Nonferrous alloys, such as titanium alloys and aluminium alloys have been increasingly used in the aerospace and automobile industries because of their high specific strengths and, therefore, high fuel efficiency [1
]. Meanwhile, the laser beam welding process has been increasingly adopted in manufacturing various engineering structures because of low distortion, high precision, and high productivity [2
]. The structures to be welded may have complex designs and consist of welds of complex shapes. For such welds, different welding positions are required when the space orientations of the welds vary during laser welding, and the laser welding parameters need be adjusted accordingly for varying welding positions to achieve optimal welding quality. Hence, it is crucial to learn about how the welding process and welding quality can be affected by the different welding positions in laser welding.
Studies on the laser welding of high-strength steel sheets showed that using a horizontal welding position was helpful in mitigating the defects of undercut and sagging, which are commonly encountered when welding in a flat welding position [3
]; moreover, higher laser power was required for a horizontal welding position than for a flat welding position [4
]. Numerical analyses using computational fluid dynamics (CFD) codes showed that gravity had no noticeable influence on the dimensions and shapes of the keyholes and weld pools when laser welding mild steel sheets in eight typical welding positions; different porosity amounts, however, were revealed for welds of different welding positions [5
]. In both mild steel and titanium alloy laser welds, different distributions and porosity amounts were found for different welding positions: the pores were distributed above the centreline in horizontal welds, but were along the centreline in flat welds. Moreover, the porosity amount in horizontal welds was greater than that in flat welds, and the excessive porosity in welds could have deteriorative effects on the weld strengths [4
Laser welding of titanium alloys has been studied ever since the birth of the laser in 1960s. The laser beams used included both the early stage CO2
laser, the Nd:YAG laser [7
], and the more recently developed fibre laser and disc laser [13
]. Experimental development and numerical modelling were carried out to study how the laser welding parameters could influence bead morphology, joint microstructure, defects formation, and mechanical behaviour. In recent years, laser beams have even been employed to join dissimilar materials, such as titanium alloy to aluminium alloy [16
]. Significant progress has already been made, based on the work already performed. However, existing research on the positional laser welding of titanium alloys is still very limited. Although the flat and horizontal laser welding of a titanium alloy has already been studied [6
], vertical welding positions haven’t been addressed to date.
Considering that the laser welding of titanium alloys in vertical positions is sometimes inevitable, this paper aims to investigate the laser welding of a titanium alloy (Ti6Al4V) in two vertical welding positions (vertical up and vertical down), and to reveal the influences of welding positions and laser welding parameters on the laser weld quality.
2. Experimental Procedures
This study used titanium alloy sheets of Ti6Al4V, which had a thickness of 3 mm. Table 1
lists the chemical compositions of the alloy in addition to its basic mechanical properties. The sheets were cut into workpieces with dimensions of 250 mm × 150 mm × 3 mm. Laser welding was implemented using a 6 kW IPG Yb-fibre laser. The core diameter of the delivery optic fibre was 0.2 mm; the collimating and focusing lenses had focal lengths of 100 mm and 300 mm respectively. The laser beam width that resulted was 0.6 mm.
Because titanium alloys are very sensitive to oxidation at high temperature, special measures were taken to protect the workpieces. The shielding scheme adopted in this study is shown in Figure 1
. It can be seen that three shielding gas flows were adopted to protect the weld pool, the high-temperature metals that had just solidified, and the underside of the workpieces. High purity (99.998%) argon was used for all laser welding experiments, and the corresponding rates for the three flows were 20 L/min, 70 L/min, and 5 L/min, respectively.
shows the two vertical welding positions, i.e., vertical up and vertical down, employed in the present study, and Table 2
presents the laser welding process parameters used to produce butt laser welds. The focal positions of the laser beam were the same for all welding experiments, with a defocusing distance of zero, i.e., the focal point was at the top surface of the workpieces.
After welding, the bead appearance, inner porosity, and mechanical properties of welds were examined to evaluate the quality of the laser joints obtained. X-ray radiography was firstly used to detect the pores in the welds. Then, metallographic samples were prepared by grinding, polishing, and etching in a solution (2 mL HF + 10 mL HNO3 + 88 mL water), based on which the weld bead appearance, undercuts, and microstructures were examined under an optical microscope. Afterwards, three mechanical testing specimens were prepared for each welding parameter setting, which were then tested under static tensile conditions. Fracture surfaces after tests were analyzed using a scanning electron microscope. With all the results obtained, the correlations between the bead appearance, inner porosity, and tensile behaviour were analysed comprehensively.