Experimental Study And Numerical Modeling Of Thermophysical Process In Laser Welding Under Low Vacuum And Its Welding Characteristics

With the development of laser technologies and optical devices,the available power for laser welding becomes higher and higher,which enables an improved penetrability.The singe-pass deep-penetration weldment makes laser welding show great advantages in manufacturing of thick components.However,weld defects,such as spatters,undercut,porosity and root sag,are easily formed,and the processing window for defect-free sound laser weld is thus very narrow when higher depth of penetration is achieved by increasing laser power.Therefore,how to increase penetration depth without sacrificing welding quality is the main challenge for application of high power laser welding.It is reported that laser welding under low vacuum or reduced ambient pressure is an effective method for improving both the penetration depth and weld quality.In view of the urgent need of developing high quality and high efficient laser welding technique for the application in the fields of aerospace and weapon equipment industries,the characteristics of laser welding under low vacuum and the corresponding thermalphysical processes were studied in this work.Firstly,the characteristics of weld formation during laser welding under low vacuum were investigated,including the effect of ambient pressure on weld geometries and porosity defects,the comparison of processing widow for laser welding of 10 mm thick high strength steel between at atmospheric pressure and under low vacuum,and full penetration laser welding of 20 mm thick high strength steel with single pass under low vacuum.It was found that weld geometry was very sensitive to reducing ambient pressure,and the penetration depth achieved under low vacuum was about two times as deep as that obtained at atmospheric pressure.There was a critical ambient pressure of about 1 k Pa,below which the weld geometry almost does not vary.3D Micro X-ray CT results revealed that low vacuum was favorable to reducing porosity defects,especially the process porosity caused by the instability of welding process.When the ambient pressure reduced from atmospheric pressure to a pressure of 10 k Pa,the number and volume of the pores in the test samples decreased by 63% and 89%,respectively.The experimental results of full penetration welding of 10 mm thick high strength steel for different ambient pressures showed that the required laser power was smaller and the processing window for sound weld was much wider under low vacuum.High quality weld joint of 20 mm thick high strength steel was achieved at a laser power of 8 k W,a welding speed of 0.6 m/min and a negative defocused distance of 10 mm using single pass laser welding under low vacuum of 1 k Pa.Secondly,the effect of ambient pressure on themophysical processes during laser welding,such as morphologies of plasma plume,interaction between laser and plasma plume,morphologies and stabilities of keyhole and inner f luid flows in the weld pool,were investigated experimentally.It was found that the keyhole with periodic expansion and contraction was very unstable at atmospheric pressure.The keyhole collapsion induced by the instability of keyhole was the reason why bubbles formed in the weld pool.At atmospheric pressure,there were two main fluid flow patterns in weld pool,i.e.,anticlockwise fluid flow induced by recoil pressure and outward fluid flow induced by Marangoni force.A flow vertex formed due to the two opposing fluid flow,trapped the bubbles in the weld pool and prevented their escape before solidification.The plasma plume showed an obvious change with a decrease in ambient pressure.The results showed that only a small plasma plume was observed around the keyhole outlet and the laser attenuation by plasma plume was very weak under low vacuum.With the decrease of ambient pressure,the stability of keyhole cross section area and keyhole depth increased significantly.Although the fluctuation of keyhole wall was also observed under low vacuum,the fluctuation amplitude was very small and the keyhole collapsion was seldom observed.There was an upward flow in the rear weld pool for lower ambient pressure.Bubbles could easily escape from the weld pool with the help of this upward flow,and hence the porosity defects were reduced in laser welding under low vacuum.Finally,a comprehensive 3D keyhole mode laser welding heat transfer and fluid flow model with the ability to consider the change of themophysical processes induced by the reduction of ambient pressure was built.The changes of boiling point,the pressure balance on keyhole wall and laser attenuation coefficient were studied numerically to examine the effect of ambi ent pressure on weld geometry and to predict the weld geometries and temperature fields for various ambient pressures.The depression of boiling point had a striking influence on heat transfer in horizontal direction,and resulted in a narrower and shorter weld pool.The pressure balance on the keyhole wall affected the vapor pressure inside the keyhole and the temperature on the keyhole wall.Although an excess pressure inside the keyhole from the bottom to the top exited at atmospheric pressure,the temperature gradient on the keyhole wall was very small.For a similar excess pressure in the keyhole at reduced ambient pressure,the temperature difference between the bottom and the top of the keyhole was much larger than the corresponding value for laser welding at atmospheric pressure.The temperature gradient on the keyhole wall drove a Marangoni convection along the keyhole wall,and finally changed the fluid flow pattern under low vacuum.The relation of the values between vapor pressure inside the keyhole and the surface tension on the keyhole wall determined the keyhole stability.The close values of vapor pressure and surface tension made the keyhole have a stronger resistance to fluctuation of vapor pressure,and finally resulted in a much stable keyhole.The laser attenuation coefficients mainly affected the laser energy absorption,thus influencing the keyhole depth and penetration depth.The weld geometries could be predicted accurately using the model by considering the above three causative parameters.The fair agreement between the calculated and the exper imentally obtained weld pool of high strength steel,aluminum alloy and commercially pure titanium with obviously different material properties at different ambient pressures indicates the validity of the physical processes considered in the simulations.