Re-melted Layer Research Articles

A novel ultrafast laser ablation fracture polishing method to simultaneously smoothing and improving the surface quality of Al2O3 ceramics with high roughness

Al2O3 ceramics are widely used in industry due to their excellent mechanical and physical properties. However, microcracks, voids and high surface roughness (Sa) on the surface of Al2O3 ceramic parts produced in conventional industrial or additive manufacturing have been one of the technical bottlenecks limiting their high-performance applications. In this paper, a novel method of ultrafast laser ablation fracture polishing (ULAFP) of Al2O3 ceramic parts is proposed for improving the surface quality and enhancing the mechanical properties of the polished surfaces. The ULAFP method is developed with an ultrafast laser, which can be set up as a picosecond (PS) or a femtosecond (FS) laser. The PS laser is used to form a remelting layer on the surface to eliminate surface void defects. Combining laser ablation and laser thermal stress-induced crack extension effects, the FS laser is used to rapidly remove the remelted layer and obtain a surface free of crack defects and finally implement FS laser layer-wise ablation to achieve smoother surface finish. The FS laser energy density is controlled at a critical state close to the ablation threshold of the material to avoid surface defects. This ULAFP can reduce the surface roughness (Sa5.254 μm) polishing of Al2O3 ceramics to 0.80 μm, while controlling the total ablation depth of the polishing process to below 100 μm maintaining the dimensional accuracy of the part and dramatically improving the mechanical properties of the polished surface. Compared with the original surface, the hardness and elastic modulus of the polished surface improved by 104.6% and 63.9%, respectively.

Effects of laser scanning overlap rate on microstructure and properties of laser surface remelting stainless steel

To study the effect of laser scanning overlap rate (LSOR) on the microstructure and properties of 304 stainless steel remelted layer (RL), nanosecond pulsed laser was used to remelt the surface of 304 stainless steel. The micro-morphology, hardness, roughness, adhesion force and corrosion resistance of RL are studied by changing the LSOR. The experimental results show that a layer of oxides mainly composed of Cr, Fe and Mn is formed on the surface of 304 stainless steel and the phase transition from α-Fe to γ-Fe after laser irradiation. Laser surface remelting (LSR) hardens the substrate surface with a hardness of 185 HV and the maximum hardness after remelting is 248.9HV. With the increase of LSOR, the surface roughness gradually increased, the adhesion force first increased and then decreased and the maximum adhesion is 26.1N. The LSOR at the turning point is 80% and the phase distribution of the RL is more uniform at this time. The maximum self-corrosion potential of the RL is −0.283V, which is positively shifted by 0.268V compared to the substrate self-corrosion potential. LSR is a promising technique to improve the surface microstructure and properties of 304 stainless steel.

Optimization of Process Parameters for the Laser Polishing of Hardened Tool Steel.

In mold making, the mold surface roughness directly affects the surface roughness of the produced part. To achieve surface roughness below 0.8 μm, the cost of surface finish is high and time-consuming. One alternative to the different grinding and polishing steps is laser polishing (LP). This study investigates and models the LP of tool steel (X38CrMoV5-1-DIN 1.2343), typical for the mold industry, having an initial rough surface obtained by electrical discharge machining. The microstructures of the re-melted layer and heat-affected zone due to the LP process were also studied. Four parameters: the laser spot size, velocity, maximum melt pool temperature and overlapping were investigated via a design of experiments (DoE) approach, specifically a factorial design. The responses were line roughness (Ra), surface roughness (Sa), and waviness (Wa). The surface topography was measured before and after the LP process by white light profilometer or confocal microscopy. DoE results showed that the selected factors interact in a complex manner, including the interactions, and depend on the responses. The DoE analysis of the results revealed that the roughness is mainly affected by the velocity, temperature and overlap. Based on a first DoE model, an optimization of the parameters was performed and allowed to find optimum parameters for the LP of the rough samples. The optimum conditions to minimize the roughness are a spot size of 0.9 mm, a velocity of 50 mm/s, a temperature of 2080 °C and an overlap of 90%. By using these parameters, the roughness could be reduced by a factor of almost 8 from 3.8 µm to approximately 0.5 µm. Observations of the microstructure reveal that the re-melted layer consists of columnar grains of residual austenite. This can be explained by the carbon intake of the electro-machined surface that helps stabilize the austenitic phase.

Determination of Cooling Rate and Temperature Gradient during Formation of Cathode Spot Craters in a Vacuum Arc

Due to the extreme thermal conditions and short lifetimes, experimental exploration of cathode spots in vacuum arcs is very difficult. The intensive heat in the cathode spot is believed to be generated by ion bombardment and by Joule heating. However, thermal conditions occurring inside the re-melted material in craters created by cathode spots are not accurately known. During the exposure to cathodic arc plasmas, an Al-Cr cathode’s surface was locally melted by successive ignition and extinction of cathode spots. The melted layer, that quickly solidified, was characterized by the formation of several thin layers with a thickness of a few micrometers that were stacked on top of each other. The corresponding solidification patterns displayed cellular and dendritic microstructures. A phase field-based model was used to simulate and determine the thermal process conditions that led to the dendritic structures observed within the re-melted layer. Different combinations of cooling rates and temperature gradients were numerical explored to determine the most probable thermal conditions under which the cathode material re-solidifies. The results showed that the material in the vicinity of the cathode spot crater re-solidified under the condition of a cooling rate of about 3 × 105 K/s and a temperature gradient of about 6 × 107 K/m. These results constitute valuable data for the validation of numerical models dedicated to cathode spot formation.

High temperature oxidation enhanced by laser remelted of CoCrFeNiAlx (x = 0.1, 0.5 and 1) high-entropy alloys

In order to further improve the oxidation resistance of CoCrFeNiAlx (x = 0.1, 0.5, and 1) high entropy alloys (HEAs), the surface of CoCrFeNiAlx prepared by vacuum arc melting is controlled by laser surface remelting (LSM) and the high-temperature oxidation properties of CoCrFeNiAlx before and after LSM is systematically studied. The results revealed that, with the increase of Al content, the parabolic velocity constant kp value gradually decreased after LSM treatment. Moreover, CoCrFeNiAl0.1 formed a continuous Cr2O3 oxide layer, CoCrFeNiAl0.5 formed an outer layer of Cr2O3 oxide film and an inner layer of Al2O3 precipitation, and CoCrFeNiAl1 formed a uniform and dense Al2O3 oxide layer on the entire area. After 100 h of oxidation, the oxide film thickness of three alloys is around 5 µm, 4 µm, and 3 µm, respectively. The improvement in oxidation resistance after LSM can be ascribed to the formation of a dense re-melted layer, uniform distribution of the elements, refinement of the re-melted layer.

Preparation and high-temperature oxidation behavior of a nanoscale remelting layer on TiBw/TC4 composites by electron beam remelting

Electron beam remelting of composites can effectively improve the strength and hardness of the material surface via regulating the geometry and microstructure of grains and reinforcements. In the present work, the microstructure and high-temperature oxidation behavior of the remelted layer of TiBw/TC4 composites with different heat inputs are systematically investigated. During the whole remelting process, the preferentially precipitated TiBw acts as heterogeneous nucleation site to accelerate the nucleation and growth of TC4 alloy, and are finally pushed to the grain boundary on account of the interfacial energy. Compared with untreated samples, many stacking faults are observed in the remelted nanoscale TiB whisker with the crystal structures of B27 and Bf. It follow as that the surface hardness increases by nearly 40% under the line energy of 96 J/mm (4 mA) because of the smaller network structure TiBw than the parent material, as well as a moderate thickness remelting layer without coarse dendrites. After electron beam remelting treatment, the oxidation resistance of the composite was significantly improved due to the segregation of aluminum on the surface of the remelting layer during oxidation.

Influence of silicon carbide mixed used engine oil dielectric fluid on EDM characteristics of AA7075/SiCp/B4Cp hybrid composites

In this study, AA7050/SiC/B4C hybrid composites were machined using the Electric Discharge Machining process, used engine oil as the dielectric fluid to obtain wealth from waste. The experiments were conducted by varying Silicon Carbide (SiC) powder concentration, electrode material (copper and brass), discharge current, pulse on time and reinforcement wt%. The Material Removal Rate (MRR), Tool Wear Rate (TWR), Surface Roughness (Ra) and machined surface hardness were recorded as responses. The inclusion of SiC particles increases the MRR because of the bridging effect, whereas the Ra value was improved due to the complete flushing of the dielectric fluid. Owing to the existence of carbon content in used engine oil, black spots were found on the machined topography. The specimens machined with a brass electrode offer better machining performance as compared with those machined with a copper electrode. The results revealed that the MRR, TWR and Ra upsurge with the intensification of discharge current and pulse on time. Owing to the absence of a re-melted layer, composites with lower machined surface hardness have a higher surface finish. The parameters were optimized using the TOPSIS method, and it was found that under used engine oil dielectric medium with a powder concentration of 3 g l−1, parametric value sets of pulse on time 45 s, current 6 A, and machined with brass electrode offers superior machining efficiency.

Machining Characteristics of Al2O3 Powder Mixed Electric Discharge Machining of AA7050/SiCp/Al2O3p Hybrid Composites

Composite materials are difficult to process under conventional machining techniques. When machined using such unconventional machining methods as the electric discharge process, the result is low machining efficiency due to the presence of reinforced particles. We sought to improve the machining characteristics of AA7050 hybrid composites by adding Al2O3 particles to the dielectric medium. Experiments were carried out using varying powder concentration, current, pulse on time, and weight percentage of composites and material removal rate (MRR), tool wear ratio, and surface roughness were recorded. Composites of different weight percentage were manufactured thru stir casting. SiCp and Al2O3p (mesh size 5 μm) were selected as reinforcing particles. Composites machined in powder mixed electric discharge machining (PMEDM) exhibit better MRR owing to the bridging effect. The modified plasma channel enhances surface quality because of the homogenous dispersal of discharge energy throughout the machined surface. Surface topography was analyzed with scanning electron microscopy. The specimen machined with EDM showed a re-melted layer and globules which were absent under PMEDM machining owing to which surface quality improves. The machining parameters were optimized using the TOPSIS technique and the specimen machined in PMEDM with parametric values 15 A current and 30 μs pulse on time offered the best machining characteristics.

Influence of assist gas on surface quality and microstructure development of laser metal processing

Laser engraving, which is a high-precision and non-contact processing technology, has been widely used to process the difficult-to-cut mould steel. However, the efficiency is still low and the influence of laser engraving on the microstructure and properties of engraved surface also needs further study. By varying the assist gas (Argon, Compressed air, and Oxygen) of nanosecond laser engraving of Cr12MoV mould steel, the material removal rate and surface quality were compared, and the mechanisms of elemental diffusion and microstructural evolution in surface remelting layer were investigated to understand the variation in micro-hardness values. The results showed that the exothermal chemical reactions significantly increased the efficiency of laser engraving, but the surface quality was deteriorated and the detachment of elements C and Cr in remelting layer was promoted. Thus, lower hardness was observed for the remelted layer in the presence of oxygen or compressed air compared to Argon, although the micro-hardness of remelted layers was all increased attributed to grain refinements. Results suggested that the use of oxygen gas during rough engraving increased the material removal rate and reduced the cost, and the use of Argon gas at final engraving steps improved the quality and mechanical properties of the engraved surface.

Properties of a rapidly solidified Ni–Nb layer prepared using a high-current pulsed electron beam

In this study, the surface of a bulk crystalline Ni–Nb powder metallurgy (PM) sample of a glass-forming composition was investigated using high-current pulsed electron-beam (HCPEB) irradiation, which also improved the surface properties. Unlike the original crystalline sample, X-ray diffraction (XRD) of the surface shows an amorphous phase following HCPEB exposure. Transmission electron microscopy (TEM) reveals that, after the Ni–Nb PM samples were exposed to 20 or 30 pulses of HCPEB irradiation, the re-melted layer contains localized nanocrystalline regions within an amorphous matrix. The scanning electron microscopy (SEM) results indicate that, after 30 pulses, a re-melted layer about 5 μm in thickness forms on the surface of the Ni–Nb PM samples. Meanwhile, a large number of cracks and craters were observed on the re-melted layer. In addition, the micro-hardness tests reveal that the micro-hardness improves significantly after HCPEB irradiation. The microhardness of the sample was the highest after the 30 pulses, which was 1.5 times that of the original sample. Furthermore, potentiodynamic polarization tests indicate that the irradiated samples of corrosion resistance were not enhanced compare to the initial samples, due to the formation of cracks providing a Cl− etching channel that accelerates the corrosion rate.

Effects of yttrium on the oxidation behavior of 304 stainless steel with coating by laser remelting in high temperature water

Effects of Y2O3 on the oxidation behavior of 304 SS with coating by laser remelting after exposed to 290 °C water containing 3 ppm O2 for 72 h were investigated. It was found that the corrosion behavior of remelted layer on 304 SS with the addition of Y2O3 was completely different from that of 304 SS and remelted layer on 304 SS without the addition of Y2O3. More Cr2O3 hematite oxides were formed around strengthening phase near the Y2O3 particles. With increasing exposure time, a little number of Fe3O4 spinel oxides were continuing formed on the surface outside the Mn/Si enrichment area. The growth of Fe3O4 spinel oxides were hindered because of the massive Cr2O3 hematite oxides around the Y2O3 particles. Some small Fe3O4 spinel oxides were formed. Finally, the types of oxide film formed on the surface of the remelted layer with the addition of Y2O3 were mainly Cr2O3 hematite oxides and a few of fine FeCr2O4 spinel oxides. The addition of Y2O3 in laser remelting layer can obviously change the corrosion behavior of 304 SS exposed to high temperature water and the related mechanisms are also discussed.

Effects of Yttrium on the pitting corrosion behavior of 304 stainless steel with coating by laser remelting

The remelted layers with or without Y2O3 were prepared for 304 stainless steel (SS) by laser remelting for improving the pitting corrosion resistance. Influences of laser remelting and Y2O3 addition on the microstructure and the pitting corrosion behavior of 304 SS have been investigated. The pitting corrosion resistance of 304 SS can be improved by laser remelting because of the fine grains were formed within the remelted layer of 304 SS after laser remelting. The addition of Y2O3 can effectively refine the grain size and eliminate the defects and inclusions in the remelted layer. The microstructure of the remelted layer was more uniform and compact that can hinder the contact between the aggressive media and the matrix. In this study, the pitting potentials of remelted layer with or without addition of Y2O3 were 0.20 V and 0.16 V respectively, which were both increased than that of the matrix (0.08 V). The polarization resistances of remelted layer with or without the addition of Y2O3 were 38 kΩ and 29 kΩ respectively, which were both increased than that of the matrix (23 kΩ). The addition of Y2O3 in laser remelting layer can obviously improve the pitting corrosion of 304 SS and the related mechanisms are also discussed.

Influence of Preheating on the Microstructure Evolution of Laser Re-Melting Thermal Barrier Coatings/Ni-Based Single Crystal Superalloy Multilayer System.

Laser surface re-melting (LSR) is a well-known method to improve the properties of atmospheric plasma-spraying thermal barrier coatings (APS TBCs) by eliminating the voids, incompletely melted particles and layered-structure. Laser energy density should be carefully selected to reduce the exposed thermal damage of the underlying single crystal (SX) matrix. Therefore, the purpose of this paper was to identify the effect of introducing induction heating to laser modifying of APS TBCs coated on Ni-based SX superalloy. The results indicated that the preheating of the substrate can lower the laser energy threshold that is required for continuously re-melting the coating. It proved that, in LSR processing of a APS TBCs/ SX matrix multilayer system, the combined method of adopting the low laser energy and preheating at elevated temperature is an effective means of minimizing the cracking susceptibility of top ceramic coating, resulting from decreasing the mismatch strain between the re-melted layer and residual APS TBCs, which can significantly improve the segmented crack condition in terms of crack dimension and crack density. Moreover, this combined method can remarkably lower heat input into an SX matrix and correspondingly the interface stored energy induced by pulsed laser thermal shock, which can effectively lower the tendency for surface recrystallization after the subsequent heat treatment.

Material interactions in laser polishing powder bed additive manufactured Ti6Al4V components

Laser polishing (LP) is an emerging technique with the potential to be used for post-build, or in-situ, precision smoothing of rough, fatigue-initiation prone, surfaces of additive manufactured (AM) components. LP uses a laser to re-melt a thin surface layer and smooths the surface by exploiting surface tension effects in the melt pool. However, rapid re-solidification of the melted surface layer and the associated substrate thermal exposure can significantly modify the subsurface material. This study has used an electron beam melted (EBM) Ti6Al4V component, representing the worst case scenario in terms of roughness for a powder bed process, as an example to investigate these issues and evaluate the capability of the LP technique for improving the surface quality of AM parts. Experiments have shown that the surface roughness can be reduced to below Sa = 0.51 μm, which is comparable to a CNC machined surface, and high stress concentrating defects inherited from the AM process were removed by LP. However, the re-melted layer underwent a change in texture, grain structure, and a martensitic transformation, which was subsequently tempered in-situ by repeated beam rastering and resulted in a small increase in sub-surface hardness. In addition, a high level of near-surface tensile residual stresses was generated by the process, although they could be relaxed to near zero by a standard stress relief heat treatment.

Surface Microstructure and High Temperature Oxidation Resistance of Thermal Sprayed NiCoCrAlY Bond-Coat Modified by Cathode Plasma Electrolysis

The surface properties of the air-plasma sprayed bond-coat have been modified by cathode plasma electrolysis (CPE). After modification, a re-melted layer without obvious pores and oxide stringers is formed, the gain size of re-melted layer is approximately 80–120 nm. It is shown, from cyclic oxidation at 1100 °C, that a thin oxide scale mainly composed of α-Al2O3 has been formed on the modified bond-coat and the oxidation resistance of the modified bond-coat has been significantly improved. Such beneficial result can be attributed to following effects: during CPE process, the plasma discharges with high temperature take place on the bond-coat surface. With plasma discharge treatment, the surface is melted and quickly re-solidified, the grain size decreases, and the pores and oxide stringers disappear. During cyclic oxidation, owing to the above modification of surface properties, the critical content of Al for selective oxidation is significantly decreased. Therefore, a continuous Al2O3 scale is formed.

Pulsed plasma deposition of Fe-C-Cr-W coating on high-Cr-cast iron: Effect of layered morphology and heat treatment on the microstructure and hardness

Abstract Pulsed plasma treatment was applied for surface modification and laminated coating deposition on 14.5 wt%-Cr cast iron. The scopes of the research were: (a) to obtain a microstructure gradient, (b) to study the relationship between cathode material and coating layer microstructure/hardness, and (c) to improve coating quality by applying post-deposition heat treatment. An electrothermal axial plasma accelerator with a gas-dynamic working regime was used as plasma source (4.0 kV, 10 kA). The layered structure was obtained by alternation of the cathode material (T1 - 18 wt% W high speed steel and 28 wt% Cr-cast iron). It was found that pulsed plasma treatment led to substrate sub-surface modification by the formation of an 11–18 μm thick remelted layer with very fine carbide particles that provided a smooth transition from the substrate into the coating (80–120 μm thick). The as-deposited coating of 500–655 HV0.05 hardness consisted of “martensite/austenite” layers which alternated with heat-affected layers (layers the microstructure of which was affected by the subsequent plasma pulses). Post-deposition heat treatment (isothermal holding at 950 °C for 2 h followed by oil quenching) resulted in precipitation of carbides M7C3, M3C2, M3C (in Cr-rich layers) and M6C, M2C (in W-rich layers). These carbides were found to be Cr/W depleted in favor of Fe. The carbide precipitation led to a substantial increase in the coating hardness to 1240–1445 HV0.05. The volume fraction of carbides in the coating notably increased relatively to the electrode materials.

Cavitation Erosion of Cermet-Coated Aluminium Bronzes.

The cavitation erosion resistance of CuAl10Ni5Fe2.5Mn1 following plasma spraying with Al2O3·30(Ni20Al) powder and laser re-melting was analyzed in view of possible improvements of the lifetime of components used in hydraulic environments. The cavitation erosion resistance was substantially improved compared with the one of the base material. The thickness of the re-melted layer was in the range of several hundred micrometers, with a surface microhardness increasing from 250 to 420 HV 0.2. Compositional, structural, and microstructural explorations showed that the microstructure of the re-melted and homogenized layer, consisting of a cubic Al2O3 matrix with dispersed Ni-based solid solution is associated with the hardness increase and consequently with the improvement of the cavitation erosion resistance.

Surface and Sub Surface Evaluation in Coated-Wire Electrical Discharge Machining (WEDM) of INCONEL® Alloy 718

Wire Electrical Discharge Machining (WEDM) is one of the most versatile and useful technological processes for cutting complex shapes made of conductive materials such as those typical of aerospace applications. With the aim to optimize process parameters, this paper studies the surface and subsurface modifications of INCONEL® alloy 718 (UNS N07718) machined by WEDM using a Zinc coated brass wire. Machining was performed under roughing and finishing conditions by setting different values of discharge energy and wire feed rate. Surface roughness of the cut surfaces was measured as well as the micro-hardness profile on polished sections. WED-machined surfaces and their cross sections were also observed by Scanning Electron Microscope (SEM) and analyzed by Energy Dispersive X-ray Spectroscopy (EDS) to evaluate possible variations of the surface chemical composition. In this investigation, results are discussed as a function of the feed rate and of the single pulse discharge energy, which is determined by duration, peak current and discharge voltage of the discharge pulse. The research demonstrates that the required surface roughness can be achieved by properly setting the feed rate and the single pulse discharge energy on the WEDM machine. Experimental results also show that under the re-melted layer thickness no significant white layer formation or thermal modification occur, indicating that the chosen set of operating parameters minimizes secondary and unwanted chemical reactions.

Investigation of effect of process parameters on multilayer builds by direct metal deposition

Multilayer direct laser deposition (DLD) is a fabrication process through which parts are fabricated by creating a molten pool into which metal powder is injected as. During fabrication, complex thermal activity occurs in different regions of the build; for example, newly deposited layers will reheat previously deposited layers. The objective of this study was to provide insight into the thermal activity that occurs during the DLD process. This work focused on the effect of the deposition parameters of deposited layers on the microstructure and mechanical properties of the previously deposited layers. It is important to characterize these effects in order to provide information for proper parameter selection in future DLD fabrication. Varying the parameters was shown to produce different effects on the microstructure morphology and property values, presumably resulting from in-situ quench and tempering of the steels. In general, the microstructure was secondary dendrite arm spacing. Typically, both the travel speed and laser power significantly affect the microstructure and hardness. A commercial ABAQUS/CAE software was used to model this process by developing a thermo-mechanical 3D finite element model. This work presents a 3D heat transfer model that considers the continuous addition of mass in front of a moving laser beam using ABAQUS/CAE software. The model assumes the deposit geometry appropriate to each experimental condition and calculates the temperature distribution, cooling rates and re-melted layer depth, which can affect the final microstructure. Model simulations were qualitatively compared with experimental results acquired in situ using a K-type thermocouple.

An investigation of the effect of direct metal deposition parameters on the characteristics of the deposited layers

Multilayer direct laser deposition (DLD) is a fabrication process through which parts are fabricated by creating a molten pool into which metal powder is injected as particles. During fabrication, complex thermal activity occurs in different regions of the build; for example, newly deposited layers will reheat previously deposited layers. The objective of this study was to provide insight into the thermal activity that occurs during the DLD process. This work focused on the effect of the laser parameters of newly deposited layers on the microstructure and mechanical properties of the previously deposited layers in order to characterize these effects to inform proper parameter selection in future DLD fabrication. Varying the parameters showed to produce different effects on the microstructure morphology and property values, leading to some tempering and aging of the steels. The microstructure of the top layer was equiaxed, while the near substrate region was fine dendritic. Typically, both the travel speed and laser power significantly affect the microstructure and hardness. Using the commercial ABAQUS/CAE software, a thermo-mechanical 3D finite element model was developed. This work presents a 3D heat transfer model that considers the continuous addition of powder particles in front of a moving laser beam using ABAQUS/CAE software. The model assumes the deposit geometry appropriate to each experimental condition and calculates the temperature distribution, cooling rates and re-melted layer depth, which can affect the final microstructure. Model simulations were qualitatively compared with experimental results acquired in situ using a K-type thermocouple.