Abstract
Laser powder bed fusion (LPBF) is a metal additive manufacturing technology, which enables the manufacturing of complex geometries for various metals and alloys. Herein, parts made from commercially pure titanium are studied using in situ synchrotron radiation diffraction experiments. Both the phase transformation and the internal stress buildup are evaluated depending on the processing parameters. For this purpose, evaluation approaches for both temperature and internal stresses from in situ diffraction patterns are presented. Four different parameter sets with varying energy inputs and laser scanning strategies are investigated. A combination of a low laser power and scanning speed leads to a more homogeneous stress distribution in the observed gauge volumes. The results show that the phase transformation is triggered during the primary melting and solidification of the powder and subsurface layers. Furthermore, the stress buildup as a function of the part height during the manufacturing process is clarified. A stress maximum is formed below the part surface, extending into deeper layers with increasing laser power. A temperature evaluation approach for absolute internal stresses shows that directional stresses decrease sharply during laser impact and reach their previous magnitude again during cooling.
Highlights
Additive manufacturing technologies offer vast possibilities for lightweight design, which appeal to medical and aerospace engineering fields, among other industry branches
It was shown that the α–βphase transformation occurs in subsurface layers and during initial melting and solidifying
Depending on the process parameters, β titanium reflections were found to a depth of 400 μm or seven layers below the surface
Summary
For the experiments’ conduction at the P07 beamline[33] at Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany, a custom-made LPBF system was used This system was designed by Uhlmann et al.[34] to enable the examination of LPBF parts, as they are being built layer by layer using high-energy synchrotron radiation. The powder bed surface was scanned by the laser with a spot size of %60 μm either longitudinally (L-Scan) or transversely (TI-Scan) regarding the incident synchrotron radiation beam; see Figure 2. A new powder layer was deposited by an automatic powder coating mechanism This recoating mechanism is designed to maintain a constant working distance from the laser to the powder bed. The transmission or longitudinal direction of the synchrotron radiation beam is denoted by LD; the layer buildup direction is denoted by BD, and the remaining axis is denoted by TD
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