Laser Alloying Advantages by Dry Coating Metallic Powder Mixtures with SiOx Nanoparticles

Abstract

Up to now, minimizing segregation of free-flowing, microscale metal powder mixtures driven by different mass density is an open challenge. In this work, effects of particle size variation on homogeneity of Al-Cu mixtures, with a density ratio of 3.3, are examined. Dry coating Al particles with 0.3 wt% fumed silica SiOx nanoparticles significantly decreases interparticle attraction. This enlarges the range of free-flowing Al particle sizes to < 20 µm. Powder mixture homogeneity is examined optically in vibrated bulk powder and thinly spread layers. From various powder mixtures, solid samples are built layer by layer with the Additive Manufacturing (3D printing) technology Laser Beam Melting in metal powder bed (LBM). Chemical homogeneity of solids is evaluated via energy-dispersive X-ray spectroscopy, backscattered electron microscopy, metallographic analysis and tensile tests. Persistent homogeneity of Al-Cu powder mixtures and LBM solids is found only with particles < 20 µm dry coated with SiOx nanoparticles. Observed segregation phenomena are explained with a decrease in particle mobility at increasing local concentration and the decreasing effectiveness of mass in smaller particles. The main effects are based on geometry, so they are expected to be transferrable to other nanoparticles, alloying components and powder bed technologies, e.g., binder jetting.

Grain Structure Evolution of Al–Cu Alloys in Powder Bed Fusion with Laser Beam for Excellent Mechanical Properties

“…Copper-enrichment resulted in increased compressive strength, while the addition of up to 5 wt.% TiB 2 entailed in improved compressive yield strength. Another in-situ alloying approach was carried out by Karg et al [23]. By mixing four different base powders the alloy composition was created homogeneously in-situ.…”

Laser-based powder bed fusion of Ti-6Al-4V powder modified with SiO2 nanoparticles

“…To improve the mixing and in situ alloying of Al-Cu powder mixtures, Karg et al modified the powder mixture with SiO 2 nanoparticles. The stabilization against segregation for small particles < 20 µm led to high homogeneity of the chemical elements in the processed specimens that was also supported by tensile tests [23]. In another work, Karg et al used drycoating with SiO 2 nanoparticles to improve the flowability, measured by the static angle of repose, and layer deposition of Al-Si powder.…”

Laser Powder-Bed Fusion as an Alloy Development Tool: Parameter Selection for In-Situ Alloying Using Elemental Powders

“…Also, if one of the elemental powders shows an irregular surface or morphology, its poor flowability may lead to heterogeneous spreading [ 13 ]. The poor flowability and density difference between elemental powders may be overcome by using finer particles (<20 μm) coated with SiOx nanoparticles, which act as spacers and reduce the attraction between powder particles [ 33 , 53 ]. Note that the simple rolling mixer used in this study is not an ideal way to produce homogeneous powder blends, so better mixers are expected to achieve a higher homogeneity in a much shorter time.…”

“…They were able to achieve chemical homogeneity after isothermal annealing at 1310 °C for 90 min. Karg et al [ 33 ] also report an improvement in chemical homogeneity after heat treatment in their in-situ alloyed aluminum alloy. Dry coating Al powder particles with SiOx nanoparticles allowed them to use a larger fraction of the powder size fraction with particles smaller than 20 µm also being usable due to reduced interparticle attraction which results in poor flowability.…”

“…Several works have shown that samples with relative densities >98% can be manufactured using mixtures of elemental powders and in-situ alloying during LPBF processing [ 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 ]. However, chemical inhomogeneity in the as-produced samples was reported by the researchers for many different materials including Ti alloys [ 24 , 25 , 29 ], Al alloys [ 30 , 31 , 33 ], high entropy alloys [ 27 ], and austenitic steels [ 32 ]. Grigoriev et al [ 24 ] reported unmelted and partially melted Al and Nb particles in the as-produced samples.…”