Study of the fusion zone and heat-affected zone microstructures in tungsten inert gas-welded INCONEL 738LC superalloy

Abstract

The fusion zone and heat-affected zone (HAZ) microstructures obtained during tungsten inert gas (TIG) welding of a commercial superalloy IN 738LC were examined. The microsegregation observed during solidification in the fusion zone indicated that while Co, Cr, and W segregated to the ␥ dendrites, Nb, Ti, Ta, Mo, Al, and Zr were rejected into the interdendritic liquid. Electron diffraction and energydispersive X-ray microanalyses using a transmission electron microscope (TEM) of secondary phases, extracted from the fusion zone on carbon replicas, and of those in thin foils prepared from the fusion zone showed that the major secondary solidification constituents, formed from the interdendritic liquid, were cubic MC-type carbides and ␥-␥Ј eutectic. The terminal solidification reaction product was found to consist of M 3 B 2 and Ni 7 Zr 2 formed in front of the interdendritic ␥-␥Ј eutectic. Based on the knowledge of the Ni-Ti-C ternary system, a pseudoternary solidification diagram was adapted for IN 738 superalloy, which adequately explained the as-solidified microstructure. The HAZ microfissuring was observed in regions surrounding the fusion zone. Closer and careful microstructural examination by analytical scanning electron microscopy revealed formation of re-solidified constituents along the microfissured HAZ grain boundaries, which suggest that HAZ cracking in this alloy involves liquation cracking. Liquation of various phases present in preweld alloy as well as characteristics of the intergranular liquid film contributing to the alloy's low resistance to HAZ cracking were identified and are discussed.

The Evolution of Cast Microstructures on the HAZ Liquation Cracking of Mar-M004 Weld

“…The induced microcracks were strongly related to inherent metallurgical factors of the Mar-M004 superalloy. As reported in previous works [20,21,24,25], constitutional liquation of MC carbides, M 3 B 2 borides, M 2 SC sulfocarbides, γ particles, and γ-γ eutectic accounts for the HAZ liquation cracking of IN 738 superalloy weld. The cracks were prone to propagate along the interfaces between the MC carbide and the matrix (Figure 5b), owing to substantial amount of MC carbides in the cast Mar-M004 superalloy.…”

“…Liquidation cracking in the HAZ is one of the most noticeable problems in welding or repair-welding of Ni-based superalloys such as IN 738 [7][8][9], Rene 80 [10][11][12][13], IN 939 [14,15], RR 1000 [16], IN 713C [17], and K465 [18]. Grain boundary liquation results from incipient melting of MC carbides, Cr-Mo borides, γ-γ eutectic and Ni-Zr intermetallics along the solidified boundaries [18][19][20][21][22][23][24]. The use of filler metals with (a) slower aging response, (b) smaller lattice mismatch between the precipitates and the matrix, (c) lowered (Ti + Al) concentrations and (d) softer weld metal have been reported to reduce the HAZ cracking susceptibility of IN 738LC welds [25].…”

Crack-Free Welding of IN 738 by Linear Friction Welding

“…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].…”

“…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 components under the influence of an axial force, generating heat at the interface between these components and resulting in the plasticizing of the material at the interface.…”

A Study of the Weldability of Gamma Prime Hardened Superalloys

“…However, γ' precipitation-strengthened alloys are very difficult to weld in a crack-free manner due to their inherent susceptibility to heat affected zone (HAZ) cracking during welding and during post-weld heat treatment [1,2,3]. Despite the difficulties regarding crack-free welding of superalloys, welding is the most essential joining and repair process for turbine parts [4].…”