Research on the Grain Boundary Liquation Mechanism in Heat Affected Zones of Laser Forming Repaired K465 Nickel-Based Superalloy

Abstract

Abstract:The damaged K465 nickel-based superalloy parts were repaired by laser forming repair technology. The cracking characteristics and grain boundary liquation in heat affected zones were investigated by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was found that the cracks originated from the heat-affected zone and extended to the repaired zone. The calculation by Thermol-Cale software showed that the larger γ 1 particles at grain boundaries partly dissolved in the γ phase, which made the solutes' concentration at the γ 1 /γ interface meet the eutectic-type liquation reaction condition of γ + γ 1 Ñ L. Then, grain boundaries liquation occurred and liquid films appeared with the temperature increasing in a rapid heating process. However, the intragranular γ 1 phase completely dissolved into the γ phase with no devotion to the liquid film. The dissolution of M 5 B 3 borides at grain boundaries could promote grain boundary liquation.

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

“…Moreover, microconstituents in lamellar form also contributed to HAZ liquation cracking, shown in Figure 5c. Solidification cracks were also found to initiate and propagate along the interfaces between the γ-γ′ colonies, which were reported in prior studies [7,10,15,18] (Figure 5d). Constitutional liquation of coarse γ′ particles at the boundaries, which do not dissolve into the matrix during heating cycles, contributes to the HAZ liquation cracking [16,21].…”

“…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].…”

Microstructure and Microhardness of Laser Metal Deposition Shaping K465/Stellite-6 Laminated Material

“…As shown in Figure 3b,c, the liquation cracking first initiated from the heat affected zone, which was near the fusion line in the pre-deposition layer. The liquidation of low melting point phase in K465 superalloy leads to the occurrence of grain boundaries liquation cracking, which was proved by many researchers like Yan et al [22] and Chen et al [6,10,11]. The liquation crack in K465 superalloy extended along several deposition layers, while extended only in the K465 single layer of laminated material.…”

“…Laser metal deposition shaping (LMDS) is an efficient way for producing components with near-net-shaped, directional solidification, and controlled porosity or chemical composition gradient [2,3], and it also knows direct laser fabrication (DLF), or laser solid form (LSF). Many LMDS nickel-based superalloys have been reported, such as Inconel 625 [4], Inconel 718 [5,6], Colmonoy 227-F [7], Rene 80 [8], IN 100 [9] et al; nevertheless, some of them (e.g., Inconel 718 [6], Rene104 [10], IN100 [9], K465 [11], DZ4125 [12], and CM247LC [13] were prone to cracking. The crack sensitivity of nickel-based superalloy increases with the increasing of aluminum and titanium elements.…”

“…However, when the contents of Al and Ti elements reach a certain level, the crack sensitivity cannot be completely eliminated [8]. Li et al proposed that the crack-free K465 sample can be formed by LSF based on preheating of the substrate [11]; however, the preheating temperature was quite high, and was difficult to realize in the practical engineering application. In addition, Hu et al [14] and Harrison et al [15] have proved the cracking behaviour by changing the chemical composition; by mixing some alloys during LMDS, the mechanical properties and formability of the nickel-based superalloy could be changed significantly [16][17][18].…”