Haiou Zhang

Fundamental study on plasma deposition manufacturing

Direct fabrication of metal or end-use-material components has become one of the most interesting fields in the R&D of materials additive manufacturing (MAM) at present. Plasma deposition manufacturing (PDM), as an emerging MAM technique derived from plasma powder surfacing, has the capacity and predominance of producing fully dense metal or high performance specialty material parts with complex shapes. This process starts with a CAD model and automatically assembles materials to a designed configuration without the need of molding or tooling. The present paper reports initial pilot study of PDM process, and some existing problems and further study are also discussed in detail.

Rapid hard tooling by plasma spraying for injection molding and sheet metal forming

Abstract Amidst the harsh competition over the development of new products around the world, rapid prototyping, especially rapid tooling methods have received widespread attention. Amongst the rapid hard tooling methods, thermal spraying can manufacture metal molds without limitation of pattern size. However, it has the disadvantage that only soft metals with low melting points such as zinc alloy can be sprayed to original mold, such as a rapid prototyping model or a natural material pattern, due to their lack of heat resistance and shrinkage of spray metals. So the wear resistance of spray tool is poor, it can be used only for trial or small-lot production. In this study, attempts were made to improve the heat resistance by using composite materials made of ceramic and metal powders as the sprayed original mold materials, and using stainless steel, tungsten carbide alloy, iron–nickel–chromium alloy with excellent wear resistance as spraying materials, respectively. The results show that injection molding spray mold and sheet metal forming spray die can be made by transferring from natural patterns and rapid prototyping models. As the durability and dimensional accuracy of the sprayed tools has sharply improved, the tools can be used for mass production.

Numerical simulation of coating growth and pore formation in rapid plasma spray tooling

Rapid plasma spray tooling (RPST) is a process that can quickly make molds from rapid prototyping or nature patterns without limitation of patterns size or material. In this paper, the process of coating growth and pore formation in RPST has been analyzed by numerical simulation. The objective of this work was to determine the porosity in plasma sprayed coatings and verify the developed computer model, which might serve for future thermal residual stress studies of plasma sprayed coatings. The analysis was divided into two steps: particle flattening and coating growth. In the analysis, a ballistic model was used for modeling the in-flight powder particles. The method allows for the calculation of off-normal spray angle, which is common in plasma spraying of engineering components. Also, a set of rules for coating growth as well as pore formation in the coating has been proposed. Based on these works, a computer program was developed to calculate the effects of process parameters, such as gun scanning velocity, spray angle, etc., on the porosity of the coating. Finally, an experiment was carried out to verify the effects of spray parameters on the porosity. The results agree with the prediction of the model.

A Node-based Smoothed Point Interpolation Method (NS-PIM) for Three-dimensional Thermoelastic Problems

A node-based smoothed point interpolation method (NS-PIM) is formulated to analyze 3-D steady-state thermoelastic problems subjected to complicated thermal and mechanical loads. Gradient smoothing technique with node-based smoothing domains is utilized to modify the gradient fields and to perform the numerical integration required in the weak form formulation. Numerical results show that NS-PIM can achieve more accurate solutions even when the 4-node tetrahedral mesh is used compared to the finite-element method (FEM) using the same mesh, especially for strains and hence stresses. Most importantly, it can produce an upper bound solution of the exact solution in energy norm for both temperature and stress fields when a reasonably fine mesh is used. Together with FEM, we now for the first time have a simple means to obtain both upper and lower bounds of the exact solution to complex thermoelastic problems.

Numerical simulation of multiphase transient field during plasma deposition manufacturing

A transient solid/liquid/vapor unified mathematical model for plasma deposition manufacturing was developed to investigate the fluid flow and heat transfer of the molten pool and deposition layer. The level-set approach was adopted to deal with the liquid/vapor interface boundary conditions, which considered surface tension gradient (the major driving forces for the melt flow), interface curvatures, buoyancy, and convection heat loss. The mixture continuum model was applied to describe melting and solidification processes at the solid/liquid interface. Moreover, the effects of main processing parameters on the thickness of the deposition layer, full depth of the molten pool and penetration depth of the substrate have been studied further. The experiments agree well with the simulation results.

Effects of scanning path on the deposition process in rapid plasma spray tooling: Modeling by homogenization theory

Rapid plasma spray tooling (RPST) is a kind of process that can quickly make a mold from rapid prototyping or nature pattern without limitation of pattern’s size or material. In a previous investigation of the authors, the process of coating growth, pore formation and its effect on coating’s properties in RPST has been analyzed numerically. The objective of this work is to calculate temperature and stress field during plasma spray process when using a different kind of scanning path. In mesoanalysis, two-scale asymptotic homogenization theory is introduced to predict the effective properties of plasma sprayed coatings with porous. Based on this, in macro process simulation, a FEM software developed system has been used to analyze the effects of gun scanning path on the temperature and stress field. The numerical examples for the four kinds of gun scanning paths are presented. In the scanning paths of s-shape (H), spire in, spire out and s-shape (≤), characteristic of the temperature field under spire out path is the best, and the maximum stress and deflection under spire out path are the smallest after the model cool. 2003 Elsevier Science B.V. All rights reserved.

Fabrication and electrochemical performance of solid oxide fuel cell components by atmospheric and suspension plasma spray

Abstract The theory of functionally graded material (FGM) was applied in the fabrication process of PEN (Positive-Electrolyte-Negative), the core component of solid oxide fuel cell (SOFC). To enhance its electrochemical performance, the functionally graded PEN of planar SOFC was prepared by atmospheric plasma spray (APS). The cross-sectional SEM micrograph and element energy spectrum of the resultant PEN were analyzed. Its interface resistance was also compared with that without the graded layers to investigate the electrochemical performance enhanced by the functionally graded layers. Moreover, a new process, suspension plasma spray (SPS) was applied to preparing the SOFC electrolyte. Higher densification of the coating by SPS, 1.61%, is observed, which is helpful to effectively improve its electrical conductivity. The grain size of the electrolyte coating fabricated by SPS is also smaller than that by APS, which is more favourable to obtain the dense electrolyte coatings. To sum up, all mentioned above can prove that the hybrid process of APS and SPS could be a better approach to fabricate the PEN of SOFC stacks, in which APS is for porous electrodes and SPS for dense electrolyte.

Simulation of microstructure evolution during hybrid deposition and micro-rolling process

Hybrid deposition and micro-rolling (HDMR) is a metal additive manufacturing process that integrates arc direct deposition manufacturing and micro-rolling. A two-dimensional cellular automata and finite volume method coupling model is developed for simulating the microstructure evolution of solidification and the dynamic recrystallization during HDMR forming. The influences of different rolling reductions on dynamic recrystallization fraction, average equivalent radius of recrystallized grains, and the area of dynamic recrystallization region are discussed. The results show that solidification microstructure consists of complete columnar dendrite. The rolling reduction plays a dominant role in determining the area of dynamic recrystallization region and the size of recrystallized grains. The average recrystallized grain size at the top position is not affected by rolling reduction, while the influence of rolling reduction on the dynamic recrystallization fraction and average radius of recrystallized grain is found to be stable, but not linear. The same qualitative and quantitative conclusions are drawn from the experimental results as well.

Numerical Simulation of Transient Multiphase Field During Hybrid Plasma-Laser Deposition Manufacturing

The hybrid plasma-laser deposition manufacturing (PLDM) process is developed based on the plasma deposition manufacturing (PDM) technology. PLDM belongs to the three-dimensional (3D) welding technology and involves the laser power as an augmented heat resource. Compared to PDM technology, the PLDM process has many advantages such as a higher power density, higher processing precision, refined microstructure, and improved mechanical performance of forming components. There exist complicated physical and metallurgical interaction mechanisms due to the combination of PLDM along with the rapid melting and solidification process. Moreover, the interaction between the laser and plasma arc also directly influences the forming quality and precision of the 3D metal components. Therefore, the proposed work is a preliminary attempt to study the transport phenomena in the PLDM process, in which the heat transfer, fluid flow, and molten powder depositing processes have been investigated in detail. The numerical study is performed by using a pressure-based finite volume difference technique after making appropriate modifications of the algorithm. The associated solid/liquid phase transformation process is involved by using an enthalpy-porosity method, and the level-set approach is introduced to track the evolution of weld surface of the deposition layer with powder feeding. An experimentally based hybrid heat input model is developed to involve the influence of the interaction of laser and arc plasma on the redistributed energy absorption by the material. Corresponding experiments of the PLDM process are performed using the same parameters as in the computations, showing a good qualitative agreement.

Mechanisms of Thermo-Solutal Transport and Segregation in High-Pressure Liquid-Encapsulated Czochralski Crystal Growth

The mechanism of dopant transport and segregation in high-pressure liquid-encapsulated Czochralski (HPLEC) grown III-V compound crystals (e.g., GaAs, InP) has been numerically studied using an integrated model, MASTRAPP. The model approximates the melt flow in the crucible as a quasi-steady-state, laminar, and axisymmetric flow, but the gas flow is considered as turbulent. Based on the physics of the growth process, a two-time-level scheme has been implemented where the dopant transport and growth are simulated at a smaller time scale while flow and temperature solutions are obtained from quasi-static calculations. Detailed numerical analyses are performed for the conditions of pure crystal rotation, pure thermally driven natural convection, and pure crucible rotation as well as for mixed flow with all of these forces present simultaneously. The dopant transport and segregation in these cases are well correlated to the corresponding melt flow pattern. Very weak radial segregation is predicted for pure crystal rotation because the resulting melt flow leads to a fairly flat solute boundary layer. The natural convection, on the other hand, produces a nonuniform boundary layer along the melt/crystal interface. This leads to a strong radial segregation with a high concentration along the central axis of the crystal. The crucible rotation has a similar effect. The combined effect of all of these flow mechanisms produces a strong radial segregation, whose extent depends on the relative strength of the driving forces. In all of these cases, strong melt flows lead to thin boundary layers that result in decreased longitudinal segregation. The predictions agree well with the experimental observations reported in the literature.