Improvement in Laser Weldability of INCONEL 738 Superalloy through Microstructural Modification

Abstract

Microstructural study of laser-beam-welded IN 738 superalloy was carefully performed to better understand the causes of heat-affected zone (HAZ) cracking and to determine an improved approach of alleviating the weldability problem. The HAZ cracks in the alloy were intergranular liquation cracks that resulted from the liquation reaction of both secondary solidification products (MC carbides and c-c¢ eutectic) and solid-state reaction products (c¢ particles) present in the preweld material. In contrast to the expectation based on Chadwick's equation, a reduction of grain boundary liquid film thickness did not produce a decrease in HAZ cracking owing to increased base alloy hardness that accompanied a preweld heat treatment designed to reduce the intergranular liquation. Moreover, a major factor limiting the effectiveness of an existing preweld heat treatment with low base alloy hardness in reducing HAZ cracking was found to be the formation of intergranular M 5 B 3 boride particles during the heat treatment. These borides can widen the HAZ brittle temperature range (BTR) during weld cooling and increase the propensity for cracking. Based on the results, a new preweld heat treatment that induces a moderate hardness and precludes grain boundary boride formation was found and was shown to produce a significant reduction in HAZ cracking in the welded alloy compared to the most effective pre-existing preweld heat treatment.

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

“…Moreover, some local sites consisted of very high Cr and Mo concentrations along with high B intensity, suggesting that fine boride particles might be formed at such sites. The presence of borides is reported to facilitate liquation at low temperature and enlarge the brittle temperature range, thus increasing the susceptibility of HAZ liquation cracking [22]. The cast Mar-M004 superalloy comprised of complex solidification products at the interdendritic boundaries.…”

“…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].…”

“…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]. Moreover, preweld heat treatments [10,14,15,[22][23][24] have been conducted to modify the microstructures of precipitate-strengthened Ni-based superalloys to decrease the sensitivity of liquidation cracking in the HAZ.…”

On Liquation and Liquid Phase Oxidation during Linear Friction Welding of Nickel-base IN 738 and CMSX 486 Superalloys

“…Fabrication of new turbine components, as well as repair of service-damaged parts, made of conventionally cast and SX superalloys usually require joining by different welding techniques. However, IN 738, like other precipitation strengthened nickel base superalloys, is very difficult to weld due to its high susceptibility to heat-affected zone (HAZ) cracking during fusion welding processes [1,2]. HAZ cracking in IN 738 is attributed to the liquation of various phases in the alloy, subsequent wetting of grain boundaries by the liquid and decohesion along one of the solid-liquid interfaces due to oncooling tensile stresses [2,3].…”

“…However, IN 738, like other precipitation strengthened nickel base superalloys, is very difficult to weld due to its high susceptibility to heat-affected zone (HAZ) cracking during fusion welding processes [1,2]. HAZ cracking in IN 738 is attributed to the liquation of various phases in the alloy, subsequent wetting of grain boundaries by the liquid and decohesion along one of the solid-liquid interfaces due to oncooling tensile stresses [2,3]. Similarly, weldability studies performed on fusion welded CMSX 486 superalloy showed that the alloy is highly susceptible to fusion zone cracking [4].…”