Influence of Preheating on the Microstructure Evolution of Laser Re-Melting Thermal Barrier Coatings/Ni-Based Single Crystal Superalloy Multilayer System

Abstract

Laser surface re-melting (LSR) is a well-known method to improve the properties of atmospheric plasma-spraying thermal barrier coatings (APS TBCs) by eliminating the voids, incompletely melted particles and layered-structure. Laser energy density should be carefully selected to reduce the exposed thermal damage of the underlying single crystal (SX) matrix. Therefore, the purpose of this paper was to identify the effect of introducing induction heating to laser modifying of APS TBCs coated on Ni-based SX superalloy. The results indicated that the preheating of the substrate can lower the laser energy threshold that is required for continuously re-melting the coating. It proved that, in LSR processing of a APS TBCs/ SX matrix multilayer system, the combined method of adopting the low laser energy and preheating at elevated temperature is an effective means of minimizing the cracking susceptibility of top ceramic coating, resulting from decreasing the mismatch strain between the re-melted layer and residual APS TBCs, which can significantly improve the segmented crack condition in terms of crack dimension and crack density. Moreover, this combined method can remarkably lower heat input into an SX matrix and correspondingly the interface stored energy induced by pulsed laser thermal shock, which can effectively lower the tendency for surface recrystallization after the subsequent heat treatment.

Effect of Coating Pre-Treatment on Surface Recrystallization of DD6 Single Crystal

“…Specifically, the water-blown TBCs exhibited no recrystallization structure after the heat experiment. The combined method of adopting low laser energy and preheating at elevated temperatures can significantly lower the heat input into the single crystal (SX) matrix and, correspondingly, the interface stored energy, which can effectively lower the tendency for surface recrystallization after subsequent heat treatment [ 23 ]. At present, research on surface recrystallization of single crystal superalloys is mainly focused on the microstructure transformation of recrystallization zones [ 24 , 25 ], the factors leading to recrystallization itself [ 26 , 27 ] and the influence of recrystallization on the mechanical properties of the alloys [ 28 , 29 ].…”

Effect of γ′ Phase Elements on Oxidation Behavior of Nanocrystalline Coatings at 1050 °C

“…The γ′ phase is the precipitation strengthened phase with a stoichiometric ration of Ni 3 Al. Al, Ti, Ta, Nb, and so on can be dissolved, which are named as γ′ phase elements [ 8 , 9 , 10 , 11 , 12 ]. With the rapid consumption of a large number of alloying element on the high-temperature environment, the original stable γ/γ′ structure will be destroyed [ 10 , 11 ].…”

“…With the rapid consumption of a large number of alloying element on the high-temperature environment, the original stable γ/γ′ structure will be destroyed [ 10 , 11 ]. Therefore, the long-term service performance of the superalloy in harsh temperatures is often related to its composition components and constitute elements [ 12 ]. Thus, in the current superalloy design, the composition of the alloy is often considered and studied to have better high-temperature oxidation resistance.…”

Percussion Drilling of Deep Holes Using Picosecond Ultrashort Pulse Laser in Ni-Based Superalloy Coated with Ceramic Thermal Barrier Coatings

“…Qi et al researched the ablation of metals coated with ceramic TBCs using nanosecond pulse fiber lasers, and found that the recast layer, edge protrusions and microcracks could not be eliminated due to the thermal effects during nanosecond laser drilling [ 16 ]. Fan et al used Nd: YAG (JK300D Nd: YAG pulsed laser, GSI, Birmingham, UK) pulsed laser to process Ni-based superalloy with TBCs, and found the maximum crack length reaches up to approximate 320 μm and the maximum crack width is approximate 50 μm [ 17 ]. In picosecond and femtosecond ultrashort pulse laser machining, these problems can be significantly improved, but for hole drilling [ 18 , 19 , 20 ], there is still a recast layer which thickness is about 5 µm after femtosecond ultrashort pulse trepanning [ 20 ].…”