Researchers from Texas A&M University, in collaboration with scientists in the Air Force Research Laboratory, have developed guidelines that allow 3D printing of martensitic steels into very sturdy, defect-free objects of nearly any shape. A paper on their work is published in the journal Acta Materialia.

Steels are made of iron and a small quantity of other elements, including carbon. Martensitic steels are formed when steels are heated to extremely high temperatures and then rapidly cooled. The sudden cooling unnaturally confines carbon atoms within iron crystals, giving martensitic steel its signature strength.

A schematic summarizing the framework of combined modeling, experiments, and fabrication to optimize the selected laser melting (SLM) additive manufacturing process parameters to achieve porosity-free bulk parts. Seede et al.

Martensitic steels have gained renewed interest recently for their use in automotive, aerospace, and defense applications due to their ultra-high yield strengths and reasonable ductility. A recently discovered low alloy martensitic steel, AF9628, has been shown to exhibit strengths greater than 1.5 GPa with more than 10% tensile ductility, due to the formation of ε-carbide phase.

In an effort to produce high strength parts with a high degree of control over geometry, the work herein presents the effects of selective laser melting (SLM) parameters on the microstructure and mechanical properties of this new steel. An optimization framework to determine the process parameters for building porosity-free parts is introduced.

—Seede et al.

To have diverse applications, martensitic steels, particularly a type called low-alloy martensitic steels, need to be assembled into objects of different shapes and sizes depending on a particular application. That’s when additive manufacturing (3D printing) provides a practical solution. Using this technology, complex items can be built layer by layer by heating and melting a single layer of metal powder along a pattern with a sharp laser beam. Each of these layers joined and stacked creates the final 3D-printed object.

However, 3D printing martensitic steels using lasers can introduce unintended defects in the form of pores within the material.

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. 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.

—Dr. Ibrahim Karaman, Chevron Professor I and head of the Department of Materials Science and Engineering

For their experiments, Karaman and the Texas A&M team first chose an existing mathematical model inspired from welding to predict how a single layer of martensitic steel powder would melt for different settings for laser speed and power. 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.

Martensite steel powder used for 3D printing. Inset shows a zoomed-in view of the steel powder. Raiyan Seede, Microstructural Engineering of Structural and Active Materials Group

After a few such iterations, their framework could correctly forecast, without needing additional experiments, if a new, untested set of laser settings would lead to defects in the martensitic steel. The researchers said this procedure is more time-efficient.

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. By combining experiments and modeling, 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.

—Raiyan Seede, first author

Seede also noted that although their guidelines were developed to ensure that martensitic steels can be printed devoid of deformities, their framework can be used to print with any other metal. He said this expanded application is because their framework can be adapted to match the observations from single-track experiments for any given metal.

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. 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 we started with a focus on 3D printing of martensitic steels, we have since created a more universal printing pipeline. 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.

—Dr. Ibrahim Karaman

Resources

• Raiyan Seede, David Shoukr, Bing Zhang, Austin Whitt, Sean Gibbons, Philip Flater, Alaa Elwany, Raymundo Arroyave, Ibrahim Karaman (2020) “An ultra-high strength martensitic steel fabricated using selective laser melting additive manufacturing: Densification, microstructure, and mechanical properties,” Acta Materialia, Volume 186, Pages 199-214 doi: 10.1016/j.actamat.2019.12.037