面向快速成形的复杂曲面重建技术研究

针对快速成形工艺特点，提出了一种基于实物的复杂曲面几何重 建的方法。由此直接生成的快速成形机优化STL接口文件，实现了反求容差与模型网格数的 最佳匹配，避免了由通用软件转换存在的错误和繁复，提高了成形设备的前处理效率，并为 反求工程与快速成形技术的有机集成提供了可靠途径。

快速成型技术中分层算法的研究与进展

根据对零件制造精度和效率的关注程度的不同,开发出了多种分层算法。在同等加工时间的情况下,根据加工精度的不同,将这些分层算法分为等层厚分层算法和适应性分层算法两类。通过对STL模型、原始CAD模型和点云数据的分析,讨论了两类分层算法的研究和发展,然后介绍了斜边分层算法和曲面分层算法等先进分层算法的原理和成果,最后讨论了快速成型分层算法的研究方向和趋势。

金属粉末激光成形过程中的检测与闭环控制研究

金属粉末激光成形技术是基于快速原型(Rapid Prototyping，RP)技术基础上发展起来的一项新型金属零件加工技术。该技术利用高能量激光束直接作用在金属粉末上，可以直接成形高熔点、形状复杂或异质材料功能梯度零件，其成形过程是按照零件几何形状，利用金属粉末逐层逐道累积成形零件的。其独特的金属零件加工特点使其得到了迅速发展，在航空、军事、医学和汽车等众多领域得到了成功应用，并发挥了重要作用。作为一项新兴技术，金属粉末激光成形技术不可避免的存在一些问题，由于激光成形过程是一个多参数影响的过程，成形过程中各参数存在较强的耦合作用，参数的不稳定往往导致成形效果难以保证，例如成形零件外形尺寸精度低、内部组织性能不佳等。这一问题，引起了国内外许多学者的关注，对加工过程中的一些重要参数与成形效果之间的关系展开研究，通过对成形过程中的一些中间过程输出参数进行实时检测，并实现闭环反馈控制，从而达到改善成形产品质量的目的。本文正是在这种背景下进行研究的，本文对熔池场几何尺寸、熔池温度场、熔覆高度与熔覆宽度实时检测进行了深入研究并实现了切实可行的检测方法，在实践中取得了较好的效果，并进行了熔覆成形过程动态辨识，研究了成形过程控制方法，提出了具有较强自适应能力的模糊PID控制方法，取得了较好的仿真效果。本课题主要研究工作有以下几个方面。 1. 激光成形过程中的熔池场包含大量的过程信息，对熔池进行实时检测，并提取熔池特征信息，为熔池场的闭环控制提供依据。为此建立了熔池场实时检测视觉传感系统，实现了图像采集、图像处理和信息提取等功能，课题成功的进行了硬件系统构建和软件系统开发。 2. 实现了红外图像比色测温法在金属粉末激光成形过程中熔池温度场检测中的应用。研究了测温原理，完成了硬件双波长图像采集系统的构建，对滤光片波长进行了选择并对选择结果进行了仿真。阐述了软件测温实现过程。 3. 激光加工过程中，对熔覆高度进行实时检测，从而实现熔覆高度闭环控制是成形高质量零件的保证。加工过程是一个多参数耦合的非线性过程，在分析激光参数对熔覆高度影响的基础上，建立利用激光工艺参数预测熔覆高度的BP神经网络模型，为实现激光加工过程熔覆高度实时预测与闭环控制打下了基础。 4. 熔覆宽度是激光成形过程中与成形效果密切相关的中间过程输出参数，该参数的实时检测为调整扫描间距，确定合理搭接率，提高熔覆表面平整度具有重要作用。提出了一种基于卡尔曼滤波技术的激光熔覆宽度检测方法。利用视觉传感系统获取激光加工过程中的熔池图像，经过图像处理求熔池宽度作为参量建立系统状态方程和测量方程，应用卡尔曼滤波原理对图像上的熔宽和熔宽变化进行状态估计，得到最小均方差条件下的熔覆宽度最佳预测值，从而减小过程噪声和测量噪声引起的熔覆宽度测量偏差，实现加工过程熔覆宽度的精确检测。 5. 成形过程动态辨识，利用阶跃响应法进行了激光参数与熔覆高度、熔覆宽度的辨识实验，建立了激光功率、扫描速度、送粉速率与熔覆高度、熔覆宽度之间的数学模型，对成形过程有了基本了解，为控制器设计与动态过程调节规范确立打下良好基础。 6. 设计了PID熔覆高度控制系统，并利用遗传算法进行了PID参数寻优，寻优获得的参数可以实现较好的控制效果，但不适于实时控制过程。为此设计了模糊PID控制系统，利用模糊逻辑算法对PID参数实现在线自动调整，控制系统具有较强的适应能力和实时处理能力，对于金属粉末激光成形过程具有较好的控制效果。

A Numerical Model of Rapid Solidification Processing of Ni-Al Alloy in Planar Flow Casting

A numerical model has been developed for simulating the rapid solidification processing (RSP) of Ni-Al alloy in order to predict the resultant phase composition semi-quantitatively during RSP. The present model couples the initial nucleation temperature evaluating method based on the time dependent nucleation theory, and solidified volume fraction calculation model based on the kinetics model of dendrite growth in undercooled melt. This model has been applied to predict the cooling curve and the volume fraction of solidified phases of Ni-Al alloy in planar flow casting. The numerical results agree with the experimental results semi-quantitatively.

Undercooling and Rapid Solidification of Nb-Si Eutectic Alloys Studied by Long Drop Tube

Niobium-silicide alloys have great potential for high temperature turbine applications. The two-phase Nb/Nb5Si3 in situ composites exhibit a good balance in mechanical properties. Using the 52 in drop tube, the effect of undercooling and rapid solidification on the solidification process and micro-structural characterization of Nb-Si eutectic alloy was studied. The microstructures of the Nb-Si composites were investigated by optics microscope (OM), X-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with X-ray energy dispersive spectrometry (EDS). Up to 480 K, deep undercooling of the Nb-Si eutectic samples was successfully obtained, which corresponds to 25% of the liquidus temperature. Contrasting to the conventional microstructure usually found in the Nb-Si eutectic alloy, the microstructure of the undercooled sample is divided into the fine and coarse regions. The most commonly observed microstructure is Nb+Nb5Si3, and the Nb3Si phase is not be found. The change of coarseness of microstructure is due to different cooling rates during and after recalescence. The large undercooling is sufficient to completely bypass the high temperature phase field.

Electron Beam Surface Treatment. Part II: Microstructure Evolution of Stainless Steel and Aluminum Alloy During Electron Beam Rapid Solidification

Surface rapid solidification microstructures of AISI 321 austenitic stainless steel and 2024 aluminum alloy have been investigated by electron beam remelting process and optical microscopy observation. It is indicated that the morphologies of the melted layer of both stainless steel and aluminum alloy change dramatically compared to the original materials. Also, the microstructures were greatly refined after the electron beam irradiation.

Computer simulation of influence of sedimentation on rapid coagulation

A computer simulation was performed to explore the features and effects of sedimentation on rapid coagulation. To estimate the accumulated influence of gravity on coagulation for dispersions, a sedimentation influence ratio is defined. Some factors possibly related to the influence of sedimentation were considered in the simulation and analysed by comparing the size distribution of aggregates, the change in collision number, and coagulation rates at different gravity levels (0 g, 1 g and more with g being the gravitational constant).

Undercooling and Rapid Solidification of Nb-Si Eutectic Alloys Studied by Long Drop Tube

Niobium-silicide alloys have great potential for high temperature turbine applications. The two-phase Nb/Nb5Si3 in situ composites exhibit a good balance in mechanical properties. Using the 52 in drop tube, the effect of undercooling and rapid solidification on the solidification process and micro-structural characterization of Nb-Si eutectic alloy was studied. The microstructures of the Nb-Si composites were investigated by optics microscope (OM), X-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with X-ray energy dispersive spectrometry (EDS). Up to 480 K, deep undercooling of the Nb-Si eutectic samples was successfully obtained, which corresponds to 25% of the liquidus temperature. Contrasting to the conventional microstructure usually found in the Nb-Si eutectic alloy, the microstructure of the undercooled sample is divided into the fine and coarse regions. The most commonly observed microstructure is Nb+Nb5Si3, and the Nb3Si phase is not be found. The change of coarseness of microstructure is due to different cooling rates during and after recalescence. The large undercooling is sufficient to completely bypass the high temperature phase field.

Rapid quantitative detection of Yersinia pestis by lateral-flow immunoassay and up-converting phosphor technology-based biosensor

Up-converting phosphor technology (UPT)-based lateral-flow immunoassay has been developed for quantitative detection of Yersinia pestis rapidly and specifically. In this assay, 400 nm up-converting phosphor particles were used as the reporter. A sandwich immumoassay was employed by using a polyclonal antibody against F1 antigen of Y. pestis immobilized on the nitrocellulose membrane and the same antibody conjugated to the UPT particles. The signal detection of the strips was performed by the UPT-based biosensor that could provide a 980 nm IR laser to excite the phosphor particles, then collect the visible luminescence emitted by the UPT particles and finally convert it to the voltage as a signal. V-T and V-c stand for the multiplied voltage units for the test and the control line, respectively, and the ratio V-T/V-C is directly proportional to the number of Y pestis in a sample. We observed a good linearity between the ratio and log CFU/ml of Y pestis above the detection limit, which was approximately 10(4) CFU/mI. The precision of the intra- and inter-assay was below 15% (coefficient of variation, CV). Cross-reactivity with related Gram-negative enteric bacteria was not found. The UPT-LF immunoassay system presented here takes less than 30 min to perform from the sample treatment to the data analysis. The current paper includes only preliminary data concerning the biomedical aspects of the assay, but is more concentrated on the technical details of establishing a rapid manual assay using a state-of-the-art label chemistry. (c) 2006 Elsevier B.V. All rights reserved.