3D Printing and Additive Manufacturing最新文献

A 3D numerical model of heat transfer and fluid flow of molten pool in the process of laser wire deposition was presented by computational fluid dynamics technique. The simulation results of the deposition morphology were also compared with the experimental results under the condition of liquid bridge transfer mode. Moreover, they showed a good agreement. Considering the effect of recoil pressure, the morphology of the deposit metal obtained by the simulation was similar to the experiment result. Molten metal at the wire tip was peeled off and flowed into the molten pool, and then spread to both sides of the deposition layer under the recoil pressure. In addition, the results of simulation and high-speed charge-coupled device presented that a wedge transition zone, with a length of ∼6 mm, was formed behind the keyhole in the liquid bridge transfer process, where the height of deposited metal decreased gradually. After solidification, metal in the transition zone retained the original melt morphology, resulting in a decrease in the height of the tail of the deposition layer.

In the present study, TiC-Fe cermets were fabricated through selective laser melting (SLM) for the first time employing pulse wave using a pulse shaping technique and regular laser pulse wave. Two samples were fabricated each with adapting pulse shaping technique and regular laser pulse wave with varied laser peak power and exposure time to obtain an optimized parameter. The pulse shaping technique proves to be an optimal method for fabrication of the TiC-Fe-based cermet. The effect of the laser peak power and pulse shaping on the microstructure development was investigated through scanning electron microscopy and X-ray diffraction analysis. Two-phased microstructures revealed the distribution of TiC and Fe. A maximum hardness and fracture toughness of 1010 ± 65 MPa and 16.3 ± 1.7 MPa m^1/2, respectively, were observed for the pulsed-shaped samples illustrating that pulse shaping can be an effective way to avoid cracking in brittle materials processed by SLM.

Spatially varying material properties allow the dynamic response of structural systems to be almost arbitrarily tailored, far beyond the first or fundamental natural frequency. Continuing advances in manufacturing technology are making it possible to achieve the necessary range of stiffness and density variations, but the design of these property distributions is a challenging task because of the complex multidimensional nature of the problem. Generative design methods based on evolutionary optimization algorithms have been successfully used to obtain solutions based on multi-material distributions. However, the applicability of these solutions is limited by their reliance on multi-material additive manufacturing (AM), which currently only offers digitally mixed acrylic polymer options that are generally unsuitable to produce functional parts. A novel structured material solution is proposed here, in which the problem domain is divided into several volume elements (voxels), each of which contains a structure whose geometrical form is altered to adjust its effective properties to desired values. The single material structural solution will be amenable for ready fabrication by the powder-based selective laser sintering and melting processes with real engineering polymer and metal systems, thereby allowing for the realization of the benefits in real-world applications. The resulting continuous design spaces are searched using a modern evolutionary algorithm, the covariance matrix adaptation evolution strategy (CMA-ES). A MATLAB implementation of this evolutionary design method, in conjunction with finite element simulations for fitness evaluation, showed good convergence for several different cantilever beam test cases when tested against several different sets of target natural frequencies. Correlations with the multi-material solutions show that the single structured material approach is on par or even better in some cases, even though the test domain was discretized into 80% fewer voxels than for the multi-material case. Furthermore, the voxel structures can be realized using current AM technologies.

Support removal is one of the thorny issues faced by laser powder bed fusion (LPBF). In particular, the efficient and safe removal of support structures from the thin-walled parts and obtaining high-quality surfaces still remains a challenge owing to their sensitivity to machining. An in-depth understanding of the material response behavior of LPBF thin-walled parts when removing support structures is necessary for overcoming this challenge. The work is divided into two parts: revealing the support removal mechanism and proposing a solution to improve the support machinability. First, the machinability of support structures on thin-walled parts with different thicknesses at different cutting depths was thoroughly investigated. Experimental investigation on cutting force, surface morphology, and deflection were carried out. The results show that cutting forces increase gradually at each cut owing to the tilt and collapse of support structures. The surface morphology is improved as the sample thickness increases but deteriorated as the cutting depth increases. Second, a novel solution of adding resin is proposed to improve the support machinability and good results have been achieved. The z-direction cutting forces for 0.3 and 0.4 mm thickness samples are reduced by 72.6% and 64.6%, respectively, and no deflection of the sample is observed after support removal. Moreover, finite element method simulations are established to further explain the support removal mechanism.

Laser welding quality forecast is highly significant during the laser manufacturing process. However, extracting the dynamic characteristics of the molten pool in the short laser welding process makes predicting of the welding quality in real time difficult. Accordingly, this study proposes a multimodel quality forecast (MMQF) method based on dynamic geometric features of molten pool to forecast the welding quality in real time. For extraction of geometric features of molten pool, an improved fully convolutional neural network is proposed to segment the collected dynamic molten pool images during the entire welding process. In addition, several dynamic geometric features of the molten pool are extracted by using the minimum enclosed rectangle algorithm with an evaluation of the performance by several statistical indexes. With regard to forecasting the welding quality, a nonlinear quadratic kernel logistic regression model is proposed by mapping the linear inseparable features to the high dimensional space. Experimental results show that the MMQF method can make an effective and stable forecast of welding quality. It performs well under small data and can satisfy the requirement of real-time forecast.

Commercially pure (cp) molybdenum (Mo) is one of the high-temperature materials of immense potential. It has a body-centered cubic (bcc) structure so it is hard to fabricate using nonequilibrium processes such as the selective laser melting (SLM) without the formation of cracks due to its inherent brittleness. This study deals with the fabrication of dense and near crack-free cp-Mo samples produced by the SLM. The laser scan strategy is adjusted from a single scan to a double scan to reduce the solidification cracks. Samples produced with a laser double scan strategy show a density of ∼99% with a hardness of ∼222 HV.

Metal three-dimensional (3D) printing technology brings several benefits to the field of high-pressure die casting of aluminum, which enhances its development. The associated conformal cooling application is already commonly used where there is a need to improve the quality of castings, increase tool life, or reduce the production cycle. However, will this technology withstand the production of a large part (∼270 × 270 × 200 mm), which will be used directly in the serial production of engine blocks? This article describes a slider with a conformal cooling case study, which was redesigned and manufactured using the laser powder bed fusion (L-PBF) method. After the slider was put into serial production of 1.0 TSI three-cylinder engine blocks, this tool was thoroughly monitored based on the temperature field by comparing the results of a simulation in SW ProCAST with reality, and furthermore examining the influence of the tool on the quality of castings. There was also an evaluation of repairs performed on the tool in the ŠKODA AUTO tool shop and the foundry. These data were compared with a serial tool. Finally, the costs to produce the slider in conventional and 3D-printed variants are compared with an outline of other possible steps for optimizing these costs. The study results show that relatively large parts can be printed and used in serial production even today. It was also confirmed that conformal cooling influenced improving tool life, and the number of repairs in ŠKODA AUTO production also decreased.