钢基表面热喷涂镍沉积过程中热力耦合场的数值模拟

利用有限元软件ANSYS中的间接耦合法,对钢基表面热喷涂镍沉积过程中的热力耦合场进行了数值模拟,对温度场及应力场的分布、基体预热温度对残余应力的影响进行了分析.结果表明:熔滴最高温度位于熔滴的中心部位,最低温度位于熔滴边缘部位;熔滴最大径向压应力值位于熔滴下表面的中心部位;提高基体的预热温度能够降低涂层/基体界面处的残余应力,但不能改变应力的分布形态.     

等离子喷涂熔滴快速撞击基体非稳态凝固行为研究进展

涂层成形过程中的缺陷含量、残余应力、沉积效率、组织结构及力学性能等指标均会随着工艺参数与基体预处理状态的不同而发生显著变化,因而需要从更加微观的角度深入理解等离子喷涂涂层的微观构筑过程,即单个熔滴的铺展凝固现象。本研究分别从熔滴凝固的类型与机理、不同因素对熔滴凝固过程的影响及凝固斑点形态的定量表征方法 3个方面详细综述了等离子喷涂熔滴撞击基体快速凝固过程的研究现状。结果表明,熔滴的铺展形态主要可以分为5类,包括圆盘型、破碎型、放射型、花瓣型及气泡型,影响铺展过程的因素主要包括熔滴特性(速度、温度、粒径、材料属性、熔化状态等)与基体状态(表面粗糙形貌、表面化学状态、吸附物及冷凝物、界面润湿性及接触热阻等)2大类,综合采用一系列参数对熔滴铺展几何形态进行表征,可实现熔滴沉积质量的定量评价。     

等离子喷涂层片形成过程流体动力学模拟

建立了等离子喷涂层片形成过程的流动、传热和凝固耦合的三维数学模型,基于计算流体力学软件Fluent,运用有限体积法（FVM）离散控制方程,流体体积跟踪法（VOF）追踪熔滴自由表面,模拟了镍熔滴撞击基底表面形成层片的流体动力学过程,并对结果可视化输出.结果表明,撞击开始时刻,压力在撞击点出现最大值,熔滴内部压力场呈＂蘑菇云＂状,撞击压力分布从撞击点向熔滴上部递减;熔滴撞击后沿径向铺展,速度矢量场出现两个对称漩涡,在基底导热和熔体流动传热综合作用下,铺展熔滴内部温度场呈＂驼峰＂状分布;熔滴铺展速度最大值滞后最大撞击压力0.03μs出现.     

环境压强对低压等离子喷涂熔滴沉积行为及涂层性能的影响

为了解环境压强对热喷涂熔滴沉积行为及涂层性能的影响机理,采用低压等离子喷涂装置在不同压强下向不锈钢SUS304基体上喷涂Ni粉末,在基体上收集一定数量的熔滴沉积物及涂层。采用光学显微镜、扫描电镜与聚焦离子束技术等对单个熔滴沉积物的形态进行观察分析,对不同压强下制备的涂层的微观组织、氧含量及结合强度等进行表征。对不同压强下基体表面吸附质的脱附/吸附、熔滴与基体间润湿能力、热传导及界面处的初期凝固等因素对熔滴在基体表面沉积行为的影响进行探讨。结果表明:单个熔滴的沉积形态由常压下的溅射状沉积到低压下的规则盘状沉积;不同压强下均可得到致密涂层,且涂层的结合强度随环境压强降低而逐渐增加,其变化趋势与单个熔滴沉积物形貌随压强变化趋势相吻合,可见熔滴的沉积行为对涂层的性能有着重要的影响。     

正弦织构化表面等离子喷涂熔滴铺展成形分析

目的 分析等离子喷涂熔滴在表面织构内的铺展、凝固规律,为了解表面织构化对等离子喷涂涂层结合机制的影响以提升涂层的结合强度提供理论辅助.方法 基于Flow 3D建立熔滴填充微织构的数值模型,研究等离子喷涂熔滴填充正弦形织构的温度场及铺展成形规律,并对比分析织构边缘凸起及织构形状的影响.同时,基于激光织构化表面等离子喷涂试验,分析不同织构对涂层形貌的影响.结果 无边缘凸起织构的散热速度比有边缘凸起织构的快约0.7μs,凹坑织构的散热速度最慢,计算结束时,其最高温依然高于熔滴液相线(1728.4 K)约44.7 K.基体最高温度约1680.5 K,其主要分布于织构边缘凸起顶端、基体平面与织构内壁的拐点以及织构内壁上的凸点位置.有边缘凸起正弦织构的凝固熔滴由中间向两侧逐渐变薄,熔滴在无边缘凸起织构外部呈规则的圆盘状,凹坑织构底部存在"空腔"特征.涂层截面形貌分析发现,有边缘凸起的织构表面的涂层形貌较优,凹坑织构涂层含有较多缺陷,形貌最差.结论 镍基涂层与织构化表面为机械结合.相比凹坑织构,正弦和直线织构更有利于提高涂层的质量.织构边缘凸起的存在对提高喷涂质量也是有利的.     

预热温度对机匣等离子喷涂铝硅可磨耗封严涂层应力场影响的数值模拟

目的 为确定机匣等离子喷涂铝硅涂层最佳预热温度,揭示不同预热温度对涂层残余应力的影响,优化基体预热温度,降低由于过大的残余应力导致涂层剥落和失效的可能性,为实际生产提供指导。方法 基于热弹塑性有限元理论,使用ANSYSWORKBENCH中稳态热和结构应力模块,建立双层等离子喷涂有限元模型。采用间接热力耦合方法对不同预热温度下的机匣等离子喷涂温度场和应力场进行模拟,分析不同预热温度对面层/黏结层/基体系统温度和应力分布的影响,重点研究了预热温度为30、50、80、120、150、180、200 ℃时涂层的温度场和应力场分布。结果 随着基体预热温度的升高,基体和涂层的温度梯度逐渐减小,面层等效应力逐渐减小,最大等效应力先减小后增大;y轴环向应力和z轴轴向应力分布及变化趋势基本相同;与y轴环向应力和z轴轴向应力相比,基体预热温度的变化对x轴径向拉应力、径向压应力的影响更大。结论 根据涂层的残余应力的分布和变化规律,等离子喷涂铝硅可磨耗封严涂层时,基体预热温度应控制在150 ℃。     

基底对等离子喷涂Mo层片形成的影响

运用扫描电镜和台阶仪研究了在不同状态下的1Cr18Ni9Ti不锈钢和纯Cu基底上大气等离子喷涂Mo层片的形成机制。结果表明:室温光滑基底上沉积的Mo层片均呈不规则飞溅花瓣状,随预热温度升高,形成的层片逐渐变得较完整;熔滴撞击铺展过程中在基底上可能会形成凹坑;基底导热率越高,层片铺展度越小;粗糙基底表面阻碍熔滴铺展,促进熔体飞溅。理论分析表明,"花瓣状"层片的形成是熔体表面张力、熔体与基底接触热阻的不均匀性和基底凹坑相互作用的结果;基底凹坑的形成主要由基底熔化所控制。     

可计及温度与层状结构影响的超高温陶瓷基复合材料涂层残余热应力理论表征模型

目的创建可计及温度与层状结构共同影响的超高温陶瓷基复合材料涂层与基体层因热不匹配导致的残余热应力的理论表征模型。方法基于经典的层合板理论与超高温陶瓷基复合材料热物理性能参数对温度的敏感性研究,引入温度和层状结构对涂层与基体层所受残余热应力的影响,形成各层残余热应力温度相关性的理论表征方法,并以ZrB_2-SiC复合材料涂层为例,利用该理论方法系统地研究了各种控制机制对残余热应力的影响及其随温度的演化规律。结果超高温陶瓷基复合材料涂层与基体层所受的残余热应力随着温度的变化而变化,涂层热膨胀系数与基体层热膨胀系数差别越大,变化幅度越大。当涂层材料热膨胀系数大于基体层材料热膨胀系数时,涂层材料遭受残余拉应力,基体层材料遭受残余压应力;随着涂层厚度的增加,涂层所受拉应力减小,而基体层所受压应力增大;当涂层材料热膨胀系数小于基体层材料热膨胀系数时,涂层材料遭受残余压应力,基体层材料遭受残余拉应力;随着涂层厚度的增加,涂层所受压应力减小,而基体层所受拉应力增大。低温下,各层所受残余热应力对层厚与每层材料组成的变化比较敏感,随着温度的升高,敏感性降低。结论对于涂层材料,应设计涂层材料的热膨胀系数小于基体层材料的热膨胀系数,使涂层遭受残余压应力,这不仅能够降低材料表面产生裂纹的危险,同时可以抑制表面已有缺陷的扩展。同时应当设计相对较小的涂层厚度,以增大涂层所受的残余压应力,降低基体层所受的残余拉应力,有效提高整体材料在不同温度下的强度性能。     

Ti-6Al-4V基体上等离子喷涂羟基磷灰石涂层的热-力耦合和残余应力

基于热弹塑性理论建立了等离子喷涂羟基磷灰石涂层的热-力耦合模型。一方面,通过引入变形对温度的影响项对经典的Fourier瞬态热传导方程进行了修正,另一方面,在考虑了温度软化效应、应变硬化效应、和应变率强化效应后,采用适用于高温、高应变率条件下的Johnson-Cook模型作为本构方程。基于上述耦合模型,采用有限元方法对涂层内的残余应力场进行了模拟,同时还模拟了基体预热温度以及涂层厚度对残余应力的影响。为了验证数值模拟的可靠性,还采用“材料去除”的实验技术测试了涂层在界面处的残余应力。结果表明：在界面边缘有明显的应力集中;适当提高基体的预热温度有利于减小残余应力;涂层的残余应力随厚度的增加而增大。     

Fe-Mn-Si记忆合金激光熔覆层残余应力的数值模拟

为分析Fe-Mn-Si记忆合金熔覆涂层的应力分布情况,利用ANSYS有限元分析软件数值模拟该涂层的温度场及应力场。模拟结果表明,当激光熔覆涂层完全冷却后,其末端基体中间区域的横向残余应力表现为残余压应力;从基体中部向两侧移动,残余压应力逐渐变为残余拉应力;沿激光扫描方向,熔覆涂层的纵向残余应力依次为压应力→拉应力→压应力。     

Wetting behavior of eutectic Al–Si droplets on zinc coated steel substrates

Transient spreading behavior, joint properties and metallurgical compositions are investigated for different hot-dip and electro-galvanized zinc coatings. The main focus is set on the effect of coating thicknesses and droplet size. While most of the droplets are observed on surfaces at room temperature, the case of pre-heated substrates is also accounted for. Both the coating thickness and the droplet size have little effect on the resulting wetting angle compared to the effect of preheating or the absence of a coating. The transient spreading behavior significantly differs for different coating types. The coating thickness affects heat transfer into the substrate during the initial stage of wetting. The metallurgical composition shows that the coating is removed over a broad interfacial area, while it accumulates at the toe of the deposited braze metal most likely due to fluid dynamic effects.     

Study of Substrate Preheating on Flattening Behavior of Thermal-Sprayed Copper Particles

In this study, the effect of substrate preheating on flattening behavior of thermal-sprayed particles was systematically investigated. A part of mirror-polished AISI304 substrates were preheated to 573 and 773 K for 10 min, and then exposed to an air atmosphere for different durations of up to 48 h, respectively. Contact angle of water droplet was measured on the substrate under designated conditions. It was found that the contact angle increased gradually with the increase of substrate duration after preheating. Moreover, smaller contact angle was maintained on the substrate with higher preheating temperature. Commercially available Cu powders were thermally sprayed onto the substrates with the same thermal treatment history as contact angle measurement using atmospheric plasma-spray technique. The splat shape had a transitional changing tendency from a splash splat to a disk one on the substrate with a short duration after preheating, while reappearance of splash splat with the increase of duration was confirmed. In general, wetting of substrate surface by molten particles may dominate the flattening behavior of thermal-sprayed particles. The occurrence of desorption of adsorbed gas/condensation caused by substrate preheating likely provides good wetting. On the other hand, the poor wetting may be attributed to the re-adsorption of gas/condensation on the substrate surface with the increase of duration. In addition, the shear adhesion strength of coating fabricated on blasted AISI304 substrate was enhanced on the once-heated substrate, but weakened with the increase of duration. The changing tendency of the coating adhesion strength and the wetting of substrate by droplet corresponded quite well with each other.     

金属熔滴连续沉积、凝固重叠工艺及实验验证

金属熔滴均匀连续沉积是一种新型的3D打印技术和快速原型(RP)技术。本文系统研究了铝熔滴连续沉积到基板表面的瞬态传热传质现象,沉积机理主要包括金属熔体的毛细现象及固液界面的传热、凝固、搭接和重熔。采用VOF方法对连续沉积的金属熔滴在水平固定铝基板表面沉积进行3D建模。针对凝固传热过程中不同参数下熔滴在铝基板表面沉积、凝固进行了研究,通过数值计算与实验验证,各种工艺参数的影响,如喷射速度、基板温度、熔滴直径与碰撞最大铺展因子的影响,显示了很好的一致性。基于上述研究,成形形貌和内部微观结构之间的定量关系进一步验证了金属熔滴连续沉积、凝固重叠工艺的可行性。此工艺为金属微沉积制造实施有效的过程控制提供指导。     

Effect of charging energy on droplet diameters and properties of high-carbon steel coatings sprayed by wire explosion spraying

The purpose of this study is to investigate the effect of charging energy on droplet diameters and the properties of high-carbon steel coatings sprayed by wire explosion spraying. With different charging energies, droplets were collected when high-carbon steel wire explosion occurred, and the coatings were sprayed on aluminum alloy substrates. Droplet diameter distributions were obtained by using a Laser Particle Size Analyzer. Coatings were characterized by observing their spreading morphologies and cross sections with a Scanning Electron Microscope (SEM) and testing their thickness, surface roughness and Vickers indentation hardness. The results revealed that when the charging energy became greater, the droplet diameter would be smaller, the diameter distribution would be more concentrated, and droplets spread more efficiently on the substrate surface. In addition, as a result of droplets with a smaller diameter, the coatings became smoother and denser as the charging energy rose. However, the coating thickness and hardness diminished with the ascension of charging energy. The recommended charging energy is 7.18 kJ to achieve better high-carbon steel coating performance.     

Effect of Substrate Concave Pattern on Splat Formation of Yttria-Stabilized Zirconia in Atmospheric Plasma Spraying

Splat morphology of yttria-stabilized zirconia (YSZ) on a microconcave-patterned substrate was investigated by both numerical and experimental approaches under a dc-rf hybrid-plasma spray condition. The spreading behavior of molten droplets on a microdimple pattern was numerically simulated in a three-dimensional form. For comparison, impact of a YSZ droplet onto a microdimple pattern of a quartz glass substrate was studied in situ utilizing thermal emissions from the droplet. Concave aspects of a substrate surface play an important role in fingering/splashing of a spreading droplet as well as convex patterns. The main mechanism that causes splashing is likely due to the slipping of a spreading droplet at the edge of concave patterns. The viscosity decrease of the spreading droplet enhances the droplet splash.     

钢基表面热喷镍冷却过程中热力耦合场的三维有限元分析

以某钢基表面热喷镍工艺为例,对凝固冷却过程进行了简化和合理假设,基于ANSYS平台,建立了热力耦合场的三维有限元模型并进行了求解,获得了应力场和温度场分布规律,分析了涂层上某些关键点的温度和应力随时间变化的情况,讨论了涂层厚度对涂层/基体界面应力的影响.结果表明,凝固冷却时,涂层温度场沿中心向外边缘逐渐降低,随时间延长呈下降趋势;基体温度场随时间的变化呈先增加后降低趋势;涂层主要承受残余压应力,随涂层厚度增加,残余应力呈增大趋势.     

高熔点金属电弧喷涂快速制模涂层变形控制研究

采用ZnAl合金作为打底层材料,通过喷涂前对基体进行预热以及在喷涂过程中对基体和涂层进行温度控制,降低喷涂过程中涂层的温度梯度,达到减小涂层内残留热应力,减小甚至消除涂层的变形,得到较厚的3Cr13不锈钢涂层。该方法可用于制造高硬度金属电弧喷涂模具,为汽车样车试制和小批量生产服务。     

Printing solder droplets for micro devices packages using pneumatic drop-on-demand (DOD) technique

A pneumatic drop-on-demand (DOD) system has been applied to solder complex and dense interconnects of modern micro electronic devices. Initial parameters of uniform droplets were first measured. Then influences of experiment parameters, such as the crucible temperature, the substrate temperature and the droplet velocity on spread of solder droplets, were investigated experimentally and theoretically. The results showed that effects of impact velocity on the spreading could be negligible and the solder spreading process was driven by capillary forces because of the low Weber (We) number of depositing droplet. The influence of initial droplet temperature on the droplet spreading was not regular. The contact diameter of solder droplets on copper substrate increased when the substrate temperature increased from 443 K to 493 K. At last, copper cables were successfully soldered to pins of a flexible circuit using the direct solder deposition and re-melting process. The soldering results showed the feasibility of the data-driven soldering technique using the pneumatic DOD deposition technology.     

Modeling of Rapid Solidification with Undercooling Effect During Droplet Flattening on a Substrate in Coating Formation

The coating deposit on the substrate in thermal spray coating process develops by solidification of individual molten particle which impacts, flattens and solidifies on the surface of the substrate. Droplet flattening and solidification typically involves rapid cooling. In this paper, a model for non-equilibrium rapid solidification of a molten droplet spreading onto a substrate is presented. Transient flow during droplet impact and its subsequent spreading is considered using the volume of fluid surface tracking method which was fully coupled with the rapid solidification model. The rapid solidification model includes undercooling, nucleation, interface tracking, non-equilibrium solidification kinetics and combined heat transfer and fluid flow as required to treat a non-stagnant splat formed from droplet flattening. The model is validated with the literature results on stagnant splats. Subsequently, using the model the characteristics of the rapidly solidifying interface for non-stagnant splat, such as interface velocity and interface temperature, are described and the effect of undercooling and interfacial heat transfer coefficient are highlighted. In contrast to the stagnant splat, the non-stagnant splat considered in this study displays interesting features in the rapidly solidifying interface. These are attributed to droplet thinning and droplet recoiling that occur during the droplet spreading process.     

铅合金熔滴冲击45钢冷基板与45钢表面熔池的对比试验

针对特殊装备领域传统钢/铅粘接结构中存在的粘接层易老化脱落、长期稳定性差的问题,提出一种熔滴沉积复合TIG焊(gas tungsten arc welding)电弧增材制造新工艺,用于制造钢/铅双金属结构。文中比较性地研究了铅合金熔滴冲击铺展45钢冷基板与45钢表面熔池后的凝固形貌,通过引入无量纲数研究了熔滴冲击速度、温度对熔滴铺展凝固形貌的影响规律,利用OM,LSCM,SEM,EDS,XRD以及显微硬度测试等方法与手段,分析了熔滴沉积复合TIG电弧增材制造钢/铅双金属试样的界面微观组织结构与显微硬度。结果表明,铅合金堆积层铺展因子随熔滴冲击速度的增加呈明显非线性增加;界面层金属间化合物主要为FeSb2和FeSn2,其厚度随电弧热输入的增加而增加;钢/铅界面处的显微硬度明显高于铅合金堆积层和45钢母材;界面层中心左侧的硬度值达到最大值,为785.3 HV,45钢一侧平均硬度为343.7 HV,铅合金一侧平均硬度为24.3 HV。