Laser shock peening is one of the most effective surface strengthening techniques, which uses laser shock-induced plastic deformation to optimize surface stress state and microstructures of target ...material. In this paper, dislocation dynamics simulation was used to investigate laser shock induced ultra-high strain rate plastic deformation of face-centered cubic (FCC) nickel and body-centered cubic (BCC) iron. Molecular dynamics was employed to calculate dislocation mobility. Based on the obtained dislocation mobility coefficient, dislocation dynamics models of nickel and iron were established. Results show that the velocity of dislocation motion increases as temperature decreases. Under ultra-high strain rate deformation, dislocation density of nickel increases while dislocation density of iron decreases as temperature rises. Moreover, iron exhibits thermal softening while nickel exhibits thermal hardening under laser shock loading. Plastic deformation dominated by dislocations is sensitive to loading direction, depending on the Schmidt factor of the slip system. The ultra-high strain rate induced by laser shock can effectively increase dislocation density by promoting dislocation multiplication and suppressing dislocation annihilation.
•LSP improves the fatigue property of Ti-6Al-4V alloy after FOD.•The improvement of fatigue property is strengthened by TSLSP-I.•TSLSP-I makes a “soft and hard” alternating energy absorption ...structure.
Two-sided and simultaneous laser shock peening (TSLSP) was used to strengthen the Ti-6Al-4V titanium alloy. The fatigue crack growth rate of the specimens was investigated by fatigue test. The results show that both direct TSLSP (TSLSP-D) and indirect TSLSP (TSLSP-I) can reduce the fatigue crack growth rate by producing high magnitude compressive residual stress. The maximum fatigue life increases by 94% (TSLSP-D) and 169% (TSLSP-I) compared with the original specimens. Moreover, the TSLSP-D results show decreased resistance to foreign object damage because of decreased plasticity while the TSLSP-I simultaneously achieves superior foreign object damage resistance and fatigue performance.
Although drilling open holes in fibre metal laminates reduces weight, it brings potential crack initiation sites. This paper investigates the effect of laser shock peening (LSP) on fatigue ...performance of open-hole titanium-based carbon-fibre/epoxy laminates (Ti-CF FMLs). A method based on multi-step simulation was proposed for the reconstruction of LSP induced residual stress field in open-hole Ti-CF FMLs. The results show that the fatigue initiation life of open-hole Ti-CF FMLs is improved by 22.1 %–81.0 % after LSP, depending on the structure of FMLs and the applied fatigue load. The improved fatigue crack initiation life attributes to compressive residual stress generated throughout the entire depth of the hole wall in outer titanium layer in Ti-CF FMLs after LSP. However, LSP leads to the formation of tensile residual stress far from the hole edge, resulting in accelerated propagation of fatigue crack. The fatigue initiation lives of open-hole Ti-CF FMLs before and after LSP were calculated by finite element simulation and theoretical method, showing good consistency.
Laser texturing has been proven to be an effective method to improve the tribological behavior of titanium and its alloys, while the hardness of the textured surface is still poor. To further ...strengthen the surface, a two-step laser processing experiment was conducted on Ti6Al4V alloy including femtosecond laser texturing and in-situ nitriding. Firstly, the sample was textured with femtosecond pulses at laser fluence of 6.37 J/cm2 and a repetition rate of 200 kHz. Then, the textured surface was micro-melted and nitrided by irradiation in a nitrogen atmosphere at a low laser fluence of 0.43 J/cm2 and a high repetition rate of 19 MHz. After nitriding, a uniform crack-free TiN coating was prepared on the top of the as-textured structures with thickness of 40–60 μm, and the surface hardness was significantly improved from 336 HV to 1277 HV. The reinforced phase TiN had microstructures of dendrites in the upper region and particles in the bottom region of the nitrided layer. The morphology analysis revealed that only a slight change in height was introduced to the textured structures by femtosecond laser nitriding and the designed functions could be maintained.
Laser shock peening of titanium alloys has been widely applied in the aerospace industry. However, little is known of the nanocrystalline formation characteristics and mechanisms. In this ...investigation, a nanocrystalline layer was formed in the duplex Ti-6Al-4V titanium alloy surface region by means of multiple pulsed laser shock peening (LSP). The phase transition and residual stress characteristics of LSP samples were analyzed with X-ray diffraction (XRD) and scanning electron microscopy (SEM). Transmission electron microscopy (TEM) was used to characterize the microstructure and morphologies. As the number of laser pulses increased for each location, higher grain refinement was observed. Micro-hardness testing showed that hardness increased with the number of pulses delivered to each location due to the formation of nanocrystalline layers and high dislocation density in the samples, and a gradient variation of the micro-hardness was obtained. In addition, mechanical twins and different dislocation configurations were formed in the α phase region while only dense dislocation tangles were observed in the β phase region after multiple laser pulse impacts.
High entropy alloys (HEAs) have attracted much attention and been considered as novel materials with wide application prospects due to their outstanding comprehensive properties. With the development ...of laser processing technology, laser additive manufacturing (LAM) is regarded as a particularly feasible way to fabricate high entropy alloys. In this study, a relatively dense equiatomic FeCrCoMnNi high entropy alloy with excellent mechanical properties is manufactured by LAM successfully. Effect of heat treatment on phase, microstructure, microhardness, residual stress and mechanical properties of as-built (AB) specimens are investigated. The results show that the AB specimen exhibits a single FCC solid solution with mixed epitaxial grown dendrite columnar grain and equiaxed grain microstructure. After heat treatment, the dendritic microstructure of AB specimen evolves into recrystallized grain structure of HT-1100 specimen (heat treated at 1100 °C), while the crystalline structure is hardly changed. The AB specimen exhibits a superior microhardness and mechanical properties to that of the corresponding arc-melting specimen. The HTed specimen has much higher ductility than that of the AB specimen without distinct sacrifice of strength.
•A relatively dense FeCrCoMnNi HEA is manufactured by LAM.•The LAMed specimen exhibits a single FCC solid solution with non-equilibrium microstructure.•The LAMed specimen has excellent microhardness and mechanical properties.•Heat treatment can improve the ductility of LAMed specimen without distinct sacrifice of strength.
Laser shock peening (LSP) is a surface modification technology to synthesize nanostructures on the surface layer of materials, thereby improving mechanical performances. In this work, a laser ...additive manufactured CoCrFeMnNi high-entropy alloy (HEA) is processed using LSP. The microstructure evolution during LSP is investigated via electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) observations. The micro-hardness distributions on cross-sections and subsurface residual stress states as functions of the laser energy are also determined. Results indicate that the LSP results in a thick hardened layer with high microhardness, and transforming the tensile stress in the subsurface into compressive stress. The microstructure apparently refines because of the formation of nanoscale grains on the surface after LSP. A novel grain refinement mechanism under ultra-high plastic strain is proposed. Surface strengthening modes are proposed to describe the relationship between strengthening behaviors and microstructure characteristics, thereby revealing the strengthening mechanism.
Extreme high-speed laser cladding (EHLA) was used to develop a pore- and crack-free TiC/IN625 coating. The surface roughness of EHLA could be as low as 6.004 µm. A more uniform dendrite size and ...composition distribution was obtained by EHLA. The mass gain of EHLA was 57.5% lower than that of the conventional laser cladding sample after hot corrosion in a mixed salt of 75%Na2SO4 + 25% NaCl at 900 ℃ for 60 h. High hot corrosion resistance of EHLA was mainly attributed to the lower surface roughness, smaller dendrite size, smaller grain size, and higher amount of low-angle grain boundaries.
•Pores- and cracks-free TiC/IN625 coatings over AISI 1045 steel were firstly prepared by EHLA.•EHLA TiC/IN625 coatings possessed much more uniform and finer microstructure.•The uniformly distributed dendrite sizes and compositions contributed to the formation of a dense oxide film.•Superior hot corrosion behavior of EHLA TiC/IN625 coatings was mainly attributed to the lower surface roughness, smaller dendrite size, smaller grain size, and more low-angle grain boundaries.
This paper investigates the mechanical properties of pure nickel with gradient microstructure produced by laser shock peening. Quasi-static tensile tests were performed on gradient nickel with ...different thicknesses. The results show that the yield strength increases while elongation decreases as the volume fraction of the gradient microstructure layer increases. Moreover, the necking in the width direction decreases as the volume fraction of the gradient layer increases. The gradient nickel produced by laser shock peening has a significant synergetic strengthening effect. The synergetic strengthening effect originates from the transformation of uniaxial stress to biaxial stress as well as stress and strain gradient produced during tensile process. Moreover, the maximum synergetic strengthening of the gradient nickel depends on the competition relationship between stress and strain gradients.
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