# Residual Stress in Laser Welding of TC4 Titanium Alloy Based on Ultrasonic laser Technology
## 3 Experimental Process
### Ultrasonic laser for Measurement of Residual Stress in Laser Welding  
#### 3.2.1 Experimental Device  
The whole ultrasonic laser experiment system included three sections: a Nd:YAG laser was used to generate the ultrasonic waves, a laser Doppler vibrometer was used to detect ultrasonic waves, a the pre-stress loading setup was used to calibrate the acoustoelastic coefficients. The whole system is presented in Figure 1. A laser pulse with a Gaussian profile was sent by the Nd:YAG laser setup (Dawa-100, Beamtech Optronics Ltd., Beijing, China), the beam was focused by the cylindrical lens as a line source with a 0.6 mm width and a 20 mm length to generate the surface waves. The main parameters of the generation laser were as follows: wavelength \(\lambda=1064\) nm, pulse width \(\tau=8\) ns, single pulse energy \(E_{0}=100\) mJ, and the maximum repetition frequency \(f=20\) Hz was adjustable within 0-20 Hz. The vibration information was detected by a laser Doppler vibrometer (Sdptop LV-S01, Sunny Optical Technology Ltd., Suzhou, China), with a He-Ne laser wavelength of 632.8 nm, frequency band of 2.5 MHz, work distance of 0.35-20 m, and a displacement resolution of 0.008 nm. The laser Doppler vibrometer had auto-focus/long-distance focus and focus point storage functions. The He-Ne laser can be manually fine-tuned by rotating the lens of the vibrometer. The detection laser was focused into a light spot with a diameter of 0.7 mm. Few laser beams are received by the photoelectric detector (Thorlabs Det10A/M, Thorlabs Inc., Newton, NJ, USA) with a wavelength of 200-1100 nm. It achieves perfect synchronization of the oscilloscope with the laser shots. An oscilloscope (Dpo4102, Tektronix Inc., Beaverton, OR, USA), with a sampling rate of 5 GS/s and frequency band of 1 GHz, was employed to store the output electrical signal from the photoelectric detector and the Doppler vibrometer after 64 repetitions, on average, in order to reduce random noise. The computer analyzed and processed the data from the photoelectric detector and oscilloscope. In the measurement experiment, two methods were adopted to avoid the interaction between the various modes of the ultrasonic signals. First, the signal detecting location was far from the exciting location. Because the propagation velocities of various waves are different in the media, sufficient distance can separate the different ultrasonic waves to better identify the transverse and longitudinal waves. In addition, the laser Doppler vibrometer has two functional options with a low-pass filter (OUTPUT LP FILTER: 500 Hz, 5 kHz, 20 kHz, and 100 kHz) and a high-pass filter (OUTPUT HP FILTER: DC is namely direct current, DC Block, 60 Hz, 1 kHz). The high-pass filter function was used, as it removes unnecessary interference signals in low frequency. OUTPUT HP FILTER: 1 kHz was set, and a high frequency wave of 1-2.5 MHz was received.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Weld No.** & **Welding Power (W)** & **Welding Speed (m/min)** & **Heat Input (k/m)** \\ \hline
1 & 2800 & 6.0 & 28 \\
2 & 2800 & 4.8 & 35 \\
3 & 2800 & 3.0 & 56 \\
4 & 2000 & 4.0 & 30 \\
5 & 2000 & 3.0 & 40 \\
6 & 2000 & 2.0 & 60 \\
7 & 1400 & 3.0 & 28 \\
8 & 1400 & 2.4 & 35 \\
9 & 1400 & 1.2 & 70 \\ \hline