Researchers are delving further into analysis of laser powder-bed fusion techniques, recently publishing their findings in ‘The role of side-branching in microstructure development in laser powder-bed fusion.’ As manufacturing of industrial parts via 3D printing and additive manufacturing processes becomes critical to an increasing number of applications today, suitable microstructures are required for quality parts.
Although 3D printing has been around since the 80s, it is still a relatively new technology—and especially to the mainstream. Benefits abound, but so do a variety of challenges. While porosity is an issue of ongoing study, here the authors address the epitaxial growth of crystals and how it affects development of microstructures.
In studying solidification microstructure, the researchers evaluated thermal parameters to better understand what types of mechanisms are behind the development of microstructures, specifically at certain areas of melt pools. During their research, the authors found that X-ray diffraction revealed two alloys made up of one face-centered (FCC) cubic phase—both in the ‘as-received powder’ and in the LPBF builds.
“Because cells can be only seen after chemical etching, the undulated surface results from the chemical perturbation. This indicates that although the high cooling rate can prohibit the formation of secondary arms, there are still solid-liquid interface instabilities in the direction orthogonal to the primary growth direction,” explained the researchers. “The presence of such side instabilities indicates that cells are in the transition from cellular to dendritic growth.”
In regards to impacts caused by porosity, the researchers noted that change in length scale was the result of the high cooling rate after the depositing of a new melt pool—with cell refinement found at fusion lines between two well consolidated weld beads and a lack of pores.
“FEA simulation confirms that adjacent melt pools effectively form a pseudo-continuous melt pool over the length scale of about 1 mm though the modulation of the laser beam causes different melt pool profile in transients between melt spots (Supplementary Movies 1 vs. 2),” stated the researchers.
“In addition, the underlying mechanisms seen in the 316L steel were also observed in the HEA, e.g. the continuous growth along the centerline and the frequent side-branching on sides of melt pools also result in two sets of thin grains and broad columnar grains in the HEA, respectively.”
“The role of side-branching is influential as it results in a ‘criss-cross’ layer microstructure and broadening of grains in the subsequent deposition in 3D printed alloys,” explained the authors. “In particular, side-branching is responsible for microstructure development when varying the scanning strategy.
“Most interestingly, the chessboard strategy with 67° rotation between layers breaks the vertical columnar grain microstructure, but it promotes both in-layer epitaxial growth and out-of-layer side branching, resulting in helical epitaxial growth. It has been shown that variations in the length-scale of microstructure correlates well with v0:25 i G0:5, and large pores cause a substantial coarsening of the microstructure due to their local thermal insulating effect.”
Laser powder-bed fusion has been the topic of many studies in recent years, from examining the effects of gas chemistry to achieving melt pool control, and studying innovative monitoring processes. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.[Source / Images: ‘The role of side-branching in microstructure development in laser powder-bed fusion’]
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