For a long time NASA has been making
efforts in developing Electron Beam Freeform Fabrication (EBF) for developing
net shape metallic components that offers benefits for suitable streamlined production
of intricate components with its ability to directly deposit materials on any
area where it is required. Many markets want to use this technique to improve
the material application efficiency by preventing the need for machining large
magnitudes of materials from wrought blocks and forgings or the development of
high detailed molds for castings.
By using EBF process for the development of
Inconel 718 components for use in high temperature structural applications is
analyzed. Inconel 718 is a commonly used superalloy for offering good
weldability that makes it suitable for the EBF process. The mechanical characteristics
of EBF deposits and the potential to customize these characteristics to
specific applications are also evaluated. Thin walls were made like that the
wall thickness consisted of width of an EBF deposit bead. Following layers were
deposited on each other to develop the wall builds. Additionally a bulk deposit
was made by developing several layers of many side-by-side EBF deposition passes.
The EBF system uses a high power electron
beam gun in vacuum. The feedstock wire is supplied from spool by following wire
feed mechanism. The gun and wire feed are installed on a gantry with the
potential of translating back and forth with single axis.
Inconel 718 wire and plates were used for EBF wall and bulk block.
Experiment
The sample plate was configured at four
corners to the EBF system support table. An electron beam gun preheated the
base plate and eradicated the surface oxides in the vicinity of the wall and
block built before deposition.
Two walls were made on the same base plate.
During the wall development, four single pass beads were deposited on top of
each other then the system was cooled for two minutes.
Tensile samples were machined from the two
wall builds that were oriented in a way that the sample length was parallel to
the wall length. Some samples machined from the block build were heat processed
o find the influence of after-EBF heat processing on the characteristics and
microstructure.
HT1 heat treatment for wrought Inconel 718
included- heating up to 1750oF for 60 minutes then air cool to room
temperature, heating up to 1325oF for 8 hours then furnace cool to 1150oF and
heating up to 1150oF for 8 hours then air cool to room temperature.
HT2 heat treatment for Inconel 718 castings
included- heating up to 2175oF for 4 hours then air cool to room temperature,
heating up to 1325oF for 8 hours then furnace cool to 1150oF and heating up to
1150oF for 8 hours then air cool to room temperature.
Results
No Inconel 718 alloy components were
vaporized to any level during the e-beam production process. Tensile characteristics
of two walls produced at dissimilar deposition travel rates were similar.
Ultimate tensile strength and yield strength showed significant difference from
sample to sample. Modulus for wall
builds was considerably lower than traditionally processed Inconel 718.
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