A. Almeida

Laser alloying of aluminium alloys with chromium

Abstract The microstructure and corrosion resistance of laser-alloyed aluminium and ANSI 7175 aluminium alloy with chromium were investigated. Surface layers alloyed with chromium contain relatively large amounts of intermetallic compounds dispersed in a matrix of α-Al. The intermetallic compound particles present needle-like morphologies, organized in a dense network or distributed radially. Al 7 Cr, Al 11 Cr 2 and α-Al phases have been identified by X-ray diffraction. The alloyed layers may contain cracks, pores, inclusions and undissolved chromium particles, depending on the chromium concentration and the particle size. However, homogeneous layers were produced by a two-step process, consisting of laser alloying followed by remelting. The second treatment eliminates porosity and refines the structure. The hardness attains a Vickers hardness of 155 HV in chromium-alloyed aluminium and exceeds 300 HV in chromium-alloyed 7175. The corrosion behaviour of the above alloys was assessed using anodic polarization techniques. Laser alloying of aluminium and 7175 with chromium improves the pitting corrosion resistance of the alloys. The effect depends on the chromium content of the alloyed layers and is more significant in 7175 alloy.

Localized corrosion of laser surface melted 2024-T351 aluminium alloy

Abstract Laser surface melting of 2024-T351 aluminium alloy with a CO 2 laser operating at 2 kW with a spot size of 1.5 mm and a substrate traverse rate of 20 mm s −1 produced a relatively thin surface-melted layer with a refined microstructure and a modified distribution of the alloying elements. The laser treatment changed both the anodic polarization behaviour and the form of localized corrosion in deaerated 3% NaCl solution. Immersion tests in the same solution under the condition of natural aeration showed that for the as-received alloy both intergranular and pitting corrosion occurred with pits distributed mainly along the rolling direction while for the laser surface melted material only pitting corrosion was present with pits distributed uniformly. This difference in corrosion behaviour as a result of laser surface melting is attributed to changes in the distribution and composition of the second-phase particles present in the alloy.

Structure and properties of Al–Nb alloys produced by laser surface alloying

Abstract Al–Nb alloys present considerable strength and hardness, being promising materials for structural applications and for coatings. This paper reports results of a study of the structure and properties of Al–Nb surface alloys produced by laser alloying. Surface alloys were produced by a two-step process: firstly, commercial purity Al substrates were alloyed with Nb by injecting an Al–25 wt.% Nb powder mixture into the melt pool generated using a CO 2 laser, and, secondly, the microstructure was refined by laser melting. In the first step, Nb concentrates near the surface of the melt pool during solidification, resulting in the production of surface layers about 700 μm thick of Al–37.6 (±3) wt.% Nb alloy. These alloys are heterogeneous and present porosity and undissolved Nb particles for all the processing parameters used. Their microstructure is formed of large dendrites of Al 3 Nb and interdendritic α-Al solid solution. Laser melting of the alloyed layers results in complete elimination of defects and homogenisation of the material. Previously undissolved Nb particles dissolve, raising the Nb concentration in the alloy and increasing the volume fraction of Al 3 Nb. The microstructure still consists of Al 3 Nb and α-Al solid solution but the dendrites are finer and the volume fraction of interdendritic α is less than 5%. The Vickers microhardness of the surface alloys is high owing to the large amount of this intermetallic compound. It varies from 480 to 650 HV, depending on the volume fraction and dendrite spacing of Al 3 Nb. Despite the brittleness of Al 3 Nb no cracks were observed, probably because of the presence of the thin interdendritic film of ductile Al solid solution, which enables better accommodation of the generated stresses.

Human mesenchymal stem cell behavior on femtosecond laser-textured Ti-6Al-4V surfaces.

AIM The aim of the present work was to investigate ultrafast laser surface texturing as a surface treatment of Ti-6Al-4V alloy dental and orthopedic implants to improve osteoblastic commitment of human mesenchymal stem cells (hMSCs). MATERIALS & METHODS Surface texturing was carried out by direct writing with an Yb:KYW chirped-pulse regenerative amplification laser system with a central wavelength of 1030 nm and a pulse duration of 500 fs. The surface topography and chemical composition were investigated by scanning electron microscopy and x-ray photoelectron spectroscopy, respectively. Three types of surface textures with potential interest to improve implant osseointegration can be produced by this method: laser-induced periodic surface structures (LIPSSs); nanopillars (NPs); and microcolumns covered with LIPSSs, forming a bimodal roughness distribution. The potential of the laser treatment in improving hMSC differentiation was assessed by in vitro study of hMSCs spreading, adhesion, elongation and differentiation using epifluorescence microscopy at different times after cell seeding, after specific stainings and immunostainings. RESULTS Cell area and focal adhesion area were lower on the laser-textured surfaces than on a polished reference surface. Obviously, the laser-textured surfaces have an impact on cell shape. Osteoblastic commitment was observed independently of the surface topography after 2 weeks of cell seeding. When the cells were cultured (after 4 weeks of seeding) in osteogenic medium, LIPSS- and NP- textured surfaces enhanced matrix mineralization and bone-like nodule formation as compared with polished and microcolumn-textured surfaces. CONCLUSION The present work shows that surface nanotextures consisting of LIPSSs and NPs can, potentially, improve hMSC differentiation into an osteoblastic lineage.

Repair and manufacturing of single crystal Ni-based superalloys components by laser powder deposition—A review

Laser powder deposition is one of the most promising methods for the repairing of the single crystal Ni-based superalloys components used in the hot-section of gas turbine engines in order to extend their lifetime and reduce their overall cost. The microstructure of Ni-based superalloys deposited on single crystal substrates of similar materials depends mainly on the materials involved, on the orientation of the deposited tracks in relation to the substrate and on the deposition parameters. In the present paper these relations are discussed and illustrated for the case of single and multiple layer depositions of NiCrAlY and Rene N4 on (100) single crystal substrates of SRR99 and CMSX-4 Ni-based superalloys. On the other hand, when the aging treatment is applied directly to the solidification microstructure resulting from laser deposition, abnormal γ/γ′ microstructures may result, due to the inhomogeneity created by alloying elements partition during solidification. Performing a homogenization annealing be...

Laser Surface Treatment of Tool Steels

Laser surface treatment (LST) is a promising technique to improve the wear and corrosion resistance of materials. In die case of tool steels, laser surface treatment is carried out preferably in the liquid state to allow for complete dissolution of alloy carbides. In this paper, the main requirements for materials used in different types of tools and the advantages of using surface engineered materials for these applications are presented. The application of laser melting to the treatment of tool steels is exemplified for AISI 420 and 440C Cr steels and sintered AISI T15 HSS. Usually, the laser melted layers contain martensite, retained austenite and carbides. In steels containing large proportions of ferrite-forming alloying elements δ-ferrite may also be observed. The laser treatment of sintered steel leads to the elimination of residual porosity. The proportion of retained austenite in laser surface melted steels is much higher than in conventionally treated steels. However, the hardness of the steel is high because the austenite is strengthened by solid solution, dislocations and the small grain size. The high volume fraction of retained austenite usually prohibits the application of tool steels in the laser treated condition. Austenite may be eliminated by double or triple tempering treatments at temperatures in the range 550 to 650 °C. During tempering, carbides precipitate within austenite and martensite, and austenite transforms to martensite. Strong secondary hardening is often observed and the temperature of the secondary hardening peak of laser surface melted (LSM) steels is higher than after conventional heat treatment.

Microstructure of Al-4.3(AT.%)Cr alloy produced by laser surface alloying

Laser alloying is a particularly effective technique to improve the corrosion and wear resistance of aluminum alloys, since it combines the controlled modification of both microstructure and chemical composition to tailor surface properties to the application requirements. The authors have performed a detailed study on the preparation of Al-Cr surface alloys by laser processing and on the structure and properties of these alloys. It was found that these alloys present a relatively high hardness and good corrosion resistance and therefore, they seem to be extremely promising surface engineered materials. In this paper, the authors will concentrate on the microstructural analysis of Al-4.3 at%Cr alloy produced by laser alloying, mainly using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Structure characterization of a laser-processed Al-Mo alloy

The surface structure of a laser-processed Al–Mo alloy has been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractory (XRD). The alloy was prepared by first laser alloying a mixture of Al and Mo powders into an Al substrate and then laser remelting the alloyed surface. Following the first laser alloying process, the needle-like equilibrium phases (Al5Mo(h) and Al5Mo(r)) are formed with a broad size ranges and distribute inhomogeneously in the α-Al solid solution matrix. This coarse structure is replaced by a finer, uniform dispersion of dendrites after the subsequent laser remelting. Four basic types of solid states precipitates are observed: (1) irregularly shaped particles constructing the dendrites and having a nearly Al5Mo stoichiometry; (2) needle-like particles which is the Al5Mo (r) phase; (3) Faceted particles having a cubic structure with a stoichiometry close to Al7Mo; (4) tiny, equi-axed particles, with a rather narrow particle size distribution and a cubic structure.

Ultrafast laser texturing of Ti-6Al-4V surfaces for biomedical applications

By controlling processing parameters such as the average fluence, number of laser pulses and beam polarization direction, different types of multiscale surface textures were produced on Ti-6Al-4V surfaces by ultrafast laser processing. The samples were textured in ambient atmosphere using an Yb: KYW chirped-pulse-regenerative amplification laser with a wavelength of 1030 nm and pulse duration of 500 fs. The wetting of simulated biological fluids as well as the human mesenchymal stem cells (hMSCs) behavior were assessed. Three types of textured surfaces were tested, consisting of: (i) Laser-Induced Periodic Surface Structures-LIPSS; (ii) nanopillars-like structures; and (iii) LIPSS overlapped to microcolumns. The laser textured surfaces present hydrophilic behavior and high affinity for HBSS (Hank’s balanced salt solution). Cell spreading and adhesion strength is reduced by the laser nanotextures as compared to a polished control surface. Cytoskeleton stretching and stress fibers were clearly observed on LIPSS while significant filopodia formation was verified on nanopillars. There was no cell proliferation on the laser nanotextured surfaces. Ultrafast laser texturing of Ti-6Al-4V surfaces is an efficient technique for increasing surface wettability, and is potentially useful as a technique to control the behavior of hMSCs by changing the cytoskeleton shape, FAPs distribution and area, and proliferation.By controlling processing parameters such as the average fluence, number of laser pulses and beam polarization direction, different types of multiscale surface textures were produced on Ti-6Al-4V surfaces by ultrafast laser processing. The samples were textured in ambient atmosphere using an Yb: KYW chirped-pulse-regenerative amplification laser with a wavelength of 1030 nm and pulse duration of 500 fs. The wetting of simulated biological fluids as well as the human mesenchymal stem cells (hMSCs) behavior were assessed. Three types of textured surfaces were tested, consisting of: (i) Laser-Induced Periodic Surface Structures-LIPSS; (ii) nanopillars-like structures; and (iii) LIPSS overlapped to microcolumns. The laser textured surfaces present hydrophilic behavior and high affinity for HBSS (Hank’s balanced salt solution). Cell spreading and adhesion strength is reduced by the laser nanotextures as compared to a polished control surface. Cytoskeleton stretching and stress fibers were clearly observed on L...

Laser Developed Al-Cr Surface Alloys: Microstructure, Mechanical and Wear Behaviour

Surface alloys with composition ranging from 10 to 20% Cr were produced by laser surface alloying. Their microstructure consists of faceted plate-like Al4Cr intermetallic compound particles dispersed in a matrix of α-Al solid solution. During remelting, heterogeneous nucleation of eutectic Al7Cr/α-Al occurred in the undercooled liquid ahead of the columnar solid-liquid interface, followed by equiaxial solidification, resulting in a microstructure formed of equiaxed cells. Al-Cr alloys present Young’s modulus and hardness values that increase with increasing volume fraction of intermetallic compounds. Wear resistance, measured in dry sliding conditions, increases with increasing load due to the protective effect of a stable mechanically mixed layer that forms at the surface of the samples and the steel counterbody. Alloys formed of equiaxed eutectic cells provide better wear resistance than those formed of large plate-like particles since a thinner, more stable and harder mechanically mixed layer is formed, which offers best protection against wear.