A Study On The Full Penetration Stability And Physical Metallurgy Of The Laser Welding Of Aeronautic Titanium Alloys

Titanium alloy is one of the most important materials and finds extensive application in aerospace industry because of its light weight, superior strength-to-weight ratio,and excellent corrosion resistance. Driven by cost and weight savings, technological progress is moving in the direction of replacing rivets and fasteners with welds. Laser welding with higher energy density can offer remarkable advantages over conventional fusion welding processes, such as minimal component distortion and high productivity, and is specifically suitable for joining aerospace structures. Therefore, it will be significant to investigate the laser welding of titanium alloy for developing the weight efficiency and improving the structure integrity of the military and commercial aerospace. The process of laser penetration welding of titanium alloy sheet involves complex reactions among the laser-plasma-material, periodical keyhole opening and closing, and melt pool flowing behavior. This process can be mapped into pool morphology and weld shape. It is well known that the mode of laser welding depend on laser power density. However, according to this study on the weld shape of BT20,TC4 and Ti-23Al-17Nb titanium alloy laser welding, the laser welding mode is not only related to the laser power density but also the heat input. Both laser power density and heat input determine the keyhole stability, melt pool behavior and weld shape of laser welding titanium alloy. The research shows that when the laser power density and heat input exceeded the threshold of full penetration laser welding of titanium alloy, hourglass shape weld was obtained. The back width to surface width ratio (Rw) reflected the full penetration stability of the laser welding process of titanium alloy sheet. According to the results in this paper, the condition for stable full penetration laser welding of BT20 titanium alloy of thickness of 2.5mm was that the Rw should be bigger than 0.4. Based on it, a parameter window of laser power -laser welding speed and laser power density-heat input threshold curves for stable full penetration laser welding were obtained. When the laser power density and heat input were near to the threshold of forming a through keyhole, unstable full penetration laser welding occurred even if the parameters were invariable. This unstable process resulted from the unstable through keyhole caused by plasma/vapor plum fluctuating periodically, which resulted in appearing alternately between partial penetration and full penetration on the weld back meanwhile the weld surface was perfect. The full penetration laser welding stability of titanium alloy is closely related to the forming of through keyhole. A physical mode of full penetration laser welding has been put forward in this paper, in which laser-welding speed should match up with drilling speed. According to energy balance on the keyhole front wall, drilling speed was related to laser power density, heat input, laser welding speed, beam diameter, materials properties and sheet thickness. A formula relating drilling speed to laser power density a forming through keyhole has been built, which can be used to evaluate the process stability of full penetration laser welding with selected welding parameters, in conjunction with the thermal efficiency in laser welding. During laser penetration welding process, titanium alloy evaporates strongly and the vapor breaks forth from keyhole up and down. The vapor flow takes the melt to move rapidly on the keyhole wall along the axis of laser beam, and at the same time the plasma/vapor plum forms over the keyhole opening, which radiates part of its power to the pool. As result, strong convection occurs in the part of pool near the keyhole opening due to the effects of vapor pressure and radiation of plasma/vapor plum, but weak convection, even laminar flow appears in the part of pool in the middle of the keyhole because of the vapor expanding. This uneven pool flow makes formation of the hourglass weld shape during full penetration laser welding of titanium alloy. Its thermal action can be simplified into a mode of one lineal heat source plus two point heat sources. And then a way of estimating the thermal efficiency of laser welding was put forward relying on the layered weld size and the lineal heat source conduction equation. The calculation results were close to the experimental ones. The weld undercut and porosity are two main kinds of defects in laser welding of titanium alloy. Adjusting welding parameters cannot eliminate the undercut, but with the active flux added in this paper, the undercut was eliminated. The microstructure of porosity showed that there were two sorts of porosity observed in welds of titanium alloy laser welding.One was small round metallurgical porosity with smooth inner wall, which results from the gas accumulating around the "pore nucleus"relative to the surface contamination. The other was lager irregular processing porosity that its inner wall existed the trace of the pool flowing, which results from the ruffle on the keyhole wall converging locally to close down the gas in the keyhole into bubbles because of the keyhole fluctuating. The processing porosity occurred easily when partial penetration or unstable-full penetration laser welding was conducted. The CO2 laser welding can break down the surface oxide film and produce little metallurgical porosity, and the YAG laser welding produces easily the metallurgical porosity and requires strict surface cleaning. The weld crystallization of titanium alloy laser welding is characterized by epitaxial growth, just the same as in joints of arc welding. The HAZ could be divided into a completely transformed zone with larger crystal, a partially transformed zone that new grains coexist with the old ones, and an untransformed zone that only heated. It is different from the HAZ of arc welding which is divided only into a coarse-grained zone and a fine-grained zone. It showed that weld shape influenced on distribution of weld grain size, growth direction and HAZ width. For a full penetration weld, the grains in the middle part of a weld were small and the grain grown face-to-face, and the HAZ was smaller than other parts of the weld. The grains in the part of upper and down weld were lager and the dendrite growth directed to the center of weld surface or back perpendicular to the fusion line, and the HAZ of this part was wide. The growth direction of grains between the middle and upper or down of weld changed from perpendicular to the axis of beam to an angle, because the direction of the largest heat dispersion in the center of weld varied withthe keyhole closing and pool melt filling back. Weld shape affected this change in grain growth direction. According to the research, the weld width and Rw affected joint ductility and fatigue life of laser welding titanium alloy. Under the research condition, the microhardness in the weld and the HAZ of laser welding BT20, TC4 titanium alloy was higher than the base metal. The joint tensile strength equaled that of the base metal, but the elongation and the fatigue life were lower than that of the base metal. The joint with a thinner weld had lower elongation than that with a wider weld. Active flux could improve joint elongation and fatigue life. The vacuum heat treatment at 650℃could reduce the joint fatigue life, but could not change the joint tensile strength. The effect of post weld heat treatment (PWHT) on elongation depends on the weld width and RW. It revealed that the PWHT specification for titanium alloy arc welding is not suitable for laser welding. The distribution of joint microhardness of intermetalics Ti-23Al-17Nb alloy laser welding presents saddle shape, in which the peak hardness was located in the HAZ and the weld microhardness was higher than that of the base metal. Under the research condition, the joint tensile properties at ambient temperature and 650℃were unacceptable, and the elongation of joints improved with the weld width increasing. The 850℃vacuum heat treatment could improved the joint tensile properties at ambient temperature and 650℃that were still lower than that of the base metal. High frequency induction preheating could enlarge the width of weld and HAZ, the joint tensile properties increased, among which tensile properties at ambient temperature could be as good as that of the base metal. Preheat combining with PWHT will be beneficial to joint integrate properties. In a word, the weld shape of titanium alloy laser welding can image the behavior of pool flowing and the process stability of full penetration laser welding. The Rw of a full penetration weld can reflect the laser power density and heat input thresholds, as well as the weld shaping condition of stable full penetration laser welding. Moreover, the weld shape influencs the homogeneity of joint grains and microstructure; as a result affects properties of the titanium alloy joints. The effects of weld width and Rw on joint properties cannot be underestimated, and more research is necessary in the future.