The unique characteristics of the laser beam compared to the normal light i.e. its directionality, monochromatcity, and brightness make it a very important tool in the surface engineering. The concentrated heat in the laser beam can precisely directed toward the processed region on the surface. The laser beam power, size, and scanning velocity are easily controlled during the different processing methods. Laser surface modifications of metallic materials, such as laser surface melting, laser surface alloying, and laser cladding are some of the most important methods to improve the mechanical properties of the processed layer on the surface. A pulsed Nd-YAG laser surface melted specimens of low carbon steel with a maximum power of 90W at different processing parameters. The rapid melt and resolidification of the treated layer produced a refinement in the microstructure. The original microstructure of this steel (Ferrite + Pearlite) has been transformed to new phases of martensitic and bainitic structures. The melted layer consists of the melted pool, the heat affected zone, and the substrate. The maximum hardness of the laser melted layer is more than 400 HV.

Key words:Laser surface Melting, Carbon steel, Microstructure, Microhardness

الملخص

Laser cladding is considered as a strategic technique , since it can yield a surface layers that compared to other hard facing techniques, have superior properties in terms, of pureness , homogeneity , hardness, bonding and microstructure .

A clad layer of bronze (6:6:3) formed on Zl-105 Aluminum alloy surface using a high power CO2 CW laser . Effects of laser scanning rates on the quality of the produced clad layer studied . The microstructure of clad layer examined using optical and scanning electron microscope. The produced phases and chemical composition of clad layer analyzed using XRD and EDS. Microhadness profile indicated a significant improvement in the hardness of the bronze coating on Zl-105 Al

nitriding is a common method for improving the hardness , mechanical properties, wear and corrosion resistance of metals. Laser nitriding of metals is an efficient process, where the irradiation of surfaces in nitrogen atmosphere leads to the fast take-up of nitrogen in the irradiated surfaces. In this paper laser surface nitriding of carbon steel carried out by melting the surface of substrate using high power CW CO2 laser with nitrogen as shrouding environment. Laser tracks were arranged as single tracks with the use of various laser powers ranging from 0.8kW to 2.0 KW , and laser scanning rates from 120mm/min to 200 mm/min. The influence of laser beam power and laser scanning rates on the microstructure and hardness profiles analysed. The laser nitride layer studied using optical microscope, and scanning electron microscope. The laser surface nitriding of carbon steel led to refinement in the microstructure and the diffusion of nitrogen caused the formation of iron nitrides on the surface. The micro hardness of the nitride surface improved to a maximum of 400HV in the present set of laser processing conditions as compared to 150HV of as received substrate as a result of the appearance of marten site in the melted pool and heat effect zon

Laser technologies have made distinguished contribution in modern industry. These have principally been realised through the important role played by lasers in advancement of manufacturing technology in areas such as welding. Laser welding has now become a very important joining technique in a wide variety of applications in the oil gas, aerospace, aircraft , medical and electronic industries.

This study presents investigations relating to the production of high quality laser welds that blend closely with surface of the welded materials, (welds termed 'net shaped'). It has not been possible to produce such welds in the past using conventional welding technology.Net shaped welds not only look attractive, but afford functional and economic benefit because no secondary machining and polishing is required.

Mild steel plates and bead-on-plate welding with a 1 kW fiber laser are usd for this investigation. Design of experiments and statistical modeling are used in the experimental study to understand parameter interactions. The independent process variables studied are power and welding speed. Experimental methods and results are detailed and results are discussed. The work shows that the laser and our work to date have made production of net shaped welds possible. The work could lead to new applications for very high geometry accuracy welding  

In this paper SiC particle reinforce aluminum metal matrix composite welded using high power continuous laser. Microstructure of laser beam welded SiC/Al MMC is characterized as functions of laser processing parameters. Results show that sound weld with little pores can be produced by laser beam welding, while aluminum carbide is formed in the weld. The size and the volume fraction of aluminum carbide are proportional to the laser power density. direction of shielding gas flow has significant effect on weld quality and weld microstructure.

    SiC particle reinforced aluminum metal matrix composites (SiCp/Al MMCs) are finding increasing applications in aerospace, automotive and microelectronic industries because of their excellent combination of high specific strength, high specific toughness, low coefficient of expansion and excellent wear resistance. However the addition of totally dissimilar reinforcements into the aluminum matrix and the great different in electrochemical properties between the aluminum metal matrix and the SiC reinforcement deteriorate the corrosion resistance and make the SiCp/6061Al MMC prone to corrosion attacks especially when servicing under electrochemical corrosive environments.

     In this dissertation, laser surface melting, laser surface alloying with Ni-Cr-B alloy powders and laser cladding with Cu-Sn-Pb-Zn bronze powders were utilized to improve the corrosion resistance of P/M SiC particles reinforced metal matrix composite (P/M SiCp/6061Al MMC) by producing corrosion resistant surface layers or metallurgical coatings with rapidly solidified and effectively refined microstructure. The microstructure of the laser surface melted, laser surface alloyed and laser clad corrosion-resistant modified surface layers or coatings were characterized by OM, SEM, XRD as functions of laser surface processing parameters and contents of alloying elements. The hardness distribution along the laser surface modified coatings’ depth direction was tested using microhardness tester. The electrochemical corrosion resistance of laser surface melted, laser surface alloyed and the laser clad SiCp/6061Al MMC were evaluated under direct current anodic polarization electrochemical corrosion test conditions in NaCl water solutions and H₂SO₄ water solutions as functions of laser processing parameters and corrosion test conditions.

Results show that:

(1) During laser surface melting of SiCp/6061Al MMC with high power CO₂ laser beam, the SiC particles are completely dissolved and the laser surface melted layer has a rapidly solidified, homogeneous and refined microstructure consisting of aluminum silicon carbide Al₄SiC₄ aluminum carbide Al₄C₃ as well as crystalline silicon particles evenly distributed in the aluminum matrix. The microstructure as well as size and volume fraction of the newly produced phases in the laser surface melted layer on substrate of SiCp/6061Al MMC depend strongly on the laser processing parameters especially the laser linear energy density. Both the hardness and corrosion resistance of SiCp/6061Al MMC under direct current anodic polarization electrochemical corrosion test conditions in 0.1M NaCl water solutions are slightly enhanced after the laser surface melting treatment.

(2) Laser surface alloying of SiCp/6061Al MMC with Ni-Cr-B alloying powders leads to the production of rapidly solidified corrosion resistant surface layers having homogeneous and refined microstructure. The laser surface alloyed SiCp/6061Al MC is composed of fine rapidly solidified nickel aluminides of Al₃Ni_2, Al₃Ni and AlNi, and chromium borides of d-CrB and Cr₂B evenly distributed in the nickel solid solution. Microstructure of the laser surface alloyed layers is strongly dependent on the laser processing parameters and the chemical composition especially boron content of the Ni-Cr-B alloying powders. The laser surface alloyed SiCp/6061Al MMC has high hardness and excellent corrosion resistance to electrochemical corrosion under direct current anodic polarization electrochemical corrosion test conditions in NaCl water solutions.

(3) By laser cladding of SiCp/6061Al MMC with Cu-6Sn-6Pb-3Zn bronze powders a rapidly solidified homogeneous dendritic bronze coating is produced on substrate of SiCp/6061Al MMC with refined microstructure consisting of fine copper-base solid solution and the interdendritic Cu₃₁Sn₈ intermetallic particles_. The laser clad corrosion resistant coating is bonded to the substrate of SiCp/6061Al MMC, metallurgically. The corrosion resistance of SiCp/6061Al MMC to the electrochemical corrosion under direct current anodic polarization electrochemical test conditions in NaCl water solutions is significantly improved after laser cladding treatment with the bronze powders.

(4) Laser surface melting, laser surface alloying and laser cladding are demonstrated to be among the promising surface modification methods for enhancing corrosion resistance of SiCp/6061Al MMC.

Keywords:  SiCp/Al Metal Matrix Composites, Laser Surface Melting, Laser

Surface Alloying, Laser Cladding, Corrosion Resistance, Microstructure

We have studied the effects of the fine structure of energy levels on the dynamics of the atom and light field. A significant influence is exhibited in the collapse-revival behavior of atomic inversion, the light squeezing, the phase of the field, and the antibunching of a light beam. When the energy difference between the doublet is large (Δ≥20g, where g is the coupling constant), the dynamics of the atomic inversion becomes identical to that of the usual Jaynes-Cummings model, as expected. However, the fine-structure effects on the phase and the antibunching of the light field exist in a much larger range. In particular, for a strong field, the normally ordered variance of the photon-number operator exhibits complicated collapse and revival patterns and the light displays strong antibunching in each revival envelope. Numerical calculations also show that this model does not produce significant squeezing due to the influence of the fine structure. 

In our study of the interaction between few energy level atom and a coherent field, we considered the effect of the excited state's fine structure on the system dynamics. Significant influence exhibited on the collapse-revival behavior of atomic inversion and light squeezing. The numerical calculations show that for weak field, the effects of fine structure may cause disappearance of the collapse and revival phenomena. Our results also show the fine structure has opposite on the light squeezing, this observed on decrease of the amount of squeezing as the average photon number increased.

We report on the first clear observation of a collisionally induced diffuse band of lithium dimer around 452.0 nm in direct single photon excitation using 337.1 nm (N2) laser radiation from an excimer laser. The band, which is being tentatively assigned to the 2 1Σ+u → X 1Σ+g transition and could only be observed at buffer gas pressures of ⩾ 75 mbar, had been theoretically pre dicted. We also report on the other violet band of Li2 around 458.0 nm, already assigned to the 2 3Πg → a 3Σ+u transition. Parametr ic dependences for the two bands are presented. Together with these continuum emissions, a number of strong atomic lithium lines are also observed. A possible correlation between the atomic and continuum emissions is presented which may help in a better understanding of the kinetics of the system.