Kaynak sonrası ısıl işlemin TIG ile kaynak yapılmış inconel 718 alaşımının mekanik özelliklerine etkisi

Superalloys have been mostly utilized in gas turbine technology with regards to their advantages of high temperature resistance characteristics. Superalloys readily satisfy the requirements including corrosion resistance, high strength at elevated temperatures, good fatigue and creep resistance which are demanded for both aerospace and land based power generation applications. The components manufactured for those fields mostly require combination of varios conventional and special processes in terms of machining, forming and joining. Tungsten Inert Gas (TIG) welding is one of the common joining processes that is applicable to superalloys. Superalloys are grouped as iron-nickel, nickel and cobalt based depending on the main alloying elements inside their structure. Each of the superalloys have face-centered cubic (FCC) matrix combined with secondary strengthening phases. Inconel 718 is a member of iron-nickel based superalloys group. Precipitation hardening is the main strengthening mechanism of Inconel 718 which is primarily strengthened by gamma double prime (γ") precipitate. Slow hardening response of gamma double prime (γ") enhance the weldability of Inconel 718 such that producing a relatively low strength and high ductility heat affected zone (HAZ) as this phase cannot form during weld cycles. This also provides adequate time for the alloy to reach desired hardness level and complete stress relief prior to hardening via precipitation of gamma double prime (γ"). However, segregation of microalloying elements particularly niobium (Nb) and formation of brittle Nb-rich Laves (Ni,Fe,Cr)2(Nb,Mo,Ti) phase is still an issue due to nonequilibrium solidification conditions during welding. Laves phase is known to be detrimental to mechanical properties. Thus, post weld heat treatments (PWHTs) are recommended for dissolving Laves phase in order to have homogenized structures and regain material properties. The aim of this study was to evaluate the influence of the type of PWHT (direct aging or solutionizing and aging) on the microstructure and mechanical properties of TIG welded Inconel 718 sheets. In this study, Inconel 718 sheets were utilized with two different thicknesses: 2 and 3,2 mm. Material was supplied in solution treated condition (between 950-990°C) per the AMS 5596 specification. Each group of specimens were welded by TIG method. Automatic welding equipment preferred for repeatitive weld joint quality. Square butt weld joint design and single V-groove butt weld joint design employed for 2 and 3.2 mm thick specimens, respectively. Constant welding paramaters applied to each group depending on the workpiece thickness. Weld joints were controlled with non-destructive test (NDT) methods including FPI and X-Ray. No remarkable welding defects have been identified by NDT methods. In order to enlighten the effect of PWHT, the welded specimens subjected to different heat treatment programs. One group was subjected to solution heat treatment at 980°C ±10° for 1 hour and subsequent aging heat treatment at 720°C±10°C for 8 hours and 620 °C±10 °C for 8,5 hours in an inert atmosphere industrial vacuum furnaces. Second group was subjected to direct aging heat treatment at 720°C±10°C for 8 hours and 620 °C±10 °C for 8,5 hours in an inert atmosphere industrial vacuum furnaces. The last group is stayed as welded to represent the weld tensile properties without heat treatment and check the influence of selected heat treatment cycles. It is to be noted that those heat treatment cycles also performed to original Inconel 718 specimens together with welded specimens in order to compare the properties of base material with its welded state. Initially, base material microstructure of Inconel 718 were examined for following conditions: as-received with no PWHT, solution (980°C) treated+aged and direct aged under optical microscope and scanning electron microscope (SEM). The grain size measurements revealed that these post-weld heat treatments did not much change the grains and 2 mm thick specimens have finer grains in all conditions. It was also observed that secondary particles were present in the austenite matrix in solution condition and could not be dissolved after subsequent heat treatments. These particles were thought to be MC type carbides. In addition, The post-weld solution heat treatment at 980°C resulted in needle-like secondary particles precipitation which is suggested to be delta (δ) in the literature. Vickers microhardness test were conducted on three groups of specimen and concluded that both post-weld heat treatments increase the hardness in same magnitude. This hardness increase is attributed to gamma prime (γ') and gamma double (γ") precipitation during aging heat treatment. However, these precipitates could not be detected during XRD analysis due to their lower volume fraction. After base material characterization, structural features of weld regions were characterized with macro and micro examinations. Weld cross sections of 2mm and 3,2 mm thick specimens in all conditions showed that weald bead size developed by amount of heat input. Heat input per unit length for 2 mm thick specimens is calculated 192 J/mm and for 3.2 mm thick specimens is calculated 419 J/mm. Thus, 3,2 mm thick specimen is all conditions represent more elongated weld pool shape. Microstructure of weld transverse section consist of dendritic fusion zone, HAZ and unaffected base material. The weld microstructure examined through the fusion zone, fusion boundary and HAZ regions. HAZ includes two subzones namely coarse grained HAZ and partially melted zone (PMZ). The dendrites in the weld fusion zone was appeared to be equiaxed while columnar dendrites were seen close to the fusion boundary. For further microstructural examination, SEM was utilized. Irregular shaped secondary particles were detected in interdendritic regions. It is suggested to be Laves phase in litetarure which further confirmed with spot EDS analysis. Laves phase was distinguished with its higher niobium concentration. However, these secondary particles could not be detected during XRD analysis due to their lower volume fraction. Laves phase formation is beacause of segragation of niobium during weld solidification. Once this phase is formed in weld metals, post weld heat treatment is critical to dissolve this undesired phase and homogenize the structure with uniform niobium distribution. According to SEM images, Laves phase was readily formed in fusion zone of the as welded and direct aged specimens. However, solution heat treatment at 980°C±10°C resulted in partially dissolution of Laves phase and subsequently formation of needle-like delta phase (δ) around nearly dissolved Laves phase. Solution heat treatment at 980°C did not achieve the free of Laves phase microstructure. The effect of PWHT on the mechanical properties of welded specimens were investigated by conducting microhardness and room temperature tensile testing. Fracture surfaces of samples failed from fusion zone further examined via stereomicroscope and SEM. Below conclusions were made accordingly; • Post weld heat treatments nearly double the hardness levels of all three zones: fusion zone, HAZ and base material. The hardness levels are identical in HAZ and base material while fusion zone is approximately 5% softer. • Comparable hardness results in fusion zone, HAZ and base material in as-welded condition could reflect that precipitation of gamma double prime ( γ") and gamma prime (γ') did not occur during weld metal cooling. • Tensile testing was performed to original samples and welded samples. It is obtained that both post-weld heat treatments increased the yield strength up to 150%, ultimate tensile strength up to 60% wheras decreased the ductility down to 140%. • The yield strength and ductility of solution+aged and direct aged welded samples are almost in the same range. The differences between yield strength and ductility are found to be lower than 5%. However, almost identical tensile strength values were obtained from the solution+aged and direct aged specimens. • When the mechanical properties of welded samples and original samples in 2 mm thick samples are compared, It was concluded that the difference of yield strength, tensile strength and ductility in same heat treatment conditions are negligible. Deformation was started from any region away from fusion zone so that almost identical yield points were obtained with original samples and welded samples. • When the mechanical properties of welded samples and original samples in 3,2 mm thick samples are compared, It was concluded that the difference of yield strength and ductility in same heat treatment conditions is more than 10%, while tensile strength is lower than 5%. Deformation was started from fusion zone where hardness is lower than the base material. This is consistent with inferior tensile properties (10% reduction in yield point) compared to base material. • Two samples as-welded (2 mm thick sheet) and solution+aged (3,2 mm thick sheet) conditions fractured from fusion zone. Fracture zones were further examined and found that microvoids were initiated at the Laves/matrix interface.