Papers by Keyword: Laser Surface Melting

Abstract: Temperature field during laser surface melting the pure Al was numerically simulated by the Laplace integral transform technique using the MathCAD software. The influences of laser power densities on the surface temperature of the pure Al were analyzed. Mathematical expressions for the temperature distribution of the heating and cooling process on the pure Al surface were obtained.

Abstract: In the present study, the effect of laser surface melting (LSM) on microstructure of AZ91D magnesium alloy was investigated with a 2kW continuous wave CO2 laser. The results indicate that both the substrate and laser melted layer mainly consisted of α-Mg and β-Mg17Al12, the amount of β-phase in laser melted layer was decreased as compared to that of the substrate. The microstructure of the laser surface melted zone consists of fine dendrites with an average grain size of less than 10m, the grain size was reduced by a factor of more than 10 times as compared to that of the substrate. The micro-hardness of the melted layer was increased to 70–85HV as compared to that of the substrate (about 53HV). Because of the grain refinement, the enhancement of the hard phase β-Mg17Al12, and the solid solution hardening of alloy elements, the micro-hardness and strength of the laser melted layer was increased significantly.

Abstract: In the present study, an attempt has been made to improve the surface hardness and wear resistance of magnesium alloy AZ91D by laser surface melting (LSM) with a 2kW continuous wave CO2 laser. The microstructure of the laser surface melted zone consists of fine dendrites with an average grain size of less than 10μm. Micro hardness of the melted zone was improved to 70-85HV as compared to 53HV of the substrate. The wear behavior of the laser surface melted layer was investigated using a ball-on-flat apparatus under dry sliding condition. Compared with the as-received AZ91D, the wear volume of the laser surface melted layer was decreased by 51%, the wear resistance of the laser surface melted layer was improved significantly.

Abstract: Laser surface melting (LSM) of aluminum alloy 6013 was conducted using a high power Nd-YAG laser under nitrogen gas with the aim of improving the stress corrosion cracking (SCC) resistance of the alloy. The SCC behavior was studied in a 3.5% NaCl solution using a slow strain rate test (SSRT). A laser-melted layer in the order of 500µm thick was produced, which consists of fine dendritic and cellular structures with some Al-Si-Cu-Fe-Mn phase particles formed at interdentritic boundaries. The results of the SSRT test showed that the susceptibility of the alloy to SCC, in terms of total elongation to failure, was decreased after the laser surface modification. This was considered to be attributed to the presence of the laser-formed AlN film, which acted as a barrier to the ingress of the corrosive Cl- into the material and enhanced the pitting resistance of the material. An examination of the fractured surface indicated that the crack propagation path, in the early stage of SCC, was along the tortuous dendrite and cell boundaries. This caused the crack path deviated from the growing direction and resulted in a longer crack length covered before failure thus increasing the overall crack propagation resistance.

Abstract: The laser surface remelting (LSR) process was successfully applied to restore localized corrosion resistance in sensitized stainless steel and also as a useful method to improve passivity of some martensitic stainless steels. The LSR process can be successfully applied to repair cracks and defects at the surface of highly thermo-mechanically loaded parts of stainless steel. The purpose of presented study was to evaluate the microstructure and properties of laser remelted surface of stainless steels. The wrought austenitic stainless steel and sintered in vacuum 316L type were studied. The laser treatment was performed with the use of high power diode laser (HPDL) and the influence of beam power of 0.7-2.1kW on the properties of the surface layer was evaluated. The geometrical characteristics and x-ray analysis of weld bead were studied as well as microhardness, surface roughness and corrosion resistance were measured. The increase of laser beam power of LSR resulted in the increase of hardness of sintered stainless steel due to the reduction of porosity and formation of fine dendritic and cellular-dendritic microstructure. The corrosion resistance of remelted surface increased for sintered materials, when remelted at 2.1kW. The wrought stainless steel revealed impairment of pitting corrosion when remelted at lower beam power rate.

Abstract: In order to improve surface properties, high chromium austenitic base heat-resistant cast steel was scanned with a 5kW continuous wave CO2 laser, the specimen was aged at the temperature of 600°C~900°C. The microstructure and phase composition of the specimen were analysed with optical microscopy, electronic microscope and X-ray diffractionse. The hardness was measured. The results show that as-cast structure of high chromium cast steel is coarse and non-homogeneous, and mainly consist of austenite, ledeburite and carbides. After laser surface melting, the section is divided into the melted zone consisted of fine austenite and carbides, the heat affected zone composed of austenite and eutectic carbides, and the base meta1. The melted zone is very fine structures with dendritic crystals, only at the bottom a cellular structure is observed. A continuous carbide network is located in the austenitic grain boundaries at the heat affected zone. Carbides particles distribute dispersed out, the hardness of melted zone increases 35% than the base metal after aging. The area and the hardness of various zones are related to the laser processing parameters. The hardening depth of melted zone and heat affected zone may be up to 200μm~300μm.

Abstract: Surface modification is a promising technique to improve wear properties of titanium and titanium alloys by modifying either the surface composition or microstructure. Laser remelting and laser nitriding of commercial purity titanium were carried out under pure argon and pure nitrogen ambient, respectively. Characterization of the laser treated surface was done by optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness tester. During laser irradiation heating, Ti exhibits a height activity and combines with N in the atmosphere of pure nitrogen forming TiN and TiN0.3, whereas Ti only transform into martensitic Ti in pure argon. The Vickers microhardnesses are greatly improved by laser remelting and laser nitriding.

Abstract: The liquid film re-growth behaviour resulting from pulsed laser surface melting (LSM) has been investigated for typical 2xxx, and 7xxx aerospace alloys, both on parent plate and friction stir welded (FSW) joints. In Zr free alloys, as a result of the high growth rate and steep thermal gradient, the melted layer re-grew with a stable front, epitaxially, from the parent subsurface grains. This caused a thin coarse grained solidified layer to form over the parent material, thermomechanically affected zone (TMAZ) and heat affected zone (HAZ), and fine columnar grains to develop over the FSW nugget zone of the same order in width as the nugget grain size. In the case of the Zr containing alloys, a very fine columnar grain structure was found over the entire surface, independent of the subsurface grain structure. This has been shown to occur by growth selection from a band of nanoscale Al grains epitaxially nucleated on Al3Zr dispersoids, at the melt solid interface, that had not fully dissolved in the melt.

Abstract: This work investigated microstructure in the surface-melted region of Ni-base single crystal superalloy CMSX-4 by using a diode laser beam as a heating source. Such processing parameters as laser power and scanning speed in laser surface melting were varied while defocusing distance and shielding gas (Ar) flow rates were fixed. Specimen surfaces were arranged parallel to the (001) of base alloy. The microstructure in the melted region was analyzed by optical microscopy and SEM. Crystal orientation of the melted region was analyzed using electron backscattered pattern analysis. The microstructure was remarkably changed when the heat input of surface melting was varied. The surface-melted region was found to solidify into a single crystal with directional dendrites that grew along the [001] directions under low heat input conditions. The surface-melted region was also a single crystal with disoriented dendrites that grew along the [100] or [010] directions under medium heat input conditions. In contrast, the melted region consisted of poly crystals with stray crystals under high heat input conditions. Such tendencies were also observed in the melted region with gas tungsten arc. These results demonstrate that the surface-melted region can solidify into a single crystal under conditions in which larger temperature gradient and higher solidification rates can be achieved.

Abstract: The laser surface-treatment methods have been quickly developed by appearing of lasers with high power beam and can increase the hardness, of the surface. A very hard, wear-resisting layer can be produced by the dispersing of ceramic grains. The essence of the technology is, that such a material (compound-phase, e.g.: metal-oxide, carbide, nitride, etc.) is added to the surface layer melted by laser, which does not solve or solves only partly in the metal-melt. This work studies the effect of the different technological parameter (such as, power of the laserbeam, motion speed, amount of the ceramic particles etc.) on the different microstructure accrued during the laser surface alloying. The desired microstructure has homogeneous carbide distribution in the matrix. But it is embarrassed by several conditions. The aim of this present work is to find out the reasons for the inhomogeneous ceramic particle distribution inside the matrix and to discover these embarrassing conditions.