Defect generation and propagation mechanism during additive manufacturing by selective beam melting

During powder bed based additive manufacturing processes a component is fabricated by locally melting of powder layers with a laser or an electron beam. The fast melting process of the stochastic powder bed induces vigorous melt pool movements which sometimes lead to faults acting as starting points for larger defects such as channels bridging many layers. Since the formation of these defects cannot be understood in the framework of a homogenized numerical approach we have developed a mesoscopic numerical model based on the Lattice Boltzmann Method for the local melting process which considers individual powder particles. The model takes into account full hydrodynamics including capillary and wetting effects. It is shown that these effects combined with the stochastic powder layer are the origin of fault formation. The numerical results are compared with experiments in order to demonstrate the predictive value of our model.     

粉末预热对电子束选区熔化成形工艺的影响

研究了电子束选区熔化成形技术中粉末预热工艺对Ti-6Al-4V合金粉末在高能高速电子束作用下抗溃散性能的影响,并对该粉末进行了三维零件成形试验以验证粉末预热在实际成形中的作用。粉末抗溃散试验结果表明,对粉末进行预热可以明显提高粉末在一定电子束束流下的抗溃散临界扫描速度,增强其抗溃散性能;钛合金粉末的三维成形试验结果表明,粉末预热能够有效地改善成形层表面的“边缘缺陷”和“结球缺陷”,提高表面质量,降低层厚;成形件显微组织观察结果表明,预热对层间结合情况有明显改善。     

Microstructure and high-temperature tensile property of TiAl alloy produced by selective electron beam melting

The microstructure and high-temperature tensile property of a Ti-47Al-2Cr-2Nb alloy fabricated via selec-tive electron beam melting (SEBM) with hatch spacings o...     

Characterization of titanium aluminide alloy components fabricated by additive manufacturing using electron beam melting

Intermetallic, γ-TiAl, equiaxed, small-grain (～2 μm) structures with lamellar γ/α₂-Ti₃Al colonies with average spacing of 0.6 μm have been fabricated by additive manufacturing using electron beam melting (EBM) of precursor, atomized powder. The residual microindentation (Vickers) hardness (HV) averaged 4.1 GPa, corresponding to a nominal yield strength of ～1.4 GPa (～HV/3), and a specific yield strength of 0.37 GPa cm³ g^?1 (for a density of 3.76 g cm^?3), in contrast to 0.27 GPa cm³ g^?1 for EBM-fabricated Ti–6Al–4V components. These results demonstrate the potential to fabricate near net shape and complex titanium aluminide products directly using EBM technology in important aerospace and automotive applications.     

YG13硼化处理后沉积气压对金刚石薄膜的影响

采用超高真空热丝化学气相沉积（HFCVD）系统,以甲烷和氢气为反应气体,在YG13（WC-13%Co）硬质合金基体上沉积金刚石薄膜。用场发射扫描电子显微镜（FESEM）和X射线衍射仪（XRD）对金刚石薄膜进行检测分析,研究YG13经950℃、3h硼化预处理后沉积气压对金刚石薄膜形貌和生长织构的影响;通过压痕法比较硼化与二步法两种预处理方法对金刚石薄膜附着性能的影响。结果表明,基体经硼化预处理后表面形成CoB、CoW2B2、CoW3B3相;当沉积温度为750～800℃,碳源浓度为3.3%时,薄膜表面形貌和生长织构随着沉积气压改变有明显的变化;硼化预处理后所得样品在1500N载荷下压痕表现出良好的附着性能,较二步法预处理更加有效地改善了膜-基附着性能。     

粉床型电子束3D打印纯钨及W-Nb合金的显微组织及开裂机制

采用粉床型电子束3D打印技术制备了纯钨及固溶强化型W-Nb合金,对2种钨基材料的显微组织和裂纹缺陷进行了分析.研究发现:二者均沿逐层累积的方向形成柱状晶,Nb元素的加入使柱状晶的平均尺寸由109.78 μm降至25.10 μm.纯钨并未发生显著的开裂现象,但是W-Nb合金内部存在沿晶界分布的微裂纹.从热应力累积和凝固过...     

Microstructure and property of sub-micro WC- 10 %Co particulate reinforced Cu matrix composites prepared by selective laser sintering

1 Introduction Selective laser sintering(SLS) is one of a few rapid prototyping(RP) techniques, which enables fabrication of three-dimensional(3D) parts with arbitrary shapes directly from metal powder with no or minimal post-processing[1?3]. In this meth…     

电子束选区熔化增材制造TiAl合金的高温硬度及氧化行为研究

钛铝合金是航空航天领域具有广阔应用前景的轻质高温合金,电子束选区熔化（EBM）增材制造技术是制备复杂结构TiAl合金有效途径,但目前对其高温性能研究较少。本文重点研究了电子束选区熔化增材制造Ti-48Al-2Cr-2Nb合金的显微组织、高温硬度及其高温氧化行为。结果表明,EBM成形Ti-48Al-2Cr-2Nb合金呈现出由等轴γ晶粒和双相区组成的独特层状组织;在800 ℃下恒温氧化100 h,表现出较低的氧化速率常数,形成的氧化膜主要由TiO2、Al2O3及TiO2 / Al2O3混合交替层组成,抗氧化性能优于传统方法制备的Ti-48Al-2Cr-2Nb合金和其他TiAl合金。此外,900℃以下,该合金具有良好的高温硬度,显微硬度随温度的升高未发生明显的下降趋势。     

Modelling and simulation of electron beam melting

In recent years, the scientific and industrial relevance of additive layer manufacturing (ALM) has grown. In the metal area of ALM, the capacity of laser technologies is noticeably limited. This is mainly due to an inertial beam deflection device, which is also referred to as a mirror galvanometer. In contrast, the electron beam technology offers high power density as well as considerable scanning rates. Therefore, electron beam melting (EBM) seems to be suitable for processing a broad variety of alloys in an economic way. In particular, the enormous scanning rates which can be realized by use of an electron beam enable an economic manufacture of high quality parts. However, profound expertise is required in order to establish EBM as an industrial production technology. By means of mathematical–physical modelling, process stability of the melting step is being increased. Moreover, by solving a detailed thermal model using the finite element method (FEM), substantial knowledge of adequate parameter settings in dependence of the utilised material is developed. Finally, a process window as a function of scan speed and beam power is developed based on experimental results.     

粉床型电子束增材制造W-3.5Nb合金孔洞缺陷及其形成机制

采用粉床型电子束增材制造(SEBM)技术制备了W-3.5Nb合金,研究了电子束线能量密度对W-3.5Nb合金孔洞缺陷形成的影响规律,分析了不同类型孔洞缺陷的形成原因.结果 表明:随着电子束线能量密度的升高,缺陷含量降低,当线能量密度升高到1.44 J/mm时,缺陷的体积含量最低为0.01％,继续增大线能量密度,缺陷的体...     

Flexible manufacturing of metallic products by selective laser melting of powder

In order to produce metallic parts directly from powder material using CAD data, the selective laser melting (SLM) process has been developed. From a series of material tests, nickel-based alloy, Fe alloy and pure titanium powders are found to be feasible for fabrication of metallic models by SLM. Finite element simulation shows stress distribution within the solid single layer formed on the powder bed during forming and some methods for avoiding defects in the products are suggested. The die for metal forming from the nickel-based alloy and the pure titanium models of bone and dental crown are demonstrated. The density of the model made by SLM is higher than 90% of the solid model. The mechanical properties of the formed model can be improved to those of the solid by post-processing.     

Performance of Ti6Al4V fabricated by electron beam and laser hybrid preheating and selective melting strategy

Electron beam selective melting (EBM) and selective laser melting (SLM) are regarded as significant manufacturing processes for near-net-shaped Ti6Al4V components. Generally, in the conventional EBM process, preheating is necessitated to avoid smoke caused by the charging of electrons. In the conventional SLM process, laser as an energy source without the risk of smoke can be employed to melt metal powder at low temperatures. However, because of the low absorption rate of laser, the powder bed temperature cannot reach a high level. It is difficult to obtain as-built TiAl4V with favorable comprehensive properties via conventional EBM or SLM. Hence, two types of electron beam and laser hybrid preheating (EB-LHP) combined with selective melting strategies are proposed. Using laser to preheat powder allows EBM to be performed at a low powder bed temperature (EBM-LT), whereas using an electron beam to preheat powder allows SLM to be performed at a high powder bed temperature (SLM-HT). Ti6Al4V samples are fabricated using two different manufacturing strategies (i.e., EBM-LT and SLM-HT) and two conventional processes, i.e., EBM at a high powder bed temperature (EBM-HT) and SLM at a low powder bed temperature (SLM-LT). The temperature-dependent surface quality, microstructure, density, and mechanical properties of the as-built Ti6Al4V samples are characterized and compared. Results show that EBM-LT Ti6Al4V exhibits a higher ultimate tensile strength (981±43 MPa) and a lower elongation (12.2%±2.3%) than EBM-HT Ti6Al4V owing to the presence of α' martensite. The SLM-HT Ti6Al4V possesses the highest ultimate tensile strength (1,059±62 MPa) and an elongation (14.8%±4.0%) comparable to that of the EBM-HT Ti6Al4V (16.6%±1.2%).     

选区激光和电子束熔化成形纯钨的组织、性能及裂纹形成机理研究

采用选区激光熔化(SLM)和选区电子束熔化(SEBM)技术成形纯钨,对比研究了两种成形方法对纯钨的宏/微观组织和力学性能的影响,利用SEM和EBSD等表征技术,分析了裂纹萌生位置及形成机制。研究结果表明,通过调控纯钨打印过程的工艺参数,可有效地减少 SLM和SEBM成形样品内部的裂纹。SLM和SEBM打印态纯钨的致密度和硬度与输入样品的能量密度呈正比例关系。与SLM相比,SEBM成形时样品内部的温度梯度更低,热应力累积更少,成形后样品中的裂纹更少。SLM打印态样品的内部裂纹多存在于搭接区域,SLM和SEBM制备过程中产生的裂纹均具有沿晶界分布的特征,而且裂纹多萌生于大角度晶界。     

Additive manufacturing of alumina parts by indirect selective laser sintering and post processing

Innovative powder preparation and post-processing techniques can be employed to obtain high density ceramic parts by means of indirect selective laser sintering. Thermally induced phase separation (TIPS) was used to produce polymer and polymer–ceramic composite particles. The effect of polymer concentration, cooling rate, stirring and alumina particles on polymer and polymer–ceramic composite particles was investigated. Homogeneous spherical alumina–polypropylene (PP) composite powder was synthesized by TIPS for selective laser sintering (SLS). Green Al₂O₃–PP component parts with a density of 34% could be produced by conventional SLS of the polymer under optimized laser power, scan speed, scan spacing and powder preheating temperature. Various post-processing techniques like pressure infiltration (PI), warm isostatic pressing (WIPing) or a combination of both were applied to increase the green density of the Al₂O₃–PP SLM parts. Infiltrating the open porosity green SLS parts with a 30 vol% alumina-powder based ethanol suspension allowed to increase the sintered density, i.e. after polymer debinding and pressureless sintering in air at 1600 °C, from 38 to 64% of the theoretical density (TD). WIPing of the SLS and SLS/infiltrated green parts at 135 °C and 64 MPa allowed raising the green density up to 93 and 83% TD and a sintered density up to 89 and 88% TD, respectively.     

Effect of processing parameters on the density,microstructure and strength of pure tungsten fabricated by selective electron beam melting

Pure tungsten samples were prepared by the selective electron beam melting (SEBM) process. The effect of the SEBM process parameters on the density, microstructure and compression strength of pure tungsten was studied. In addition, the influence of substrate preheating temperature during SEBM was studied. A processing window for additive manufacturing of pure tungsten by SEBM was preliminarily determined. Pure tungsten samples with relative density of 99.5% and without obvious pores and microcracks were successfully fabricated. The as-built pure tungsten samples showed strong columnar grain structures. Compression strength along the columnar grains in the build direction was measured to be 1560 MPa. Fracture occurred predominantly along the columnar grain boundaries by decohesion, in addition to brittle transgranular fracture. Refinement and strengthening of the columnar grain boundaries are expected to improve the compression strength of the SEBM-fabricated pure tungsten.     

微弧氧化改性电子束选区熔化Ti6Al4V合金的腐蚀与磨损性能（英文）

为了分析微弧氧化电压对Ti6Al4V(TC4)合金腐蚀和磨损性能的影响,分别在400、420和450 V对电子束选区熔化(SEBM)制备的TC4样品进行微弧氧化表面处理。结果表明,随着氧化时间和温度的增加,亚稳态锐钛矿型TiO₂逐渐转变为金红石型TiO₂。MAO膜表面形貌主要是尺寸分布均匀的大量微孔,仅在450V电压下出现裂纹和>10μm的孔隙;MAO膜厚度与施加电压呈正相关。MAO膜的耐腐蚀性能和磨损性能与其相成分、表面微孔尺寸分布及膜厚有关;当MAO电压为420 V时,腐蚀电流密度最小(0.960×10^-7 A/cm²),阻抗最大(7.17×10⁵Ω·cm²),耐腐蚀性能最好;相同载荷条件下,涂层的摩擦因数及磨损量均大于基体的;随着MAO施加电压的增加,MAO膜的磨损机制由磨粒磨损转变为粘着磨损,且450V电压时粘着磨损加剧,摩擦因数也最大,为0.821。     

Microstructures of Rene 142 nickel-based superalloy fabricated by electron beam melting

Rene 142, a commercial, columnar grained, gas turbine airfoil Ni-based superalloy, has been fabricated from a pre-alloyed, atomized powder by additive manufacturing using electron beam melting. Monolithic components having [0 0 1] oriented, columnar grain structures exhibited a creep-optimized 59% volume fraction of cuboidal, coherent, γ′-phase precipitates averaging 275 nm on the side, and with γ/γ′ channel widths ranging from 25 to 75 nm. Transmission electron microscopy, utilizing bright and dark field imaging of optimally oriented γ/γ′ interfaces showed prominent misfit coherency strains as δ-fringe patterns. Corresponding hardness measurements also indicated the possibility of creep strength comparable with the commercial alloy. The notable feature of this study was the monolithic development of desirable superalloy properties without conventional, multi-step heat treatments.     

SLM打印制备WC-16%Co硬质合金工艺与样品特性研究

本研究深入探索了由激光选区熔化法(SLM)制备的WC-16%Co硬质合金的显微结构及机械性能,并对硬质合金打印样品进行后续真空热处理,研究真空热处理对打印样品显微结构及机械性能的影响。研究结果表明在SLM打印过程中WC晶粒产生快速和不均匀的生长,打印样品的显微结构在平行于激光方向(垂直截面)具有层状结构特征。打印过程中WC发生分解并与Co形成η相。真空热处理可以提升打印样品致密度以及硬度,但无法消除打印件中的η相以及在垂直截面上的层状结构,同时造成WC晶粒的显著长大。