选择性激光烧结预热温度的自适应控制研究

SLS成形过程中 ,粉末预热温度是决定制件精度、强度的重要因素。当零件截面形状发生突然变化时 ,粉末预热温度需要随之变化 ,否则成形件将严重变形甚至不能使用。原有SLS设备采用人工干预实现变预热温度调节 ,劳动强度大且工艺参数不好控制。本文提出一种粉末预热温度的模型参考自适应控制方法 ,使预热温度随着具体切片几何形状变化而变化 ,真正实现生产过程的自动化、智能化。     

基于分区域控制的大型SLS装备预热策略研究

粉床的预热是激光选区烧结(SLS)加工过程中一个非常重要的环节,预热均匀性对零件的成形效率和精度有着重要影响。针对大型SLS装备(2 m×2 m),由于粉床区域面积较大,粉床全场温度均匀性难以达到要求等问题,提出了分区域预热温度控制方法,建立基于辐射能系数的数值分析模型,搭建了实验平台,以尼龙12作为试验材料,开展预热试验研究,验证了数值模型的准确性,获得了大型SLS装备的最佳预热策略。试验结果表明,以尺寸为2 m×2 m尼龙12粉床为控制对象,使全场温度差控制在5℃,满足了工艺指标要求,为实现大型SLS装备研制、成形精度控制等关键问题提供技术支撑。     

选区激光烧结成型设备预热装置的研究

选区激光烧结(Selective laser sintering,SLS)成型设备中热能的传递与吸收是整个零件成型过程中极其重要的环节,热能的均匀分布和有效利用是决定SLS成型优劣的重要因素.然而目前通过选区激光烧结制造的零件普遍存在精度较低,表面质量较差,强度不够等问题,研究表明粉末的预热温度是影响制件精度和强度的重要因素.通过建立辐射系统的数学模型,并对选区激光烧结成型辐射换热进行分析,分析预热温度场的均匀度,进而设计了能提供均匀温度场的新型预热装置,为烧结粉末提供良好预热条件.     

氧化锆零件激光选区烧结/冷等静压复合成形技术

氧化锆陶瓷材料以其优异的性能在工业生产中具有极大的应用前景,但由于脆性大、硬度高等原因,复杂形状氧化锆零件往往难以成形和加工。为了获得复杂形状氧化锆陶瓷零件,通过溶剂沉淀法将粘接剂尼龙12覆膜至纳米氧化锆粉末的表面,然后对覆膜后的粉体进行激光选区烧结(Selective laser sintering, SLS)成形,并通过传统的冷等静压(Cold isostatic pressing, CIP)技术对SLS零件进行致密化处理,同时满足氧化锆初坯成形时形状复杂度和密度的要求。通过试验得出在激光能量密度为0.415 J/mm2时,获得的SLS陶瓷件密度较大,对不同激光能量密度制备的SLS陶瓷件进行保压压力为200 MPa的冷等静压致密化处理,根据热脱脂机理以及粘接剂的TG曲线,分别制定了SLS/CIP试样的热脱脂工艺,最后对脱脂试样进行高温烧结,在后续处理的各环节,氧化锆零件的密度仍受SLS成形的影响,但该影响逐渐减弱,SLS/CIP/FS成形件最大相对密度和维氏硬度分别达到了97%和1180 HV1,已接近“模压-烧结”的致密氧化锆陶瓷的性能,在试样断口的扫描电子显微镜(SEM)分析基础上,对氧化锆复合成形的微观演变进行了研究。虽然最终烧结件密度和硬度仍有待提高,但是提出了一种极具潜力的氧化锆零件近净成形工艺方法,为制造高性能复杂形状的陶瓷零件奠定了基础。     

SLS快速成形的高精度伺服系统机理研究

讨论和分析了SLS快速成形伺服系统的闭环和半闭环控制,并提出了利用双闭环控制实现伺服系统的高精度控制。研究表明,SLS快速成形的双闭环控制能够获得较好的控制稳定性并能进一步提高原型件的成形精度。     

基于试验的SLS成型预热温度场研究

预热是选区激光烧结成型工艺的重要部分.结合试验,得到了同一水平面上的烧结密度变化规律和预热温度与烧结密度的关系,并得到了粉床表面预热温度场.结果表明,预热温度在成型缸内部区域较良好,而在靠近缸壁的区域较低.这可用于指导工艺参数的调整.SLS预热稳态温度场进行研究,这对于提高烧结件性能、减少翘曲变形具有重要意义,同时对提高加热的均匀性和零件的成型精度也具有重要意义.     

面向机器人成形加工可视化系统的三维形貌检测技术

将傅立叶变换轮廓法用于机器人等离子熔射成形过程中零件的三维形貌检测，目的是在满足一定精度要求的前提下，对机器人成形加工过程中被加工体进行现场检测和实时控制，并对成形过程工艺参数进行反馈修正，提高成形精度。详细论述了该方法的测量原理，零件形状与所对应的相位之间的关系，建立了三维形状测量的系统构架，最后给出了测量结果并提出了误差来源和改进措施。     

SLS成形件的收缩模型和翘曲模型

快速成形的精度是快速成形技术发展和扩大其应用领域的关键,工艺误差是影响成形精度的关系之一,而成形件收缩和翘曲对其工艺精度起决定性的作用,通过建立SLS成形过程的收缩模型研究了SLS加工中成形件的收缩形式以及收缩规律,同时研究了收缩的不均匀性所引起的翘曲,建立了成形件的翘曲模型,发现烧结收缩与烧结程度密切相关,翘曲量与烧结收缩率成正比,与零件线尺寸成正比,这些研究结果为克服SLS加工收缩引起的误差和收缩补偿提供了依据,为减少翘曲变形提供了一种方法。     

气割几何形状精度的控制

气割的质量是指金属的金相组织变化、材料的物理性能和零件的几何形状精度。其中前两者与材料和切割方法有关,目前都能掌握。至于零件的几何形状精度,至今仍是影响气割质量的主要原因。气割几何形状精度包括:切割断面的几何尺寸和外形的控制。它们受下列因素影响:气割嘴的式样和尺寸、氧气流量、氧气的纯度、预热焰的氧与乙炔比     

选择性激光烧结快速成型制件翘曲变形的研究

针对翘曲变形对选择性激光烧结(SLS)成形精度的影响很大的问题,通过在HRPS-ⅢA型成型机上进行快速成型试验,找出产生翘曲变形的根本原因,得到了激光扫描速度和激光功率、粉末预热温度等工艺参数对翘曲变形的影响规律,并研究了减小翘曲变形的有效措施。     

选择性激光烧结激光能量密度与温度补偿规律研究

选择性激光烧结（SLS）成型预热温度从成型缸中间到缸壁呈降低趋势,由于预热温度不均匀,导致SLS成型件密度不均匀。针对这一问题,通过增加激光能量密度对预热温度不足进行了补偿。通过试验建立了激光能量密度差△E与预热温差△T的经验公式,并比较了温度补偿前后烧结件的机械强度。实验结果表明,通过增加激光能量密度对预热温度不足进行补偿,提高了产品机械强度。     

SLS翘曲变形计算与扫描方式优化

选择性激光烧结工艺(selective laser sintering,SLS)加工中所产生的翘曲变形严重影响了成型件的加工精度和SLS技术的应用与推广。本文在研究SLS成型误差的基础上,分析造成SLS成型件翘曲的主要原因与产生的机理,构建了一种新的翘曲变形的力学模型。并以此模型为基础,分析了现有SLS扫描方式存在的不足,提出一种新的间隔式激光光栅扫描方式。通过实验验证,该扫描方式可以有效地降低SLS成型件的翘曲变形,提高SLS成型件的加工精度,而且能大大提高加工速度,降低加工能耗。     

Influence of process parameters on part shrinkage in SLS

Shrinkage, one of the typical phenomena in the selective laser sintering (SLS) process, affects the final dimensional accuracy of SLS products. We investigated the relationship between the shrinkage and the process parameters of SLS in order to improve dimensional accuracy. According to the characteristic of SLS, the following process parameters are considered: layer thickness, hatch spacing, laser power, scanning speed, temperature of working environment, interval time and scanning mode. A neural network model on the relationship between the processing parameters and shrinkage was built on the basis of a series of experiments. The experimental investigation results show that the neural network model is possible to be used to predict the effects of the process parameters on the shrinkage with reasonable accuracy and to analyze the relationship between the shrinkage and the process parameters of SLS quantitatively. So it is suitable to apply neural networks approach to study the SLS process. This model will allow us to produce the SLS parts with the desired quality attributed by selecting the appropriate parameter values prior the processing. This paper proposes a promising approach to improve the accuracy of the SLS part.     

Building Porous Biopolymeric Microstructures for Controlled Drug Delivery Devices Using Selective Laser Sintering

The capability to build parts with predetermined porous microstructure and dense walls using powdered biomaterials makes selective laser sintering (SLS), one of the more flexible rapid prototyping (RP) process, a strong candidate for building biodegradable controlled release drug delivery devices (DDD). The objective is to design a varying-porosity circular disc with outer region being denser acting as diffusion barrier region and inner more porous region acting as drug encapsulation region. This is to achieve a zero-order of release over a desired duration of time in drug administration.A key study in this paper was to determine the influence of critical SLS process parameters namely, laser power, laser scan speed and part bed temperature on dense wall formation and control of the porous microstructure of SLS-fabricated parts built with biomaterials. The physical characteristics of the fabricated devices were investigated through microstructure examination using the scanning electron microscope (SEM). Two biodegradable polymers, namely Polycaprolactone (PCL) and Poly (-L) Lactic Acid (PLLA), were investigated. For PCL varying-porosity disc, the laser power is set at 3 W (inner region) - 4 W (outer region), the scan speed at 5080 mm/s and the part bed temperature at 40°C. For Poly(-L) Lactic Acid (PLLA), the laser power is set at 12 W, the scan speed at 1270 mm/s and the part bed temperature at 60°C. With the set of SLS parameters tabulated for specific polymers, polymeric matrix with specific porosity can be fabricated as drug delivery devices.     

Thermo-mechanical coupled 3D-FE modeling of heat rotary draw bending for large-diameter thin-walled CP-Ti tube

The heat rotary draw bending of large-diameter thin-walled (LDTW) commercial pure titanium (CP-Ti) tube is a highly nonlinear thermo-mechanical coupled physical process. Developing a reliable finite element (FE) model for this process is an effective way to investigate the heat loading and the complex bending behaviors. In this study, considering the characteristics of multi-die constraints and local heating, a thermo-mechanical 3D-FE model was established for preheating and heat bending of LDTW CP-Ti tube in terms of both accuracy and efficiency. First, using the static implicit algorithm, a preheating model was developed to predict the temperature distribution of bending tools. In this model, the key issues such as the full-sized geometry modelling, thermal interaction definition, and automatic heating control were solved to increase the simulation accuracy and efficiency. Then, introducing the predictions of preheating model and using the dynamic explicit algorithm, a thermo-mechanical coupled 3D-FE model was established for the heat bending simulation via the geometry modelling simplification, temperature definition of bending tools, realization of non-uniform temperature distribution, etc. Considering the temperature history of bending tools and wall thickness changing of bent tube, the reliability of preheating model and heat bending model was verified by several experiments. The results showed that the maximum relative errors of both predicted temperature and wall thickness changing degree were less than 9 %. Based on the reliable models, the effects of preheating temperature on the temperature distribution of bending tools and wall thickness changing of tube were numerically evaluated. The established model provides the scientific basis for the prediction and control of bending qualities of the heat RDB process, and the modeling method is also of general significance to the other heat-aided forming process.     

新的激光快速成形方法及应用

针对目前主要激光快速成形方法的优缺点,提出一种基于三维实体边界分割的激光快速成形新方法.该方法将叠层实体制造(LOM)法主要进行激光二维轮廓线扫描和选择性激光烧结(SLS)法能够加工多种覆膜粉末材料的优点有机结合,能够克服SLS法加工效率低,成形件致密度、硬度、精度和表面粗糙度等性能较低的缺点.利用自主开发的快速成形系统,采用热像仪测温与Ansys模拟激光能量输入模型相结合的方法对工艺参数进行确定,对基于三维实体边界分割激光快速成形方法进行应用研究,实现三维实体的快速成形及金属零件的快速铸造,并通过无模具快速铸造的手段获得金属齿轮铸件.     

基于CAM的螺旋桨四轴加工方法研究

为降低螺旋桨加工成本,提出了一种新的加工方法－分割式加工方法。将螺旋桨的芯轴和叶片分离,用数控车床加工芯轴,用四轴加工中心加工叶片。加工完成后用配合的方式将叶片安装在芯轴上,从而避免了使用昂贵的五轴数控加工中心加工螺旋桨。设计结果表明,采用该方法加工出的螺旋桨零部件,其精度和装配精度可与五轴数控机床加工的性能相当。     

颗粒尺寸及组合对SLS碳化硅预制体精度的影响

采用热法包覆方法制备了不同颗粒组合的碳化硅陶瓷粉末,并利用选区激光烧结(SLS)工艺制备了碳化硅陶瓷预制体,研究了颗粒尺寸及组合对激光烧结陶瓷预制体成形精度的影响规律。结果表明：颗粒尺寸及组合对激光烧结陶瓷预制体成形精度的影响显著,预制体翘曲量随颗粒尺寸的增大而减小,表面粗糙度和尺寸误差均随颗粒尺寸的增大而增大;相对于单一颗粒,由双尺寸颗粒组合制备的预制体的翘曲量减小了11%~50%,表面粗糙度Ra值减小了12%~20%,尺寸误差减小了38%~79.6%。