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Tuesday, June 24, 2008 - 11:00 AM

Microstructural Characterisation of Laser Deposited Alloy 718

D. J. Marks, TWI, Rotherham, United Kingdom; L. Smith, TWI, Cambridge, United Kingdom; D. Clark, University of Birmingham, Birmingham, United Kingdom

Direct metal laser deposition (DMLD) offers many advantages as both an additive manufacturing and repair technique. The extent to which process parameters can be controlled during DMLD allows optimisation of conditions during manufacture and repair. However, the relationship between deposition parameters and microstructure needs to be defined, and then modelled, for material optimisation to be achieved.

Samples of Alloy 718 were deposited using a series of different laser power, frequency and feed-rate, to explore the relationship between process parameter and microstructure. Samples were deposited as a series of six passes, built up into a triangular cross section. By varying the pass spacing, the aspect ratio of the triangle was altered, allowing variations in cooling rate to be explored. Comparison was also made between as-deposited and heat treated samples.

The use of a field emission gun scanning electron microscope (FEG-SEM) equipped with electron backscattered detection (EBSD) allowed detailed microstructural characterisation. This revealed finely spaced dendrite arms within the deposits, indicating a high cooling rate during solidification. Chemical mapping of niobium highlighted the extent of segregation to interdendritic regions, which contain significant quantities of secondary phases and particles. Given the close dentrite spacing, it was considered that beneficial second phases, such as γ′′ should be comparatively well distributed throughout the deposits. Average grain size measurements revealed no significant trend between the locations within each deposit, or between as-deposited and heat treated samples. Crystallographic analysis indicated some evidence of texture development within the deposits.

Following an ageing heat treatment, as expected, there was little diffusion of the niobium, even when there are such small distances between the interdendritic regions and dendrite cores. This result suggests that any nucleation and growth of secondary phases during heat treatment (with the exception of γ′) would be confined to the niobium-rich interdendritic regions.


Summary: Direct metal laser deposition (DMLD) offers many advantages for both manufacture and repair of superalloy components, allowing control of both deposit geometry and microstructure. To define what constitutes an optimum deposit, microstructural characterisation is required. Alloy 718 DMLD samples manufactured using different laser power, frequency, feed-rates and post deposition heat treatment, were examined to explore the relationship between process parameter and microstructure. A field emission gun scanning electron microscope (FEG-SEM) equipped with electron backscattered detection (EBSD) was used to characterise the deposits. Finely spaced dendrite arms observed within the samples indicated high cooling rates, and chemical mapping of niobium highlighted the extent of segregation to the interdendritic regions. Given such finely spaced dendrites it was considered that beneficial second phases, such as g², would be comparatively well distributed throughout the deposits. Crystallographic information revealed some evidence of texture development. Analysis following a two-stage ageing treatment suggested that any nucleation and growth of secondary phases during heat treatment (with the exception of g¢) would be confined to the niobium-rich interdendritic regions.