Mitigating cracking in laser powder bed fusion of Ti-48Al-2Cr-2Nb via introducing massive β phase

The high flux combined with the high energy of the monochromatic synchrotron radiation available at modern synchrotron facilities offers vast possibilities for fundamental research on metal processing technologies. Especially in the case of laser powder bed fusion (LPBF), an additive manufacturing technology for the manufacturing of complex-shaped metallic parts, in situ methods are necessary to understand the highly dynamic thermal, mechanical, and metallurgical processes involved in the creation of the parts. At PETRA III, Deutsches Elektronen-Synchrotron, a customized LPBF system featuring all essential functions of an industrial LPBF system, is used for in situ x-ray diffraction research. Three use cases with different experimental setups and research questions are presented to demonstrate research opportunities. First, the influence of substrate pre-heating and a complex scan pattern on the strain and internal stress progression during the manufacturing of Inconel 625 parts is investigated. Second, a study on the nickel-base superalloy CMSX-4 reveals the formation and dissolution of γ' precipitates depending on the scan pattern in different part locations. Third, phase transitions during melting and solidification of an intermetallic γ-TiAl based alloy are examined, and the advantages of using thin platelet-shaped specimens to resolve the phase components are discussed. The presented cases give an overview of in situ x-ray diffraction experiments at PETRA III for research on the LPBF technology and provide information on specific experimental procedures.

Revealing dynamic processes in laser powder bed fusion with in situ X-ray diffraction at PETRA III.

Powder production by gas atomization of γ‐TiAl based alloys typically yields a highly nonequilibrium material regarding the occurring phases and their microstructural appearance. In particular, the equilibration of the powder and the associated phase transformations during heating are of great importance for the subsequently applied densification techniques. The present work employs in situ high‐energy X‐ray diffraction to investigate how this thermodynamic equilibration manifests itself in the resulting phase distribution, the ordering behavior of the disordered α and β phase, both evidenced in the powder, and the change of the γ lattice parameters during heating of a Ti–46.3Al–2.2W–0.2B (at%) powder up to 850 °C. Complementary microstructural characterization of the gas‐atomized powder and the heat‐treated material condition reveals that the temperature exposure predominately affects the dendritic parts of the microstructure, especially when the α phase is transformed into γ phase with small embedded grains of α2 and βo.

On the Temperature‐Induced Equilibration of Phase Distribution and Microstructure in a Gas‐Atomized Titanium Aluminide Powder

Ball milling treatment is a low-cost and achievable large-scale production method for preparing the composite powder used for the laser powder bed fusion (LPBF) process. For the ball-milled composite powder, good powder spreadability and composition homogeneity are essential for obtaining the LPBF-fabricated part with good forming quality. In this article, the Nb and B nanoparticles modified TiAl4822 composite powder with a nominal composition of Ti-47.53Al-1.98Cr-2.81Nb-0.15B are prepared via ball milling. It is found that the physical properties of the composite powder are more sensitive to the milling time than the milling speed and ball-to-powder ratio. Long-time milling is inclined to cause the formation of broken particles, poor powder flowability, and high oxygen contamination. Besides, the process maps between the ball milling energy E t and particle size distribution width, the static angle of repose, composition inhomogeneity, and oxygen content are established, respectively. Results show that when the applied E t is insufficient or excessive, the degree of sphericity or composition homogeneity always worsens. In these cases, deposition defects such as line or area defects are more prone to occur, due to the enhanced particle rearrangement resistance induced by the addition of nanoparticles. 

Ball Milling Treatment of Nb and B Nanoparticles Modified TiAl4822 Composite Powder and Its Effect on Powder Bed Quality and Powder Spreadability in Additive Manufacturing

Raimund Krenn

Papers

In situ investigation of the rapid solidification behavior of intermetallic γ-TiAl-based alloys using high-energy X-ray diffraction