Titanium alloys have been widely used in aerospace, automotive and medicine due to its low density, high specific strength and good corrosion resistance. In order to meet the requirements for lightweight and high performance, laser aided additive manufacturing (LAAM) technique, which is developed based on the principle of ‘discrete + accumulation’, has gradually shown broad application prospect in the fabrication of titanium structural parts. Typical microstructure of LAAM titanium alloys is epitaxially growing columnar grain. This microstructure characteristic brings about a strong <001> texture, which is generally accepted as the main reason for anisotropic mechanical properties and even reduction in strength.
In a new paper published in the International Journal of Extreme Manufacturing (IF: 10.036), a joint team of researchers, including Dr. Shang Sui, Dr. Youxiang Chew, Dr. Fei Weng, Dr. Chaolin Tan and Dr. Zhenglin Du from Singapore Institute of Manufacturing Technology, and Dr. Guijun Bi from Guangdong Institute of Intelligent Manufacturing, adopted the in-situ Ni alloying method to tailor the microstructure of LAAM Ti-6Al-4V alloy and enhance its strength. The intrinsic mechanisms of Ni addition on AM-built Ti–6Al–4V alloy is well established, which is conducive to promoting the development of customized titanium alloys for AM.
Ni additive demonstrates the following four effects on the microstructure of LAAM Ti-6Al-4V alloy: (i) With increasing Ni content, the morphology of prior-β grain of LAAM Ti-6Al-4V-xNi gradually changes from columnar to equiaxed, while the size decreases first and then increases. (ii) More globular α phase forms as the Ni content increases. (iii) Ni additive induces the precipitation of Ti2Ni nanoparticle. (iv) Ni additive increases the content of β phase, while decreases the content of α phase, since it is a β-stabilizing element. Consequently, the yield strength and tensile strength of LAAM Ti-6Al-4V-xNi alloys increases linearly with the increase of Ni content, while the ductility shows an opposite trend.
One of the lead researchers, Dr Guijun Bi, said, “Laser aided additive manufacturing (LAAM) technique, which is characterized by powder feeding, can tailor the content of chemical elements in real-time to realize in-situ alloying and has remarkable advantages in clarifying the alloying elements effect and developing customized alloys”.
The Ni element has a great tendency of enlarging compositional supercooling in titanium alloys, normally leading to severe macroscopic segregation in castings. The first author Dr Shang Sui explained, “the cooling rate in the LAAM process can reach 103-105 °C/s, significantly inhibiting the macroscopic elements segregation. Thus, the eutectoid element Ni, which is not desirable for the castings, may be appropriate for LAAM-processed titanium alloys via controlling its content.”
Dr Fei Weng illustrated, “Similar to Ni element, Cu is also a eutectoid element and is undesirable for cast titanium alloys. However, recent publication in Nature demonstrates that Cu addition can promote ultrafine grain formation and result in high strength in additively manufactured titanium alloys.”
Two powder hoppers were utilized to adjust the Ni content in the fabricated blocks. Dr Chaolin Tan further indicated, “such kind of process is also conducive to manufacturing heterostructured materials.”
The underlying mechanisms for microstructural evolution and strength enhancement are investigated in detail. It is indicated by termination mass migration theory that the element diffusion between α and β phases is the main driving force for globularizing α phase. Additionally, a modified strengthening model was applied to quantitatively compute the yield strength. Quantitative analysis demonstrates that the strengthening effects of both the refined α lath phase and solid solutes dominate the contribution to overall yield strength of the LAAM-built Ti-6Al-4V-xNi (x=0, 1.1, 1.7, 2.5 wt. %) alloys.
Moreover, Ni is a β-stabilizing element, and will be enriched in the β phase. The large amount of solid-solution Ni atoms in the β phase increase its strength significantly while reducing the material deformability. Considering that the α phase and the β phase are arranged alternately in the titanium alloys, the deterioration of the deformability of the β phase limits the deformation of the α phase, resulting in the decrease in elongation.
Dr Youxiang Chew said, “These findings can accelerate the development of additively manufactured titanium alloys.”
International Journal of Extreme Manufacturing (IF: 10.036) is a new multidisciplinary, double-anonymous peer-reviewed and fully open access journal uniquely covering the areas related to extreme manufacturing. The journal is devoted to publishing original articles and reviews of the highest quality and impact in the areas related to extreme manufacturing, ranging from fundamentals to process, measurement and systems, as well as materials, structures and devices with extreme functionalities.
International Journal of Extreme Manufacturing
Study of the intrinsic mechanisms of nickel additive for grain refinement and strength enhancement of laser aided additively manufactured Ti–6Al–4V
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