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Powder Processing and Advanced Additive Manufacturing of Materials
Featured research (4)
This study reports the successful crack-free fabrication of the non-weldable high γʹ Ni-superalloy IN 738 by laser powder bed fusion. The as-fabricated texture was composed of columnar grains with preferred orientation along <100> direction. Scanning electron microscopy and X-ray diffraction analysis revealed the presence of M(Ti, Ta, W, Mo, Nb)C and M(Cr, Mo, W)23C6 carbides along the grain boundaries. A heat treatment used for IN 738 castings yielded a bimodal distribution of γʹ precipitates with a primary γʹ volume of 25% and size of ∼226 nm and secondary γʹ with a volume of 43% and a size of ∼88 nm. The carbides observed in the as-built condition were maintained. The heat treatment increased the hardness from 408 HV to 487 HV. The specimen exhibited an excellent room temperature yield strength, ultimate tensile strength, and elongation of 1010 MPa, 1444 MPa, and 13%, respectively. The coupons showed yield strength of 560 MPa and 388 MPa, the ultimate tensile strength of 765 MPa and 538 MPa, and an elongation of 17% and 14% at 850 °C and 927 °C, respectively. Finally, fracture analysis was used to better understand fracture behavior.
Electron beam wire fed (EB-WF) additive manufacturing (AM) can be utilised for cost-effective part repair in the aerospace industry and, especially for titanium alloys, the vacuum processing effectively mitigates high temperature contamination for enabling reliable high-performance. This research advanced EB-WF additive processing and simulation for depositing Ti–6Al–4V thin walls (3 mm in thickness) that emulates repair of damaged fan and compressor blades. The main focus of this research was to understand the effect of the initial substrate microstructure and residual stress profile on the final deposit properties. The results revealed that the EB-WF additive repair process for depositing Ti–6Al–4V yielded minimal distortion (<350 μm) and residual stresses (<150 MPa (0.15σys)). The initial residual stress states of the substrate were found to have a negligible effect on the final residual stress profiles, from the stress relaxation effect during EB-WF AM. Although significant variance in the microstructure for each substrate condition was present after deposition, their mechanical properties were similar. Deposited test specimens had tensile yield and ultimate strength values ranging between 800-830 MPa and 860–880 MPa, respectively. The similar mechanical properties of the interface were correlated with the microstructural features such as layer bands and titanium alpha (α) colonies.
The as-built surface morphology on micro-scale parts produced by laser powder bed fusion (PBF-LB/M) considerably influences their mechanical behavior. Femtosecond pulsed laser was utilized to selectively ablate surface regions of stainless steel microstruts produced by PBF-LB/M, allowing the fabrication of micro-scale specimens for tensile and surface characterization of its intrinsic or "roughness-free" mechanical behavior. The study fabricates 280μmx400μm cross-sectioned specimens from vertically built PBF-LB/M microstruts of 500μm nominal diameter. The new micromachining strategy utilized in this work was designed to ensure similar roughness at the edges and faces of the specimens, the average Ra are 1.2±0.4μm and 0.9±0.2μm, respectively, as well as to impart low curvature at edges, i.e. ρ=3±1μm. In addition, the micromachined surfaces revealed an increased average hardness from 276±11HV to 340±14HV at near-surface depths <3.6μm and similar bidirectional compressive residual stresses in laser incidence and scan directions, i.e. −371±65MPa and −429±41MPa, respectively. The effect of surface morphology and hardened surface layer on tensile elongation are negligible, as the average yield strength, ultimate tensile strength, and uniform tensile strain obtained for the micromachined specimens are comparable to those obtained from electropolished counterparts.
Considerable surface roughness, dimensional deviation, and non-uniform microstructure are a few of the characteristics found on thin or micro-scale features fabricated via laser powder bed fusion (LPBF) that yield inferior and/or inconsistent mechanical properties. Femtosecond laser micromachining can aid in fabricating micro-scale parts with ultra-high dimensional precision. In this work, the surface and tensile behavior of microstruts of 500μm nominal diameter micromachined with Gaussian laser pulses of <100fs duration are characterized. Roughness parameters such as Ra=0.9±0.2μmand Rz=3.4±1.3μm are achieved on the micromachined faces. Surface-associated grains are successfully ablated with negligible microstructural damage to the microstruts. As a result, the average uniform strain under quasi-static tensile loading is measured as 54±2% compared to 42±1% for the as-built microstruts. Uniform and non-uniform deformation strain portions are separated analytically and characterized primarily via in-situ imaging. Progressive degradation of the surface and dimensional variance is observed on the micromachined test specimens. Post necking initiation, ablation-associated asperities on the micromachined surfaces evolve into notches, leading to tensile failure.