Development of lightweight high strength Ti-6Al-4V-xCr-yNi alloys by μ-plasma powder additive manufacturing process

Abstract

This thesis reports on development of high-strength and lightweight Ti-6Al-4V-xCr-yNi alloys by μ-plasma powder additive manufacturing (μ-PPAM) process for aerospace, shipbuilding, and marine industries. Thermo-calc simulated phase diagram was used to design Ti-6Al-4V-xCr-yNi alloys. The experimental investigation was conducted in two stages. Thirty pilot and 27 main experiments were conducted making single-layer depositions of Ti-6Al-4V-xCr-yNi alloys to identify optimum values of μ-PPAM process parameters for manufacturing their multi-layer depositions. Microstructure, formation of phases, phase orientation maps, phase fraction, tensile properties, fracture toughness, and abrasion resistance were studied for multi-layer deposition samples of Ti-6Al-4V-xCr-yNi alloys. Corrosion behaviour Ti-6Al-4V-xCr-yNi alloys were studied in an aqueous solution of sodium chloride by evaluating potentiodynamic polarization, electrochemical impedance spectroscopy, morphology, and topography of their corroded surfaces and corrosion mechanisms. Tribological behaviour Ti-6Al-4V-xCr-yNi alloys was studied at different loads by evaluating the wear rate, morphology of wear track, and subsurface deformation. A thermo-calc simulated phase diagram revealed that 5 at.% of chromium and nickel are their upper limits that can be added to Ti-6Al-4V alloy without formation of any brittle phase and without increasing its β-transus temperature. Parametric combination of 319W as μ-plasma power, 2.9 g/min as mass flow rate of feedstock powder, and 47 mm/min as deposition head travel speed that yielded uniform and continuous deposition having combination of maximum deposition efficiency, minimum aspect ratio, and minimum dilution was identified as the optimum combination for manufacturing multi-layer deposition of Ti-6Al-4V-xCr-yNi alloys by the μ-PPAM process. Presence of chromium and nickel in Ti-6Al-4V-xCr-yNi alloys formed Cr2Ti, and Ti2Ni as the intermetallic phases whose melting point is higher than Ti-6Al-4V alloy. It led to accumulation of more gas bubbles thus imparting higher porosity. Results indicated that addition of chromium and/or nickel refined the grains of β-Ti and α-Ti phases of Ti-6Al-4V-xCr-yNi alloys. The α-Ti phase grains are refined due to formation of intermetallic phase Cr2Ti in Ti6Al4V5Cr and Ti6Al4V2.5Cr2.5Ni alloys and Ti2Ni in Ti6Al4V5Ni alloy.