Additive manufacturing(AM) has gained popularity due to the possibility of fabricating near net shapes of complex geometries with limited material waste and post processing. Haynes 282 is a precipitation strengthened Ni-based superalloy that is a good candidate for AM due to its weldability that primarily comes from its low volume fraction of gamma prime (γ’) phase. Haynes 282 has applications in high temperature industrial gas turbine engines due to its thermal stability and high temperature creep resistance.
However, there are fundamental challenges related to the development of AM fabrication processes that need to be understood. Electron beam powder bed fusion(EBM-PBF) processing can result in large thermal gradients and rapid temperature cycling. The resulting microstructure is a columnar morphology commonly seen in AM that differs significantly from conventionally processed wrought alloys. In this work, microstructural heterogeneities in EBM-PBF Haynes 282 were systematically characterized as a function of process parameters such as build height, scan length, and scan velocity. Here, the effect of these process parameters on microstructure features such as γ’ and MC carbides were investigated.
A Haynes 282 pyramidal build geometry that was characterized in this study was fabricated in an ARCAM Q10 (GE Additive) PBF-EB machine using a pre-alloyed powder. The specimen was deposited as part of a group of 12 builds on a Type 304 austenitic stainless steel base plate with set platform temperature of 1000 ⁰C and a chamber vacuum pressure of 0.7 Pa. A bilinear rastering scan strategy was used with 200 Ám hatch spacing and a rotation of 67.5 ⁰ between layers.
To fully understand the process-property-structure relationship, location specific characterization was done using Vickers hardness testing, electron backscatter diffraction (EBSD), and scanning electron microscopy (SEM). Site specific Vickers hardness indents were done over approximately 6,200 points, with a spacing of 150 Ám and a 200 g load. EBSD was performed to quantify the texture of the grains along the build direction. SEM analysis was used to characterize the γ’ precipitates and carbide morphology, size and distribution along the build direction.
Results and Discussion:
Maps of the microhardness measurements indicated a 4% increase in the hardness in the build direction. The average hardness from the top was measured to be 327 HV while the bottom of the build was 315 HV. EBSD data analysis showed large columnar grains that have a strong texture in the <001> orientation, parallel to the build direction. Preliminary SEM imaging of the γ’ precipitates showed a change in the morphology of the precipitates from cuboidal to spherical along the build direction. Image analysis of γ’ size revealed a decrease in size along the build direction.
Site specific characterization was done to examine the microstructural and mechanical properties of this unique pyramidal geometry of EBM-PBF Haynes 282. The main conclusions from this work are as follows:
• EBSD analysis revealed the presence of columnar grains growing along the <001> orientation parallel to the build direction.
• Gamma prime size decreased along build direction and morphology of gamma prime precipitates was also observed to have changed from a cuboidal to a spherical shape.
• The hardness was found to increase by 4% in the build direction.
Keywords: Superalloy, Haynes 282, Gamma prime, Additive Manufacturing, EBM-PBF, Process-Structure-Property Relationship