Development of processing strategies for the additive layer manufacture of aerospace components in Inconel 718.

Additive Layer Manufacturing, ALM, of metal components has been steadily
developed over the past decade. Further work is necessary to understand the
metallurgical response of alloys to user defined processing parameters to establish the
robustness of individual ALM systems and how the response affects microstructure
and mechanical properties. This thesis addresses several areas to this end for the
Nickel-Iron superalloy Inconel 718; Identification of key process variables for two types of commercially
available additive systems (powder-bed EOS M270 and blown-powder
Trumf DMDSOS). Development of a processing theme for Inconel 718 on the EOS M270 to
build simple 3D shapes. The melting response to user defined variables. Analytical modelling of the melt pool geometry and the local solidification
conditions (i.e. cooling rate, temperature gradient, and isotherm velocity). A microstructural investigation of the as-deposited grain structure. Simple mechanical testing.
Statistical Design of Experiments (DOE) in the form of Central Composite Design
(CCD) was used extensively to minimise experimental effort throughout the project.
For the investigation of the EOS M270, processing maps were produced to identify
a processing window where fully-dense, pore-free parts were obtained with the key
variables being beam velocity and offset distance between adjacent melted lines. This
was necessary as Inconel 718 had no defined processing conditions at the time of the
investigation. Test samples were built and their microstructure investigated for a range of processing conditions. The grain structure of all samples was seen to consist of fine,
dendritic, columnar grains orientated with a strong <001> fibre texture aligned
perpendicular to the horizontal layers being melted.
The DMD505 investigation considered single track thin walls deposited over a
range of laser powers and beam velocities. The grain structures obtained varied across
the process window but did not fit into classical 'equiaxed' or 'columnar'
morphologies. 'Mixed' microstructures consisting of long grains which are
continuous across melted layer boundaries and short discontinuous grains were
observed at high laser powers and beam velocities. This corresponded to fluctuations
in the top surface of the weld tracks. At lower powers, long continuous grains were
observed along the total wall height. As velocity decreases there is a change to
elongated grains which are contained within a single melted layer. Melt pool geometry and solidification conditions were modelled using analytical
heat transfer equations which showed good agreement (±lO%) with experimental
results for geometry and cooling rate for both processes. The shape of the melt pool is
shown to influence heavily the resulting grain structure.
Other materials and processing issues in ALM are considered such as surface
roughness and thermally induced stress. These are discussed in relation to material
response to user defined processing parameters and a material's thermal and physical
properties which are related by underlying heat transfer equations. Material selection
charts are used to compare the properties of different engineering alloys which in turn
can be used as a basis for parameter selection during processing.