Tomography of Laser Powder Bed Fusion Maraging Steel

Abstract

The presence of defects in additive manufactured maraging steel is a widespread problem as its dependence on processing parameters significantly influences it. Using X-ray computed tomography, along with optical microscope data limited to 2D images, quantifies the internal porosity present on a compact tension sample typically employed in fatigue testing. The primary goal of this research is to analyse the pores obtained after the fabrication of a compact tension sample and their main definition parameters, such as sphericity, aspect ratio, surface, and volume, and obtain validation of which method is valid for each of the parameters analysed. The current study aims to enhance the understanding of defects in maraging steel samples through non-destructive 3D analysis. Conventional 2D analyses are limited to surface measurements, providing incomplete information. The proposed method will provide a comprehensive understanding of the defects inside the maraging steel sample, thereby improving the reliability of this material for further applications. This study will contribute to academic and industrial communities by providing a novel approach to analysing maraging steel samples and, ultimately, developing improved materials for various applications. The study's findings reveal that most pores are produced by gases that are trapped in the fabrication process, and keyhole pores only appear near the surface.

Keywords: X-ray computed tomography; additive manufacturing; laser powder bed fusion; maraging steel; porosity.

Figure 1

(A) LPBF CT specimen orientation pattern (red lines are weld beads’ orientation, and the blue line is the crack plane), (B) diagram of LPBF process, and (C) welding beads of the additive manufacturing maraging steel sample.

Figure 2

Flow charts of (A) pore thresholding obtention and (B) reconstruction of 3D pores through the separated slides provided after X-ray CT.

Figure 3

(A) CT sample section after X-ray tomography was reconstructed in FIJI; (B) porosity in the volume analysed and main porosity types.

Figure 4

Sample X-ray CT output cross-section of a parallelepiped core. (A) Segmented image, (B) original image, and examples of pore volumes: (C) 1474 µm³, (D) 1236 µm³, (E) 583 µm³, and (F) 2285 µm³.

Figure 5

(A) Pores presented in the sample volume analysed, where blue dots show the pores and red lines indicate the specimen’s boundaries; (B) porosity projection of the hole’s volume on the X-Y plane, where the colour bar represents the height of each pore.

Figure 6

(A) The percentage of porosity of the sample obtained through X-ray CT and (B) segmented image with the coordinate axis used in the previous image and pores are shown in a different colour.

Figure 7

Relation between main definition parameters of pores measured through X-Ray CT (3D) or OM (2D), each blue circle is a different pore. (A) Aspect ratio vs. circularity in 2D, (B) aspect ratio vs. sphericity in 3D, (C) aspect ratio vs. area in 2D slides, (D) aspect ratio vs. volume in 3D, (E) circularity vs. area in 2D slides, and (F) sphericity vs. volume in 3D.