Prediction of Welding Mode for the Effect of Plasma Gas Flow Rate on the Mechanical and Microstructural Characteristics of Plasma Arc Welded Titanium Alloy Joints

In the present investigation, the effect and role of plasma gas flow rate on the formation of microstructure, weld defects and plasma modes during plasma arc welding of Ti6Al4V titanium alloy were studied using microscopic examinations, energy dispersive spectroscopic analysis, tensile tests and microhardness measurements. Plasma gas flow rate influences the arc pressure, arc constriction and stability. The transformation of plasma arc from conduction mode to keyhole mode causes severe changes to the microstructural characteristics of the titanium welds. This transformation takes place with small variations of PGFR. The formation of excess weld metal out of weld bead observed at the bottom of the weld is attributed by the formation of keyhole. The macrostructural examination shows that the weld geometries increases and weld defects such as lack of penetration and porosity decreases with increase in the PGFR. The microstructural examination shows that there are various phases formed during variation in PGFR. Acicular α and Widmanstätten α were observed considerably in all the welded joints. Strength of the welded joints and hardness increases with increase in plasma gas flow rate. In the joint welded with 1 L/min, there is formation of α-case which is an oxygen rich brittle subsurface structure and found detrimental to the ductility of the joints.