Characterization of Interfacial Corrosion Behavior of Hybrid Laminate EN AW-6082 ∪ CFRP.

The corrosion behavior of a hybrid laminate consisting of laser-structured aluminum EN AW-6082 ∪ carbon fiber-reinforced polymer was investigated. Specimens were corroded in aqueous NaCl electrolyte (0.1 mol/L) over a period of up to 31 days and characterized continuously by means of scanning electron and light microscopy, supplemented by energy dispersive X-ray spectroscopy. Comparative linear sweep voltammetry was employed on the first and seventh day of the corrosion experiment. The influence of different laser morphologies and production process parameters on corrosion behavior was compared. The corrosion reaction mainly arises from the aluminum component and shows distinct differences in long-term corrosion morphology between pure EN AW-6082 and the hybrid laminate. Compared to short-term investigations, a strong influence of galvanic corrosion on the interface is assumed. No distinct influences of different laser structuring and process parameters on the corrosion behavior were detected. Weight measurements suggest a continuous loss of mass attributed to the detachment of corrosion products.

Interface-Mediated Twinning-Induced Plasticity in a Fine Hexagonal Microstructure Generated by Additive Manufacturing.

The grain size is a determinant microstructural feature to enable the activation of deformation twinning in hexagonal close-packed (hcp) metals. Although deformation twinning is one of the most effective mechanisms for improving the strength-ductility trade-off of structural alloys, its activation is reduced with decreasing grain size. This work reports the discovery of the activation of deformation twinning in a fine-grained hcp microstructure by introducing ductile body-centered cubic (bcc) nano-layer interfaces. The fast solidification and cooling conditions of laser-based additive manufacturing are exploited to obtain a fine microstructure that, coupled with an intensified intrinsic heat treatment, permits to generate the bcc nano-layers. In situ high-energy synchrotron X-ray diffraction allows tracking the activation and evolution of mechanical twinning in real-time. The findings obtained show the potential of ductile nano-layering for the novel design of hcp damage tolerant materials with improved life spans.

Pyrometric-Based Melt Pool Monitoring Study of CuCr1Zr Processed Using L-PBF.

The potential of in situ melt pool monitoring (MPM) for parameter development and furthering the process understanding in Laser Powder Bed Fusion (LPBF) of CuCr1Zr was investigated. Commercial MPM systems are currently being developed as a quality monitoring tool with the aim of detecting faulty parts already in the build process and, thus, reducing costs in LPBF. A detailed analysis of coupon specimens allowed two processing windows to be established for a suitably dense material at layer thicknesses of 30 µm and 50 µm, which were subsequently evaluated with two complex thermomechanical-fatigue (TMF) panels. Variations due to the location on the build platform were taken into account for the parameter development. Importantly, integrally averaged MPM intensities showed no direct correlation with total porosities, while the robustness of the melting process, impacted strongly by balling, affected the scattering of the MPM response and can thus be assessed. However, the MPM results, similar to material properties such as porosity, cannot be directly transferred from coupon specimens to components due to the influence of the local part geometry and heat transport on the build platform. Different MPM intensity ranges are obtained on cuboids and TMF panels despite similar LPBF parameters. Nonetheless, besides identifying LPBF parameter windows with a stable process, MPM allowed the successful detection of individual defects on the surface and in the bulk of the large demonstrators and appears to be a suitable tool for quality monitoring during fabrication and non-destructive evaluation of the LPBF process.

Pandora's Box-Influence of Contour Parameters on Roughness and Subsurface Residual Stresses in Laser Powder Bed Fusion of Ti-6Al-4V.

The contour scan strategies in laser powder bed fusion (LPBF) of Ti-6Al-4V were studied at the coupon level. These scan strategies determined the surface qualities and subsurface residual stresses. The correlations to these properties were identified for an optimization of the LPBF processing. The surface roughness and the residual stresses in build direction were linked: combining high laser power and high scan velocities with at least two contour lines substantially reduced the surface roughness, expressed by the arithmetic mean height, from values as high as 30 µm to 13 µm, while the residual stresses rose from ~340 to about 800 MPa. At this stress level, manufactured rocket fuel injector components evidenced macroscopic cracking. A scan strategy completing the contour region at 100 W and 1050 mm/s is recommended as a compromise between residual stresses (625 MPa) and surface quality (14.2 µm). The LPBF builds were monitored with an in-line twin-photodiode-based melt pool monitoring (MPM) system, which revealed a correlation between the intensity quotient I2/I1, the surface roughness, and the residual stresses. Thus, this MPM system can provide a predictive estimate of the surface quality of the samples and resulting residual stresses in the material generated during LPBF.

Peritectic titanium alloys for 3D printing.

Metal-based additive manufacturing (AM) permits layer-by-layer fabrication of near net-shaped metallic components with complex geometries not achievable using the design constraints of traditional manufacturing. Production savings of titanium-based components by AM are estimated up to 50% owing to the current exorbitant loss of material during machining. Nowadays, most of the titanium alloys for AM are based on conventional compositions still tailored to conventional manufacturing not considering the directional thermal gradient that provokes epitaxial growth during AM. This results in severely textured microstructures associated with anisotropic structural properties usually remaining upon post-AM processing. The present investigations reveal a promising solidification and cooling path for α formation not yet exploited, in which α does not inherit the usual crystallographic orientation relationship with the parent ß phase. The associated decrease in anisotropy, accompanied by the formation of equiaxed microstructures represents a step forward toward a next generation of titanium alloys for AM.

Inducing Stable α + ß Microstructures during Selective Laser Melting of Ti-6Al-4V Using Intensified Intrinsic Heat Treatments.

Selective laser melting is a promising powder-bed-based additive manufacturing technique for titanium alloys: near net-shaped metallic components can be produced with high resource-efficiency and cost savings [...].

An Assessment of Subsurface Residual Stress Analysis in SLM Ti-6Al-4V.

Ti-6Al-4V bridges were additively fabricated by selective laser melting (SLM) under different scanning speed conditions, to compare the effect of process energy density on the residual stress state. Subsurface lattice strain characterization was conducted by means of synchrotron diffraction in energy dispersive mode. High tensile strain gradients were found at the frontal surface for samples in an as-built condition. The geometry of the samples promotes increasing strains towards the pillar of the bridges. We observed that the higher the laser energy density during fabrication, the lower the lattice strains. A relief of lattice strains takes place after heat treatment.

The role of surface and subsurface point defects for chemical model studies on TiO2: a first-principles theoretical study of formaldehyde bonding on rutile TiO2(110).

We report a systematic investigation of the effects of different surface and subsurface point defects on the adsorption of formaldehyde on rutile TiO(2)(110) surfaces using density functional theory (DFT). All point defects investigated--including surface bridging oxygen vacancies, titanium interstitials, and subsurface oxygen vacancies--stabilize the adsorption significantly by up to 56 kJ mol(-1) at a coverage of 0.1 monolayer (ML). The stabilization is due to a decrease of the coordination (covalent saturation) of the surface Ti adsorption sites adjacent to the defects, which leads to a stronger molecule-surface interaction. This change in the Ti is caused by the removal of a neighboring atom (oxygen vacancies) or substantial lattice relaxations induced by the subsurface defects. On the stoichiometric reference surface, the most stable adsorption geometry of formaldehyde is a tilted η(2)-dioxymethylene (with an adsorption energy E(ads)=-125 kJ mol(-1)), in which a bond forms to a nearby bridging O atom and the carbonyl-O atom in the formaldehyde binds to a Ti atom in the adjacent fivefold coordinated lattice site. The η(1)-top configuration on five-coordinate Ti(4+) is much less favorable (E(ads)=-69 kJ mol(-1)). The largest stabilization is exerted by subsurface Ti interstitials between the first and second layers. These defects stabilize the η(2)-dioxymethylene structure by nearly 40 kJ mol(-1) to an adsorption energy of -164 kJ mol(-1). Contrary to popular belief, adsorption in a bridging oxygen vacancy (E(ads)=-86 kJ mol(-1)) is much less favorable for formaldehyde compared to the η(2)-dioxymethylene structures. From these results we conclude that formaldehyde will bind in the η(2)-dioxymethylene structure on the stoichiometric surface as well as in the presence of Ti interstitials and bridging oxygen vacancies. In the light of these substantial effects, we conclude that it is essential to include all the types of point defects present in typical, reduced rutile samples used for model studies, at realistic concentrations to obtain correct adsorption sites, structures, energetic, and chemi-physical properties.

Molecular imaging of reductive coupling reactions: interstitial-mediated coupling of benzaldehyde on reduced TiO2(110).

We report the first visualization of a reactive intermediate formed from coupling two molecules on a surface-a diolate formed from benzaldehyde coupling on TiO(2)(110). The diolate, imaged using scanning tunneling microscopy (STM), is reduced to gaseous stilbene upon heating to ∼400 K, leaving behind two oxygen atoms that react with reduced Ti interstitials that migrate to the surface, contrary to the popular expectation that strong bonds in oxygenated molecules react only with oxygen vacancies at the surface. Our work further provides both experimental and theoretical evidence that Ti interstitials drive the formation of diolate intermediates. Initially mobile monomers migrate together to form paired features, identified as diolates that bond over two adjacent five-coordiante Ti atoms on the surface. Our work is of broad importance because it demonstrates the possibility of imaging the distribution and bonding configurations of reactant species on a molecular scale, which is a critical part of understanding surface reactions and the development of surface morphology during the course of reaction.

Vapour-phase gold-surface-mediated coupling of aldehydes with methanol.

Selective coupling of oxygenates is critical to many synthetic processes, including those necessary for the development of alternative fuels. We report a general process for selective coupling of aldehydes and methanol as a route to ester synthesis. All steps are mediated by oxygen-covered metallic gold nanoparticles on Au(111). Remarkably, cross-coupling of methanol with formaldehyde, acetaldehyde, benzaldehyde and benzeneacetaldehyde to methyl esters is promoted by oxygen-covered Au(111) below room temperature with high selectivity. The high selectivity is attributed to the ease of nucleophilic attack of the aldehydes by the methoxy intermediate-formed from methanol on the surface-which yields the methyl esters. The competing combustion occurs via attack of both methanol and the aldehydes by oxygen. The mechanistic model constructed in this study provides insight into factors that control selectivity and clearly elucidates the crucial role of Au nanoparticles as active species in the catalytic oxidation of alcohols, even in solution.

In situ ambient pressure studies of the chemistry of NO2 and water on rutile TiO2(110).

The adsorption of NO(2) on the rutile TiO(2)(110) surface has been studied at room temperature in the pressure range from approximately 10(-8) torr to 200 mtorr using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Atomic nitrogen, chemisorbed NO(2), and NO(3) were formed, each of which saturates at pressures below approximately 10(-6) torr NO(2). Atomic nitrogen originates from decomposition of the NO(x) species. For pressures of up to 10(-3) torr, no significant change in the NO(x) surface species occurred, suggesting that environmentally relevant conditions with typical NO(2) partial pressures in the 1-100 ppb range can be modeled by ultrahigh vacuum (UHV) studies. The chemisorbed surface species can be removed by in situ annealing in UHV: all of the NO(x) species disappear around 400 K, whereas the N 1s signal associated with atomic nitrogen diminishes around 580 K. At higher pressures of NO(2) (p(NO(2)) > or = 10(-6) torr), physisorbed NO(2) and adsorbed water, which was likely due to displacement from the chamber walls, appeared. The water coverage grew significantly above approximately 10(-3) torr. Concurrently with co-condensation of water and NO(2), the population of NO(3) species grew strongly. From this, we conclude that the presence of NO(2) and water leads to the formation of multilayers of nitric acid. In contrast, pure water exposure after saturation of the surface with 200 mtorr NO(2) did not lead to a growth of the NO(3) signals, implying that HNO(3) formation requires weakly adsorbed NO(2) species. These findings have important implications for environmental processes, since they confirm that oxides may facilitate nitric acid formation under ambient humidity conditions encountered in the atmosphere.

McMurry chemistry on TiO(2)(110): Reductive C=C coupling of benzaldehyde driven by titanium interstitials.

Selective reductive coupling of benzaldehyde to stilbene is driven by subsurface Ti interstitials on vacuum-reduced TiO(2)(110). A combination of temperature-programmed reaction spectroscopy and scanning tunneling microscopy (STM) provides chemical and structural information which together reveal the dependence of this surface reaction on bulk titanium interstitials. Benzaldehyde reductively couples to stilbene with 100% selectivity and conversions of up to 28% of the adsorbed monolayer in temperature programmed reaction experiments. The activity for coupling was sustained for at least 20 reaction cycles, which indicates that there is a reservoir of Ti interstitials available for reaction and that surface O vacancies alone do not account for the coupling. Reactivity was unchanged after predosing with water so as to fill surface oxygen vacancies, which are not solely responsible for the coupling reaction. The reaction is nearly quenched if O(2) is adsorbed first-a procedure that both fills defects and reacts with Ti interstitials as they migrate to the surface. New titania islands form after reductive coupling of benzaldehyde, based on scanning tunneling microscope images obtained after exposure of TiO(2)(110) to benzaldehyde followed by annealing, providing direct evidence for migration of subsurface Ti interstitials to create reactive sites. The reliance of the benzaldehyde coupling on subsurface defects, and not surface vacancies, over reduced TiO(2)(110), may be general for other reductive processes induced by reducible oxides. The possible role of subsurface, reduced Ti interstitials has broad significance in modeling oxide-based catalysis with reduced crystals.

Surface-mediated self-coupling of ethanol on gold.

The transformation of ethanol to its carbonyl compounds, namely acetaldehyde, ethyl acetate, acetic acid, and ketene, occurs on Au(111) with O-containing Au nanoparticles formed as a result of Au atom release upon ozone exposure. The product distribution strongly depends on the surface oxygen coverage. Ethoxy and acetate are identified as two key reaction intermediates during the oxidation of ethanol. The formation of acetaldehyde is due to the deprotonation of ethoxy, which can be further oxidized into acetate. The low-temperature formation of the ester, ethyl acetate, proceeds via the coupling of acetaldehyde with excess surface ethoxy. These reaction pathways appear relevant to heterogeneous processes catalyzed by supported gold nanoparticles, thus providing further insight into the mechanistic origin of gold-mediated oxidation of alcohols.

Selectivity control in gold-mediated esterification of methanol.

The Midas touch: The low-temperature transformation of methanol to methyl formate, formaldehyde, and formic acid is promoted by atomic oxygen adsorbed on metallic gold (see picture). The reactions occur with O-containing Au nanoparticles formed on Au(111) upon oxidation with ozone at 200 K; the facile esterification to methyl formate occurs well below room temperature.