Laser material deposition (LMD) is a process in which a laser beam creates a melt pool on the surface of a substrate while a powder nozzle delivers a powdered additive into the melt pool, where the powder material then melts. A dense, metallurgically bonded layer forms once the melt solidifies. The velocity of powder particles in LMD is one of the key factors that influence the amount of particles’ energy absorption (along their individual trajectories) through the laser beam on the way between the powder nozzle and the substrate. The amount of energy that is absorbed by the powder particles above the melt pool determines the temperature on the substrate surface and, hence, influences the formation of the melt pool and the deposition of the powder material. Data about the particle velocity are, therefore, essential for modeling the LMD process—not only for physical simulation where a particle’s energy input can be integrated along its trajectory, but also in an experimental environment of process parameter studies, where understanding the interdependencies between the process parameters is crucial. In this study, a setup using a high-speed camera and an illumination laser is used to measure the velocity of powder particles in the powder gas jet of a coaxial powder nozzle. Several parameters that are known to influence the particle velocity are varied: Feed gas rate, shielding gas rate, nozzle geometry (width of annular gap), and powder mass flow rate. In this study, four different powder types are used. The influence of these process parameters on the particle velocity is measured. Four different methods for tracking individual particles and calculating the velocity distribution within the powder gas jet are used and compared: Manual frame-by-frame particle tracking and manual evaluation from multiple exposures in single frames, as well as particle tracking velocimetry and particle image velocimetry (PIV), which incorporate region-of-interest boxes into the algorithm. Sufficient accordance as to the measurement results is found in comparing the four methods. Further, using the highly automatable PIV method, the influence of main process parameters on particle velocity is measured. Out of the examined parameters, the feed gas rate is found to have the most immediate impact with a linear correlation to particle velocity. A correlation between different powder particle size distributions and measured particle velocities is shown as well.

中文翻译：

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同轴粉末喷嘴的粉末气体喷嘴中用于激光材料沉积的粒子速度测量

激光材料沉积（LMD）是一种过程，其中激光束在基板表面上形成熔池，而粉末喷嘴将粉状添加剂输送到熔池中，然后粉末材料在该熔池中熔化。一旦熔体凝固，便形成致密的冶金结合层。LMD中粉末颗粒的速度是影响粉末在喷嘴和基材之间通过激光束的能量吸收量（沿其各自的轨迹）的关键因素之一。熔池上方的粉末颗粒吸收的能量大小决定了基材表面的温度，因此影响熔池的形成和粉末材料的沉积。因此，有关粒子速度的数据为 LMD过程建模必不可少的–不仅对于可以沿其轨迹整合粒子能量输入的物理模拟，而且对于过程参数研究的实验环境（了解过程参数之间的相互依赖性至关重要）也至关重要。在这项研究中，使用高速照相机和照明激光的装置用于测量同轴粉末喷嘴的粉末气体喷嘴中粉末颗粒的速度。已知会影响粒子速度的几个参数会发生变化：进料气体速率，保护气体速率，喷嘴几何形状（环形间隙的宽度）和粉末质量流量。在这项研究中，使用了四种不同的粉末类型。测量这些工艺参数对粒子速度的影响。使用和比较了四种不同的方法来跟踪单个粒子并计算粉末气体射流内的速度分布：手动逐帧粒子跟踪和单帧多次曝光的手动评估，以及粒子跟踪测速和粒子图像测速（PIV），它将兴趣区域框合并到算法中。通过比较这四种方法，可以找到与测量结果完全一致的结果。此外，使用高度自动化的PIV方法，可以测量主要工艺参数对颗粒速度的影响。从检查的参数中，发现进料气速率具有最直接的影响，并且与颗粒速度呈线性关系。