Study On The Process And Porosity Formation Mechanism Of Laser Deep Penetration Welding Of Medium Thick Plate Of Mild Steel

Mild steel is widely used in industry because of its good mechanical properties.In addition,with the further research and development of laser technology,mild steel laser welding method has also been popularized.However,the welding mechanism of medium thick plate is complex,and the defects such as spatter,pit and porosity are easy to appear in the welding seam,which seriously affects the welding quality;On the other hand,in the deep penetration welding of thick plate of mild steel,the weld is easy to produce porosity and weaken the effective working section of the weld seam,resulting in stress concentration,greatly reducing the quality and performance of the product.Therefore,it is not only of great theoretical significance but also of scientific significance for the industrial welding production of mild steel to deeply study the process characteristics and pore formation mechanism and characteristics of laser deep penetration welding of medium thick plate of mild steel.In this paper,this project studies the process characteristics,microstructure and mechanical properties of medium thick plate of mild steel.Furthermore,through the analysis of the mechanism and characteristics of the formation of pore in the weld,a better method and welding process parameters to restrain pores are obtained,as well as good welding effect and quality.At first,according to the"I"type cross-section morphology,the theory of 3D heat source model was set up.The temperature field distribution and thermal cycle curve were calculated by Sysweld simulation software,and the relationship between the temperature field and the formation of pores and the microstructure of joints was obtained.The second,exploratory experiments on the formation mechanism of pores in mild steel were designed to study the influence of protective gas types and surface oxides on the formation of pores in the state of penetration and non-penetration,observe the porosity in the longitudinal section,and measure the element distribution of the pore wall and the surrounding weld by EDS.The results show that the metallurgical pores are mainly H₂pore and CO pore,and the H₂pores are round in shape with rough inner wall and shallow depth.The content of oxygen element is much,but the content of carbon element is almost not.The CO pore is striped worm shape,with deep pore wall,more carbon and oxygen elements,and carbon segregation phenomenon exists.The shape of keyhole pores is irregular,and there are many folds in the inner wall,and there are traces of metal scouring.CO₂protective gas is used to effectively inhibit the generation of pores;The surface oxide can enhance the laser absorption,the molten pool flow is intense,which is conducive to bubble escape,and also can reach a larger depth of penetration.In the state of penetration,the probability of forming pores is small,mainly because the bubbles can escape from the upper and lower part of the molten pool.The effect of welding parameters on porosity formation and weld formation of laser penetration welding was studied by single factor experiment.With the power condition,the number of pores first decrease and then increase.With the increase of velocity,the number of pore increased,but the stomata decreased first and then increased.From negative defocus of-4mm to positive defocus of 4mm,the number of pores presents a"V"shape distribution,and the porosity shows an upward trend.If the welding parameters are not appropriate,it is easy to cause uneven forming,splashing,pits and hump defects on the surface of the welding parts.In addition,laser power represents the size of the energy,forming the"X"type,"I"type,"cone"and"ellipse"type cross section morphology,which successively represent the three different states of transition penetration,moderate penetration and only penetration.Finally,the results show that under the welding conditions of power 3100W,speed 10mm/s and defocus of 0mm/s,the porosity is lower and the weld is smooth formed.The relationship between microstructure and microhardness of the joint was also established.The equiaxed crystal consists of polygonal proeutectoid ferrite.The columnar crystal at the edge of the weld is consisted of ferrite.The heat affected zone is mainly consisted of ferrite,bainite and martensite.The region with more ferrite content has a lower hardness,while the region with higher martensite and bainite content has a higher hardness,and the joint presents an"M"shaped hardness distribution.It was found that only the fracture of 20mm/s joints in the speed group appeared in the weld zone with a strength of 310MPa,and the fracture zone of weld joints under other parameters appeared in the base metal zone with a strength close to that of the base metal.