The widespread application of metal additive manufacturing (AM) is limited by the ability to control the complex interactions between the energy source and the feedstock material. Here we develop a generalizable process to introduce nanoscale grooves to the surface of metal powders which increases the powder absorptivity by up to 70% during laser powder bed fusion. Absorptivity enhancements in copper, copper-silver, and tungsten enables energy efficient manufacturing, with printing of pure copper at relative densities up to 92% using laser energy densities as low as 82 J/mm³. Simulations show the enhanced powder absorptivity results from plasmon-enabled light concentration in nanoscale grooves combined with multiple scattering events. The approach taken here demonstrates a general method to enhance the absorptivity and printability of reflective and refractory metal powders by changing the surface morphology of the feedstock without altering its composition.

Powders
Two copper powders were used in the study. A commercially available copper powder was purchased from LPW/Carpenter Additive (99.95% purity). A second set of copper powder was obtained from Lawrence Livermore National Laboratories (LLNL) (99.99% purity). This powder was made at LLNL through gas atomization using C10100 powder stock. Eutectic AgCu was purchased from LPW/Carpenter Additive (28.1 w% Cu 71.9 w% Ag). Pure W powder with a mean diameter of 45 µm was purchased from Tekna.

Etching Powders
To etch the Cu and AgCu powders, a solution of FeCl3, HCl, and ethanol at a ratio of ~70g:50ml:150ml was prepared. For 100 g of smooth, as-purchased copper powder, 25 ml of acetic acid was added to a 250 ml Erlenmeyer flask and the native copper oxide layer on the powder was etched for 5 min, during which the solution turned blue due to the dissolution of the copper oxide layer; stirred at 400 RPM using a magnetic stir bar for 4 min and allowed the powder to sediment for 1 min. The acetic acid solution was removed using a pipette and 100 ml of the FeCl3 etching solution was added to the flask. The solution was covered with paraffin and stirred for 1, 5, or 10 h at 400 RPM. The flask was then rested for 5 min to allow the powder to sediment. The FeCl3 solution was discarded using a pipette. The powder was washed in fresh ethanol 8 times, by centrifuging in a 50 ml tube at 100 RPM for 60 s. The powders were poured onto a 6-inch petri dish to dry for 5 h. After drying, the powders were sieved using <75 µm mesh in a vibratory sieve shaker (FRITSCH ANALYSETTE 3 PRO). The yield from this process was about 90%, producing 90 g of nanotextured powder for 100 g of as-purchased powder. The AgCu powder was etched for 1 hr.

To etch the tungsten powder, H2O2 30% was used. For 100 g of as-purchased tungsten powder, 20 ml of H2O2 30% was added to a 500 ml Erlenmeyer flask and stirred at 600 RPM using a magnetic stir bar. The flask was manually agitated to fully distribute the powders within the acid. Within 2-3 min, an exothermic reaction released water vapor. The flask was allowed to cool for 5 min. The remaining liquid was initially light yellow due to soluble tungsten oxide species in solution; upon cooling, the color changed to a characteristic dark blue associated with tungsten pentoxide crystallization. The liquid products were removed with a pipette and the etching procedure was repeated for a minimum of three times and a maximum cumulative etching time of 1 hr. The powders were washed, cleaned, and dried similarly to the Cu and AgCu powders.

Absorptivity Experiments
To measure the absorptivity of the powders, a custom calorimetry experimental setup was built to fit on the build plate of a commercial metal 3D printer (Aconity Mini 3D). The printer was equipped with a 200 W and 1070 nm Yb-doped fiber laser. Copper substrates of C10100 purity (99.99%) were machined with 2 mm thickness and a recessed area of 4×4 mm² and 50 µm depth. This recessed area was filled with copper powder, and the depth set the powder layer thickness used in the calorimetry experiments. The laser was scanned on the powder in the form of a 4 mm line scan. Scanning was performed at a power of 175 W and two speeds of 100 and 656 mm/s. At least 3 experiments were performed per scanning condition for each powder type. During the laser scanning, the temperature of the copper substrate and powder was collected using a type-K thermocouple spot-welded to the back of the substrate. The absorptivity was calculated as the ratio of energy required to raise the sample (powder+substrate) temperature to the measured value relative to the laser energy input from the scanning.

Printing
All printing experiments were performed using a low volume, custom-built laser powder bed fusion system at the Advanced Manufacturing Laboratory at LLNL. The metal 3D printing system was equipped with a Yb-doped fiber 1070 nm wavelength scanning laser with a maximum power of 1 kW. The 2.5 cm build plate and approximately 10 mm build height allowed for low volume prints using up to ~200 g of copper powder. For each powder system, 6 mm diameter cylinders were printed using laser powers ranging from 100 to 500 W and laser scanning speeds of 300 and 600 mm/s. The layer size and hatch spacing were 50 µm and 80 µm, respectively, for all prints. The build chamber was prepared by pumping to 10⁻³ Torr and purging with argon to atmospheric pressure while maintaining an oxygen concentration less than 100 ppm.

Nano and Micro X-Ray Tomography
Synchrotron X-ray nanotomography was performed to characterize the surface features and obtain a 3D representation of a single etched powder particle. The experiments were performed at the Stanford Synchrotron Radiation Lightsource (SSRL) Beamline 6-2C. A 7 kV beam was used, which produced a 15 nm pixel resolution. The pixels were binned by 2 during image acquisition, resulting in an effective pixel size of 30 nm. Two images were collected (and averaged) at 0.5 degree increments over a range of 180 degrees. Ten reference images were taken, averaged, and used for background correction in the tomography images. The process and reconstruction were performed using an algebraic reconstruction technique (ART) with 20 iterations in TXM Wizard, an open-source software developed at SSRL. The slices were reconstructed and visualized for surface feature profiling using the software Dragonfly.

X-ray microtomography was also performed to characterize the porosity in the printed cylinder structures using a Sky Scan 1273 X-ray microscope. Pixel sizes ranged from 4.25 to 5 µm in different scans of the printed cylinders. The slices were reconstructed and visualized for relative density measurements using Dragonfly. Grayscale 3D images were segmented using a watershed transform. Edges between two areas of interest, porous and solid regions, were identified using a Sobel edge detection method. Seeds for areas of interest were manually chosen using histographic segmentation, and a watershed transform enabled segmentation of pores and solid aspects of the prints. Relative density measurements were defined as the volume fraction of the solid regions in the cylinder.

EM Wave Simulations
EM wave simulations were performed using commercial finite element software (COMSOL Multiphysics) to probe mechanisms of the enhanced optical absorption in nanotextured Cu powder. The actual surface profile of Cu05 powder was extracted from a cross-section of reconstructed images acquired via x-ray nanotomography. For parametric studies, the textured powder particle surface was also locally approximated to two-dimensional grooves on a flat and semi-infinite Cu substrate. A normally or obliquely incident plane wave was assumed in the simulations. The absorption enhancement factor was calculated by the ratio of the absorption cross-section of the textured surface to that of the flat surface with the same physical area. Optical constants for Cu used in the EM wave simulations were obtained from Palik. 

Ray Tracing Simulations
COMSOL Multiphysics software and its geometrical optics module were used to perform ray tracing simulations to compute ray trajectories on a powder bed system. This approach is effective for modeling electromagnetic wave propagation when the ratio of the incident wavelength to the characteristic dimension in the model is less than 0.1. While propagating, waves are assumed to be locally plane, and the effects of diffraction at edges and corners in the geometry are typically neglected. Beds of as-purchased powder and nanotextured powder were considered in a geometric optics framework. Here, the individual nanotextured powder particles were treated as as-purchased powders, but with some fraction of their surfaces ascribed some absorption enhancement factor. To compute ray trajectories, a ray release boundary condition was initialized to specify the initial position and direction of rays. The rays were released from an area with the direction (0,0,-1) orthogonal to the powder bed. 5×10⁴ rays were released with a uniform polarization of (1,0,0). Specular reflection boundary conditions were used, such that the angle of incidence (AOI) of the incoming ray matched that of the reflected ray, with respect to the surface normal vector. The classical implementation of Snell's Law and Fresnel equations were used to determine the absorption of the laser at each time step. 

As each ray strikes a surface, it is absorbed a certain amount. The amount of the ray that is absorbed is a function of its polarization state and AOI with respect to the surface it is striking. For nanotextured powder, if a ray strikes a powder surface position.