Microstructural development during additive manufacturing of biomedical grade Ti-6Al-4V alloy by three-dimensional binder jetting: material aspects and mechanical properties

Additive manufacturing (AM) of biomedical materials provides enormous opportunities to fabricate multifunctional and structurally designed frameworks for tissue engineering, such as dental implants and bone substitutes. Despite several advantages of the binder jet 3D printing technology over other AM methods, for example, no limitations in materials selection, high materials recycling efficiency, no thermal stress development, no need to support materials, and the possibility of fabrication of printing functionally graded materials, the fabrication of biomedical-grade titanium alloys with high-density, fine microstructure, and low pickup of impurities is still challenging. This work presents the effects of powder particle size and 3D printing conditions on the microstructural features and mechanical properties of Ti-6Al-4V alloy. The formation of large and inter-aggregate pores during binder jetting is demonstrated and discussed. Design and selection of particle size distribution with a mean diameter of similar to 20 mu m and large span and positive skewness are proposed to minimize binder-induced powder aggregation and fabricate green parts with a density of 65 +/- 1% PFD (pore-free density). Dilatometric studies under a partial pressure of argon (0.1 bar) determine that sintering just above the alpha/beta transus temperature (similar to 980 degrees C) provides a high strain rate to remove pores, but high-temperature sintering (>= 1250 degrees C) is required to attain 97% PFD. The successful fabrication of high-density Ti-6Al-4V parts (>= 96% PFD) with the microstructure comparable to metal injection molding (MIM) titanium parts (approximate to 100 mu m alpha grains + beta lattes) is demonstrated. The tensile strength and elongation values fall in the range of 880 +/- 50 MPa and 6 +/- 2% (depending on the processing condition), which is comparable with metal injection molded parts and superior to the laser powder bed fusion technology concerning ductility. The content of carbon (<0.02 wt.%) and nitrogen (0.01 wt.%) also falls in the standard region of metal injection molding. However, oxygen pickup during sintering moderately increases the oxygen content (for 30-50%) over the standard level. The concentration of interstitials entrapped in the metal is comparable to that of parts manufactured by the powder bed fusion process, but the mechanical properties are better matched with the commercial titanium alloy. The fabrication of the titanium alloy as per the ASTM F2885 standard provides an excellent opportunity for the binder jetting process to develop custom-made biomaterials.

Automated summary

Additive manufacturing of biomedical materials provides opportunities for tissue engineering. However, the fabrication of high-density biomedical-grade titanium alloys with fine microstructure is challenging. This study investigates the effects of powder particle size and 3D printing conditions on the microstructural features and mechanical properties of Ti-6Al-4V alloy, and proposes a particle size distribution design to minimize powder aggregation and achieve high-density green parts.