Numerical simulation of the effects of the impact velocity on the particle deposition characteristics in cold gas dynamic spraying

In this study, the effects of the impact velocity on the particle deposition characteristics in cold gas dynamic spraying （CGDS） of 304 stainless steel （SS） on an interstitial free （IF） steel substrate are numerical simulated by means of a finite element analysis （FEA）. The results have illustrated that when the particle impact velocity exceeds a critical value at which adiabatic shear instability of the particle starts to occur. Meanwhile, the fatten ratio and impact crater depth （or the effective contacting area ） increase rapidly. The particle-substrate bonding and deposition mechanism can be attributed to such an adiabatic shear deformation induced by both the compressive force and the slide friction force of particle. The critical velocity can be predicted by numerical simulation, which is useful to optimize the CGDS processing parameters for various materials.

Progress in the application of cold gas dynamic spraying to repairing continuous casting molds

A new continuous casting mold repairing method--cold gas dynamic spraying （CGDS） is introduced. The study investigates the advantages of the CGDS process regarding repairing operation, such as convenient, in-situ repairation,little heat delivery, microstructural and dimensional stability and other special applications. Microstructure and mechanical properties of the copper alloy coating, nickel coating, ceramic composite coating, and their interface to the substrates ,which are usually used in repairing operation have been researched by means of optical microscopy （ OM）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and micro-hardness tests. Experimental results have demonstrated the relative density of the copper alloy coating is as high as 98.7%, and that no obvious difference can be observed between the CrZrCu substrate and the Cu alloy coating in terms of microstructures; thus the interface is quite difficult to be identified. The bonding strength and micro-hardness of the Cu alloy coating reach up to 37 MPa and 310 HV0.2 ,respectively. The interface between the copper alloy coating and the nickel coating is either zigzag or wave shaped, and the cohesion is relatively good. As-sprayed nickel coating is dominated by severely deformed particles,and the relative density is up to 98.5%. Complete recrystallization occurred after annealing at 900℃ for one hour,while its micro-hardness remains as high as 124.1 HV02. All these results have indicated that CGDS is a promising technology for repairing the continuous casting mold and that its future development is prosperous as well.

Microstructure and micro-hardness of the Inconel 718 superalloy coating prepared by cold gas dynamic spraying

Inconel 718 superalloy coating was prepared on a martensitic steel substrate by cold gas dynamic spraying （ CGDS ）. Microstructure and micro-hardness of the coating were characterized by means of scanning electronic microscope （SEM） ,energy dispersive spectrometer （EDS）, and micro-hardness and nano-indentation tests. The results have indicated that the coating＇ s thickness can reach more than 300 um, and there exists a good interfacial cohesion between the coating and the substrate. The quantity and size of the defects at the bottom, middle and top areas increase gradually. There is no significant change to the original microstructure of the Inconel 718 superalloy particle after the process of CGDS ;and an obvious torsional deformation on the particle＇ s profile is produced while little deformation at the center part occurs. The micro-hardness of the coating at the bottom, middle and top areas decreases in turn. Compared with the bottom area, the micro-hardness of the middle and top areas decreases by about 10% and 21%, respectively. The nano-hardness of the coating is much lower than the original particle,which decreases by about 13.5% at the bottom area and 28% at the top area,respectively. The distribution of micro-defects is an important factor to the micro-hardness of the coating.

Influence of the preheated substrate temperature on the microstructure deposition behavior of the 304 stainless steel coatings deposited by cold gas dynamic spraying

The effects of the substrate temperature on the deposition and microstructure of the 304 stainless steel （SS） cold gas dynamic spraying （CGDS） coatings were investigated. It was found that the higher substrate temperature could increase the deposition rate of the 304 SS particles,even the oxide films existed at the interface. There was a critical oxide film with a thickness of 3 -4 um which could be destroyed by the impacted particles. The micro-hardness and microstructure of the 304 SS coatings under different substrate preheating temperatures were almost the same.

Study on the 316L stainless steel coatings and their properties by cold gas dynamic spraying

316L stainless steel coatings were prepared by cold gas dynamic spraying （CGDS） on a carbon steel. A new heater of sprayed powders was added to the traditional facility. The results show that the coating quality is obviously improved, due to better interface bonding and reduced porosity.

Numerical simulation of particle tracks in the cold gas dynamic spraying process

In this study,the distribution behavior of the particle flow field in cold gas dynamic spraying （CGDS） was simulated through the Computational fluid dynamics （CFD） method. Traces of the particles with different diameters in the gas flow field were analyzed, and effects of fiat and sphere substrates on the particle tracks were also compared .Simulation results indicate that different escaping directions of particles flow with the two substrates. These investigations. gave instructions on how to design the powder recovery and dusting machines in a CGDS system.

Effects of standoff distance on the supersonic flow fields in cold gas dynamic spraying

In this study,the effects of standoff distance （SOD） on the supersonic flow fields （including gas and particle flow fields） during cold gas dynamic spraying （ CGDS ） are investigated by means of computational fluid dynamics （CFD）. The variation of velocity,temperature,pressure and density with different SoD is elucidated through the analysis of the distribution properties of the flow fields. It is found that the shock waves in front of the substrate remarkably influence the gas and particle flow fields. The wave system of expansion waves and pressure waves come into being continuously. The velocity of gas reaches the super.,;onic speed at the position of the Mach disc, while it decreases sharply when the gas flow crosses the Mach disc. The optirnal SoDs are 40 mm for 1 um particles and 50 mm for both 5 um and 22 um particles.

Elastic Modulus of 304 Stainless Steel Coating by Cold Gas Dynamic Spraying

304 stainless steel coating was deposited on the IF steel substrate by cold gas dynamic spraying （CGDS）, and the elastic modulus of the 304 stainless steel coating was studied. The elastic modulus of cold sprayed 304 stain- less steel coating was measured using the three-point bend testing and the compound beam theory, and the other me- chanic parameters （such as the equivalent flexural rigidity and the moment of inertia of area） of the coatings were also calculated using this compound beam theory. It is found that the calculated results using the above methods are accu- rate and reliable. The elastic modulus value of the cold sprayed 304 stainless steel coating is 1. 179 X 105 MPa, and it is slightly lower than the 304 stainless steel plate （about 2 X 105 MPa）. It indicates that the elastic modulus of the cold sprayed coatings was quite different from the comparable bulk materials. The main reason is that the pores and other defects are existed in the coatings, and the elastic modulus of the coatings also depends on varies parameters such as the feed stock particle size, porosity, and processing parameters.

Fracture behavior of 304 stainless steel coatings by cold gas dynamic spray

304 stainless steel coating was deposited on the Interstitial-Free steel substrate by cold gas dynamic spraying （CGDS）. Three-point bending test of the cold sprayed 304 stainless steel coating was tested by SHIMADZU electro-hydraulic servo-controlled fatigue testing machine and the fracture morphology was examined by scanning electron microscopy. The results showed that the fracture behavior of the cold sprayed 304 stainless steel coating was brittleness fracture. The crack in the coating occurred in the interfaces between particles and the crack extended to the internal of the coating with the increase of the load. When the crack has extended to the combination interface between coating and substrate, the crack extended to the two sides. The microstructure and mechanical property of the cold sprayed 304 stainless steel coating have been optimized after heat treatment.

冷气动力喷涂技术制备涂层的研究进展

冷气动力喷涂技术是在室温或有限加热的温度下利用高压气体产生高速气流喷涂金属、合金以及复合涂层的新兴表面工程技术。综述了国内外冷喷涂领域的研究现状,总结了冷喷涂技术制备纯金属涂层、合金涂层以及复合涂层的研究进展,尤其是制备纳米结构涂层的最新成果,主要包括纳米涂层的结构、性能、特点及应用。

冷喷涂304不锈钢涂层的弯曲力学行为研究

采用冷喷涂(CGDS)技术在IF钢基体上制备304不锈钢涂层。用SHIMADZU液压伺服疲劳试验机对304不锈钢涂层样品进行三点弯曲实验,用扫描电子显微镜来研究冷喷涂304不锈钢涂层的断裂行为。结果表明:冷喷涂304不锈钢涂层的断裂行为为脆性断裂;裂纹萌生于涂层表面,随着载荷和力矩的增加,裂纹向涂层内部扩展,裂纹在涂层中是V字型的;当裂纹扩展到涂层和基体的结合界面处时,裂纹向两侧横向扩展,相邻裂纹扩展相互贯通后,涂层和基体发生开裂或脱落。

碰撞速度对冷喷涂粒子沉积行为影响的数值模拟研究

采用有限元数值模拟方法研究了冷喷涂过程中304不锈钢粒子的碰撞速度对IF钢基板的碰撞和沉积变形行为的影响。结果表明,当粒子速度高于一个在产生绝热失稳变形并发生沉积的临界速度时,粒子的扁平率、有效接触界面积均显著增加。粒子沉积过程中的剪切失稳行为是由撞击过程中产生的压力与摩擦力共同作用的结果。这个临界速度可以由数值模拟计算得到,用于优化多种材料的冷喷涂工艺参数。

冷气动力喷涂技术修复连铸结晶器应用研究进展

推介了一种全新的结晶器修复技术———冷气动力喷涂技术,介绍了该技术的优势和特点。对结晶器修复用铜、镍和铜—镍涂层的界面以及陶瓷涂层复合涂层的组织结构与力学性能进行了研究。结果表明,铜合金涂层的致密度达到98.7%,铜涂层与铜基板在显微组织上没有明显不同;结合强度和显微硬度(HV0.2)分别为37 MPa和310;铜涂层与镍涂层的界面为曲折波纹状,结合良好;冷喷态镍涂层主要由严重变形颗粒构成,致密度达到98.5%,在900℃下退火1 h发生完全再结晶,显微硬度(HV0.2)仍保持124.1,表明冷喷涂技术是一种具有潜力的结晶器修复技术,并对冷喷涂技术修复连铸结晶器的前景进行了展望。

基板预热温度对冷喷涂涂层组织及沉积行为的影响

采用冷气动力喷涂方法在不同温度的基板表面制备了304不锈钢涂层,研究了基板温度对冷喷涂涂层组织及沉积特性的影响。试验结果表明,不同基板温度制备的冷喷涂涂层组织及显微硬度相同,随基板温度的升高,涂层的沉积效率升高;基板表面的氧化膜存在一个临界厚度尺寸,在临界尺寸下,氧化膜易被撞击粒子击破清除,氧化膜的临界厚度为3～4μm。

冷喷涂Inconel718高温合金涂层组织及微观硬度

采用冷喷涂技术在马氏体钢表面制备了Inconel 718高温合金涂层,利用SEM、显微硬度计和纳米压痕仪分析了涂层组织和微观硬度。结果表明,涂层厚度可达300μm以上,与基底界面结合良好,底部、中部和顶部涂层微观缺陷数量和尺寸依次增多;冷喷涂沉积过程不会对Inconel 718高温合金粒子的原始组织结构产生显著影响,沉积后粒子的外部轮廓发生了明显的扭曲变形,而心部变形较小;涂层底部、中部和顶部的显微硬度依次降低,与底部涂层相比,中部和顶部涂层显微硬度分别降低10%和21%;涂层平均纳米硬度较原始粒子显著降低,底部和顶部涂层分别降低13.5%和28%;涂层内部微观缺陷分布是影响涂层整体硬度的重要因素。

冷喷涂工艺中粒子流轨迹的数值模拟

采用计算流体力学(CFD)技术模拟研究了冷气动力喷涂技术中粒子流的流场分布特性。通过计算,分析了不同粒子直径下粒子在气体流场中的轨迹,比较了平面基板和球形基体对粒子流轨迹的影响。模拟结果清楚给出了两种条件下粒子流的流动方向和速度大小,为冷喷涂系统粉末回收与除尘装置的设计提供了理论指导。

喷嘴出口与基板间距离对冷喷涂超音速流场的影响

采用计算流体力学(CFD)模拟研究了冷气动力喷涂技术中喷嘴出口与基板距离(SoD)对超音速流场(包括气体流场和粒子流流场)的影响,通过计算流场的分布特性,分析了流场中速度、温度、压力和密度的变化规律。模拟结果发现,由于基体前激波的存在,极大影响了气体和粒子流流场。气流形成连续交替的膨胀波、压缩波波系,在马赫盘处气体速度达到超声速,越过马赫盘以后速度下降很大。SoD为40 mm时,1μm粒子流流场达到最优;SoD为50 mm时,5μm和22μm粒子流流场达到最优,此时粒子流对基体的轰击速度最大。