# A Comparative Study on the Laser Welding of Ti6 Al4V Alloy Sheets in Flat and Horizontal Positions
## 1 Introduction  
Titanium alloys have been widely used in many industrial fields, such as aerospace and aircraft, because of their superior properties [1]. Meanwhile, lasers are applied to weld such titanium alloy components and achieve high quality at a high speed thanks to its high brightness and power availability [2]. For components with complex shapes, the weld tracks are generally not straight lines but complicated two-dimensional or three-dimensional curves (e.g., girth welds of pipelines), which lead to changes in the welding positions during laser welding. Such changes in welding positions may result in fluctuations in welding quality because of the different actions of gravity for various welding positions, which then necessitates the adjustment of welding parameters accordingly. As a basis to optimize the laser welding parameters for varying welding positions, it is therefore necessary to study the influence of welding position on weld quality. Some research has already been done on the effects of welding position in the laser welding of steels. [PERSON] et al. [3] indicated that employing the 2G (horizontal) welding position (with the laser beam perpendicular to the direction of gravity) could mitigate the welding defects of undercuts and sagging in laser welding of 13-mm-thickhigh strength steel plates. Such defects occurred commonlywhen using the 1G (flat) welding position (with the laser beam in the same direction with gravity). Such an improvement in quality was attributed to a more balanced state for the weld pool between the surface tension, recoil pressure, and gravity. [PERSON] et al. [PERSON] et al. (2017) compared the process window and porosity distribution when laser welding 10-mm-thick 30 CrMnSiA ultrahigh strength steel plates using flat and horizontal positions, and found that higher line energies were required to reach the same penetration depth in the horizontal position than that for the flat position. In addition, the pores in the horizontal position laser welds were located in the upper part of the weld, while those in the flat position were in the weld center. [PERSON] et al. [PERSON] et al. (2017) studied the laser welding of 20-mm-thick low carbon steel plates in eight different welding positions, and found that the welding position had little influence on the weld bead shape and the fluid flow features of weld pools, but could lead to different levels and positions of porosity. Besides laser welding, the effects of the welding position have also been studied for other welding processes, such as gas metal arc welding (GMAW), hybrid laser-arc welding (HLAW), and electron beam (EB) welding. [PERSON] and [PERSON] and [PERSON] and [PERSON] (2018) numerically investigated the fluid flow during gas-metal-arc fillet welding for different various joint configurations (L and V shapes) and welding positions (tilting angles), and revealed that workpiece orientation and welding configuration could affect the free surface profile of the weld pool significantly, which might in turn affect the strength of the welds. [PERSON] et al. [PERSON] et al. (2018) numerically studied the molten pool behaviors in gas metal arc welding (GMAW) of a 10-mm-thick V-groove steel plate with various welding positions (flat, overhead, and vertical). Different trends were found for humping, overflow, and lack of penetration. [PERSON] et al. [PERSON] et al. (2018); [PERSON] et al. (2018); [PERSON] et al. (2018) investigated the molten pool behavior for all-position narrow gap GMAW of 25-mm-thick carbon steel plates and found that the molten pool surfaces had different shapes when welding in flat, vertical down, and overhead welding positions due to the different actions of gravity. [PERSON] et al. [PERSON] et al.