Researchers from the City University of Hong Kong (CityU) have successfully developed a super-strong, highly ductile and super-light titanium-base alloy using Additive Manufacturing. Their findings have been published in a paper, ‘In situ design of advanced titanium alloy with concentration modulations by additive manufacturing’, in the scientific journal Science.
The research team was led by Professor Liu Chain-Tsuan, University Distinguished Professor in the College of Engineering and Senior Fellow of CityU’s Hong Kong Institute for Advanced Study (HKIAS). Dr Zhang Tianlong, a postdoc in the Department of Materials Science and Engineering (MSE), conducted the experiments. President Way Kuo of CityU also contributed to the published paper. Their findings anticipated opening up a new pathway to design alloys with unprecedented structures and properties for various structural applications.
Dr Zhang notes that most people consider AM as a revolutionary technology that can produce machine parts to complex shapes within just one step. “However, we unveiled that it has important potential in designing materials, rather than simply designing geometries,” he stated.
Metallurgists may think that a lack of uniformity in alloy components is undesirable because it leads to bad properties, such as brittleness. One of the key issues in the AM process is how to eliminate this inhomogeneity during the fast cooling of AM parts. But Dr Zhang’s previous modelling and simulation study found that a certain degree of heterogeneity in the components can actually produce unique and heterogeneous microstructures that enhance the alloy’s properties.
“The unique features of Additive Manufacturing provide us with a greater freedom in designing microstructures,” Dr Zhang added. “Specifically, we have developed a partial homogenisation method to produce alloys with micrometre-scale concentration gradients with the aid of 3D printing, which is unachievable by any conventional methods of material manufacturing.”
The research team’s proposed method involves the melting and mixing of two different alloys: titanium alloy powders and stainless steel powders, using Laser Beam Powder Bed Fusion (PBF-LB). By controlling parameters like laser power and scanning speed during the AM process, the team successfully created the non-uniform composition of the elements in the new alloy in a controllable way.
“In addition to the use of Additive Manufacturing, the composition of the two-powder mixture is another key to creating the unprecedented lava-like microstructures with a high metastability in the new alloy,” said Professor Liu. “These unique microstructures give rise to the supreme mechanical properties, allowing the alloy to be very strong but ductile, and lightweight.”
While AM stainless steel generally has a microstructure of 7.9 g/cm3, the new alloy is only 4.5 g per cubic centimetre, resulting in around 40% lighter weight. In their experiments, the titanium alloy with lava-like microstructures exhibited a high tensile strength of approximately 1.3 GPa with a uniform elongation of about 9%. It also had an excellent work-hardening capacity of over 300 GPa, which guarantees a large safety margin prior to fracture and is useful in structural applications.
“These excellent properties are promising for structural applications in various scenarios, such as the aerospace, automotive, chemical, and medical industries,” commented Professor Liu. “As the first team to use 3D printing to develop new alloys with unique microstructures and properties, we will further apply this design idea to different alloy systems to further explore other properties of the new alloys.”
Other researchers who participated include Dr Luan Junhua, Dr Wang Anding and Dr Kong Haojie. Other collaborators are Professor Huang Zhenghua from Guangdong Academy of Sciences and Dr Wang Dong from Xi’an Jiaotong University.
The research was supported by CityU, HKIAS, the National Key Research and Development Program of China, the National Natural Science Foundation of China, Guangdong Academy of Sciences, and the US National Science Foundation.