**1. Introduction**

Nickel-based Inconel 625 alloy has numerous applications in the aeronautics, aerospace, marine, chemical, and petrochemical industries [1,2]. The alloy is generally used in a medium temperature range (250–593 ◦C) for structural applications requiring high strength and excellent corrosion resistance, and in a high temperature range (over 593 ◦C) for applications calling for outstanding creep resistance (ASTM E139-11, ASTM B444). These service properties can be achieved by conventional manufacturing technologies such as forging, rolling, or extrusion [3–6]; laser powder bed fusion (LPBF) additive manufacturing (AM) technology, however, offers numerous advantages over conventional manufacturing, more specifically in terms of its ability to fabricate parts with near net shapes, unique designs, added functionalities, low buy-to-fly ratios, and high productivity [7–10]. Moreover, LPBF is capable of producing fully functional parts directly from metal powder without the need for specialized tooling and intermediate processing steps.

It should nevertheless be noted that complex heat effects, which occur during LPBF and are related to highly localized multiple melting–remelting of powder and of underlying bulk materials, differ from those seen during conventional casting and welding. They are also responsible for strongly non-equilibrium heat and mass transfer and solidification phenomena leading to grain refinement, texture development, and the formation of unusual metastable phases [11–13]. High residual stresses resulting from a combination of significant temperature gradients and high cooling rates represent

another peculiarity of LPBF; to avoid distortions and cracking, printed parts must therefore be subjected to post-processing stress relief heat treatment before they can even be removed from the build plate [14,15].

To mitigate the undesirable effects of LPBF processing on the microstructure of parts (columnar structure and precipitation formation), various post-processing heat treatments have been proposed [12,13,16,17]. These treatments frequently differ from those recommended for conventionally processed IN625 alloy parts, because of the previously mentioned structural features related to LPBF processing. For example, only a small amount of recrystallized structure is found in the LPBF IN625 alloy at 980 ◦C (~0.8 Tm) [12], while in the conventionally deformed (ε = 0.4) IN625 alloy, annealing in the 900–980 ◦C temperature range results in full recrystallization [18]. A fully recrystallized structure has been observed in the LPBF IN625 alloy only at temperatures higher than 1100 ◦C [12,13,16]. Thus, numerous studies on LPBF-fabricated alloys have aimed to find an original sequence of post-processing heat treatments, which can include solution treatment, homogenization annealing, aging, etc., in order to render the service properties of LPBF parts comparable or superior to those of conventionally manufactured alloys of similar compositions.

An excellent combination of outstanding corrosion resistance and superior creep resistance, as well as the relatively high tensile strength of nickel-based IN625 alloy (up to 600 ◦C), make it an interesting choice for aerospace applications. It has been shown that post-processing annealing of LPBF IN625 alloy can significantly improve its room temperature ductility as compared to its as-built state [13,19–21]. However, the assessment of mechanical properties cannot solely be limited to room temperature testing, especially for materials dedicated for service at elevated temperatures. In this context, it is known that conventionally processed nickel-based superalloys face the risk of embrittlement at temperatures higher than 600 ◦C, and that thermal treatments can affect this mechanical behavior either positively or negatively [22].

It has been shown, for example, that at 538 ◦C, the mechanical resistance and the elongation to failure of an IN625 alloy that was electron beam-melted and then hot isostatically pressed (HIP, 1120 ◦C, 100 MPa, 4 h) were close to those of wrought IN625 alloy [23]. At 760 ◦C, however, as compared to its wrought counterpart, the laser powder-fused IN625 alloy (HIP under the same conditions as above) manifested significantly lower ductility, but similar mechanical resistance [16,24]. Notwithstanding the preceding, such information is very limited, which makes it difficult to compare the tensile properties of printed and wrought IN625 parts. The outstanding creep resistance of wrought IN625 alloy favors its use at elevated temperatures, but, as was the case with the tensile properties, we could not find any publicly available information on the creep properties of 3D-printed IN625 alloy.

Unlike IN625 alloy, LPBF IN718 alloy, as a precipitation-hardened alloy with a higher mechanical resistance at elevated temperatures [25], has been covered by many studies [26–30]. It was shown that LPBF IN718 alloy manifested a high build-orientation-related anisotropy of its creep properties, caused by preferentially oriented distributions of dendrites and precipitations formed during LPBF processing [26]. Furthermore, the application of the solution (980 ◦C, 1 h) and aging (718 ◦C/8 h + 621 ◦C/10 h) heat treatments recommended by the AMS5662 specifications for forged and welded IN6718 alloy to the LPBF IN718 alloy led to lower creep rupture times, compared to the as-built state [26]. It was shown that this property degradation stems from the replacement of particle-shaped δ phase precipitates located in the interdendritic regions of the as-built alloy by needle-shaped δ phase precipitates located in the equiaxed structure of the solution-treated and aged alloy.

For the same LPBF IN718 alloy, limited data are available on its microstructure and mechanical behavior after post-processing HIP treatments. Similarly to the above-mentioned influence of the δ phase morphology, it was shown in Reference [28] that at 650 ◦C, the HIP-treated alloy (1200 ◦C, 103 MPa, 4 h) exhibited a lower creep rupture time compared to the as-built state. Note that this comparison was flawed, since the materials in both states were tested under the same stress of 650 MPa (~0.8 of YS for the as-built alloy), which put the HIP material under less favorable conditions, since HIP reduces the mechanical resistance characteristics of the printed material. Nevertheless, it was assumed

that the needle-like δ phase grain boundary precipitates found in crept HIP-treated samples could be a cause of lower creep lifetimes, but their origins were not clear; it was uncertain whether the precipitation took place during HIP or if it occurred during creep testing at 650 ◦C. A comparison of the HIP LPBF IN718 alloy and the conventional hot-rolled IN718 alloy showed that the former manifested shorter creep lives than the latter under the same testing conditions [28].

It was also shown in Reference [27] that the application of the HIP conditions recommended for the wrought IN718 to the LPBF IN718 alloy (1180 ◦C, 175 MPa, 4 h [31]) triggered three concurrent phenomena, namely, microstructure homogenization, δ phase dissolution, and the formation of coarse carbide precipitates. When these phenomena were combined, they significantly improved the creep rupture time at 650 ◦C and 550 MPa, as compared to what was obtained in the as-built state.

Based on the above-mentioned observations, two main objectives can be established for future work: (a) Building a comprehensive database of the mechanical behaviors of LPBF IN625 alloy over a wide temperature range; for this study, the application of heat treatments recommended for the wrought alloy of the same composition was considered reasonable as a first approximation; (b) establishing a correlation between the mechanical properties of LPBF IN625 alloy and its microstructure (size of structural elements, nature and morphology of precipitates), with the ultimate goal of optimizing the post-processing conditions for this material.

This work focused on the first objective. The tensile and creep behaviors of laser powder-fused IN625 alloy subjected to stress relief (SR) annealing, solution treatment (ST), and hot isostatic pressing (HIP) were studied. The tensile behavior was studied in the 25 to 871 ◦C temperature range (68 to 1600 ◦F), with this range corresponding to the widest service diapason recommended for IN625 alloy [32]. The creep behavior was studied at 760 ◦C (1400 ◦F) under various stresses, this temperature corresponding to the onset of the high temperature embrittlement phenomenon observed in our previous work [16]. The correlation between the mechanical behavior and the structural features will form the core of the next publication.

It should be noted that since LPBF IN625 alloy in its as-built condition is characterized by a significantly high level of residual stresses and a strongly heterogeneous microstructure, it is not suitable for practical use, and was therefore excluded from consideration in this study. Meanwhile, the microstructure and the mechanical properties of the as-built LPBF IN625 alloy at room and elevated temperatures can be found elsewhere [16,24].

Note also that in this work, the exact heat and HIP treatment conditions have been omitted and the tensile testing stress values measured normalized to protect proprietary partner information.
