Wenda Tan

Understanding the particle capture mechanism in the laser directed energy deposition additive manufacturing process provides a foundation for improving productivity and reducing material loss. Capturing this phenomenon is challenging due to its highly transient time interval. To analyze the impact behavior of spherical Ti-6Al-4V powder, a custom laser directed energy deposition system was designed to control the deposition of individual particles. Synchrotron X-ray imaging at 24 kHz monitored the impact dynamics of individual powder particles. A multi-physics model based on the Smoothed-Particle Hydrodynamics scheme supported the analysis by providing the temperature and velocity fields of the molten pool. Results revealed that in laser directed energy deposition, surface tension forces dominated the powder–molten pool interaction. Hydrophobic Ti-6Al-4V powder particles had an equilibrium contact angle of 121 degrees with their molten state. Furthermore, Ti-6Al-4V powder particles required a high impact velocity (6 to 12 m/s) to transition from oscillation to submergence behavior. Lastly, oscillating powder particles that impacted near the location of the laser beam exhibited faster melting, highlighting the contribution of the laser beam to the melting mechanism. This work lays the foundation for investigations of powder particle impact in laser directed deposition and supports the validation of numerical models of powder–molten pool interactions.

Abstract This study investigates domain decomposition strategies for high-performance smoothed particle Galerkin (SPG) modeling of single-grit diamond scribing of silicon carbide fiber-reinforced silicon carbide (SiC f /SiC) composites. Domain decomposition enables massively parallel processing (MPP) by partitioning the SPG model into subdomains, each assigned to a processor for parallel execution. The SPG model without domain decomposition is first validated against experimentally measured scribing forces and is used as the baseline. The computational time and force prediction error associated with three domain decomposition strategies are evaluated to identify the trade-off between scaling efficiency and numerical stability. Three principles for optimal domain decomposition are identified. First, subdomains should contain similar numbers of particles to ensure balanced workload across processors and maximize computational efficiency. Second, the number of subdomains should be selected to balance computational speedup and numerical stability. Too few subdomains underutilize available computational resources, while too many increase inter-subdomain communication and may amplify numerical discrepancies and force fluctuations. Third, subdomain boundaries should avoid high-deformation and high-contact regions. Aligning subdomains parallel to the cutting direction confines active deformation within fewer subdomains, reduces data exchange, and improves both accuracy and computational efficiency. Based on these findings, we envision that GPU-accelerated high-performance computing (HPC) architectures with thousands of processing cores can enable large-scale multi-grit grinding simulations using SPG modeling.

Pore closure during hot isostatic pressing (HIP) is often accompanied by localized microstructural refinement in additively manufactured (AM) Ti alloys. However, the mechanism pathway from thermo-mechanical deformation to dynamic recrystallization (DRX) remains difficult to quantify. Here, a coupled finite-element and phase-field (FE-PFM) framework is developed to link pore-scale inelastic deformation to DRX near pore-affected regions. The FE model resolves HIP-driven pore shrinkage using an elastic-plastic-creep constitutive law and provides spatially and temporally varying inelastic strain-rate histories. These histories are then supplied to a multiphase-field DRX formulation in which dislocation density evolves via a Kocks-Mecking model and drives stored-energy-controlled nucleation and grain-boundary migration. The simulations reproduce key experimental trends of pore closure and pore-associated equiaxed grain clusters. Finite-element results show that densification arises from the combined action of creep and plastic deformation. Parametric studies at 50, 100, and 175 MPa further demonstrate that, while pore closure increases with pressure, the contribution of plastic deformation is regulated by competition between sustained stress concentrations and creep-driven stress redistribution. Moreover, recrystallization is governed by the integrated thermo-mechanical history, specifically the temporal overlap between the availability of potential nucleation sites and the nucleation kinetics, rather than by peak strain rate or pressure magnitude alone. Together, these results provide a mechanistic basis for HIP process optimization aimed at achieving simultaneous densification and microstructural refinement.

Abstract Hierarchical porous copper (Cu) structures are promising for applications in filtration, catalysis, and lightweight components due to the strength, durability, and permeability of Cu. However, one key challenge in manufacturing is achieving precise control of the pore size distribution within these structures. This study introduces a novel approach that integrates the space-holder technique with binder jetting additive manufacturing to fabricate Cu structures with both meso- and macro-scale porosity. In the initial stage, sodium chloride (NaCl) powder was used as a removable space holder within a Cu powder matrix to fabricate samples. NaCl in the samples was later dissolved by water to create meso-scale pores. The final porosity could be tuned by adjusting the volume percentage of NaCl within the mixed powder. In the second stage, the Cu-NaCl mixture was used to print samples using the binder jetting technique. The meso-scale pores were formed through the removal of the NaCl space holder, while the macro-scale channels were directly printed according to specific design specifications. The porous structures were characterized with optical microscopy and X-ray micro-CT scanning. The pore morphology and distribution were measured. This study demonstrates a versatile and scalable method for producing porous Cu structures with customizable porosity levels and pore size distributions. This method is tailored to cater to various industrial applications. Additionally, it holds the potential for optimization and application to other metal systems, highlighting its broad applicability in manufacturing advanced porous materials with multifunctional capabilities.

Abstract Laser keyhole welding with multiple lasers in proximity has been an emerging technology in industrial applications. Compared with the commonly used single-laser system, the application of multi-laser welding provides different heat distributions to enable additional processing parameter manipulations, which can be particularly useful for certain applications. While experimental studies have revealed the keyhole geometries and molten pool dimensions in the multi-laser welding process, the dynamic behavior of multiple coexisting keyholes and the resultant fluid flow within the molten pool have not been well understood. In this work, a multi-physics numerical model is used to predict the transient and nonuniform distributions of laser absorption, temperature, and flow velocity in the tri-laser welding process. Two power settings that lead to different keyhole geometries are investigated. A detailed discussion is given to elucidate the effects of laser drilling within the high-temperature molten pool and its impact on enhanced overall laser absorption of the molten pool. A comparative analysis between single-laser and tri-laser keyhole welding is given to evaluate the significance of different driving forces acting on the molten pool to the keyhole dynamics.

Abstract Laser keyhole welding of dissimilar metals has been widely used in industrial applications. One critical challenge for this process is the formation of intermetallic compounds (IMCs) that undermine the electrical and mechanical properties of the joints. Compared with the commonly used linear contours, welding with spiral contours can provide larger areas of joining and hence higher allowable loading. This can be particularly useful for certain applications. In this research, laser welding experiments with different spiral contours were performed, and the chemical composition, microstructure, and mechanical properties of the joints were characterized. Three spiral distances were used in the experiments. As the spiral distance was changed from 0.1 mm to 0.3 mm and 0.5 mm, the average Cu concentration in the upper region of the joints was decreased, lower amounts of IMCs were found in the joints, and the joints were capable of sustaining higher mechanical loading.

Conspectus Additive Manufacturing (AM) technology produces three-dimensional components in a layer-by-layer fashion and offers numerous advantages over conventional manufacturing processes. Driven by the growing needs of diverse industrial sectors, this technology has seen significant advances on both scientific and engineering fronts. Fusion-based processes are the mainstream techniques for AM of metallic materials. As the metals go through melting and solidification during the printing processes, the final microstructure and hence the properties of the printed components are highly sensitive to the printing conditions and can be very different from those of the feedstock. It is critical to understand the process-microstructure-property relationship for the accelerated optimization of the processing conditions and certification of the printed components. While experimentation has been used widely to acquire a mechanistic understanding of this subject matter, numerical modeling has become increasingly helpful in achieving the same purpose. In this Account, the authors review their ongoing collaborative effort to establish a multiphysics modeling framework to predict the process-microstructure-property relationship in fusion-based metal AM processes. The framework includes three individual modules to simulate the dominating physics that dictate the process dynamics and microstructure evolution during printing as well as the responses of the printed microstructure to specific mechanical loadings. The process model uses the material properties and processing conditions as the inputs and simulates the laser-material interaction, multiphase thermo-fluid flow, and fluid-driven powder motion. It has successfully revealed the physical causes of depression zone shape variation as well as powder motion during the laser powder bed fusion process. The microstructure model uses the thermal history of the printing process and the material chemistry as the inputs and predicts the nucleation and growth of multiple grains in the multipass and multilayer printing processes. It has been used to understand the effects of inoculation and thermal conditions on grain texture evolution. The property models use microstructure data from simulations, experimental measurements, or statistical analyses as the inputs and leverage various computational tools to predict the mechanical response of the AM materials. These models have been used to quantitatively evaluate the effects of grain structure, residual strain, and pore and void defects on their properties and performance. While this and many other modeling works have significantly grown our collective knowledge of the process-microstructure-property relationship in fusion-based metal AM processes, efforts should be further invested in developing advanced theories and algorithms for the governing physics, leveraging data-driven approaches, accelerating simulation speed, and calibrating/validating models with controlled experimental measurements, among other aspects.

This paper proposes a smart security check system of urban rail transit based on the current demand for developing security checks considering the inefficient efficiency, mismatched mode, and poor performance of security checks. This paper analyzes the crucial technologies, which include differentiated security check mode, process and facility optimization, and security and ticket check integration. A simulation model of Tianruncheng Station of Nanjing Metro is established to verify the effectiveness of the key technologies. The proposed method provides a theoretical basis for developing a smart security check system.

Urban rail transit is in the stage of rapid development in China. Many cities planned and are building new rail transit lines. The access to new urban rail transit lines will affect the distribution of passenger flow and passenger travel time in the existing urban rail network. This paper takes Nanjing Metro Line 4, which opened in 2017, as an example to study the influence of new line access operation on passenger flow distribution, and passenger travel time of the urban rail system. Firstly, the influence of new line access on passenger flow distribution and passenger flow in short-term operation is analyzed. Then, the influence of new line access network on passenger travel time and its reliability after short-term and long-term operation reaches the stable stage is analyzed, and the rule is summarized. The research results show that access to the new line will enhance the total passenger flow of the subway system and ease the congestion of crowded stations. It can also improve the travel time reliability to CBD during peak hours, reduce the travel time reliability to CBD during non-peak hours, and reach a stable stage two years after the new line accesses the network.