A Research On The Formation Mechanism Of The Keyhole In Deep Penetration Laser Welding Based On An Analog Detection Technology Of The Dynamic Keyhole

Compared with the traditional welding methods,deep-penetration laser welding has many significant advantages and has been widely used.But at the same time,it still has many shortcomings,such as the weld sag,porosity,easy to produce spatter and so on.These shortcomings restrict the further development of laser welding technology.They are closely related to the keyhole behavior caused by material melting,evaporation and molten pool flow during welding.Therefore,it is of great meaning to study the formation and maintenance mechanism of the keyhole in deep-penetration welding laser process for optimizing welding technology and improving weld quality.Firstly,an analog detection technology of the dynamic keyhole is adopted in this paper.It uses gas jets to create a keyhole in liquid materials,and films its formation and maintenance process by high-speed cameras.Compared with the keyholes obtained by "sandwich" observation method,it is found that there are many similarities in morphology,dynamic behavior and influence of changing process parameters.And in the actual laser machining process,the behavior of the keyhole is mainly caused by the gas-liquid interaction between the pressure in the keyhole and the molten pool,which is basically consistent with the dynamic stress of the keyhole in the simulation experiment in this paper.Therefore,the results obtained by this method meet the requirements of similarity principle and have guiding significance for the study of the formation and maintenance mechanism of keyholes in the actual laser deep penetration welding process.Secondly,through the analysis of the result of the simulation experiments,the formation and maintenance mechanism of the laser keyhole is deduced.It is considered that the recoil pressure caused by material evaporation is the main driving force of the keyhole formation,and it decreases with the increase of the keyhole depth during the whole keyhole formation process.At the same time,surface tension,hydrostatic pressure and other factors that can close the keyhole are increasing,so during the formation of keyholes,the change of depth will be fast first and then slow.Then,the bending of the wall surface during the maintenance of the keyhole was observed.After calculation and analysis,it was concluded that the internal pressure of the keyhole was different at different depths.Therefore,the keyhole is briefly divided into three areas along the depth direction to analyze its maintenance mechanism: the top region,the middle region and the bottom region.The top area is near the keyhole outlet,where is maintained mainly by surface tension of the molten pool and the recoil pressure generated by material evaporation.In the middle of the keyhole,the recoil pressure generated by the evaporation of the front material and dynamic pressure brought by the impact of the material vapor on the rear side together maintain the keyhole opening.At the bottom of the keyhole,due to the existence of multiple reflection and absorption mechanisms,the material evaporation at the bottom of the keyhole is relatively more than the other regions.Recoil pressure and dynamic pressure of gas are the main factors to maintain the opening of the bottom of the keyhole,but the surface tension of the molten pool here,hydrostatic pressure,fluid dynamic pressure,etc.,restrict the further expansion of the keyhole.And then,according to the mechanism of keyhole maintenance process,the causes of keyhole oscillation and bubble formation are analyzed.The keyhole will shrink rapidly under the joint effect of factors such as surface tension,hydrostatic pressure when the pressure inside the keyhole decreases instantly.Then,the keyhole is re-formed through a new round of “drilling” under the action of the laser and completing an oscillation.In the process of the keyhole oscillation,when the longitudinal contraction rate is less than the transverse,the keyhole will be transverse“cut off”,part of the gas will be wrapped by the liquid and forming bubbles.It will form pores in the weld if it can’t be opened again of discharged in time.Finally,according to the analysis and calculation results,it is considered that in the process of laser deep fusion welding,the pressure at the outlet of the keyhole is the environmental pressure,and its internal pressure must be greater than the environmental pressure.And in the direction of depth,the internal pressure increases with the increase of the keyhole depth.But there is a pressure drop in the keyhole,near the bottom,which is also where the keyhole to be “cut off” to form bubbles during oscillation.