Abstract
The high thermal gradients experienced during manufacture via selective laser melting commonly result in cracking of high γ/γ′ Nickel based superalloys. Such defects cannot be tolerated in applications where component integrity is of paramount importance. To overcome this, many industrial practitioners make use of hot isostatic pressing to ‘heal’ these defects. The possibility of such defects re-opening during the component life necessitates optimisation of SLM processing parameters in order to produce the highest bulk density and integrity in the as-built state.In this paper, novel fractal scanning strategies based upon mathematical fill curves, namely the Hilbert and Peano-Gosper curve, are explored in which the use of short vector length scans, in the order of 100μm, is used as a method of reducing residual stresses. The effect on cracking observed in CM247LC superalloy samples was analysed using image processing, comparing the novel fractal scan strategies to more conventional ‘island’ scans. Scanning electron microscopy and energy dispersive X-ray spectroscopy was utilised to determine the cracking mechanisms.Results show that cracking occurs via two mechanisms, solidification and liquation, with a strong dependence on the laser scan vectors. Through the use of fractal scan strategies, bulk density can be increased by 2±0.7% when compared to the ‘island’ scanning, demonstrating the potential of fractal scan strategies in the manufacture of typically ‘unweldable’ nickel superalloys.
Highlights
1.1 Selective Laser Melting Selective laser melting (SLM) utilises a computer-controlled scanning laser beam to manufacture complex components by melting metallic powder in a layer-by-layer fashion, direct from a 3D computer model
Cracking is multi-directional in the Peano-Gosper samples, this is to be expected due to the addition of a scan vector that is at a 65° angle to the X or Y axis
Novel fractal scan strategies based upon the Hilbert and Peano-Gosper curves have been developed and experimentally tested via SLM
Summary
1.1 Selective Laser Melting Selective laser melting (SLM) utilises a computer-controlled scanning laser beam to manufacture complex components by melting metallic powder in a layer-by-layer fashion, direct from a 3D computer model. Ni alloys are of significant interest to SLM users for their performance in the high-temperature, load-bearing environments found in the aero and industrial gas turbine industry They are the materials of choice for hot gas path components due to their excellent creep and thermal fatigue strength, oxidation resistance and hot corrosion resistance [9]. CM247LC [10] was initially developed as a directional solidification nickel alloy for the investment casting of turbine blades, designed for high creep strength It is a low carbon derivative of MAR-M247 with reduced Zr and Ti content, and tightened control on Si and S. Novel scanning strategies based on fractals are developed as a method of reducing the residual stress build-up responsible for cracking in CM247LC samples manufacture by selective laser melting. No data is available on how fractal scan strategies influence integrity post SLM, the in-situ methods for reducing thermal gradients in SLS demonstrated by Ma et al [18] and
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