γʹ and γ″ co-precipitation phenomena in directly aged Alloy 718 with high δ-phase fractions

Co-precipitation of γ′ and γ′′ is the main strengthening mechanism that provides superior high-temperature strength in directly aged Alloy 718 aerospace parts. Control of their morphology, fraction, and configuration might allow exposure to more demanding operation environments in next-generation aircraft engines. The density of geometrically necessary dislocations introduced during hot deformation has been shown to significantly affect the co-precipitate morphology of γ′ and γ′′ in materials free of the δ-phase. However, the combined effects of geometrically necessary dislocation density and lower Nb content due to higher δ-phase fractions on co-precipitation behaviour and strengthening remain unknown. We verify these effects by hardness testing as a proxy for high-temperature strength in materials with 4.1 % δ-phase fraction. Deformation at 950 °C yields a remarkable increase of 12 % in hardness after direct ageing, explained by the prevalence of complex co-precipitate configurations. Deformation at 1000 °C decreases the δ-phase fraction and geometrically necessary dislocation density but achieves up to 19 % volume fractions of γ″, leading to a predominance of monoliths and duplet co-precipitates and a better direct ageing response. Atom probe microscopy reveals the flux of elements during co-precipitation. We recommend a δ-annealing treatment before the final forging step for manufacturing stronger Alloy 718 aerospace parts

The role of parent austenite grain size on the variant selection and intervariant boundary network in a lath martensitic steel

This study investigated the influence of parent austenite grain refinement on the intervariant boundary network (population and connectivity) in a lath martensitic steel. Parent austenite grain refinement revealed a progressive reduction in the fraction of the 60° misorientation boundaries in martensite, which was linked to a decrease in the 60° / [110] intervariant boundary population. The phenomenological theory of martensite crystallography demonstrated that the variant selection mechanism altered from the 3-variant clustering (V1V3V5) in the coarse parent austenite towards the 4-variant clustering (V1V2V3V4) in the fine parent austenite grain, due to the change in the lattice parameter of the parent and daughter phase in which the martensite transformation occurs, as measured using in situ neutron diffraction. The change in the variant clustering arrangement with the parent austenite grain refinement led to a progressive promotion of 60° / [111] and 10.5° / [011] intervariant boundaries at the expense of 60° / [110] martensite intervariant boundaries. Subsequently, the connectivity of low energy {110} tilt intervariant boundaries gradually increased through the refinement of parent austenite grain size, eventually reducing the high energy {110} twist boundary connectivity. This change improved the impact toughness of martensite produced from the fine-grained austenite as the weak connectivity of high energy boundaries delays the coalescence of voids, promoting ductile fracture.publishedVersio

A comprehensive study on meltpool depth in laser-based powder bed fusion of Inconel 718

One problematic task in the laser-based powder bed fusion (LB-PBF) process is the estimation of meltpool depth, which is a function of the process parameters and thermophysical properties of the materials. In this research, the effective factors that drive the meltpool depth such as optical penetration depth, angle of incidence, the ratio of laser power to scan speed, surface properties and plasma formation are discussed. The model is useful to estimate the meltpool depth for various manufacturing conditions. A proposed methodology is based on the simulation of a set of process parameters to obtain the variation of meltpool depth and temperature, followed by validation with reference to experimental test data. Numerical simulation of the LB-PBF process was performed using the computational scientific tool “Flow3D Version 11.2” to obtain the meltpool features. The simulation data was then developed into a predictive analytical model for meltpool depth and temperature based on the thermophysical powder properties and associated parameters. The novelty and contribution of this research are characterising the fundamental governing factors on meltpool depth and developing an analytical model based on process parameters and powder properties. The predictor model helps to accurately estimate the meltpool depth which is important and has to be sufficient to effectively fuse the powder to the build plate or the previously solidified layers ensuring proper bonding quality. Results showed that the developed analytical model has a high accuracy to predict the meltpool depth. The model is useful to rapidly estimate the optimal process window before setting up the manufacturing tasks and can therefore save on lead-time and cost. This methodology is generally applied to Inconel 718 processing and is generalisable for any powder of interest. The discussions identified how the effective physical factors govern the induced heat versus meltpool depth which can affect the bonding and the quality of LB-PBF components

Microstructure and mechanical properties of Ti6Al4V alloys fabricated by additive friction stir deposition

Microstructure and mechanical properties of Ti6Al4V alloys fabricated by additive friction stir depositio

A comprehensive investigation of abrasive barrel finishing on hardness and manufacturability of laser-based powder bed fusion hollow components

AbstractOne of the main issues of laser-based powder bed fusion (LB-PBF) parts is surface quality and dimensional deviations, which require post-processing. Conventional post-processing such as turning and milling cannot machine internal surfaces and therefore is not suitable for hollow components. In this paper, Ti–6Al–4 V components with different hollow shapes were printed by LB-PBF and post-processed by centrifugal barrel finishing (CBF). Samples were printed based on Taguchi L18 design of experiments (DoE) on the (L18: 21 × 33) matrix and polished in abrasive solution by porcelain triangular media 2 × 2 mm. The effect of process parameters including rotation direction, speed, time and volumetric percentage of abrasive on hardness and manufacturability, including surface quality, material removal rate (MRR) and dimensional deviation, are discussed. The novelty of this work is the application of this process to clean both the internal and external surfaces of LB-PBF parts, where previously it has only been investigated for external surfaces. This paper scrutinized the performance of the CBF on internal geometries, and it was shown for the size of the investigated components, the hexagonal hollow achieved the highest maximum removal rate over the square and circular hollows. In addition, the effect of CBF on plastic deformation and microstructural characterization has been investigated to find the effect of this process on work hardening. The results of this study also show that the rotational speed and the volumetric percentage of the abrasive directly drive the MRR. A higher rotational speed increases the slope of the sliding path and the sliding speed between printed parts and abrasive media, which causes higher cutting and grinding, MRR and media wear rate.</jats:p