Weld Formation Mechanism And Microstructural Thermal Stability Of Laser Welding Of Inconel 617

Solid-solution strengthened Ni-base superalloy Inconel 617?UNS N06617?is widely used in the Advanced Ultra Super Critical?A-USC?and Very High Temperature Reactor?VHTR?.Welding is an essential part of processing technology in the process of thermal power and nuclear power equipment manufacturing.Ni superalloy is very difficult to be welded,which is an important factor to restrict its application.The laser welding produces a deeper and narrower weldment even in the low heat input condition and is used in the field of aerospace,nuclear and automotive because of the higher power and concentrated heat-resource.Microstructures,mechanical properties and microstructural thermal-stability of the laser welded joint of Ni superalloy Inconel 617 were investigated.Laser welding of 5 mm-thick Inconel 617 without filler wire was studied.With increase of the laser power,heat input decreased and its geometry transformed from Y-type to I-type.Meanwhile,the faster cooling rate of fusion zone resulted in the finer dendrites,the smaller and less precipitates and higher hardness of fusion zone.11 mm-thick Inconel 617 was jointed by the narrow gap laser welding with filler wire.No lack of fusion,pores and solidification cracks were observed in the weld metal.Compared with heat input of TIG welding of 11mm-thick Inconel 617,heat input of narrow gap laser welding with filler wire was reduced by60⁷0%.The remelting fine grained regions were observed in the inter-layer regions.M₆C,M?C,N??M=Cr,Ni and Ti?and M₂₃C₆ carbides precipitated in the weld metal.There were a lot of liquation regions in the inner grains and at the grain boundaries?GBs?in HAZ and a small number of liquaton cracks at GBs.The hardness of HAZ was higher than that of base material?BM?because the dislocation density and transgranular M₂₃C₆ carbides in HAZ were more than that in BM.Liquation cracking in the fiber laser welding Inconel 617 joint was systemically investigated.There two type hot crackings in HAZ:mixed mode hot cracking and liquation cracking and the hot crackings in HAZ was mainly dominated by liquation cracking.In laser welding,the constitutional liquation of M₂₃?C,B?₆ carbides and the high-angle boundaries in BM produced the continuous GB?Cr,Mo?-rich liquid film at GBs.The continuous GB liquid film was teared by the welding thermal stress,and the Liquation cracking was produced.In laser welding,HAZ liquation crackings decreased by increasing heat input,the preheating temperature,defocusing amount and by using the right heat treatment before welding.The high heat input reduced mixed mode cracking,even fully eliminate mixed mode cracking,while preheating reduced mixed mode cracking to some extent.The intergranular liquid film produced mainly by the constitutional liquation of M₂₃?C,B?₆ particles was observed to migrate in HAZ of laser welded Inconel 617.The negative coherency strain caused by the lattice misfit between the element Mo and the matrix was sufficient to drive LFM in HAZ.LFM tended to occur in HAZ with the high heat input due to the thin initial liquid film.LFM tended to occur on the GB with the fine particles.Liquation crackings were reduced in the weld with the high heat input and the ST?1100°C/1 h?condition,because the thin liquid film was obtained by LFM.A lot of M₂₃C₆ carbides and?'precipitated in HAZ exposed at 750°C,and M₂₃C₆carbides only precipitated in HAZ exposed at 850°C and 950°C.The high dislocation density and residual stress resulted in the precipitation of the finer transgranular M₂₃C₆carbides and?'after exposed at 750°C and 850°C,and the fast dissolution of the fine M₂₃C₆carbides leaded to less precipitation of M₂₃C₆ carbides at 950°C.With increasing exposure time,M₂₃C₆ carbides dissolved into matrix and?'phase coarsened.The precipitates in weld metal thermally exposed were studied.After long-term thermal exposure at 750°C,there were a large number of the M₆C type carbide(?₁-M₆C,?₂-M₁₂C),M₂₃C₆ carbide,the dispersed?'and a small quantity of Ti?C,N?carbide and TCP-?phase in the weld metal.M₆C and M₂₃C₆ carbide precipitated in the weld metal at 850°C and 950°C.In addition,no TCP and?'phases were observed.The reaction of solute atoms?Mo,Cr?and C in matrix,the interconversion of M₆C and M₂₃C₆,and the transformation of?Cr,Ni??C,N?in as-welded metal and decomposition of Ti?C,N?resulted in the precipitation of M₆C and M₂₃C₆carbides.With the exposure temperature increasing,the content and quantity of the M₆C carbide decreased and M₂₃C₆ carbides coarsened.With long-term exposure at 750°C and 850°C,the content of transgranular nanoscale M₂₃C₆ carbides decreased and the content,while the size of M₆C carbides increased.With long-term exposure at 950°C,the content of M₂₃C₆ carbides decreased and M₆C carbides did not change.With the exposure time increasing,the relationship between the rodlike M₆C carbides and matrix?was transformed from coherent or semi-coherent to non-coherent.The Ostwald ripening and davies-nash-stevens?LSEM?of M₆C carbides resulted in an increase in size of M₆C and M₂₃C₆ carbides.An increase in size of M₂₃C₆ carbides was attributed to the Ostwald ripening of M₂₃C₆ carbide.The coarsening rate of M₂₃C₆ carbides was lower than that of the GB M₂₃C₆ carbides.During the long-term thermal exposure at 750°C,?phase precipitated on the defined crystal planes and kept the orientation relationship with matrix:?111?_?//?0001?_?,[110]_?//[11^-2 0]_?and?111?_?//?01^-1 1?_?,[011]_?//[11^-2 0]_?.The growth pattern of the lamellar structures prompted the nucleation and growth?phase.With increase in the thermal exposure time,the size of?'phases increased.An increase in size of?'phases resulted from the Ostwald ripening of?'phases.The mismatch between?'phases and matrix?was small,and then the interface of?'/?was coherent.?'phases kept the globular morphology and had good thermal stability.At the initial stage?0⁵00 h?of thermal exposure,the precipitation of the GB M6C and M23C6 carbides and the transgranular dispersed M23C6 carbides and?'phases result in both a decrease in impact toughness and an increase in the room temperature strength and hardness.After thermal exposure??500 h?,the room temperature strength decreased due to the coarsening of the discrete M23C6 and M6C carbides at GBs.A lot of discrete M23C6carbides and an increase in space of M23C6 carbides result in an increase of the impact toughness of weld metal,while an increase in content and size of M6C carbides resulted in a decrease of the impact toughness of weld metal.The mechanical properties were not affected by a small number of?phase in the weld metal.