High Power Continuous Wave CO2 Research Articles

The formation of trinitite-like surrogate nuclear explosion debris (SNED) and extreme thermal fractionation of SRM-612 glass induced by high power CW CO2 laser irradiation

We describe a new approach to the bench top production of surrogate nuclear explosion debris by employing high power continuous wave CO2 laser irradiation. High surface temperatures >2,500 K can be rapidly attained, allowing virtually any combination of materials to be fused into a glassy matrix that can display high levels of elemental fractionation. Examples of the laser fused glasses will be presented and compared to trinitite nuclear explosion glass along with the elemental fractionation effects that were induced in the NIST glass standard SRM-612 by this method.

Crack separation mechanism in CO2 laser machining of thick polycrystalline cubic boron nitride tool blanks

An improved method for cutting thick polycrystalline cubic boron nitride (PCBN) tool blanks is explored because current methods of pulsed Nd:YAG laser cutting and wire electrical discharge machining (EDM) are constrained by low speed and low precision. We present a CO2 laser/waterjet (LWJ) process to cut 4.8-mm-thick PCBN tool inserts by a crack separation mechanism. In LWJ, the PCBN blank is locally heated using a high-power continuous wave CO2 laser to cause phase transition from cubic to hexagonal followed by water quenching to generate thermal stresses and form boron oxide leading to increased brittleness, subsequent cracking, and material separation. A 23 fractional design of experiment (DOE) approach was employed to determine the factors of laser power, cutting speed, and waterjet pressure on the responses of phase transformation depth, taper, and surface roughness. A numerical heat flow model, based on Green’s function, was used to calculate the temperature distributions along the depth. Surface profilometer, scanning electron microscopy, and Raman spectroscopy were utilized to analyze the phase transformation and crack zones. Results from LWJ compared with pulsed Nd:YAG laser and laser microjet™ methods indicate LWJ cuts 30 times faster; this was attributed to a nonconventional material removal (crack separation) mechanism. When LWJ was compared against nitrogen-assisted CO2 laser cutting, improved cut quality (less taper and smaller heat-affected zone) was observed due to a greater control on phase transformation and crack propagation. DOE analysis revealed laser power and waterjet pressure, and the interactions among them are more significant factors than others.

Effect of Y<sub>2</sub>O<sub>3</sub> Addition on the Microstructure and Corrosion Behaviour of Laser Cladding of Al Powder on ZM5 Magnesium Alloys

The aim of this work was to improve the surface layer properties of the Mg-Al-Zn cast ZM5 magnesium alloys by melting and preplaced Al powder or Al+Y2O3 powder on the surface. Laser processing was carried out using high power continuous wave CO2 laser. The resulting surface layers were examined using metallographic light microscopy, scanning electron microscopy, X-ray diffraction. The corrosion resistance was detected by potendiodynamic polarization test. The morphology of the Al cladding coatings are dendrities. And the coatings have excellent corrosion resistance compared to the untreated ZM5 substrate. The enhanced corrosion resistance coating doped with Y2O3 was attributed to the refined microstructure induced by Y2 O3

MICROSTRUCTURE AND WEAR RESISTANCE OF LASER-TREATED Ni–P–Al2O3 ELECTROLESS DEPOSITION COMPOSITE COATING

A uniform Ni–P–Al2O3 electroless plating layer was produced on medium carbon steel and then scanned by high-power continuous wave CO2 laser beam. The entire coating is divided into three regions: laser-strengthened region, transition zone, and heat-affected zone. The strengthened coating shows good performance characterized by uniform components and refined microstructures. The main phase is comprised of Ni3P with some non-equilibrium phases. The reinforced particles, Al2O3 , are homogeneously dispersed in the coating. Its abrasive resistance also increases because of the Al2O3 particles.

Laser assisted composite surfacing of materials for improved wear resistance

In the present study, a high power continuous wave CO2 laser has been used for development of titanium boride/diboride dispersed aluminium matrix composite on aluminium, a graded SiC dispersed α-Fe matrix composite on mild steel and TiN dispersed α-Ti matrix composite on Ti6Al4V. A detailed study of the microstructures and phases developed in the composite layer has been investigated and correlated with process parameters. A significant improvement in microhardness and wear resistance properties was recorded. The mechanism of properties improvement is discussed in detail.

Microstructure and properties of laser cladding TiC/TiAl composite coatings on γ-TiAl intermetallic alloy

To improve the wear resistance of the γ-TiAl intermetallic alloy, Ti–Al–TiC alloy powders were prepared on the surface of TiAl intermetallic alloy and then treated by a high power continuous wave CO2 laser. TiC reinforced TiAl matrix composite coatings have been successfully synthesised. The microstructure, phase composition, microhardness and wear resistance of the laser cladding composite coatings were investigated by optical microscope, scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy and pin on disc wear test. The results show that the morphology of reinforced TiC homogeneously disperses in TiAl matrix and changes from dendritic TiC into short bar and particle shape with increasing the scanning rate. Also, the microhardness is enhanced by an increase in the scanning rate. The average microhardness of the composite coating under laser scanning rate of 600 mm min–1 is 1·8 times of the substrate. The laser cladding composite coatings have better wear resistance than original TiAl alloy, and the wear mechanisms are discussed in detail.

Development of wear resistant composite surface on mild steel by laser surface alloying with silicon and reactive melting

The present study concerns laser surface alloying with silicon of mild steel substrate using a high-power continuous wave CO 2 laser with an objective to improve wear resistance. The effect of surface remelting using nitrogen as shrouding environment (with and without graphite coating) on microhardness and wear resistance has also been evaluated. Laser surface alloying leads to formation of a defect free microstructure consisting of iron silicides in laser surface alloyed mild steel with silicon and a combination of silicides and nitrides when remelted in nitrogen. Carbon deposition prior to remelting leads to presence of a few martensite in the microstructure. A significant improvement in microhardness is achieved by laser surface alloying and remelting to a maximum of 800 VHN when silicon alloyed surface is melted using nitrogen shroud with carbon coating. A detailed wear study (against diamond) showed that a significant improvement in wear resistance is obtained with a maximum improvement when remelted in nitrogen atmosphere followed by carbon coating.

Direct laser cladding of Co on Ti–6Al–4V with a compositionally graded interface

In the present study, attempts have been made to fabricate Co layers on the surface of Ti–6Al–4V substrate with a compositionally graded interface by direct laser cladding. Laser processing is carried out by pre-placing the powder (or powder blends) on the substrate, and melting it using a high power continuous wave CO 2 laser with Ar as shrouding gas. A compositionally graded interface is developed by applying powder blends of Ti to Co at a ratio of 90:10 near to Ti–6Al–4V substrate to 10:90 prior to development of Co layer. A defect-free microstructure is developed with the presence of Ti 2Co and TiCo and Co 2Ti at the interface. The volume fraction of individual phase was found to vary with the depth from the Co-clad zone. A significant improvement in microhardness is achieved at the interfacial region. Uniform corrosion resistance increases along the graded interface, but the pitting corrosion resistance is marginally deteriorated. Direct laser clad layer possesses a better biocompatibility than that of as-received Ti–6Al–4V sample.

Laser cladding of Ni-based alloy on copper substrate

The laser cladding of Ni1015 alloy on Cu substrate was prepared by a high power continuous wave CO 2 laser. Its microstructure was analyzed by optical microscope (OM), scanning electron microscope (SEM), and X-Ray diffraction (XRD). The average microhardness of the cladding coating was Hv 280, which was almost three times of that of the Cu substrate (Hv 85). OM and SEM observations showed that the obtained coating had a smooth and uniform surface, as well as a metallurgical combination with the Cu substrate without cracks and pores at the interface. With the addition of copper into the nickel-based alloy, the differences of thermal expansion coefficient and melting point between the interlayer and cladding were reduced, which resulted in low stresses during rapid cooling. Moreover, large amount of (Cu, Ni) solid solution formed a metallurgical bonding between the cladding coating and the substrate, which also relaxed the stresses, leading to the reduction of interfacial cracks and pores after laser cladding.

Modeling of high power laser interaction with APS deposited FeCrTiC

The experiments of laser glazing plasma-sprayed FeCrTiC composite coatings with a high power continuous wave CO 2 laser were simulated with a numerical code ‘Fusion 2-D’. The glazing experiments were carried out using power density of q=8 kW cm −2 and a beam scan speed of v=2.5–10 mm s −1. The surface temperature of the coatings was monitored with a CCD camera during the treatment. These experimental data were directly compared with the temperature calculated theoretically. The numerical code also enabled us to calculate the depth of the laser-melted zone. The code was finally applied to simulate the laser glazing experiments with different operational parameters to achieve the desired glazing effect.

Laser surface alloying of copper with chromium

Abstract Laser surface alloying (LSA) of Cu with pre-deposited Cr has been carried out using a high power continuous wave CO 2 laser. Following LSA, the surface microstructure and composition have been correlated with the laser processing parameters. It is found that a judicious selection of laser power density ( Q ) and scan speed ( v ) (or interaction time, t i ) is essential to obtain a defect-free and uniform alloyed zone (AZ) for a given pre-deposit thickness ( t z ). The AZ-depth is directly related to Q and t i . Microstructurally, the AZ consists of dispersed Cr-rich particles in the Cr-alloyed Cu-rich solid solution. Rapid quenching in the present LSA routines has extended the solid solubility of Cr in Cu to about 4 at.%. The total Cr content in the AZ varies inversely with Q and t i for a given t z . Finally, a process optimization diagram has been constructed to predict the microstructure and composition of the AZ for the present system for different conditions of LSA.

Novel stable resonator for large-volume TEM00 mode operation

A novel stable resonator is developed to realize stable operation at a TEM00 mode with a large beam diameter. The output coupler has a circular partial-reflection region in the center surrounded by an antireflection region. The transverse mode is mainly selected by the partial-reflection region. An amplified diffraction beam is extracted from the outer antireflection region. By combining part of the ‘‘core’’ beam transmitted from the partial-reflection region with the amplified diffraction beam, a diffraction-limited beam with the M2 factor of 1.3 is obtained from a high power continuous wave CO2 laser as predicted theoretically.

Microstructural characterization and In Situ transmission electron microscopy analysis of laser-processed and thermally treated Fe-Cr-W-C clad coatings

Rapidly solidified microstructures of Fe-Cr-W-C quaternary alloy deposited on low-carbon steel by laser cladding were investigated. The clad-coating alloy, a powder mixture of Fe, Cr, W, and C with a weight ratio of 10:5:1:1, was processed with a high-power continuous wave CO[sub 2] laser. The laser processed microstructure comprised fine primary dendrites of a face-centered cubic (fcc) austenitic [gamma] phase and interdendritic eutectic consisting of a network of pseudohexagonal M[sub 7]C[sub 3] carbides rich in Cr in an fcc austenitic [gamma] phase. The interlamellar spacing in the eutectic matrix was about 20 nm. The relatively high microhardness, about 900 kg[sub f]/mm[sup 2], of such fine microstructures is attributed to the formation of complex ternary carbides uniformly distributed in the eutectic matrix. In situ transmission electron microscopy (TEM) of thermally treated clad coatings revealed that transformation of the primary [gamma] phase to body-centered cubic (bcc) ferrite ([alpha] phase) commenced after heating at 843 K for about 7 minutes. Phase change of the interdendritic [gamma] austenite to a bcc [alpha] ferrite occurred after about 30 minutes of hold period at 843 K. Transformation of the M[sub 7]C[sub 3] carbides did not occur even after heating at 843 K for about 3.2 hours.more » The growth of a thin M[sub 2]O[sub 3] (M = Fe, Cr) oxide scale was detected after heating at 843 K for approximately 24 minutes. After cooling gradually to room temperature, the softened (723 kg[sub f]/mm[sup 2]) microstructure consisted of primary dendrites with a bcc [alpha] ferrite crystal structure and interdendritic ternary eutectic of untransformed M[sub 7]C[sub 3] carbides in [alpha] ferrite.« less

Evolution of the structure of ultrafine SiC-laser-formed powders with synthesis conditions

Ultrafine SiC powders in the nanometre range (less than 100 nm) have been synthesized by laser-driven reactions. Mixtures of SiH 4 and C 2H 2 (eventually with He) have been irradiated by a high power continuous-wave CO 2 laser in a cross-flow configuration cell. By varying the reaction conditions (mainly flow rates of the reaction gases) amorphous to crystalline powders of various sizes were formed. The powders were investigated by 29Si solid state nuclear magnetic resonance. This technique, contrary to X-ray diffraction, is mainly sensitive to the local order of small regions of the materials. A comparison of the different spectra obtained provides information on the phase composition, α-SiC or β-SiC of the materials, which seems different for these small powders from what is usually observed.

NMR characterisation of Si-based ultrafine laser formed powders

Ultrafine SiC, Si 3N 4 and Si/C/N powders in the nanometric range (<100nm) have been synthesized by laser driven reactions. Mixtures of SiH 4 C 2H 2 (or CH 3SiH 3), SiH 4 NH 3 and SiH 4 CH 3NH 2 NH 3 have been irradiated by a high power continuous wave CO 2 laser in a crossflow configuration cell. By varying the reaction conditions (flow rates and ratios of the reacting gases, laser power), amorphous to crystalline powders of variable size were formed. The powders were investigated by 29Si and 13C solid state NMR. A comparison of the different spectra obtained provides information on the local structure and composition of the materials.

Laser-fused refractory oxides for optical coatings

Abstract Laser sintering was carried out using a high power continuous-wave CO 2 laser to prepare pellets of zirconia (ZrO 2 ), hafnia (HfO 2 ) and yttria (Y 2 O 3 ) mixed oxides as starting materials in the deposition of optical coatings. Hardened recrystallized pellets appeared to have been formed during laser treatment. X-ray diffraction analysis revealed a monoclinic-to-tetragonal phase transformation in the binary system while the ternary system was found to have a mixture of two crystalline phases. Cross-sectional scanning electron microscopy showed two isothermal crystalline regions in the ternary system. The optical inhomogeneity was low in the films deposited from the laser-fused pellets, but the absorption at a wavelength of 351 nm increased with increasing HfO 2 content. The films deposited from laser-fused pellets were analysed by electron spectroscopy for chemical analysis and found to be stoichiometric and homogeneous.

Microstructure evolution and nonequilibrium phase diagram for Ni-Hf binary alloy produced by laser cladding

Synthesis of nonequilibrium Ni-Hf binary alloys were carried out using laser cladding technique. In this process mixed powder in the ratio of Ni-26 wt% Hf was delivered using a screw feeder into a melt pool of the substrate, generated by a high power continuous wave CO 2 laser beam. The microstructure of the claddings thus produced were investigated using optical, scanning and transmission electron microscopy and X-ray microanalysis techniques. Due to the inherent rapid cooling rate associated with the process of laser cladding process, some nonequilibrium hereto unreported phases formed in the claddings. There is also an extension in the solid solubility of Hf in the terminal α phase as compared to the equilibrium Ni-Hf binary phase diagram. This paper investigates this solid solubility extension, the evolution of the microstructure in the claddings in the system and also characterizes the metastable phases formed in terms of crystal structure and microchemistry. A nonequilibrium phase diagram for Ni-Hf binary alloy is recommended based on the micro-chemistry and differential thermal analysis data.

Effect of laser-processing parameters on the formation and properties of a stellite hardfacing coating

Large-area surface modification using a high power continuous wave CO 2 laser is discussed. The results of the effect of the laser-processing parameters on the properties of the coatings are presented. A hardfacing layer of Stellite 6 alloy (trademark, Cabot Corporation, Kokoma, IN) was melted onto type 304 and type 410 stainless steels by laser energy. Complete fusion and metallurgical bonding could be achieved with proper laser-processing parameters. A typical dendritic structure was observed. Dilution of the coating by the substrate was dependent on the energy density of the laser process and was shown to affect the hardness of the coatings.