# Residual Stress in Laser Welding of TC4 Titanium Alloy Based on Ultrasonic laser Technology  
[PERSON]  
1 College of Sciences, Northeastern University, Shenyang 110819, China; [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (E.Z. ); [EMAIL_ADDRESS] (Y.G. )1 College of Sciences, Northeastern University, Shenyang 110819, China; [EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (E.Z. ); [EMAIL_ADDRESS] (Y.G. )1 College of Sciences, Northeastern University, Shenyang 110819, China; [EMAIL_ADDRESS] (Y.Z. )1  
surfaces, even in harsh environments. Ultrasonic laser technology has been widely used in defect detection [8; 9] and residual stress measurement [10; 11; 12; 13]. Many researchers have been attracted by ultrasonic laser technology because of its outstanding advantages in residual stress measurement. [PERSON] et al. [10] measured surface residual stress in steel bars with different heat treatments processes using a laser-induced ultrasonic surface wave. [PERSON] and [PERSON] [11] discussed surface wave dispersion produced by residual stress. The results showed that diffraction correction can be introduced to improve the precision of surface wave dispersion measurements. Based on the laser ablation mechanism, [PERSON] et al. [12] measured residual stress using the surface skimming longitudinal waves generated by a laser. Although the method damaged the sample surface, the measurement precision was high. [PERSON] et al. [13] determined the residual stress in autofrettaged steel specimens using laser-generated creeping longitudinal waves. [PERSON] and [PERSON] [14] measured the residual stress in a 7075-T651 aluminum sample surface-treated with low plasticity burnishing using an ultrasonic laser. [PERSON] et al. [15] used laser-induced surface waves to determine the velocity distribution around the laser welded joint of an aluminum alloy sheet, from which the profile of the principal residual stress was calculated based on acoustoelastic theory. [PERSON] et al. [16] designed a special optoacoustic transducer for ultrasonic pulse excitation and detection. The technology was applied to the residual stress measurement of electronic welded stainless steel specimens. The results were consistent with conventional approaches. The simplified finite element model (FEM) was used by [PERSON] and [PERSON] [17] to determine the sensitivity and capability of ultrasonic laser technology for residual stress measurement. The possibility of depth profiling was demonstrated for depths below the surface up to 0.3 mm for the particular case studied. Many scholars have researched laser welding and heat treatments for stress relaxation after welding. [PERSON] et al. [18] used the laser Nd:YAG (Neodymium Yttrium Aluminum Garnet) welding machine to determine the optimal welding parameters of dual-phase steel, and heat treatment was used to reduce the hardness of the HAZ. [PERSON] et al. [19] researched the effects of laser welding parameters on penetration and microstructure characterization of a DP1000 steel butt joint. The results indicated that the process parameters have a significant effect on the weld penetration, and the fusion zone of the butt joints exhibited greater hardness than the base metal and HAZ of butt joints. [PERSON] et al. [20] investigated the weldability during 10 kW high-power fiber laser welding of 304 stainless steel with three kinds of shielding gases. The appropriate shielding gas should be selected according to the application demands and welding requirements. [PERSON] et al. [21] reported a feedback mechanism for rapid identification of optimal laser parameters during welding of AlMg\({}_{5}\) coupons using real-time monitoring by high-speed imaging. The optimization of the Nd:YAG laser welding process parameters for sealed small titanium tube ends was presented by [PERSON] et al. [22]. The effects of the laser welding parameters on the melted length were analyzed and optimized using the Taguchi and regression analysis method.