The cold gas dynamic spraying of commercially pure titanium coatings was investigated. Specifically, the relationship between several key cold spray parameters on the quality of the resulting coatings was studied in order to gain a more thorough understanding of the cold spray process. To achieve this goal, three distinct investigations were performed.;The first part of the investigation focussed on the effect of propelling gas, particularly helium and nitrogen, during the cold spraying of titanium coatings. Coatings were characterised by SEM and were evaluated for their deposition efficiency (DE), microhardness, and porosity. In selected conditions, three particle velocities were investigated such that for each condition, the propelling gasses temperature and pressure were attuned to attain similar particle velocities for each gas. In addition, a thick and fully dense cold sprayed titanium coating was achieved with optimised spray parameters and nozzle using helium. The corresponding average particle velocity was 1173 m/s.;The second part of the investigation studied the effect of particle morphology (spherical, sponge, and irregular) and size distributions (mean particle sizes of 20, 29, and 36 μm) of commercially pure titanium on the mechanical properties of the resulting cold sprayed coatings. Numerous powder and coating characterisations were performed. From these data, semi-empirical flow (stress-strain) curves were generated based on the Johnson-Cook plasticity model which could be used as a measure of cold sprayability. Cold sprayability can be defined as the ease with which a powder can be cold sprayed. It was found that the sponge and irregular commercially pure titanium powders had higher oxygen content, poorer powder flowability, higher compression ratio, lower powder packing factor, and higher average particle impact velocities compared to the spherical powders. XRD results showed no new phases present when comparing the various feedstock powders to their corresponding coatings. For all feedstock powder morphologies, it was observed that the larger the particle size, the higher the temperature generated on impact. For the spherical powders, the higher the temperature generated on impact, the lower the stress needed to deform the particle. In addition, as the kinetic energy of the impacting particle increased, the flow peak stress decreased while the final strain increased. Furthermore, higher final flow strains were associated with higher coating ΔHV 10 (between the coatings and the feedstock powders). Similar relationships are expected to exist for the sponge and irregular feedstock powders. Based on porosity, the spherical medium powder was found to have the best cold sprayability.;The final part of the investigation focussed on the effect of substrate surface roughness and coating thickness on the adhesion strength of commercially pure titanium cold sprayed coatings onto Steel 1020, Al 6061, and Ti substrates. Adhesion strength was measured by tensile/pull tests according to ASTM C-633-01 standard. Through-thickness residual stresses of selected coatings were measured using the modified layer removal method (MLRM). In addition, mean coating residual stresses were calculated from MLRM results. It was found that adhesion strength increases with increasing substrate surface roughness and decreases with increasing coating thickness. Furthermore, mean coating residual stresses were correlated with adhesion strength and it was suggested that higher adhesion strengths are associated with higher mean compressive stresses and a higher probability for adiabatic shear instability to occur due to the higher particle impact velocities. In general, it was found that under similar cold spray conditions and substrate surface preparation method, adhesion strength was strongest for commercially pure titanium coatings deposited onto Al 6061, followed by Ti, then Steel 1020.
展开▼
机译：研究了商业纯钛涂层的冷气动态喷涂。具体而言，研究了几个关键的冷喷涂参数与所得涂层质量之间的关系，以便更全面地了解冷喷涂工艺。为了实现这一目标，进行了三项不同的研究。研究的第一部分集中在钛涂层冷喷涂过程中推进气体（特别是氦气和氮气）的影响。用SEM表征涂层，并评估其沉积效率（DE），显微硬度和孔隙率。在选定的条件下，对三种粒子速度进行了研究，以便针对每种条件调整推进气体的温度和压力，以使每种气体获得相似的粒子速度。此外，通过优化的喷涂参数和使用氦气的喷嘴，获得了厚而密实的冷喷涂钛涂层。相应的平均粒子速度为1173 m / s。;研究的第二部分研究了商业应用的粒子形态（球形，海绵状和不规则形）和粒度分布（平均粒度为20、29和36μm）的影响纯钛对所得冷喷涂涂层的机械性能的影响。进行了许多粉末和涂层表征。从这些数据，基于Johnson-Cook可塑性模型生成了半经验流动（应力-应变）曲线，该曲线可用于衡量冷喷涂性。冷喷涂性可以定义为粉末可以被冷喷涂的难易程度。已经发现，与球形粉末相比，海绵和市售不规则纯钛粉末具有较高的氧含量，较差的粉末流动性，较高的压缩比，较低的粉末堆积系数和较高的平均颗粒冲击速度。当将各种原料粉末与其相应的涂层进行比较时，XRD结果表明不存在新的相。对于所有原料粉末形态，观察到粒径越大，冲击产生的温度越高。对于球形粉末，撞击时产生的温度越高，使颗粒变形所需的应力越小。另外，随着冲击粒子的动能的增加，流动峰值应力减小，而最终应变增大。此外，较高的最终流动应变与较高的涂层ΔHV10（在涂层和原料粉末之间）有关。对于海绵和不规则原料粉末，预期存在相似的关系。根据孔隙率，发现球形介质粉末具有最佳的冷喷涂性能。研究的最后部分集中在基材表面粗糙度和涂层厚度对商业纯钛冷喷涂涂层对Steel 1020的附着强度的影响上， Al 6061和Ti衬底。根据ASTM C-633-01标准通过拉伸/拉力测试来测量粘合强度。使用改进的层去除法（MLRM）测量所选涂层的全厚度残余应力。另外，从MLRM结果计算出平均涂层残余应力。已经发现，粘合强度随着基材表面粗糙度的增加而增加，并且随着涂层厚度的增加而降低。此外，平均涂层残余应力与粘附强度相关，并且表明较高的粘附强度与较高的平均压缩应力相关联，并且由于较高的颗粒碰撞速度而发生绝热剪切不稳定性的可能性较高。通常，发现在相似的冷喷涂条件和基材表面制备方法下，对于在Al 6061上沉积的商业纯钛涂层，其次是Ti，然后是Steel 1020，其粘合强度最强。
展开▼
