尼龙6粉末选区激光烧结试件平面尺度对应力和变形的影响分析

针对选区激光烧结的局部加热和冷却过程,试件平面尺度变化对温度场、应力和变形都有着直接影响。通过建立烧结过程的瞬时温度场和应力应变模型,结合不同面能量密度,对试件平面尺度与应力和变形的关系进行分析。结果表明,对于正方形试件,不同面能量密度下,当平面尺度由5 mm×5 mm增至30 mm×30 mm时,试件端部表面应力增幅为20.5%～36.2%,且变形量也从0.11 mm增大到2.95 mm;而试件中部的应力变化量较小。不同平面尺度的烧结实验结果显示,当烧结试件平面尺度由5 mm×5 mm增大到30 mm×30 mm时,其变形量由0.14 mm增至1.74 mm;平面尺度较小(5 mm×5 mm、10 mm×10 mm)时,烧结试件为四边翘曲变形,较大平面尺度下多呈现为中凸变形形态。     

不同温度下受压封隔器胶筒有限元分析

利用ANSYS软件中超弹性材料Mooney-Rivlin建立胶筒轴对称有限元模型,分析受压胶筒在温度载荷作用下应力和变形沿筒长分布规律。研究结果表明,温度对受压胶筒应力分布具有一定的影响:室温(25℃)时,上胶筒应力沿筒长均匀分布,中间胶筒上下两端应力较小,中部应力较大,下胶筒底端外表面应力最大,为19.693 MPa,内表面应力最小,为3.984 MPa;随着温度升高,中间胶筒应力分布趋于均匀,下胶筒底端外表面应力增大,100℃时该应力最大值为19.716 MPa。胶筒变形随温度载荷升高沿筒长非线性变化,上胶筒顶端变形小,且沿筒长变形非线性增大,中间胶筒变形最大,为9.17 mm,下胶筒受温度影响较小,其变形较小且变化较为平缓。     

难加工材料硬态干车削中切削参数对残余应力的影响

本文对切削参数和残余应力之间的关系的进行研究.针对某型号高强高硬钢在硬态干车削过程,研究了各切削参数对已加工表面残余应力及残余应力层深分布的影响.结果发现,不同的切削参数条件下,工件已加工表面残余应力可以为拉应力也可以为压应力,残余应力作用层深度为300μm左右.对残余应力影响较大的切削因素为切削速度和进给量,切深对残余应力影响较小,切削参数选择低速低进给时,容易得到有利于提高工件疲劳寿命的表面残余压应力.     

不同工艺下GH4169镍基高温合金电弧增材制造热力场数值模拟

采用Simufact Welding软件建立电弧增材制造GH4169镍基高温合金的有限元模型,对该模型进行了试验验证,模拟分析了不同沉积路径(单向沉积、往复沉积)和不同层间冷却时间(0,60,120 s)下多层单道成形过程中的热力场及变形量。结果表明:模拟得到电弧增材制造过程中的热循环曲线和残余应力与试验结果相吻合,相对误差分别小于9%和3%,验证了该模型的准确性;与单向沉积路径相比,往复沉积路径可以改善成形件热分布和应力场分布的均匀性,应力场和热积累的影响范围较小,温度和残余应力较低,变形对称性较好;随着层间冷却时间的延长,热积累、残余应力和变形量均降低,但降低幅度逐渐减小。采用往复沉积路径且层间冷却60 s时,电弧增材制造GH4169镍基高温合金的成形质量较好。     

单晶硅超精密磨削过程的分子动力学仿真

对内部无缺陷的单晶硅超精密磨削过程进行了分子动力学仿真,从原子空间角度观察了微量磨削过程,解释了微观材料去除、表面形成和亚表面损伤机理,并分析了磨削过程中的磨削力和磨削能量消耗。研究表明:磨削过程中,在与磨粒接触的硅表面原子受到磨粒的挤压和剪切发生变形,堆积在磨粒的前方,当贮存在变形晶格中的应变能超过一定值时,硅的原子键断裂,即完成了材料的去除;随着磨粒的运动,磨粒前下方的硅晶格在磨粒的压应力作用下晶格被打破,形成了非晶层,非晶层不断向前向深处扩展,造成了单晶硅亚表面的损伤;同时部分非晶层原子在压应力的作用下与已加工表层断裂的原子键结合,重构形成已加工表面变质层。     

环形精密谐波传动柔轮的承载特性研究

为了研究谐波齿轮中环形柔轮负载状态下变形程度和应力分布,首先在波发生器和环形柔轮之间应用了更为贴合实际的过盈装配模型,解决了大变形装配的难题,并通过实验验证了该方法的有效性。在此基础上对谐波齿轮在空载和额定负载两种工作状态下的受力和变形状况进行了有限元数值计算。最后,通过数据处理对空载和额定负载下柔轮齿根和柔轮圆环中性层的变形和应力分布规律进行了研究。结果表明柔轮圆柱壳体中性层分布应力值较小,柔轮齿根处的分布应力最大且主要为切向应力。相对于空载,额定负载下柔轮的最大变形和最大应力增量分别为空载时的0.46%和93.6%,说明负载对柔轮的变形影响较小但对应力的影响较大。有限元数值仿真的结果与相关文献基本吻合,说明在模型中对柔轮应用过盈装配是成功的,这为研究实际工作条件下谐波齿轮传动性能提供了一种更准确的方法。     

热沉位置对钛合金薄板焊接残余应力的影响

采用切条应力释放法测量钛合金TC4薄板对接试件选用不同的热沉位置分别进行动态控制低应力无变形(DC-LSND，dynamically controlled low stress no-distortion)钨极氩弧焊(TIG)时试件中的纵向残余应力和纵向残余塑性应变的分布。测量结果表明，热源与热沉之间的距离是重要的工艺参数之一，该参数对焊接应力和变形的控制效果有很大影响，适当的热沉位置是动态控制低应力无变形钨极氩弧焊实现低应力无变形焊接效果的一个必要条件。热源与热沉之间的高温金属处于力学熔化状态、无力学抗力时，热源与热沉之间距离的增加有助于降低焊接残余应力，减小纵向塑性变形。在所选用的焊接条件下，动态控制低应力无变形钨极氩弧焊焊接时热源与热沉相距30mm，近缝区的不协调应变较小，控制焊接应力与变形的效果较好。     

精密切削钛合金TC4表面残余应力的模拟研究

本文建立钛合金正交切削热一力耦合有限元仿真模型,采用自适应网格(ALE)技术及Johnson-Cook强化模型和剪切失效模型定义切削.通过对TC4切削过程的有限元模拟,分析了刀具刃口半径及材料的变形对已加工表面残余应力分布和表面质量的影响,并提出在切削钛合金时需采用较小的切削深度、较高的刀具锋锐度、较高的切削速度进行切削,以得到较好的表面加工质量.     

激光表面淬火工艺对表面硬度影响实验研究

采用激光表面淬火工艺可以有效提高金属材料表面层的硬度和耐磨性,可以在表面层形成残余压应力,从而提高材料的抗疲劳性能。采用实验方法研究激光束的直径、移动速率、功率等对表面淬硬层深度及硬度的影响,具有一定的意义。实验结果表明,当激光光斑直径和扫描速度不变时,表面最大硬度随着激光功率的增加先增加后降低,存在一个最佳功率值;当其他参数不变时,在一定范围内淬硬层的深度随着扫描速度的增加而降低;两道扫描路径之间的搭接区域硬度较小,且存在较小的残余拉应力,是整个结构的薄弱环节。     

镍基合金涂层和扭力轴的扭转特性仿真

为了得到修复再制造后扭力轴基体和镍基合金熔接涂层的力学性能,分析了不同扭转弧度对熔接涂层和基体力学性能的影响,得出熔接涂层和基体的应力、应变随扭转弧度的变化规律,并讨论模型轴向外表面和径向内表面应力、应变随扭转弧度的变化规律。研究结果表明:修复后的模型力学性能达到原有力学性能的90%;随着扭转弧度的增大,扭力轴基体和熔接涂层的应力、应变均逐渐增大;对于模型轴向外表面,最大应力出现在涂层上,并且由涂层中心向四周的结合界面扩散,熔接涂层应变是先小于基体应变,然后再大于基体应变;对于模型径向内表面,熔接涂层的应力和应变始终大于基体的应力和应变。     

Two-dimensional modeling of sintering of a powder layer on top of nonporous substrate

Selective laser sintering (SLS) of a two-component metal powder layer on the top of multiple sintered layers by a moving Gaussian laser beam is modeled. The loose metal powder layer is composed of a powder mixture with significantly different melting points. The physical model that accounts the shrinkage induced by melting is described by using a temperature-transforming model. The effects of the porosity and the thickness of the atop loose powder layer with different numbers of the existing sintered metal powder layers below on the sintering process are numerically investigated. The present work will provide a better understanding to simulate much more complicated three-dimensional SLS process.     

Two-dimensional modeling of sintering of a powder layer on top of nonporous substrate

Selective laser sintering (SLS) of a two-component metal powder layer on the top of multiple sintered layers by a moving Gaussian laser beam is modeled. The loose metal powder layer is composed of a powder mixture with significantly different melting points. The physical model that accounts the shrinkage induced by melting is described by using a temperature-transforming model. The effects of the porosity and the thickness of the atop loose powder layer with different numbers of the existing sintered metal powder layers below on the sintering process are numerically investigated. The present work will provide a better understanding to simulate much more complicated three-dimensional SLS process.     

Temperature and stress analysis and simulation in fractal scanning-based laser sintering

A model for calculating the evolution of temperature and thermal stresses within a single metallic layer formed on the powder bed using different scanning patterns in selective laser sintering (SLS) was proposed. The model allows for the non-linear behavior of thermal conductivity, specific heat, and elastic modulus due to temperature changes and plastic deformation capabilities with a bilinear isotropic hardening behavior. The effect of laser scanning patterns on temperature, residual thermal stresses and distortion were investigated. It was shown that the distortion and transient stresses of a layer processed by a moving laser beam is decreased with fractal scanning pattern.     

Scan pattern, stress and mechanical strength of laser directly sintered ceramics

A finite element model was developed to simulate the influence of laser scan patterns (laser jump patterns, ratio of length to width and laser scan angle) on the temperature and stress distributions in the process of laser direct sintering ceramics. The effects of laser scan patterns on the stresses in the laser directly sintered ceramic samples were investigated by simulation. Different ceramic samples were prepared by the laser sintering process for the four-point bending strength measurement. The simulation results consist with the experimental ones. The results in the present research can be used to optimize process route for the laser direct sintering process and provide a guidance for selecting the laser processes parameters for high mechanical strength ceramic components.     

激光直接烧结成形多层金属薄壁件的温度场有限元模拟

为掌握多层粉末烧结过程中激光熔池的加热冷却规律以及各烧结层之间的相互影响,综合考虑热传导、热辐射和热对流以及材料的高度非线性,基于ANSYS平台建立了多层金属薄壁件的三维温度场有限元模型,利用APDL语言编程实现模拟中激光热源的移动,采用"单元生死"技术描述粉末材料动态增长过程。模拟结果表明:在现有工艺参数下,烧结获得的熔深在0.15mm以上,熔宽在0.61mm左右,烧结成形件与基体以及层层之间搭接牢固;成形件中,与x方向的热梯度相比,z方向热梯度占绝对优势,这说明成形件在冷却过程中热量的散失以堆积方向为主。将模拟结果与实验结果进行了对比,实验结果较好地验证了模拟结果。     

FEA simulation of thermal processes during the direct metal laser sintering of Ti64 titanium powder

With regard to the fact that laser sintering belongs to the high-temperature processes in which metal particles are sintered by a high-power laser, forming a homogenous structure, it is necessary and important to know the characteristics and the mechanism of these thermal processes. A high-power laser system produces three forms of heat that include convection, conduction, and radiation. These thermal processes affect the formation of internal stresses and tension that lead to deformations and rapidly influence the resulting quality, dimensions, density, micro-structure, and mechanical properties of fabricated parts. In response to this fact, it was important to analyse these heat transfer methods instantly during the direct metal laser sintering (DMLS) process simulation and subsequently monitor the parameters and settings of the sintering equipment in order to obtain acceptable manufacture outputs intended for further use. This work is focused on the creation of a FEA simulation model and the simulation of thermal processes across an object during and after the sintering process in the cooling stage, when it is important to consider a laser beam trajectory, temperatures of individual elements affected by the laser beam, and current laser energy in time. A 3D FEA simulation model was created in order to represent actual behaviour of a part during the sintering process. The simulation model consisted of two sub-models, particularly the building platform model with the dimensions of 250 mm × 250 mm × 22 mm, with stainless steel as the selected material, and the model of individual layers of sintered titanium powder with the dimensions of 10 mm × 10 mm × 0.03 mm. The total number of used layers was 12, which represents the total thickness of 0.36 mm. Applied power was P = 170 W. The simulation as such was carried out using the FEA software, Simulia Abaqus supported on the Windows x86-64 platform, which uses an integrated solver to make thermal and mechanic calculations. The calculations included also the impact of the protective argon atmosphere located in the process chamber. Mutual impact between individual layers was also considered. The simulation results were confronted with the results of already performed experimental studies of other scientific works, with the compliance and confirmation of assumptions being on a very good level.     

激光熔覆裂纹产生机理及控制方法分析

激光熔覆是利用激光辐射使熔覆材料和基体表面同时熔化并快速凝固、从而显著改善基体材料物理性能的一种新的工艺方法。熔覆工艺的选择不当会使熔覆层产生裂纹,降低熔覆制件抗疲劳的能力。本文研究了高能激光束和材料的作用特点以及熔覆层的应力类型,分析了熔覆层裂纹的形成机理,建立了热应力的计算模型,从熔覆材料和基体材料的选择、数值模拟技术预测以及利用熔覆层应力的作用特点改进熔覆工艺三个方面阐述了裂纹的影响因素和控制方法。     

直接金属激光烧结成形的机理及实验研究

探讨了直接金属激光烧结成形机理，分析了烧结参数与激光输入能量之间关系。在室温惰性气体环境下，进行了不同匹配参数下314不锈钢金属粉末的直接激光烧结成形实验，测试了成形件的微观组织结构和机械性能，研究了工艺参数与直接金属激光烧结成形件的微观组织结构及宏观机械性能的关系。为制定合理的激光烧结工艺参数提供了依据。     

粉末薄层选区激光烧结温度场数值模拟与实验研究

单层粉末的激光烧结温度场对三维结构熔结成型有着直接影响。通过对工程塑料粉末选区激光烧结过程能量作用形式的分析,构建了粉体与金属基板界面接触热阻的导热模型和扫描烧结的移动热源模型;利用有限元法对功率为10～25W,光斑直径为0.24mm的激光逐行扫描烧结过程的温度场进行了数值模拟;通过温度场分布模拟和激光烧结实验得到了单层厚度为0.5mm的PA6粉末行扫描间距的优化区间为0.3～0.4mm,获得了熔结质量较好、厚度均匀的平板形烧结物,为进一步优化工艺参数提供了基础。     

Numerical simulation of the thermal history multiple laser deposited layers

Multilayer direct laser metal deposition is a fabrication process in which the parts are fabricated by creating a molten pool into which metal powder particles are injected, and a layer is laid down by moving the pool. Height is added by creating additional layers on top of the first layer. During fabrication, a complex thermal history is experienced in different regions of the build. The thermal history includes the reheating process for previously deposited layers caused by subsequently deposited layers. The objective of this study is to provide insight into the thermal history during the direct laser deposition process. Using the commercial ABAQUS/CAE software, a thermomechanical 3D finite element model was developed. This work presents a 3D heat transfer model that considers the continuous addition of powder particles in the front of a moving laser beam using ABAQUS/CAE software. The model assumes the deposit geometry appropriate to each experimental condition and calculates temperature distribution, cooling rates, and remelted layer depth which can affect the final microstructure. Model simulations were qualitatively compared with experiments results acquired in situ using a K-type thermocouple.