Crack-Free Welding of IN 738 by Linear Friction Welding

Abstract

Abstract. Inconel 738 (IN 738), like other precipitation-hardened nickel-base superalloys that contain a substantial amount of Al and Ti, is very difficult to weld due to its high susceptibility to heat-affected zone (HAZ) cracking during conventional fusion welding processes. The cause of this cracking, which is usually intergranular in nature, has been attributed to the liquation of various phases in the alloy, subsequent wetting of the grain boundaries by the liquid and decohesion across one of the solid-liquid interfaces due to on-cooling tensile stresses. In the present work, crack-free welding of the alloy was obtained by linear friction welding (LFW), notwithstanding the high susceptibility of the material to HAZ cracking. Gleeble thermomechanical simulation of the LFW process was carefully performed to study the microstructural response of IN 738 to the welding thermal cycle. Correlation between the simulated microstructure and that of the weldments was obtained, in that, a significant grain boundary liquation was observed in both the simulated specimens and actual weldments due to non-equilibrium reaction of second phase particles, including the strengthening gamma prime phase. These results show that in contrast to the general assumption of LFW being an exclusively solid-state joining process, intergranular liquation is possible during LFW. However, despite a significant occurrence of liquation in the alloy, no HAZ cracking was observed, which can be partly related to the nature of the imposed stress during LFW. IntroductionIN 738 is a nickel-based superalloy with a significant volume fraction of an ordered intermetallic Ni 3 (Al,Ti) γ' phase, possessing an excellent high-temperature strength and remarkable corrosion resistance. This makes it a suitable material for manufacturing of hot-section components in aeroengines and in power generation turbine applications where they are exposed to severe operating conditions. Welding techniques are usually employed for both the fabrication and repair of these components. IN 738, like other precipitation-hardened nickel-base superalloys that contain a substantial amount of Al and Ti, is very difficult to weld due to its high susceptibility to heataffected zone (HAZ) cracking during welding and strain age cracking during post-weld heat treatment (PWHT) [1]. The cause of this cracking during conventional fusion welding processes, which is usually intergranular in nature, has been attributed to the liquation of various phases in the alloy, subsequent wetting of the grain boundaries by the liquid and decohesion across one of the solid-liquid interfaces due to on-cooling tensile stresses. Detailed mechanism of grain boundary liquation cracking in the alloy has been provided elsewhere [2].To address the problem of liquation cracking in weldments, a recent trend has involved the use of supposedly solid-state welding techniques, such as, friction welding for joining crack susceptible structural alloys. Friction welding generally involves the rubbing together of two com...

Tracking heat-affected zone cracking susceptibility in standard and modified heat treated IN 738 superalloy welds

“…Figure 3 shows the microstructure of the recommended manufacturer standard SHT preweld IN 738 which consists of coarse primary γ' precipitates, fine spherical secondary γ' precipitates of about 0.1 μm in diameter and other solidification products that formed during casting. This microstructure is consistent with previously reported investigation [10][11][12]14]. In general, the excellent high temperature strength of nickel based superalloy is known to be strongly dependent on the morphology, distribution and size of the γ' precipitates in the matrix.…”

Linear and rotary friction welding review

“…16 In the case of IFW'd superalloy tube (Fig. 36), [89][90][91] the residual tensile stresses in the hoop direction are the largest near the inner wall, while the largest compressive stresses are in the axial direction near the outer wall of the tube. On the whole, the weld developed large von Mises residual stresses (Fig.…”

“…In the work of Ola et al, 70,87 possible liquation of various phases during LFW of superalloys, i.e. CMSX-486 single crystal superalloy 70 and IN738 [89], was investigated using the Gleeble thermomechanical simulation of the process to identify local melting of the g-g ′ eutectic constituents. Significant grain boundary liquation was observed in both the Gleeble specimens and the HAZ of actual joints due to non-equilibrium reaction of second-phase particles, including the strengthening g ′ phase.…”

“…This is associated with constraint stresses from the welding jig. 89 As for other process parameters, such as rotational speed, oscillation frequency and amplitude, and even friction time, they appear to be less important for the development of residual stresses as they affect heat input during welding and not while cooling.…”

“…f Residual stress (y direction) profiles for LFW'd Ti64 sample with both synchrotron measurements and contour method 16

36 Representative residual stresses in IFW'd IN718 tube joint: a equivalent stress, von Mises, b X-direction stress, S11, c Y-direction stress, S22 and d Z-direction stress, S33. e Axial residual stresses for IFW 720Li Ni-based superalloy 89 compared to c .

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