Plenty of 3D printing research has been completed over the years on metal materials, which are decidedly strong. One adjective we also hear when describing metal 3D printing materials is ductile, meaning the ability to deform under tensile stress. Usually we talk about metal as being either one or the other, but typically not both, and certainly not in terms of 3D printing. But thanks to a team of researchers from Stockholm University in Sweden, the University of Birmingham in the UK, and China’s Zhejiang University, a new metal 3D printing technique can be used to manufacture metals that have both excellent ductility and strength, compared to metal parts made using more traditional manufacturing methods.
“Strength and ductility are natural enemies of one another, most methods developed to strengthen metals consequently reduce ductility,” explained Dr. Leifeng Liu, the project head and an AMCASH research fellow at the University of Birmingham. “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.”
The joint research team recently published the findings on their new SLM method, which involves a popular stainless steel, in a paper, titled “Dislocation network in additive manufactured steel breaks strength-ductility trade-off,” in the Materials Today journal; co-authors include Dr. Liu, Qingqing Ding, Yuan Zhong, Ji Zou, Jing Wu, Yu-Lung Chiu, Jixue Li, Ze Zhang, Qian Yu, and Zhijian Shen.
According to the paper’s abstract, “Most mechanisms used for strengthening crystalline materials, e.g. introducing crystalline interfaces, lead to the reduction of ductility. An additive manufacturing process – selective laser melting breaks this trade-off by introducing dislocation network, which produces a stainless steel with both significantly enhanced strength and ductility. Systematic electron microscopy characterization reveals that the pre-existing dislocation network, which maintains its configuration during the entire plastic deformation, is an ideal ‘modulator’ that is able to slow down but not entirely block the dislocation motion. It also promotes the formation of a high density of nano-twins during plastic deformation. This finding paves the way for developing high performance metals by tailoring the microstructure through additive manufacturing processes.”
The researchers were able to optimize the process parameters during 3D printing to achieve their results, which could help accelerate the technology toward manufacturing strong and ductile heavy-duty parts.
The research team’s ultrafast cooling rate, which ranges from about 1,000°C to 100,000,000°C per second, is why their new method of 3D printing strong, ductile materials works. Until the advent of 3D printing technology, this high level of cooling was not possible to achieve in bulk metal production processes. When metal materials are cooled down that rapidly, they end up in what’s known as a non-equilibrium state, which makes it possible to produce unique microstructures, such as the sub-micro dislocation network. According to the research paper, this network is the main factor behind these improved mechanical qualities.
“This work gives researchers a brand new tool to design new alloy systems with ultra-mechanical properties,” said Dr. Liu. “It also helps metal 3D printing to gain access into the field where high mechanical properties are required like structural parts in aerospace and automotive industry.”
Dr. Chiu, who is in charge of the Centre for Electron Microscopy at the University of Birmingham’s School of Metallurgy and Materials, set up a micro and nano material testing system inside the university’s electron microscopes. This system, which allows users to analyze a metal sample’s mechanism and performance in-situ during mechanical tests, helps identify effective microstructural features for better properties, as well as help researchers better understand the mechanisms.
As metal 3D printing continues to quickly progress toward widespread industrial application, metal 3D printed products have come under intense scrutiny and skepticism. The technology is not affordable to all, and metal powder 3D printing can cause defects, which deteriorate a product’s mechanical properties. Because the industry’s annual global revenue is estimated to hit over $20 billion by 2025, these types of research projects into the qualities of 3D printing materials are increasingly important.
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