Investigation of Distortion, Porosity and Residual Stresses in Internal Channels Fabricated in Maraging 300 Steel by Laser Powder Bed Fusion

Abstract

The use of parts containing internal channels fabricated by laser powder bed fusion (LPBF) in maraging steel is gaining attention within industry, due to the promising application of the material in molds and forming tools. However, LPBF processing has issues when it comes to unsupported channels, leading to defects that can result in a limited performance and shortened component life. The present study aims to provide a detailed evaluation of the metallurgical effects arising from the LPBF printing of channels made of maraging 300 steel. The results show that channel distortion occurs due to the lack of support, associated with increased roughness at the top part of the channel profile caused by partial melting and loosening of the powder. Statistical analyses showed that distortion is significantly affected by channel length. A high level of porosity derived from a lack of fusion was observed in the region above the channel and was attributed to layer irregularities caused by the absence of support, with a predominance of large and irregular pores. Residual stresses, always of a tensile nature, present a behavior opposed to that of distortion, increasing with increases in length, meaning that higher levels of distortion lead to an enhanced effect of stress accommodation/relief, with porosity having a similar effect. All these phenomena, however, did not seem to affect crystallographic orientation, with a nearly random texture in all cases, most likely due to the energy input used and to an optimized laser scanning strategy. These findings are vital to increase the amount of attention paid towards the design of internal channels, especially with those with the purpose of coolant circulation, since distortions and poor surface finishing can reduce cooling efficiency due to a defective fluid flow, while porosity can have the same effect by hindering heat transfer. Residual stress, in its turn, can decrease the life of the component by facilitating cracking and wear.

Keywords: distortions; internal channels; laser powder bed fusion (LPBF); maraging 300 steel (18Ni300); porosity; residual stresses.

Figure 1

Geometry of part manufactured by LPBF (dimensions in millimeters).

Figure 2

Build plate design for the LPBF printing of channel specimens.

Figure 3

Out-of-scale schematic indication of the locations of incidence of the X-ray beam for identifying phases and measuring residual stresses.

Figure 4

Indication of the region of interest in channel specimens for SEM analyses.

Figure 5

Surface deviations between as-printed (3D scans) and designed samples: (a) t1; (b) t2.

Figure 6

Porosity distribution in specimen “t2”, with volumetric pore classification.

Figure 7

Distributions of pore diameter, compactness and sphericity as functions of pore volume in specimen “t2”.

Figure 8

Distributions of pore sphericity, compactness, volume and diameter along the height of specimen “t2”.

Figure 9

Low-magnification SEM image showing the aspect of pores observed in specimens.

Figure 10

Summary of residual stress values, with their respective errors.

Figure 11

Representative SEM images of the bulk microstructure of as-printed maraging 300 steel: (a) specimen “t1”; (b) specimen “t2”; (c) upper inner surface of the channel in specimen “t2”; (d) detail from “d” showing differences between consolidated and overhanging features on the surface of the channel.

Figure 12

High-magnification EBSD maps of specimen “t1” showing phase distribution (a,d), pattern quality (b,e) and Kernel average misorientation (c,f) of the upper outer specimen surface (a–c) and of the upper inner channel surface (d–f). In phase maps, blue corresponds to ferrite and red to austenite. The building direction corresponds to the height of the images.

Figure 13

High-magnification EBSD maps of specimen “t2” showing phase distribution (a,d), pattern quality (b,e) and Kernel average misorientation (c,f) of the upper outer specimen surface (a–c) and of the upper inner channel surface (d–f). In phase maps, blue corresponds to ferrite and red to austenite. The building direction corresponds to the height of the images.

Figure 14

Diffractogram of as-printed maraging steel.

Figure 15

Low-magnification IPF maps in the X (a,d), Y (b,e) and Z (c,f) directions of the upper outer specimen surface (a–c) and of the upper inner channel surface (d–f). The building direction corresponds to the height of the images.

Figure 16

Pole figures: (a) upper outer specimen surface; (b) upper inner channel surface.