# Residual Stress in Laser Welding of TC4 Titanium Alloy Based on Ultrasonic laser Technology
## 3 Experimental Process
### Preparation of Experimental Specimens  
To ensure welding quality, preparatory work is necessary. The national standard GB/T3621 was applied, and anything beyond standard scratches, indentation, dents, and cracks were not allowed on the surface of the sample. The sample should be treated before welding and the treatment solution composition was HNO\({}_{3}\) (30%-40%), HF (3%), and water (the rest). The temperature of the treatment solution was 20-30 \({}^{\circ}\)C, and the treatment time was 3-5 min. Then, the sample was flushed with cold water for 3 min, and dried in a drying box at 82-116 \({}^{\circ}\)C. Sandpaper was used to polish the oxide layer on the surface to be welded of the sample. Until the sample surface is polished into silver white metallic luster, acetone is used to wipe it clean. Laser welding is a high precision welding method, so there are certain requirements for the working environment. The room temperature should be kept at 15-30 \({}^{\circ}\)C, and the relative humidity should not exceed 60%. The voltage fluctuation of the welding power supply should be less than 10% to ensure the stability of the laser. In order to ensure the stability of the specimens in the welding process, a special fixture should be used to fix the specimens. On the back of the fixture, a protective gas passage should be reserved to prevent the back of the weld from being oxidized. The laser welding equipment used in the experiment was a HL3006D-type HAAS-LASER YAG laser produced by TRUMPF (Stuttgart, Germany). The highest stable output power was 3 kW, the wavelength was 1064 nm, and the diameter of the spot after focusing was 0.6 mm. The light guiding and focusing system was composed of a circular polarizer, a beam expander, an optical fiber, and a focusing lens. In the process of welding, the six axis manipulator produced by KHI (Kawasaki, Japan) was used for precise positioning, and the relative motion of the laser beam and work piece was realized using an operating platform. Many parameter settings can be changed during laser welding, such as welding power, welding speed, welding angle, heat input, and inert gas flow, but in most cases, the individual effect on the residual stress was not clear. Since the relationship between the process parameters in laser welding is complex, in this paper, only welding power, welding speed, and heat input were changed. The material was a TC4 titanium alloy plate with a yield strength of 860 MPa, a density if 4.5 g/cm\({}^{3}\), an elastic modulus of 110 GPa, and a Poisson ratio of 0.34. The nominal chemical composition of the alloy is shown in Table 1.  
Continuous laser welding was used in this experiment. The weldment size was 200 mm \(\times\) 50 mm \(\times\) 2 mm and the welding method was butt welding. Argon was used as a protective gas. The protective gas was composed of three parts: the coaxial protective gas for protecting the melting pool, the front protective gas (for protecting the surface of the newly solidified weld, and the back protective gas for protecting the back of the weld. Considering the use of laser energy, the defocusing amount was set to zero. The range of welding power and welding speed was chosen as wide as possible while ensuring the welding quality of the products. Table 2 shows the different combinations of welding speed and welding power used in this paper. Heat input is defined as welding power/welding speed. The heat input represents the laser energy absorbed by the weld per \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Material** & **Al** & **V** & **Fe** & **Si** & **C** & **O** & **N** & **H** & **Ti** \\ \hline TC4 & 5.5–6.8 & 3.5–4.5 & 0.3 & 0.15 & 0.1 & 0.2 & 0.05 & 0.015 & Balance \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The composition of TC4. unit length, and it plays a decisive role in the distribution in welding residual stress.