Study On Physical Mechanism And Process Of Low Power Pulse Laser Regulated Arc Welding For Titanium Alloy

Titanium alloy welded structural parts have been widely applied in military equipment relates to national security,due to its high specific strength and good corrosion resistance,such as the T joint of the rear fuselage wall and the main bearing of the landing safety on fighters.In recent years,with the acceleration of the modernization process of military equipment manufacturing,it is urgently required to shorten the development and production cycle of the equipment,which correspondingly demands to improve the the efficiency and stability of welding of titanium alloy structural parts.However,the traditional arc welding method prossesses the disperse heat energy distribution and weak penetration force,leading to limited welding speed and large welding molten pool,and the traditional welding method are susceptible to parameter variations,causing burn through easily.To solve these problems,this thesis proposes a low power pulse laser regulated arc hybrid heat source to improve the welding speed and reduce the sensitivity of parameter variation,to achieve an efficient and stable titanium alloys welding process.In this thesis,the properties of low power pulse laser regulated arc hybrid heat source are characterized.The high speed welding process of titanium alloy structural parts by hybrid heat source are conducted.Causes of burn through,undercut and ridge defect are revealed.The arc physical properties of hybrid heat source is characterized,the mechanism of action time characteristics and action spatial characteristics by pulse laser are studied.The nature of formation controlling and welding stability improvement by pulse laser effect are revealed.Hybrid heat source and arc heat source in term of welding energy consumption and speed are compared.Physical properties of hybrid heat source regulated heat source effect are verified by the welding process of titanium alloy T joint structural parts.The main contents and conclusions of this paper are as follows:(1)Characterization of pulse laser regulated arc hybrid heat source.In this part,the energy density,plasma flow state,plasma ionization degree of the central conduction region of the hybrid heat source with laser action(hybrid heat source at peak phase),the action time characteristics and action spatial characteristics of the hybrid heat source before and after the pulse laser action are studied.The results show that the energy density of central conductive region of the hybrid heat source at peak phase increases from 2.4×103 W/cm2 to 1.8x105 W/cm2 which is about 75 times that of the single arc(TIG current 150 A,laser pulse energy 12 J,pulse width 3 ms,welding speed 500 mm/min).Compared with the TIG heat source,the Reynolds number of the central conductive region of the hybrid heat source at peak phase significantly reduces,while the ionization degree of Ti particles is significantly increases.By adjusting the laser pulse width,pulse frequency and pulse energy adjustment,it is possible to control the laser action duration,the peak hybrid heat source duty cycle,the interval time of adjacent cycle and the energy transfer behavior of the hybrid heat source.The time dependent property of the hybrid heat source is controlled.By the periodic compression and defection of the hybrid heat source before and after the pulse laser action,the position of the hybrid heat source can be controlled,and the spatial properties of the hybrid heat source can be controlled.(2)Designing welding process of titanium alloy by pulse laser regulated arc hybrid heat source.In this part,for the burn through,undercut and ridge weld during the welding process,the high speed welding process is designed,and the influence of the parameters on the weld formation is clarified.The results show that for 1 mm thick titanium alloy butt welding,laser power between 80?120 W,arc power between 500?700 W,can lead to good weld formation.For 2 mm thick titanium alloy plate butt joint high speed welding can be realized(laser power 650 W,arc power 3800 W,heat source spacing 1.5 mm,welding speed 2070 mm/min).By adjusting the distance of the heat source,the control of the penetration state of the weld is realized,and the burn through caused by the instability of the molten pool is effectively suppressed.By adjusting the heat source distance and the defocusing,the "ridge" weld appearance during welding process with filler wire is effectively improved.It is found that the energy matching degree of the hybrid heat source welding and the peak hybrid heat source duty cycle affect the stability of the welding process.Coupling heat source welding achieves the effect of a long-time action molten pool with a peak-coupled heat source and a base-coupled heat source for a long time,which is the essential to improve welding stability.Finally,the root welding process of 10 mm thick titanium alloy sheet with 1 mm blunt edge was realized(laser pulse energy 20 J,pulse frequency 20 Hz,arc current 200 A,heat source spacing 2 mm,welding speed 400 mm/min).(3)Regulation mechanism and application of pulse laser regulated arc hybrid heat source.This part reveals the mechanism of action time and action spatial during the hybrid welding process,based on the analysis of the keyhole dynamic behavior and flow state of the molten pool.The hybrid heat source welding of T joint is developed.Hybrid heat source and arc heat source welding are compared in term of welding energy consumption and speed.The results show that the mechanism of time regulation of hybrid heat source is realized by the regulation of the keyhole existence time and its growth rate in the middle and late statges,which means to adjust the action time of the peak hybrid heat source on the time scale,leading to the peak hybrid heat source and the base hybrid heat source alternately acts on the molten pool,and the overheating of the overall molten pool is effectively suppressed and the volume and weight of the molten pool is reduced.The spatial regulation of hybrid heat source is to realize by the regulation of the keyhole generation position and keyhole wall forces,which means to adjust the action spatial of the peak hybrid heat source on the spatial scale,resulting the overheating of the local molten pool is effectively surppressed,by weakening the heat of the metal at the bottom of the keyhole,increasing the surface tension of the bottom metal,and weakening the surface energy of the molten pool.The density gradient and surface tension gradient improve the surface molten pool flow state.The wall penetration welding of T joint structural part is realized(arc power 1900 W,laser power 550 W,welding speed 350 mm/min),and tensile strength of the joint is no less than 94.2%of the base metal.Compared with TIG welding,welding speed is increased to 1.75 times by hybrid heat source,while the welding heat input is 41.3%less.To butt welding of 2 mm thick titanium alloy with similar energy comsumption the welding speed of the hybrid heat source is increased to 7.9 times whena well formed welded joint obtained,and the tensile properties are comparable to those of the base metal.To weld 1.2 mm thick titanium alloy welding,the hybrid heat source energy consumption is 171 J/mm,which is 75.3%if TIG welding energy consumption of 227 J/mm,hybrid heat source welding speed can can improve 5.75 times than TIG welding.Low-power pulsed laser-controlled arc-coupled heat source welding technology can achieve a low-energy,high-efficiency welding process for titanium alloys.