The effects of particle size distribution on the rheological properties of the powder and the mechanical properties of additively manufactured 17-4 PH stainless steel

Highlights

• Moderate changes in the size lead to significant changes in powder performance.

• Static mechanical properties are robust against moderate changes in particle size.

• Better control of powder is required to precisely know the effects of particle size.

Abstract

It is well known that changes in the starting powder can have a significant impact on the laser powder bed fusion process and subsequent part performance. Relationships between the powder particle size distribution and powder performance such as flowability and spreadability are generally known; however, links to part performance are not fully established. This study attempts to more precisely isolate the effect of particle size by using three customized batches of 17-4 PH stainless steel powders with small shifts in particle size distributions having non-intersecting cumulative size distributions, designated as Fine, Medium, and Coarse. It is found that the Fine powder has the worst overall powder performance with poor flow and raking during spreading while the Coarse powder has the best overall flow. Despite these differences in powder performance, the microstructures (i.e., porosity, grain size, phase, and crystallographic texture) of the built parts using the same process parameters are largely the same. Furthermore, the Medium powder produced parts with the highest mechanical properties (i.e., hardness and tensile strength) while the Fine and Coarse powders produced parts with effectively identical mechanical properties. Parts with good static mechanical properties can be produced from powders with a wide range of powder performance.

Introduction

Many additive manufacturing (AM) technologies rely on powders for feedstock material. Changes to the powder including chemistry, powder size, morphology, etc., from different batches, vendors, or reuse may impact the manufacturing process. Thus, there are significant measurement science needs for AM powder characterization and performance [1], [2], [3], [4], [5], [6]. Besides chemistry, one of the fundamental powder characteristics is the particle size distribution (PSD). Literature studying the effects of PSD on powder and part performance has been summarized in recent reviews [3], [5], [7]. For the purpose of this study we focus on findings for metals-based laser powder bed fusion (LPBF). There are some general notions about what makes a good or acceptable powder for LPBF. One criterion is the powder should have a high, uniform packing density in the powder bed. Packing density is typically determined by measurements of undisturbed powder and agitated powder (e.g., apparent and tapped density), and the difference is quantified by ratios or indexes (e.g., Hausner and Carr) which are empirically correlated with good and bad performing powders. It is generally accepted that achieving a uniform and consistent packing density is linked to flowability and spreadability [3], [8], [9]. However, there is little consensus on how to best to define and quantify these properties. Funnel flow tests (e.g., Hall and Carney), powder rheometers, and rotating drum devices are most commonly applied to quantify flowability [10], [11], [12]. There are a lack of common tests and metrics to describe spreadability [13], [14], [15]. Additionally the spreading mechanism influences the powder performance (e.g., blade versus roller systems [16]). Vock et al. [3] summarize three general findings consistent across the literature: a narrower PSD increases flowability, larger particles improve flowability, and increasing moisture content decreases flowability.

Links between powder performance and part performance are difficult to surmise. Here we focus on mechanical properties. Often there may be differences in powder performance with no change in mechanical properties or there may be changes in mechanical properties unrelated to powder performance (e.g., solely attributed to changes in chemical composition) [3], [8], [17], [18], [19], [20]. It should be noted that the powder performance has an effect on the window of acceptable process parameters; however, there can be a wide window of process parameters which produces similar results for different powders [19], [21], [22], [23]. Relationships between final part density, PSD, and flowability have been observed: higher density correlates with wider PSDs, finer (i.e., smaller particle size) PSDs, more spherical powders, and increased flowability [3], [5], [8], [9], [23], [24]. Relationships between mechanical properties, PSD, and flowability are fewer: increased mechanical properties occur for narrower PSDs and increased flowability [3], [5], [25], [26]. These relationships are based on relative changes in the PSD and require compromises. For example, a fine, narrow PSD, expected to have good density and mechanical properties, may in fact have poor density and properties because of decreased flowability. One aspect of powder performance not discussed yet that may provide a bridge between powder and part performance is the powder-laser interaction, as determined by such properties as laser absorptivity and powder bed thermal conductivity [3]. However, it is unclear how important these are to the final part performance (e.g., [27]).

Isolating the effect of powder performance on mechanical properties is difficult because additive manufacturing appears to be more sensitive to variations in chemical composition than traditional manufacturing processes. The material used in this study, stainless steel (17-4), exhibits a wide range of mechanical properties [28], [29], [30], [31]. LPBF 17-4 is not always fully martensitic like traditionally manufactured 17-4 [28], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]. Depending on the chemistry and processing history, the material may contain significant amounts of retained austenite. The Cr and Ni equivalent values on the Schaffner diagram [36], [40], which account for the influence of several elements, have been used to explain why some powders result in high amounts of retained austenite. Higher Ni equivalent values and lower Cr/Ni equivalent ratios indicate austenite will be more stable. Higher nitrogen content in nitrogen atomized powders, resulting in a higher Ni equivalent value, is believed to be the main cause of high amounts of retained austenite in LPBF 17-4 [32], [33], [34], [35], [36]. Quantifying the amount of retained austenite is challenging using surface measurement techniques because the austenite transforms to martensite from mechanical loading including grinding and polishing. Phase volume fractions are further complicated by the low c/a ratio of the crystal lattice in body centered tetragonal (BCT) martensite, which appears nearly body centered cubic (BCC) from X-ray and electron backscatter diffraction (XRD and EBSD, respectively). Some have reported that AM 17-4 is primarily BCC-Fe ($\delta$-Ferrite) [31], [41], [42], [43] based on secondary EBSD signals such as image quality. This type of analysis has been used in traditionally manufactured multi-phase steels [44], [45] where a higher dislocation density along with fine sub-grain microstructures of martensite results in a lower image quality compared to $\delta$-Ferrite. However, some caution should be used with such techniques, because martensitic microstructures are hierarchical with varying dislocation densities depending on the transformation sequence. This can result in different categories of martensite within a single material (e.g., coarse and traditional lathes) [46]. The amount of coarse and fine BCC/BCT grains varies within AM 17-4 depending on chemistry and thermal history [42], [47]. Further work is required to understand martensitic transformations in AM 17-4.

There are a couple of studies on LPBF 17-4 which focused on powder size and morphology effects on the part performance [21], [43]. Irrinki et al. [21] studied three water atomized powders with varying size distributions compared to a single gas atomized powder. They found the process (i.e., energy density) could be tuned to produce similar microstructures, densities, strength, and hardness with slightly lower elongation to failure for the water atomized powders compared to the gas atomized powder. Higher energy densities were required to produce dense parts for the water atomized powders compared to the gas atomized powders, and the energy density required to produce dense parts increased with increasing powder size for the water atomized powder [21]. Auguste et al. [43] studied two gas atomized powders with one powder having a narrower size distribution. They saw differences in mechanical properties that they attributed to differences in microstructure (coarse and fine grained microstructures). The differences in microstructure were then attributed to differences in chemical composition which caused different solidification paths and phase transformations. The influence of the powder size distribution was not discussed as a contributor to the different in part performance.

The current study aims to provide additional experimental data to draw links between powder performance and mechanical properties. The study differs from other studies in that it focuses on moderate shifts in the PSD with powders that have non-intersecting cumulative size distributions rather than powders from different manufacturing processes, different vendors, significant changes in the distribution shape, and/or large changes in the PSD. The intention is to isolate the effect of particle size on powder performance and part performance.