The characteristics of molten pools provide valuable insight into the complexity of the metal additive manufacturing process, which has a significant influence on the quality of the parts built using this process. In our work, we develop a three-dimensional multiphysics finite element model of a selective laser melting (SLM) process on a Ti–6Al–4V alloy. The dynamic characteristics of the molten pool are studied by multiphysics simulation with consideration of phase transitions, recoil pressure, surface tension, and Marangoni effect. The results show the time-evolution of temperature distribution, flow field, and surface morphology of a single track during the SLM process. The recoil pressure caused by evaporation plays a significant role in molten pool dynamics and induces a depression at the head of the molten pool. As a result of the backward Marangoni flow, the material is shifted to the tail region and a vortex is generated. In addition, a protrusion is presented at the middle and start points of the scanned track, while a depression is formed at both sides and at the terminal point. The simulation and the experimental results on the surface morphology of the molten track during the SLM process are in good agreement. Furthermore, it is found that metal evaporation may take place not only on the surface of the molten pool but also inside it, due to the drop in pressure. This is a significant contributor to the formation of porosity in SLM parts.

Interface disbond in thermal barrier coatings (TBCs) is one of the key issues that cause their premature failure. In general, blind hole defects are often used as substitutes in transient thermography. The linear laser fast scanning thermography (LLFST) method was developed in this study and combined with several post-processing algorithms to accurately detect blind hole defects in TBCs. Through numerical simulation and experimental verification, a unique thermal response characteristic of blind holes in the cooling phase, namely a distinct “tailing” phenomenon, was summarized and utilized to recognize small defects. Validation tests indicated that blind holes with diameters of 1, 2, and 3 mm and artificial disbonds with diameters of 2 and 3 mm in TBCs are detected with high efficiency.

Quantitatively measuring a dynamic liquid surface often presents a challenge due to high transparency, fluidity and specular reflection. Here, a novel Transmission-Lattice based Geometric Phase Analysis (TLGPA) method is introduced. In this method, a special lattice is placed underneath a liquid to be tested and, when viewed from above, the phase of the transmission-lattice image is modulated by the deformation of the liquid surface. Combining this with multi-directional Newton iteration algorithms, the dynamic deformation field of the liquid surface can be calculated from the phase variation of a series of transmission-lattice images captured at different moments. The developed method has the advantage of strong self-adaption ability to initial lattice rotational errors and this is discussed in detail. Dynamic 3D ripples formation and propagation was investigated and the results obtained demonstrated the feasibility of the method.

As a revolutionary new technique, laser-engineered net shaping (LENS) is a layer additive manufacturing process that enables accurate, rapid and automatic repair of industrial component damage. In the laser repair (LR) process or in service, surface cracks can appear, which have a detrimental effect on the repair quality and the mechanical performance; therefore, the surface crack detection of repaired components has attracted much attention. Laser spot thermography is an important nondestructive testing method with the advantages of non-contact, full-field and high precision, which shows great potential in the crack detection of repaired components. The selection of thermographic process parameters and the optimization of thermal image processing algorithms are key to the success of the nondestructive detection. In this paper, the influence of material properties and thermographic process parameters on the surface temperature gradient is studied based on the simulation of laser spot thermal excitation, and the selection windows of thermographic process parameters for iron-based and nickel-based alloys are obtained, which is applied to the surface crack detection of repaired components. To improve the computational efficiency of thermal images, the Prewitt edge detection algorithm is used in the thermal image processing, which realized fast extraction of cracks with a high signal-to-noise ratio (SNR), and the detection sensitivity of crack width can reach 10 μm. To further study the influence of surface roughness on the thermographic detection, repair layers with and without polishing process are characterized, which show that the Prewitt edge detection algorithm is well applicable to crack detection on surfaces with different roughness level.

As an important branch of metal additive manufacturing, laser repair has broad application potential in the aerospace, automobile and ship building industries. In the process of laser repair, metal powder is introduced into the laser action area and heated to a molten state to form a molten pool, which moves on the component surface according to the preset scanning path. Additionally, the temperature of the repaired component changes dynamically with the movement of the molten pool, leading to the time-evolution of stress and deformation. Therefore, online deformation measurement for the repair process is essential for understanding the evolution mechanism of stress and deformation in laser repair. However, extreme environments such as ultra-high temperature, strong laser radiation and metal powder splashing exist during the laser repair, which bring great challenges for dynamic optical measurement. In this paper, an online deformation measurement system based on orthogonal sampling moiré for laser repair environment is developed, which is applied to measure the deformation field of the metal substrate during laser repair. In the measurement, laser repair is performed on a cantilever beam substrate and orthogonal grating is prepared on the side surface of the cantilever beam. The real-time grating images are recorded by an optical imaging system, in which a group of filter components are used to obtain optical images with high signal-to-noise ratio. Finally, the deformation field of the substrate during laser repair is calculated with the orthogonal sampling moiré method. The results show that the scanning path in the repair process has a significant influence on the distribution of residual deformation, which offers reference for optimizing parameters of laser repair.

Although three-dimensional (3D) morphology measurement has been widely applied on the macro-scale, there is still a lack of 3D measurement technology on the microscopic scale. In this paper, a microscopic 3D measurement technique based on the 3D-geometric phase analysis (GPA) method is proposed. In this method, with machine vision and phase matching, the traditional GPA method is extended to three dimensions. Using this method, 3D deformation measurement on the micro-scale can be realized using a light microscope. Simulation experiments were conducted in this study, and the results demonstrate that the proposed method has a good anti-noise ability. In addition, the 3D morphology of the necking zone in a tensile specimen was measured, and the results demonstrate that this method is feasible.