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Featured Application: This paper presents the in-situ laser polishing of four typical alloys in the selective laser melting (SLM) process. Selective laser melting (SLM) is a layer by layer process of melting and solidifying of metal powders. The surface quality of the previous layer directly affects the uniformity of the next layer. If the surface roughness value of the previous layer is large, there is the possibility of not being able to complete the layering process such that the entire process has to be abandoned. At least, it may result in long term durability problem and the inhomogeneity, may even make the processed structure not be able to be predicted. In the present study, the ability of a fiber laser to in-situ polish the rough surfaces of four typical additive-manufactured alloys, namely, Ti6Al4V, AlSi10Mg, 316L and IN718 was demonstrated. The results revealed that the surface roughness of the as-received alloys could be reduced to about 3 μm through the application of the laser-polishing process, and the initial surfaces had roughness values of 8.80–16.64 μm. Meanwhile, for a given energy density, a higher laser power produced a laser-polishing effect that was often more obvious, with the surface roughness decreasing with an increase in the laser power. Further, the polishing strategy will be optimized by simulation in our following study. [ABSTRACT FROM AUTHOR]

Featured Application: This paper presents a laser ultrasonic testing (LUT) method for the inspection of internal hole defects in a Ti-6Al-4V part produced by additive manufacturing (AM). The LUT system achieved a resolution in sub-millimeter scale, demonstrating its significant potential in the quality evaluation of additive manufactured part. For a non-contact, non-destructive quality evaluation, laser ultrasonic testing (LUT) has received increasing attention in complex manufacturing processes, such as additive manufacturing (AM). This work assessed the LUT method for the inspection of internal hole defects in additive manufactured Ti-6Al-4V part. A Q-switched pulsed laser was utilized to generate ultrasound waves on the top surface of a Ti-6Al-4V alloy part, and a laser Doppler vibrometer (LDV) was utilized to detect the ultrasound waves. Sub-millimeter (0.8 mm diameter) internal hole defect was successfully detected by using the established LUT system in pulse-echo mode. The method achieved a relatively high resolution, suggesting significant application prospects in the non-destructive evaluation of AM part. The relationship between the diameter of the hole defects and the amplitude of the laser-generated Rayleigh waves was studied. X-ray computed tomography (XCT) was conducted to validate the results obtained from the LUT system. [ABSTRACT FROM AUTHOR]

Laser polishing is a widely used technology to improve the surface quality of the products. However, the investigation on the physical mechanism is still lacking. In this paper, the established numerical transient model reveals the rough surface evolution mechanism during laser polishing. Mass transfer driven by Marangoni force, surface tension and gravity appears in the laser-induced molten pool so that the polished surface topography tends to be smoother. The AlSi10Mg samples fabricated by laser-based powder bed fusion were polished at different laser hatching spaces, passes and directions to gain insight into the variation of the surface morphologies, roughness and microhardness in this paper. The experimental results show that after laser polishing, the surface roughness of Ra and Sa of the upper surface can be reduced from 12.5 μm to 3.7 μm and from to 29.3 μm to 8.4 μm, respectively, due to sufficient wetting in the molten pool. The microhardness of the upper surface can be elevated from 112.3 HV to 176.9 HV under the combined influence of the grain refinement, elements distribution change and surface defects elimination. Better surface quality can be gained by decreasing the hatching space, increasing polishing pass or choosing apposite laser direction. [ABSTRACT FROM AUTHOR]

• A method of 3D reconstruction for LPBF parts by light field camera is proposed. • Simple and high-precision calibration is achieved by the consistency method. • A LFEPI-Unet is designed for accuracy disparity reconstruction. • Accurately mapping of disparity, depth, and 3D spatial coordinates is established. • First-time application for in situ 3D reconstruction and monitoring of LPBF parts. The assessment of output quality in the laser powder bed fusion process entails the analysis of the three-dimensional morphology of the manufactured parts, which plays a crucial role in quality control to prevent metallurgical defects. This study proposes a novel approach of three-dimensional reconstruction for laser powder bed fusion parts using a light field camera. Initially, a consistency method for high-precision camera calibration was explored to determine the virtual camera pixel size and coefficient of effective focal length of light field camera. Subsequently, the model based light field epipolar-plane image Unet is designed to obtain disparity information without any data augmentation techniques. By leveraging both binocular and light field optical paths, a robust mapping relationship of the disparity, depth, and visually coherent three-dimensional contour was established. Finally, the three-dimensional contours of laser powder bed fusion parts were reconstructed by the established mapping with a single exposure of the light field camera. The proposed method enables three-dimensional contour visualization of laser powder bed fusion parts from multi-view perspectives, which facilitates in situ monitoring of the spattering distance, three-dimensional spatial points and lines of the printed layers and contours of printed parts. With a calibration error of 50 μm and resolution of 180 μm for the light field camera (Lytro illum camera), the maximum relative and absolute errors between reconstructed three-dimensional spatial points were 4.33 % and 260 μm respectively when compared with theoretical dimension, and 0.5 % and 210 μm respectively when compared with manufactured dimension. The maximum root-mean-square error of reconstructed three-dimensional contour surfaces was found to be 1.55 mm. This study represents the first application of a light field camera for in situ three-dimensional reconstruction of laser powder bed fusion parts, which notably can be obtained from only a single round of calibration and exposure, enabling seamless monitoring of their surface morphology and facilitating online quality control. These advancements have vast potential for enhancing high-end additive manufacturing. [ABSTRACT FROM AUTHOR]

In laser powder bed fusion (LPBF) processes, critical defect issues such as porosity limit the quality of the fabricated part. This work investigates the mechanism of pore evolution during the laser post-processing of an LPBF part. A laser beam was utilized to scan the top surface of a Ti–6Al–4V alloy part. The morphologies of the internal layers as well as the distribution of the porosity inside the Ti–6Al–4V sample were experimentally measured and compared using micro-computed tomography (micro-CT). It was found that the void fraction was significantly decreased, from 2.51% to 0.77%, by laser post-processing. A multi-physics coupled finite element model based on the Level-set method was built to analyze the evolution mechanism of the pore. This model simulated the effects of the energy density, Marangoni flow and mass transfer on the pore evolution. Typical processes were studied in detail, and a table summarizing the pore evolution under different post-processing parameters is given. Image 1 • Internal morphologies of Ti–6Al–4V sample measured by micro-CT. • A Level-set model is built to analyze the evolution mechanism of porosity. • Mass transfer on the bubble surface is the essence of pore evolution. • Main factor impacting gas bubble rising is from Marangoni effect rather than buoyancy. [ABSTRACT FROM AUTHOR]