Abstract
Laser welding is widely used in titanium alloy welding due to its high energy density, small heat affected zone, and rapid processing ability. However, problems with laser welding, such as deformation and cracking caused by residual stress, need to be resolved. In this paper, the residual stress in laser welding of TC4 titanium alloy was studied using an ultrasonic laser. The residual stress in titanium alloy plates is considered a plane stress state. A pre-stress loading method is proposed and acoustoelastic coefficients are obtained. Based on the known acoustoelastic coefficients, the transverse and longitudinal residual stresses in laser welding are measured using an ultrasonic laser. The results show that longitudinal residual stress is greater than the transverse stress. The distribution regularity of the residual stress is similar to normal welding, but the tensile stress zone is much narrower. Then, the influence of heat input and welding speed on residual stress is discussed. With increasing heat input, the welding zone widens, and the peak value of the residual stress increases. A higher welding speed should be chosen when the welding power is constant. This research has important significance for the measurement and control of residual stress in the laser welding process.
1. Introduction
The high temperature mechanical properties, high strength-to-weight ratio, and good corrosion resistance of titanium and titanium alloys have led its diversified and successful application in a variety of fields with demanding performance and reliability requirements, such as in the aerospace, automotive, medical, nuclear, petrochemical, and power generation industries [1,2]. Common titanium alloy welding methods include tungsten inert gas welding, electron beam welding, and laser welding. Laser welding is a dominant welding method because of its concentrated energy density, small heat affected zone (HAZ), minimal welding deformation, and fast welding speed. Laser welding technology is suitable for welding of TC4 titanium alloy [3,4].
5. Conclusions
An ultrasonic laser technique was used to measure the residual stress in laser welding of TC4 titanium alloy. A pre-stress loading method was proposed and the acoustoelastic coefficients of TC4 titanium alloys were first obtained. On the basis of known acoustoelastic coefficients, the longitudinal and transverse welding residual stresses were measured with an ultrasonic laser technique. The results showed that the longitudinal residual stress was obviously larger than the transverse stress. The longitudinal residual stress was high tensile stress near the welding seam, and the maximum value of longitudinal residual stress was about 420 MPa, which was half of the yield strength. Then, the effect of the process parameters on the features of the residual stress was discussed. The results showed that heat input is the dominant parameter determining laser welding residual stress. With increasing heat input, the welding zone widened and the peak value of the residual stress increased gradually. Higher welding speeds should be chosen as much as possible when the welding power is constant. Heat treatment can reduce the maximum of the longitudinal residual stress by about 80% and lead to the redistribution of stress.
