Dynamic evolution of powder stream convergence with powder feeding durations in direct energy deposition

Highlights

• Dynamic evolution of the powder stream convergence was measured successfully.

• Nozzle channel characteristics changes with powder feeding duration are elucidated.

• Scouring erosion of powder particle cause nozzle channel characteristics to change.

• Contributions of nozzle characteristics changes on powder divergence are analyzed.

• The deposition height will gradually decreases with long-time feeding powder.

Abstract

During direct energy deposition (DED), changes in the powder feeding behavior significantly affect the deposition process. In this study, a high-speed photography method was developed to observe the dynamic evolution of the powder stream for various powder feeding durations, and the synchronous evolution of the inner channel characteristics of the nozzle were also measured. Combined with powder stream simulations, the contribution of the evolution of each channel characteristic on the powder divergence was discussed. The results show that as the powder feeding duration increased, the inner channel characteristics of the nozzle changed dynamically, which led to the powder stream divergence. For a powder feeding duration of 60 h, the nominal powder spot size increased by 41.4% at the plane 15 mm below the nozzle exit. As the powder feeding time increased, the inner wall roughness, inner channel diameter, and the exit size of the nozzle also increased owing to the scouring erosion from the high-speed powder particles. Subsequently, changes in the inner wall roughness and inner channel diameter of the nozzle tended to be smaller owing to the formation of a work hardening layer. It was also demonstrated that the changes in the inner wall roughness of the nozzle had a negligible effect on the powder stream divergence, an increase in the inner channel diameter of the nozzle extended the maximum divergence range of the particle trajectory at the nozzle exit, and the expansion of the nozzle exit size reduced the constraint causing the allowed trajectory range of the ejected powder particles to increase. The deposited height gradually decreased as the powder feeding duration increased, resulting in a 38.7% decrease after 60 h.

Introduction

Direct energy deposition (DED) is an additive manufacturing technology that can be applied for the direct manufacturing of the metal parts with complex shape, the preparation of coatings on existing geometries and the rapid repair of damaged components [1,2]. It has also been used for the fabrication of functional-gradient material [3,4], composite materials [5,6] and the rapid repair of damaged components. Compared with traditional manufacturing methods, the DED shows several advantages including shorter manufacturing cycle, lower manufacturing cost and high performance [7,8].

In the powder-based laser additive manufacturing process, the flow behavior of powder has important influence on the interaction of laser, powder and melt pool, and it also determined the deposition quality. Therefore, the flow behavior of powder in both the powder spreading [9,10] and powder feeding [11,12] has attracted researcher's attention. During DED, a laser beam is scanned on the substrate according to the planned deposition path to create a melt pool, and the metal powders are injected into the melt pool synchronously through the nozzle [13]. Therefore, the powder feeding behavior by the nozzle is a key factor during DED. The studies of Tan et al. [14] and Zekovic et al. [15] have proved the different interaction positions of powder and melt pool greatly affect the geometric of deposited layer, so the preset deposited layer height can be obtained by controlling interaction condition. Tabernero et al. [16] and Wang et al. [17] indicated that the convergence of the powder stream would have a significant attenuation effect on the laser energy during DED. Cong et al. [18] increased the interaction efficiency of powder stream and melt pool by an ultrasonic vibration-assisted DED method, and improved the deposited quality. Besides, Toyserkani et al. [19] revealed the influence of the interaction between powder distribution and melt pool on the deposited geometry. Arrizubieta et al. [20] alleviated the over-high phenomenon and obtained more homogeneous deposition geometry by control instantaneously the mass flow rate at the nozzle exit. Because of the importance of powder stream to the deposition process, many studies have been focused on the concentration distribution of the powder stream by simulation and experiment, Pinkerton et al. [21] and Liu et al. [22] indicated that the morphology of powder flow at different distance from the nozzle exit will present different forms. Furthermore, some investigations found that the nozzle channel structure and particle's parameters have important influence on powder stream morphology [23, 24]. Besides, Liu et al. [25] studied the trajectory of the particles in the nozzle inner channel, and found that collision is an important factor that causes the powder trajectory to diverge below the nozzle. Kovaleva et al. [26] indicated that velocity recovery coefficient in the particle-wall collision is one of the reasons for the convergence of powder stream. Based on the understanding of the powder stream, some process models were built to investigate the deposition geometry and thermal behavior [12,27].

From the above, it can be seen that most of previous studies in DED focused on the static or ideal state of the powder stream feeding behavior with specific powder feeding parameters or different nozzle structures. However, in the case of long-time continuous powder feeding, especially at high powder feeding rates, the feeding duration will mean the complex interaction process of nozzle channel parameters and a large number of high-speed particles is constantly going on. The gas-solid two-phase flow composed of high-speed particles and carrier gas will seriously erode and damage the inner channel characteristics of the nozzle (including the inner wall roughness, inner channel diameter, exit size, etc.). As a result, the state of the powder stream will change continuously and affect the deposition quality. Actually, some material processing technology with high-speed powder feeding have presented the tendency. In the abrasive machining, Deng et al. [28] and Sun et al. [29] found the erosion appeared at nozzle's exit after long-time feeding powder, and the nozzle channel was enlarged. Related studies about erosion wear have been conducted in the abrasive flow and cold spraying fields. It was proved there are many factors influence the erosion wear. The most severe erosion wear occurs at impact angles between 20 and 30° [30]. The particle shape influences erosion rate more than particle size, and the particle velocity is one of the most important factors in erosion experiments [31]. The self-sustaining oscillation of the flow rate at the nozzle exit during cold spraying is conducive to thermomechanical softening and continuous erosion of the nozzle, finally, the exit of nozzle will become thinner and be destroyed [32]. Obviously, the change of the nozzle channel will cause a significant change in the powder feeding behavior. Fanicchia et al. [33] have demonstrated the mass distribution of powder flow has important influence on the deposition profile in spraying process. It indicated that the change of the nozzle channel finally would affect the deposition profile of spraying. So, the deterioration of the deposition quality caused by nozzle eroded may also occur in all material processing technology with high-speed powder feeding. In the fields of cold spraying and abrasive flow, the nozzles are usually made of wear-resistant materials and the carrier gas is supersonic. However, compared with spraying technology, the speed of powder particles in DED is much lower. Therefore, the erosion wear of the nozzle inner channel by powder particles is usually ignored, and no further in-depth research has been be conducted. Furthermore, to obtain high thermal conductivity and reflectivity, the nozzles used in DED are usually made of copper, which has poor wear resistance. Therefore, even if measurements and simulations of the powder feeding behavior have been reported in those publications, they may not be suitable for the analysis of a long-time deposition process.

To reveal the dynamic evolution of powder stream and nozzle channel characteristics during the long-time deposition process, a novel research method is proposed in this study. Through observe and characterize the powder stream with a long-time feeding, the evolution of powder stream convergence and nozzle inner channel characteristics with feeding duration will be obtained. Furthermore, combine with the simulation of powder stream, it will be discussed how the evolution of nozzle inner channel characteristics gradually change the convergence of powder stream under long-time feeding powder. This study will be an important basis for understanding the long-time continuous deposition process, and it also has the important reference significance to other material processing technologies with high-speed powder feeding.