Objective Recently, with the continuous development of laser additive manufacturing, the demand for metal powder is increasing. Vacuum induction melting gas atomization is a new technology that combines vacuum induction smelting technology and inert gas atomization technology. Systems based on this technology exhibit low oxygen content, good sphericity, and high yield. Therefore, this technology is suitable for preparing high-performance powder materials with various particle size requirements. The atomizer is instrumental to vacuum induction melting gas atomization. Compared with the free-fall atomizer, the close-coupled atomizer has a compact structure with a guide pipe that allows metal to melt and flow out. Consequently, a small flight distance and a high atomization efficiency are achieved. Currently, research on close-coupled vacuum induction melting gas atomization mainly focuses on the study of the particle size distribution and properties of a certain section of the particle size, while research on the particle size distribution and properties of the entire section of the particle size is relatively few. Moreover, there is a lack of theoretical guidance for powder production. In this study, the effects of atomizing pressure and superheat on the size distribution and properties of the powder particles in the process of close-coupled vacuum induction melting gas atomization were studied. Further, reasons for the change in the powder particle size and properties were analyzed. Based on different powder size requirements of various industries, this study will play a guiding role in powder preparation.
Methods Herein, the vacuum degree reached 5 Pa, and the argon gas with 99.99% purity was used for atomization. The single variable control method was used to study the effects of atomizing pressure and superheat on the particle size distribution and surface morphology, respectively, during the close-coupled gas atomizing process of Fe-Cr alloy powder. After cooling, 500-g powder was randomly weighed and screened using a standard vibrating screen. The screen mesh was selected as 100 mesh (150 μm), 150 mesh (105 μm), 270 mesh (53 μm), and 500 mesh (25 μm). Then, an electronic scale was used to weigh the mass of each particle size section. A small amount of powder was randomly selected to measure the cumulative distribution using a laser particle size analyzer. The powder morphology was observed using a scanning electron microscope. The bulk density and fluidity testers were used to measure the bulk density and fluidity. Laser cladding technology was used to clad the powder onto 3Cr13 stainless steel, and the macromorphology and rockwell hardness of the laser cladding layer were studied.
Results and Discussions Results show that when other atomization parameters are the same, the cumulative distribution curves of the powder move to the left and then move to the right when the atomizing pressure is further increased to 4.2 MPa. The median particle size of the corresponding powder first decreases and then increases. The yield rate of the powder with a size of -105 μm first increases and then decreases (Fig. 3). Both the powder fluidity and bulk density first increase and then decrease (Table 2). The stability of the atomization process is improved by increasing superheat. At 150 ℃ superheat, the metal shows a “metal tumor” at the end of the tube, which halts the atomization process. When the superheat is increased to 200 ℃ and 250 ℃, the liquid steel viscosity decreases, the fluidity increases, and the cumulative distribution curves of the powder move to the left. When the superheat is further increased to 300 ℃, the cumulative distribution curves of the powder move to the right. The median particle size of the corresponding powder first decreases and then increases. Moreover, the yield of the fine powder first increases and then decreases (Fig. 6). Both the powder fluidity and bulk density first increase and then decrease (Table 3).
Conclusions When the atomizing pressure and gas velocity are increased, the breakability of the alloy liquid column is improved. When the atomizing pressure is increased to 4.2 MPa, the gas pressure is increased significantly more compared with the gas velocity. Moreover, increasing the gas pressure results in an increase in the negative pressure at the end outlet of the liquid guide tube, which increases the metal melt flow. The broken melt is affected by a decrease in the gas energy per unit volume. The increased degree of gas flow disorder causes the collision and fusion of droplets, thus forming a coarse powder. Thus, the median particle size increases. When the superheat is increased, the surface tension in the metal droplets is decreased and the contact angle between the gas-liquid interface is decreased. Consequently, the thickness of the film formed by the initial crushing is decreased, optimizing the crushing effect. When the superheat is increased to 300 ℃, the cooling and solidification times of the metal melt and broken into droplets is longer. The cooling speed differs for droplets with different sizes. During the falling process, some small droplets combine into large ones and some droplets collide or agglomerate with the powder in the cooling process to form rods or agglomerate into a coarse powder. Therefore, the median particle size of the powder increases. The laser cladding layer of the alloy powder prepared using the vacuum induction melting gas atomization technology shows good properties, and the hardness of the cladding layer is HRC54--HRC57.
