Abstract
This article outlines a detailed study of solution treatments and delta precipitation treatments carried out on laser-directed energy deposited (DED) alloy 718 specimens. Two different sets of DED process parameters were used in high and low energy conditions that yield different microstructural features to study the effect of process parameters on delta precipitation. These two conditions were subjected to solution treatment at 1010 °C and 1050 °C each for 1 h, which improved homogeneity and altered grain texture with introduction of annealing twins. The as-built and solution-treated specimens served as the initial reference condition for subsequent delta processing treatments (DPT) performed at three temperatures of 850 °C, 900 °C, and 950 °C to study the effect of short- and long-term exposures ranging from 1 to 48 h. When as-built specimens were subjected to DPT, interdendritic delta precipitates were observed at Nb-rich regions. In contrast, solution-treated specimens under short-term exposure to DPT resulted in intergranular delta phase precipitates whereas under long-term exposures to DPT yielded predominantly intragranular delta precipitates, which grew denser and longer with increased time of treatment. For longer exposure times of 24 and 48 h, a continuous film of intergranular delta phase was noticed. The morphology, location, and volume fraction of delta phase precipitates studied in this research are imperative for designing the performance of alloy 718 built by DED process.
Highlights
Alloy 718 is a nickel, iron-based superalloy which is primarily used in aerospace, space, petrochemical, and oil and gas industries due to its superior high-temperature properties and corrosion resistance, with service temperatures varying between − 200 and 650 °C
Δ-precipitation occurs in alloy 718 between the temperature range of 800 and 1000 °C, and the corresponding thermal treatment designed to achieve δ-phase saturation in the alloy is sometimes called delta processing treatment (DPT) [3]
Uniaxial tensile testing showed increase in both yield and tensile strength for wrought specimens subjected to solution treatment at 1100 °C/water quenched (WQ), and DPT performed at 900 °C/8 h/WQ as the mode of δ-precipitation was predominantly intergranular [4]
Summary
Alloy 718 is a nickel, iron-based superalloy which is primarily used in aerospace, space, petrochemical, and oil and gas industries due to its superior high-temperature properties and corrosion resistance, with service temperatures varying between − 200 and 650 °C. Many material properties of importance in aerospace applications, such as tensile strength, creep resistance, fatigue strength, and ductility, depend on precipitation strengthening and grain stability, which in turn is affected by morphology, distribution, and volume fraction ( Vf) of δ-phase in alloy 718. Uniaxial tensile testing showed increase in both yield and tensile strength for wrought specimens subjected to solution treatment at 1100 °C/water quenched (WQ), and DPT performed at 900 °C/8 h/WQ as the mode of δ-precipitation was predominantly intergranular [4]. Low-cycle fatigue properties showed no direct correlation with the amount of δ-phase Vf in wrought alloys [13], the bimodal distribution of grains that resulted due to uneven δ-precipitation decreased the fatigue life. Short intergranular δ-phase precipitates have been reported to be beneficial in enhancing the tensile strength, ductility, creep life, stress-rupture, and notch-rupture properties of alloy 718. The study reveals the differences in precipitation patterns of δ-phase based on DED processing conditions and prior solution treatments through comprehensive microstructural analysis
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