Mechanism Of Solidification Cracking In Narrow-gap Laser Welding With Filler Wire For High-strength Steel

Heavy section high-strength steels have been used widely in the manufacture of naval ships,and welding is a key process during its manufacturing.In recent years,the narrow gap laser welding with filler wire has the common advantages of narrow gap welding and laser welding at the same time:energy concentration,low heat input,low deformation,fast welding speed,and low filler.It has been favored by researchers in the welding of thick-walled components,the development prospect is broad.However,In the single-layer filling test of 10Ni5CrMoV steel narrow gap laser welding with filler wire,as the aspect ratio of the weld increases,solidification cracking will occur in the center of the weld.However,the main cause of solidification cracking is still unclear.Therefore,the paper mainly focuses on the solidification cracking problem in narrow gap welding and studies its formation mechanism and proposes countermeasures,which has important theoretical significance and application value for realizing high-efficiency and high-quality welding of large-thickness and high-strength steels.20mm thick 10Ni5CrMoV steel is used as the main test material.Firstly,the effects of various process conditions(laser power,welding speed,wire feeding speed,oscillation pattern,groove bottom width)on weld formation and cracking sensitivity are studied.The research results show that when the wire feeding speed is 6m/min,the laser power is 5-6kW and the welding speed is 0.45-0.6m/min,the cracking sensitivity is high;At the welding speed of 0.8m/min,the wire feeding speed and the groove bottom width have little effect on the cracking sensitivity;the influence of different oscillation patterns on the cracking is not obvious.In addition,by calculating the aspect ratio and fusion ratio of the weld under different process conditions,both of them are not necessarily related to the formation of cracking.Focus on the formation mechanism of solidification cracking.The results show that the size and position of the bulge region in the weld are closely related to the solidification crack,and the width Wmax of the bulge region and the distance d from the weld surface will change the solidification sequence of each position of the weld,thus determining the position of the final solidification point of the weld.If the final solidification point is inside the weld,and the surrounding liquid phase has solidified,and the final solidification point is opened under the action of stress and no excess liquid phase is replenished to form cracking.Therefore,the concept of bulging angle is proposed to measure the degree of bulge.By measuring the size of the bulge region and combining the cracking condition,the cracking criterion based on the bulging angle β is obtained.When the bulging angle is 30°<β<60°,solidification cracking will occur,and no cracking will occur in other cases.1000MPa grade high-strength steel was used for narrow gap welding to prove the reliability of the theoretical analysis.The results show that cracking occurs when 20°<β<40°,indicating that the range of different steel grades is different,30°<β<60° only for 10Ni5CrMoV steel.On the basis of the cracking formation mechanism,it is proposed to adjust the welding process to change the weld bulging angle β to suppress the solidification cracking.Firstly,the relationship between process-bulging angle-cracking was established,and the process range of β<30° was obtained:laser power 3.8-4.5kW,welding speed 0.36-0.6m/min,wire feeding speed 6.2-7.5m/min or laser power 6.5-8kW,welding speed 0.75-1.0m/min,wire feeding speed 6.2-8m/min,and the process range of β>60° is:laser power 6.5-8kW,welding speed 0.6-0.75m/min,sending wire speed 4-5.5m/min.Finally,the above process interval is verified,which conforms to the solidification cracking criterion.