Abstract: For a long time, the deep penetration of laser beam welding (LBW) of ultra-thick titanium alloys used for super large-sized equipment has always been a significant challenge in engineering fields due to the dispersion of most laser beam energy in air. In this study, we designed the good matching of laser beam and welding parameters at a high power of 30 kW and a low ambient pressure of 0.3 Pa, and the deep weld penetration (95 mm) of Ti-6Al-4V alloy ultra-thick plates was first time achieved via vacuum LBW (VLBW). It was much larger than ever reported in titanium alloy VLBW joints. During VLBW, the plasma plume was gradually inhibited with the decrease in ambient pressures and basically disappeared below 101 Pa. The lower ambient pressure improved the stability of the molten pool, and enhanced the energy absorption inside the keyhole, thereby largely increasing the weld penetration. After VLBW, the contents of the impurity gas elements (N, H, and O) in the joint hardly changed. A relatively uniform welding temperature and cooling rate throughout the thickness resulted in a relatively homogeneous microstruc-ture in the joint, leading to uniform hardness, strength, and toughness throughout the thickness. The secondary α( αs ) and α_ martensite precipitated in the heat-affected zone and fusion zone, respectively. They increased the αs / βand α_ / βinterfaces, resulting in their higher microhardness values than those in the base material (BM). In this case, the VLBW joint reached 987 MPa in strength, achieving equal strength welding to the BM. Furthermore, the impact energy (28 J) and fracture toughness (82.4 MPa m1/2 ) of the joint both could reach over 90% of the BM, achieving excellent strength-toughness matching. This study provides a new way for the high-property deep penetration welding of ultra-thick titanium alloys for large-sized components.

Key words: Titanium alloys, Ultra-thick plates, Vacuum laser beam welding, Microstructure, Mechanical properties