How does nickel-based superalloy 625 corrode?
Nickel-based superalloy Inconel 625 has excellent mechanical properties and corrosion resistance and has been used in key applications such as aerospace and marine industries. However, its poor processability and high plastic deformation load requirements have always troubled manufacturers. Therefore, many studies have explored additive manufacturing of Inconel 625 to overcome these issues. Researchers from the University of Porto in Portugal summarized the direct laser deposition (DLD) technology of Inconel 625 and its composite materials.
DLD IN625 mechanical properties
The elongation of wrought IN625 heated at 760 °C decreased abnormally, although the authors did not provide any reason for this decrease. However, the observed sharp decrease in elongation of deformed IN625 can be attributed to the strain rate applied in the tensile test. Some authors mention this behavior for similar alloys tested at 600 °C. Several other authors also demonstrated this behavior of tensile properties during tensile tests at similar temperatures.
High-melting solute elements (molybdenum, niobium) have low diffusion coefficients in the nickel matrix; therefore, their presence in the solid solution increases the creep strength of the alloy, while chromium, although having a smaller hardening coefficient, has a lower hardening coefficient due to its presence in IN625. High weight fraction still contributes significantly to solid solution strengthening. Carbon is also an effective solid solution strengthening element and a source of primary and secondary carbide strengthening. IN625 has a face-centered cubic (FCC) crystal structure similar to other nickel-based superalloys that resists phase transition from room temperature to the melting point. Furthermore, the FCC structure has a lower rate of thermal activation processes that influence the creep deformation phenomenon. Resistance to external surface degradation caused by oxidation and thermal corrosion is mainly controlled by chromium, aluminum and titanium. Due to the complex shape of most IN625 parts, over-machining coupled with the difficult-to-machine nature of the alloy caused considerable difficulties during the manufacturing process. Additionally, the forming of IN625 requires higher power during the forming process (approximately 4 times that of carbon steel).
Forged IN625 has high mechanical properties at room temperature. However, its high-temperature mechanical properties set it apart from steels of equivalent strength. ASTM B443 specifies minimum requirements for sheet/plate and ASTM B564 specifies requirements for IN625 forgings. ASTM E8 and ISO 6892 are standard methods for uniaxial tensile testing of metallic materials at room temperature, while ASTM E21 and ISO 6892 provide methods for tensile testing of metals at elevated temperatures for forging and AM IN625. ASTM E292 and ASTM E740 Standard test methods for determining breaking strength (high temperature) and residual strength of notched specimens under tensile loading. The above test methods are suitable for both conventionally manufactured and additively manufactured IN625 materials. Furthermore, in the case of certain tests (e.g., tests against ASTM E740), AM material testing may encounter certain limitations, such as specimen thickness or specimen geometry.
Over the past few years, there have been several efforts on AM of the IN625 using DLD. Mechanical properties reported in various studies are summarized, and mechanical property specifications for conventionally manufactured IN625 are also provided for reference. All properties shown here are without subsequent post-processing heat treatment.
