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Abstract
In situ X-ray-based measurements of the laser powder bed fusion (LPBF) additive manufacturing process produce unique data for model validation and improved process understanding. Synchrotron X-ray imaging and diffraction provide high resolution, bulk sensitive information with sufficient sampling rates to probe melt pool dynamics as well as phase and microstructure evolution. Here, we describe a laboratory-scale LPBF test bed designed to accommodate diffraction and imaging experiments at a synchrotron X-ray source during LPBF operation. We also present experimental results using Ti-6Al-4V, a widely used aerospace alloy, as a model system. Both imaging and diffraction experiments were carried out at the Stanford Synchrotron Radiation Lightsource. Melt pool dynamics were imaged at frame rates up to 4 kHz with a ∼1.1 μm effective pixel size and revealed the formation of keyhole pores along the melt track due to vapor recoil forces. Diffraction experiments at sampling rates of 1 kHz captured phase evolution and lattice contraction during the rapid cooling present in LPBF within a ∼50 × 100 μm area. We also discuss the utility of these measurements for model validation and process improvement.
Highlights
INTRODUCTIONLaser Powder Bed Fusion (LPBF), known as Selective Laser Melting or Laser Beam Melting, is a rapidly developing additive manufacturing technology that provides significant design flexibility relative to conventional manufacturing techniques and enables the production of highly complex parts at minimal added cost for low-volume production. In a LPBF process, a high power (∼100’s of W) continuous wave (CW) laser selectively scans over a thin metal powder layer, generating a melt pool that rapidly solidifies to create a two-dimensional solid layer adhered to the substrate or part beneath it
In a Laser Powder Bed Fusion (LPBF) process, a high power (∼100’s of W) continuous wave (CW) laser selectively scans over a thin metal powder layer, generating a melt pool that rapidly solidifies to create a two-dimensional solid layer adhered to the substrate or part beneath it
Melt pool morphology has been investigated via in situ measurements of the pool depth using inline coherent imaging, an interferometry-based technique.16. While these approaches based on optical methods provide important information about the dynamics of the LPBF process, they are limited to surface imaging only and cannot provide information about bulk material behavior
Summary
Laser Powder Bed Fusion (LPBF), known as Selective Laser Melting or Laser Beam Melting, is a rapidly developing additive manufacturing technology that provides significant design flexibility relative to conventional manufacturing techniques and enables the production of highly complex parts at minimal added cost for low-volume production. In a LPBF process, a high power (∼100’s of W) continuous wave (CW) laser selectively scans over a thin metal powder layer, generating a melt pool that rapidly solidifies to create a two-dimensional solid layer adhered to the substrate or part beneath it. Melt pool morphology has been investigated via in situ measurements of the pool depth using inline coherent imaging, an interferometry-based technique.16 While these approaches based on optical methods provide important information about the dynamics of the LPBF process, they are limited to surface imaging only and cannot provide information about bulk material behavior. Kenel et al performed in situ X-ray microdiffraction of rapid solidification in Ti-64 under well-defined cooling conditions with 1 ms time resolution, providing additional insight into the fundamental solidification behavior of this alloy under LPBF-like conditions.26 They performed cyclic heating and cooling to elucidate microstructural evolution induced by thermal behavior similar to what occurs in a multi-layer build, though the thermal boundary conditions in their experiment represent a somewhat different case than what is present in an LPBF build. We report initial X-ray imaging and diffraction experiments using this instrument at the Stanford Synchrotron Radiation Lightsource (SSRL)
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