Improving the microstructure, mechanical properties & process route in selective laser melting of nickel-superalloys

Wang, Xiqian (2018). Improving the microstructure, mechanical properties & process route in selective laser melting of nickel-superalloys. University of Birmingham. Ph.D.

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Abstract

Selective Laser Melting (SLM) was used to develop a manufacturing route for high temperature aero-engine components from the Ni-superalloys CM247LC, focussing on improving the microstructure, mechanical properties, and processing route. A statistical design of experiments approach was applied to determine the optimum processing parameters leading to the least structural defects. High-speed imaging was used to observe the melt pool during SLM. Microstructural investigations showed that certain elements were selectively evaporated, then condensed in the form of particles. These were then re-incorporated within the build. Cracks and pores were found in SLM-processed samples and these were sometimes associated with these condensed particles. Residual stresses, developed within SLMprocessed samples, were measured using neutron diffraction, highlighting the role of the scanning strategy on the residual stress development.

The solidification microstructures formed in SLM-processed samples were characterised using analytical scanning and transmission electron microscopy. Cells, with identical orientation and 700 nm in width containing a high density of very small γʹ (up to 20 nm), were observed. Cell boundaries and grain boundaries were found to contain high densities of dislocations, Hf/Ti/Ta/W-rich precipitates and γ/γʹ eutectic containing larger particles of γʹ up to about 50 nm. The cooling rate derived from the cell size was estimated at 106 K/s, but the cooling rate, derived from the size of γʹ within grains was estimated as 104 K/s based on Jominy end-quench test.

SLM-processed samples also showed high yield strength due to their fine microstructures, alongside poor ductility resulting from the presence of cracks. Post-SLM heat treatments were used to reduce the extent of cracking and porosity by Hot Isostatic Pressing (HIPping) and also to promote the precipitation of γʹ. These treatments improve the ductility in vertically built samples, but the ductility in horizontally built samples remains low. Though SLM-processed samples subjected to post-processing heat treatments showed poor creep strength, this was improved by HIPping. A novel approach for netshape SLM/HIP processing was assessed for manufacturing a blisk using powder CM247LC or dual materials.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):