There are many different types of stainless steel, classified by four basic types: austenitic, ferritic, martensitic, and duplex. Martensitic is a type of steel alloy characterized by high hardness and toughness and is often used in applications that require corrosion resistance. Because it is cheap, lightweight, and high strength, martensitic steel is used in many industries, including aerospace, automotive, and defense.
Researchers from Texas A&M University and the Air Force Research Laboratory are developing ways to be able to use 3D printing of martensitic steel. According to Dr. Ibrahim Karaman, Chevron Professor and head of the Department of Materials Science and Engineering, says, “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."
Martensite is formed by heating the steel to very high temperatures and then rapidly cooling it (via air, oil, water). But in order to have more diverse applications, 3D printing (also called additive manufacturing) can provide a means. 3D printing works by building an object layer by layer. Using this technology allows scientists to build complex items one layer at a time by using a laser to heat a layer of metal powder. When these layers are stacked together and joined, they produce a 3D object. Using lasers to heat the metal, however, can cause defects in the metal, called pores.
"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," said 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."
The researcher’s approach to determining how many defects were created by the laser was simple. They used a model that’s used by welders. They used different speeds and powers of lasers and then examined the changes in the melted powder.
"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," said Raiyan Seede, a graduate student in the College of Engineering and the primary author of the study. "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."
The findings of the study are published in Acta Materialia.