We used ultrahigh-speed synchrotron x-ray imaging to quantify the phenomenon of vapor depressions (also known as keyholes) during laser melting of metals as practiced in additive manufacturing. Although expected from welding and inferred from postmortem cross sections of fusion zones, the direct visualization of the keyhole morphology and dynamics with high-energy x-rays shows that (i) keyholes are present across the range of power and scanning velocity used in laser powder bed fusion; (ii) there is a well-defined threshold from conduction mode to keyhole based on laser power density; and (iii) the transition follows the sequence of vaporization, depression of the liquid surface, instability, and then deep keyhole formation. These and other aspects provide a physical basis for three-dimensional printing in laser powder bed machines.

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Additive manufacturing currently facilitates new avenues for materials discovery that have not been fully explored. In this study we reveal how additive manufacturing can be leveraged to produce dispersion strengthened (DS), multi-principal element alloys (MPEA) without the use of traditional mechanical alloying or chemical reactions. This new processing technique employed resonant acoustic mixing to coat an equiatomic NiCoCr powder with nano-scale yttrium oxides. Then, through laser powder bed fusion (L-PBF), the coated powder was successfully consolidated into 99.9% dense parts. Microstructural analysis confirmed the successful incorporation and dispersion of nano-scale oxides throughout the build volume. Furthermore, high temperature mechanical testing of the DS alloys showed significant improvements in strength and ductility over the baseline NiCoCr. As a result, this recently discovered processing route opens a new alloy design and production path that is synergistic between additive manufacturing and dispersion strengthening, possibly enabling a new generation of high-performance alloys. Additive manufacturing (AM) techniques have broadened many aspects of component design, enabled part count reduction, and decreased commissioning time for prospective NASA hardware and industrial applications 1. Currently, the bulk of metallic AM research has been conducted on traditional alloys 2-4. Yet, AM facilitates new avenues for materials discovery that have not been fully explored 5,6. For laser powder bed fusion (L-PBF), the finely focused, laser melting of powder is analogous to welding and is thus problematic for alloys that are difficult to weld. Unfortunately, many of the alloys used for ultra-high temperature applications (>1000 °C) fall within this criterion. Therefore, there currently exists a need for high temperature alloys that can be produced through L-PBF or similar AM processes. One alloy system that has shown promise is the multi-principal element alloy (MPEA) class. The discovery and growth of this interesting new class of alloys, coined "High Entropy alloys", has coincided with the development of AM. High entropy alloy development has led to the identification of a wide range of MPEAs, such as the ternary alloy NiCoCr, which has demonstrated impressive mechanical properties over a wide range of temperatures and stresses 7,8. Recent studies have also presented favorable results from producing the NiCoCr alloy using AM 9. The success of fabricating the "Cantor alloy" (NiCoCrMnFe) and its derivatives through AM may result from the small gap between their solidus and liquidus temperatures 10 , thereby, reducing the risk of heat affected zone (HAZ) cracking and lowering residual stresses 11. Unfortunately, the phase simplicity that enhances the manufacturability of alloys such as NiCoCr will also limit their high temperature mechanical properties. An additional strengthening mechanism must be introduced, such as dispersion strengthening (DS). Dispersion strengthening, primarily through the use of oxides,...