element model for predicting the residual stresses that result from the LSP of aluminum alloy Al2024-T351 samples of interest for aeronautic industry in order to optimize the laser treatment to increase the fatigue life of the material. In order to correlate the simulation results with experimental data, three different laser shock processing strategies (pulse sequences) were performed on fatigue specimens and their fatigue life were compared. The starting points of cracks were identified by means of optical and scanning electron microscope examinations and a correlation with the maximum tensile stress regions predicted by the numerical model has been established.     

The influence of coatings on subsurface mechanical properties of laser peened 2011-T3 aluminum

High power Q-switched laser systems are currently being developed for use in a process known as laser shock processing or laser peening which results in significantly improved fatigue properties in aluminum components. An ablative, sacrificial coating such as paint or metal foil is used to protect the aluminum component from surface melting by the laser pulse, which adversely affects fatigue life. This paper, using nano-indentation, analyzes the effect of the paint and foil coatings on the shock wave propagation into the aluminum specimen and the resulting change in mechanical properties versus depth. Near the surface, hardness was found to be increased by the laser peening, however this process decreased the measured elastic modulus. The laser pulse energy density and properties of the foil including its adhesion to the aluminum alloy were found to influence the change in surface mechanical properties.     

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.     

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.     

On the application of laser shock peening for retardation of surface fatigue cracks in laser beam‐welded AA6056

The present study aims to investigate the extent to which the fatigue behaviour of laser beam‐welded AA6056‐T6 butt joints with an already existing crack can be improved through the application of laser shock peening. Ultrasonic testing was utilized for in situ (nondestructive) measurement of fatigue crack growth during the fatigue test. This procedure allowed the preparation of welded specimens with surface fatigue cracks with a depth of approximately 1.2 mm. The precracked specimens showed a 20% reduction in the fatigue limit compared with specimens without cracks in the as‐welded condition. Through the application of laser shock peening on the surfaces of the precracked specimens, it was possible to recover the fatigue life to the level of the specimens tested in the as‐welded condition. The results of this study show that laser shock peening is a very promising technique to recover the fatigue life of welded joints with surface cracks, which can be detected by nondestructive testing.     

The effect of excimer laser surface treatment on the pitting corrosion fatigue behaviour of aluminium alloy 7075

An excimer laser (KrF) operating at a wavelength of 248 nm was used to modify the surface microstructure of 7075-T651 aluminium alloy. The aim was to improve both the corrosion resistance and the pitting corrosion fatigue resistance of the alloy by means of laser surface melting (LSM). The microstructure and the phases of the modified surface structure were analysed, and the corrosion behaviour of the untreated and the laser-treated specimens were evaluated by immersion test. The fatigue resistance of the 7075 alloy has been presented in the form of S/N curves.A microscopical examination and the transmission electron microscopy (TEM) study revealed that LSM caused a reduction both in number and size of constituent particles and a refinement of the grain structure within the laser melted zone. As a result, the corrosion resistance of the aluminium alloy was improved. There was a significant reduction in the number of corrosion pits and shallow attack occurred. The fatigue test results showed that under dry fatigue conditions, the total fatigue life of the laser treated specimens, in which the crack initiation period is of considerable significance, was lower than that of the untreated specimens. However, after shot peening, the fatigue life of the laser treated specimens was recovered. This was primarily attributed to the elimination of surface defects, but also be in part, due to the introduction of compressive residual stresses in the surface layer of the specimen. The fatigue resistance of the shot peened laser-treated specimens, tested in 3.5 wt% NaCl solution with 48 hrs prior immersion, was greater than the untreated specimens with an increase of two orders of magnitude in fatigue life. This was primarily due to the elimination of surface defects and the reduction of corrosion pits.     

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。     

Studies on fatigue life enhancement of pre-fatigued spring steel specimens using laser shock peening

SAE 9260 spring steel specimens after enduring 50% of their mean fatigue life were subjected to laser shock peening using an in-house developed 2.5 J/7 ns pulsed Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) laser for studying their fatigue life enhancement. In the investigated range of process parameters, laser shock peening resulted in the extension of fatigue life of these partly fatigue damaged specimens by more than 15 times. Contributing factors for the enhanced fatigue life of laser peened specimens are: about 400 μm thick compressed surface layer with magnitude of surface stress in the range of −600 to −700 MPa, about 20% increase in surface hardness and unaltered surface finish. For laser peening of ground steel surface, an adhesive-backed black polyvinyl chloride (PVC) tape has been found to be a superior sacrificial coating than conventionally used black paint. The effect of repeated laser peening treatment was studied to repair locally surface melted regions and the treatment has been found to be effective in re-establishing desired compressive stress pattern on the erstwhile tensile-stressed surface.     

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.