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.
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. Figure 2 provides an overview of the mechanical properties reported in various studies and also provides mechanical performance specifications for conventionally manufactured IN625 for reference. All properties shown here are without subsequent post-processing heat treatment.
DLD IN625 meets/exceeds the ultimate tensile strength (UTS), yield strength (YS) and elongation (%) of conventionally manufactured IN625 products. However, additively manufactured IN625 is generally less ductile than conventionally manufactured IN625, with typical elongation values ranging between 60% and 65%. The DLD build has higher percentages of lower elongation in the UTS, YS, and XY directions, and lower percentages of higher elongation in the UTS, YS, and Z directions. Isotropic behavior is evident in the mechanical properties of the DLD construct. Most DLD builds exhibit UTS and YS within 820-880 MPa and 460-600 MPa, respectively, in the XY (horizontal) direction and 730-840 MPa and 370-510 MPa, respectively, in the Z (build) direction. In recent studies, reported elongation values ranged between 29% and 45% in the XY direction and between 40% and 56% in the Z (build) direction. The directional dependence of the properties is mainly attributed to the characteristic columnar structure produced by the DLD process, which results in changes in the mechanical properties with reference to the direction of tensile loading during tensile testing. The Laves phase is formed at the end of solidification and is distributed between dendrites.
Comparing the mechanical properties of IN625 by Selective Laser Melting (SLM) and Direct Laser Deposition (DLD) it was concluded that with appropriate parameters and post-heat treatment, DLD has better strength and elongation of more than 40% compared to SLM structures . It has also been reported that DLD builds have relatively lower elongation and strength (approximately 30%) and smaller changes in elongation in the XY and Z directions compared to heat-treated SLM builds with minimal anisotropic behavior. big.
