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Title: Development of processing strategies for the additive layer manufacture of aerospace components in Inconel 718
Author: Deffley, Robert James
ISNI:       0000 0004 2742 5770
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2012
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available