Effect Of Welding Thermal On The Microstructure And Defects In Fused Zone Of Nickel-based Single Crystal Superalloys

Nickel-based single crystal superalloy has excellent high-temperature mechanicalproperties, which was the main material of aero-engine turbine blades, blades wasdamaged due to wear、erosion and other reasons in service which often used to weldingprocess to repair, in order to reduce maintenance costs. In this paper, electron beam andTIG heat source were used to welding nickel-based single crystal superalloys DD407.The effects of different welding heat source and process parameters on the fused zoneappearance were studied, the characteristics of microstructure of fused zone metal underdifferent welding heat effect were analyzed by optical microscopy and spectroscopytests, and the distribution characteristics、influencing factors and formation reasons ofstray grains and cracking which were the two kinds of defects of solidifiedmicrostructure were discussed in depth.The results were shown as follow: the fused zone of nickel-based single crystalsuperalloys DD407was formed in the different regions of several growth direction dueto the function of preferential growth and heat flow under the effect of welding heat.Under the TIG thermal effects, when the current decay time was zero, fused zone didnot exist the planar microstructure, with the current decay time increase to2seconds ormore, there was a clear planar growth area in microstructure, and the fused zone wascrystallized. Increase the current decay time, which was not conducive to maintainingthe integrity of single crystal of fused zone.The formation of stray grains in fused zone microstructure of nickel-based singlecrystal superalloys DD407was caused by constitutional supercooling of the front ofsolidification interface under the effect of welding. The degree of constitutionalsupercooling was largest in the different crystal junction, so it easy to form stray grainsand destroy the integrity of single crystal of fused zone. The stray grains of fused zonewere grew with the morphology of equiaxed dendritic and columnar dendritic. Underthe effect of welding, the factors that affect integrity of single crystal of fused zone wasmainly welding heat input. Using the process parameters of small welding current andlarge welding speed can reduce the content of stray grains of fused zone, which is moreconducive to maintaining the integrity of single crystal of fused zone.It was found through the process, the nickel-based single crystal superalloysDD407has a higher sensitivity to hot cracking in weld solidification process, the mainlyexpression as follows: under the electron beam thermal effects, the fused zone would mainly produce longitudinal cracks, transverse cracks, intergranular cracks. Under theTIG thermal effects, the fused zone would mainly produce crater cracks, intergranularcracks. And the crack often gathered in stray grains area. Under the electron beamthermal effects, when other process conditions unchanged, with the increase of beamcurrent, the content of solidification cracking in fused zone increased first and thendecreased. Under the TIG thermal effects, with the current decay time increase, thecontent of solidification cracking in fused zone increased.