Investigation of Impact Behavior of TIG Welded Inconel 718 at Aircraft Engine Operating Temperatures. Yağız Uzunonat a,

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1 Investigation of Impact Behavior of TIG Welded Inconel 718 at Aircraft Engine Operating Temperatures Yağız Uzunonat a, 1 Anadolu University, School of Transportation, Turkey a yuzunonat@anadolu.edu.tr Keywords: Inconel 718, TIG Welding, Impact Resistance, Hardness, Welding Microstructure. Abstract. In this work, Charpy notch pendulum impact test was performed on non-welded and TIG (Tungsten Inert Gas) welded Inconel 718 specimens at three different temperatures as 20 o C, 500 o C and 700 o C. After the completion of tests, SEM (Scanning Electron Microscopy) inspection and EDX (Energy-Dispersive X-Ray Spectroscopy) analysis were performed for the microstructural examination of the specimens. Hardness measurements on the rupture zone of selected welded and non-welded specimens were also performed to make a better approach to interpretation of the impact behavior of material. Inspections indicated that hardness values in the heat affected zone of welded specimens dramatically decreased and they displayed higher ductility during fracture than non-welded samples due to partial softening of structure. The reason of further decrease in impact resistance of welded specimens was explained as the precipitation and coarsening of γ and carbide phases in the interdendritic regions with increasing temperature. Introduction Hardening mechanism of Inconel 718 depends on the precipitation of γ and γ intermetallic phases during aging. Although γ increases structural endurance, it is less compatible with γ matrix when compared to γ phase in terms of lattice structure. Inconel 718 has a good ductility due to FCC austenitic γ matrix and a high strength based upon the precipitation of γ phase. This condition makes Inconel 718 a successful material in high temperature structural applications. Rapid grain growth and γ δ transformation as a result causes a decrease in the material properties in the applications above 650 o C. Phase creates a notch effect in matrix structure and majorly influences the rapid fall of properties due to its needle-like or laminated structure [1]. Even there are many studies about microstructure and mechanical properties of raw Inconel 718 material, effects of welding process to these properties have rarely been examined. Welding is a frequently applied maintaining process to structural gas turbine elements in aviation technologies. One of the major problems in post-welding properties of nickel based alloys is embrittlement. Welded nickel based alloys can show brittle or ductile fracture behavior especially under impact loads [2]. With this study, alloy properties of TIG welded gas turbine fastening components under impact loads were analyzed and possible treatments after welding process were informed. Equipment and Experimental Procedure Inconel 718 high pressure turbine fastening bolts were used for all tests as specimen. Since the specimens are actual gas turbine engine components, the real operating environment can be simulated more accurately. TIG welding machine was used and the device runs with 18.4 a current and 0.18 mm/s feed rate during the process. Device voltage is between V and rate is determined by the device itself during welding. Argon flow rate was specified as 19 l/min. Notches are 1 mm depth and were opened parallel to impact fracture line and a 0.30 mm standard copper electrode was used in EDM machine. Heraus MR-170 mini laboratory furnace was used for heating of the specimens. The heated specimens were removed from furnace and mounted to impact machine and then test was started. The required

2 time for this procedure was determined as 20 seconds. Cooling rates of the specimens were measured with thermocouple during this process and furnace temperatures were determined as 790 o C and 550 o C. Specimens were heated for 45 minutes to obtain the uniform temperature distribution of specimen cross-section before impact test. Impact tests were performed by considering their weldability and behavior under different critical loads at different critical temperatures. 9 welded and 9 non-welded Inconel 718 specimens were prepared and Charpy impact test was performed at 20 o C, 500 o C and 700 o C. The reason for selecting 700 o C as top temperature there is a rapid fall in material properties in the applications above 650 o C, thus 700 o C was selected as top limit temperature to observe the changes in material structure precisely. In aircraft engines, Inconel 718 fastening bolts are widely used between o C in standard operating conditions, 500 o C was selected as the average temperature of this range. Finally, 20 o C is selected for the examinations at room temperature. After impact tests, specimens were prepared for the microstructural and hardness analyses. Results and Discussion For the facilitation in comparison of experimental data, specimens were numbered and separated into two groups as Welded (W) and Non-Welded (NW). Average of fracture values for each temperature of specimens were used in figures. Impact test results of welded and non-welded specimens for three different temperatures were given in Table 1. Table 1. Fracture values of specimens for different temperatures Specimen No Process W W W W W W W W W Temperature ( o C) Impact Resistance (kgm/cm 2 ) Specimen No Process NW NW NW NW NW NW NW NW NW Temperature ( o C) Impact Resistance (kgm/cm 2 ) It is seen in Fig. 1, welded specimens show obviously higher impact resistance than non-welded specimens. Hardness reduction in the microstructure after the welding process causes the material to show ductile behavior. This explains that hardness can vary directly with tensile loads and inversely with impact loads. The overall load was applied instantly because of the implementation method of impact test, ductility increase in material improved the impact absorption and fracture rates of specimens. Based on Fig. 1, impact rates of the specimens can be analyzed in two states as o C and o C. In first state, overheating of γ caused further impact resistance reduction in welded specimens. Precipitation of γ and coarsening of NbC phases in interdendritic regions of welded specimens are main factors of this decrease. In second state, γ δ transformation accelerates decrease of impact resistance of all specimens.

3 Figure 1. Average impact resistances of specimens Thereby, both γ δ transformation and interdendritic precipitation and coarsening of this phase were included to fracture mechanism, decrease in the properties of welded specimens was higher. δ phase is in the form of needle-like or lamellar particles and accelerates the decrease in mechanical properties by creating a secondary notch effect [3-5]. Figure 2. SEM image of W3 (welded / 700 o C) specimen In Fig. 2, similar to γ phase, the formation and then precipitation in interdendritic regions of new phases can be observed with the effect of increasing temperature. These new structures are carbides and expressed as MxCy. Since phases are brittle, they coarsen and reduce the ductility of the material at dendrite boundaries at impact test application temperatures and help the fracture process to continue over the subject regions after welding. Mo, Ti and Nb are strong carbide forming metals among Inconel 718 structural elements. Indicated regions in Fig. 2 are rich from the content of molybdenum and titanium according to EDX analysis in Fig. 3. White and black regions were determined as Mo6C and TiC respectively as a result of research carried out in the literature [6-8].

4 Figure 3. EDX analysis results of W3 (welded / 700 o C) specimen a - white region, b - black region Hardness measurements of the samples subjected to impact testing were made in the direction parallel to the fracture line, 100 micrometers inside from the surface and is made of 0.8 mm intervals. Six hardness values obtained from the different samples and their specimen numbers are given in Table 2. Table 2. Results of hardness inspection Specimen No. Hardness [HRC] W W W NW NW NW As it can be seen clearly from Fig. 4, obvious difference between the hardness of welded and non-welded specimens was originated from the dissolution of γ during welding. γ phase accumulates in the dendrite boundaries during cooling and softens the main matrix. Dislocations due to instant application of impact load and deformation hardening due to plastic strain are main factors of the fluctuation in hardness values of non-welded specimens [9-10]. Figure 4. Hardness values of selected specimens Since the measurements were started from the EDM notch, rates at 0.8 mm in welded samples are lower than those in other parts. Fracture didn t occur in the line where initially opened notch was

5 located, so deformation hardening caused by impact did not activate itself in this region. As seen in the graph, W9 and NW18 specimens showed lower hardness properties by the reason of γ δ transformation at 700 o C. Conclusion TIG welded Inconel 718 specimens showed higher fracture resistance than non-welded specimens contrary to the behavior exhibited under loading conditions like tension or bending. Upon analyzing the fracture modes, welded specimens also indicated more ductile behavior. Hardness reduction in the microstructure after the welding process causes the material to show ductile behavior. This explains that hardness can vary directly with tensile loads and inversely with impact loads. The overall load was applied instantly because of the implementation method of impact test, ductility increase in material improved the impact absorption and fracture rates of specimens. Investigations in the literature in consistency with experimental results of study proved that hardness in the welding zone falls significantly. This state was another factor to increase the ductility of the material. Consequently, by the aid of post-weld aging heat treatment subsequent to solution treating either residual stresses of weld process can be eliminated or maximum strength of material can be regenerated. References [1] W.J. Mills and L.A. James: Transactions of the ASME. Journal of Engineering Materials and Technology, Vol. 107 (1985), p [2] D. Choi, et al.: Journal of Materials Technology, 113 (2007), p [3] C. Radhakrishna and R.K. Prasad: Materials at High Temperatures, 12 (1994), p [4] S. Kalluri, K. Rao, G. Halford and M. McGaw: The Minerals, Metals & Materials Society, (1994), p [5] D. Cai, W. Zhang, P. Nie, W. Liu and M. Yao: Materials Characterization, 58 (2007), p [6] Metals handbook (9th edition), ASM (American Society for Metals), 6 (1983), p [7] J. Gordine: Welding Journal Research Supplement, 49 (1970), p [8] K. Easterling: Introduction to the physical metallurgy of welding (2nd edition), Butterworth-Heinemann, (1992). [9] D. Dye, O. Hunziker and R.C. Reed: Acta Materialia, 49 (2001), p [10] S. Kou: Journal of the Minerals, Metals and Materials Society, 55 (2003), p Authors background Your Name Title* Research Field Personal website Yağız Uzunonat Assistan Professor Mechanical Engineering