Qun Bo Fan

Full-Scale Numerical Simulation of Plasma-Sprayed Functionally Gradient Materials

To develop novel and advanced thermal barrier coatings, full-scale numerical simulation of plasma-sprayed functionally gradient materials is conducted in this paper, including the prediction of basic parameters at the nozzle exit, simulation of three dimensional simulation of the plasma jet, modeling of the interaction between the plasma jet and the particles, calculation of flight trajectories and temperature history of flying metal and ceramic particles, the interaction between the molten particles and the substrate, as well as the deposition process of the coatings. Various complex phenomena, such as turbulent effects with chemical reactions in the plasma jet, dispersion status of the particles onto the substrate, and the composition distribution of the functionally gradient materials, are fully taken into account. The numerical simulation results are found to be in good agreement with experimental evidence.

Research of the Anti-Penetration Mechanism for Titanium Alloys Armor Based on 3D Optical Scanning Technology

To reveal the anti-penetration mechanism of titanium alloys armor quantitively, the volumes of the front crater region, the ductile hole-enlargement region, as well as the back caving region of Ti-6Al-4V alloy and β20C alloy were measured via 3D Optical scanning technology. The experimental results show that β20C alloy, of which the volume fraction of ductile hole-enlargement region is 51%, 10% larger than that of Ti-6Al-4V alloy, presents a better ballistic performance. It is found that the ballistic performance is closely related to the volume of ductile hole-enlargement region and a relatively larger ductile hole is beneficial to improve the ballistic performance. However, the ballistic performance shows no improvement with increasing the volume of front crater region.

Validation of the Tabulated Johnson-Cook Model for a Dynamic Compression Simulation

MAT224 is a tabulated version of the Johnson-Cook model in LS-DYNA. Compared with the original Johnson-Cook material, MAT224 was developed to simulate the dynamic response of a material by just defining the effective stress as a function of effective plastic strain at different strain rates and temperatures, thus avoiding the tedious parameter fitting procedures in the traditional Johnson-Cook model. However, the stability and precision of solution is strongly dependent on the effective stress versus effective strain curves in MAT224, and unreasonable curve data will lead to warnings or errors in the process of solution. In the current study, a two-dimensional axisymmetric finite element model for the Ti-6Al-4V titanium alloy under dynamic compression was built, and MAT224 was employed. By investigating the effects of the curve numbers, strain ranges, data points, as well as changing tendencies, on the simulation results, the stability and the reliability for MAT224 are systematically studied.

Experimental Study and Numerical Simulation of the Tensile Test for GH4169 Coated with YSZ Coating at High Temperature

In this study, high temperature tensile test was conducted on the specimen of superalloy GH4169 coated with wt8%-YSZ under the conditions of 950°C and 80MPa. By using finite element (FE) simulation method, the failure initiation and evolution of the metal substrate and YSZ coating were predicted. It was found that stress concentration originally occurred in the YSZ ceramic top coating, causing an axial stress and triggering severe debonding failure in the center region of the specimen at t=180s. With increased load, further interfacial debonding failure of the residual coating occurred due to the presence of oblique tensile stress and at t=900s, and only a few residues can be seen at the arc transitional region. Subsequently, the metal substrate was subjected to uniform tensile deformation and finally ruptured at t=76min with apparent necking. In addition, it was notable that YSZ coating can relieve stress significantly (nearly 40% lower), thus helping prolong the substrates service life in the same environment. Simulated results were consistent with observed behavior.

Modified Quartet Structure Generation Set Reconstruction of Finite Element Model for Co-Continuous Ceramic Composites

Co-continuous ceramic composites have a complicated topology structure which makes it much more difficult for finite element model reconstruction. In this paper, the two-dimensional co-continuous ceramic composites finite element model is reconstructed by a modified quartet structure generation set method which modified the generation parameters based on quartet structure generation set (QSGS) method, and a numerical simulation at high strain rate is accomplished. The content mainly contains: (1) The distribution features of metal phase and ceramic phase of real co-continuous ceramic composites SEM image is calculated by mathematical statistics to determine the parameters that control the reconstruction such as volume fraction, core distribution probability and directional growth probability; (2) Two phase volume fraction and 2-point correlation function of the reconstructed finite element model is calculated as the quality assessment parameters, which verify the reconstructed finite element model are in allowable error range compared with the real SEM image; (3) Numerical simulation at high strain rate is carried out using the reconstructed finite element model. The failure behavior of co-continuous ceramic composites at high strain rate is analyzed, validates the reconstructed finite element model meets the requirements of numerical calculation.

The Correlation between Microstructure and Dynamic Mechanical Property of Heat Treated Β20C Titanium Alloy Based on Different Forging Processes

As a new material, the microstructure of β20C titanium alloy can match well with property by forging process. However, the microstructure of billet is inhomogeneous in actual forging. For ensuring microstructures homogenization, two forging processes are designed. Process 1 is large deformation above the phase transition (T=1050°C, 70% deformation) and small deformation in two-phase region (T=860°C~890°C, ≤40% deformation). Process 2 is small deformation above the phase transition (T=1050°C, 40% deformation) and large deformation in two-phase region (T=860°C~890°C, 50%~60% deformation). Then microstructures are observed and dynamic compressive strength and the critical fracture strain of samples are test after solid-solution treatment. It turns out that the homogeneity of microstructure of process 2 is improved by heat treatment. The microstructure is lamellar microstructure with 1650MPa dynamic strength and 15% critical fracture strain through “Process 1 + 840°C 1h/FC” while the microstructure is equiaxed microstructure with 1650MPa dynamic strength and 20% critical fracture strain through “Process 2 + 840°C 1h/FC”. In conclusion, the microstructure of large deformation in two-phase region can accumulate more deformation energy which is beneficial for dynamic recrystallization.

Modeling Dynamic Behaviors of Reticulated Ceramic/Metal Composites (RCMC)

Reticulated ceramic/metal composites (RCMC) are characterized by the ceramic skeleton and metallic matrix both continuous in three-dimensional space, and this special structure makes full use of the ceramic phase’s high strength and the metal phase’s toughness. Correspondingly, the deformation and damages mechanisms of RCMC under dynamic load conditions are very complicated and the related studies are quite important. In the current study, the dynamic properties of RCMC are investigated by using finite element methods based on the factitious microstructures of SiC/Al composites. The ceramic/metal interface is induced by employing a “constrained-tie-break” method, so as to study the effects of interface when suffering from a dynamic impact loads. Effects of the phase distribution, shape, ratio, as well as the size are systematically analyzed. The methods proposed in this paper would be helpful for predicting the dynamic behaviors of RCMC, and developing new ceramic/metal composites.

Modeling the Cracking Process of the YSZ Thermal Barrier Coating under the Thermal Shocking Loads

For low thermal conductivity and high corrosion resistance, yttria stabilized zirconia (YSZ), as a top coat (TC), is widely used in thermal barrier coatings (TBCs), and the micro-structure of the TC has significant effects on it thermal shock resistance. Combining digital image processing technique with finite element mesh generation methods, finite element (EF) models based on actual microstructures of plasma sprayed YSZ thermal barrier coatings are built in this paper, so as to simulate the coating’s dynamic failure process when suffering thermal shocking loads. The cracking process is revealed by calculating both the stress and strain evolutions within the coating. Based on the proposed method, the effects of porosity and distribution are further studied. The simulation results agree well with the experimental observation, indicating that the cracks are mainly caused by pore connectivity, which promotes the growth of cracks. This work is expected to be helpful to establish the quantitative relationship between the TBCs porosity and the coating’s service performance.

Atomistic Simulation of Defect Energy in Pyrochlores and its Effect on Disorder

To enhance the high-temperature stability of zirconate pyrochlore structures, one has to focus on their transformation to the disordered state, fluorite. An atomistic simulation calculation is presented in this paper to predict the propensity of rare earth zirconate pyrochlores to transform to fluorite at high temperature. By detailed calculation of defect formation energy of cation antisites and Frenkel pair, as well as their interactions, the mechanisms of disorder transformation are ascertained. The results show that the tendency of cation disorder is less than the anion’s and disorder transformation will accelerate in advanced stage. The calculation of defect energy in pyrozirconates with different cation on the A site have proved helpful in unraveling their different order-disorder transformation tendency.

Perturbation Molecular Dynamics Simulation of Thermal Conductivity of Zirconia

Thermal conductivity of zirconia and yttrium stabilized zirconia (YSZ) is calculated with perturbation molecular dynamics method (PMD). The results showed that thermal conductivity of YSZ is lower than that of pure zirconia and PMD is a very effective way in thermal conductivity calculation for ceramics. In higher temperature region, the calculated values show a different tendency with the experiment ones, which is because that photon conduction is not considered in PMD method. By taking photon effects into account, the calculated thermal conductivity curve is found fairly well coherent with measurements. In addition, the thermal expansion of zirconia is also presented by calculating the volumes at different temperatures. The results and methods in this paper have been proved to be very helpful in further design of new ceramic materials in the field of thermal barrier coatings (TBCs).