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

Abstract

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, 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 ~20 µm 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 a/b tarsus (~1050 °C) provides a high strain rate to remove pores, but high-temperature sintering (³1250 °C) is required to attain 97 % PFD. The successful fabrication of high-density Ti-6Al-4V parts (³96 % PFD) with the microstructure comparable to metal injection molding (MIM) titanium parts (»100 µm α grains + β lattes) is demonstrated. The tensile strength and elongation fall in the range of 880±50 MPa and 6±2 %, depending on the processing condition. The content of carbon (<0.02 wt.%) and nitrogen (0.01 wt.%) also falls in the standard region of metal injection molding parts. 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.