Inconel 738 is one of the widely used nickel-based superalloys in high-temperature applications, especially in land-based and aerospace gas turbine engines. This paper reports the feasibility of direct laser metal deposition (LMD) of Inconel 738. Cracks evolved during deposition at the substrate/deposit interface and within the deposit along high angle grain boundary for scanning speed of 6 and 12 mm/s due to the intense residual stress and incipient melting. Results showed liquation cracking due to low melting crack boundary γ′ and significant micro-segregation of Al and Ti along the crack boundaries. By maximizing the energy density and by reducing the scanning speed to 3 mm/s, crack-free single wall specimens were successfully manufactured. Microstructural evolution of primary, secondary, grain boundary γ′, MC carbides, and M2B borides in the as-deposited and heat-treat specimens are discussed. Mechanical properties and microstructural development were investigated using tensile testing and scanning electron microscopy. Energy dispersive spectroscopy confirmed significant micro-segregation on various elements along the interdendritic and grain boundaries. X-ray diffraction validated the presence of the observed carbides and boride in the as-deposited and heat-treated samples.

Direct laser metal deposition of Inconel 738

The liquation cracking behavior of IN738LC superalloy during low power Nd:YAG pulsed laser welding

The role of γ′ precipitate particles, which is the principal strengthening phase of most of the precipitation hardened nickel base superalloys, on weld heat affected zone (HAZ) cracking was investigated. The HAZ microstructures around welds in a commercial nickel-based superalloy IN 738LC were simulated using Gleeble thermomechanical simulation system. Microstructural examination of the simulated HAZs and those present in actual tungsten inert gas (TIG) welded specimens showed that γ′ particles persisted during heating to the welding simulation temperatures, where they reacted with the surrounding γ matrix producing liquid film by a eutectic-type reaction, which subsequently infiltrated the grain boundary regions. The on-cooling ductility of the alloy was significantly reduced as shown by the comparatively low ductility recovery temperature. Correlation of simulated HAZ microstructures with hot ductility properties of the alloy revealed that HAZ cracking resistance was damaged by liquation reaction involving the γ′ precipitate particles. The results support previously reported observations which indicated that γ′ precipitate particles could contribute to HAZ microfissuring in superalloys through liquation reaction besides the generally reported rapid solid-state re-precipitation of γ′ phase.

On the role of liquated γ′ precipitates in weld heat affected zone microfissuring of a nickel-based superalloy

The heat affected zones (HAZs) of low and high heat input laser welds of a newly developed superalloy, ATI Allvac 718Plus, were studied. Low heat input welds suffered significant HAZ grain boundary liquation cracking, while no cracking was observed in spite of a more extensive HAZ intergranular liquation in the higher heat input welds. Combination of lower welding stresses generated during cooling, and relaxation of these stresses by thick intergranular liquid were suggested to be the factors that contributed to the absence of cracking in the high heat input welds. Further, healing of some of the HAZ cracks in lower heat input welds by fusion zone interdendritic liquid occurred through liquid backfilling.

Effect of heat input on heat affected zone cracking in laser welded ATI Allvac 718Plus superalloy

The diffusional coherency strain theory and its implications for chemically induced migration of liquid films and grain boundaries are reviewed and critically examined. The critical experiments in metals and oxides show the validity of this theory. The observed characteristics of this phenomenon: misfit dislocations at the initial boundary location; migration direction; faceting; dependence on grain boundary structure, on boundary curvature, and on temperature are also consistent with the coherency strain theory. Other theories are also described, but are mostly inconsistent with the observations, and there is very little experimental evidence for the validity of any of them. There was experimental evidence that the coherency strain theory is also valid for discontinuous precipitation. The significance of lattice diffusion at small distances in the order of the lattice spacing is examined. The related phenomena of diffusion induced recrystallisation and surface corrugation are also described. Fina...

R.G. Thompson

Papers

Liquid film migration (lfm) in the weld heat affected zone (haz) of a ni-base superalloy