【作者】 刘彦学；

【导师】 袁晓光；

【作者基本信息】 沈阳工业大学 ， 材料加工工程， 2008， 博士

【摘要】 镁合金易氧化、不耐磨等是其应用受到制约的主要原因之一。目前解决该问题的方法是在其表面形成保护层,例如:微弧氧化膜,各类化学镀层等,但这类方法均存在保护层薄、耐磨性差等问题。热喷涂技术也可以在镁合金表面形成保护层,有效地提高镁合金表面的耐磨性,也能提高耐腐蚀性。但是,热喷涂过程中存在的高温,易造成镁合金与涂层界面的氧化问题,涂层也缺乏致密性等,限制了热喷涂技术的应用。冷喷涂技术是在较低温度下形成涂层的一项新技术,可以避免喷涂过程中的镁合金氧化,且涂层产生的是压应力,涂层结合强度高,致密性强。因此,冷喷涂技术是解决镁合金表面易腐蚀、不耐磨问题的可选择技术。为此,本文针对镁合金表面的冷喷涂技术开展了研究工作,旨在开发该技术并为实际应用奠定技术基础。本文研究工作主要包括以下内容:首先,设计了Zn-Al和Al-Si-Fe两个用于冷喷涂的系列合金,目的是在镁合金表面建立防腐和耐磨涂层,同时,也在改善镁合金表面耐热性方面发挥作用。合金粉末采用超音速气体雾化技术制备。粉末特征和粒度分布分析表明,粉末形状多为球形或近球形,粒度分布比较集中,小于400目的粉末占80%以上,适合于冷喷涂工艺。建立了冷喷涂过程中,喷嘴内气体和气固两相流流动的物理和数学模型,利用计算软件模拟了气体压力分布和气体、粉末在喷嘴内的流动速度。通过模拟结果分析发现,传统喷嘴结构存在进一步改进的可能性。为了提高喷涂效率,根据喷嘴内气体和粉末的流动机理,以及动力学原理,对喷嘴结构进行了进一步的优化设计。通过增加出口管面积和出口管锥角的方式,设计了新的喷嘴结构。通过模拟计算,新结构的喷嘴可以提高气体和粉末的加速效果,喷出速度得到一定程度的提高。定点喷涂试验结果表明,采用新结构的喷嘴,可以使喷涂粉末的利用率提高10%以上。系统地试验研究了Zn-Al和Al-Si-Fe合金粉末冷喷涂工艺,确定了适合两类合金粉末的冷喷涂工艺参数。分析了喷涂层组织特点和涂层合金元素扩散规律。发现镁合金表面冷喷涂Zn-Al和Al-Si-Fe合金粉末存在着适宜的工艺参数。Al-Si-Fe合金粉末合适的喷涂工艺参数为:喷涂气体压力为2.1MPa,喷涂气体温度为550℃,喷涂距离35mm;ZA20合金粉末喷涂气体压力2.15MPa,喷涂气体温度300℃,喷涂距离40mm;ZA27合金粉末喷涂气体压力2.1MPa,喷涂气体温度330℃,喷涂距离20mm。涂层组织保持快速凝固的组织特征,涂层内部组织致密,涂层与基体间是机械结合。经热处理后,涂层更加致密、均匀,涂层和基体中均发生合金元素的互扩散现象,基体向涂层方向的扩散量要大于涂层向基体方向的扩散量;随着温度的提高和时间的延长,基体和涂层之间的扩散程度提高,但是当温度提高到300℃,时间延长到3h后,其扩散层变化微小。经扩散处理后,涂层与基体的结合方式由机械结合变为冶金结合,结合强度得到提高。在中性盐雾腐蚀和电化学腐蚀中,Zn-Al和Al-Si-Fe合金冷喷涂层均表现出良好的耐腐蚀性能。ZA20合金涂层及ZA27合金涂层的耐蚀性能均好于AZ91D镁合金基体。ZA20合金的腐蚀失重量约为AZ91D镁合金的0.055倍,ZA27腐蚀失重量约为AZ91D镁合金的0.072倍。AZ91D镁合金的自腐蚀电位为-1.7V左右,ZA27合金冷喷涂涂层自腐蚀电位为—0.2V左右,Al-Si-Fe合金冷喷涂层的自腐蚀电位为—1V左右,远高于基体合金,且冷喷涂层的极化曲线中出现明显的钝化区间,因而它能很好地保护基体,提高基体的耐蚀性。冷喷涂层和AZ91D镁合金具有不同的腐蚀机制,ZA27合金冷喷涂层发生了轻微的均匀腐蚀,而AZ91D镁合金发生了严重的局部腐蚀,冷喷涂层的均匀腐蚀危险性要远小于AZ91D镁合金的局部腐蚀。Al-Si-Fe合金冷喷涂层也存在局部腐蚀,但腐蚀程度较轻。磨损试验表明,涂层的耐磨性远远高于镁合金的耐磨性。在润滑条件下AZ91D镁合金的磨损失重量约为ZA20合金的5.92倍,是ZA27合金涂层的11倍;AZ91D镁合金厚度磨损减少量约为ZA20合金涂层的1.88倍,是ZA27合金涂层的2.2倍。在干磨损条件下,当磨损时间为50min时,ZA27合金涂层的磨损失重仅是固态挤压AZ91D镁合金的16%。更多还原

【Abstract】 The application of magnesium alloy is restrained by their easy oxidizing and wearing. A coating on the surface of magnesium alloy can solve these problems,such as minute arc oxidized film,all kind of plated film.But these films have some similar problems that the film is very thin and easy wearing.Hot spray technology can form a coat on the magnesium alloy surface that improves the oxidizing and wear resistance of magnesium alloy.But the high temperature during spray process will cause oxidizing in the interface between magnesium alloy and coat,the coat is a low compactness too.Cold spray technology is a new technique that can set up a coat under low temperature and can avoid magnesium alloy oxidized.During spray process,compressive stress exerts in the cold spray coat,which can produce high strength and compactness in coat.Therefore,cold spray technology can solve the problems of magnesium alloy surface oxidizing and wearing.In this paper,we have systematically studied the technology of a coat on the magnesium alloy surface by cold spray technology,in order to set up a technique basis for its application.The paper major research works include the following contents.First,Zn-Al and Al-Si-Fe series alloy chemical compositions have been designed for cold spray,the aim is raising the corrosion resistance and the wear-resistance and improving heat-resistance of magnesium alloy surface.The alloy powders are prepared with the supersonic gas atomization equipments.The results show that alloy powder has globular or near globular shape,narrow distribution of grain size and powder amount less than 400 meshes reaching 80%,which can completely meet the requirement for cold spray.The physical and mathematical model for describing gas and gas-particle two-phases flowing in nozzle has been set up during cold spray process;gas pressure distribution and gas-powder two-phase flowing velocity in nozzle have been simulated with the software. The results show that the traditional nozzle structure has some possibility for further improvement.In order to improve the spray effect,the supersonic nozzle has been designed on the basis of fluid mechanics and aerodynamics.With the consideration of the nozzle design theory and viscosity boundary layer theory,the nozzle structure has been optimized.New nozzle structure increases the area and cone angle of pipe outlet.The calculating simulations results show that new structure nozzle can raise acceleration effec! of gas and powder,powder spraying velocity from nozzle increased.The utilization ratio of spray powder can be increased more than 10%under fixed site spray test.Through studying the cold spray process on the magnesium alloy surface with rapidly solidified Zn-Al and Al-Si-Fe alloy powder,the suitable technology parameters have been determined.The characteristic of coat microstructure and diffusion law of alloying elements has been analyzed.The appropriate parameters of cold spray Al-Si-Fe alloy powders on magnesium alloy surface are that:the pressure of spray gas to alloy powder is 2.1MPa,the temperature of spray gas is 550℃and the spray distance is 35mm.ZA20 alloy powder appropriate parameters are pressure of spray gas 2.15MPa,spray gas temperature 300℃,spray distance 40mm.ZA27alloy powder appropriate parameters are pressure of spray gas 2.1MPa,spray gas temperature 330℃,spray distance 20mm.The coat has the characteristics of rapidly solidified and compacted microstructure.The bonding way between coat and substrate alloy is mechanical joint.The diffusion treatment has been conducted on the coated samples with different heat treatment process.The results show that coat microstructure becomes more compact and homogeneous after heat treatment,accompanied with the diffusion between coating and substrate alloy.Moreover, the diffusion quantities diffused into coating from substrate alloy are more than that of diffused into substrate alloy from coating.As temperature is rising and time increasing,the diffusion degree between coat and substrate alloy improves.However,the diffusion layer varies little when the temperature rises to 300℃and the time increases to 3h.The bonding way changes from mechanical joint to metallurgical joint;the bonding strength gets raised after diffusion treatment.The corrosion behavior of the coats has been studied.The Zn-Al and Al-Si-Fe alloy coat performs better corrosion resistance than that of the substrate magnesium alloy in both salt mist corrosion and electrochemical corrosion.The corrosion weight loss of ZA20 and ZA27 alloy coat is 0.055 and 0.072 times respectively than that of AZ91D magnesium alloy.The self-corrosion potential of AZglD magnesium alloy,ZA27 and Al-Si-Fe alloy coat are about -1.7V,-0.2V and -1V respectively,moreover,polarization curves of ZA27 and Al-Si-Fe alloy coat have a clear passivation region,which can protect the substrate from being corroded.AZ91D magnesium alloy and cold spray coats have different corrosion mechanism,which the cold spray coat of ZA27 alloy takes slight homogeneous corrosion and AZ91D magnesium alloy is heavy local corrosion.The danger of homogeneous corrosion for cold spray coat is far inferior to the local corrosion for AZ91D magnesium alloy.Al-Si-Fe alloy coat takes local corrosion also,but the degree of corrosion is very light.Wear test shows that the wear-resistance of cold spray coat is far better than that of AZ91D alloy.AZ91D magnesium alloy weight loss under lubricating wear condition is 5.92 and 11 times respectively than that of ZA20 and ZA27 alloy coat.The wear thickness of AZ91D magnesium alloy is 1.88 and 2.2 times respectively than that of ZA20 and ZA27 alloy coat.Under dry wearing condition,the wear weight loss of ZA27 alloy coat is 16% than that of AZ91D magnesium alloy.更多还原