DOI: 10.5061/dryad.3j9kd51td
title: High absorptivity nanotextured powders for additive manufacturing

description: 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.
description: <strong>Powders</strong><br>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.
 <strong>Etching Powders</strong><br>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 &lt;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.
 <strong>Absorptivity Experiments</strong><br>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.
 <strong>Printing</strong><br>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. <strong>Nano and Micro X-Ray
 Tomography</strong><br>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. <strong>EM
 Wave Simulations</strong><br>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.  <strong>Ray Tracing
 Simulations</strong><br>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.
description: # High Absorptivity Nanotextured Powders for Additive Manufacturing
 [https://doi.org/10.5061/dryad.3j9kd51td](https://doi.org/10.5061/dryad.3j9kd51td) Open Source datasets for the manuscript titled "High Absorptivity Nanotextured Powders for Additive Manufacturing". This dataset contains all the raw data for analysis conducted in the paper. It contains raw data from calorimetry experiments (Calorimetry.zip), nanoindentation experiments (nanoindentation.zip), and both lab-scale X-ray tomography results (all files titled "Tomography*"). ## Description of the data and file structure #### Calorimetry Calorimetry.zip contains CSV files from calorimetry experiments. All calorimetry data was collected using a home-built setup using a National Instruments DAQ and was converted from LVM to CSV. See Methods for additional detail on data collection. Data contains files with titles formatted as follows: Copper powder naming format: manufacturer_controlOrEtchedPowder_etchingConcentration_etchTIME_laserPower_laserSpeed_testNumber.csv Comprehensive examples: 1. lpw_control_200W_100mmps_001.csv - lpw - copper powder purchased from LPW. control - indicates this is a control experiment. Powders were not etched. 200W - laser power in watts designated by "W". 100mmps - laser speed in millimeters per second designated by "mmps". 001 - test number 1. 2. lpw_etoh_10x_1hr_200W_656mmps_004.csv lpw - copper powder purchased from LPW. etoh - indicates this is an etched experiment with ethanol as the solvent. No other solvents are used. 10x - indicates concentration. This is the exact solution described in the methods. No other concentration was used. 1hr - 1 hour of etching. 200W - laser power in watts designated by "W". 100mmps - laser speed in millimeters per second designated by "mmps". 004 - test number 4. 3. lllnl_control_175W_656mmps_001.csv lllnl - copper powder produced by LLNL (Lawrence Livermore National Lab). control - indicates this is a control experiment. Powders were not etched. 200W - laser power in watts designated by "W". 100mmps - laser speed in millimeters per second designated by "mmps". 001 - test number 1. Copper-Silver (AgCu) and Tungsten (W) powder naming format: element_controlOrEtchedPowder_laserSpeed_laserPower_testNumber.csv Comprehensive examples: 1. W_control_175mmpersec_200W_01.csv - W - tungsten powder. control - indicates this is a control experiment. Powders were not etched. 200W - laser power in watts designated by "W". 100mmps - laser speed in millimeters per second designated by "mmps". 01 - test number 1. 2. Ag_control_175mmpersec_200W_01.csv - Ag - silver powder. control - indicates this is a control experiment. Powders were not etched. 200W - laser power in watts designated by "W". 100mmps - laser speed in millimeters per second designated by "mmps". 01 - test number 1. * The CSV files in Calorimetry.zip contain columns for time (milliseconds) and temperature (Celsius). * Any files ending in ".lvm" are readable via Notepad and can be converted to ".csv" using "name_change.m". * They can be processed using the MATLAB file called analyze_calorimetry.m, which implements calculations from Trapp et al., Appl. Mater. Today (2017) ([https://doi.org/10.1016/j.apmt.2017.08.006](https://doi.org/10.1016/j.apmt.2017.08.006)). Test numbers may start greater than 001, but there are at least three tests per sample type. #### Nanoindentation Nanoindentation.zip contains CSV files from nanoindentation experiments. Indentation data is collected using a KLA iMicro Nanoindenter as described in the Methods. File names are formatted as follows: test_00x.csv where "00x" indicates the indentation number. For example, "test_003.csv" indicates the 3rd indentation experiment. Columns in the CSV files include: * load - in millinewtons * depth - in micrometers * time - in seconds Data can be processed using "analyze_indentation.m" which implements Oliver and Pharr, JMR (2004) ([https://doi.org/10.1557/jmr.2004.19.1.3](https://doi.org/10.1557/jmr.2004.19.1.3)). #### Tomography Raw tomography data from synchrotron X-ray imaging at SLAC and lab-scale X-ray imaging (at LLNL) is in the form of STL files, raw figure slices to be reconstructed with software of choice, and ORSSession files. The latter two file types can be opened and processed using the free academic version of DragonFly. Files are named with the following format: Tomography_Cu10_100W_300mmps-timestamp_00x where: * Cu - indicates copper. * 10 - indicates the etch time. * 100W - indicates the laser power in watts as "W". * 300mmps - indicates laser speed in millimeters per second. * 00x - here x indicates the index for large datasets that had to be split into multiple zip files. Once unzipped, these datasets (image slices) can simply be concatenated by copying them into the same directory. ## Sharing/Access Information All data is available here. If anything appears to be missing, please contact the corresponding author. ## Code/Software Each zip file includes the relevant MATLAB script for processing all CSV or LVM data types as indicated above. Comments are provided throughout the scripts for 1) loading raw data, 2) processing and performing relevant calculations, and 3) creating and exporting figures and cleaned/processed datasets.