Study On Laser MAG Arc Composite Welding Process And Joint Drop Hammer Impact Performance Of Marine Steel

Ships are inevitably subjected to impact loads(such as explosive shock waves and high-speed fragments)in actual service,leading to local plastic deformation and even fracture of the ship structure.The mechanical properties of welded joints become the main influencing factor on the load-bearing capacity of the ship structure.For the welding of marine steel,traditional arc welding methods have disadvantages such as low welding efficiency and large deformation after welding.Laser arc hybrid welding,a new welding method,is widely used in the manufacturing industry due to its advantages such as high welding efficiency,low welding heat input,and small post weld deformation.At present,research on the static mechanical properties of welded joints is relatively in-depth both domestically and internationally,but few scholars have conducted research on the impact resistance of welded joints under dynamic impact loads.This article conducted laser MAG arc hybrid welding process tests on AH36 marine steel butt joints and T-joints with plate thicknesses of 6mm and 8mm.The optimized welding process parameters were used to prepare the butt joints and T-joints,and the impact tests of the joints under dynamic load were carried out using a drop hammer impact equipment.By using acceleration collection and high-speed photography methods,the loading data and fracture time of butt joints and T-joints during the impact process were observed and analyzed,and the influence of different process parameters on the impact fracture energy of joints was studied.The microstructure and fracture morphology of different parts of the joint were characterized using analysis and testing techniques such as SEM and OM.The influence of different process parameters on the microstructure transformation and drop hammer impact performance of the joint was studied,and the failure mechanism of the joint under impact load was explored.A numerical simulation model of 8mm T-joint drop hammer impact was established using LS-DYNA finite element software,and the drop hammer impact process of the T-joint was numerically calculated.The main conclusions are as follows:1)The influence of the main process parameters of laser MAG arc hybrid welding on the weld formation was studied using a controlled variable experimental method.The results showed that when the laser power was 4.0k W,the welding current was 235A-255 A,and the welding speed was 1.2m/min-1.5m/min,single-sided welding and double-sided forming of6 mm butt joints could be achieved;When the laser power is 5.5k W-5.8k W,the welding current is 275A-295 A,and the welding speed is 1.0m/min-1.2m/min,it can achieve single-sided welding and double-sided forming of 8mm butt joints,resulting in beautiful weld formation without defects such as spatter and undercut.2)The optimal welding process parameters range obtained through process optimization for the T-joint is: laser power 3k W,welding current 275A-295 A,defocusing amount+10mm;When the plate thickness is 6mm,the matching welding speed is0.8m/min-1.0m/min;When the plate thickness is 8mm,the matching welding speed is0.6m/min-0.8m/min.At this time,there are no obvious defects on the weld surface and the transition at the weld toe is smooth.3)The results of the butt joint drop hammer impact test indicate that the critical height of the 6mm joint is 550 mm,and the critical height of the 8mm joint is 1000 mm.The impact fracture positions of the butt joint are all located in the base metal with or without weld reinforcement,and the welding heat input has little effect on the impact performance of the joint.4)The results of the T-joint drop hammer impact test indicate that the critical impact height of the T-joint drop hammer impact with a plate thickness of 6mm is 290mm;As the welding heat input increases from 6.2k J/cm to 8.2k J/cm,the width and grain size of the coarse grain zone on the fracture side of the joint increase by 7% and 16% respectively,while the impact fracture energy decreases by 8%;The critical height of the drop hammer impact on the T-shaped joint with a plate thickness of 8mm is 510mm;As the welding heat input increases,the width and grain size of the coarse grain zone on the fracture side of the joint increase by 21% and 56%,respectively,while the impact fracture energy decreases by33%.5)The main form of impact fracture of T-shaped joints is ductile fracture.Under different heat input conditions with a plate thickness of 6mm,there is no significant difference in the microstructure of the T-shaped joint fracture surface,and the fracture surface presents a large number of dense dimples with continuous network distribution;The micro morphology of the fracture surface of the 8mm thick T-joint is mainly tear dimple.The smaller the heat input is,the larger and deeper the dimple is.As a result,the lower the heat input is,the higher the fracture toughness of the joint is,and the greater the fracture energy is required.6)The simulation results of the drop hammer impact of the T-shaped joint indicate that as the load increases,the weld toe of the joint cracks first,followed by fracture at the base metal.From the analysis of the fracture path,the current material model simulation calculation results are consistent with the experimental results.