1 Introduction.
Additive manufacturing is one of the most promising technology trends across several industries. The ability to produce parts that are close to the net shape of the final product with no material waste, coupled with the new possibilities for product designers and engineers to develop and produce highly complex and functional parts, has led industry experts to call this technology revolutionary. In particular, the additive manufacturing of metal parts has great potential for industries such as the automotive or aerospace industries.
The process most commonly used for 3D printing metal parts is called Selective Laser Melting (SLM). The laser beam selectively melts the metal material represented in the form of a fine metal powder. The powder is completely melted in the process, which results in the powder material consolidating completely, reaching almost 100% material density (1). The final product is then built layer by layer, as shown in Figure 1(2).
Therefore, the quality of the metal powder mainly determines the quality of the later product. The key parameters for high-quality metal powders are particle size distribution and particle shape (3). The particle size distribution has a direct impact on the fluidity of the metal powder and its potential to provide uniform powder bed density. These two factors are necessary for the high resolution of the final product geometry, which requires an extremely thin layer of powder material.
In addition, particle shape mainly affects the flow characteristics and accumulation of metal powders. Spherical particles are more likely to line up and stack together more efficiently than asymmetric particles. The efficient arrangement of the particles is essential for the high density of the bulk powder and for the final density of the printed metal part (4). For the production of metal powders, there are different processes, and the gas atomization process is one of them. The advantages are high productivity, the ability to control particle characteristics, and good uniformity of the particles produced (5,6). This technology uses air, steam, or inert gas to produce powder from molten metal. The metal material is melted in a furnace. The molten metal is then transferred to the ladle and tundish to create a stream of molten metal.
This gas stream is broken down into powder by the impact of the nozzle gas jet (see Figure 2)(7). In addition to the surface tension of the molten metal, viscosity is critical for the melt to form spherical droplets from the nozzle outlet. The atomizing medium (e.g. gases such as nitrogen, argon or helium), more precisely the kinetic energy of the medium, is the driving force of the atomization process and needs to overcome the viscous forces of the molten metal that resist deformation on the one hand, and the surface energy that prevents the free surface from being generated on the other hand (9). The viscosity of the molten metal determines the breakage of the droplets from the pool of molten material (10). The higher the viscosity of the molten metal, the more likely it is that the droplets will crack near the tip, regardless of whether the elongated liquid line that subsequently separates will break. For lower viscosity fluids, the droplets are formed by capillary necking and clamping near the tip of the short tip (11). Pre-measuring the viscosity of different metal melts in the laboratory helps metal powder manufacturers to perfectly design process parameters such as nozzle diameter, gas flow and velocity or process temperature.
2 Experimental Setup.
The purpose of the experiments shown in this report is to compare two different alloys for the production of metal powders used in additive manufacturing. The two alloys exhibit different properties during the production process. These differences may be due to the different viscosity levels of the two alloys in the molten state. Therefore, the purpose of the measurement is to determine the temperature correlation as well as the shear rate correlation of the viscosities of the two alloys.
2.1 Sample The sample analyzed is typically used for additive manufacturing and has the following chemical composition (the number indicates the maximum weight %): Table 1: Chemical composition of the two samples.
2.2 Measurement Procedure.
Insert the sample into the furnace at room temperature. From there, the sample was heated to 1650 at a heating rate of 15 kmin. Hold constant for another 30 minutes at 1650 degrees Celsius to reach temperature equilibrium in the sample. The measuring hammer is then placed into the molten sample and the viscosity is measured at a constant shear rate of 25 s-1 for 30 minutes. The next step is a shear rate ramp from 1 s-1 to 73 s-1. Then, cool the sample to 1550 at a cooling rate of 5k min and repeat the procedure. The same measurements are also made at 1500 °C, as this temperature range is important for the production process of metal powders. With 24% h2 and 97The mixture of 6% AR is purged from the sample at room temperature to reduce oxidation effects that can affect the measurement.
2.3 Measuring device.
Measurements were made on Anton Paar's furnace rheometer system (FRS) (see Figure 3). The device is capable of fast and accurate temperature control over a wide temperature range and consists of a furnace with a highly constant temperature gradient and a DSR head for accurate rheological measurements. The air bearing of the DSR head allows the measurement of very low viscosities as well as oscillation measurements. The measuring system consists of a concentric cylinder system with a CC30 cup and a CC19 pendulum made of Al2O3 with a profiled surface.
3 Results and Discussion.
The aim was to compare the rheological behavior of the two samples mentioned above. Therefore, viscosity at different temperatures and the dependence of viscosity on shear rate at constant temperature are of interest. Figure 4 shows the viscosity of the two samples at four different temperatures, with the W360 showing significantly lower viscosity over the entire temperature range. In addition, the measurement signal from the sample W360 is much more stable compared to the W722. Table 2 summarizes the average viscosity of the two samples at each temperature.
Figure 5 shows the slope of the shear rate and the shear rate-related viscosity of the two samples at all temperatures. Sample w360 exhibits almost Newtonian behavior, as does w722 at 1650. At 1500 and 1550, the behavior of the W722 changes to shear thinning at low shear rates. The increase in viscosity at higher shear rates is mainly due to Taylor vortices in the measurement gap due to low viscosity samples.
4 Summary. Two samples. The viscosity of the W360 is much smaller than that of the W722, and the measurement signal is more stable. In addition, the shear rate dependence varies, with the W722 exhibiting shear thinning behavior, while the W360 exhibiting almost Newtonian behavior. Customers use rheological comparisons to gain a deeper understanding of sample behavior at process temperatures. From the experience of customers, W360 is very problem-free in production, and the process parameters are quite well known. However, the W722 often causes the nozzles of the gas atomization system to clog and the powder produced has more quality issues.
From the above results, it can be seen that the higher viscosity of W722 may cause these problems. The solution now may be to further increase the processing temperature to ensure low viscosity or to adjust the nozzle diameter to affect the shear rate applied to the sample and take advantage of the shear thinning effect of the sample. Regardless, the measurement results support the further development of high-quality steel powders for additive manufacturing, which will support disruptive future developments across multiple industries.