Liuhe Li

Bio: Liuhe Li is an academic researcher from Beihang University. The author has contributed to research in topics: Plasma-immersion ion implantation & Ion implantation. The author has an hindex of 16, co-authored 72 publications receiving 1640 citations. Previous affiliations of Liuhe Li include City University of Hong Kong & Shanghai Jiao Tong University.

Effects of heat treatment combined with laser shock peening on wire and arc additive manufactured Ti17 titanium alloy: Microstructures, residual stress and mechanical properties

Abstract: Wire and arc additive manufactured (WAAM) metal parts usually contain large columnar grains and detrimental tensile residual stress, affecting their mechanical performance. In this work, a post-treatment method combining heat treatment (HT) and laser shock peening (LSP) was employed to alter the microstructure and mechanical properties of WAAM Ti17 titanium alloy. The results show severe plastic deformation was induced in the surface layer, which, in turn, led to a high-level surface compressive residual stress (~−763 MPa) by combination treatment of HT and LSP. Meanwhile, high-density dislocations and mechanical twins were observed in coarse α phases after treatment by laser shock wave, and gradually evolved into refined α phases. The elongation of samples was significantly improved by 15% while ensuring original ultimate tensile strength (UTS, 1153 ± 13 MPa) after HT and LSP treatment. This combined HT and LSP method helps enhance the mechanical performance of WAAM parts through changing their microstructure and residual stress distribution.

Laser shock peening induced fatigue crack retardation in Ti-17 titanium alloy

Abstract: Laser shock peening is an advanced surface treatment technique of great interest introducing beneficial compressive residual stress and further enhancing fatigue crack propagation resistance of metallic components. In this study, fatigue crack propagation and subsequent retardation of Ti-17 titanium alloy under laser shock peening are presented. Varying degrees of fatigue crack retardation were observed after peening with pulse energy of 20 J and 30 J. The fatigue life was increased up to 2.4 times that of the unpeened counterpart. The fatigue arrests were observed in the deceleration zone after peening, showing different angles with the fatigue crack path as the peening energy varied. The fatigue crack retardation mechanism based on the plastic zone size and crack propagation energy density drop at the crack tip was further discussed, and a crack tip energy density criterion was proposed to quantitatively understand the fatigue crack retardation.

X-Ray Diffraction

Abstract: Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

Substrate-free gas-phase synthesis of graphene sheets

Abstract: A substrate-free gas-phase synthesis apparatus and method that is capable of rapidly and continuously producing graphene in ambient conditions without the use of graphite or substrates is provided. Graphene sheets are continuously synthesized in fractions of a second by sending an aerosol consisting of argon gas and liquid ethanol droplets into an atmospheric-pressure microwave-generated argon plasma field. The ethanol droplets are evaporated and dissociated in the plasma, forming graphene sheets that are collected. The apparatus can be scaled for the large-scale production of clean and highly ordered graphene and its many applications. The graphene that is produced is clean and highly ordered with few lattice imperfections and oxygen functionalities and therefore has improved characteristics over graphene produced by current methods in the art. The graphene that is produced by the apparatus and methods was shown to be particularly useful as a support substrate that enabled direct atomic resolution imaging of organic molecules and interfaces with nanoparticles at a level previously unachievable.

FTIR Spectroscopy for Carbon Family Study

Abstract: Fourier transform Infrared (FTIR) spectroscopy is a versatile technique for the characterization of materials belonging to the carbon family. Based on the interaction of the IR radiation with matter this technique may be used for the identification and characterization of chemical structures. Most important features of this method are: non-destructive, real-time measurement and relatively easy to use. Carbon basis for all living systems has found numerous industrial applications from carbon coatings (i.e. amorphous and nanocrystalline carbon films: diamond-like carbon (DLC) films) to nanostructured materials (fullerenes, nanotubes, graphene) and carbon materials at nanoscale or carbon dots (CDots). In this paper, we present the FTIR vibrational spectroscopy for the characterization of diamond, amorphous carbon, graphite, graphene, carbon nanotubes (CNTs), fullerene and carbon quantum dots (CQDs), without claiming to cover entire field.

History of diamond-like carbon films — From first experiments to worldwide applications

Abstract: Diamond-like carbon (DLC) films combine several excellent properties like high hardness, low friction coefficients and chemical inertness. The DLC coating material can be further classified in two main groups, the hydrogenated amorphous carbon (a-C:H, ta-C:H) and the hydrogen free amorphous carbon (a-C, ta-C). By adding other elements like metals (a-C:H:Me) or non-metal elements like silicon, oxygen, fluorine or others (a-C:H:X), several modifications of the properties can be adjusted according to application requirements. First reports on hard amorphous carbon films were published in the 1950s and about 20 years later there began worldwide intensive research activities on DLC. In the following years the number of publications increased continuously and the importance for industrial applications became more and more evident. Several deposition techniques were applied to prepare a-C:H, ta-C, metal containing a-C:H:Me and non-metal containing a-C:H:X coatings. In parallel the structure and deposition mechanisms of DLC coatings were extensively studied. An essential obstacle for a broad industrial application was the high compressive stress level in a-C:H films causing delamination and limiting the film thicknesses. With metal based intermediate layer systems most adhesion problems could be solved satisfactorily and thus from the mid-1990s the pre-conditions for a broad application especially in the automotive industry were given. With modified a-C:H:X and a-C:X coatings a considerable friction reduction or surface energy adjustments could be achieved.