Challenges in Additive Manufacturing of Space Parts: Powder Feedstock Cross-Contamination and Its Impact on End Products

Abstract

This work studies the tensile properties of Ti-6Al-4V samples produced by laser powder bed based Additive Manufacturing (AM), for different build orientations. The results showed high scattering of the yield and tensile strength and low fracture elongation. The subsequent fractographic investigation revealed the presence of tungsten particles on the fracture surface. Hence, its detection and impact on tensile properties of AM Ti-6Al-4V were investigated. X-ray Computed Tomography (X-ray CT) scanning indicated that these inclusions were evenly distributed throughout the samples, however the inclusions area was shown to be larger in the load-bearing plane for the vertical specimens. A microstructural study proved that the mostly spherical tungsten particles were embedded in the fully martensitic Ti-6Al-4V AM material. The particle size distribution, the flowability and the morphology of the powder feedstock were investigated and appeared to be in line with observations from other studies. X-ray CT scanning of the powder however made the high density particles visible, where various techniques, commonly used in the certification of powder feedstock, failed to detect the contaminant. As the detection of cross contamination in the powder feedstock proves to be challenging, the use of only one type of powder per AM equipment is recommended for critical applications such as Space parts.

Keywords: Additive Manufacturing; Ti-6Al-4V; cross contamination; microstructure; space applications; tensile properties.

Figure 1

Horizontal, vertical, and inclined orientations of static tensile specimens: Z is the building direction, and X is the recoating direction.

Figure 2

Geometry of static tensile specimens [20].

Figure 3

Schematic representation of: (a) the projections on the P_xy, P_yz, P_xz planes of the defect detected via X-ray CT (adapted from [23]); and (b) the regions of analysis defined for each tensile sample from Batch 1 built in X and Z directions.

Figure 4

SEM images of the fracture surface of a tensile specimen in the Z orientation: (a) overview of the fracture surface; (b) close-up view of cracks; (c) close-up view of cracked inclusion with a brittle appearance; (d) close-up view of inclusion with a brittle appearance.

Figure 5

Strength values of non-contaminated (Batch 2) and contaminated (Batch 1) Ti-6Al-4V, in the stress relief annealed condition (■ tensile strength, ● yield strength).

Figure 6

Fracture elongation of non-contaminated (Batch 2) and contaminated (Batch 1) Ti-6Al-4V, in the stress relief annealed condition.

Figure 7

X-ray CT images of a contaminated (Batch 1) tensile sample built in Z direction showing (a) high density inclusions (white particles); and (b) distribution of these defects in the part, coloured according to their associated volume.

Figure 8

X-ray CT of a tensile sample from Batch 1 built in Z direction showing (a) the porosity (dark regions); and (b) distribution of these defects in the part, coloured according to their associated volume.

Figure 9

Box plot of area of the projections of the inclusions of samples built in X and Z directions, in the yz, xz and xy planes.

Figure 10

Microstructure of the horizontal plane of Batch 1 Ti-6Al-4V tensile specimen, in the stress relieve annealed condition showing: (a) an overview of the microstructure with an embedded particle; and (b) the referred particle at higher magnification.

Figure 11

Microstructure of the vertical plane of Batch 1 Ti-6Al-4V tensile, in the stress relieve annealed condition showing: (a) an overview of the microstructure with an embedded particle; and (b) the referred particle at higher magnification.

Figure 12

Microstructure of the horizontal plane of Batch 2 Ti-6Al-4V tensile specimen, in the stress relieve annealed condition, showing (a) an overview; and (b) microstructural features in more detail.

Figure 13

Microstructure of the vertical plane of Batch 2 Ti-6Al-4V tensile specimen, in the stress relieve annealed condition, showing (a) an overview; and (b) microstructural features in more detail.

Figure 14

Back-scattered SEM image of Batch 1 powder, highlighting the sphericity of the particles.

Figure 15

X-ray CT slice image of the powder from Batch 1, showing high density inclusions, visible as bright particles.