Research On Laser Welding And The Mechanism Of Liquation Crack Of 718 Superalloy

Due to the advantage of the excellent mechanical properties, oxidation resistance and thermal corrosion resistance, the 718 superalloy has been widely applied in aerospace, aviation, petrochemical industry and energy industry. In order to improve the production efficiency and reduce the cost, welding technology has been gradually applied in the connection and repairment of 718 alloy components. Laser welding has been the first choice of the 718 alloy welding and repairing technology due to the advantages of high energy density, small heat input compared with the traditional welding. CO2 laser welding with filler and no filler were carried out to wrought 718 superalloy with defferent grain size. Laser welding characteristics of 718 alloy, microstructure,mechanical propertiesand the mechanism of the liquation crack were researched in this paper by optical microscope, scanning electron microscopy and energy dispersive and other analysis methods. The experimental results showed that:Good weld joints with a “nail” shape on cross section can be got by CO2 laser welding and change the laser welding process parameters will affect the weld formation. Based on the research of the laser welding process of alloy 718, it was concluded that the appropriate welding process parameters: laser Power = 6 ~ 7 kW, welding speed = 1 ~ 1.5 m/min. The microstructure of the weld zone is typical solidification structure and the low melting eutectic phase(?+Laves) with rich Nb was found between the interdendritic segregation. The blocky carbides were found dispersion distribution on the parent metal austenitic matrix and discontinuous precipitated phase were found on the grain boundary, mainly for MC type carbide,?phase and a small amount of Laves phase. The grain boundary of the HAZ zone was coarser than the base metal due to precipitated phase on the grain boundary partially melted or disappear.The microhardness of the no filler laer welding joint was lower than the baser metal. The microhardness of laser filler repair welding joint was higher than the base metal. The postweld heat treatment can obviously increase the tensile strength of the joint at room temperature and at high temperature, but it will reduce the plastic of the welding joint. The tensile fracture of welding joint at room temperature was ductile fracture and at high temperature, the tensile fracture was brittle fracture and fracture occurred in the weld zone.The liquation cracks of heat affected zone were found to be relationship with the chemical composition of the base metal, the grain size and the welding stress during the laser welding. The precipitated phase with Nb?Mo and B on the austenite grain boundary melted and constitutional liquation by the welding heat input was the major cause of liquation crack. The baser metal with different guain sizes has different sensitivity to the liquation crack, the greater the more prone to the liquation. The laser welding to simulate the actual repair weling of 718 alloy found that with the increase of the welding stress, the total length of liquefied cracks increased.