Unique crystallographic texture formation in Inconel 718 by laser powder bed fusion and its effect on mechanical anisotropy

Recrystallization-based grain boundary engineering of 316L stainless steel produced via selective laser melting

Microstructural evolution and mechanical performances of selective laser melting Inconel 718 from low to high laser power

The microstructure evolution of Inconel 718 (IN718) superalloy fabricated by selective laser melting (SLM) and wrought process during subsequent heat treatment process and their corresponding mechanical properties were comparatively studied in our work After the same solution heat treatment at 1080 °C, the grain size of the SLM-IN718 keeps unchanged and also its dislocation subgrain structure is retained, whereas the grains of wrought IN718 is coarsened obviously Due to the existence of dislocation subgrain structure and higher residual stress in the SLM-IN718 matrix, the precipitation of γ'' strengthening phase is improved a lot during the subsequent aging process in the SLM-IN718 specimen compared to that in wrought IN718 specimen Consequently, the hardness and strength of SLM-IN718 are significantly enhanced, which far exceed that of wrought IN718 By adjusting the aging treatment, the precipitation of γ'' strengthening phase can be further optimized and the corresponding mechanical properties of SLM-IN718 are improved significantly Especially, the elongation of SLM-IN718 at room temperature is significantly increased to 29% with only a little decrease in the strength

Comparative study on the microstructure evolution of selective laser melted and wrought IN718 superalloy during subsequent heat treatment process and its effect on mechanical properties

Homogenization is one of the critical stages in the post-heat treatment of additive manufacturing (AM) component to achieve uniform microstructure. During homogenization, grain coarsening could be an issue to reserve strength, which requires careful design of both time and temperature. Therefore, a proper design of homogenization becomes particularly important for AM design, for which work hardening is usually no longer an option. In this work, we discovered an intriguing phenomenon during homogenization of suction-cast and AM Inconel 718 superalloys. Through both short and long-term isothermal heat treatments at 1180 °C, we observed an abnormal grain growth in the suction-cast alloy but continuous recrystallization in the alloy made by laser powder bed fusion (LPBF). The grain size of AM samples keeps as small as 130 μm and is even slightly reduced after homogenization for 12 h. The homogeneity of Nb in the AM alloys is identified as the critical factor for NbC formation, which further influences the recrystallization kinetics at 1180 °C. Multi-type dislocation behaviors are studied to elucidate the grain refinement observed in homogenized alloys after LPBF. This work provides a new pathway on microstructure engineering of AM alloys for improved mechanical performance superior to traditionally manufactured ones.

A comparative analysis of Inconel 718 made by additive manufacturing and suction casting: Microstructure evolution in homogenization

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1369&context=mse_pubs

Improved plasticity of Inconel 718 superalloy fabricated by selective laser melting through a novel heat treatment process

Horizontal-direction Inconel 718 alloy cylindrical rods were fabricated by selective laser melting (SLM) and subsequently subjected to solution or homogenization plus double aging (SA or HA) treatments. Lever arm creep tests were performed with the creep tensile axis perpendicular to the building direction of the samples with a constant stress of 650 MPa at 650 °C. Scanning and transmission microscopy and electron backscatter diffraction were used to clarify the microstructure of the SA and HA samples before and after creep. The experimental results revealed that the average grain sizes of the SA and HA samples were basically the same before creep, and larger γ′ and γ″ strengthening phases existed in the HA sample. The grains tended to grow, and the higher dislocation density within grains and lower local misorientation values close to the grain boundaries occurred in the HA sample during creep, which contributed to the longer creep rupture life in comparison with the SA sample. The fracture surfaces of the SA and HA samples displayed intergranular features, and the creep voids and micro-cracks were formed around the δ or Laves phases at the grain boundaries. The creep fracture mechanism was also discussed.

Study on the microstructure and creep behavior of Inconel 718 superalloy fabricated by selective laser melting

In this study, Inconel 718 alloy cylindrical rods with their axes normal or parallel to the building directions were fabricated by selective laser melting (SLM), then homogenized and double aged. Creep tests of the heat-treated samples were conducted under a constant stress of 650 MPa at 650 °C. The microstructures of the as-built, heat-treated and post-creep horizontal and vertical samples were characterized using scanning and transmission electron microscopy and electron backscatter diffraction. The experimental results showed that both equiaxed grains and columnar dendrites exist in the horizontal and vertical samples before and after creep. The average grain sizes remained nearly unchanged during creep. The larger grain size and reduced regions of transverse grain boundaries, as well as the promoted precipitation of the γ″ phases at the creep test condition contributed to the longer creep rupture life (~120 h) of the vertical sample. Stress concentration occurring especially at the transverse grain boundaries led to the final failure by intergranular fracture for the two building orientational samples. The anisotropy of creep behavior of the SLMed Inconel 718 alloy could not be eliminated by the present heat treatment schedule.

Shi Jingfang

Papers

Improved plasticity of Inconel 718 superalloy fabricated by selective laser melting through a novel heat treatment process

Study on the microstructure and creep behavior of Inconel 718 superalloy fabricated by selective laser melting

Microstructure and creep anisotropy of Inconel 718 alloy processed by selective laser melting