Numerical and experimental investigation of multilayer SS410 thin wall built by laser direct metal deposition

The influence of thermal history on the microstructures and properties of a multilayer stainless steel 410 (SS410) thin wall built by laser direct metal deposition (LDMD) process was investigated experimentally and numerically. Thermal history at two specified points in the substrate was measured by thermocouples during the process. A three-dimensional (3D) finite element model was developed to study the thermal history of the deposited material for the laser direct metal deposition of multilayer thin wall. The simulated and measured thermal history indicated that the absorption and loss of heat tended to be close to equilibrium when the deposited material reached a certain height during the LDMD process. Different microstructure regions were formed due to the different thermal history the material experienced. The hardness distribution along the height centerline of the thin wall was measured. The results indicated that thermal history had an important effect on the microstructure, and consequently on the final properties.     

Porous structures fabrication by continuous and pulsed laser metal deposition for biomedical applications; modelling and experimental investigation

The use of porous surface structures is gaining popularity in biomedical implant manufacture due to its ability to promote increased osseointegration and cell proliferation. Laser direct metal deposition (LDMD) is a rapid manufacturing technique capable of producing such a structure. In this work LDMD with a diode laser in continuous mode and with a CO₂ laser in pulsed modes are used to produce multi-layer porous structures. Gas-atomized Ti-6Al-4V and 316L stainless steel powders are used as the deposition material. The porous structures are compared with respect to their internal geometry, pore size, and part density using a range of techniques including micro-tomography. Results show that the two methods produce radically different internal structures, but in both cases a range of part densities can be produced by varying process parameters such as laser power and powder mass flow rate. Prudent selection of these parameters allows the interconnected pores that are considered most suitable for promoting osseointegration to be obtained. Analytical models of the processes are also developed by using Wolfram Mathematica software to solve interacting, transient heat, temperature and mass flow models. Measured and modelled results are compared and show good agreement.     

Numerical investigation on flow process of liquid metals in melt delivery nozzle during gas atomization process for fine metal powder production

Based on volume of fluid (VoF) interface capturing method and shear-stress transport (SST) k-ω turbulence model, numerical simulation was performed to reveal the flow mechanism of metal melts in melt delivery nozzle (MDN) during gas atomization (GA) process. The experimental validation indicated that the numerical models could give a reasonable prediction on the melt flow process in the MDN. With the decrease of the MDN inner-diameter, the melt flow resistance increased for both molten aluminum and iron, especially achieving an order of 10² kPa in the case of the MDN inner-diameter ≤1 mm. Based on the conventional GA process, the positive pressure was imposed on the viscous aluminum alloy melt to overcome its flow resistance in the MDN, thus producing powders under different MDN inner-diameters. When the MDN inner-diameter was reduced from 4 to 2 mm, the yield of fine powder (<150 μm) soared from 54.7% to 94.2%. The surface quality of powders has also been improved when using a smaller inner-diameter MDN.     

Performance and wear characteristics of ceramic, cemented carbide, and metal nozzles used in coal–water–slurry boilers

Ceramics, cemented carbides, and metals were prepared to be used as nozzles in CWS boilers. CWS burning tests in a boiler with these nozzles were carried out. The erosion wear resistance of these nozzles was compared by determining their erosion rates and hole diameter variation. Results showed that the life of the ceramic nozzles is about 30 times high than that of the metal nozzles. The wear types at the nozzle wall surface differed in various positions. The nozzle center wall section suffers form abrasive impact under low impact angles, and the damage at the center wall mainly occurs by plowing and plastic deformation for metals, and by polishing action for carbides and ceramics. The primary wear mechanisms at the exit of ceramic nozzle exhibited thermal shock damage with chipping owing to the greater thermal stresses.     

金属熔焊成形三维温度场数值模拟与分析

针对三维金属熔焊技术经历多次重叠热循环,层间等待时间直接影响成形质量和焊缝微观组织及可能引起的成形缺陷,建立了基于Sysweld求解器的单道多层直线往返堆积路径有限元模型,并进行加工试验验证. 结果表明,数值模拟技术可准确得到金属熔焊复杂的温度场及循环曲线,随层数的增加,热量累积,热源后方热影响区不断扩大,需设置层间等待时间保证成形质量. 等待时间大于30 s时,焊缝区经历明显的温度循环,成形质量较好. 等待时间越长,焊缝微观组织晶粒越均匀细密.     

粉末共注射成形充模流动过程前沿位置及场分布的数值模拟

对粉末共注射充模流动过程进行数值分析和实验验证。采用实验和数值拟合的方法确定芯/壳层界面的厚度,并运用改进的控制体积法对芯、壳层喂料前沿进行追踪。采用有限元和有限差分法对控制方程组进行数值求解,用Matlab进行程序开发,获得芯、壳层充模过程中的熔体前沿分布以及温度场和压力场的分布情况。将模拟结果与实验结果进行对比分析,发现在充填初期,模拟的喂料前沿位置与实验较为吻合,但随着充填的进行,两者偏差增大,其原因可能是在模拟过程中没有考虑注射坯的收缩。     

基于Fluent的熔融金属填充焊接冲蚀孔形成过程数值模拟

针对熔融金属填充焊接工艺过程中液流冲蚀形成小孔的过程进行数值模拟研究. 分析确定该过程的控制方程和边界条件,建立二维轴对称模型. 利用Fluent软件,从物理过程出发建立凝固熔化模型以及热源模型. 利用VOF模型追踪金属相与气相界面,获得液流在不同的焊接条件下,冲蚀形成小孔的过程. 并通过获得的流场,温度场分析形成不同形貌小孔的原因. 利用锡铅合金为试验材料在堆焊过程中得到的焊缝截面形貌,试验结果表明,模拟计算得到结果与试验结果基本吻合.     

Analysis of plastic flow localization under strain paths changes and its coupling with finite element simulation in sheet metal forming

Formability of sheet metal is usually assessed by the useful concept of forming limit diagrams (FLD) and forming limit curves (FLC) represent a first safety criterion for deep drawing operations. The level of FLC is strongly strain path dependent as observed by experimental and numerical results and therefore non-proportional strain paths need to be incorporated when analyzing formability of sheet metal components. Simulations using finite element method allow accurate predictions of stress and strain distributions in complex stamped parts. However, the prediction of localized necking is a difficult task and the combination of forming limit diagram analysis with finite element simulations often fail to give the right answer, if complex strain paths are not included in these predictions.