Researchers at Texas A&M University, USA, in collaboration with scientists at the US Air Force Research Laboratory, have reportedly developed a process that allows the Additive Manufacturing of martensitic steels into sturdy, defect-free objects of nearly any shape.
Martensitic steels naturally lend themselves to applications in the aerospace, automotive and defence industries, among others, where high-strength, lightweight parts need to be manufactured without adding to part cost. However, for these and other applications, the metals must to be able to be built into complex structures with minimal loss of strength and durability.
“Strong and tough steels have tremendous applications but the strongest ones are usually expensive – the one exception being martensitic steels that are relatively inexpensive, costing less than a dollar per pound,” stated Ibrahim Karaman, Chevron Professor I and head of the Department of Materials Science and Engineering. “We have developed a framework so that 3D printing of these hard steels is possible into any desired geometry and the final object will be virtually defect-free.”
Although the procedure developed was initially for martensitic steels, researchers from the Texas A&M said they have made their guidelines general enough so that the same AM pipeline can be used to build intricate objects from other metals and alloys. The findings of the study were reported in the December issue of Acta Materialia.
Martensite steels and metal Additive Manufacturing
Martensite steels are formed when steels are heated to extremely high temperatures before being rapidly cooled. The sudden cooling unnaturally confines carbon atoms within iron crystals, giving martensitic steel its signature strength. To meet the needs of a diverse field of applications, martensitic steels, particularly low-alloy martensitic steels, must be capable of being formed into a wide range of shapes and sizes depending on requirements.
Additive Manufacturing is the ideal solution for the production of complex-shaped components, but the AM of martensitic steels using laser-based processes such as Laser Powder Bed Fusion (L-PBF) can introduce defects such as pores within the material.
Additive Manufacturing is the ideal solution for the production of complex-shaped components, but the AM of martensitic steels using laser-based processes such as Laser Powder Bed Fusion (L-PBF) can introduce defects such as pores within the material.Click to Tweet
“Porosities are tiny holes that can sharply reduce the strength of the final 3D printed object, even if the raw material used for the 3D printing is very strong,” explained Karaman. “To find practical applications for the new martensitic steel, we needed to go back to the drawing board and investigate which laser settings could prevent these defects.”
Predicting and preventing defects in additively manufactured martensitic steels
For their experiments, Karaman and the Texas A&M team first used an existing mathematical model inspired by one used in welding to predict how a single layer of martensitic steel powder would melt at different laser speed and power settings. By comparing the type and number of defects they observed in a single track of melted powder with the model’s predictions, they were able to change their existing framework slightly so that subsequent predictions improved.
After a few such iterations, it was stated that their framework could correctly forecast – without additional experiments – whether a new, untested set of laser settings would lead to defects in the martensitic steel builds. This procedure is more time-efficient, the researchers explained:
“Testing the entire range of laser setting possibilities to evaluate which ones may lead to defects is extremely time-consuming, and at times, even impractical,” stated Raiyan Seede, a graduate student in the College of Engineering and the primary author of the study. “By combining experiments and modelling, we were able to develop a simple, quick, step-by-step procedure that can be used to determine which setting would work best for 3D printing of martensitic steels.”
Seede also noted that, although their guidelines were developed to ensure that martensitic steels can be additively manufactured devoid of deformities, their framework can be used for the AM of any other metal.
“Although we started with a focus on 3D printing of martensitic steels, we have since created a more universal printing pipeline,” Karaman said. “Also, our guidelines simplify the art of 3D printing metals so that the final product is without porosities, which is an important development for all type of metal Additive Manufacturing industries that make parts as simple as screws to more complex ones like landing gears, gearboxes or turbines.”
Further contributors to the study included Austin Whitt and Raymundo Arróyave from the Department of Materials Science and Engineering; David Shoukr, Bing Zhang and Alaa Elwany from the Department of Industrial and Systems Engineering; and Sean Gibbons and Philip Flater from the Air Force Research Laboratory, Florida. The research was funded by the Army Research Office and the Air Force Research Laboratory.