Effect Of Processing Craft On The Microstructure And Property Of Inconel 718 Alloy

Inconel 718 alloy is processed into various components used in various environments due to its excellent property.As Inconel 718 alloy with different processing craft has different microstructure and property,the appropriate processing craft is required for each service environment.In a word,formulating the proper processing craft of Inconel 718 alloy is quite significant for its application.Deformation is an important means of optimizing the microstructure and improving the properties in the nickel-based alloy.The previous works mainly focus on hot compression and creep behavior.However,there are few literatures on the consequence of cryogenic deformation on the microstructural evolution of Inconel 718 alloy.In particular,previous studies demonstrate a big controversy in the effect of cold rolling on the precipitation behavior of the intermediate phases in Inconel 718 alloy.Furthermore,the previous method for investigating the precipitation behavior of the intermediate phases is rather simplex,confined to XRD and metallographic observation.Hence,it is necessary to do several more in-depth researches in this field.As a high-energy-density fusion welding technique,electron beam welding(EBW)has been extensively employed in the joining of Fe-Ni based alloy.Compared with the conventional welding techniques,EBW shows outstanding advantages in terms of a narrower fusion zone(FZ)/heat affected zone(HAZ)and a less distortion of the material being welded,which is ascribed to its lower heat input.Moreover,due to the high energy density of EBW,it enables high welding speeds to be realized and makes it possible to reduce total welding time by making single-pass welds,even in heavy section materials.However,it has a major limitation in the form of segregation of Nb in the interdendritic regions leading to Laves phase formation and consequent mechanical property deterioration of the fusion zone.In addition,this alloy suffers from facile heat affected zone(HAZ)intergranular microfissuring.Extensive studies by previous researchers have been devoted to the heat input and post-weld heat treatment in pursuit of excellent microstructure and properties.Nevertheless,the base metal along with the welding speed is also a key factor which influences the weldability.Therefore,it is essential to implement the research with respect to the effect of the base metal and the welding speed on the weldability of electron beam welded Inconel 718 alloy.In this work,we investigate isothermal/isochronal precipitation behavior of intermediate phases in various degrees cold-rolled Inconel 718 alloy as well as the influence of the base metal and the welding speed on the microstructure and property of electron beam welded Inconel 718 alloy via differential scanning calorimetry(DSC),optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD)and Vickers microhardness.The conclusions can be formulated as follows:(1)For the cold-rolled Inconel 718 alloy samples with isochronal DSC measurements,the values of the onset temperature(TOnset1),the peak temperature(TPeak1)and the end temperature(TEnd1)of γ"-phase precipitation decrease as the degree of cold rolling is raised from 0 pet to 70 pet;while the values of the onset temperature(TOnset2),the peak temperature(TPeak2)and the end temperature(TEnd2)of δ-phase all first increase and then decrease as the degree of cold rolling is raised from 0 pct to 70 pct.(2)For the cold-rolled Inconel 718 alloy samples isothermally aged at 800℃ for 8h,cold rolling accelerates γ"→δ transformation.While for the cold-rolled Inconel 718 alloy samples isothermally aged at 950℃ for 6h,cold rolling promotes δ-phase precipitation and the morphology of δ-phase changes gradually from needle shape to short rod/globular shape with the degree of cold rolling increasing.(3)A coarser grain structure of the base metal(equivalent to a higher heat treatment temperature applied on the base metal)or a higher welding speed demonstrates a significant increase in HAZ cracking susceptibility.Additionally,increasing the grain size of base metal would result in the weld shape changing from a stemless wine glass shape to a nail head shape with a larger penetration-to-width ratio.(4)Decreasing the welding speed yields an enhanced microsegregation in the region near the center of weld pool,thus more Laves phase is observed in this region.Also,it is found that the center of weld pool shows a more severe microsegregation than the adjacent zone of the fusion line.