1,721,078 research outputs found
Replacement of hard chromium plating by thermal spraying - Problems, solutions and possible future approaches
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Wear and corrosion behaviour of HVOF-sprayed WC-CoCr coatings on Al alloys
No full textWC-CoCr coatings were HVOF-sprayed onto an AA6082T6 substrate. Thickness values between 50 Î1⁄4n and 150 Î1⁄4n were produced by stepwise increase of the number of torch scans. This increase makes the coatings not only thicker, but also denser. This is due both to peening effects and by modifications to the splat formation mechanism, investigated by FIB. Thanks to such densification, the hardness, the wear and impact resistance and the corrosion protectiveness of the layers increase with the number of torch scans. The largest improvement occurs from 2 to 3 torch scans. These coatings were also compared to anodized films: cermets have superior wear and impact resistance but offer less corrosion protection. Copyright © 2009 ASM International® All rights reserved
Heat treatment effects on the corrosion resistance of some HVOF-sprayed metal alloy coatings
No full textThe present study evaluates the effects of a 600 °C, 1 h heat treatment on the corrosion resistance of threeHigh Velocity Oxygen Fuel (HVOF) flame-sprayed alloy coatings: a Co–28Mo–17Cr–3Si (similar to Tribaloy-800) coating, a Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe (Diamalloy-4006) coating and a Ni–32Mo–16Cr–3Si–2Co (similar to Tribaloy-700) coating. Electrochemical polarization tests and free corrosion tests wereperformed in 0.1 M HCl aqueous solution. The corrodkote test (ASTM B380-97R02) was also performed, toevaluate the coatings qualitatively. The heat treatment improves the corrosion resistance of the Co–28Mo–17Cr–3Si coating and of the Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe coating by enhancing their passivationability. The precipitation of sub-micron sized secondary phases after the treatment may produce galvanicmicrocells at intralamellar scale, but the beneficial contribution provided by the healing of the very small but dangerous interlamellar defects (normally present in thermal spray coatings but not detectable usingordinary scanning electron microscopy) prevails. The effect on Ni–32Mo–16Cr–3Si–2Co coatings is moreambiguous: its sensitivity to crevice corrosion is worsened by the heat treatment
Effect of surface roughness and industrial heat treatments on the microstructure and mechanical properties of Ti6Al4V alloy manufactured by laser powder bed fusion in different built orientations
Full text linkThe emergence of powder bed fusion in recent years has made it one of the most demanded additive manufacturing technologies for Ti6Al4V alloys in the biomedical and aerospace industries due to its ease of part fabrication with complex geometry. However, Ti6Al4V components require a post-processing treatment to optimize their mechanical properties for engineering applications. The present study offers an analysis of the effects of different industrial heat treatments (704 °C for 120 min, 740 °C for 130 min) on the microstructure and tensile properties of Ti6Al4V samples manufactured by laser powder bed fusion in different orientations (Z, 45°, XY and XZ). These heat treatments were selected to improve the mechanical properties of the as-built material and to obtain samples representative of real industrial applications. SEM observations illustrated that the α’ martensite grown in the columnar parent β-grains was converted into an α+β mixture after both heat treatments. EBSD showed that the newly formed α phase maintained, inside the parent β grains, the same orientation relationship as the as-built α′ martensite. However, on a macroscopic scale, the α phase exhibited no preferential orientation. The heat-treated samples, which exhibited approximately 10% lower ultimate tensile strength and yield strength than the as built samples but a 10–13% higher elongation, were practically isotropic in their mechanical response. What little anisotropy was left was mainly attributed to few elongated interlamellar pores. The sandblasting process was also investigated; it did not affect the static mechanical properties of the samples but it reduced the Ra values by 25%. Finally, Vickers microhardness vs. strength relationships were studied by considering the α-phase orientation
Residual stress in laser-based directed energy deposition of aluminum alloy 2024: simulation and validation
Full text linkSimulations of laser-based directed energy deposition of metals have received increasing interest aimed at reducing the experimental effort to select the proper processing condition for the repair or overhaul of actual components. One of the main issues to be addressed is the evaluation of the residual stress, which may lead to part failure under nominal loading. In this frame and specifically relating to aluminum alloys, few works have been developed and validated. This lack of knowledge is addressed in this paper: namely, the proper approach to simulate the activation of the deposited metal is discussed in case of single deposition and is shifted to a case of multiple depositions over a substrate. The validation of the predicted residual stress is made by comparison with the actual stress resulting from X-ray diffraction
Effect of the Distance from Build Platform and Post-Heat Treatment of AlSi10Mg Alloy Manufactured by Single- and Multi-Laser Selective Laser Melting
Full text linkIn the present study, AlSi10Mg samples produced by selective laser melting (SLM) were studied. Samples were machined from two types of bars obtained through different methods: either single laser (SL) or multiple laser (ML) machine setup. The bars were built perpendicular to the platform, which was pre-heated at 150 °C (working temperature), up to a height of 300 mm. The effect of the distance from the platform on the mechanical properties was investigated through tensile samples in as-built condition and after unconventional heat treatments (U-HT). Tensile strength changed by 80 MPa along the Z-axis (build direction) for SL case and by 100 MPa for ML case in the as-built samples. Vickers microhardness revealed an analogous gradient. This was correlated to a gradient in intra-granular precipitates' distribution along the Z-axis, as revealed by scanning electron microscopy (SEM). An unconventional heat treatment at 175 °C for 6h slightly improves the mechanical strength; higher temperature treatments at 200 and 225 °C for the same duration cause a progressive decrease in strength with an increase in elongation. The amount and size of the precipitates and the thickness of eutectic Si change with the heat treatment temperature, justifying the mechanical behavior
Ti6Al4V-ELI Alloy Manufactured via Laser Powder-Bed Fusion and Heat-Treated below and above the β-Transus: Effects of Sample Thickness and Sandblasting Post-Process
Full text linkTi6Al4V-ELI is the most-used lightweight alloy in the aerospace industrial sector thanks to its high mechanical strength and corrosion resistance. The present paper aims, firstly, to evaluate the effects induced by different heat treatments, which were performed above and below the β-transus temperature on Ti6Al4V-ELI samples manufactured via Laser Powder-Bed Fusion in different orienta-tions (XZ, XY, Z and 45◦ ). The first set of tensile samples and bars were heat-treated at 1050◦ C × 1 h, while the second and third set were heat-treated at 704◦ C × 120′ following the AMS2801 standard specification, and at 740◦ C × 130′ . These heat treatments were chosen to improve the as-built mechanical properties according to the ASTM F3001 and also ASTM F2924-14 standard specifications. Optical and SEM measurements reveal primary, secondary and tertiary α-laths below the β-transus, while above this temperature, the microstructure varies in relation to the sample’s thickness. Secondly, this work analyzed the results obtained after a sandblasting process, which was performed on half of all the available heat-treated tensile samples, through XRD and Vickers microhardness measurements. XRD analysis also highlighted the presence of α2-Ti3 Al and TiAl3 precipitates and the microstructural change in terms of the α-phase
Defect-Correlated Vickers Microhardness of Al-Si-Mg Alloy Manufactured by Laser Powder Bed Fusion with Post-process Heat Treatments
Full text linkLaser powder bed fusion is an additive manufacturing process characterized by different advantages like the manufacture of samples with complex geometry without the use of tools and/or molds. Generally, the manufactured samples are characterized by high tensile strengths which, however, can be affected by the presence of defects due to the unoptimized process parameters. In a large applications field, a low density of the as-built AlSi10Mg samples is a very important parameter to considered, e.g., due to both the loss of the tensile strengths correlated with a premature failure of the samples and the increase in time and costs associated with the manufacturing process. In addition, different post-process heat treatments can increase these effects leading to an ineffective manufacturing process. In this scenario, the present work shows the analysis of spherical and lack-of-fusion pores induced by the laser powder bed fusion process on the AlSi10Mg samples and their variations after different heat treatments (direct aging and T6). At the same time, the influence of pores on the Vickers microhardness and the tensile properties has been studied in the same AlSi10Mg samples (bars and billets) that were printed with single- and double-laser machine setup. Different process parameters were also analyzed and compared. The study was supported by the microstructural and pore analysis performed by optical microscopy along the XZ plane (build direction) and the XY plane. Finally, the greatest effects of pores were observed on the Vickers microhardness values; in fact, two different relationships between microhardness and density variation are discussed. The T6 heat treatment leads to a rounding of the pores already formed in the as-built samples and to a formation of new small pores. Graphical Abstract: [Figure not available: see fulltext.
HVOF-sprayed WC-CoCr coatings on Al alloy: Effect of the coating thickness on the tribological properties
No full textThe microstructure, the micromechanical properties, the wear behaviour and the impact resistance of WC-CoCr cermet coatings, deposited onto an aluminium alloy substrate by High Velocity Oxygen-Fuel (HVOF) flame-spraying, were examined as a function of the coating thickness, which was varied between 50 mu m and 150 mu m by performing different numbers of scans of the HVOF torch in front of the substrate. The coatings became denser and significantly harder as the number of torch scans increased: the analysis of single WC-CoCr splats by combined SEM and Focused Ion Beam (FIB) techniques enabled the interpretation of the mechanisms underlying this phenomenon. In accordance to such densification, the sliding wear resistance increased with the number of torch scans, as abrasive grooving and brittle failure mechanisms were progressively suppressed. The resistance to cyclic impact was also enhanced. In comparison to anodised films, the WC-CoCr coatings appeared much more resistant against wear and cyclic impact; specifically, three torch scans seem enough to produce a coating having suitable characteristics. (C) 2009 Elsevier B.V. All rights reserved
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