# Application of Pulsed Laser-TIG Hybrid Heat Source in Root Welding of Thick Plate Titanium Alloys
## 1 Introduction  
Titanium alloys, which have already been considered as a kind of key developing structural materials with great strategic significance by a number of the world's dominant military powers, have been used in substitution aluminum alloys and steels in the aerospace, ship and chemical industries, etc., because they possess many good characteristics such as high strength to weight ratio, corrosion resistance and excellent weld-ability [1; 2]. In recent years, due to the need for nation space science development, remarkable advances have been made in large-tonnage launch vehicles, spaceports and other equipment, and we have come a long way in production capacity and technology level of titanium alloys. The demand for titanium alloys with complex structural components with large volume and thickness is constantly increasing [3; 4]. The advanced welding technology, which is considered as the common technique in components manufacturing, is one of the most attractive problems in the joining field of titanium alloys [5]. As we all know, multilayer multi-channel welding is a commonly used technology in conventional jointing of a thick mild plate. A stable formation of continuous weld in the first layer during the welding process is important to achieve welded joints with high quality and productivity [6]. Recently, several different welding methods have been developed to jointing titanium alloy. However, each of these methods has its own significant imperfection. Arc welding, which is the earliest method developed for titanium welding, has a low productivity. The electron beam welding has a limitation on the overall dimensions of the weld workpieces due to there the necessity of making a vacuum in the working chamber [7]. Laser welding, which is considered as a high-efficiency method, is the most promising jointing method for thick plates [8]. [PERSON] et al. reported that a satisfactory titanium alloy joint can be achieved at high weldingspeed by 1.2 KW laser welding process [9]. However, the laser welding process requires greater precision in the positioning of the pieces to be jointed. Thus, the researchers are stimulated to look for another welding process that has the ability to penetrate deeper into the workpiece and consequently reduce total heat input. Laser-arc hybrid heat source, which possesses many new and unique merits, such as improved manufacturing stability, efficiency, quality and reduced energy consumption, is a mixed high energy density laser with an arc of good gap tolerance forming an efficient welding method [10; 11; 12]. The addition of a laser beam can improve the arc stabilities, whereas the arc can also play an efficient role in melting welding [13]. Thus, when the laser-arc hybrid welding was introduced, it attracted much attention from researchers in titanium alloy manufacturing. [PERSON] suggested that the technology of 5 KW laser-TIG welding with filler wire of titanium alloys with a thickness of 5 mm has been developed [14]. By studying the 1.5 KW laser-TIG hybrid welding process of thick plate, [PERSON] reported that a satisfactory weld can be obtained and the tension of the weld is higher than that of the TA15 alloy [15]. By carrying out 2.15 KW laser-MIG on titanium, [PERSON] et al. indicated that the weld appearance is sensitive to changes in the gap [11]. [PERSON], et al. published a paper in 2008, in which titanium alloy with 2.0 mm was joined successfully by pulsed laser-TIG hybrid welding [16]. However, most of the literature is focused on the larger energy laser or thin plate welding process; for root welding of thick plate titanium alloy, the literature is extremely limited [17]. In this study, root welding experiments were carried out on thick plate titanium alloys, and an experimental system was developed to implement the laser-TIG hybrid welding process. The authors investigated the regulatory effect of the pulsed laser on the thermal distribution and mechanical situation in the molten pool.