High temperature mechanical properties of cryogenically cooled alloy 718 weldments
TL;DR: In this paper, the micro-segregation of niobium and Laves phase formation during Gas Tungsten Arc (GTA) welding of alloy 718 was studied by employing different cooling techniques which resulted in weld cooling rates ranging from 43.2°C/s to 508.7°C /s.
Abstract: The micro-segregation of niobium and Laves phase formation during Gas Tungsten Arc (GTA) welding of alloy 718 were studied by employing different cooling techniques which resulted in weld cooling rates ranging from 43.2 °C/s to 508.7 °C/s. The weld cooling rates were achieved by the combinations of modified pulse wave form, argon and helium shielding gases, copper heat sink and liquid nitrogen cooling. A combination of compound current pulse (CCP) waveform with helium shielding and liquid nitrogen cooling resulted in lesser niobium segregation and discontinuous, finer Laves particles in the interdendritic regions compared to that of the conventional constant current GTA weld metal. This process yielded better ageing response and improved high temperature mechanical properties of the weldments.
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TL;DR: Inconel 718 is a nickel-ferrous-chromium based superalloy extensively used in the aerospace sector at elevated temperature up to 650°C due to its better mechanical properties and weldability as discussed by the authors.
51 citations
31 Jan 2020-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the effects of gas composition and filler wires on the laves phase formation were studied in detail for welding of Inconel 718 with two different shielding gases, namely argon (Ar) and argon with a 5-vol% hydrogen mixture (ArH) and two fillers.
Abstract: Gas Tungsten arc welding (GTAW) process was employed for welding of Inconel 718 with two different shielding gases, namely argon (Ar) and argon with a 5 vol% hydrogen mixture (ArH) and two fillers viz., ERNiCrMo-10 and ERNiCrMo-4. The effects of gas composition and filler wires on the laves phase formation were studied in detail. The results revealed that hydrogen addition through ArH shielding gas mixture resulted in better grain refinement in the welds than pure Ar. The hydrogen addition induced a steep thermal gradient in the weld, which lowered the segregation of elements like Niobium (Nb) and Molybdenum (Mo) at the interdendritic regions. The laves phase formation in Mo-rich filler addition welds was minimized due to restriction of Nb segregation by Mo at the interdendritic region. Tensile test results indicated that the strength and ductility of the joints of both autogenous and filler added welds of Ar were higher than the ArH shielded welds. In the case of filler added welds, higher Mo content filler exhibited better tensile properties in both shielding gas combinations due to solid solution strengthening of Mo. Nano-sized hydrogen assisted cracks observed in ArH autogenous welds caused a reduction of strength and ductility.
25 citations
TL;DR: In this article , the effect of TIG welding of thin butt joints made of 1.0 mm thick sheets (Inconel 718) in laboratory conditions has been investigated and it was shown that the most favourable quality of the sheets was obtained when welding arc linear energy was restricted within the range of approximately 45 J/mm to 80J/mm.
1 citations
01 Jul 2023-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper , the Superalloy-718 joints were developed by deploying an advanced variant of GTAW popularly known as InterPulse gas tungsten compressed arc welding (IP-GTCAW), and the joints were subjected to the post weld heat treatment (PWHT) cycles of direct aging (DA), solution annealing at 980 °C and 1065 °C followed by aging (980STA) respectively.
Abstract: The techniques of InterPulse magnetic compression and high frequency pulsation of arc in gas tungsten arc welding (GTAW) were deployed to minimize intermetallic laves phase evolvement in fusion zone (FZ) of Superalloy-718 welds and enhance the precipitation strengthening of joints. The Superalloy-718 joints were developed by deploying an advanced variant of GTAW popularly known as InterPulse gas tungsten compressed arc welding (IP-GTCAW). The joints were subjected to the post weld heat treatment (PWHT) cycles of direct aging (DA), solution annealing at 980 °C and 1065 °C followed by aging (980STA and 1065STA) respectively. The tensile properties and hardness of joints were evaluated. The microstructures of joints were analyzed using optical (OM) and scanning electron microscopy (SEM). The elemental analysis of Laves phase and dendrite core of FZ of joints was performed by energy dispersive spectroscopy (EDS). The tensile fractured surfaces were analyzed using SEM. Results showed that the Superalloy-718 joints developed using IP-GTCAW showed better response to PWHT than GTAW joints because of the greater refining of FZ and evolvement of finer and discrete Laves phase in FZ. The 980STA joints exhibited superior tensile properties than DA and 1065STA joints. It is correlated to the greater dissolution of Laves phase in nickel austenitic (γ) matrix resulting in more niobium (Nb) accessible for the precipitation strengthening of FZ. The 1065STA joints showed slightly higher hardness of FZ than 980STA joints because of the almost complete dissolution of Laves phase in FZ. However, the tensile properties of 1065STA joints are lower than 980STA joints because of severe grain growth in FZ and BM. The evolvement of microvoids at the interface of Laves phase/weld matrix leading to development and coalescence of microcracks in FZ on tensile loading is main mechanism responsible for the premature fracture of joints.
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TL;DR: In this paper, the authors used an integrated analytical approach to generate a constitution diagram for INCONEL 718* in the temperature range associated with solidification, where the important features of the diagram are an austenite (γ)/Laves phase eutectic which occurs at ≈19.1 wt pct Nb and a Laves phase which contains ≈22.4 wt nb. The distribution coefficient for Nb was found to be ≈0.5.
Abstract: As part of a program studying weldability of Ni-base superalloys, results of an integrated analytical approach are used to generate a constitution diagram for INCONEL 718* in the temperature range associated with solidification. Differential thermal analysis of wrought material and optical and scanning electron microscopy, electron probe microanalysis, and analytical electron microscopy of gas tungsten arc welds are used in conjunction with solidification theory to generate data points for this diagram. The important features of the diagram are an austenite (γ)/Laves phase eutectic which occurs at ≈19.1 wt pct Nb between austenite containing ≈9.3 wt pct Nb and a Laves phase which contains ≈22.4 wt pct Nb. The distribution coefficient for Nb was found to be ≈0.5. The solidification sequence of INCONEL 718 was found to be (1) proeutectic γ, followed by (2) a γ/NbC eutectic at ≈1250°C, followed by (3) continued γ solidification, followed by (4) a γ/Laves phase eutectic at ≈1200°C. An estimate of the volume fraction eutectic is made using the Scheil solidification model, and the fraction of each phase in the eutectic is calculatedvia the lever rule. These are compared with experimentally determined values and found to be in good agreement.
335 citations
TL;DR: In this article, diffusion bonding and laser welding are considered for joining a wide range of materials of interest in the aerospace industry, as well as in many other industrial applications, and offer remarkable advantages over conventional fusion welding processes.
Abstract: Advanced materials generally require novel joining techniques. Developments in new materials research should be conducted hand in hand with work on weldability and joining capacity aspects. Sound joint quality for any new material has always been considered a milestone in a research and development scheme for a new material, particularly in terms of widespread applications. Better understanding of the microstructure-mechanical properties relationships of the bonded or welded joints will feed back to the materials development activities both in conventional and new materials areas. The two joining processes diffusion bonding and laser welding are considered in this literature review, since these processes are capable of joining a wide range of materials of interest in the aerospace industry, as well as in many other industrial applications, and offer remarkable advantages over conventional fusion welding processes. Of particular interest is the ability to join the more difficult aerospace alloys wi...
290 citations
TL;DR: In this article, the authors investigated the precipitation of the metastable intermetallic phases (γ and γ) in the commercial nickel base superalloy, INCONEL 718, over a wide range of aging temperatures.
Abstract: Some aspects of the precipitation of the metastable intermetallic phases —γ″ and γ″—in the commercial nickel base superalloy, INCONEL 718, have been investigated over a wide range of aging temperatures. It has been confirmed that the spherical γ″ particles and the ellipsoidal γ″ particles evolve predominantly through homogeneous nucleation. Precipitation of the former does not appear to precede that of the latter in this alloy. The tetragonal distortion associated with the γ″ particles has been found to increase with increasing precipitate size. It has been observed that at certain temperatures, physical association between precipitates of the two types occurs frequently, leading to the development of different composite precipitate morphologies. During coarsening, the precipitate size has been found to depend linearly on the cube root of the aging time for γ′ as well as γ″ particles.
258 citations
TL;DR: In this article, microstructural characterisation of super austenitic stainless steels was conducted using light optical microscopy, scanning electron microscopy and quantitative image analysis, and the quantitative chemical information was obtained through electron probe microanalysis.
Abstract: Super austenitic stainless steels are often welded using high Mo, Ni base filler metals to maintain the corrosion resistance of the weld. An important aspect of this processing is the weld metal dilution level, which will control the composition and resultant corrosion resistance of the weld. In addition, the distribution of alloying elements within the weld will also significantly affect the corrosion resistance. Dissimilar metal welds between a super austenitic stainless steel (AL-6XN) and two Ni base alloys (IN625 and IN622) were characterised with respect to their dilution levels and microsegregation patterns. Single pass welds were produced over the entire dilution range using the gas tungsten arc welding process. Microstructural characterisation of the welds was conducted using light optical microscopy, scanning electron microscopy, and quantitative image analysis. Bulk and local chemical compositions were obtained through electron probe microanalysis. The quantitative chemical information w...
150 citations
TL;DR: The physical metallurgy of cast and wrought alloy 718 is discussed in this paper in terms of the importance of Nb on the phase reactions found in Alloy 718, and the role or influence of nb is first noted during the solidification of alloy 7 18 as the large atoms Nb, MO, and Ti segregate to the interdendritic regions where the Nb rich Laves phase forms.
Abstract: The physical metallurgy of cast and wrought alloy 718 is discussed in terms of the importance of Nb on the phase reactions found in alloy 718. The role or influence of Nb is first noted during the solidification of alloy 7 18 as the large atoms Nb, MO, and Ti segregate to the interdendritic regions where the Nb rich Laves phase forms. The Laves phase must be solutioned at high temperatures and the Nb must be distributed throughout the dendrite areas. The normal phases found in alloy 718 are the MC, TiN, delta, Laves, ?/’ and ?/. The y and ?/ are the main strengthening phases and their precipitation behavior is determined primarily by the amount of Nb, the temperature and time of exposure. Any incomplete homogenization of the cast or wrought material will produce non-uniform precipitation of the delta, 1/’ and f phases during working or aging heat treatments. The longtime stability of alloy 718 is related to the stability of the 1/’ phase which transforms to delta and ?/ with increasing time and temperatures. Eventually the f phase will solution and the delta phase becomes the terminal phase in the 718 system. An aCr phase is found in the grain boundaries in long time exposures in the 1100°-13500F temperature range. The Mzs C6 carbide phase normally found in Ni base alloys is not found in alloy 7 18.
122 citations