Using complex shapes and precise structural parts is becoming a strong trend in modern advanced manufacturing. However, traditional manufacturing technology hardly achieves the complex geometric parts directly. Selective laser melting (SLM) is an advanced manufacturing technology for metallic materials, enables production parts with complex geometry combined with the enhancement of design flexibility. The cooling rate of molten pool can reach 103~106 K/s during the SLM process. In this case, the solid solubility of the alloying elements in the matrix can be greatly enhanced. Aluminum alloy has been widely used in industry. At present, the strength of SLM-formed aluminum alloys is far lower than that of high-strength aluminum alloys obtained from a traditional process. It is necessary to develop high-strength aluminum alloy composition based on SLM technical characteristics. The present study is devoted to design high-strength AlSiMg1.5 aluminum alloy specifically for SLM using the local structure model based on the liquid-solid structural compatibility of the alloy and the technical characteristics of the liquid quenching in SLM. The effect of the ageing treatment on the microstructure, the hardness, and the compressive properties of the SLM-formed AlSiMg1.5 alloy was systematically studied. Almost completely dense samples were obtained by adjusting the parameters of SLM process. When the ageing temperature was 300 ℃, the super-solid solution Si precipitated and grew in the island-like Al-rich structure, and the reticular Si-rich structure decomposed and spheroidized gradually with the increases of ageing time of SLM-formed AlSiMg1.5 samples. In this case, the hardness and the strength of the samples decreased, but the elongation increased significantly. The microstructures of the SLM-formed AlSiMg1.5 samples did not change obviously when the ageing temperature was 150 ℃. But the hardness and yield strength of the samples significantly increased first and then decreased slightly. The maximum microhardness and compressive yield strength of SLM-formed AlSiMg1.5 samples aged at 150 ℃ were (169±1) HV and (453±4) MPa, respectively, and the elongation of samples exceeds 25%. In this study, a special Al91.0Si7.5Mg1.5 (mass fraction, %) aluminum alloy specifically for SLM with excellent formability and mechanical properties was designed.

应用“团簇+连接原子”模型，基于合金液-固局域结构相容性和金属选区激光熔化(SLM)工艺熔体急冷的技术特性，设计高Mg含量SLM专用AlSiMg1.5合金新成分，系统研究时效温度和时间对SLM成形AlSiMg1.5合金显微组织和力学性能的影响。结果表明，通过调整工艺参数，可获得近乎全致密的SLM成形样品。当时效温度为300 ℃时，随着时效时间的延长，SLM成形样品岛状富Al组织中过固溶Si逐渐析出长大，网格状富Si组织逐渐分解球化，样品的硬度和压缩屈服强度逐渐降低，塑性明显增加。当时效温度为150 ℃时，不同时效时间下SLM成形样品的显微组织没有发生明显变化，但硬度和屈服强度随时效时间的延长先增大后略有降低。SLM成形AlSiMg1.5样品经150 ℃时效处理后的最大显微硬度和压缩屈服强度分别为(169±1) HV和(453±4) MPa，样品延伸率超过25%。本工作设计获得了成形性和力学性能优异的SLM专用铝合金新成分Al_91.0Si_7.5Mg_1.5 (质量分数，%)。

The packing density of the powder layer plays a key role in the final quality of the parts fabricated via powder-bed-based (PBB) additive manufacturing. This paper presents a combined experimental and computational modeling study on the scraping type of powder-spreading process, in order to understand the fundamental mechanisms of the packing of the powder layer. The deposition mechanisms at the particulate scale, including particle contact stress and particle velocity, are investigated, using the discrete element method, while the macro-scale packing density is validated by experiments. It is found that there is a stress-dip at the bottom of powder pile scraped by the rake. This stress-dip makes the powder particles uniformly deposited. Three kinds of deposition mechanisms dominating the powder spreading process are identified: cohesion effect, wall effect, and percolation effect. The cohesion effect, which leads to particle agglomerations and thus reduces the packing density, becomes stronger with the decrease of particle size. The wall effect, which leads to more vacancies in the powder layer, becomes stronger with the decrease of layer thickness or the increase of particle size. The percolation effect exists in bimodal powder particles, which leads to particle segregation within the powder layer and thus reduces the packing density. The three kinds of deposition mechanisms compete with each other during the powder-spreading process and make combined effects on the packing density of the powder layer. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd.

The urgent need for lightweight, high-accuracy, and personalized products has led to the rapid development of additive manufacturing. Selective laser melting (SLM), which is a very promising additive manufacturing technique, has attracted remarkable attention. The mechanical properties of SLM parts are highly related to the formation of pores and cracks. In this work, SLM parameters for AlSi10Mg alloy were optimized, and the SLM AlSi10Mg sample with a high relative density of 99.63% was obtained. The SLM sample exhibited good properties, including an ultimate tensile strength (UTS) of approximately 478 MPa, a total elongation of 8%, and an average hardness of 122 HV along the horizontal direction. However, due to a high cooling rate, an inhomogeneous microstructure with refined grains and a Si network was obtained. To achieve a homogeneous microstructure and further improve the elongation of the SLM samples, the effect of heat treatments on the microstructure and mechanical properties of the SLM samples along the horizontal direction was analyzed. After the heat treatments, the strength of the samples changed significantly and the elongation was significantly improved. Further, after a solid solution treatment at 540oC for 1 h, the UTS significantly decreased to approximately 246 MPa and the elongation increased to more than 22%. For the sample annealed at 236oC for 10 h, a UTS of approximately 368 MPa and elongation of approximately 17% were obtained. Moreover, the sample subjected to ageing at 130oC for 4 h exhibited a high strength similar to the level of the SLM sample, the elongation was increased to approximately 11.9%, and the hardness was approximately 133 HV which is 10% higher than that of the SLM sample. The improved performance of the aged samples can be attributed to the combination of solution strengthening, microstructural refinement, and precipitation strengthening. The results show that low-temperature ageing is the optimized heat-treatment method for SLM samples with fine microstructures.

This work presents a novel modeling framework combining computational fluid dynamics (CFD) and cellular automata (CA), to predict the solidification microstructure evolution of laser powder bed fusion (PBF) fabricated 316 L stainless steel. A CA model is developed which is based on the modified decentered square method to improve computational efficiency. Using this framework, the fluid dynamics of the melt pool flow in the laser melting process is found to be mainly driven by the competing Marangoni force and the recoil pressure on the liquid metal surface. Evaporation occurs at the front end of the laser spot. The initial high temperature occurs in the center of the laser spot. However, due to Marangoni force, which drives high-temperature liquid flowing to low-temperature region, the highest temperature region shifts to the front side of the laser spot where evaporation occurs. Additionally, the recoil pressure pushes the liquid metal downward to form a depression zone. The simulated melt pool depths are compared well with the experimental data. Additionally, the simulated solidification microstructure using the CA model is in a good agreement with the experimental observation. The simulations show that higher scan speeds result in smaller melt pool depth, and lack-of-fusion pores can be formed. Higher laser scan speed also leads to finer grain size, larger laser-grain angle, and higher columnar grain contents, which are consistent with experimental observations. This model can be potentially used as a tool to optimize the metal powder bed fusion process, through generating desired microstructure and resultant material properties.

Laser directed energy deposition (DED) involves complex physical processes, and the trial and error examinations are time consuming and cost expensive. The research paradigm can be reshaped using advanced phenomenological models via computing the spatiotemporal variations of the build features. In this work, multi-layer and multi-track laser DED of Ti-6Al-4V were systematically explored on multiple scales including the 1D track, the 2D layer and the 3D full build considering the complex transport of energy, mass, and momentum in the moving freeform molten pool. The results showed that convex, near-flat, and wavy builds were generated using gradually larger hatch spacings. The profiles of individual tracks and layers were extracted through the unique advantages of the model. The individual tracks exhibited various patterns and rotated with specific inclinations to form distinct layer profiles. The net increments of the deposit generated upon the printing of a new track during the continuous deposition process showed that the smaller hatch spacing caused higher overlap rate of horizontally adjacent tracks but lower remelting rate of vertically adjacent tracks in neighboring layers. The 3D numerical model was validated with corresponding experiments for various process conditions. The scientific findings can provide useful insights for further researches of DED.

Selective laser melting (SLM) is an attractive rapid prototyping technology for the fabrication of metallic components with complex structure and high performance. Aluminum alloy, one of the most pervasive structural materials, is well known for high specific strength and good corrosion resistance. But the poor laser formability of aluminum alloy restricts its application. There are problems such as limited processable materials, immature process conditions and metallurgical defects on SLM processing aluminum alloys. Some efforts have been made to solve the above problems. This paper discusses the current research status both related to the scientific understanding and technology applications. The paper begins with a brief introduction of basic concepts of aluminum alloys and technology characterization of laser selective melting. In addition, solidification theory of SLM process and formation mechanism of metallurgical defects are discussed. Then, the current research status of microstructure, properties and heat treatment of SLM processing aluminum alloys is systematically reviewed respectively. Lastly, a future outlook is given at the end of this review paper.