Differences in the high temperature oxidation behaviour of Inconel 718® fabricated by additive manufacturing and conventional routes
摘要
The high-temperature oxidation behaviour of Ni55-Cr21-Nb5.50-Mo3.30-Ti1.5-Al0.80-Fe Inconel 718® fabricated by selective laser melting (SLM) additive manufacturing (AM) was compared with conventionally wrought (W) material at 900 and 1000 °C in laboratory dry air. Oxidation in all cases followed the parabolic rate law; however, the (AM) alloy exhibited consistently higher oxidation kinetics. At 900 °C, the parabolic rate constant kp for the (AM) alloy was 32 × 10⁻³ mg²·cm⁻⁴·h⁻¹ compared to 27 × 10⁻³ mg²·cm⁻⁴·h⁻¹ for the (W) alloy. At 1000 °C, the (AM) alloy reached 126 × 10⁻³ mg²·cm⁻⁴·h⁻¹, while the (W) alloy showed 79 × 10⁻³ mg²·cm⁻⁴·h⁻¹, confirming a faster degradation rate for the (AM) condition at both temperatures. The activation energies for the oxidation of the (AM) specimens were QAM=-74 kJ mol− 1 K− 1 and QW=-58 kJ mol− 1 K− 1 for the (W) samples, also indicating that oxidation was more feasible for the SLM specimens. The oxide scales formed also differed depending on the manufacturing route. The (AM) specimens developed thicker and more multilayered scales containing 3 oxide phases namely, Nb₂O₅, Cr₂O₃, and spinel-type (Ni, Fe)Cr₂O₄, whereas the (W) alloy primarily formed only two oxides which were Nb₂O₅ and Cr₂O₃. In addition, both materials exhibited intergranular oxidation zones (IOZ), but these were deeper in the (AM) samples. Ti- and Al-rich oxides were observed at grain boundaries in the (AM) alloy, while mainly Al-rich oxides were detected in the (W) material. These results indicate that microstructural features characteristic of additive manufacturing, such as elemental segregation and residual porosity, reduce the oxidation resistance of Inconel 718® at elevated temperatures.