Research on metal additive manufacturing has found a way to print 3D steel free of porosity. Of all the combinations and variants of modern steel, martensite shines because of its high strength, ductility, relatively low weight, and cost-effective production. However, 3D printing of complex structures can have an effect on the strength and durability of any material, so, the researchers from Texas A&M University find a solution for this.
The research team developed a set of guidelines and parameters that made it possible to manufacture low alloy martensite additives (AF9628) into defect-free parts without sacrificing geometric freedom. AF9628 usually shows strength greater than 1.5GPa with a tensile ductility of 10%, so matching this using SLM will prove difficult.
The challenge of 3D metal printing
The fusion of metal bed powder, while offering unmatched design freedom, can result in the formation of damaged pores in the parts produced called porosity. According to Dr. Ibrahim Karaman, the author of this study, porosity can sharply reduce the strength of 3D printed parts, even if the raw material is strong.
Develop a framework for AM martensites
To prevent the formation of defects during AM, the team first chose a computational, inexpensive welding-inspired mathematical model, the Eagar-Tsai model, to predict the melting pond geometry from a single layer of martensitic powder for various laser settings. They compare the results of the model predictions with the actual defects of the experiments, and tweak the models to predict the next layer better. After many improvements and iterations, the team tested various process parameters, and formed an SLM processing map for AF9628. Geometric criteria to precisely determine the maximum distance between hatch lines are also developed, ensuring the team avoids defects caused by inadequate fusion between layers.
Raiyan Seede, a graduate student and co-author of this study, explains: “Testing the entire range of possible laser settings to evaluate which ones can cause damage is very time consuming, and sometimes, even impractical. By combining experimentation and modeling, we can develop simple, fast, step-by-step procedures that can be used to determine which settings are most suitable for 3D printing of martensitic steel. “
Using their framework, 3D researchers scored martensite with a tensile strength of 1.4GPa and an extension of 11%. According to the study, the tensile strength of martensitic steel is the highest reported to date for all 3D molded alloys – an impressive achievement. The research team then developed a clever approach to process parameter improvements so that it also applies to other metals and alloys.
Karaman concluded, “Although we started with a focus on 3D printing of martensitic steel, we have since created more universal printing pipelines. Also, our guidelines simplify 3D metal printing art so that the final product is without porosity, which is an important development for all types of metal additive manufacturing industries that make components as simple as screws become more complex such as landing gear, gearboxes or turbines. “
Further details of the research and findings can be found in a paper titled ‘Very high strength martensitic steel made using selective laser melting additive manufacturing: Densification, microstructure, and mechanical properties‘. This was co-written by Raiyan Seede, David Shoukr, Bing Zhang, Austin Whitt, Sean Gibbons, Philip Flater, Alaa Elwany, Raymundo Arroyave, and Ibrahim Karaman.
Researchers from Texas A&M University have previously studied the science behind 3D printing technology with a number of studies. Earlier this year, a research team from the University examined various factors that can have an impact on the success of FDM print, i.e. nozzle diameter and layer thickness. Last year, different researcher ensembles from the University the developed bioink enables printing of 3D scaffold for protein transportation for therapeutic purposes.
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The picture shown shows the melee martensit powder. Picture via Raiyan Seede.
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