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It is reported that researchers from the University of Pennsylvania, Stanford University, and Lawrence Livermore National Laboratory (LLNL) have reported the latest research on high-absorption nano-textured powders for metal 3D printing. The paper was published in Science Advances under the title "High absorptivity nanotextured powders for additive manufacturing".
Metal additive manufacturing (AM) has a wide range of application potential in the healthcare, aerospace, automotive, and energy industries. To date, the application of metal additive manufacturing has been limited to a small number of weldable materials that can be stably printed without extensive post-processing, such as stainless steel, AlSi10Mg, some nickel superalloys, and titanium alloys. These materials can be easily printed with commercial laser powder bed fusion (LPBF) systems, which typically use a near-infrared (wavelength of 1060 to 1080 nm) laser to scan over a metal powder layer to induce melting and fuse the powder. Repeating this process layer by layer results in a net-shaped three-dimensional (3D) printed structure. However, due to the photothermal properties of powder raw materials, free-form printing of highly reflective and refractory metals that are difficult to weld has been limited.
In their research, the researchers developed an etching process for the production of modified metal powder feedstocks, in particular for increasing the absorption rate. Nanoscale grooves are introduced into the surface of the metal powder, resulting in up to 70% higher powder absorption during the laser powder bed melting process. Wet chemical etching techniques are used to modify the surface of conventional metal powders to produce nanoscale surface features. Using in-situ calorimetry experiments, electromagnetic (EM) simulations of the surface of individual powder particles, and ray tracing simulations of powder beds, the researchers demonstrated that the absorption rate was increased due to the increased local absorption on the nanoscale features of the powder.
While the quality of the printed part is affected by many factors other than the absorption rate, scientists have demonstrated that these surface modified powders can be used to print high-purity copper and tungsten metal structures using lower power (100 to 500W) laser metal 3D printing systems. Increased absorption of copper, copper-silver, and tungsten enables energy-efficient manufacturing, with laser energy densities as low as 83 joules per cubic millimeter to print pure copper with a relative density of up to 92%. The method developed here makes it possible to print highly reflective and refractory pure metals that are difficult to weld while requiring the same energy as commercial printing alloys. The simulation results show that the plasmon resonance light aggregation in the nanoscale groove is combined with multiple scattering effects to enhance the total powder absorption. The method used in this paper demonstrates a general method to enhance the absorbency and printability of reflective and refractory metal powders by altering the surface morphology of the raw material without changing the composition of the raw material.
Etching produces nanoscale surface structures
Researchers prepared nano-textured copper powder, copper-silver powder and tungsten powder through batch solution process. Nano-textured copper powder was produced by etching purchased copper powder (LPW Technology Co., Ltd., 99.95% purity) and homemade copper powder (Lawrence Livermore National Laboratory (LLNL), 99.99% purity) using FeCl3, hydrochloric acid, and ethanol solutions. The results of LPW copper powder are mainly reported here.
Fig. 1.Surface topography changes of textured powders before and after etching.
The nano-textured surface improves powder absorption
Calorimetry experiments performed with a laser power of 175 W and a 1/e2 beam diameter of 60 μm showed that the absorption of the nanotextured powder was improved compared to the powder at the time of purchase. The effective absorption rate of Cu00 powder Aeff was 0.172 at 100 mm/s and 0.219 at 656 mm/s.
Figure 2.Experimental and simulated absorption enhancement of textured powders.
Nano-textured powder improves printing at low power
To evaluate the feasibility of high-absorption nanotextured powders for LPBF, the researchers printed cylindrical structures. Scientists quantify the relative density (i.e., solid volume fraction) as a function of the laser scanning parameters and combine it into the volumetric energy density. Studies have shown that at high power (more than 200W for stainless steel), the interaction between the laser and the powder becomes less important because the beam simply stays at the top of the melt pool and does not interact with the powder. The researchers' results are consistent with this observation, as the improvement in print quality is most pronounced at lower energy inputs.
Figure 3.Changes in the relative density and XCT density of the print volume.
Of the printing conditions explored, the scientists observed the greatest improvement in print quality using Cu10 powder, which can be quantified by relative density measurements. This improvement is most pronounced at low energy densities, where the surface structure of the powder is expected to play a greater role in the light-matter interaction. However, the absorption rate measurements showed that Cu05 had a higher absorption rate than Cu10. In addition to the absorption rate, the print quality is also affected by other factors. The higher absorption rate of Cu05 may cause the powder to be discharged from the laser path due to recoil pressure. This may result in more ablation or splashing and may manifest as a missing fusion defect observed here.
Despite these current limitations in understanding basic powder dynamics, the scientists demonstrated the utility of highly absorbing powders by printing structures including a 50 mm long triple periodic minimal surface at 100 W and 300 mm/s scan speed (Figure 4). Nano-textured powders reduce the energy density required to print copper to a level similar to that required for printing stainless steel and titanium alloys.
Figure 4.Low energy density printing of copper and example structures using textured powders.
The scientists demonstrated that the absorption rate of metal powder feedstock can be improved by self-evolving surface texture in etching solutions without alloying or using high-absorption nanoparticle additives. The increase in absorption is attributed to the localization of the incident light at nanoscale grooves on the powder surface, which are smaller than or equivalent to the laser wavelength and cause resonance. High absorption powders can be started printing at low energy densities (83 J/mm3). Under these printing conditions, a relative density of (≥0.92) copper was printed (not previously reported). The powder developed here can be used for printing in commercial LPBF systems at medium power (~400 W). These imperfectly structured powders deviate from the idealized, smooth spherical form sought after when creating the powder feedstock, but improve the photothermal efficiency and print quality of the manufacturing process. The general approach of the study exploits defects on the surface of the raw material to improve the interdependence between the laser and the material without modifying the laser or material composition.
Paper Links:
High absorptivity nanotextured powders for additive manufacturing
SCIENCE ADVANCES 4 Sep 2024 Vol 10, Issue 36
DOI: 10.1126/sciadv.adp0003
Reprinted by Chen Changjun of the Yangtze River Delta G60 Laser Alliance