Study On Plasma Physical Characteristics And Welding Defects During Laser-MAG Hybrid Welding Of 42CrMo Steel

42CrMo steel has excellent mechanical properties and technological properties.It is used to make gears,rear axles,connecting rods and other parts with great load in industry.With the great increase of mechanical load capacity and heat load capacity when the piston is working,the steel piston made of 42CrMo steel gradually began to replace the traditional aluminum piston.However,because of the high carbon content and the content of alloy elements in 42CrMo,the welding performance of the alloy is poor,and cracks and pores are easy to be found in the weld.It seriously affects the wide application of 42CrMo in industry.At present,the welding methods mainly used for 42CrMo are arc welding,resistance welding,friction welding and electronic beam welding.Fiber laser-MAG hybrid welding technology overcomes the disadvantages of laser welding and arc welding,and combines the advantages of both,more and more research and application have been conducted in recent years.The plasma both inside and outside of the keyhole,the dynamic behavior of keyhole and molten pool,the formation mechanism and control methods of cracks and pores in the welding process of 42CrMo steel with laser-MAG hybrid welding method,have been studied theoretically and experimentally in this dissertation,which provides the experimental basis and theoretical guidance for the stable welding process and good weld quality of the laser-MAG hybrid welding of 42CrMo steel in industry.The main research work is as follows:(1)The plasma signal and characteristic parameters,such as electronic temperature and electron density,that inside and outside keyhole during the laser welding,leading laser laser-MAG hybrid welding and leading arc laser-MAG hybrid welding of 42CrMo steel were detected,calculated and analyzed.The results showed that the plasma spectrum was not detected during the laser welding with side blowing,but plasma spectrum can still be detected in the two kinds of hybrid welding.According to the plasma spectrum,for laser-MAG hybrid welding,when leading laser,the effect of laser on arc initiation and arc suppression of MAG arc is more obvious than that of leading arc,and the arc can be drawn deeper along the ionization path to the keyhole.The plasma average temperature of leading laser laser-MAG hybrid welding is about 700K lower than that of leading arc laser-MAG hybrid welding,and the plasma density is also slightly lower.The maximum values of plasma density in the two kinds of hybrid welding are all near the openings of the keyhole.(2)The laser-MAG hybrid welding of 7mm thick 42CrMo steel with V groove were conducted.The flow of the molten pool and the dynamic behavior of the keyhole were observed with a high-speed video camera.The information on the variations in the weld temperature was collected using an infrared(IR)thermal imaging system.The influence of process parameters,such as laser-arc distance,defocusing distance,laser input energy-arc input energy ratio,relative positions of the laser beam and arc,angle of incidence of the laser beam and shielding gas flow rate,on the formation of porosity were investigated.The results showed the flow of the molten pool,the keyhole stability,the weld cooling rate and the bubble escaping rate as a result of different parameters could all affect pores formation.The orthogonal test results of 5mm thick 42CrMo steel showed that,the laser power and the welding speed were two key process parameters to reduce the pores,a relatively high laser power and a relatively low welding speed favored the reduction of pores,fewer pores formed with positive defocus than with zero and negative defocus,the defocusing distance and the wire feed speed had limited potential to reduce pore but had optimal values.An ideal pore-free weld was obtained for the optimal welding process parameters.(3)Aimed at the problem of hot crack easily occurred at lap joint of ring weld of 42CrMo steel piston,a welding test to simulate the circular welding on 42CrMo steel was performed to compare and investigate the possibility of hot crack formation at the weld joints in three welding processes:laser welding,leading laser laser-MAG hybrid welding,and leading arc laser-MAG hybrid welding.A high-speed camera is used to observe fluid flow in the molten pool and weld solidification during the welding process to calculate the solid-liquid boundary movement rate(VSL).An IR thermal imaging device is used to collect the information on the variations in the weld temperature during this period to calculate the solid-liquid boundary temperature gradient(GSL).The result showed that leading laser laser-MAG hybrid welding process has a smaller VSL and a smaller GSL compared with leading are laser-MAG hybrid welding process and laser welding process.Additionally,the metal in the molten pool has superior permeability while flowing toward the dendritic roots and can compensate for the inner-dendritic pressure balance.Therefore,leading laser laser-MAG hybrid welding process has the lowest hot cracking susceptibility.(4)The microstructure and properties of welded joint of 42CrMo steel by laser-MAG hybrid welding were investigated.The results of metallographic examination show that the base metal is a tempered sorbite and contains a small amount of ferrite.The weld composition is mainly acicular martensite,and lath martensite has not been found.The microhardness test shows that the highest hardness area is in the heat affected zone,which indicates that the tempered martensite is not formed in the heat affected zone.The hardness at the bottom of the weld is greater than the hardness at the top of the weld.The cause of this difference is due to the effect of electric arc,which lead to the cooling rate is relatively low in the weld pool at the top of the weld.At the same time,because of the addition of welding wire,under the neutralization of ER80S-G welding wire with low carbon content,the carbon content in the upper part of the weld decreases,resulting in a decrease in hardness.The result of scanning electron microscope observation and energy spectrum analysis of the porosity in the weld seam showed that,the pores are formed by the collapse of keyholes,the shape of the pores is very irregular,the inner wall is not smooth and has traces of liquid metal scouring,the wall components of round pore and pear shaped pores are approximately the same,all belong to invasive pores.The result of scanning electron microscope observation of cracks in welds showed that,all cracks are cracked along the grain boundary of the columnar crystal,and the fracture have the characteristics of intergranular liquid film separation,that is,there is a thin layer of liquid film when cracking.