Laser Remelting Process Research Articles

Microstructural evolution of the amorphous layers on Mg-Zn-Ca alloy during laser remelting process

Mg-Zn-Ca bulk metallic glasses (BMGs) have been regarded as new biodegradable materials for orthopedic implants, while size constrains limit their biomedical applications. In this study, Mg-based amorphous layers with the nominal composition of Mg65Zn30Ca5 have been successfully produced using the laser remelting method. The influences of laser scanning speed on the microstructural characteristics of Mg-Zn-Ca amorphous layers during laser remelting process were investigated systematically by experiments and numerical simulations. Amorphous phase could be formed when the laser scanning speed was kept above 1000 mm/min. The remelted layer mainly consisted of a gradient structure: amorphous-crystalline composite layer, transitional layer and crystalline substrate. The increase of the laser scanning speed was benefit to improve the corrosion resistance of the remelted layer due to the formation of amorphous structure. Analysis of the temperature distributions and related thermal profiles simulated using the finite element method indicated that the crystallization could be suppressed with the increase of the laser scanning speed, which would shorten the residence time and leads to higher cooling rate in this process as well.

Friction wear property of laser surface processed Ni-based amorphous alloy coatings

A Ni–Fe–B–Si–Nb amorphous alloy was deposited on a steel substrate surface via a laser cladding process, and a laser cladding plus laser remelting process. The wear behavior of the laser processed samples and the bulk metallic glass (BMG) sample with the same nominal composition were tested using a pin-on-disc type testing machine. The nano-mechanical properties of the samples were measured with a nano-characterization system. The friction wear tests showed that deep grooves and wear debris were formed on the worn surface of the laser cladded coating, while only shallow grooves for the laser remelted coatings. The friction coefficients of laser remelted coatings and BMG were lower than the laser cladded coating. The wear mass losses of the laser remelted coating were less than the BMG when the laser remelting scanning speed was higher than 6 mm/min. The nano-hardness and elastic modulus of the remelted coating is higher than that of the laser cladded coating. Also, they increase with the increasing laser scanning speed with 1227.9 HV and 277.4 GPa when the remelting scanning speed is 8 m/min. Based on the nano-indentation and friction wear tests results, it was found that the friction wear properties of the laser cladded coating, laser remelted coatings and BMG related well to the ratio of H3/E2. A higher value of H3/E2 can lead to a better wear resistance property.

Effect of tempering on laser remelted AISI H13 tool steel

The effect of tempering temperature on surface hardened AISI H13 tool steel by laser remelting process using an Yb-fiber laser has been investigated. Single remelting tracks were produced in argon environment at different laser power and scan speed in the range of 400–600 W and 200–1600 mm/min respectively, maintaining the laser spot diameter fixed at 3 mm. Their effects on geometrical aspects, microstructure and microhardness were analyzed considering the thermal history of molten pool recorded using an IR pyrometer. Microstructure was found to be associated with cooling rate and melt pool lifetime, estimated from the temperature signal. Thereafter, remelted surface with 30% overlapping tracks was generated and subjected to 1 h tempering cycle at different temperatures in 500–900 °C range followed by hardness and wear tests. Hardness was retained fully up to 500 °C and significant softening occurred at 600 °C and 700 °C. At 800 °C and higher temperatures, effects of laser remelting were impaired completely, but substrate got hardened through martensite formation in a conventional manner. Wear resistance followed the trend of hardness. Changes in microstructure and formation of various phases were found to be the reasons behind the modifications in hardness.

Quantitative cellular automaton model and simulations of dendritic and anomalous eutectic growth

A quantitative cellular automaton (CA) model for dendritic and anomalous eutectic growth has been developed. Solid/liquid(SL) interface curvature plays an important role on the dendritic and eutectic growth kinetics. In order to improve the accuracy of calculated interface curvature, a height function method was used in this research. Simulation results showed that the precision of the interface curvature has been significantly improved. And the quantitative ability of present CA model has also been improved through the simulations of equilibrium shapes. Using the quantitative CA model, directional solidification of dendritic and eutectic growth has been investigated, as well as the anomalous eutectic growth during laser remelting process. For anomalous eutectic, it is found that the very fine lamellar eutectic growing at much smaller cooling rate is the reason to observe the lamellar to anomalous transition.

Cellular Automaton Simulation of the Growth of Anomalous Eutectic during Laser Remelting Process.

Anomalous eutectic morphologies were observed during laser remelting of a Ni-Sn powder bed, where it was sandwiched between a lamellar eutectic at the bottom of melt pool. That is, the anomalous eutectic growth mechanism can be divided into two processes: one is the lamellar to anomalous transition (LAT); the other is the anomalous to lamellar transition (ALT). The thermal distribution at the bottom of melt pool is simulated by the finite difference method. It is found that the cooling rate at the bottom of melt pool is a linear function of time. A cellular automaton (CA) model is developed to simulate the anomalous growth. Simulation results show that the mechanism of the LAT is that one phase overgrows the other followed by subsequent nucleating of the other phase. The mechanism of the ALT is the competitive growth between the anomalous and lamellar eutectic; as the cooling rate increased, the lamellar eutectic is more competitive.

Evolution of solidification microstructure and dynamic recrystallisation of Inconel 625 during laser solid forming process

This study investigated the evolution of solidification microstructure and dynamic recrystallisation (DRX) during the laser solid forming of the Ni-based Inconel 625 superalloy. The as-deposited microstructure mainly showed epitaxially grown columnar grains with fine equiaxed grains between them. These fine equiaxed grains were formed by the discontinuous DRX (DDRX) and continuous DRX (CDRX) processes, which were induced by the cyclic thermal stress resulting from the repeated laser deposition. The bulging of pre-existing grains and sub-grain rotation were the main mechanisms of the DDRX and CDRX phenomena, respectively. Additionally, after the occurrence of DRX, the dislocations were released and there was no distortion in the recrystallised grains. Coarse equiaxed grains were present in the top zone of the deposit; these grains were formed by the columnar-to-equiaxed transition during the solidification of the molten pool after the end of the laser re-melting and deposition process.

Enhanced micro-hardness and wear resistance of Al-15Si/TiC fabricated by selective laser melting

Abstract High relative density (>98%) Al-15Si alloy samples were fabricated by selective laser melting (SLM) with laser re-melting process. The micro-hardness and wear resistance of Al-15Si alloy samples were investigated in this study. The addition of 5 wt% TiC improved the micro-hardness of SLM-produced Al-15Si alloy, which had a remarkable micro-hardness of 172.8 Hv. After the heat treatment (annealing or quenching), the hardness of all tested samples decreased significantly, while the Al-15Si/TiC composites with the laser re-melting kept the higher hardness values (145 Hv). This sample presented the lower coefficient of friction (0.42) and wear rate (2.75 × 10 -5 mm3 N-1 m-1) than other conditions. Based on the results, the addition of 5 wt% TiC with laser re-melting in SLM process, together with annealing treatment afterwards, was a feasible approach to obtain a high micro-hardness and good wear resistance of Al-15Si alloys.

Effect of Bionic Unit Shapes on Solid Particle Erosion Resistance of ZrO2–7wt%Y2O3 Thermal Barrier Coatings Processed by Laser

Inspired by the coupling phenomena in biological systems, to improve the solid particle erosion resistance of Thermal Barrier Coatings (TBCs), different kinds of bionic units were made on the coating surfaces using Bionic Coupled Laser Remelting (BCLR) process. The NiCoCrAlYTa/ZrO2–7wt%Y2O3 double-layer structured TBCs were prepared by air plasma spraying. The microstructure, microhardness and phase composition of the as-sprayed and bionic specimens were examined. The solid particle erosion behaviors of bionic specimens as function of bionic unit shape were investigated. The results indicated that the bionic specimens had better erosion resistance than the as-sprayed specimen. The specimen with striation and grid bionic units had the better erosion resistance, while the dot showed the worse. The bionic units were characterized by the dense columnar crystal structure and the high hardness, which are the main reasons for improving the erosion resistance. Under the synergistic action of the shear stress and normal stress on the protrusive coating surface, the erosion failure of the as-sprayed TBCs was proved to be the fracture and spallation of the splats. By contrast, the spallation of segmented bionic unit occurred in the overlapping area between the adjacent laser irradiation, and the erosive unit surface presented the clear and deep furrows, which revealed that the erosion failure mechanism of bionic TBCs was dominated by brittle and some ductile erosion. These results showed more opportunities for bionic application in improving the solid particle erosion resistance of components in the windy and sandy environment.

Effect of cooling rate on residual stress and mechanical properties of laser remelted ceramic coating

Mild steel substrates were coated with commercially available alumina and chromia powders using the powder flame spraying process. The top layers of the flame sprayed coatings were remelted using a 2 kW fiber laser. Thermo-cycles of the laser remelting process were monitored on-line using an infrared pyrometer. Cooling rates were varied using different laser scanning speeds. Surface morphology, microstructure and phases of the laser treated and as-sprayed coatings were investigated using optical microscopy, scanning electron microscopy, X-ray diffraction and X-ray tomography. Surface residual stress of the as-sprayed and laser treated coatings was measured using X-ray diffraction. The inherent defects like porosity and inter-lamellar boundary diminish to a great extent upon laser remelting. Surface residual stress of the remelted coatings tends to increase with increase in cooling rate. Surface crack density of the laser treated coating was reduced appreciably when coatings were preheated prior to laser remelting.

Mechanical and Microstructural Evolutions of the Laser Deposited Titanium Layers under Various Shielding Atmospheres

Direct laser melting process is a kind of prototyping process whereby a 3-D part is built layer-wise by melting the metal powder with laser scanning. The properties of laser melted layer are found to be strongly dependent upon the types of shielding gas used. In this study, the effects of shielding gases on the mechanical and microstructural properties of the deposited titanium layers have been investigated. The laser remelting process has also been implemented at various shielding atmospheres to investigate the changes in surface roughness and hardness of deposited layers. The characterization of laser processing parameters, such as laser power, scan rate, gas flow rate, powder layer thickness, beam spot size and hatch distance, is proved to be useful in controlling the mechanical and microstructural properties of the deposited layer.

Application of laser remelting process on the Zr-Cu based alloy composite

In this study, a 1.0 mm thick (Zr48Cu32Al8Ag8,Ta4)Si0.75 alloy composite (AC) plate was laser remelted (LR) by employing the Nd:YAG laser technique. To fully remelt the plate, a two-sided welding process with preselected parameters was employed. The microstructure, mechanical (microhardness), thermal properties, and corrosion resistance (potentiodynamic polarization) of the laser remelted alloy composite (LR-AC), the Zr-Cu AC (with the crystalline structure), and the Zr-Cu based bulk metallic glass composite (BMGC, the same composition as the AC. The amorphous matrix is produced by a rapid cooling rate) were then investigated and compared.The test results show that after the laser remelting processes, the LR-AC consisted of an amorphous matrix and reinforced phase, Ta, in the remelted areas, which is similar to the BMGC; however, the volume fractions and grain size of the micro-sized reinforced phase, Ta, in the LR-AC were reduced. When compared to those of the BMGC, the GFA indices of LR-AC were slightly improved. For the hardness tests, LR-AC showed a similar hardness to that of the BMGC; however, a higher value was obtained when compared to that of the AC. Furthermore, the LR-AC exhibits a lower Icorr (a slower corrosion rate), which is attributed to the amorphous matrix and less micro-sized Ta content, eliminating corrosion occurring between the Ta and the matrix of the alloy.

Influence of laser re-melting and vacuum heat treatment on plasma-sprayed FeCoCrNiAl alloy coatings

FeCoCrNiAl high entropy alloy coatings were prepared by supersonic air-plasma spraying. The coatings were post-treated by vacuum heat treatment at 600 and 900 °C, and laser re-melting with 300 W, respectively, to study the influence of different treatments on the structure and properties of the coatings. The phase constitution, microstructure and microhardness of the coatings after treatments were investigated using X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. Results showed that the as-sprayed coatings consisted of pure metal and Fe-Cr. The AlNi3 phase was obtained after the vacuum heat treatment process. A body-centered cubic structure with less AlNi3 could be found in the coating after the laser re-melting process. The average hardness values of the as-sprayed coating and the coatings with two different temperature vacuum heat treatments and with laser re-melting were 177, 227, 266 and 682 HV, respectively. This suggests that the vacuum heat treatment promoted the alloying process of the coatings, and contributed to the enhancement of the coating wear resistance. The laser re-melted coating showed the best wear resistance.

A cellular automaton simulation of W–Ni alloy solidification in laser solid forming process

An alloy cellular automaton (CA) model is developed for the microstructure simulation in directional solidification and laser solid forming (LSF) process. The CA model's capture rule was modified by a limited neighbor solid fraction (LNSF) method. A multiscale two-dimensional model is presented for simulating laser remelting process, which is the same as LSF without the addition of metallic powders into melt pool. The metallurgy process in melt pool was simulated, including temperature distribution, pool shape and solidification of microstructure. The microstructure evolution of tungsten–nickel alloy (W–Ni) during LSF is also simulated by present CA model.

Effect of unit size on thermal fatigue behavior of hot work steel repaired by a biomimetic laser remelting process

AISI H13 hot work steel with fatigue cracks was repaired by a biomimetic laser remelting (BLR) process in the form of lattice units with different sizes. Detailed microstructural studies and microhardness tests were carried out on the units. Studies revealed a mixed microstructure containing martensite, retained austenite and carbide particles with ultrafine grain size in units. BLR samples with defect-free units exhibited superior thermal fatigue resistance due to microstructure strengthening, and mechanisms of crack tip blunting and blocking. In addition, effects of unit size on thermal fatigue resistance of BLR samples were discussed.

The Fabrication of Porous Si with Interconnected Micro-Sized Dendrites and Tunable Morphology through the Dealloying of a Laser Remelted Al-Si Alloy.

Coral-like porous Si was fabricated through the dealloying of a laser remelted as-cast AlSi12 alloy(Al-12 wt % Si). The porous Si was composed of interconnected micro-sized Si dendrites and micro/nanopores, and compared to flaky Si, which is fabricated by direct dealloying of the as-cast AlSi12 alloy. The structure of the porous Si was attributed to the dendritic solidification microstructure formed during the laser remelting process. The micropore size of the porous Si decreased from 4.2 μm to 1.6 µm with the increase in laser scanning velocity, indicating that the morphology of porous Si could be easily altered by simply controlling the laser remelting parameters. The coral-like porous Si provided enough space, making it promising for high-performance Si-based composite anode materials in lithium-ion batteries. The proposed hybrid method provides a straightforward way of tuning the porous structure in the dealloyed material.

Improvements in Microstructure and Wear Resistance of Plasma-Sprayed Fe-Based Amorphous Coating by Laser-Remelting

Amorphous coating technology is an attractive way of taking advantage of the superior properties of amorphous alloys for structural applications. However, the limited bonds between splats within the plasma-sprayed coatings result in a typically lamellar and porous coating structure. To overcome these limitations, the as-sprayed coating was treated by a laser-remelting process. The microstructure and phase composition of two coatings were analyzed using scanning electron microscopy with energy-dispersive spectroscopy, transmission electron microscopy, and x-ray diffraction. The wear resistance of the plasma-sprayed coating and laser-remelted coating was studied comparatively using a pin-on-disc wear test under dry friction conditions. It was revealed that the laser-remelted coating exhibited better wear resistance because of its defect-free and amorphous-nanocrystalline composited structure.

Effects of laser remelting process on the microstructure, roughness and microhardness of in-situ cold sprayed hypoeutectic Al-Si coating

In this paper, Al-6vol.% Si coating was in-situ manufactured by cold spraying from powder mixture of pure Si powder in fraction 25vol.% with a pure gas-atomized Al powder, mean particle sizes being of 16μm and 41μm respectively. Nitrogen was chosen as the spraying and carrier gas to avoid oxidation. The image analysis results show that the Si content in the deposits (6vol.%) is much lower than that in the feedstock powder (25vol.%). Due to the high roughness and low bond strength between the Al matrix and the Si particles, a laser remelting process was used to improve these shortages. Results show that the remelted surface layer presents a fine microstructure and a high microhardness. X-ray diffraction analysis indicates that the Al peaks move to the right side (large lattice parameter) after laser remelting process. It could be attributed to two factors: (1) formation of supersaturated Al (Si); (2) large tensile residual stress, due to the high cooling rate. In addition, the laser remelted samples exhibit a lower surface roughness than the corresponding as-sprayed samples.

Improved corrosion resistance of air plasma sprayed WC-12%Co cermet coating by laser re-melting process

WC-12%Co cermet coating was deposited on AISI 321 stainless steel by air plasma spraying. The coated samples were re-melted by CO2 laser beam at 100mm/min travel speed. The microstructural features after laser re-melting treatment demonstrated an appreciable decrease in the concentration of micro-defects. The significant improvement in the corrosion resistance and barrier properties of the laser re-melted coating was also evident from potentiodynamic polarization and impedance spectroscopy analyses. The laser re-melted coated samples had a lower corrosion current density (2.1μA/cm2) and higher charge transfer resistance (1.4kΩ-cm2) in combination with high coating resistance (40.30kΩ-cm2).

The Study of High Temperature Oxidation Performance of Thermal Barrier Coatings Prepared by Plasma Spraying and Laser Remelting

The CoCrAlYTaSi/YSZ thermal barrier coating was prepared on the surface of GH586 by plasma spraying and laser remelting process. The coating prepared by plasma spraying, YSZ have no phase transformation. After remelting, YSZ also have no phase transformation, but the grain of the preferential growth orientation changed. Plasma spraying coating is mainly composed of t and t' phase, all the t phase transform into t' phase and recrystallization after remelting. After remelting, the substrate grain and γ' phase size are obviously grown up, leading to the decrease of high temperature oxidation resistance of substrate. In the process of oxidation, no obvious phase transformation are taken place in the plasma spraying coating, and the coating after laser remelting, tetragonal-monoclinic phase transformation happened.

Restrained TGO growth in YSZ/NiCrAlY thermal barrier coatings by modified laser remelting

Abstract Thermal barrier coatings (TBCs) are widely applied in high temperature gas turbines and jet engines to protect the superalloy components. Due to oxygen transfer through topcoat and the oxidation of bond-coat alloy, a thin layer of thermally grown oxide (TGO) will generate between the ceramic topcoat and the metallic bondcoat of TBCs after long-term operation. Cracking and spalling are often observed at the topcoat-TGO and TGO-bondcoat interfaces since the thickening of TGO layer may lead to strong tensile stress. Previously, laser remelting process has been developed as an effective way in restraining the TGO growth. In the present research, preheating laser remelting and sol-gel repairing laser remelting were respectively employed to further decelerate the TGO growth by reducing the number density and average size of vertical cracks in the topcoat. After 100 h isothermal oxidation, the TGO growth rate of the modified laser remelted specimens are much lower than the conventional laser remelted specimens, which shows the potentiality of preheating and sol-gel repairing as effective promotions of laser remelting process in restraining the TGO growth.