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Particle and Powder Characteristics of Metal Feedstock for Powder-bed Additive Manufacturing

2024-10-15Application Note

This study underscores the critical role of metal powder characteristics in additive manufacturing, specifically the optimization of particle size distribution, particle shape, bulk density, and flowability to achieve superior quality in 3D printed powders.

 

Product Bettersizer 2600, HFlow 1
Industry Additive Manufacturing
Sample Alloy Metal Powders 
Measurement Type Particle Size, Particle Shape, Bulk Density, Flowability
Measurement Technology Laser Diffraction, Powder Characterization

 

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Introduction 

In recent years, 3D printing has emerges as a pivotal technology in the Industry 4.0 era, experiencing rapid advancements that traditional powder manufacturing processes can no longer accommodate. Metal powders for 3D printing require stricter control over parameters such as particle size, particle shape, bulk density and fluidity compared to other powder materials, in order to ensure high performance and consistency of printed components. The quality of these metal powders directly affects the stability of the printing process and the reliability of the resulting parts.

 

1.1 Impact of particle size and distribution

In powder-bed fusion additive manufacturing, the layer-by-layer stacking process is a unique mode for manufacturing 3D printed parts, making the thickness of the powder layer a critical factor influencing the performance of the final product. For laser powder bed fusion (LPBF), the typical powder size ranges from 10 to 50 µm, supporting high-precision printing. In general, the smaller the powder particle size, the thinner the powder layer, and the greater the densification of the formed part, as demonstrated in Figure 1.[1,2] However, fine particles are prone to agglomeration, which can lead to reduced powder bed density and fluidity. A wider particle size distribution (PSD) enhances packing density by allowing smaller particles fill the voids between larger ones, but this can also reduce flowability. Therefore, optimizing particle size and PSD is essential to improving powder bed packing and flowability.

 

Fig1-Schematic-of-AM-powder-spreading-process-with-a-recoater-blade

Figure 1. Schematic of AM powder spreading process with a recoater blade

 

1.2 Impact of particle shape

Particle sphericity significantly affects performance in the 3D printing process. Ideally, powders used in 3D printing should have a high degree of sphericity to promote good flow and achieve uniformity at higher layer densities. Conversely, irregular powder particles impede flowability, leading to increased gaps and voids in the powder bed, as illustrated in Figure 2.

 

Fig2--Packing-of-regularand-irregular-particles

Figure 2. Packing of regular (spherical) and irregular particles

 

1.3 Impact of powder bulk density and flowability

Powder bulk density and flowability are key parameters that affect both the packing density of the printed layer and the density of the molded part. PSD critically influences bulk packing density by determining the efficiency of void filling. An optimal PSD facilitates maximum interparticle contact and minimizes voids, enhancing packing density; in contrast, a sub-optimal PSD can lead to significant void formation and reduced density due to the entrapment of fine particles in the gaps between coarse particles, resulting in a wedging effect.[4] As shown in Figure 2, higher bulk density of the powder correlates with greater density in the resulting part during the molding process, while low packing density leads to porosity and rough surface deformation.[4]

 

Good flowability ensures uniform spreading of the powder during the printing process, preventing build-up and uneven layering, while improving surface finish and dimensional accuracy of the part. Therefore, optimizing both powder bulk density and flowability is key to ensuring the quality and efficiency of 3D printing.

 

Fig3-Schematic-illustration-of-the-LPBF-process-with-high-and-low-powder-bed-density

Figure 3. Schematic illustration of the Laser Powder Bed Fusion (LPBF) process with high and low powder bed density

 

This application note presents a thorough investigation of the particle and powder properties of four alloy metal powders. It employs laser diffraction technology to measure particle size distribution and dynamic imaging to assess particle shape using the Bettersizer 2600, along with evaluations of bulk density and flowability. (See Fig. 4 for an image of the Bettersizer 2600). By examining how microscopic particle characteristics influence macroscopic powder properties, this study offers valuable insights for analyzing the physical properties of powders used in 3D printing.

 

Fig4-Bettersizer-2600-with-dynamic-imaging-module-PIC-1-and-wet-dispersion-unit-BT-902

Figure 4. Bettersizer 2600 with dynamic imaging module PIC-1 and wet dispersion unit BT-902

 

Experimental Procedures

The particle size distribution of powder particles was measured using the Bettersizer 2600 laser diffraction analyzer. The principle of laser diffractionrelies on detectors capturing the scattering patterns of individual particles when the laser beam strikes the particles in the sample cell, as shown in Figure 5. Utilizing Mie scattering theory, the scattered light signals are converted into particle size distribution information.

Fig5-Setup-of-the-Bettersizer-2600-laser-system

Figure 5. Setup of the Bettersizer 2600 laser system

 

The PIC-1 module of the Bettersizer 2600 is employed to characterize particle shape using dynamic imaging. As shown in Figure 6, this method employs two LED lights to illuminate the dispersed particles as the sample flows through the sample cell. Two cameras, one with 0.5x magnification and the other with 10x magnification, capture clear images of each particle.

 

Fig6--Setup-of-the-PIC-1-dynamic-imaging-system

Figure 6. Setup of the PIC-1 dynamic imaging system of Bettersizer 2600.

 

The flow rate and bulk density of metal powders are measured using an HFlow 1 flowmeter funnel in accordance with ASTM B213 and ASTM B212 standards. The HFlow 1 can be set up with Hall (2.5 mm orifice), Carney (5 mm orifice) and Gustavsson (2.5 mm orifice) funnels. In this case, flowability is determined by measuring the time it takes for 50 g of powder to pass through a Hall funnel. Bulk density is assessed by measuring the density of the powder that naturally falls through the same Hall funnel into a 25 ml measuring cup.

 

Figure7--Setup-of-HFlow-1

Figure 7. Setup of HFlow 1 with Hall, Carney and Gustavsson funnels 

 

Result and discussion 

 

3.1 Particle size and distribution

The results of the particle size analysis are presented in both Figure 8 and Table 1. The D50 values for samples #1-4 are 42.62 µm, 37.75 µm, 37.57 µm, and 34.68 µm, respectively. Notably, samples #2 and #3 show similar D50 values, indicating comparable particle sizes. Sample #1 has a relatively larger particle size, exhibiting coarser characteristics, while sample #4 has the smallest particle size, suggesting finer particles. Additionally, the Span values, calculate using the equation: Span=(D90-D10)/D50), reveal that sample #2 has a narrower particle size distribution compared to the other three samples.

Fig8--Particle-size-distribution-of-4-samples

Figure 8. Particle size distribution of 4 samples

 

Table 1. Results of particle size measurement

Powder Name

D10(μm)

D50(μm)

D90(μm)

Span

Sample #1

22.95

42.62

70.59

1.093

Sample #2

23.15

37.75

57.90

0.925

Sample #3

24.51

37.57

56.21

0.917

Sample #4

20.92

34.68

55.78

1.008

 

3.2 Particle shape

More than 20,000 particles from each sample were measured over time using the Bettersizer 2600 equipped with both the PIC-1 and BT-802 wet dispersion units. Figure 9 displays images of individual particles from the four test powders captured during shape evaluation. The circularity (C) of all samples is detailed in Table 2. Circularity is calculated automatically by the Bettersize software using the equation,

Circularity-equation

where A is the projected area of a particle and the perimeter of the projected area. This metric provides a measure of the similarity of the particle to a sphere, with a value of 1 indicating a perfect circle. The value Cx denotes the circularity of particles at x% accumulation. As shown in Table 2, sample #3 exhibits poorer circularity, while the remaining three samples demonstrate similar circularity values.

Figure 9. Particle images from PIC-1

Figure 9. Particle images from PIC-1

 

Table 2. Circularity evaluation of four samples.

Powder Name

Number of detected particles

C10

C50

C90

Sample #1

21357

0.874

0.935

0.957

Sample #2

23985

0.886

0.938

0.956

Sample #3

23063

0.867

0.928

0.918

Sample #4

27105

0.870

0.932

0.960

 

3.3 Flowability and bulk density

Table 3 illustrates the relationship among particle size, particle shape, bulk density, and flowability characteristics of the four samples. Comparing samples #1 and #2, it is clear that these samples exhibit similar sphericity; as particle size decreases, both bulk density and flowability also decline. In the comparison of samples #2 and #3,while both have the same particle size; sample #3's poorer sphericity results in lower packing density due to the effects of particle filling and bridging, while its flowability decreases due to increased interparticle friction. Consequently, it can be concluded that the packing properties and flowability of powders are influenced by both particle size and shape. To better understand these relationships and maintain quality control of the feedstock, it is essential to investigate particle size distribution, particle shape, bulk density, and flowability.

 

Table 3. Summary of particle size, particle shape, bulk density and flowability

Powder Name

D5(μm)

C50

Bulk density (kg/cm3)

Flowrate (s/50g)

Sample #1

42.62

0.935

4.22

21.2

Sample #2

37.75

0.938

4.13

22.3

Sample #3

37.57

0.928

4.02

25.8

Sample #4

34.68

0.932

4.05

26.1

 

Conclusion 

This study underscores the critical role of metal powder characteristics in additive manufacturing, specifically the optimization of particle size distribution, particle shape, bulk density, and flowability to achieve superior quality in 3D printed powders. The methodologies introduced, including laser diffraction and dynamic imaging using the Bettersizer 2600, along with the HFlow-1 funnel flowmeter, provide a solid foundation for enhancing powder properties and advancing the capabilities of 3D printing technology.

 

References 

[1] K. Abd-Elghany, D.L. Bourell, Property evaluation of 304L stainless steel fabricated by selective laser melting, Rapid Prototype. J. 18 (2012) 420–428. 

[2] Gibbons, D.W., Govender, P. & van der Merwe, A.F. Metal powder feedstock evaluation and management for powder bed fusion: a review of literature, standards, and practical guidelines. Prog Addit Manuf 9, 805–833 (2024). 

[3] Panwisawas, C., Tang, Y.T. & Reed, R.C. Metal 3D printing as a disruptive technology for superalloys. Nat Commun 11, 2327 (2020). 

[4] A.K.H Kwan, K.W. Chan, V. Wong. A 3-parameter particle packing model incorporating the wedging effect. Powder Technology 237, 172-179

 

About the Author

Bettersize-application-engineer-Perfil-Liu

 Perfil Liu

Senior Application Engineer @ Bettersize Instruments

 

Bettersizer 2600

 Laser Diffraction Particle Size Analyzer


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