| The high power and quality laser beam has developed rapidly in recently years,which makes the welding thickness of the metal material reach to 30 mm in a single pass of autogenous laser welding process.The high power laser welding is widely applied for lager thickness of major structure in energy,nuclear facilities and marine engineering,for its extremely quality standards,long stability and reliability of the weld.However,the high power laser welding is limited by the root weld defects,such as periodic humps or dropping out at the root of the metal pool in the welding process,which makes the weld quite poor.Because the weld is hardly formed in a single pass of the current laser welding of the thick metal,all the scientific researchers and the engineering technicians are hope to find technological breakthroughs on maximum thickness of laser welding process for the first time.The thick-plate laser welding is always the focus of the welding field at the home and abroad in recent ten years.The systematic and comprehensive study on welding experiments and mechanism are carried out,which are showed as follows:Firstly,an thick-plate laser welding method using additional electromagnetic force is proposed to prevent the weld defects,such as root periodical humps and dropping-out,in this paper.The continuous and upward electromagnetic force,which is generated by the single-directon high current and external magnetic field,is used to ensure the formation of the weld and keep the stability of the welding process.The high speed camera and the ―sandwich‖ welding method are used to capture the transient and dynamic flow behaviors of the melted metal pool in this paper.For the first time,the transient and direct flow behavior and evolution of both the keyhole and the metal pool,which are from upper part,longitudinal section and the root in the welding process.The dynamic flow behavior and evolution include transient dropping out process of the root metal pool,the transient evolution process of the metal pool from its formation to dynamic stability,the transient flow behavior of the inner pool,and the static welding process with no root dropping out.(1)It is found that the main force,which affect the stability of thick-plate laser welding,are the recoil pressure from the vapor of the keyhole,the dynamic and downward flow pressure,oscillating pressure at the rear pool and the downward gravity.The key and decisive reason,leading to the root humps and the dropping-out,is that the root surface tension at the root pool is not enough to balance the downward force above.In addition,the intense oscillation of the keyhole,due to the internal wall of the local keyhole intensely vaporized by laser radiation,drives the metal liquid flow at a high speed and aggravates the instability of the laser welding process.(2)It is also found that the root bulge always existed in laser welding process,even the laser welding process with supporting electromagnetic force.The reason,to keep the welding process static and the good weld formation,is that the root metal pool is supported by the electromagnetic force and no more drop down.It is concluded that the key factor to improve the weld quality is the sustainability and stability of the single direction electromagnetic force upward.Based on the research of the dynamic an d transient flow behavior,we comprehensively and deeply analysis the force,making the welding process instability.Therefore,we get an idea that if the thick plates could be welded by the laser in a single pass,there must exist an external force to sup port the metal pool,coupled with the surface tension.Then,based on the experiments of the high speed camera and the ―sandwich‖ method on the dynamic and transient flow behavior of both keyhole and the melted metal pool,the mechanism research is carried out using electromagnetic force,which focus on inhibiting root dropping-out,keeping the welding process static and assuring the weld formation.(1)There is a delta-shaped region,which is just below the keyhole and at the front of the liquid pool.The material at the delta-shaped region is melted by the mode of heat conduction,when the laser move continually a certain distance.Two parameters of ? and H,which is used to evaluate the important effect of the delta-shaped region on the welding process stability,have the crucial impact on the dynamic flow pressure at the root pool in the thick-plate laser welding.The most direct result of the dynamic flow pressure on the liquidity of the metal pool is the bulge at the front of the pool.(2)It is the oscillation of the keyhole,for local keyhole being intensely vaporized by laser radiation,is the direct reason for the bulge growing up,aggravating the balance of the root surface and leading to dropping-out.(3)In the thick-plate laser welding process with electromagnetic force,the significance of the upward electromagnetic force is to prevent root metal pool from dripping down and keep the root bulge dynamic,continual and sustained always.The metal liquid at the bulge in the front could return to the metal pool,supported by both the electromagnetic force and the root surface tension,which is vital to maintain the welding process stable,dynamic,and balanced.If the root dropping-out could be prevented,after the laser move forward,then rear metal pool solidify continually,and the weld could form finally.The weld formation could also controlled by adjusting the magnitude of the electromagnetic force.The morphology of the weld could be improved at the rear metal pool by the electromagn etic force and the quality of the weld could be optimized in a certain range.Next,in order to deeply understand the significate influence of the electromagnetic force on the welding process,the distribution of the magnetic field is simulated in three-dimension mode in this paper.It is found that the direction of magnetic induction line at the region of the metal pool change to an opposite,which mean that the direction of the electromagnetic force is upward at the middle and root weld,but downward at the upper weld.when the magnetization direction of the magnet is changed in vertical direction or the magnet is posited in a 45-degree angle,the distribution of the magnetic field is calculated again.If the magnet is posited in a 45-degree angle,all the direction of the magnetic induction line stay the same and exist no opposite,which mean the electromagnetic force is always upward in the welding process.If the magnet is magnetized in different direction,the direction of the magnetic induction line also keep same direction and the density of the magnetic field increases.Based on the results of the simulation,it is concluded that the distribution of the magnetic field could be optimized to control the magnitude and the direction more accurate as required,which is benefit to perfect the influence of stable welding mechanism of the influence of the electromagnetic force on the weld formation.Finally,the systematic and comprehensive welding experiments on the morphology of the weld are implemented in laser welding of different thick 316 stainless steel,including the influence of the laser power on the weld humps morphology of 10-15 mm stainless steel,the inhibiting effect of the upward electromagnetic force on root dropping out in the welding proces s of 16-30 mm thick metal materials,and four welding process comparison experiments: autogenous laser welding process,laser + high current welding process,laser + magnetic field welding process and laser + current +magnetic field welding process.(1)It is found that the laser power has a direct effect on the forming process of the root humps,the weld quality is sensitive to the magnitude of the laser power,and the adjustable arrange of welding parameters to obtain weld with no defects is relatively smal l and narrow.But the laser welding process with the additional electromagnetic force could minimize the input value of laser power in laser welding of thick plates,and prevent the periodic root humps to maintain welding stability and weld formation.(2)The four comparison experiments are used to verify the fact that the real supporting force on the melted metal pool is the electromagnetic force,not the high current or magnetic field alone.The electromagnetic force is space volume force and exist in t he while melted metal pool,not a surface force just at the root surface.It is also concluded that the electromagnetic force is little influenced by the hysteresis and demagnetization effect.(3)The electromagnetic force could increase the concretion rate of crystal and decrease the growth rate of the tiny crystal,which is beneficial to homogenize and refine the grains and improve the mechanical property of the weld. |
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