High Power Diode Laser Cladding Research Articles

Analysis and modeling of melt pool morphology for high power diode laser cladding with a rectangle beam spot

Dynamic behavior of a melt pool reflects the internal heat transfer, convection and mass addition in laser cladding, and it is closely related to the complex geometry and quality of the cladding layer. A machine vision system was built to directly detect and measure the melt pool morphology in wide-track laser cladding by high power diode laser (HPDL) with a rectangle beam spot (RBS). The captured images were processed with image processing technology to obtain the geometry profile and the characteristic parameters of the melt pool such as width (Wm), length (Lm) and area (Am). A full factorial experiment with pre-placed powder method was conducted to obtain the correlation between the geometrical characteristics and the process parameters. The effect of the process parameters including laser power, powder thickness and scanning speed on the Wm, Lm and Am were analyzed in detail. A nonlinear fitting model was used to fit the relationship between the process parameters and the geometrical characteristics. And a mathematically defined curve was adopted to describe the melt pool geometry profile. The above model was confirmed by all the experiments. The results indicated that the Wm, Lm and Am will increase with increasing laser power, while increasing scanning speed will cause the decreasing of the melt pool size. Moreover, the Wm and Lm can be predicted by the process parameters, and the melt pool contour profile can be well described as the curve.

Effect of the remelting scanning speed on the amorphous forming ability of Ni-based alloy using laser cladding plus a laser remelting process

Amorphous coatings exhibit prominent properties, including high hardness and good wear resistance properties. In this study, Ni-based amorphous composite coatings with a composition of (Ni0.6Fe0.4)68B18Si10Nb4 were prepared on a mild steel substrate via high power diode laser cladding plus laser remelting processes. The microstructure of the coating was studied using X-ray diffraction (XRD) and a scanning electron microscope (SEM). The effect of the remelting scanning speed on the amorphous forming ability of the Ni-based alloy was studied systematically by experimental and numerical simulation methods. The results indicated that there was no amorphous phase formed when the scanning speed was 4m/min and 5m/min. The amorphous phase was observed when the remelting scanning speed was 6m/min. The amorphous phase volume fraction increased with an increase in the remelting scanning speed. The fraction was found to be approximately 64% when the remelting speed was 8m/min. The simulation results indicate that the increase in the remelting scanning speed may lead to lower residence time and higher cooling rate of the melted pool. As a result, high remelting scanning speed can be concluded to be a necessary condition for the formation of the amorphous phase when using the laser cladding plus laser remelting process.

Corrosion behavior of Hastelloy C22 coating produced by laser cladding in static and cavitation acid solution

The Hastelloy C22 coatings on Q235 steel substrate were produced by high power diode laser cladding technique. Their corrosion behaviors in static and cavitation hydrochloric, sulfuric and nitric acid solutions were investigated. The electrochemical results show that corrosion resistance of coatings in static acid solutions is higher than that in cavitation ones. In each case, coating corrosion resistance in descending order is in nitric, sulfuric and hydrochloric acid solutions. Obvious erosion-corrosion morphology and serious intercrystalline corrosion of coating are noticed in cavitation hydrochloric acid solution. This is mainly ascribed to the aggressive ions in hydrochloric acid solution and mechanical effect from cavitation bubbles collapse. While coating after corrosion test in cavitation nitric acid solution shows nearly unchanged surface morphology. The results indicate that the associated action of cavitation and property of acid solution determines the corrosion development of coating. Hastelloy C22 coating exhibits better corrosion resistance in oxidizing acid solution for the stable formation of dense oxide film on the surface.

Effect of mechanical polishing on corrosion behavior of Hastelloy C22 coating prepared by high power diode laser cladding

Hastelloy C22 coatings were prepared on mild steel by high power diode laser cladding technique. In order to investigate the effect of mechanical polishing on corrosion behavior, the coatings with original surface (coating C1) and new surfaces after mechanical polishing away of 0.3mm (coating C2) and 0.6mm (coating C3) in depth were studied. Their microstructures and compositions were measured, and corrosion resistance in seawater was studied. The planar, cellular and dendritic solidifications were found from the coating/substrate interface to the top surface of coatings. In coating C1, smallest size primary solidification containing mainly Ni and largest quantity of eutectic networks composed of Cr, Mo and W are dominant, and harmful carbide M(Cr, W)2C is found. While Mo6Ni6C phase exists only in coatings C2 and C3. The open circuit potential and electrochemical impedance spectroscopy of coating C1 are higher within two days, and then lower later than the other two coatings. Coatings C2 and C3 display larger stable radius of capacitive loop and lower corrosion current density owing to higher contents of Cr and Mo, as well as no harmful carbide at surface, which reveals higher corrosion resistance and lower corrosion rate than coating C1. Consequently, properly mechanical polishing of original coating surface is beneficial for improving its corrosion resistance.

High power diode laser cladding of Fe–Co–B–Si–C–Nb amorphous coating: Layered microstructure and properties

Fe–Co–B–Si–C–Nb amorphous composite coating about 0.6mm in thickness was fabricated by using a one-step laser cladding method. Microstructures and phases were investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and electron probe microanalysis. Microhardness and wear resistance tests were conducted to evaluate the mechanical properties of this coating. Near the interface of coating/substrate, the region showed a layered microstructure, which could be generally categorized into three layers: layer I (columnar dendrites phase), layer II (equiaxed dendrites phase) and layer III (amorphous-particle composite phase). The main reason for this layered structure was due to the difference in chemical compositions of the three layers. With regard to mechanical properties, the microhardness and wear resistance of the Fe–Co–B–Si–C–Nb amorphous composite coating also exhibited layered characteristics. The mean value of the microhardness for layer I, layer II and layer III was 729, 680 and 1245HV, respectively. The friction coefficient of the transitional layer III was 0.28 times lower than that of the substrate under the same sliding friction condition.

Structure and corrosion resistance properties of Ni–Fe–B–Si–Nb amorphous composite coatings fabricated by laser processing

Ni-based amorphous composite coatings with a composition of Ni40.8Fe27.2B18Si10Nb4 have been prepared on a mild steel substrate by high power diode laser cladding plus laser remelting processes. The microstructure of the coating was studied by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). Polarization tests and electrochemical impedance spectroscopy (EIS) measurements were employed to study corrosion behavior of the coatings in 3.5% NaCl solution. The corrosion resistance properties of bulk metallic glasses (BMGs) ribbon with same nominal composition and 316L stainless steel were also measured for comparison. It was found that the laser remelted coatings exhibit dense amorphous structure matrix embedded by some crystal phases. Due to the existence of grain boundaries, the Ni-based amorphous composite coating exhibits worse corrosion resistance than BMGs ribbon. While the laser remelted coating shows better corrosion resistance properties than 316L stainless steel and laser cladded coating. A capacitive loop and a straight line are shown in the Nyquist plot, indicating that a Warburg element can be drawn in the equivalent electrical circuit representative of the laser remelted coating.

Effect of Heat Treatment on Niobium Segregation of Laser-Cladded IN718 Alloy Coating

An IN718 alloy coating was prepared by high-power diode laser cladding. 980STA standard heat treatment and direct aging (DA) were employed to improve the properties of this coating. The niobium segregation in the as-deposited coating and the heat-treated coating had been investigated using a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDAX). The results showed that 980STA standard heat treatment improved the microhardness of the coating significantly, and Laves concentration was reduced from 30.6 vol pct to about 11.4 vol pct after 980STA. The niobium concentration in Laves of the 980STA-treated coating was higher than that of DA-treated coating and as-deposited coating. Only a small portion of niobium in the coating was precipitated in the form of γ″ during the 980STA heat treatment. The rest of niobium was the alloying element for solid-solution strengthening and the constituent element of Laves. The niobium segregation facilitated the formation of Laves.

Amorphous structure evolution of high power diode laser cladded Fe–Co–B–Si–Nb coatings

Fe–Co–B–Si–Nb coatings were fabricated on the surface of low carbon steel using high power diode laser cladding of [(Fe0.5Co0.5)0.75B0.2Si0.05]95.7Nb4.3 amorphous powders at three different scanning speeds of 6, 17 and 50m/s. At each scanning speed, laser power was optimized to obtain low dilution ratio. Scanning electron microscopy, X-ray diffraction, transmission electron microscopy with energy dispersive spectrometer and electron probe micro analysis were carried out to characterize the microstructure and chemical composition of the cladded coatings. Differential scanning calorimetry was also carried out to investigate the fraction of the amorphous phase. The results showed that dilution ratio and scanning speed were the two main factors for fabricating Fe–Co–B–Si–Nb amorphous coating by high power diode laser cladding. Low dilution ratio was crucial for the formation of amorphous phase. When the dilution ratio was low, the fraction of amorphous phase in the cladded coatings increased upon increasing the scanning speed.

Microstructure and wear properties of laser processed Ni based amorphous matrix coatings

Ni40·8Fe27·2B18Si10Nb4 (at-%) coating was deposited on mild steel substrate using high power diode laser cladding followed by laser remelting process. The phase composition and microstructures of the as cladded coating and remelted coating were analysed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The microhardness and wear properties of the coatings were also tested and analysed. It was observed that the remelted coating has an amorphous matrix embedded with some dendrites. The remelting of the coating increased the hardness of as cladded coating by ∼25%. The remelted coating exhibited a characteristically lower friction coefficient (0·21) than the as cladded coating (0·27) and the substrate (0·31). Significant increase in wear resistance was found for the remelted coating compared with the as cladded coating and the substrate under identical conditions.

Dilution effect on the formation of amorphous phase in the laser cladded Ni–Fe–B–Si–Nb coatings after laser remelting process

Ni–Fe–B–Si–Nb coatings have been deposited on mild steel substrates using high power diode laser cladding. Scanning laser beam at high speeds was followed to remelt the surface of the coatings. Different laser cladding powers in the range of 700–1000W were used to obtain various dilution ratios in the coating. The dilution effect on the chemical characterization, phase composition and microstructure is analyzed by energy dispersive spectroscopy, X-ray diffraction and scanning-electron microscopy. The microhardness distribution of the coatings after laser processing is also measured. The results reveal that Ni-based amorphous composite coatings have successfully been fabricated on mild steel substrate at low dilution ratio when the cladding power was 700W, 800W and 900W. While at high laser power of 1000W, no amorphous phase was found. The coatings with low dilution ratio exhibit the highest microhardness of 1200HV0.5 due to their largest volume fraction of amorphous phase.

Modeling of the Off-Axis High Power Diode Laser Cladding Process

Off-axis high power diode laser (HPDL) cladding is commonly used for surface quality enhancement such as coating, part repairing, etc. Although some laser cladding models are available in literature, little has been reported on the modeling of powder flow and molten pool for a rectangular beam with side powder injection. In this article, a custom-designed flat nozzle delivers the powder material into a distinct molten pool formed by a HPDL with a rectangular beam. A powder model is first presented to reveal the powder flow behavior below the flat nozzle. Key parameters such as nozzle inclination angle, rectangular beam profile, shielding gas flow rates, and powder feed rate are incorporated so that spatial powder density, powder velocity, and temperature distribution are distinctly investigated. Then in order to describe thermal and fluidic behaviors around the molten pool formed by the rectangular beam, a three-dimensional self-consistent cladding model is developed with the incorporation of the distributed powder properties as input. The level set method is adopted to track the complex free surface evolution. Temperature fields and fluid motion in the molten pool area resulting from the profile of rectangular beam are distinctly revealed. The effect of continuous mass addition is also embedded into the governing equations, making the model more accurate. A HPDL cladding with little dilution is formed and the simulated results agree well with the experiment.

High power diode laser cladding

The characteristics of new high power diode lasers make them ideal for several materials processing operations. One such operation being laser cladding. A 1.2 kW diode laser has been used to successfully clad ferrous metals with 25% Cr, 10% Ni deposits. The effect of laser power, cladding speed, powder delivery rate and method were investigated. The resulting clad layers were examined and characterised using optical and electron microscopy, hardness testing and x-ray analysis in order to determine the quality of the clad. Results indicated that extremely good clad layers were produced which were free from porosity and well bonded to the substrate material. In a single pass, the clad area was also significantly larger than those produced by other types of laser. Additionally, the high quality of the clad was also indicated by the very low levels of dilution of clad material and the fact that the metallurgical structure of the clad was finer than that achieved with conventional cladding operations. The overall conclusions of this work were that cladding with the high power diode laser is an extremely effective process, even compared to other laser cladding alternatives, and that large area, high quality clad deposits can be successfully deposited.