The laser surface remelting (LSR) treatment was performed to Al - 2.0 wt% Fe alloy with a 2 kW Yb-fiber laser (IPG YLR-2000S). The substrate and laser-treated material characterization w ere executed using different techniques . Among them, the microstructure was analyzed by optical microscope, SEM, low-angle X-ray diffraction (LAXRD) and the corrosion test was made in aerated solution of 0.1 M H2SO4 at a temperature of 25°C ± 0.5°C. As result was shown , the micrograph of LSR-treated material displaying can be a fine cellular structure and the existence of certain nano - porosities and a similar to a nano-dendritic growth was observed too. The characteristic of melted zone was constituted of metastable phases according to the result of x-rays and the behavior corrosion as a result of the LSR-treated sample, which it was shown to be more resistant to corrosion than the untreated sample. A comparative study was carried out of the cyclic polarization of the laser-treated and untreated samples, demonstrating that the reduction and oxidation reverse peaks were not observed and being the cyclic polarization behavior wa s of irreversible character in both samples, however, the LSR-treated sample propitious the passivity on the surface also reduc ed the corrosion phenomena. Wherefore, this type of laser-treated alloy can be applied in the aerospace, aeronautic and automobilist industries.
The aluminum oxide or alumina has important properties, because the alumina is produced during the laser surface remelting (LSR) treatment, and this technique does not suit within the traditional techniques. It is a modern technique that is being used recently, therefore it is important to focus on this work. As well, the authors Nakajima et al. [
The laser surface treatment of metals is a process where a small surface volume of materials is melted instantly by a laser beam and cooled rapidly, thereby producing very fine microstructure with an improved wear and corrosion resistance. Laser surface melting (LSM) was reported by authors Lee et al. [
In this study, the microstructural and corrosion resistance analysis of the hypereu- tectic Al-2.0 wt% Fe alloy LSR-treated was performed and this was the aim of this research, where the substrate and laser-treated material were characterized using different techniques. Among them, the microstructure was analyzed by optical microscope, SEM, low-angle X-ray diffraction (LAXRD) analysis and the corrosion test was performed in aerated solution of 0.1 M H2SO4 at a temperature of 25˚C ± 0.5˚C. In the electrochemical study, the following tests were carried out: open circuit potential, the polarization resistance, the corrosion current, determination of the corrosion rate and finally cyclical polarization for the untreated and laser-treated samples. The importance of this research is due that this LSR-treated alloy presented metastable phases, which between them, what are of greater importance are alumina and nitrite, the presence of a fine cellular structure and the existence of certain nano-porosities, a high corrosion resistance and a large plateau of passive zone, and therefore this type of laser-treated alloy can be applied in the aerospace, aeronautic and automobilist industries.
Al-2.0 wt% Fe alloy was prepared with commercially pure raw materials, being that the aluminum has 99.76% purity, containing 1.54 wt% of iron, and other elements with quantities below 50 PPM. The casting assembly used in solidification experiments consists of a water-cooled mold and the heat was extracted only from the bottom, promoting vertical upward directional solidification. This apparatus was used to obtain cylindrical casting, with dimensions of 6 cm diameter and 10 cm length. The preparation of samples for the analysis consisted in sectioning the cylindrical casting into several longitudinal slices, and milling the surfaces to improve parallelism. Each piece was sanded with 1200# SiC sand paper and then sand-blasted was applied to reduce the surface reflectivity, thus increasing the laser energy absorption coefficient.
Laser Surface Remelting Treatment SpecificationsThe laser surface remelting (LSR) treatment was performed with a 2 kW Yb-fiber laser (IPG YLR-2000S) that operates at wavelength of 1.07 μm. The laser beam is coupled to a 160 mm focusing lens (optical head). For this optical system the focused beam diameter was 100 μm. The laser output power was 600 W and the scanning speed was 40 mm∙s−1. For this experiment, the sample was positioned 3 mm above the focus (out of focus), which results in a 600 μm diameter laser beam, which was designed by Riva et al. [
After the laser processing, different regions were identified on the cross-section of the sample, through the SEM micrographs analysis: laser melted zone (LMZ), the interface between the treated region and the substrate, and unaffected region (substrate), which was studied by Pariona et al. [
The substrate and laser-treated material characterization was performed using different techniques. Therefore, the microstructure was analyzed by a Shimadzu SSX-550 scanning electron microscope (SEM) and an Olympus BX-51 optical microscope (OM) with a QColor 3 digital camera for the image capture. The SEDS-500 energy dispersive spectroscopy (EDS) equipment was used for semi-quantitative analysis and it was coupled to SEM. As well, low-angle X-ray diffraction (LAXRD) analysis were recorded at a scan speed of 0.2˚min−1, using a Lab XRD-6000 diffractometer (minimum detection >1%). The corrosion test was performed in aerated solution of 0.1 M H2SO4 at a temperature of 25˚C ± 0.5˚C. Working electrodes of surface-treated and untreated samples were prepared with epoxy resin to expose a top surface. Corrosion potentials (Ecor) were measured using Autolab PGSTAT 30 potentiostat system connected to a microcomputer. The samples were cut with diamond disk. For the cross-section analysis them were sanded (600 up to 1200 #), then polished with diamond paste (1 μm) and colloidal silica. The chemical attack with 0.5% HF was also made on these samples for analysis microscopic.
To better understand the corrosion process that occurs of the laser-treated Al-2.0 wt% Fe alloy, firstly a study of the micrographs of samples in both, as much on the surface and as well as on the cross-section were executed, the details of the changes that suffers when was subjected to RSL-treatment sample and whose results will be shown to follow. Soon it will be discussed the influence of micrograph on the result of the electrochemical study. As well, in the electrochemical study, the following tests were carried out, among them, open circuit potential (OCP), the polarization resistance, the corrosion current (Icor), determination of the corrosion rate and finally cyclical polarization for untreated and laser-treated samples.
In this work, all untreated surface of Al-2.0 wt% Fe alloy was covered with an arrangement of multiple weld fillets, in order to analyze this treatment. Micro-porosity has been observed on the surface of the laser-treated sample and more preferably in the region on the weld fillet. Besides that, the protuberance on this surface was observed, which corresponds to the region on the weld fillet, a similar result of this microstructure was given by authors Sun et al. [
The first region (
These overlapping lines are most notorious in Al-2.0 wt% Fe alloy that in Al-1.5 wt% Fe alloy [
On the other hand, in the interface between the treated region and the substrate was not observed clearly the feature of the heat affected zone (HAZ), it has been studied by other authors, between them, Bertelly et al. [
tated on the grain-boundary of the substrate, which may be verified in
The characteristic of the melted zone is mainly due to the high cooling rate imposed during RSL-treatment, ASM [
The phases formed on the surface of LSR-treated and untreated samples were analyzed using LAXRD technique, as described in the Materials and Methods section.
LAXRD analysis revealed the presence of Al2O3 and AlN phases in most of the diffraction peaks of the treated sample (see
In this study, the high energy applied in the laser-treatment, allied to the fact that it was carried out in a suitable environment, for the formation of oxides and nitrides, favored the characteristics of high hardness, wear and corrosion resistance of the samples LSR-treated in acidic or alkaline media, as has also been reported by Patnaik [
In addition to the simple metal phases shown in
The micropolarization of ±10 mV around corrosion potential promotes a perturbation in the equilibrium potential, giving the appearance of an anodic and cathodic current in the electrochemical cell circuit. This technique of electric current versus applied
potential was conducted to the laser-treated and untreated samples, and the result is shown in
The anodic polarization curves are shown in
As a consequence of the application of the open-circuit corrosion potentials, linear micropolarization and of the anodic polarization curves and the result is summarized in
Electrochemical methods make use of measurable electrical properties (current electrical, potential differences, accumulation interfacial charge, among others), from phenomena in which a redox species interacts, physically and/or chemically with other components of the medium, or even with interfaces. Luther et al. [
A comparative study was carried out of the cyclic polarization to the laser-treated and untreated samples, with the purpose of studying the behavior of these materials in the same electrolyte solution.
Region | Ecorr (V) | Rp (KΩ) | βa (V/dec) | βc (V/dec) | Icorr (A/cm2) | Corrosion rate (mm/year) |
---|---|---|---|---|---|---|
Treated Surface | −0.577 | 22.1 | 0,.086 | 0.117 | 9.37e−7 | 0.07 |
Untreated Substrate | −0.629 | 2.08 | 0.066 | 0.172 | 9.95e−6 | 0.78 |
these reactions are of irreversible character at this condition.
The cyclic polarizations for the untreated specimen in H2SO4 aerated solution is shown in
With the linear increase in the potential, the thickness of the barrier layer increased with anodizing time; in other words, the thickness increased with anodizing potential. After the inversion potential, there is a sharp drop of the anodic current up to +0.50 V and, thereafter, reaching low values of currents in potential close to 0.0 V.
The cyclic polarization curve of
According to this result, the LSR-treated sample showed clearly the passive zone. However, the LSR-treated sample results in reduction of the current density, and this fact indicates a lower corrosion rate and therefore it takes an improvement in the corrosion resistance, this fact can be verified, otherwise, the corrosion rate according to
According to this study, the following conclusions were made, then:
1) The melted zone is mainly due to high cooling rate imposed during RSL-treatment sample;
2) Therefore, the melted zone was constituted of metastable phases and by LAXRD analysis revealed the presence mainly of Al2O3 and AlN phases;
3) These phases contributed in the microstructural modification, favored the characteristics of high hardness and corrosion resistance of the LSR-treated sample in sulfuric acidic;
4) The polarization resistance has been increased about 11 times for the laser-treated sample relative to the untreated sample, implying directly on the corrosion resistance of the material;
5) In result of the electrochemical study, it was also observed, in ten-fold decrease, the corrosion current after the laser-treatment had occurred;
6) In the technique of cyclic voltammograms, the reduction and (re)oxidation reverse peaks/waves were not observed in both samples in the same potential range and so demonstrating that these reactions are of irreversible character at this condition;
7) The LSR-treated sample showed clearly a wide passive zone. However, the LSR- treated sample results in reduction of the current density, and this fact indicates a lower corrosion rate and therefore it acts in improvement in the corrosion resistance;
8) The electrochemical parameters that correspond to the LSR-treated material showed better results than the untreated material. Therefore, this type of laser-treated alloy can be applied in the aerospace, aeronautic and automobilist industries;
9) To better understand the efficiency of the treated material laser, other studies are being carried out, such as, microhardness, study of the roughness by atomic force technique, electrochemical impedance spectroscopy, Raman spectroscopy and numerical simulation by finite element using the Marangoni phenomenon and optimized by Multigrid technique.
This work was entirely financed by CNPq (Brazilian National Council for Scientific and Technological Development), Fundação Araucária (FA), CAPES (Federal Agency for the Support and Evaluation of Postgraduate Education), and FINEP (Research and Projects Financing Agency). We also thank to LABMU-UEPG.
Pariona, M.M. and Micene, K.T. (2017) The Alumina Film Nanomorphology Formed to Improve the Corrosion Resistance of Al-2.0 wt% Fe Alloy as Result of the Laser Surface Melting Technique Applied. Advances in Chemical Engineering and Science, 7, 10-22. http://dx.doi.org/10.4236/aces.2017.71002