Papers by Author: G. de Vasconcelos

Abstract: In this work low carbon steel samples were coated by tungsten carbide (WC), and then the properties of the obtained coatings were evaluated, such as morphology, hardness, phase composition and dimension of the layer. The results indicate the formation of a new phase, composed of iron carbide in the base metal, a more dense coating has been obtained when the WC powder was sprayed directly on to the steel and then irradiated with the laser beam.

Abstract: This work presents the evaluation of the use covering with carbon black to replace the graphite covering used in previous work, aiming to eliminate the grinding step to the process. The results of this treatment were characterized by optical microscopy, scanning electron and hardness. The micro hardness achieved was near to 760 HV _0.05, coefficient of friction of 0.2. Scanning electron microscopy showed that the extent of the treated area by laser reaches near to 90μm, and the thickness coating of carbon black is about to 2μm. The graphite can be substituted by carbon black, acting as coupling of the laser beam radiation, and as a solid lubricant, according to REIS (2009).

Abstract: This study investigate the laser surface hardening in tool steel, avoiding the high reflectivity of the metal to the CO2 radiation by covering the surface sample with black carbon. Hardening process using graphite was effective in many works in the word, although, the particles size of graphite powder was in the range of some micrometers. In order to reduce the particle size, the carbon black was used in the laser thermal treatment of 4340 steel. The microhardness change to 850HV, compared to the substrate of 250HV and the friction coefficient ranged 0.2 and 0.3, because of the covered surface by carbon black. These thin coating of carbon black reduced the wear rate near to a hundred in comparison to the surface without carbon coating.

Abstract: The purpose of this paper is the evaluation of the use of a low power CO2 laser beam (50W) to promote the thermal treatment of AISI M2 high-speed steels and evaluate the tribological properties of the graphite coating that takes place on the steel surface after laser irradiation. In order to minimize the steel surface reflection at approximately 90% of the incident radiation wavelength (10.5 μm), an incident radiation absorber layer was applied to the steel surface sample. This coating aims to absorber the incident heat and transfer part of heat to the steel surface. As results the surface presented high hardness and a thin coating of graphite on the surface. The micro-structural changes, occurred on the steels surface, resulting from heat transfer from the absorbing layer, were evaluated through optical microscopy (MO), scanning electron microscopy (MEV), micro-hardness essays (MH), X-ray analysis (XRD), Raman spectroscopy and tribological testers on the graphite coating. MO analysis showed on the transversal section of the heated affected zone a brighter layer than the sample’s core, 30 micrometers thickness, in the surface sample a black coat 10 micrometers thickness. By MH analysis this brighter layer presented hardness approximately 30% superior than the regions without treatment, and by Raman spectroscopy it was evaluated the graphite coating. The XRD analysis on the surface sample reveals an increasing of the martensite and iron-carbide phase. The experimental results of pin-on-disk tests on the graphite coatings reveal a reduced coefficient friction as compared to the original surface.

Abstract: In this study, the results of the feasibility of sintering green compacts of metallic powder of MoSi2 by a CO2 laser beam as the heating source has been investigated. The main advantage of this technique is to promote a dense material in a reduced time when compared to the conventional sintering process. In order to sintering the MoSi2 powder, green compacts of 6mm of diameter and 1.6mm thickness were produced in a steel die in a uniaxial press at 100MPa and after, isostatic pressed at 350MPa. The micrograph of the samples exposed to the laser radiation performed by scanning electron microcopy (SEM) reveal the efficiency of the sintering process and the X-ray diffraction of the powders confirmed the presence of single phase after and before laser processing. The average microhardness of these compacts reached near to 700 Hv0.2 in the cross section to the laser irradiation, indicating the all sintering of the green compact.