J.X. Li

Bio: J.X. Li is an academic researcher from University of Kentucky. The author has an hindex of 1, co-authored 1 publications receiving 160 citations.

Enhancement of fatigue and corrosion properties of pure Ti by sandblasting

Abstract: Commercially pure titanium was sandblasted with SiO2 particles of 200–300 μm in diameter. It was found that the sandblasted samples exhibited an increase in fatigue strength by 11% over that of the untreated samples. The peak subsurface compressive residual stress produced by sandblasting was measured by XRD to be around 480 MPa. Three distinct regions were observed in the sandblasted samples, namely the severely deformed surface layer, the region deformed mainly by twinning, and the substrate. After recovery treatment below 300 °C, the surface layer of the sandblasted samples was transformed into a nano-crystalline structure, and its corrosion resistance was significantly improved.

Review of shot peening processes to obtain nanocrystalline surfaces in metal alloys

Abstract: The importance of application of shot peening process to obtain nanocrystal surface is presented with an inclusive clarification of actual state of the art. Description of different shot peening methods which have proved to be able to create nanocrystallised layers is presented. Then the available microstructural characteristics of nanocrystal thin layers obtained with different processes are depicted. In addition, the influence of the process is reviewed on material behaviour under different loading conditions. On this basis, some possible addresses for future research in this field are drawn and underlined.

Effect of structure evolution induced by ultrasonic peening on the corrosion behavior of AISI-321 stainless steel

Abstract: A nanocrystalline surface layer was produced on an AISI-321 stainless steel by severe plastic deformation via ultrasonic peening (UP). The microstructural evolution of the surface layer was characterized by means of X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM). The volume fraction of strain-induced α-martensite as a function of the effective strain ( e ¯ ) was evaluated quantitatively using XRD and magnetic measurements. Considering the e ¯ magnitudes and the TEM data obtained, it is concluded that a grain refinement of austenitic structure passes ahead of the α-martensite formation, particularly in the top surface layer. The nanocrystalline austenitic grain structure (mean grain size ∼ 15 nm) was observed at e ¯ = 0.45 , while the startup of the strain-induced martensitic transformation was revealed at the strain extent of 0.62. The nanostructured surface layer formed after straining to e ¯ = 0.8 already contains mainly the martensite nanograins characterized by an average size of about 10 nm. Grain size increased gradually up to 60 nm within the layer containing both austenite and martensite phases at a depth of about 30 μm from the treated surface. Both the microhardness behavior of the stainless steel surface and its corrosion performance in 3.5% NaCl solution can be enhanced by the UP. They are shown to be in correlation with: (i) the grain refinement process and (ii) the increase in the volume fraction of strain-induced α-martensite.

Titanium in biomedical applications—properties and fabrication: a review

Abstract: Ti and Ti-based alloys have unique properties such as high strength, low density and excellent corrosion resistance. These properties are essential for the manufacture of lightweight and high strength components for biomedical applications. In this paper, Ti properties such as metallurgy, mechanical properties, surface modification, corrosion resistance, biocompatibility and osseointegration in biomedical applications have been discussed. This paper also analyses the advantages and disadvantages of various Ti manufacturing processes for biomedical applications such as casting, powder metallurgy, cold and hot working, machining, laser engineering net shaping (LEN), superplastic forming, forging and ring rolling. The contributions of this research are twofold, firstly scrutinizing the behaviour of Ti and Ti-based alloys in-vivo and in-vitro experiments in biomedical applications to determine the factors leading to failure, and secondly strategies to achieve desired properties essential to improving the quality of patient outcomes after receiving surgical implants. Future research will be directed toward manufacturing of Ti for medical applications by improving the production process, for example using optimal design approaches in additive manufacturing and investigating alloys containing other materials in order to obtain better medical and mechanical characteristics.

A numerical model of severe shot peening (SSP) to predict the generation of a nanostructured surface layer of material

Abstract: Generation of a surface layer of material characterized by grains with dimensions up to 100 nm by means of severe plastic deformation is one of the most interesting methods to improve the mechanical behaviour of materials and structural elements. Among the ways to obtain a surface layer with this characteristic, shot peening is one of the most promising processes, since it is applicable to very general geometries and to all metals and metal alloys without high-tech equipments. Notwithstanding the fact that the ability of shot peening to obtain nanostructured surfaces by using particular process parameters (mainly high impact energy and long exposure time) is proved, deep knowledge of the correct choice of quantitative values of process parameters and their relation to the grain size and the thickness and uniformity of the nanostructured layer is still lacking. In this paper a finite element model of severe shot peening (SSP) is developed with the aim of predicting the treatment conditions that lead to surface nanocrystallization. After having assessed the accuracy of the model as regards mesh parameters and constitutive law of the material, the results are discussed and interpreted in terms of induced residual stresses and surface work hardening. A method to assess the formation of nanostructured layer of materials based on the value of the equivalent plastic strain is developed. The comparison with experimental results allow to affirm that the model is a useful tool to predict the generation of a nanostructured surface layer by shot peening and to relate the peening parameters with the treated surface layer in terms of residual stresses, work hardening, and depth of the nanostructured layer.