Laser Processing Conditions Research Articles

Laser processing of an aluminium AA6061 alloy involving injection of SiC particulate

In the present laser processing work, the powder injection technique was investigated as a method for producing a surface metal matrix composite (MMC) containing large SiC particulates (SiCp) (105–150 μm). This size is known to enhance the wear resistance of bulk aluminium-based composites. The effects of the laser-processing conditions, the powder feeding rate and the surface situations necessary to produce a well incorporated MMC on the surface were studied, and the microstructure examined. In previous work, laser processing involving the preplacement of SiCp was developed to create an AI-SiCp (≤45 μm) MMC layer on aluminium alloy surfaces. Some of these ideas were used in conjunction with the injection process in the present work to enhance the surface-wear resistance. The wear resistance of an MMC obtained by a single laser track with the injection technique was determined and compared with the base alloy and the MMC layer produced by the preplacement technique.

Microstructural and microhardness characteristics of laser-synthesized Fe-Cr-W-C Coatings

The effects of laser-processing parameters on the microstructure and microhardness of Fe-Cr-W-C quaternary alloy coatings were investigated experimentally. The coatings were developed by laser processing a powder mixture of Fe, Cr, W, and C at a weight ratio of 10:5:1:1 on a low-carbon steel substrate using a 10 kW continuous wave CO2 laser. Depending on the processing parameters, either hypoeutectic or hypereutectic microstructures were produced. The hypoeutectic microstructures comprised primary dendrites of nonequilibrium face-centered cubic (fcc) austenite γ phase and eutectic consisting of pseudohexagonal close-packed (hcp) M7C3 (M = Cr, Fe, W) carbides and fcc γ phase. The hypereutectic microstructures consisted of hcp M7C3 primary carbides and eutectic similar to that in the hypoeutectic microstructures. The formation of hypoeutectic or hypereutectic microstructures was influenced by the alloy composition, particularly the C concentration, which depends on the amount of powder delivered into the melt pool and the extent of substrate melting. The enhancement of the lattice parameter of the γ phase is associated with the significant dissolution of alloying elements and lattice strains resulting from rapid quenching. The higher hardness of the hypereutectic microstructures is principally attributed to the formation of hcp M7C3 primary carbides. The relatively lower hardness of the hypoeutectic microstructures is related to the presence of y phase in the primary dendrites, excessive dilution from the base material, and relatively low concentrations of Cr and C. The results provide insight into the significance of laser-processing conditions on the composition and hardness of Fe-Cr-W-C alloy coatings and associated solidification characteristics.

Laser processing to createin-situ Al-SiCp surface metal matrix composites

When SiC particulate (SiCp) is preplaced on an aluminium alloy surface, a molten zone can be formed in the aluminium specimen by laser processing, and there is a possibility of producing anin-situ Al-SiCp metal matrix composite (MMC) layer on the surface, which will modify the surface properties. Under specific laser processing conditions in the present work, a smooth and continuous Al-SiCp MMC layer was developed on the surface with well-distributed and embedded SiCp in the layer. In most cases, the SiCp partially dissolved in the liquid and reprecipitated during the solidification. The dissolution of the SiCp is discussed, and the precipitate in the present work is identified as Al4SiC4. The thickness of the Al-SiCp MMC layer was limited to 30–50 Μm when SiCp was preplaced on the specimen. The mechanism of both the formation of the Al-SiCp and the limitation of the MMC layer thickness in this process was studied.

Laser Crystallized Polysilicon Thin Films and Applications

AbstractPulsed excimer-laser crystallization of amorphous silicon on non-crystalline substrates is an important processing technique for large-area polycrystalline silicon films and devices. Interest stems, in large part, from proposals to use polycrystalline silicon on glass in large-area electronic applications, such as flat-panel active matrix displays and two-dimensional imaging systems. The polycrystalline silicon is envisioned to increase the functionality and reduce costs over the current circuits that use amorphous silicon. Also, it is found that laser-crystallized polycrystalline silicon exhibits some interesting materials properties, such as a sharp peak in the average grain size with large lateral grain growth as a function of excimer laser energy density. The average grain size increases with increasing laser fluence and peaks on the order of several microns or two orders of magnitude larger than the film thickness. The grain size then decreases with further increases in laser fluence. This peak in grain size is accompanied by a similar peak in the Hall electron mobility. This is a significant relationship for devices since the grain structure has a substantial influence on electrical properties. But to the detriment of device parameters, this large lateral grain growth occurs over a very arrow range of laser fluences and is accompanied by a corresponding peak in the surface roughness of the films. These relationships between laser processing conditions, materials properties, and device parameters force a compromise between large grain size for high mobility and homogeneity of material for uniformity of device characteristics. A window does exist in process parameter space where good-quality devices with uniform characteristics have been obtained. In addition, these attributes have been achieved under conditions that yield good polycrystalline silicon and good amorphous silicon devices on the same wafer within a mm of one another, allowing for hybrid polycrystalline and amorphous silicon circuits.

The effect of surface condition on acoustic emission during welding of aluminium with CO2 laser radiation

The fast Fourier transform spectrum of acoustic emission during CO2 laser welding of Al 1100 shows frequency components in the 3-9 kHz range that can be identified with the presence of a keyhole and correlate with penetration. In addition, a study of the effect of anodization and surface pretreatment of Al 1100 with excimer laser radiation has shown that acoustic emission at 9-10 kHz arises from burning off of surface oxide. A comparison of these results with those predicted from simple thermal and fluid dynamical models yields good agreement with theory. These results indicates that acoustic emission over specific frequency ranges may be highly diagnostic of laser processing conditions.

高出力ＹＡＧレーザによる炭素鋼の表面硬化処理

A high-power YAG laser was applied to the surface hardening of carbon steels containing 0.18∼0.54wt%C, and the relationships between laser processing conditions and surface hardening were investigated by hardness and microstructure.The structure of the hardened zone underwent complete martensitic transformation in all of the carbon steels tested, and its hardness increased with greater carbon content. Under identical irradiation conditions, the hardened zone expanded with increasing carbon content. A hardened zone extending from the surface to a depth of 1.0mm was obtained at a laser power of 1.0kW and a scanning speed of 1mm/s.It was found that in the surface hardening of carbon steels, a high-power YAG laser can be used to control the hardened zone by selecting the appropriate irradiation conditions.

Laser Transformed SiC Thin Films

ABSTRACTSilicon carbide has excellent physical and electronic properties for use in devices when higher temperatures or higher power densities are required. We have investigated a direct laser conversion technique to create electrical conductors on the high band-gap silicon carbide. Thin films of silicon carbide (SiC) were sputter deposited on AI2O3, SiO2, and Si substrates using a SiC target with an RF planar magnetron. These films were irradiated at 308 nm with multiple 15 ns excimer laser pulses creating 0.5 to 2 mm wide electrically conducting paths. Both the irradiated and unirradiated films were evaluated as a function of substrate type, deposition temperature, finish, stoichiomelry, annealing temperature, sputter gas, film thickness, and laser processing conditions. The lowest resistivity films, originally 10 ohm-m, were calculated to be 160 μohm-m obtained after irradiation, which compares to a value of 50 μohm-m obtained after irradiating bulk SiC. The films were characterized using XPS, SIMS, AES, SEM, and Raman spectroscopy. We were able to characterize the composition of the films and conducting traces, the surface oxide, the critical binding energies, the lattice structure, and the morphology of the microstructure. Models for the phase transformations and conductivity have been formulated.

Laser processing of plasma-sprayed R-Ba-Cu-O coatings

Thick Y-Ba-Cu-O and Er-Ba-Cu-O coatings were deposited by plasma spraying onto nickel substrates. These plasma-sprayed coatings were laser melted to modify their microstructures. The effects of primary processing conditions, such as linear energy and number of passes, on microstructural modifications were assessed. The microstructure of the as-sprayed coatings was largely transformed to produce a fine Y2O3 dispersion in a Ba-Cu-O matrix. A very low level of coating/substrate interactions can be maintained by appropriate laser processing conditions.

Metal-ceramic joining by laser

Within the scope of increase in cutting tools efficiency, a CW CO 2 laser beam has been used for butt welding sintered tips containing cobalt and tungsten carbide to a medium carbon steel core. After optimization of the laser processing conditions, full penetrating, sound and homogeneous welds are observed. Hardness is found to be nearly constant across the entire weld bead cross-section. Micrographic observations in the same area show a fine dendritic microstructure; microanalysis (EDS) as well as X-ray diffraction experiments reveal that dendrites contain mainly iron and cobalt, while tungsten has segregated in the interdendritic zone where complex (Fe-Co-W) rich carbides are likely to form

Control of EL2 concentration and dislocation density in liquid encapsulated Czochralski-grown GaAs using laser processing

The authors report the first study of laser processing of GaAs under stoichiometry control. A series of experiments was performed to investigate the influence of laser processing on the electronic and structural properties of liquid encapsulated Czochralski-grown GaAs. A thermal treatment was used to convert samples from semi-insulating to conducting behaviour by severely reducing the EL2 concentration in the material. These samples were subjected to both laser processing and standard thermal annealing under stoichiometry control. Laser-assisted processing resulted in much stronger EL2 recovery rates than for standard thermal annealing and the tendency was more pronounced for arsenic-rich stoichiometry than with gallium-rich stoichiometry. Indeed, selected laser processing conditions were found to yield significantly higher sample resistivities than those of the starting material. Etch pit densities were studied as a function of the laser processing conditions and were found to be a minimum for near-stoichiometric conditions. A simple defect model is proposed and used to explain these experimental results.

Microstructure and Phase Selection in Laser Treatment of Materials

Microstructure formation during the laser treatment of materials is discussed so as to enable one to design appropriate alloys and laser processing conditions which will produce a microstructure that has optimum properties. In order to predict microstructures for different laser processing conditions, the theoretical models for single phase and two-phase eutectic growth under a wide range of solidification rates have been developed. Specific emphasis is placed on the new physics that become important under rapid solidification conditions of laser processing. Based on these models, the selection of a specific microstructure under given laser processing conditions has been established. These results are then used to construct microstructure/ processing condition maps for the laser treatment of materials. These diagrams theoretically predict the variation in the microstructure along the depth of the solidified pool as a function of laser scanning velocity.

Interaction of titanium carbides with alloying elements in steel: E.V. Sokolova et al, Poroshkovaya Metallurgiya, No 1, 1991, 68–72, (In Russian)

Laser surface melt hardening of a pearlitic grey iron was carried out using a 1 kW continuous wave CO2 laser, without preheating of the specimens and without prior graphite coating of the surface. It was shown that, with suitable laser processing conditions, it is possible to obtain melt areas without cracks in either single- or multiple-track melting experiments. In the case of single-track melting, the microstructure of the melted zone was composed of dendritic austenite and interdendritic ledeburite with a hardness of about 700 HV. For multiple-track melting, the austenite is partially transformed into cementite and martensite in overlapping regions, leading to an increase in the hardness.

Characteristics of the formation of a radiation structure by focusing a multichannel-laser beam

The positions of the planes with different states of the structure of the radiation in the far-field zone of focusing optical systems were determined theoretically and experimentally for multichannel lasers as a function of the focal lengths of the focusing systems and of the distance from the exit window of a laser resonator due to the focusing system. An analysis was made of the influence of the radiation structure on the formation of the zones of the laser interaction with steel under different laser processing conditions.

The Effect of Oxygen in the Si Substrate on Mo, W, Ti, and Co Silicide Growth by Infrared Laser Heating

This study of the effect of implanted oxygen in the Si substrate was accomplished using an IR heating method and a combination of different materials analysis techniques. Principally, Auger electron spectroscopy combined with depth profiling was implemented to investigate the composition of the reacted metal‐Si systems as well as the relative movement of the oxygen during silicide formation. Our systematic study of these four metal‐Si systems yielded some interesting results. First, for the three metals Mo, W, and Ti, we observed basically inhibited metal‐Si reactions at laser processing conditions that yielded completely reacted metal silicides without implanted oxygen. Second, the evolution from inhibited reactions through partial, metal‐rich silicides and finally to completely reacted metal silicide formation at high temperatures was observed and characterized. Last, a distinctly different response to the presence of oxygen was observed for the Ti samples as compared to the Mo and W samples. In the Ti sample, the oxygen accumulated at the cap layer Si/Ti interface whereas the oxygen accumulated at the metal/Si substrate interface for the Mo and W cases. The effect of the implanted oxygen on the formation of was negligible as compared to the Mo, W, and Ti systems.

Microstructure and tribology of laser mixed Fe/Ti/C multilayered films on AISI 304 stainless steel

An excimer laser was used to mix Fe/Ti/C multilayered films on 304 stainless steel substrates. The samples were processed at both 1.1 and 1.7 J/cm2 with the number of pulses at each position varied between 1 and 10. Composition, microstructure, phase evolution, and tribological properties were observed to correlate with total laser fluence. Increases in Fe concentration from substrate interdiffusion and loss of C content were observed with increasing total laser fluence. The best tribological properties were observed in films possessing a combination of an amorphous or Fe3C phase plus fine grain TiC following processing at low and intermediate total laser fluence. At higher total fluences a combination of α-Fe, Fe3C, and fine-grain TiC was observed along with degradation in the wear and friction properties. Under optimum laser processing conditions the modified surface had a friction coefficient under dry sliding conditions reduced by a factor of 2 relative to uncoated 304 stainless steel and was significantly more wear resistant. These improvements in wear and friction appear to be related to the reduced chemical reactivity of the amorphous and carbide phases and to the influence of microstructure on improved mechanical properties.

Microstructure of laser clad Ni- Cr- Al- Hf alloy on a γ′ strengthened ni- base superalloy

Alloys and coatings for alloys for improved high temperature service life under aggressive atmo-spheres are of great contemporary interest. There is a general consensus that the addition of rare earths such as Hf will provide many beneficial effects for such alloys. The laser cladding technique was used to produce Ni-Cr-AI-Hf alloys with extended solid solution of Hf. A 10 kW CO2 laser with mixed powder feed was used for laser cladding. Optical, scanning electron (SEM) and scanning transmission electron (STEM) microscopy were employed to characterize the microstructure of alloys produced during laser cladding processes. Microstructural studies revealed grain refinement, considerable in-crease in solubility of Hf in the matrix, Hf-rich precipitates, and new metastable phases. The size and morphology of γ′ (Ni3Al) phase were discussed in relation to its microchemistry and the laser processing conditions. This paper will report the microstructural development in this laser clad Ni-Cr-AI-Hf alloy.

Laser surface melting of a pearlitic grey cast iron

Laser surface melt hardening of a pearlitic grey iron was carried out using a 1 kW continuous wave CO 2 laser, without preheating of the specimens and without prior graphite coating of the surface. It was shown that, with suitable laser processing conditions, it is possible to obtain melt areas without cracks in either single- or multiple-track melting experiments. In the case of single-track melting, the microstructure of the melted zone was composed of dendritic austenite and interdendritic ledeburite with a hardness of about 700 HV. For multiple-track melting, the austenite is partially transformed into cementite and martensite in overlapping regions, leading to an increase in the hardness.

Laser Cladding with a Ni-Fe-Cr-Al Alloy

ABSTRACTAn investigation is reported of the laser surface.cladding of an alloy of nominal composition Ni-4Fe-30Cr-6A1 (wt%) on a mild steel substrate, using a continuous powder feed of the elemental powder mixture into the melt pool. The effect of laser processing conditions has been investigated in relation to clad dimensions, bonding, dilution level, hardness and microstructural features. A 2 kW CW C02laser was used working at 1.6 kW power and 2.5 mm beam diameter at different traverse speeds ranging from 1.8 to 25 mm/s: clad layers were produced in the range of 0.15-3 mm thick. Fine microstructures were obtained (corresponding to the rapid solidification rates). Good metallurgical bonding was observed between the clad layer and substrate within the range of specific energies, 50-130 J/mm2. For all the conditions studied, no cracking was observed in single tracks; low specific energy conditions gave little porosity.

Laser-processed electrodes consisting of amorphous nickel-niobium-platinum group metal surface alloys on niobium

Laser processing was applied to a specimen composed of a bulk niobium substrate covered by electrodeposited nickel and platinum group metals in order to form amorphous surface alloys having excellent electrocatalytic activities. The bulk niobium substrate acts as an electric conductor instead of thin amorphous alloys of a high electric resistance. Laser processing leads to alloying of electrodeposited metals with niobium, and hence the niobium content of the laser-processed layer is determined by the laser-processing conditions. A wide area is vitrified only when the niobium content was 35–40 at.% in spite of the fact that nickel-niobium alloys containing 25–65 at.% niobium become amorphous by conventional methods for preparation of amorphous alloys such as melt spinning. Vitrification of a wide area is performed by overlapped traverses by the laser beam and often results in crystallization in the heat-affected zone in the previously vitrified phase. Thus the laser processing must be carried out so as to form the surface alloy having a very high glass-forming ability, and the appropriate laser-processing conditions must be determined for each alloy family. The laser-processed amorphous alloy electrodes exhibit excellent electrocatalytic activities when their surfaces are activated chemically.

Surface structures produced in 1C?1.5Cr and 0.38C-Ni-Cr-Mo steels by high-power CO2 laser processing

The effects of surface high-power laser processing were studied for 1C-1.5Cr and 0.38C-Ni-Cr-Mo steels by changing the incident power density and laser-material interaction time. Both melted and solid-state transformed regions were produced, and studied by means of depth-selective surface Mossbauer measurements, X-ray diffraction analyses, metallogmphy and microhardness measurements. The results are compared with those previously obtained by laser surface-melting of OAC carbon steel, and are discussed with reference to the carbon content in the base alloys as well as the conditions of laser processing.