Additive manufacturing of graded microstructures for aeroengine components

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Attard, Bonnie ORCID: https://orcid.org/0000-0002-1270-7332 (2022). Additive manufacturing of graded microstructures for aeroengine components. University of Birmingham. Ph.D.

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Abstract

The microstructure formed after laser powder bed fusion (L-PBF) depends on the thermal gradient and solidification rate with the complex physical interactions of the melt pool and heat flow also impacting the solidifying structure. Due to the layer-by-layer methodology applied in L-PBF, modifying these factors is possible and an area of growing research. Thus, the microstructure itself can be seen as a variable tool to be controlled through processing parameters, allowing L-PBF to be harnessed to move towards the generation of functionally graded microstructures and the formation of components with structures which are fully optimised for their application. This work explores the feasibility of additively manufacturing tailored Inconel 718 microstructures by varying process parameters and scanning strategies, to modify the mechanical properties and performance. This investigation focuses on controlling the heat input and thermal history through process parameter manipulation; notably the heat input parameters (power, scan speed, and hatch spacing) and island scanning parameters (island size, shift, and island overlap). Preliminary investigations into the use of base plate heating at 400°C with the objective of moving from columnar to equiaxed growth were also made.

The changes in preferred orientation, morphology and grain size were fully characterised and compared using scanning electron microscopy, electron back scatter diffraction and X-ray diffraction. The solidification cell size was quantified to estimate the effect of the process parameters on the cooling rates. Both grain size and preferred orientation were successfully tailored through scanning parameter modification and separately through base plate heating. Based on these obtained microstructures, graded microstructures were generated using subtle modifications in the island strategy.

The viability of using scanning strategies to modify the mechanical properties of Inconel 718 through variations of microstructure was then investigated in more detail. Mechanical testing indicated that the scanning strategy can be used to generate functionally graded specimens; larger grains and more [001] preferred alignment resulting in lower Young’s modulus and strength or smaller grains with a reduced preferential [001] orientation along the build direction increased the Young’s modulus and strength. Digital image correlation showed that strain in graded structures was concentrated in highly textured areas. The ductility was limited by the entrainment of thin Al oxides folded in the melt pool which were investigated in more detail. Further investigation into the high temperature isothermal oxidation resistance of the microstructurally variant components was made. The mass gain behaviour was found to be broadly similar between the different conditions after 504 h and comparable to wrought Inconel 718 but following sub-parabolic oxidation kinetics possibly attributable to the formation of an underlying Nb rich layer.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):