Metal powder recyclability in binder jet additive manufacturing

The recyclability of 316L stainless steel powder in the binder jetting process has been determined. The powder characterization results demonstrated a 22% increase in the number of coarse particles (> 30 µm) and an 18.2% reduction in the number of small particles (< 10 µm) after recycling up to 16 times. A few elongated and irregular-shaped particles were found after recycling, possibly due to particle agglomeration during handling and sieving. A negligible increase in the oxygen content by 0.036% was detected in the recycled powder. The density of sintered parts produced using recycled powder was approximately 1.5% lower than when using fresh powder due to the changes in the particle size distribution and the flowability of the powder caused by the changes in morphology. Final parts built using fresh and recycled powder showed similar hardness (155 ± 3 HV and 165 ± 9 HV) and yield strength (206 ± 16 MPa and 192 ± 10 MPa), respectively

Selective laser melting and tempering of H13 tool steel for rapid toolingg applications

H13 components with a relative density of ∼99% were additively manufactured using the selective laser melting (SLM) process. The highest density part (relevant density 99%) with the lowest level of porosity (<0.01%) was made with a volumetric energy density of 760 J/mm3 (152 W laser power, 100 mm/s scanning speed, 40 μm hatch spacing, and 50 μm layer thickness). Wrought and additively manufactured samples underwent tempering at 550, 600, and 650 °C for 2 h followed by furnace cooling. Additively manufactured samples and wrought H13 samples that were austenitized followed by water quenching were martensitic with similar microhardness values of 708.4 ± 25.0 HV and 708.1 ± 12.6 HV, respectively. A tempered martensitic structure was observed in SLM-manufactured and tempered samples. Samples that were additively manufactured and tempered at 550 °C showed higher microhardness (728.5 ± 28.2 HV) than non-tempered SLM-manufactured samples due to an upward shift in the secondary hardening phase. Tempering at 600 and 650 °C resulted in coarsening of the carbides and martensite, which led to a reduction in microhardness. Additively manufactured samples maintained higher microhardness values than wrought H13 samples at all tempering temperatures, likely because of higher dislocation density, finer grains present, and higher volume fraction of carbide nanoparticles

Additive Manufacturing of ODS Steels Using Powder Feedstock Atomized with Elemental Yttrium

This study investigates the microstructure and mechanical properties of an austenitic ODS steel produced by the Laser Directed Energy Deposition (LDED) process using powder feedstock atomized with elemental yttrium. The Microstructure of the samples was characterized by electron microscopy, and mechanical properties were measured using a tensile test and nanoindentation. Further, the thermal stability of the LDEDproduced ODS steels were evaluated. As-printed samples showed a cellular structure with Si-Mn-Y-Oenriched nanoparticles that were found to be amorphous. After 100 hours at 1000°C in an argon atmosphere, a partially recrystallized microstructure with a decrease in the number density of Y-O-enriched nanoparticles with crystalline structure was revealed. The as-printed (600 W, 600 mm/min) samples exhibited an ultimate tensile strength of 774 MPa and an elongation at a break of 22%. A lower ultimate tensile strength of 592 MPa and higher elongation of 42% was measured after 100 hours at 1000°C.Mechanical Engineerin

In-situ manufacturing of ODS FeCrAlY alloy via laser powder bed fusion

Gas-atomized Fe–24Cr–8Al–0.5Y (wt.%) powder was used as a feedstock in a laser powder bed fusion process with nitrogen atmosphere. Formation of Al–Y–O-enriched nanoparticles with diameters of 10–100 nm implied in-situ precipitation of Al–Y–O-enriched nanoparticles within the ferritic matrix developing an oxide dispersion strengthened FeCrAl alloy without any mechanical alloying. Thermodynamics and kinetics of the internal oxidation during rapid solidification of laser powder bed fusion process are discussed

Effects of atomizing media and post processing on mechanical properties of 17-4 PH stainless steel manufactured via selective laser melting

Water-atomized and gas-atomized 17-4 PH stainless steel powder were used as feedstock in selective laser melting process. Gas atomized powder revealed single martensitic phase after printing and heat treatment independent of energy density. As-printed water atomized powder contained dual martensitic and austenitic phase regardless of energy density. The H900 heat treatment cycle was not effective in enhancing mechanical properties of the water-atomized powder after laser melting. However, after solutionizing at 1315 degrees C and aging at 482 degrees C fully martensitic structure was observed with hardness (40.2 HRC), yield strength (1000 MPa) and ultimate tensile strength (1261 MPa) comparable to those of gas atomized (42.7 HRC, 1254 MPa and 1300 MPa) and wrought alloy (39 HRC, 1170 MPa and 1310 MPa), respectively. Improved mechanical properties in water-atomized powder was found to be related to presence of finer martensite and higher volume fraction of fine Cu-enriched precipitates. Our results imply that water-atomized powder is a promising cheaper feedstock alternative to gas-atomized powder

Oxide Dispersion Strengthened Nickel Based Alloys via Spark Plasma Sintering

Oxide dispersion strengthened (ODS) nickel based alloys were developed via mechanical milling and spark plasma sintering (SPS) of Ni–20Cr powder with additional dispersion of 1.2 wt% Y2O3 powder. Furthermore, 5 wt% Al2O3 was added to Ni–20Cr–1.2Y2O3 to provide composite strengthening in the ODS alloy. The effects of milling times, sintering temperature, and sintering dwell time were investigated on both mechanical properties and microstructural evolution. A high number of annealing twins was observed in the sintered microstructure for all the milling times. However, longer milling time contributed to improved hardness and narrower twin width in the consolidated alloys. Higher sintering temperature led to higher fraction of recrystallized grains, improved density and hardness. Adding 1.2 wt% Y2O3 to Ni–20Cr matrix significantly reduced the grain size due to dispersion strengthening effect of Y2O3 particles in controlling the grain boundary mobility and recrystallization phenomena. The strengthening mechanisms at room temperature were quantified based on both experimental and analytical calculations with a good agreement. A high compression yield stress obtained at 800 °C for Ni–20Cr–1.2Y2O3–5Al2O3 alloy was attributed to a combined effect of dispersion and composite strengthening

Selective laser melting of 304L stainless steel: Role of volumetric energy density on the microstructure, texture and mechanical properties

The role of volumetric energy density on the microstructural evolution, texture and mechanical properties of 304L stainless steel parts additively manufactured via selective laser melting process is investigated. 304L is chosen because it is a potential candidate to be used as a matrix in a metal matrix composite with nanoparticles dispersion for energy and high temperature applications. The highest relative density of 99 % ± 0.5 was achieved using a volumetric energy density of 1400 J/mm3. Both XRD analysis and Scheil simulation revealed the presence of a small trace of the delta ferrite phase, due to rapid solidification within the austenitic matrix of 304L. A fine cellular substructure ranged between 0.4–1.8 μm, was detected across different energy density values. At the highest energy density value, a strong texture in the direction of [100] was identified. At lower energy density values, multicomponent texture was found due to high nucleation rate and the existing defects. Yield strength, ultimate tensile strength, and microhardness of samples with a relative density of 99 % were measured to be 540 ± 15 MPa, 660 ± 20 MPa and 254 ± 7 HV, respectively and higher than mechanical properties of conventionally manufactured 304L stainless steel. Heat treatment of the laser melted 304L at 1200 °C for 2 h, resulted in the nucleation of recrystallized equiaxed grains followed by a decrease in microhardness value from 233 ± 3 HV to 208 ± 8 HV due to disappearance of cellular substructure

Oxide dispersion strengthened 304L stainless steel produced by ink jetting and laser powder bed fusion

This paper discusses the fundamentals of a novel hybrid method to synthesize oxide dispersion strengthened (ODS) 304 L stainless steel (SS) alloy using a modified laser powder bed fusion (LPBF) machine. Previously, ODS metal matrix composites have been produced by LPBF via ball-milling, which is expensive to scale. Here, we selectively dope yttria nanoparticles into a SS matrix by jetting a precursor chemistry onto the SS substrate prior to laser conversion and consolidation. The new alloy shows good room temperature mechanical properties. Microstructures are studied using electron microscopy, energy dispersive spectroscopy and electron backscatter diffraction

Selective laser melting of austenitic oxide dispersion strengthened steel: Processing, microstructural evolution and strengthening mechanisms

Oxide dispersion strengthened (ODS) alloys exhibit superior mechanical properties due to the presence of nano-sized thermally stable oxide particles. However, manufacturing of ODS alloys is very complex and composed of numerous time consuming steps such as mechanical alloying, which is one of the main barriers toward the widespread application of ODS alloys. Light mixing of 304L stainless steel powder with sub-micrometer size yttria particles was coupled with selective laser melting (SLM) to produce 304L ODS nanocomposite. The added yttria was dissolved in the matrix due to the high intensity of the laser and altered the rheological properties of the melt and caused balling effect. The SLM 304L ODS alloy presented cellular substructure with a uniform dispersion of yttrium silicate (Y–Si–O) spherical nanoparticles, range 10–80 nm. As a result, the SLM 304L ODS alloy showed a high ultimate tensile strength of ~700 MPa, ductility of ~32% and microhardness of ~350 HV. The underlying mechanism for this strength and ductility improvement are identified. This study provides deep insight into an alternative method of producing ODS alloys with fewer steps and capable of manufacturing complex design geometries