不同基板预热温度对激光金属沉积成形过程热应力影响的研究

研究不同基板预热温度对激光金属沉积成形过程热应力的影响,对于降低成形过程的热应力,抑制成形过程裂缝的产生,减小成形过程试样和基板的翘曲变形具有非常重要的意义.根据有限元分析中的"单元生死"技术,编程建立了基板预热对激光金属沉积成形过程热应力影响的三维多道多层数值模拟模型,详细分析了基板未预热和分别预热到200℃、300℃、400℃、500℃ 、600℃时对沉积成形过程Von Mise'S热应力、X方向、Y方向以及Z方向热应力的影响.在与模拟过程相同的参数下,利用镍基合金粉末分别在基板未预热和分别预热到300℃ 、400 ℃、500℃、600℃ 时进行了成形试验,试验的结果跟数值模拟结果吻合较好.     

激光金属沉积成形基板预热温度的控制

为了降低激光金属沉积成形过程中试样和基板间的温度梯度,减小和抑制成形过程的热应力,提高试样的成形质量,提出并设计了一种用于激光金属沉积成形的基板预热系统.其中基板预热温度的控制由智能比例微分积分控制器以及计算机串口温度采集反馈控制来完成.利用自行研制的激光金属沉积成形设备和基板预热系统进行了成形实验,实验结果表明该基板预热系统温度控制良好,可以满足实际应用的需要.     

激光熔覆成形预热基板设计及试验研究

为了降低激光熔覆成形过程中熔覆层热应力和减少裂纹的产生,以热传导理论和激光快速成形理论为依据,进行了预热基板的结构和控制设计;并进行了不同预热温度下单道熔覆和平面熔覆的试验研究.结果表明,通过智能PID控制器可对预热温度进行良好控制,通过热电偶温度采集模块可以实现对基板预热温度的连续、实时检测;在基板预热条件下进行激光熔覆成形可以显著改善试样的成形质量,并有效降低成形过程的热应力,减少了熔覆层裂纹的产生.     

扫描方式对激光金属沉积成形过程热应力的影响

为改善金属试样的成形质量,降低沉积过程的热应力,研究不同扫描方式下激光金属沉积成形过程中热应力的动态分布规律具有非常重要的意义。根据有限元中的“单元生死”技术,通过ANSYS参数化设计语言 (ANSYS parametric design language, APDL) 编程研究了沿长边平行往复、沿短边平行往复以及层间正交变向平行往复等填充扫描方式对整个成形过程热应力的影响,详细探讨了各种扫描方式下Von Mises热应力、x方向热应力、y方向热应力以及z方向热应力的动态分布规律,并分析热应力产生和分布的原因。在与模拟过程相同的条件下,实际成形试验所得结果与模拟结果吻合较好。     

钛合金激光直接成形过程中热力耦合场的数值模拟

为控制成形过程的热应力,根据有限元法中的生死单元技术,利用ANSYS参数化设计语言编程实现对多道多层激光金属沉积成形过程三维温度场和应力场的数值模拟,并对熔池与粉末、激光与粉末的相互作用进行能量补偿,更加准确地计算成形过程中温度场和应力场的动态变化,得到成形过程中模型温度场、温度梯度、热应力场和残余应力的分布规律.结果表明,成形件不同层上的各节点虽然被激活的时间不一样,但它们具有相似的温度变化规律;试样内的温度梯度主要沿z轴方向分布,熔池区的温度梯度非常大,但其他方向不明显;瞬态热应力集中在温度梯度变化较大的区域,这与热应力形成的机理是一致的;通过对成形件中各方向的残余应力分析,从温度梯度的角度总结各方向残余应力变化规律,侧面验证残余应力的形成机理.通过相同工艺参数下的试验验证,证明上述分析与实际情况是基本吻合的.     

扫描路径对选择性激光熔化成形质量的影响

基于选择性激光熔化技术,研究了不同扫描路径对Ti6Al4V成形试样的表面粗糙度、残余应力、翘曲变形的影响。结果表明:无论是长边扫描还是短边扫描,上表面的粗糙度值要明显小于侧表面的粗糙度值,和扫描方向相平行的侧表面的粗糙度值要小于和扫描方向相垂直的侧表面的粗糙度值。采取短边扫描的成形件和基板之间的残余应力值更小,且成形件和基板的最大翘曲变形相较于长边扫描减小了约35%。在SLM成形前,采取对基板进行适当预热的方式可以减小成形过程中成形件和基板之间的温度梯度,从而达到减小残余应力的目的。采取短边扫描且基板预热温度为300℃的成形件达到了99.4%的近全致密,采取短边扫描的成形件的致密度要高于采取长边扫描的成形件的致密度。实验结果表明,采取短边扫描的SLM成形件的成形质量更好。     

激光立体成形TC4热-变形行为数值分析

本文针对激光立体成形过程中的热应力变形问题,首先利用原位测量平台监测了一次沉积和二次沉积单壁墙过程中的温度场和基板变形历史,然后基于测量结果校验了有限元热力耦合模型,最后分析了激光立体成形过程中热-变形的演化规律。结果表明,模拟结果与实验结果相符。随着沉积层数增加高温区面积不断增加,且熔池前沿温度梯度明显大于熔池尾部。相比一次沉积过程,二次沉积过程中熔池温度明显较高,且升温速率更快。另外,初始沉积阶段的高温度梯度和冷却阶段的高冷速是诱导热应力和变形的主要因素。     

激光选区熔化钛铝合金裂纹形成机理及抑制研究

裂纹是激光选区熔化成形TiAl合金时最常见、破坏性最大的一种缺陷。以Ti-47Al-2Cr-2Nb合金粉末为成形材料,利用X射线衍射、扫描电子显微镜、X射线能谱分析及残余应力检测等检测方法,研究SLM成形TiAl合金过程中裂纹的形成机理及抑制方法。研究结果表明:SLM成形TiAl合金的裂纹为冷裂纹,具有穿晶断裂的特征。由于SLM成形过程中的快速加热冷却,成形件内部残余应力高于材料的抗拉强度,导致了裂纹的形成。裂纹多起源于试样侧面边缘粉末黏结、缺口等缺陷存在的地方,此外,合金试样内部含有大量的α2脆性相及孔隙,也有利于裂纹产生和拓展。随着基板预热温度逐渐提升到300℃,一方面温度梯度减小使得材料热胀冷缩减弱,试样内部的残余应力由(267.2±13.4) MPa降低到(172.6±8.6) MPa;另一方面,预热导致β相稳定性提高,α2脆性相的含量减少,最终使得试样中的裂纹数量和尺寸明显得到控制,试样的相对致密度也由87.64%上升到93.84%。     

复杂薄壁零件激光快速成形过程的热力耦合场数值模拟

研究直接激光成形过程中叶片温度场与热应力场的关系,改善成形质量.以空心叶片为例,根据激光测量仪测得的散点图,用圆弧逼近得到叶片的近似轮廓.利用有限元软件MSC.Marc建立Ti-6Al-4V(TC4)圆弧逼近的空心叶片的有限元模型,并对激光直接成形过程进行数值模拟,得到叶片在成形过程以及冷却过程中的温度场和热应力场.随着成形的进行,由于热量的积累,成形叶片的温度不断升高,已成形部位的热应力随着成形过程的热循环不断变化,小曲率半径处的热应力大,且热循环对热应力变化影响较小;在直接激光成形结束后,整体温度开始下降,各处的热应力都逐渐升高.通过分析温度场和热应力场的分布情况,得到不同曲率半径对温度场和热应力场的影响规律.基于模拟结果,总结的规律和试验经验,提出改善成形质量的有效方法.并利用激光直接成形技术,加工出模拟分析模型的实物零件,验证模拟和结论的正确性.     

CST液黏传动摩擦副表面织构技术研究

可控起动传动装置(Controlled Start Transmission, CST)为重型刮板输送机提供了一种软起动、过载保护和多机功率平衡的可靠方法。液黏摩擦传动单元(Hydro-viscous Drive Unit,HDU)是CST装置的核心,它包括多组摩擦副。摩擦副的可靠性和耐磨性能直接决定了CST装置的整体性能。以粉末冶金和激光表面工程技术为基础,提出以仿生非光滑表面来改善和提高摩擦副机械耐磨性能的新思路。利用激光直接沉积和激光脉冲钻孔技术,制备了具有不同非光滑表面形态和参数的非光滑表面试样。通过表面形貌分析和摩擦磨损试验发现:激光直接沉积过程的基板预热(600℃),有助于提高激光沉积试样的表面质量,包括表面形貌、裂纹抑制控制以及沉积材料与基板结合位置的晶粒细化等;与光滑试样相比,具有非光滑表面形态的试样具有更好的耐磨性能,在试验条件下,当小孔间距等于1 mm,小孔直径等于0.8 mm时,材料表面具有最好的耐磨性能。该研究为CST装置液黏传动摩擦副的未来研究和试制提供了新的方法。     

Investigation on the process of laser surface melting using two sequential scans

Laser melting as a flexible and economical surface treatment technology is of significant value to industries. Many efforts have been taken on purpose to gain such desired machining goals and reliable service performances; yet, beneficial and efficient melting strategy is still needed. In studies of some other processes such as multi-pass laser cladding, it is found that the preheating effect of a previous scan can contribute to achieve better results like increasing dilution and improving thermal stress status. Hence in this investigation, for laser surface melting with single track, a helpful preheating process is performed by proposing an approach of using two sequential scans and the effects of preheating laser are further clarified. To this end, a three-dimensional non-linear, indirectly coupled thermal?Cstructural model for the sequential combined process is established accounting for temperature dependency of material properties. Then, investigations are carried out to examine temperature and residual stress fields under different preheating laser power and interval distance. The results show that this melting strategy can improve the residual stress distribution state. In addition, a high thermal stress distributes between two scans and the tensile stress component is released by rapid two-time fusions. Besides, the different combinations of preheating parameters take diverse effects on heat-affected and -remelted zone as well as residual stress distribution.     

铀表面铝镀层热应力的有限元分析

对铀表面磁控溅射沉积铝镀层的热应力进行了热弹塑性有限元分析。结果表明：镀层内的热应力较大,达到铝的屈服强度,镀层界面两侧存在明显应力梯度,试样侧边存在由于边缘效应引起的应力分布不均匀性,至侧边约4倍镀层厚度处,应力分布不均匀性逐渐消失;沉积温度升高,界面塑性应变明显增大,镀层弹性模量和泊松比对镀层界面热应力和塑性应变的影响较小,而屈服强度的影响较大,减薄镀层厚度有利于改善镀层界面的热应力和塑性应变。     

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.     

Interfacial Stress and Failure Analysis for Piston Ring Coatings under Dry Running Condition

A two-dimensional thermomechanical finite element model was developed to analyze the sliding process of a piston ring with coating sliding on cylinder liner under dry running condition. Thermal and mechanical effects were considered simultaneously in the model. The aim of the current work is to study the mechanisms of scuffing, failure, and seizure occurrence in a piston ring-liner system. It is shown that coating thickness plays an important role in the thermal and mechanical stress status at the contact area, coating bulk body, and interface of the coating and piston ring substrate. The coating thickness also exhibits a significant influence on the temperature rising at the contact area and interface of the ring coating and substrate, which could cause failure at the interface of the coating and substrate before it happens at the contact surface under some specific conditions. The results also show that thinner coating thickness in some specific range could have a higher possibility of cracking or failure. Furthermore, it is found that the thermal loading is the key cause of scuffing or failure of the piston ring coating.     

Cr12MoV钢电子束熔覆前的预置涂层工艺热应力仿真

以镍基粉末为例,利用ANSYS对Cr12MoV钢电子束熔覆处理前的等离子喷涂的工艺过程进行数值模拟,采用间接耦合的方法,对热喷涂冷却过程进行温度场和应力场耦合分析.结果表明：粉末熔滴温度随着冷却过程的进行而向边缘逐渐下降;基体温度先急速升温最大值后缓慢下降,下降趋势渐趋于熔滴温度下降的趋势;熔滴主要承受压应力,最大值位于熔滴的下表面中心位置;离熔滴中心轴线逾远,承受的拉应力逾大;基体合适的预热温度有利于降低熔滴径向应力.     

Thermomechanical finite element analysis of slider-disk impact in magnetic storage thin film disks

Oblique impact of a slider with a rotating disk in a hard disk drive was analyzed using the finite element method. A three dimensional, thermomechanical, impact model was developed to study the mechanical and thermal response during the impact of a spherical slider corner with a rotating disk. The model was validated by comparing the finite element results with analytical solutions for a homogeneous glass substrate disk. Impact penetration, stress and incurred flash temperature were obtained for various normal impact velocities. The effects of material layers on the disk were also investigated by introducing layers with different material properties and thicknesses. It was found that for a rounded slider corner and a critical normal impact velocity of 0.03 m/s studied in this work, the layers have insignificant effects on the mechanical response and small but predictable effects on the flash temperature.     

覆膜铁熔融覆膜过程界面传热与熔膜填充行为研究

覆膜铁是金属包装领域的新型复合材料。成品覆膜铁的界面结合强度和水透性取决于覆膜过程中高聚物薄膜熔融层的厚度和流动铺展行为。但覆膜过程复杂而迅速，且受钢板表面微观形貌影响，熔融层膜厚及其流动铺展行为难以精细调控。针对此，基于分形理论表征与重构基板带钢表面微观形貌；基于广义Maxwell模型试验建立聚合物膜的黏弹性本构关系；建立了考虑基板带钢表面微观形貌的覆膜过程有限元仿真模型，并试验验证了模型的准确性。研究表明，熔融层膜厚主要受基板带钢初始预热温度影响，界面填充率则主要受覆膜辊压力影响。高聚物膜熔融层厚度和界面填充率随基板带钢粗糙度降低以及基板带钢和覆膜辊初始预热温度增加而增加，且辊压力增加也导致界面填充率增加。这些规律被定量化描述，为精细化调控熔融层膜厚及其流动铺展行为提供了理论依据。