Papers by Author: Vincent Ji

Abstract: Laser cladding of a Ni based powder on Cu-Ni-Al or cast iron was performed with a 4kW continuous Nd: YAG laser. The Cu-Ni-Al and cast-iron substrates are used for their thermal properties in glass mold industry. But the issue of these materials is their lack of resistance on corrosion and abrasion. The role of the Ni based alloy is to protect the mold without affecting its thermal properties (Heat Affected Zone (HAZ)). The purpose of this research is to produce a well bonded Ni based melted powder without pores or cracks on a non-planar surface (curvilinear section). An investigation of the impact of the processing parameters, power (1500-3200 W), scanning speed (2.5-10 mm/s) and powder feeding rate (24.5-32.5 g/min) on the bonding quality, the porosity propagation and HAZ appearance is performed. The used methods are neutronography, Scanning Electron Microscopy, Energy Dispersive Spectroscopy and Electron BackScatter Diffraction (EBSD). These multi-scale techniques are obviously complementary. Neutronography is a well-adapted non-destructive method to observe the porosity in the volume thanks to the contrast between materials. EBSD analysis allows us to analyze the microstructural evolution of the coating notably by observing the dendrites growth. This same method also permits to observe the HAZ nature according to the laser cladding parameters. Those methods allowed to optimize the processing parameters in a way to obtain perfect bonding, to avoid porosity propagation and to limit the HAZ emergence.

Abstract: In glass industry, laser cladding is an innovative surfacing technique allowing to deposit a layer of nickel to protect glass mold against corrosion, abrasion and thermal fatigue. This method (powder fusion by projection), well known in additive manufacturing represents a real technological leap for the glass industry. But during laser cladding of Ni-based powder on gray cast iron, cracks can be observed for some process conditions. These cracks are often due to the Heat Affected Zone that creates structural stresses linked to the development of a martensitic structure in the ferritic matrix of the lamellar graphite cast iron. The aim of this work is to observe the impact of laser cladding (without substrate pre-heating usually employed to limit cracking) on the coating behavior but also on the flake-graphite cast iron substrates. The microstructure and the mechanical properties were studied (SEM and microanalysis, microhardness) around the interface cladding/substrate. Also, the impact of the processing parameters (power P (1500-2300 W), scanning speed v (2.5-10 mm/s) and powder feeding rate PFR (24.5-32.5 g/min) was studied by using the ANOVA (ANalysis Of VAriance) technique. It has been observed that laser cladding on graphite cast iron is possible without cracks by limiting the linear energy induced by the process. Also, an optimization of the processing parameters (P, v, PFR) in order to obtain the industrial expected geometry of the coating has been proposed.

Abstract: Strength and ductility are the two most important mechanical properties of a structural material. However, they are often mutually exclusive. In this study, a 6 wt. % TiB₂ nanoparticle reinforced 7075Al (i.e. TiB₂/7075Al) composite was designed and produced by the processing route combining casting, friction stir processing, hot extrusion and T6 heat treatment. The result of tensile testing demonstrates that the as-processed composite sample presents an ultimate tensile strength of 677 MPa and a total elongation to failure of around 15 %, being higher than any Al or Al based materials ever reported. The typical microstructure contains the TiB₂ reinforcement nanoparticles uniformly distributed in the equiaxed Al grain matrix (2 μm in average grain size). In addition to the dispersed nanoprecipitates of the 7075Al (Al-Zn-Mg-Cu) matrix, the integrated TiB₂ nanoparticles are systematically decorated by a shell corresponding to (Zn_1.5Cu_0.5)Mg. This finding challenges our understanding and opens a door for further enhancing strength and ductility being easily scalable for industrial applications.

Abstract: High strength, low density, and excellent corrosion resistance are the main properties that make titanium attractive for a variety of applications. The phase structures and phase transitions of titanium, which are of tremendous scientific and technological interest, have attracted a great deal of attention for many years. In addition to hexagonal close packed α-Ti, high temperature phase β-Ti with body-centered cubic structure and ω-Ti with the hexagonal structure of high-pressure phase, the face-centered cubic structure, which is not in the P-T diagram of titanium, is observed in ultrathin films. In the present paper, the Ti films prepared by magnetron sputtering on MgO(111) single crystal substrate were investigated by means of X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscope (HRTEM). The results showed that the Ti films grow epitaxial with a face centered cubic (fcc) structure even the thickness is up to about 50nm. With the thickness increases, the Ti films transformed to hexagonal close packed (hcp) structure and showed an epitaxial growth along (002)_hcp-Ti direction. The results show that the onset thickness of fcc-hcp structure transformation is 50-100nm. The temperature and power of sputter affect the formation of fcc-Ti.

Abstract: In order to investigate the high temperature oxidation behavior of AISI 430 stainless steel in atmosphere with humidity, the thermogravimetric analysis (TGA) has been conducted for different duration under air with variable absolute humidity (from 0% to 5%) at 800 °C and 900 °C. XRD has been used to determine residual stresses in each layer and in substrate. It has been found that the oxidation kinetics and residual stresses were affected by the water vapor. After oxidation, the oxide scale composed of an inner Chromia (Cr₂O₃) layer and an outer Mn_1.5Cr_1.5O₄ spinel layer, while breakaway oxidation happened with the introduction of water vapor at 900 °C. The residual stresses in each of oxide layer are in compression and their levels varied with oxidation conditions.

Abstract: The objective of this work was to investigate high temperature oxidation behavior of AISI 430 stainless steel, which was proposed to use as interconnector in the planar solid oxide fuel cells (SOFCs). The oxidation of the alloy has been conducted at 700°C, 800°C and 900°C for 12h-96h by thermal gravimetric analysis (TGA) system. The oxide surface morphology, cross-section microstructure and the chemical composition of the oxide scales were performed by FEG-SEM and EDX. The X-ray diffraction (XRD) was used to identify the oxide phases formed on the alloy and to determine the residual stress in the scale. It has been found that the oxide scale composed of a inner Cr₂O₃ layer and an outer Mn_1.5Cr_1.5O₄ layer. The residual stresses in both oxide layers are compressive and the residual stress evolutions in the two layers are different according the oxidation temperature.

Abstract: X-ray diffraction method has been used to analyze the residual stress distribution in weld region of an X70 pipeline steel before and after heat treatment. The welding process has been realized by industrial arc welding with circular weld seams. The effect of heat treatments on the level and the distribution of residual stresses were investigated. Stress distribution was characterized by relative high compressive stresses in weld seam just after welding. However, residual stress relaxation phenomenon was observed in weld region after heat treatments due to microstructure restoration and recrystalization. Optical microscope observation and Vickers hardness measurements were also realized as complementary microstructure characterization techniques.

Abstract: X-ray diffraction method has been used to analyze the residual stress distribution across the joint of Al-Si-12Cu alloy just after welding process. On the other hand, isothermal heat treatments have been applied in order to study the residual stress evolution in this material. Optical microscope observation and Vickers hardness measurement have been carried out as complementary microstructure techniques. The different zones of welded joint have been analyzed, where the fusion zone was characterized by dendritic structure and different to heat affected zone. We have found a softening phenomenon in welded joint after heat according to microhardness results.

Abstract: Oxygen ion diffusion coefficient was measured, for the first time, in oxide films grown by the high temperature oxidation of the AISI 304 austenitic stainless steel. The steel samples were polished and then oxidized in synthetic air in order to grow the chromia oxide (Cr₂O₃) film. The oxygen diffusion experiments in the oxide films were performed by using the¹⁸O isotope as oxygen tracer. The isotope diffusion studies were performed from 750 to 900^o C, in Ar +21%¹⁸O₂ atmosphere and the oxygen ion diffusion profiles were established by secondary ion mass spectrometry (SIMS) analysis. From the¹⁸O diffusion profiles, the bulk, effective and grain boundary diffusivities were determined. Using Wagner´s theory, it is shown that, under the used experimental conditions, the oxygen ion diffusion is large enough to assure the growth rate of the chrome oxide films formed by the oxidation of the AISI 304 stainless steel.

Abstract: In order to investigate the role of oxygen diffusion in the oxidation process of the AISI 439 ferritic stainless steel, oxygen ion diffusion coefficients were determined, for the first time, in oxide films formed by the oxidation of this steel. Steel samples were firstly oxidized from 750^o C to 900^o C, in synthetic air, in order to grow oxide films mainly made up of chrome oxide; the oxygen diffusion experiments were then performed using the stable isotope ¹⁸O as oxygen tracer. The introduction of the ¹⁸O in the oxide film was performed by means of the gas-solid isotopic exchange method, in the temperature range of 750-900^o C, in Ar+21%¹⁸O₂ atmosphere. The ¹⁸O diffusion profiles were established by secondary ion mass spectrometry (SIMS). Parabolic oxidation constants calculated by means of Wagner´s theory, using the oxygen ion diffusion coefficient determined by our experimental process, are greater than oxidation constants previously determined in oxidation experiments from 850 to 950º C, in air, which indicates that the oxygen ion diffusion is large enough to assure the growth rate of the oxide film formed by the oxidation of the AISI 439 steel in these temperatures.