Songzhe Xu

1. Ni-based superalloys, owing to their superior high-temperature mechanical performance, microstructural stability, and corrosion resistance, are widely regarded as key structural materials for ai...

During the Laser Powder Bed Fusion (L-PBF) process, the occurrence of defects such as cracks and pores significantly reduces the mechanical properties and service life of the parts. Therefore, in-depth research on the non-equilibrium metallurgical processes in L-PBF is of great importance. In this study, utilizing synchrotron radiation X-ray imaging technology, we investigated the formation and movement mechanisms of pores in Hastelloy X during the L-PBF process. By adjusting processing parameters, we observed the pore behaviour within the molten pool under different keyhole modes and identified that higher laser power deepens the vapour depression, while higher scanning speed generally yields a shallower, more stable keyhole by reducing keyhole-wall oscillations. The study further confirmed three types of keyholes formed under different process parameters, along with their corresponding pores distribution characteristics. By tracing pores, we revealed the melt pool flow dynamics within the Hastelloy X melt pool during L-PBF. The observational results and schematic diagrams of the two-dimensional keyhole model presented a solid theoretical basis and practical reference for optimizing L-PBF process parameters by linking keyhole morphology or stability (depth and oscillation) with pore formation, escape, and entrapment, thereby reducing porosity and enhancing material density and component performance.

The quality of metal powder is essential in additive manufacturing (AM). The defects and mechanical properties of alloy parts manufactured through AM are significantly influenced by the particle size, sphericity, and flowability of the metal powder. Gas atomization (GA) technology is a widely used method for producing metal powders due to its high efficiency and cost-effectiveness. In this work, a multi-phase numerical model is developed to compute the alloy liquid breaking in the GA process by capturing the gas–liquid interface using the Coupled Level Set and Volume-of-Fluid (CLSVOF) method and the realizable k-ε turbulence model. A GA experiment is carried out, and a statistical comparison between the particle-size distributions obtained from the simulation and GA experiment shows that the relative errors of the cumulative frequency for the particle sizes sampled in two regions of the GA chamber are 5.28% and 5.39%, respectively. The mechanism of powder formation is discussed based on the numerical results. In addition, a discrete element model (DEM) is developed to compute the powder flowability by simulating a Hall flow experiment using the particle-size distribution obtained from the GA experiment. The relative error of the time that finishes the Hall flow in the simulation and experiment is obtained to be 1.9%.

In this work, a novel internal heat absorption technology using inorganic material rods is employed during the solidification process of steel ingots, aiming to control their solidification and improve the quality of the final product. The study investigates the effect of the insertion depth of inorganic materials on the solidification microstructure and macrosegregation of 2.5-ton 42CrMo ingots. The mechanical properties of samples from the product are also tested. A numerical simulation model for casting 2.5-ton ingots is established and implemented in Ansys Fluent fluid simulation software, with inorganic material rods set at different preset depths. The simulation explores the physical processes of the melting and floating of inorganic materials in molten steel, as well as their effects on the temperature and flow fields of the material. The results show that deeper insertion of inorganic materials (200 mm from the hot top) reduces the tendency for macrosegregation compared to that at the insertion depth of 100 mm. Specifically, the positive segregation area decreases by 10.35%, while the negative segregation area decreases by 15.32%. Moreover, deeper insertion results in a significant refinement of the solidification microstructure, ultimately enhancing the mechanical properties of the products machined from the ingots (i.e., the yield strength increased by 4.7%). The numerical simulation results indicate that as the placement depth of inorganic materials in the ingot mold increases, the cooling effect becomes more significant, the flow area of molten steel initiated by the inorganic materials expands, and the linear velocity of the double-circle flow increases. This further explains why the solidification quality of the ingots improves with the increasing placement depth of inorganic materials.

A solar chimney is a typical device to harness naturally available energy resources to power ventilation inside buildings. A comparative computational fluid dynamics study of the flow and thermal mechanics inside solar chimneys is performed in this study. We utilize a variational multiscale formulation to model the combined turbulent/laminar flow regimes presented in the natural ventilation problem in the sense of large eddy simulation. Nitsche type weak enforcement of Dirichlet boundary conditions is integrated into the numerical framework to address the excessive mesh resolution requirement in flow and thermal boundary layers. Numerical methodology is verified and validated against experimental data in a model room with a solar chimney, and good agreement between the present results and the reference data is obtained. Finally, the thermofluid characteristics are investigated in a building equipped with different solar chimney designs. Particular emphases are placed on the effects of attached building spaces to the flows within the chimneys. The results indicated that the complex and realistic building space in this paper reduces the turbulence entering the solar chimney inlet and, thus, increases the air flow rate by up to 48.9% compared with the frequently used standalone chimney simulation paradigm. In addition, the thermal comfort indices are presented. With increased air flow rate in the chimney, the overall thermal comfort in the building is likely to be decreased, suggesting the necessity in the future studies to consider thermal comfort as an optimization objective.

We develop a parallel finite element computational framework based on the level set method and adaptive octree meshes for free‐surface flows. The discretized governing formulations are stabilized using a residual‐based variational multiscale method. We redistance the level set field and restore mass conservation after each time step. The octree mesh is adaptively refined according to the interface location, and the load is rebalanced across processes. The adaptive octree‐based level set method is verified and validated using a canonical problem of dam break without obstacle, and a mesh convergence study is carried out in this problem. The surface tension is further considered via a continuum surface force model in the method and validated using a problem of single bubble rising in ambient liquid. A parallel scaling analysis of this method is also performed for the bubble rising problem. The energy equation and Marangoni effect are finally considered in the method and validated using a problem of thermocapillary droplet migration with and without gravity. This work illustrates the ability of the framework to accurately model and predict free‐surface flows under various scenarios.