<p>An existing thermal model for laser melting and additive
manufacturing (AM) was expanded to include phase transformation and hardness
predictions for an alloy steel and coupled to experimental results. The study was performed on AISI 8620, a
popular case-hardening, steel to understand microstructural and property
effects for potential repair applications.
The experimental samples were
polished, etched with nital and picral for comparison, imaged, and Vicker’s microhardness
was taken at 0.5 and 0.2 kg loads. The
etched images revealed a transformation zone slightly larger than the melt zone
in all cases including a gradient in transformation along the outer edges of
the transformation zones. The microhardness
measurements revealed that the lower energy cases provided a higher hardness in
the melted region even after tempering due to multiple passes. But the overall hardness was higher than what
is to be expected of a fully martensitic structure in AISI 8620. The phase transformation model qualitatively
shows a similar microstructure where molten regions turn completely to
martensite. The model also predicts a
transformation zone larger than the melt pool size, as well as the
transformation of pearlite but not ferrite near but not in melt pool. This observation is experimentally verified
showing a heat affected zone where pearlite is clearly transformed but not
ferrite outside the transformation zone comprised of complete martensite. The hardness model predicts a lower hardness
than the experiments but is similar to what is expected based on published
Jominy End Quench tests. The cases in
the regime dominated by conductive heat transfer show good agreement with the predictions
of melt pool shape and hardness by the thermal model. However, at higher powers and lower speeds,
the fluid flow influenced the shape of the melt pool and the hat transfer in
its vicinity, and the model was less accurate.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12735371 |
| Date | 29 July 2020 |
| Creators | Matthew L Binkley (9182462) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/MICROSTRUCTURE_DEVELOPMENT_IN_MULTI-PASS_LASER_MELTING_OF_AISI_8620_STEEL/12735371 |
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