查看更多>>摘要：A novel method named low frequency electro-magnetic stirring assisted near-liquidus squeeze casting (LFEMS-NSC) was developed for preparing Mg-RE alloy castings. The grain refining effect in commercially pure Mg (CP-Mg), peritectic Mg-Zr alloy and eutectic Mg-Gd alloy were comparatively analyzed among conventional gravity cast, squeeze cast and LFEMS-NSC. The results showed that the LFEMS-NSC exhibited effective grain refining efficiency in CP-Mg and Mg-Gd alloys. With changing conventional gravity casting or squeeze casting to LFEMS-NSC, the grain morphologies of CP-Mg were transformed from coarse columnar into fine equiaxed grains. The average grain size of CP-Mg was refined from ~10 mm to ~232 μm, where the grain refinement efficiency was comparable to that of adding Mg-Zr master alloy. The application of LFEMS-NSC to Mg-Zr and Mg-Gd alloy systems also show considerable grain refining effect, and it show different effectiveness when the solute content varies. The grain refinement mechanism of LFEMS-NSC is mainly because of the enhanced survival rate of free chill crystals and enhanced heterogeneous nucleation rate through melt pre-treatment by LFEMS and applied pressures during solidification.
查看更多>>摘要：During the laser based manufacturing process such as metal additive manufacturing (AM) and laser cladding, enhancing or inhibiting liquid metal flow within the laser-induced melt pool provides a promising approach to tune material microstructure and the resulting mechanical properties. However, the effect of convection flow, specifically the dominant Marangoni convection flow in the melt pool, on the as-solidified material microstructure is still vague. This study aims to reveal convection-modified grain evolution in the laser melting process, e.g., the selective laser melting AM. We use a process-microstructure model and systematically design comparative simulation cases (with and without convection flow) to identify the effects of convection flow on melt pool geometry, solidification conditions, and as-solidified grain structure formation. The model is validated by secondary electron images and electron backscatter diffraction of the laser melted IN625 alloy provided by NIST Additive Manufacturing Benchmark Test Series. It is found that the strong Marangoni convection flow can widen the melt pool and notably affect the solidified microstructure in terms of grain growth directions and bulk nucleation events. This study provides a quantitative basis for controlling the as-solidified microstructure by manipulating the convection flow in the laser-induced melt pool.
查看更多>>摘要：Inspired by the physical nature and functionality of morphable surfaces, this paper develops the polishing tools with such surface characteristics. The surface textures of morphable polishing tools can be tuned and adapted according to the stage and requirements of the targeted surface finish and material removal rate. To investigate the performance of morphable surfaces in polishing, morphable polishing tools with labyrinth, dimple and hybrid textures were fabricated by curing silicone materials with different Young's moduli. The morphable polishing tools can be produced with fast responsiveness and reversibility by controlling the air pressure in the tool cavity. Fixed-spot polishing experiments with the developed morphable tools were performed on a copper surface, and the surface morphology, surface roughness and removed volume of polished copper were measured with a laser confocal microscope. Furthermore, static and dynamic polishing forces, polishing pressure distribution and interfacial friction coefficient between the tool and workpiece were obtained and benchmarked in polishing experiments regarding the labyrinth, dimple and hybrid tools. The results show that under the investigated polishing parameters, the roughness of polished surface is the smallest with the dimple tool, higher with the hybrid tool and the labyrinth tool. The material removal volume, static and dynamic polishing forces under different tool offsets are larger in the polishing processes with a dimple tool. In addition, the theoretical and experimental study indicates that the tool surface texture significantly affects the polishing pressure distribution. Labyrinth tools show denser pressure distribution owing to the finer textures of a smaller characteristic length, while dimple tools show sparsely distributed pressure owing to the coarser surface texture.
查看更多>>摘要：Process parameters in machining are predominantly selected by following manual tuning procedures. Using data from the system and dedicated performance indicators combined with learning-based approaches enables automating these procedures while reducing the costs of the machining process. This study investigates efficient data-driven approaches for autonomous parameter selection in turning. The number of experimental trials for finding optimal process parameters is reduced by incorporating expert knowledge and transferring knowledge between different tasks. The turning process costs are modeled using Gaussian process models, and the selection of informative experiments is achieved by Bayesian optimization. In this study, all tested methods using expert knowledge or transfer of knowledge reduced the number of experiments by at least 40% compared to a standard approach for parameter selection based on Bayesian optimization without expert knowledge, confirming the efficiency of the applied methods.
查看更多>>摘要：The two additive manufacturing processes Powder Bed Fusion (LPBF) and Directed Energy Deposition (DED) have different geometrical resolutions and production flexibilities, making their hybridization attractive. LPBF microstructure displays fine grains, with weak preferential crystal orientation. DED generates a highly textured and inhomogeneous microstructure with equivalent grains diameters ranging from a few micrometers to over a millimeter. The microstructure of the hybrid LPBF-DED sample is the addition of these two microstructures with an interface free from cracks or particular pores. The effect of this strong heterogeneity of the hybrid microstructure on mechanical behavior is analyzed by tensile tests instrumented with local strain gauges, others using digital image correlation method and finally on samples tested inside a scanning electron microscope. This multi-scale characterization showed that the difference in the elastic properties causes the localization of the strain field and generates a plastic incompatibility at the interface. An optimized heat treatment leads to isotropic and homogeneous hybrid microstructure, with a larger DED grain size. It leads to identical plasticity mechanisms during tensile tests and lowers the strain gradient around the interface.
查看更多>>摘要：During the fabrication of AlSi10Mg alloys by laser powder bed fusion (LPBF), the high-power density incident on the melt pool locally results in rapid heating and cooling rates, which leads to residual stress inevitably. Since the residual stress can lead to defects in the LPBF-built parts, it is necessary to reduce them by post-process treatments. However, the conventional heat treatments (HT) will weaken the mechanical properties while removing the residual stress, which is still one of the major challenges in additive manufacturing. This work proposes a deep cryogenic treatment (DCT) to reduce the residual stress, while maintaining the mechanical performance. The residual stress were measured by the hole-drilling strain-gauge method. The mechanical properties and microstructure were investigated using hardness measurements, tensile tests, scanning electron microscopy, and electron back-scattered diffraction. The results indicate that this treatment can relieve up to 72.7% of the residual stress, which is equivalent to the 76.5% reduction achieved by heat treatment. Moreover, this treatment can improve the tensile strength by 245 MPa and hardness by 56.72 HV0.2 in comparison to the heat-treated samples. It is suggested that plastic deformation occurred at the interfaces between Al matrix and eutectic-Si networks due to the discordant volume shrinkage responsible for the stress relief. Thermal stress was evaluated to verify the occurrence of the plastic deformation. The present work proves that the proposed DCT is an effective method to simultaneously reduce the residual tensile stress and maintain the strength and elongation in the samples built by additive manufacturing.
查看更多>>摘要：Additive manufacturing (AM) technologies have seen rapid growth in the past decade. Achieving high-quality consistency and accuracy remains a challenge in the fabrication of large-format metallic parts using the directed energy deposition AM processes. An efficient dimension correction strategy is required to prevent build failure during the AM process. In this paper, a laser line scanning sensor was integrated into a robot-based laser aided additive manufacturing (LAAM) system to realise the on-machine measurement of the part geometry. With this system, an in-process adaptive dimension correction strategy was proposed. The dimensional deviations in the intermediate layers could be corrected immediately after they were detected during the LAAM process, thus avoiding excessive dimensional deviation leading to build failure. A tool-path generation process for dimension correction was proposed which did not rely on traditional time-consuming CAD reconstruction. Only 3D point cloud was used directly, enabling the quick response of the LAAM system in restoring the dimensional accuracy. The deviated surface could be automatically filled up, and the subsequent deposition processes were resumed after each cycle of the dimension correction. To facilitate the proposed dimension correction strategy, a Robot Operating System (ROS)-based software platform was developed. Experimental comparisons between the part built with and without correction were conducted. The results demonstrated a significant improvement in dimensional accuracy when the correction strategy was applied.
查看更多>>摘要：Selective laser melting (SLM) of NiTi shape memory alloys (SMAs) was studied systematically for the resulting density and surface roughness. It was found that high laser energy density could not guarantee dense SLM NiTi SMAs. Furthermore, the phase composition and transformation temperatures varied remarkably with delicate change of laser power and scanning speed. Mechanical tests suggested that the SLM NiTi alloys made under the optimal process parameters (70 W in laser power and 105 mm/s in laser scanning speed) had an ultimate tensile strength of 788 MPa, an elongation of 7.43% in austenite state. Bending shape memory test revealed that 100% recovery rate was obtained in a sequent bending test (45°→90°→180°→270°→360°), and the bending deformation was fully recovered in ten-time cycle bending test at 180°, and recovery rate was 97% after ten-time cycle bending test at 360°. The excellent bending shape memory properties was attributed to high-density dislocations and Ti2Ni nanoprecipitates which impeded the occurrence of plastic deformation effectively. The findings in this study demonstrated that SLM NiTi parts had excellent shape memory properties and was promising for industrial applications.
NSTL
Elsevier