A breakthrough in high-strength 3D printing martensitic steel technology! This technology is the result of a collaboration between the Texas A & M University School of Engineering and scientists from the US Air Force Research Laboratory and may be used in the aerospace, automotive, and defense industries.
For thousands of years, metallurgists have been carefully adjusting the composition of steel to enhance its performance. To this day, there is a product called martensitic steel that stands out in its steel category because of its higher strength and more cost-effectiveness.
Martensitic steel is very suitable for aerospace, automotive, and defense industries that need to manufacture high-strength, lightweight parts without increasing costs. However, for these and other applications, the metal must be built into a complex structure while minimizing the loss of strength and durability. Researchers at Texas A & M University, in collaboration with scientists in the Air Force Research Laboratory, have now explored a process that can 3D print martensitic steel to very strong, defect-free parts of almost any shape.
A low alloy martensitic steel, AF9628, due to the formation of ε-carbide phase, its strength is greater than 1.5 GPa, and its tensile ductility exceeds 10%. The effect of selective laser melting (SLM) on the structure and mechanical properties of this new type of steel was studied. An optimized process for determining the manufacturing process parameters of non-porous parts is introduced. This process utilizes the low-cost Eagar-Tsai model and calibrates it through a single-track experiment to predict the geometry of the molten pool. In order to avoid the porosity caused by fusion in the printed parts, a geometrical standard for determining the maximum allowable hatch spacing has also been developed. Using this process, full-density samples can be successfully manufactured on various process parameters, so that the AF9628 SLM process diagram can be constructed. The printed samples showed a tensile strength of up to 1.4 GPa, which is the highest strength reported to date in any 3D printed alloy, with an elongation of up to 11%. While maintaining full density, the flexibility shown in the selection of process parameters also provides the possibility of local microstructure improvement and parameter optimization to improve the mechanical properties of printed parts.
Ibrahim Karaman, professor of Chevron I and head of the materials department, said: "Tough steel has a wide range of applications, but the strongest steel is usually expensive. One exception is the relatively cheap martensitic steel, Pounds cost less than one dollar. "" We have developed a process so that these hard steels can be 3D printed into any desired geometry, and the final object is almost free of defects. "
Although the originally developed process was for martensitic steel, researchers at Texas A & M said they have made the technology versatile enough so that the same 3D printed pipeline can also be used to build complex objects from other metals and alloys. The results of this study were published in the "Acta Materialia" magazine in December 2019, entitled "An ultra-high strength martensitic steel fabricated using selective laser melting additive manufacturing: Densification, microstructure, and mechanical properties"
Steel is made of iron and a small amount of other elements (including carbon). When the steel is heated to an extremely high temperature and then rapidly cooled, martensitic steel is formed. The sudden cooling naturally confines the carbon atoms in the iron crystals, giving martensitic steel its signature strength.
In order to have multiple uses, it is necessary to assemble martensitic steel, especially a type called low-alloy martensitic steel, into objects having different shapes and sizes according to specific applications. At that time, additive manufacturing (often referred to as 3D printing) provided a practical solution. Using this technology, a single layer of metal powder can be heated and melted in a pattern by using a high-energy laser beam to build complex parts layer by layer. All these layers connected and stacked print the final 3D printed object.
However, 3D printing of martensitic steel using lasers can cause defects in the form of pores in the material.
Karaman said: "The pores are tiny holes. Even if the raw materials used for 3D printing are very strong, they can greatly reduce the strength of the final 3D printed object." At the source, research which laser settings can prevent these defects. "
For their experiments, Karaman and the Texas A & M team first selected an existing mathematical model inspired by welding to predict how a single layer of martensitic steel powder would melt under different laser speed and power settings. By comparing the type and number of defects they observed in a single molten powder with the predicted value of the model, they can slightly change their existing parameters to improve subsequent predictions.
After several such iterations, if a new set of untested laser settings will cause defects in the martensitic steel, then their process can be correctly predicted without additional experiments. Researchers say this will save time.
"Testing the entire range of laser settings to assess which settings may cause defects is very time-consuming and sometimes unrealistic," said Raiyan Seede, a graduate student in the School of Engineering and lead author of the study. . "By combining experimentation and modeling, we were able to develop a simple, fast, step-by-step procedure that can be used to determine which setting is best for 3D printing of martensitic steel."
Seede also pointed out that although guidelines have been developed to ensure that non-deformed martensitic steel can be printed, its process can be used to print with any other metal. He said that this extended application is because their frames can be adapted to the monorail experimental observations of any given metal.
Karaman said: “Although we started by focusing on 3D printing of martensitic steel, we have since created a more general printing solution.” “In addition, our guide simplifies the process of 3D printing metal, so that the final product does not Blowholes are an important development for all types of metal additive manufacturing industries, from simple parts such as screws to more complex parts such as landing gear and gearboxes or turbines. "
Other contributors to this research include Austin Whitt and Raymundo Arróyave of the Department of Materials Science and Engineering. David Shoukr, Bing Zhang and Alaa Elwany of the Department of Industrial and Systems Engineering; Sean Gibbons and Philip Flater of the Florida Air Force Research Laboratory.