Control and qualification of titanium welds

Abstract

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The study was aimed at controlling the weld geometry of thin-plate titanium and one of its alloys (Ti-6Al-4V) by ultrasonic means and qualiFying the metals in the as-welded condition in terms of their grain sizes and mechanical properties. The alignment and symmetry of the weld pools were successfully tested by using ultrasonic shear waves. The grain sizes at the weld fusion zone were found to be related to their ultrasonic attenuation by a mathematical relationship. The temperature effect in locating weld pool radii in titanium was found at temperatures up to 600 °C. The ultrasonic velocity decreased as the temperature increased and the square of temperature affected the rate of change of the ultrasonic velocity. After compensation for the temperature effect, the maximum location error of the weld pool radius was 17 % which was comparable to previous measurement using different techniques.A positive relationship was seen between weld geometry (penetration depth and weld width) and heat input. A welding spectrum for titanium and its alloys of different thicknesses was obtained. Back shielding gas was beneficial in obtaining good welds. Both heat input rate and cooling rate were found to affect the grain size of the weld, with the cooling rate being the dominant factor. The grain size exhibited a Hall-Petch effect on mechanical properties, such as the tensile properties and fracture toughness of the weld. The phase transformation positively contributed to better mechanical properties in most cases, whilst the presence of interstitials worsened tensile properties. A system was developed in this study to utilise the above information and data for possible real-time and closed-loop control of the TIG welding process to give a desirable weld. Specifically, a process control data base was built up using software and a knowledge-based system for acceptable welding parameters, which were determined by acceptable penetration depth, grain size and mechanical properties. An algorithm was successfully written which relates the ultrasonic signal to the penetration depth of the weld. A hardware control circuit was built which took in the ultrasonic signal and converted it to a driving signal to change the welding speed and thereby change cooling rate