SHS reaction of Ti/Al multilayers and resistive heating used for joining of Ti-6Al-4V alloy

Abstract

This paper describes the experiment of joining of the Ti-6Al-4V alloy with a freestanding Ti/Al multilayer foil used as a filler material. The joining process relied on an exothermic reaction of Ti/Al multilayers backed with a resistive heating. The DC electric current (1.6 kA/cm2) was maintained for 2 to 6 min in order to determine the effect of the bonding time on microstructure and mechanical strength of the obtained connection. Visual observations and measurements of the temperature of the joint indicated that the reaction accompanied by emission of light starts already after few tens of seconds of the current heating (thermocouple read-out: ~250 °C). The microstructure investigations showed that, independently of applied resistive heating times, their central parts are built of randomly oriented equiaxed crystallites, confirming that the Ti/Al multilayer foil had undergone self-propagating high temperature synthesis (SHS). The sides of the joints were separated from the filler material with layers of Ti3Al intermetallic phase of the thickness increasing with the processing time. The hardness of the Ti-6Al-4V alloy near the TiAl filler material after 2 min and 4 min of joining process time is still comparable with that measured for as-supplied material. It slightly decreases after 6 min indicating that the latter time is a threshold for the base material overheated during such experiments. The shear tests of Ti-6Al-4V/(Ti/Al)/Ti-6Al-4V joints helped to estimate their strength at ~300 MPa for all processing times, proving it as fast and reliable method for joining parts of Ti-6Al-4V alloy.