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In additive manufacturing processes using laser beam melting high thermal gradients are generated, inducing residual stresses within the parts that may lead to deformations and, in worst cases, cracks. One of the materials that is the most sensitive to residual stresses is the Ti-6Al-4V alloy. In the present study, we focus on the various parameters that may control the genesis and build-up of the residual stress states. Dwell time and thermal conductivity, both influencing the heat evacuation, were studied. Higher thermal evacuation was find to rises residual stresses within the part. Then, the reliability of the energy density as a comparison parameter was investigated. Samples with the same energy densities but different power and scanning speed were elaborated. Energy density was shown as a non-reliable parameter to compare different processed parts.

The understanding of long-term aging of aeronautical materials, in particular aluminium alloys used in the fuselage and structure of aircraft is of extreme importance for airline fleets. In this work, a plate from an old aircraft (Breguet) was retrieved and studied in terms of microstructure and mechanical properties. A comparison was made between this naturally-aged alloy and a modern alloy on which different artificial aging conditions were applied. The old alloy exhibits a precipitation of θ-Al2Cu at grain boundaries and of Ω-Al2Cu on dispersoids. This non-expected nanostructure for an alloy in T4 state was attributed to the heat that the plate experienced during the aircraft cycles. However, it is shown that this aging is reversible (after a solution treatment). Moreover, the very long time of outdoors exposure seems to have caused intergranular corrosion causing the early failure during tensile tests on some of the specimens. The artificial aging (low temperature, 100°C for up to 10,000h) applied on the modern 2017A alloy did not allow to reproduce the nanostructure of the old plate, meaning that isothermal conditions for artificial aging might not be appropriate in this case.

Titanium alloys are widely used in many applications thanks to their good corrosion resistance and to their high specific modulus. However, at temperatures above 450°C, oxidation must be taken into account to improve life assessment of components. Especially for long-term exposures, oxidation leads to an oxide scale along with an oxygen enrichment in the metal below this oxide scale. In this study, the oxidation behaviour of Ti– 6Al–4V is investigated between 450 and 600°C for a maximum duration of about 6800 h. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) are used to characterize the oxide layer and the layer affected by oxygen uptake. The local oxygen content is quantified. Vibration tests are performed to determine elastic properties on thin specimens with different thicknesses affected by oxygen enrichment. Creep tests are also carried out to quantify the influence of this oxygen uptake on viscoplastic behaviour. An oxygen-enriched zone occupying 5% of the cross-section of a specimen is sufficient to induce detectable changes in its creep behaviour.

The sequential-coupled thermo-mechanical model and direct cyclic technique are employed to investigate laser welding and low-cycle fatigue residual stress, respectively. The effects of residual stress on fatigue properties and the relaxation behaviour are analyzed. The simulation results highlight the strong dependence of laser welding residual stress on constitutive models and how low-cycle fatigue residual stress is influenced by the initial laser welding residual stress. Furthermore, the simulation indicates that residual stress redistributes and relaxes in the weld and heat-affected zone after low-cycle fatigue. To validate the accuracy of the simulation, the neutron diffraction experiment is carried out, and the experimental data are consistent with the simulation results. [ABSTRACT FROM AUTHOR]

The particular roles of grain morphology and defects, controlled using laser‐scan strategies, on the mechanical properties and the fatigue behavior of 316L stainless steel are investigated. Microstructural characterization and X‐ray tomography analysis was performed to understand the genesis of polycrystalline microstructure and defects. Tensile and fatigue tests were performed to analyze the effect of defect population and microstructural properties on plasticity and damage mechanisms during monotonic and cyclic loading. The effect of the grain‐size and shape and type of defect was carefully investigated to evaluate the mechanisms driving the mechanical behavior under quasi‐static and fatigue loading. It is shown that the laser‐scan strategy determines the anisotropy in the plane perpendicular to the building direction. Moreover, contrary to the existing literature, for 316L obtained by AM, the grain size and shape does not affect the mechanical properties, and LoF defects drive the fatigue life, independent of the defect/grain size ratio. [ABSTRACT FROM AUTHOR]

To better understand the properties of polycrystals at a microscopic scale during cyclic mechanical loading we have measured the relationship between grain orientations, their positions inside the sample and their internal stresses. In this work, in-situ 3DXRD technique was performed on over hundred grains during the stress-induced martensitic transformation in a Cu-Al-Be shape memory alloy. Information about the position, orientation, and stress field was obtained for each austenitic grain. These results have been used to develop a procedure that allows automatic processing for a large number of grains, matching them during loading and leads to a quantitative stress field. A strong heterogeneity of stress state between the grains at the surface and in the volume is evident.

X‐Light is an open‐source software that is written in Python with a graphical user interface. X‐Light was developed to determine residual stress by X‐ray diffraction. This software can process the 0D, 1D and 2D diffraction data obtained with laboratory diffractometers or synchrotron radiation. X‐Light provides several options for stress analysis and five functions to fit a peak: Gauss, Lorentz, Pearson VII, pseudo‐Voigt and Voigt. The residual stress is determined by the conventional sin2ψ method and the fundamental method. [ABSTRACT FROM AUTHOR]

To study the influence of the microstructure of cast iron on the adhesion of an epoxy coating, ferritic, pearlitic and austempered samples were prepared in as-received, polished and oxidised states. A pull-off test (dry adhesion) was performed before immersing in water while the cross-cut test was made after 24 days of exposition in distilled water (wet adhesion). X-rays were combined with optical microscopy (LOM) and scanning electron microscopy (SEM) for surface analysis. The adhesion of the epoxy coating on the cast iron surface firstly depends on the roughness of the surface; polished samples showed high adherence by comparison with as-received samples. On the oxidised samples, the surface oxide significantly improves the adhesion of the coating in both dry and wet states for all three sample microstructure. The presence of carbide in the structure was observed to decrease adherence. [ABSTRACT FROM AUTHOR]

The microstructure of a high-purity Al-6.5Si-1Fe (wt pct) alloy after solidification at various cooling rates was investigated. In most of the cases, the monoclinic beta-AlFeSi phase was observed as long and thin lamellae. However, at a very slow cooling rate, Fe-bearing precipitates with Chinese script morphology appeared together with lamellae. Further analysis showed all these Chinese script precipitates correspond also to the monoclinic beta phase. This finding stresses that differentiating second phases according to their shape may be misleading. [ABSTRACT FROM AUTHOR]

Using high resolution powder neutron diffraction data, we show that there is a distinct anomaly in the structural noncentrosymmetry around the magnetic transition point TN for bulk and nanoscale BiFeO3. It appears that the structural noncentrosymmetry - which gives rise to the ferroelectric polarization - is suppressed anomalously by ~1% (of the average noncentrosymmetry at above the magnetic transition) in the bulk sample and by ~12% in the nanoscale sample as the magnetic transition is approached from higher temperature. This observation shows that the multiferroic coupling improves in the nanoscale sample which is expected to brighten the application prospects of nanoscale BiFeO3 in nanospintronics-based sensor devices.PACS Nos. 75.70.Cn, 75.75.-c [ABSTRACT FROM PUBLISHER]