Additive manufacturing of pure Ti with superior mechanical performance, low cost, and biocompatibility for potential replacement of Ti-6Al-4V

Customized titanium 3D parts with good mechanical performance have attracted considerable attention for application in structural industries. In this study, α-Ti materials with superior tensile strength and good ductility were fabricated via additive manufacturing (AM). To improve the mechanical properties, core-shell structured Ti-(N) powder was used to introduce nitrogen atoms into selective laser melting (SLM) fabricated Ti materials. The microstructural analysis of as-fabricated Ti materials revealed an α microstructure with various grain morphologies such as irregular/massive grains, equiaxed grains, and refined acicular martensite grains (α՛) when different energy densities (E) from 83 J/mm3 to 278 J/mm3 were applied. Furthermore, we observed that grain refinement occurred with increasing solute nitrogen atoms. The mechanical properties were evaluated based on interrelated aspects, including nitrogen content and energy density, by changing the scanning parameters. Our results showed that as the nitrogen content in the as-fabricated Ti materials increased, a significant improvement in tensile strength and hardness was observed. The yield strength (σys) and ultimate tensile strength (σUTS) increased up to 1072 MPa and 1126 MPa, respectively, when the nitrogen content was approximately 0.5 wt.%, which was approximately three times that of SLM-processed pure Ti (∼0.01 mass% N). The increment in yield strength was further discussed mainly in terms of solid solution strengthening using the Labusch model and grain refinement strengthening using the Hall-Petch equation. This calculation raises the ability to quickly predict the yield strength of Ti materials with nitrogen solid solution produced by selective laser melting.

Refined grain formation behavior and strengthening mechanism of α-titanium with nitrogen fabricated by selective laser melting

High oxygen-content titanium and titanium alloys made from powder

Titanium powder metallurgy materials with solid solution nitrogen elements were developed by using pure Ti powder with high nitrogen contents (Ti-(N)), which were prepared via heat treatment from 640 °C to 800 °C in a N2 gas atmosphere. Ti2N compound layers were formed at the Ti-(N) powder surface as a shell, and solid solution nitrogen elements were also detected in the matrix of this powder. Ti-(N) powder was consolidated by spark plasma sintering (SPS), where Ti2N compounds were completely decomposed, and hot extrusion was then used to fabricate fully dense titanium with solid solution nitrogen elements. X-ray diffraction analysis showed that the lattice constant of α-Ti along the c-axis clearly increased in proportion to the nitrogen content due to the nitrogen solid solution behavior. Tensile test results revealed that the powder metallurgy titanium with 0.52 mass% nitrogen exhibited a 0.2% yield stress of 813 MPa and 31% elongation, which were remarkably superior to the strength-ductility balance of pure titanium with a 0.2% yield stress of 305 MPa and 25% elongation. In this study, the Hall-Petch equation and Labusch model were used to quantitatively evaluate the mechanism for the α-Ti grain refinement and solid solution strengthening effects of Ti-(N) powder metallurgy materials.

Strength-ductility improvement of extruded Ti-(N) materials using pure Ti powder with high nitrogen solution

Quantitative strengthening evaluation of powder metallurgy Ti–Zr binary alloys with high strength and ductility

Solid solution strengthening effect by oxygen (O) and nitrogen (N) atoms of α-titanium (Ti) materials was quantitatively evaluated using Labusch model by consideration of the experimental data. When using Labusch model to predict solid solution strengthening improvement, an application of the isotropic strains by solute elements is generally assumed to estimate Fm value, which is the maximum interaction force between the solute atoms and dislocations. It is, however, difficult to exactly calculate Fm value for α-Ti materials with O and N solute atoms because the anisotropic strains are induced in α-Ti crystal with hcp structure by these elements. In this study, Fm value was experimentally derived from the relationship between 0.2% yield stress and solute elements (O and N atoms) content of Ti sintered materials. As a result, the strengthening improvement was proportional to c/Sf (c: soluted atom content, Sf: Schmid factor), and its factor of proportionality of Ti-O and Ti-N materials was 4.17 × 10 and 3.29 × 10, respectively. According to this analysis, it was clarified that Fm value of Ti-O and Ti-N materials was 6.22 × 10 and 5.21 × 10, respectively, and then the estimated strengthening improvement by using these values was significantly agreed with the experimental results of Ti sintered materials with O and N solution atoms.

Quantitative Analysis on Light Elements Solution Strengthening in Pure Titanium Sintered Materials by Labusch Model Using Experimental Data

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Ductility Improvement Mechanism of Ti–6Al–4V+O Sintered Material

Ductility Improvement Mechanism of Ti-6Al-4V + O Sintered Material

https://www.jstage.jst.go.jp/article/jjspm/65/7/65_407/_pdf

Mizuki Fukuo

Papers

Quantitative Analysis on Light Elements Solution Strengthening in Pure Titanium Sintered Materials by Labusch Model Using Experimental Data

Quantitative strengthening evaluation of powder metallurgy Ti–Zr binary alloys with high strength and ductility

Ductility Improvement Mechanism of Ti–6Al–4V+O Sintered Material

Ductility Improvement Mechanism of Ti-6Al-4V + O Sintered Material

Quantitative Analysis on Light Elements Solution Strengthening in Pure Titanium Sintered Materials by Labusch Model Using Experimental Data