A great deal of work is involved in optimizing materials for additive manufacturing. Porosity is a consistent problem in metal 3D printing, and scientists spend a lot of time studying each metal material to try to minimize or eliminate flaws. In a paper entitled “Microstructure, Solidification Texture, and Thermal Stability of 316 L Stainless Steel Manufactured by Laser Powder Bed Fusion,” a team of researchers examines 316 L stainless steel using techniques including scanning and transition electron microscopy, diffraction methods and atom probe tomography.
Porosity can be eliminated by controlling the laser power and laser scanning speed during the 3D printing process, the researchers point out.
“The final properties are governed by the microstructure of the material,” they continue. “The microstructure of the LPBF material is formed under the conditions of high temperature gradients and solidification rates, far from the ones of conventional materials. This results in the formation of a nonequilibrium microstructure with a unique set of properties. Epitaxial nucleation of cellular colonies has commonly been observed, which results in the solidification texture and anisotropic mechanical properties of LPBF materials.”
The study, which was conducted over several years, focuses on the metallurgical aspects of the material, as well as its microstructure. The formation of a cellular structure in a molten pool was discussed in relation to the thermal gradient and solidification rate. The correlation between the primary cell spacing and hardness was also discussed in relation to additive manufacturing process parameters and the presence of porosity.
Several experiments were carried out with the stainless steel material. Specimens were additively manufactured using a Phenix Systems PM 100 machine. For the microstructural analysis, parameters of 50 W laser power and a 120 mm/s laser scanning speed were used because they provided the lowest porosity. Microstructural analysis was performed using optical and electron microscopy methods.
Several conclusions were reached. The as-built microstructure of the stainless steel consists of colonies of cells, and the boundaries between the cells are not regular high-angle grain boundaries, but rather dislocation structures of 100-300 nm in thickness. The size of the cells in the colonies depends on the manufacturing conditions, and may vary within a single track.
“The segregation of elements on the cell boundaries is presumably a function of the solidification conditions, and it may vary in AM 316 L manufactured at different laser powers and scanning speeds,” the researchers state. “Primary cell spacing is the key parameter that controls strength, following the Hall–Petch relationship. In many cases, deviations from the Hall–Petch relationship can be explained by variations of the primary cell spacing through the LPBF material and porosity.”
Solidification texture was formed by colonies of cells that grew through several layers. The texture was controlled by the manufacturing strategy. Cells within colonies were stable up to 800-900°C, after which point they disappeared. The disappearance of the cells resulted in a decrease in hardness. Colony growth was not significant until 1050 °C.
“Nanoscale oxide particles probably form from surface oxide, or due to oxygen pick up during manufacturing,” the researchers continue. “They are stable and do not coalesce or change shape after heat treatment up to 1050 °C. The contribution of these nanoscale particles to hardness of LPBF 316 L material seems to be insignificant, since after heat treatment the hardness of LPBF 316 L steel approached values typical for conventional coarse-grained material.”
Authors of the paper include Pavel Krakhmalev, Gunnel Fredriksson, Krister Svensson, Igor Yadroitsev, Ina Yadroitsava, Mattias Thuvander, and Ru Peng.
Discuss this and other 3D printing topics at 3DPrintBoard.com or share your thoughts below.
Subscribe to Our Email Newsletter
Stay up-to-date on all the latest news from the 3D printing industry and receive information and offers from third party vendors.
You May Also Like
Biden Admin Announces Flurry of Reshoring Actions, Including Council on Supply Chain Resilience
Yesterday, I posted about the over $6 billion in new funding for US advanced manufacturing related to batteries and semiconductor packaging announced by the Biden administration to be awarded in...
Biden Admin Announces Over $6 Billion in Funding for Battery and Chip Advanced Manufacturing in November
In a development that has become routine for the Biden administration, the White House announced a total of more than $6 billion in new funding opportunities for advanced manufacturing applications...
America Makes and NCDMM Funding Now Tops $729M for American 3D Printing and Beyond
In a significant move to advance the U.S. manufacturing sector, the National Center for Defense Manufacturing and Machining (NCDMM) and America Makes have recently announced substantial ceiling increases on their...
US Army Reserve Test 3D Printed Explosives
In October 2023, US Army Reserve Soldiers assigned to the 102nd Training Division (Maneuver Support) attended a two-week Combat Engineer Reclassification Course at Fort Leonard Wood in Missouri, where they...
Upload your 3D Models and get them printed quickly and efficiently.