Study On The Morphology Of Keyhole In Laser Deer Penetration Welding Of Optical Fiber

High power fiber laser has the advantages of small size,light weight,good beam quality under extremely high power,high processing flexibility and low operating cost,which has attracted the attention of many scholars in the field of global welding.However,there are still many defects in laser deep penetration welding,such as weld depression,spatter,hump and so on.Generally speaking,the physical process of laser welding is the conversion process from laser light energy to material heat energy.The abovementioned welding defects are related to the melting,evaporation,melt flow,solidification phase transformation and other phenomena during the energy conversion process.The keyhole is the main place where laser and welding material interact and realize energy conversion in deep penetration welding.Therefore,it is of great significance to understand the physical process of laser deep penetration welding and optimize the welding process.In this paper,the formation process,the 3-D shape and the energy coupling on the wall of the keyhole are studied.The experiment is carried out by means of adjusting the laser output time to millisecond level,and the "sandwich" method is used for auxiliary verification.It is found that the formation process of keyhole is very fast,and the complete formation time of keyhole is millisecond level.In low speed welding,the formation process of keyhole includes three stages: rapid increase,slow increase and basic stability.In high-speed welding,the formation process of keyhole only increases rapidly.The rapid formation of keyhole has little to do with welding speed,welding power and defocusing amount.However,the formation time of keyhole is not longer than the laser welding characteristic time(spot diameter divided by welding speed),and the decrease of welding characteristic time with the increase of welding speed is the reason that keyhole only has a rapid formation process in high-speed welding.The experiment was carried out by means of fast freezing in the molten pool.The keyhole morphology of laser welding Cu-Hf alloy can be divided into two parts: vapor pressure maintaining area(the diameter is on the order of millimeter)and laser direct acting area(the diameter is the same as the spot diameter);the keyhole morphology of laser welding low carbon steel is approximately circular,its diameter is the same as the vapor pressure area of the keyhole in Cu-Hf alloy;increasing laser power or reducing welding power The connection speed can improve the closing time of keyhole in the molten pool of low carbon steel.Further analysis shows that the main reason for the hole to remain in the molten pool of low carbon steel is that the steam pressure in the hole keeps the hole from collapsing.The ultra depth of field microscope was used to observe the front wall of Cu-Hf alloy keyhole which was preserved by the fast freezing method of molten pool.It was found that there was an obvious structure of concentric elliptical ring.The direct observation method was used to verify that the roughness of the front wall of the 3-D keyhole obtained by the two experimental methods was basically the same,and with the increase of laser welding power,the roughness of the hole wall was significantly improved.Combined with two experimental methods,a 3-D keyhole model is established and small spheres with different diameters are added to represent the influence of different roughness on laser absorption rate.It is found that with the increase of roughness,the absorption rate of keyhole wall for laser increases significantly.For example,when the roughness is increased from 0.5 ?m to 1 ?m,the absorption rate of pore wall is increased from about 45% to more than 60%.