Study On Measurement Of Temperature Field And Weld Seam’s Characteristics In Laser Welding Of TA15Titanium Alloy

As an excellent structural materials in aerospace industry, titanium alloy has many advantages in reducing the weight of structure, improving material utilization rate and reducing cost. Because it has many characters, such as light weight, high strength-to-weight ratio, corrosion resistance and high temperature resistance. The application of laser welding makes these advantages highlight further more. Based on "keyhole"effect, deep penetration laser welding has many advantages, such as high energy density, concentrate heating, little heat loss, large depth-to-width ratio of weld, low residual stress of weld, and high precision of welding. However laser welding process is a fast non-uniform heat transfer process, large temperature gradient will exist near the weld, and there will be varying degrees of residual stress and deformation inevitably in post-weld structure. These problems will directly affect the quality and performance of use of welded structure. Molten character parameters and thermal cycling parameters can be controled through the control of heat input and cooling method of the welding process, which is conductive to achieving the prediction and control of the welding results. Temperature field distribution of welding can reflect complex welding thermal process, so studying welding temperature field it is very important for controling welding quality.In this paper, high-speed camera and infrared thermal imaging system were used to do real-time monitoring of laser welding process of TA15titanium alloy. Video and image results from monitoring were analyzed and calculated; relationship between welding parameters and molten pool character, temperature field character and thermal cycling character in laser welding of TA15titanium alloy were investigated; simple three-dimensional temperature field reconstruction was realized by using a self-developed software; then profile morphology, microstructure distribution, and appearance quality, defects of final weld were analyzed.（1） During laser welding of TA15titanium alloy, the length and width of surface and back molten pool increase along with heat input, while other welding parameters keep the same; types of shielding gas has modest impact on the shape and size of surface molten pool, He as shielding gas, width and length of back molten pool are larger than that of Ar, while heat input is low; Ar as shielding gas, width and length of back molten pool are larger than that of He. while heat input is high. （2） While laser power keep the same, the maximum temperature in surface temperature field increase along with decreasing of welding speed; the maximum temperature in back temperature field changed irregular when welding speed decreases.（3） While laser power keep the same, the relationship of surface thermal cycling character parameters and heat input as following:（a） the time above freezing point increases along with the increasing of heat input;（b） the average heating rate of molten pool decreases while the heat input increases;（c） the average cooling rate of800～500℃and800～300℃decreases while the heat input increases, and ωc-8/5is much larger than ωc₈/3.（4） Thermal images of molten pool can be got by means of high-speed camera monitoring, after analysis and calculation, the three-dimensional welding temperature field can be restructed.（5） When heat input is large, heat-affect zone is relatively wide, grains refines gradually in the direction along fusion line to base metal, heat-affect zone of phrase transition is obvious; when heat input is small, heat-affect zone is relatively narrow, it is difficult to identify heat-affect zone of phrase transition. Sheet is more sensitive to heat input than thick. When heat input increases, the likelihood of welding defects increases, and as heat input increases, it is prone to discover defects, while He as shielding gas.