In material development, strength and ductility can be considered as opposite ends of a rope. As the level of one property improves, the quality of the other lessens. So it is in the interest of metallurgists to challenge this balance, and find ways to make metals that are strong, yet easy to work with.
A recent study from the University of Birmingham in the UK has found that 3D printing can instil ultra-mechanical properties in a popular steel alloy. Focusing on selective laser melting (SLM) the researchers, in collaboration with Stockholm University and Zhejiang University, China, have made a discovery that could usher in the “the next generation of structural alloys.”
Dr. Leifeng Liu et al’s research performs systematic electron microscope analysis of the molecules in melted and solid 3D printed steel.
The rapid cooling rate required to deposit metal layer by layer (between 1000 °C and 100 million°C per second) means that the crystalline structure of solid 3D printed steel is irregular, “dislocated”, instead of uniform, as it would be when processed conventionally. The ultrafast cooling process creates a so-called non-equilibrium state in the alloy – a solid state in which molecules are non uniform.
The discovery in this research is that dislocated molecules retain a pre-existing network structure throughout solidification, which functions to modulate irregularities in the material. By tuning this structure, metallurgists could effectively “program” steel molecules to make high-strength, ductile product, suitable for use in high-value, high-performance applications.
A new tool for alloy design
Dr. Liu is a research fellow in Birmingham’s Advanced Materials Characterization and Simulation Hub (AMCASH). In a statement about the the research Dr. Liu explains, “The 3D printing technique is known to produce objects with previously inaccessible shapes and our work shows that it also provides the possibility to produce the next generation of structural alloys with significant improvements in both strength and ductility,”
“This work gives researchers a brand new tool to design new alloy systems with ultra-mechanical properties.”
Dislocation network in additive manufactured steel breaks strength–ductility trade-off is published online in the journal Material Today. It is co-authored by Leifeng Liu, Qingqing Ding, Yuan Zhong, Ji Zou, Jing Wu, Yu-Lung Chiu, Jixue Li, Ze Zhang, Qian Yu, and Zhijian Shen.
Nominate the best research of the year in the 3D Printing Industry Awards 2018.
Featured image shows a 3D printed steel part for nuclear fusion test reactor. Photo by Dr. Leifeng Liu/University of Birmingham