# A Comparative Study on the Laser Welding of Ti6 Al4V Alloy Sheets in Flat and Horizontal Positions
## 2 Experimental Procedures  
Titanium alloy (Ti6 Al4V, annealed) sheets 3 mm in thickness were used in the study, with the chemical composition listed in Table 1. The sheets were cut into rectangular workpieces of approximately 150 mm \(\times\) 300 mm. Argon with a purity of 99.998% was used as a shielding gas in all instances.  
Welding trials were performed using an IPG Photonics YLS-6000 (6 kW) Yb-fiber laser (IPG Photonics, Oxford, MS, USA) with an output wavelength of 1070 \(\pm\) 10 nm. Table 2 details the collimating and  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Elements** & **Al** & **V** & **Fe** & **C** & **N** & **H** & **O** & **Ti** \\ \hline Contents, wt \% & 5.8 & 4.0 & 0.2 & 0.05 & 0.03 & 0.011 & 0.19 & Balance \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of the titanium alloy Ti6 Al4V sheets (wt %).  
focusing units used and the resulting calculated laser beam profile characteristics. For all the trials performed, the process head was mounted on a 6-axis articulated arm robot (Reis Robotics, Obernburg, Bavaria, Germany).  
A schematic diagram of the laser welding setup is shown in Figure 1. Three flows of shielding gas were employed in welding. The main flow was used to protect the molten weld pool, with a rate of 20 L/min. The trailing shielding flow was used to prevent the high temperature metals that just solidified from oxidation, with a rate of 70 L/min. The root shielding flow was to protect the back surface of a specimen, with a rate of 5 L/min.  
The two welding positions studied (i.e., flat and horizontal) are shown in Figure 2. For each welding position, two sets of laser welding parameters were chosen, as listed in Table 3. In all welding trials, the focal position of the laser beam was located on the top surface of the workpiece.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Delivery fiber core diameter, mm & 0.2 \\ Beam parameter product, mm\(\cdot\)mrad & 6 \\ Collimating unit focal length, mm & 100 \\ Focusing unit focal length, mm & 300 \\ Beam width, mm & 0.6 \\ Rayleigh length, mm & 0.94 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Optic combinations used in the butt welding trials.  
Figure 1: Schematic diagram of the laser welding setup.  
Figure 2: Two welding positions studied in the study.  
Close fitting butt welds were produced with welding conditions listed in Table 3. After welding, the specimens were firstly inspected using X-ray radiography to detect any porosity in the welds. Secondly, metallographic samples were cut from the specimen, then ground, polished and finally etched in a solution of 2 mL of HF + 10 mL of HNO\({}_{3}\) + 88 mL of water. The shapes and dimensions of the welds were observed and the weld profile defects were measured with an optical microscope (OM) (Olympus, Tokyo, Japan). For clarity, Figure 3 details the definition of the parameters and defects relating to weld profiles. Thirdly, static tensile tests were carried out to evaluate the mechanical properties of the welds, and fracture surfaces were examined with scanning electron microscope (SEM) (FEI Company, Hillsboro, OR, USA) to determine the fracture features. Finally, the influences of the two welding positions on the weld morphologies, porosity levels, and mechanical properties were analyzed.