Linking In Situ Melt Pool Monitoring to Melt Pool Size Distributions and Internal Flaws in Laser Powder Bed Fusion

Claudia Schwerz,Lars Nyborg

doi:10.3390/met11111856

Abstract

In situ monitoring of the melt pools in laser powder bed fusion (LPBF) has enabled the elucidation of process phenomena. There has been an increasing interest in also using melt pool monitoring to identify process anomalies and control the quality of the manufactured parts. However, a better understanding of the variability of melt pools and the relation to the incidence of internal flaws are necessary to achieve this goal. This study aims to link distributions of melt pool dimensions to internal flaws and signal characteristics obtained from melt pool monitoring. A process mapping approach is employed in the manufacturing of Hastelloy X, comprising a vast portion of the process space. Ex situ measurements of melt pool dimensions and analysis of internal flaws are correlated to the signal obtained through in situ melt pool monitoring in the visible and near-infrared spectra. It is found that the variability in melt pool dimensions is related to the presence of internal flaws, but scatter in melt pool dimensions is not detectable by the monitoring system employed in this study. The signal intensities are proportional to melt pool dimensions, and the signal is increasingly dynamic following process conditions that increase the generation of spatter.

Highlights

Laser powder bed fusion (LPBF) is an additive manufacturing technology that consists of iteratively spreading a thin layer of powder on a build platform and selectively melting regions of the powder bed by means of a laser source until an entire component is built.The melt pool formed upon the interaction of the laser with the substrate and powder bed is the basic unit of this manufacturing process; fundamental research in LPBF has the investigation of the melt pool as a starting point [1,2]
The LPBF process is mapped for Hastelloy X considering fusion modes and volume fraction of internal flaws
The melting modes are determined based on melt pool geometry: process parameters that generate semicircular melt pools operate in conduction mode (Figure 1A), and process parameters that generate melt pools with depression operate in keyhole mode (Figure 1B)

Summary

Introduction

Laser powder bed fusion (LPBF) is an additive manufacturing technology that consists of iteratively spreading a thin layer of powder on a build platform and selectively melting regions of the powder bed by means of a laser source until an entire component is built.The melt pool formed upon the interaction of the laser with the substrate and powder bed is the basic unit of this manufacturing process; fundamental research in LPBF has the investigation of the melt pool as a starting point [1,2]. Melt pool geometries have been mapped in the LPBF process space, both by measurements on cross-sections of single tracks [8,9], and by direct observation of melt pool by in situ high-speed, high-energy X-ray imaging [10], experimentally showing that the key process parameters determining melt pool dimensions are laser power and scan speed. Other factors such as shield gas flow rate [11] and the defocusing distance [12,13] influence melt pool geometry and can even alter the fusion mode

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Conclusion