Abstract:The main objective of this investigation is to study the influence of arc constriction current frequency (ACCF) on tensile properties and microstructural evolution of aerospace Alloy 718 sheets (2 mm in thickness) joined by constricted arc TIG (CA-TIG) welding process. One variable at a time approach of design of experiments (DOE) was used, in which ACCF was varied from 4 to 20 kHz at an interval of 5 levels while other parameters were kept constant. The joints welded using ACCF of 4 kHz exhibited superior tensile properties extending joint efficiency up to 99.20%. It is attributed to the grain refinement in fusion zone leading to the evolution of finer, discrete Laves phase in interdendritic areas. An increase of ACCF above 12 kHz caused severe grain growth and evolution of coarser Laves phase in fusion zone. Alloy 718 welds showed more obvious tendency for Nb segregation and Laves phase formation at higher levels of ACCF due to the slower cooling rate. The volume fraction of Laves phase was increased by 62.31% at ACCF of 20 kHz compared to that at 4 kHz, thereby reducing the tensile properties of joints. This is mainly due to the stacking of heat input in weld thermal cycles at increased levels of ACCF.

Key words: constricted arc TIG welding; arc constriction current frequency; Alloy 718 sheet; tensile properties; microstructural evolution; Laves phase