Characterizing the effects of laser control in laser powder bed fusion on near-surface pore formation via combined analysis of in-situ melt pool monitoring and X-ray computed tomography.

Abstract

Near-surface or sub-surface pores are critical to the structural integrity of additively manufactured (AM) metal parts, especially in fatigue failure applications. However, their formation in laser powder bed fusion is not well-understood due to the complex processes happening near the surface, which are challenging to monitor. A lack of high-fidelity data hinders understanding of the process and its effects. It is not well-known that problems with laser control parameters such as galvanometer acceleration and laser power on/off delay can form near-surface pores in laser powder bed fusion (LPBF) AM processes, and we investigated the characteristics of these pores in this research. We also demonstrate the capabilities and processes of combined studies using in-situ melt pool images and ex-situ X-ray computed tomography (XCT) images. Using the National Institute of Standards and Technology (NIST) Additive Manufacturing Metrology Testbed (AMMT), varying laser control schemes were implemented while in-situ coaxial melt pool images were acquired during the build of Nickel superalloy parts. A combination of time-stepped digital commands, in-situ coaxial melt pool monitoring images (≈ 8 μm/pixel), and ex-situ high-resolution XCT images (≈ 3.63 μm/voxel) were demonstrated. Advanced image analysis methods were used to characterize the pores found in terms of size and shape distribution and spatial location. XCT images, in high correspondence to melt pool images, clearly show the effects of the laser control parameters. We present the complete analysis chain of AM command, in-situ melt pool imaging, ex-situ XCT acquisition, and image analysis. Possible near-surface pore formation mechanisms are explained through the comparative image analysis. The approach of compiling combined analyses based on time-stepped digital commands, in-situ monitoring results, and ex-situ XCT measurement through image analysis enables observation and categorization of the different near-surface pore formation mechanisms stemming from laser and scan control.