【作者】 唐文勇；

【导师】 陈清华；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2014， 博士

【摘要】 冷气体动力学喷涂简称冷喷涂，是近几十年来发展起来的一种新型材料表面改性技术。冷喷涂中，喷涂材料均存在一个临界沉积速度，只有撞击速度超过其临界沉积速度的粒子才能沉积在基板上并形成涂层。全面、系统地分析冷喷涂过程中粒子速度和温度的影响因素并对这些因素进行优化，可有效提升涂层性能和沉积效率，并对冷喷涂设备的开发和冷喷涂技术更为广泛的应用提供重要的指导意义。影响粒子速度的因素有很多，如喷管结构、气体进口参数、粒子物性参数、形貌与尺寸、喷涂距离等。本论文予以考虑的送粉气流，在现有的冷喷涂研究中并未得到重视。高压冷喷涂系统中，喷涂粉末是由一定流量的低温送粉气流携带并轴向注入预混腔内，与经预热的高温主气流混合成载气体，低温送粉气流进入喷管后会对粒子加速产生影响。本论文通过数值模拟与试验分析相结合的研究方法，对高压冷喷涂中送粉气流对喷管内气体流场及粉末沉积的影响进行了深入地研究。首先，利用计算流体力学方法，对考虑送粉气流时冷喷涂超音速流场进行数值模拟。研究了与送粉气流相关的因素：送粉压差、送粉管径与喷管喉部直径之比及预混腔长度对冷喷涂超音速内气体流动特性及粒子速度和温度的影响。送粉气流以射流形式注入到主气流内，随送粉压差、喷管喉部直径及预混腔长度不同，喷管内气体呈现不同的流动特性。当预混腔长度为0时，送粉气流进入喷管后会卷吸主气流，随送粉压差增加，喷管内低温送粉气流所占比率显著增加，喷管喉部之前产生气体回流现象，使得两股气流充分换热，最终导致喷管内气体有效温度下降，因此粒子撞击基板的速度和温度随送粉压差的增加而显著下降。保持送粉压差不变，增加喷管喉部直径，可使进入喷管的送粉气流所占比率下降。然而，此时喷管喉部前气体回流现象消失，位于喷管轴心区域的低温送粉气流与周围的高温主气流换热效果差，导致喷管渐扩段内轴心区域气体速度和温度较低，位于这一区域的粉末粒子仍然由低温、低速气流携带，粒子撞击速度和温度得不到有效的提升。喷管前的预混腔不仅对粒子在进入喷管前进行预热，同时还可提高送粉气流与主气流的换热效果，使得两股气流在进入喷管时能充分换热，因此在送粉压差一定的情况下，适当增加喷管喉部直径的同时使用一定长度的预混段可有效地提削弱送粉气流对粒子速度和温度的不利影响。其次，提出了一种新型送粉方式，设计了基于机械能和重力的送粉器，并开发了可精确测量和控制送粉压差的高压冷喷涂实验系统。有别于典型的高压冷喷涂系统，所开发的冷喷涂系统通过压力平衡的设计，使得送粉器内的压力和粉末注射管的压力始终平衡状态。在粉末落入送粉管后，可以根据试验需要单独引入送粉气流。同时，本送粉方式为实现高压冷喷涂粉末输送不依赖送粉气流提供一种可行方案。最后，利用开发的高压冷喷涂设备制备了不同参数下铝基板铜涂层，讨论了与送粉气流相关的因素对涂层厚度及沉积效率的影响。实验结果显示，在特定实验喷涂参数下，送粉压差由0.05MPa增加到0.2MPa，涂层厚度和粉末沉积效率下降幅度超过50%；送粉压差不同时，增加主气流温度对涂层厚度及粉末沉积效率的提升程度不同。送粉压差一定时，通过增加喷管喉部直径和预混腔长度，可有效消弱送粉气流对粉末沉积带来的不利影响，大幅提升粉末沉积效率。更多还原

【Abstract】 Cold gas dynamic spraying (CGDS, often referred to as simply ‘cold spraying’) isa relatively new technique used to deposit materials onto the surface of a substrate. Incold spraying, for a given spray material, there is a critical particle velocity; coatingformation by the particle impact velocity above its critical velocity Thus,comprehensively and systematically analyzing the influence factors of particleacceleration in the process of cold spraying and optimizing those factors can improvethe quality of coating and the deposition efficiency effectively, and provide a crucialguide for the rapid development of the cold spraying equipment as well as the wideapplication of the cold spraying process.There are many factors that affect the particle velocity, such as: Laval nozzlestructure, gas inlet parameters, particle physical property parameter, morphology andsize, standoff distance and so on. Beyond that there are still many factors that not beentaken into account. The effect of powder carrier gas on the particle deposition wasstudied in this paper, which has not been taken into account enough. While, in theexisting high pressure cold spraying system, powder is carried by a certain flow ratepowder carrier gas, and inject into the prechamber before the nozzle in radial direction.After enters into the nozzle the low temperature powder carrier gas will lead to thedecreasing of the effective temperature in the nozzle, which would case the decreasingof the coatings quality and deposition efficiency. In this paper, the effect of the powdercarrier gas on the particle deposition was studied thoroughly through theoreticalanalysis, numerical simulation and experimental analysis.1. Numerical investigation on the cold spraying supersonic flow field with apowder carrier gas is carried out using a computational fluid dynamics (CFD) program,FLUENT. The effects of the Pressure differential between powder carrier gas and mainpropulsion gas (P), the ratio of the nozzle throat diameter on the powder injectordiameter (Rd) and the length of the prechamber (Lp) on the gas flow field, the velocityand temperature of particle are studied in this study. The gas flow characteristics insidethe nozzle are quite different under different parameters (P, Rd, Lp). In the condition ofthe Lp=0, the proportion of the powder carrier gas in the nozzle increase significantlywith increasing of the pressure differential, and the gas backflow phenomenon appearbefore the nozzle throat. The gas effective temperature of gas in the nozzle drop significantly because of the gas backflow and the large proportion of the powder carriergas. So that the particle impact velocity and temperature decrease remarkably withincreasing of the pressure differential. The proportion of the powder carrier gas in thenozzle decrease when a nozzle with large throat size is used under a certain pressuredifferential. While the mixing of the two gas flow streams in the converging sectionbecomes weak as the diameter ratio Rdincreases. As a result, the injected cold carriergas along the centerline will cause a lower temperature and lower velocity of the gasflow around the centerline in the divergent section of the nozzle. Thus, increasing thediameter ratio cannot significantly increase the velocity of the gas flow and the impactvelocities of the particles on the centerline when there is no prechamber in front of thenozzle. The prechamber in front of the nozzle not only preheat the particle, but alsoimprove the heat transfer between the two gas flow streams. Appropriately increasingthe nozzle throat diameter and elongating the prechamber to30~50mm can effectivelyweaken the adverse effect of the powder carrier gas on the particle velocity andtemperature.2. A new powder feeding method is put forward, a powder feeder based on themechanical energy and gravity is designed. Moreover, a high pressure cold sprayingsystem is established, in which the pressure differential between powder carrier gas andmain propulsion gas can be measure and controlled accurately. Unlike the traditionalhigh pressure cold spraying system, the pressure between the powder feeder and thepowder feed pipe is always balance because of the designing of the pressure balance inthe developed cold spraying system. In the powder feed process, the powder outflowfrom the powder feeder depending on the mechanical energy and the powder gravityrather than the powder carrier gas. The powder carrier gas is introduced into the powderfeed pipe according the requirement of the cold spraying tests after the powder fallinginto the powder feed pipe. Furthermore, the powder carrier gas is unavoidable in thetraditional high pressure cold spraying powder feed process, which cause a negativeinfluence of powder deposition in a certain degree; while, the developed high pressurecold spraying system provide a solution to solve this problem completely, because ofthat the powder carrier gas is not necessary in the current powder feed process.3. Copper coatings on aluminum substrate are fabricated experimentally by thedeveloped high pressure cold spraying system; the effects of the parameters associatedwith powder carrier gas on the coating thickness and deposition efficiency are discussedin this paper. The experimental results show that the effect of powder carrier gas on the coating thickness and deposition efficiency is significant. Under the specific sprayingparameters in the test, with increasing of the pressure differential from0.05MPa to0.2MPa, the decline range of the coating thickness and deposition efficiency are more than50%; The degree of promotion of the coating thickness and powder depositionefficiency with increasing of the temperature of main propulsion gas is different whenthe pressure differential is different. By increasing the nozzle throat diameter andprechamber length can effectively weaken the negative influence of powder carrier gason powder deposition, and improve the powder deposition efficiency sharply in the caseof a certain pressure differential.更多还原