Objective Laser cladding technology is widely used in coating deposition, worn surface repairing, and direct fabrication due to advantages, such as high deposition accuracy, small heat-affected zone, metallurgical bonding, and minimal dilution with the substrate. It is a surface modification technology for thin coating/layer fabrication with improved surface properties or surface defect refurbishment by forming highly resistant gradient coatings/layers on a substrate. Recently, the innovations in diode laser cladding significantly support remanufacture, and more stringent requirements have been put forward on the forming accuracy and quality. There is inadequate research on forming mechanisms in the diode laser cladding process; just depending on numerous process tests or human experience cannot effectively improve accuracy or control quality under unfavorable conditions, such as heat accumulation and stress concentration. To improve the laser cladding technology level, the development of on-line and high-speed monitoring and process controlling system is necessary. In particular, studies on powder melting behavior play a paramount role in forming mechanisms. Therefore, for fore-put powders, the characteristic behavior of the melting process was detected, and the investigation of the thermal physical characteristics was analyzed. Then, the equations of heat balance to describe the melting process were established. Finally, a model to analyze the thermal physical process in diode laser cladding process was set up.
Methods To analyze the interaction process between the diode laser and fore-put powders, first, an optical acquisition system with a high-speed camera was set up to observe the powder melting process; second, based on the analysis of the collected high-speed camera data, the characteristic behavior of the cladding process was analyzed; and finally, according to the analysis of the powder melting process, the models of thermophysical processes in different characteristic stages were set up, and the heat-balance equation for each characteristic stage was established.
Results and Discussions There are typical physical phenomena in the powder melting process, starting from the laser irradiation of the powder layer to the formation of the cladding layer (Fig. 4). The cladding process can be divided into four stages according to the existing physical phenomena (Fig. 5). 1) Initial stage: under the laser heat source radiation, the powder particles adhere to each other and gather to form particle agglomerates. 2) Melting stage one: the powder particles are melted to form liquid metal pellets with sizes of 1.27--1.90 mm. 3) Melting stage two: the liquid metal pellets agglomerate to form a larger liquid metal ball with a diameter of 3.80--7.25 mm, surrounded by a few liquid metal pellets with diameters of <2.54 mm. 4) Forming stage: numerous liquid metal pellets and powder particles merge into large liquid metal balls in the central area of the laser action to form a liquid metal pool with a diameter of 9.52--12.50 mm and spread over the substrate. The forming shape mainly depends on the duration of each characteristic stage in the laser cladding process with fore-put powders (Figs. 3 and 6). In addition, the energy balance equation for each stage can be established. From the heat transfer process perspective, the influence of laser power on the powder melting process was analyzed, and the mechanism by which the laser parameters incident the forming quality was clarified.
Conclusions In this study, the melting process of fore-put powders was detected by a high-speed camera and divided into four typical stages. Based on the influence of laser irradiation on the duration of each characteristic stage, the duration of each of characteristic behavior ①, ②, and ③ increased under the same laser irradiation. This result shows that the transformation of the powders from solid to liquid droplets does not require much energy; however, to realize the movement, convergence, and fusion penetration of the liquid droplets, certain laser energy is required. The process is slow with a lower laser power, which affects the spread of the liquid droplets and forming morphology. A thermophysical model that can describe the dynamic thermal interaction behavior between the laser and fore-put powders has been established. Through the analysis of the model, the thermophysical interaction behavior at different characteristic stages can be explained. According to the analysis of the powder melting process, when the laser power increases or the defocus amount decreases, the rate at which the powder melts increases, and numerous liquid metal drops are formed. The diameter of the liquid metal drops and spread rate increases. As the radius of solid powder particles decreases or the reflectivity of liquid metal drops, the duration of ① and ② reduces; also, the wetting angle decreases, and the spreading performance improves.
