Determining the mechanisms of dissolution and isothermal solidification kinetics during transient liquid phase bonding of commercially pure titanium using a pure copper interlayer

Abstract

Brazing of Ti with pure Cu interlayers has become increasingly important in efforts to reduce the cost of manufacturing in the aerospace industry. A detailed understanding of the dissolution behaviour and kinetics of isothermal solidification in Ti/Cu joints was determined to promote the further development of this brazing technology. During heating Ti-Cu intermetallic compounds form at the solid/liquid interface which impede diffusion of copper into the base metal but quickly consume to pure Cu foil. Temperatures above the melting point of the Ti2Cu-TiCu eutectic (960 °C) are needed before significant liquid phase forms and dissolution of the base metal accelerates once heating above the Ti rich peritectic of 1005 °C occurs. Maximum dissolution and gap widening occurs just as the braze temperature of 1050 °C is reached, followed by a rapid isothermal solidification (IS) rate in the first five minutes of the isothermal hold. Following this rapid IS stage, the rate becomes constant with the square root of braze time. The experimental results and predictions from this work indicate that decreasing the Cu interlayer thickness from 100 to 25 μm reduces the isothermal solidification time from 226 to 7.56 min respectively. Brazing with a 50 μm gap for 75 min created a dense homogenized joint with a centerline concentration of 3 wt% Cu.