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 ~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 α/β transus temperature (~980 °C) provides a high strain rate to remove pores, but high-temperature sintering (≥1250 °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 (≈100 μm α grains + β 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 (