| Carbon fibre reinforced resin composite is a kind of important structural material for making large aircraft,which has many advantages such as low density,higher specific strength and so on.Invar steel has an extremely low coefficient of thermal expansion and favorable stability,making it an ideal material for the mould used to form aerospace composite parts.Welding is an indispensable process in the manufacturing process of Invar steel moulds,which has a decisive influence on the final performance of the mould.However,the domestic research on the welding process of Invar steel is still in infancy,and the related research is scarce,which seriously limits the use of Invar mould in actual production.Compared with high-energy beam welding techniques such as laser welding,multi-layer arc welding has significant cost advantages when connecting thickness plates,and is currently widely used in the processing of composite parts forming moulds.In this paper,multi layer multi pass welding(MLMPW)and multi layer weave bead welding(MLWBW)process are used to study the automatic welding technology of 19.05 mm thick Invar steel for aircraft composite parts,solving the welding heat process emphatically,and analysing the influence of heat accumulation effect on welding process.In this paper,the technology experiment of MLMPW and MLWBW for Invar steel with thickness of 19.05 mm is designed by common welding parameters,such as welding current,welding voltage,number of layers and number of channels.The results show that the weld appearance of the two welding processes is relatively good when Invar steel is made of 1 mm blunt edge,60 degree groove and copper plate gasket.Compared with MLMPW,MLWBW is relatively simplified,and the weldment undergoes fewer thermal cycles,which can effectively improve post-weld deformation and reduce welding defects.The thermal process finite element modeling of MLMPW and MLWBW is carried out for thickness Invar steel mould.According to the actual size and working condition of the member,the finite element geometric model is established.The spatial discretization of 100 mm×50 mm×19.05 mm specimens is carried out.Besides,meshing is performed using a strategy of transition meshes.The related thermal physical parameters and the double ellipsoidal heat source model are confirmed to calculate.The effects of different interlayer temperature on the thermal process of two welding processes are calculated and analysed by using the MSC.MARC software.Combined with the temperature field simulation results of MLMPW and MLWBW,the heat accumulation effects of the two welding processes are studied.It is found that the heat source of MLMPW is applied more frequently,so the heat accumulation effect of welding is more obvious than MLWBW.The microstructure of Invar steel welding is investigated to explore the microscopic mechanisms affecting the welding performance.The research results show that in the MLMPW process,the weld undergoes multiple heating and cooling,and the subsequent weld has a secondary heat treatment effect on the front weld,which is easy to weaken the quality of the previous weld.The Heat Affected Zone(HAZ)width of the MLWBW joint is narrower than that of the MLMPW joint,and the grains of weld bead are finer.Finally,the influence of thermal accumulation effect on the mechanical properties of Invar steel is studied.The microhardness and tensile test of the automatic welding of Invar steel are carried out to evaluate the effect of the heat accumulation on the mechanical properties of the welded joint.For the MLWBW joints,the weld heat input significantly affects the weld grain size,further increasing the average hardness value of the weld zone and the heat affected zone.Both MLMPW and MLWBW processes are capable of obtaining welded joints with good forming,less weld defects and excellent mechanical properties.The MLWBW simplifies the welding process and improves the welding production efficiency. |
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