The paper details experimental data for the fatigue behaviour of aeroengine alloy Ti-6Al-2Sn-4Zr-6Mo following wire electrical discharge machining (WEDM), with minimum damage generator technology and optimised trim pass strategies. Comparative results for flank milled samples are also given together with associated micrographs detailing workpiece subsurface integrity and fracture initiation. Despite a marginally higher S-N response for the milled specimens compared to the wire machined samples when subject to a finishing regime, linear regression statistical analysis suggested no significant difference in performance at the 5% level based on slope responses. Micrographs showing sample crack initiation sites and fatigue crack growth paths suggest a 40- 50 μm altered zone in fracture surfaces for milled specimens with fatigue striations defining the crack path. For WEDM surfaces, crack initiation was in some cases due to defects below the machined surface, with secondary cracks probably due to local stresses.
Superhydrophobic surfaces, which mimic the anti-wetting capabilities of the lotus leaf (Nelumbo nucifera) and similar naturally-occurring surfaces have been subject of significant interest in the engineering field for many years; in particular to create self-cleaning and anti-bacterial surfaces. A droplet of water landing on a superhydrophobic surface will attach itself to foreign debris and slide away. There have been a significant number of reports of ultra-short pulsed lasers being used to generate such surfaces, and rather fewer reports that use much lower cost nanosecond systems, which is the approach we have chosen to follow, in order to develop a low-cost process suitable for large surface areas. Flat sheets of the commonly used stainless steels 304 and 316 were textured using a nanosecond pulsed fibre laser operating at 1064nm, generating surface grooves of typical dimension 10-30 µm deep by 30 µm wide, dependent on the fluence. Quantitative analysis of the wettability of the laser micro-structured surfaces was carried by measuring the static contact angle of a droplet of deionized water with a volume in the microliter range. The transition between hydrophilic to hydrophobic was periodically monitored. Such laser structured metal surfaces are hydrophilic immediately after processing, transitioning to super hydrophobic in some cases, displaying static contact angles larger than 130°, however this can take up to several months. Our work has concentrated on speeding up this transition and to this end we have studied the impact of the laser processing and environmental parameters, and we have demonstrated that these can have significant impact on the wettability transition.
Surface engineering, through either texturing or surface modification, for enhanced functionality is currently one of the most emerging research topic/technologies. This is mainly related to its huge commercial potential in terms of added value or product customisation, with applications extending from tooling, to mould & die, medical, food & drink, aerospace and others. With the recent advances in EDM generator design and process control, a number of commercial solutions were released by leading technology providers where it is deemed possible to apply a certain surface finish/structure, resulting in enhanced functionality (e.g. reduced friction or reduced/controlled adhesion) via the EDM process itself and without the need for a secondary operation. This paper investigates the surface integrity effects for this modified surface structuring concept (3DS). Experimental results showed that it was possible to reduce the friction coefficient (consequently ejection forces in the injection moulding process, for example) by as much as 60%, compared to samples machined using standard EDM settings while both having similar Ra values. 3DS samples however showed slightly lower fatigue performance which is likely to be the result of surface micro-cracking which appeared to be more prominent on these, compared to the standard EDM samples.
The research focused primarily on the effects of WEDM on the surface / subsurface integrity of Ti-6Al-2Sn-4Zr-6Mo titanium alloy and Udimet 720 nickel base superalloy, and encompassed measurement of surface roughness and contamination, workpiece microhardness, recast layer thickness, residual stresses and fatigue performance. Associated productivity and manufacturing costs were also evaluated in respect of blade root slot machining in aerospace turbine / compressor discs. Preliminary tests results showed that machined surfaces with Ra of ~0.5μm, less than 2μm of recast layer and almost neutral residual stresses were obtained following a roughing and four finishing passes. In a subsequent phase it was possible to reduce the number of finishing passes into only 2 while maintaining similar surface integrity figures, where also it was possible to wire machine firtree slots geometrical accuracy of ±7μm. This phase also entailed extensive assessment of the impact of pulse shape on surface integrity when using similar pulse energies. Fatigue performance tests proved no statistically significant differences between WEDM and flank milling at 5% level. The research also investigated the use of coated wires and oil based dielectric compared to standard brass wire and de-ionised water, where significant differences in productivity were recorded.
Following a brief review of recent minimum damage EDM pulse generator developments, experimental data is presented for workpiece productivity & integrity when WEDM Udimet 720 nickel based superalloy and Ti-6Al-2Sn-4Zr-6Mo titanium alloy, using Cu core coated wires (ZnCu50 and Zn rich brass). Up to a 70% increase in productivity was possible compared to when using uncoated brass wires with the same operating parameters. Surfaces measuring ∼ 0.6 μm Ra, with near neutral residual stresses and almost zero recast were produced following two trim passes. Cross-sectional micrographs of specimens following rough machining (∼180mm2/min) showed the recast to be < 7 μm thick (up to 11 μm for uncoated wire), with comparable results for both alloys. Surface cracking, when evident, was restricted to within the recast layer.
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