Closed-loop temperature controlled solid-state additive manufacturing of Ti-6Al-4V with forging standard out-of-plane tensile properties

Abstract

Here, we present the first demonstration of additive friction stir deposition of Ti-6Al-4V with closed-loop temperature control, wherein the print head rotation rate is adjusted in real-time to maintain a constant tool temperature during deposition. The as-printed Ti-6Al-4V is fully dense, achieving forging standard tensile properties along both the in-plane and out-of-plane directions. The measured yield strength, ultimate tensile strength, and elongation are 903 MPa, 1000 MPa, and 17% along the in-plane direction, and 948 MPa, 1024 MPa, and 16% along the out-of-plane direction. With closed-loop temperature control, the microstructure is found to be comparable along the thickness direction, contrasting the previous work performed under parameter-controlled conditions. For all the deposition conditions explored in this work, the deposited Ti-6Al-4V exhibits a lamellar microstructure with alpha laths forming inside prior beta grains, indicating higher peak temperatures than the beta transus. The increase of a pseudo heat index leads to an increase in the prior beta grain size and a decrease of the out-of-plane yield strength. Deposition of Ti-6Al-4V is seen to modify the microstructure of the substrate near the interface: the original equiaxed microstructure is replaced by lamellar and bimodal microstructures. We finally discuss the possibility of forming equiaxed microstructures in the as-deposited Ti-6Al-4V and provide evidence for different types of microstructures forming in the same deposition layer, which may be caused by the thermal gradient during reheating.