Laser Surface Melting Research Articles

Effect of laser surface melting pretreatment on the growth behavior and mechanical properties of microarc oxidation coating on Ti6Al4V alloy

Porous TiO2 ceramic coatings with high contents of Si and P were fabricated on a Ti6Al4V alloy using hybrid processing of laser surface melting (LSM) and microarc oxidation (MAO). The effect of LSM pretreatment on the growth behavior and mechanical properties of the MAO coating was studied. The results showed that the MAO coating on LSM pretreatment Ti6Al4V (LSM-MAO) had quite different surface morphologies, thickness, and compositions compared to the MAO coating on as-received titanium alloy. Grain refinement and dislocation in the LSM-treated Ti6Al4V alloy provided extra energy and more numbers of discharge channels during the MAO process. LSM–MAO coating exhibited a more uniform pore distribution, relatively small pores, and larger thickness compared to the MAO coating. The LSM-MAO coating had higher hardness and better wear resistance than the MAO coating. LSM is deduced to be an appropriate and efficient pretreatment method for achieving a high-quality MAO coating on titanium alloy.

Effect of laser surface melting on surface integrity of Al−4.5Cu composites reinforced with SiC and MoS2

Abstract Two types of composites were prepared with Al−4.5Cu alloy as a matrix using stir casting method. One was reinforced with 10 wt.% of SiC and 2 wt.% of MoS2. The other was reinforced with 10 wt.% of SiC and 4 wt.% of MoS2. Their surfaces were remelted using a CO2 laser beam with an objective to study the influence of laser surface melting (LSM). The topography, microhardness, corrosion resistance and wear resistance of the laser melted surfaces were studied. Overall surface integrity after LSM was compared with as-cast surface. LSM enhanced the microhardness and wear resistance of the surface in each case. Porosity of the laser melted surface was low and corrosion resistance was high. Thus, LSM can be conveniently applied to enhancing the surface integrity of the aluminium composites. However, there is an optimum laser specific energy, around 38 J/m2 in this study, for obtaining the best surface integrity.

Effects of laser surface melting on crystallographic texture, microstructure, elastic modulus and hardness of Ti−30Nb−4Sn alloy

Abstract The biocompatibility of orthopedic implants is closely related to their elastic modulus and surface properties. The objective of this study was to determine the effects of cold rolling, recrystallization and laser surface melting (LSM) on the microstructure and mechanical properties of a biphase (α″+β) Ti−30Nb−4Sn alloy. X-ray diffraction (XRD) texture analysis of the cold-rolled substrate revealed the [302]α″//ND texture component, while analysis of the recrystallized substrate showed the [302]α″//ND and [110]α″//ND components. The β-phase texture could not be directly measured by XRD, but the presence of the [111]β//ND texture component was successfully predicted by considering the orientation relationship between the α″ and β phases. Nanoindentation measurements showed that the elastic modulus of the cold-rolled substrate (63 GPa) was lower than that of the recrystallized substrate (74 GPa). Based on the available literature and the results presented here, it is suggested that this difference is caused by the introduction of crystal defects during cold deformation. The combined nanoindentation/EBSD analysis showed that the nanoindentation results are not affected by crystal orientation. LSM of the deformed alloy produced changes in hardness, elastic modulus and crystallographic texture similar to those produced by recrystallization heat treatment, creating a stiffness gradient between surface and substrate.

Influences of laser surface melting on microstructure, mechanical properties and corrosion resistance of dual-phase Cr–Fe–Co–Ni–Al high entropy alloys

In this paper, a laser surface melting (LSM) method was utilized on CrFeCoNiAlx (x = 0.5, 0.6, 0.7) high entropy alloys (HEAs) with FCC + BCC structure, and the effects of LSM on their microstructure, mechanical properties and corrosion behavior were systematically investigated. It is intriguing that, a layer with BCC + B2 structure formed on the surface of CrFeCoNiAl0.6 and CrFeCoNiAl0.7 substrates after LSM, while a monolithic FCC layer formed on that of CrFeCoNiAl0.5 substrate under the same condition. LSM influences the mechanical properties of CrFeCoNiAlx HEAs to varying degrees including hardness and tensile properties decided by the phase formation in laser-treated surfaces and the substrates. In addition, compared to the substrates, obvious increase in self-corrosion potential and pitting potential were found in the laser-treated surface layers of CrFeCoNiAl0.6 and CrFeCoNiAl0.7. Interestingly, the corrosion resistance of laser-treated surface layers has positive correlation with Al content, while that of the substrates shows negative correlation with the concentration of Al, which is possibly resulted from the difference in corrosion mechanism. This work provides a new route for significantly improving surface properties of dual-phase HEAs, which would be very beneficial to their further application as advanced materials.

Real–time prediction of penetration depths of laser surface melting based on coaxial visual monitoring

Laser surface melting (LSM) is an important laser processing technique that can be used in laser conduction welding, laser remelting, and defect elimination. Penetration depths are essential to determine LSM qualities. To enhance the ability to predict penetration depths, a real-time prediction method based on coaxial visual characteristics and the convection state of a molten pool is reported. First, the coaxial visual characteristics of the molten pool are analyzed, and its image is divided into convection, transition, and boundary regions. Based on these region characteristics, the convection state can be indicated. Then, the relationships among process parameters, coaxial visual characteristics, and penetration depths are presented. Finally, we attempt to predict the penetration depths based on these coaxial visual characteristics. The results show that the convection region characteristics are more sensitive to the changes in process parameters. The oscillation amplitude of the convection region can be considered monitored objects to indicate the convection state. The convection region can predict the penetration depths better than the entire molten pool. Furthermore, this work will be beneficial to understanding the experimental phenomenon and enhance the monitoring ability of other laser processing techniques, such as laser alloying, laser cladding, and laser-based additive manufacturing.

Effect on corrosion behaviour of the surface of aluminium 4.5 Cu with bamboo leaf ash composites by laser treatment

Aluminium composites have various applications in engineering thanks to their greater strength with lightweight, formability and ductility. However, the properties of the Aluminium composites such as wear, corrosion, hardness, and fatigue resistance are critical at near-surface regions. In this present research work, the effect on the surface of Aluminium 4.5Cu with Bamboo Leaf Ash composite by laser surface melting was done, wherein the fabrication was done by stir-casting with bamboo leaf ash reinforced particles with 2 and 4 weight percentage. By using the laser machine, the composite’s surface was melted. Microstructure and corrosion behaviour of composites before surface melting and after laser surface melting was inspected and the outcomes were compared. It was perceived that, the diminishment of porosity in the grain refinement. The corrosion resistance of composites had improved remarkably by Laser Surface Melting. The Corrosion mechanism was analysed by observing the morphology of the corroded surface of composites. Henceforth the Laser Surface Melting will be applied appropriately to boost the surface properties of aluminium composites.

Laser-melted surfaces fabricated on MAO films in Al and Si electrolytes

In order to improve the functions of microarc oxidation (MAO) films, the current work investigated the effects of the laser surface melting (LSM) process on MAO films fabricated in different electrolytes. A series of processing experiments was carried out on the composite films. In detail, MAO films using aluminum (Al) and silicon (Si) electrolytes were prepared on a Ti–6Al–4V alloy surface and then treated by using the LSM process. At the same time, the properties of the films, such as phases and morphology, were characterized and analyzed by X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results showed that moderate-intensity plasma was more easily obtained on the silicon MAO (Si-MAO) films than on the aluminum MAO (Al-MAO) ones. Similarly, a higher laser power was necessary to liquefy or gasify alumina (Al2O3). Compared with the defective structure of Al-MAO film after LSM, the structure of the Si-MAO film was almost free of defects. The film morphologies showed that an MAO film with low porosity and a smooth surface can be obtained after LSM treatment using proper parameters. In conclusion, the composite silicate system process using the silicon electrolyte had better modification performance.

Fiber Laser Surface Melting of a NiTi Superelastic Alloy: Influence on Structural and Mechanical Properties

The surface melting of a NiTi superelastic alloy using a high-power laser Yb:Fiber was investigated. The influence of this process on the microstructural and mechanical properties was also examined. The reference material was a 3 mm nitinol strip with a homogeneous austenitic B2 phase. For the laser surface melting process, input fluences were applied from 17.5 to 45 J/mm2. The morphology of the structure and the chemical composition of several regions were determined by optical microscopy, scanning electron microscopy, dispersive energy spectra, and X-ray diffraction techniques. The mechanical properties, such as modulus of elasticity and hardness, were determined using nanoindentation and microindentation techniques. The greatest surface finishing of the fusion zone was observed for the condition 35 J/mm2. Three well-defined regions (fusion zone (FZ), heat-affected zone (HAZ), base metal (BM)) could be observed and dimensions of grain size, width, and depth of the melted pool were directly affected by the laser fluence. The geometry of the molten pool could be controlled by the optimization of the laser parameters. High laser fluence caused preferential volatilization of nickel, dynamic precipitation of intermetallic phases, including Ti2Ni, Ni3Ti, and Ni4Ti3, as well as solubilization of TiC in the matrix, which led to grain refinement. Thus, high laser fluence is a suitable technique to enhance mechanical properties such as hardness and Young’s modulus.

Effect of Laser Surface Melting on Microstructure and Performance of Super 13Cr Stainless Steel

Effect of Laser Surface Melting on Microstructure and Performance of Super 13Cr Stainless Steel

Strengthening of the Surface Properties of Al-4.5%Cu/BLA Composites by Laser Applications

Aluminium composites were used widely due to merits like high strength to weight ratio, electrical and thermal conductivity. In this paper, the influence of the Laser Surface Melting on exterior surface of Al-4.5% Cu alloy metal matrix composite reinforced with 2 and 4 wt.% of Bamboo Leaf Ash particles produced by stir casting. The composite’s surface was melted using a laser machine. The microstructure and the hardness of as-cast and laser melted surface of composites were studied. The results of the composites have been compared before and after laser treatment. The grain size on surface of composites was reduced after laser treatment and also observed that porosity was minimized. The hardness of surface composites was enhanced desirably after laser treatment. Thus, re-melting the surface by laser can be suitably useful to strengthen the surface properties of the Aluminium composites.

Stiffness and hardness gradients obtained by laser surface melting of an aged β-Ti alloy

Components with variable stiffness have been attracting interest because of their potential for use in biomedical implants. Here, a β-titanium alloy aged under different conditions was subjected to laser surface melting (LSM). The high cooling rate typical of LSM avoided α-phase precipitation in the fusion zone (FZ) and heat-affected zone (HAZ) as the α precipitates were solid-solutioned into the β phase, producing a surface layer with full β phase over the aged substrate and reduced stiffness and hardness. The decrease in stiffness occurred independently of the aging condition used while the decrease in hardness was greater for the sample aged at the lower temperature as this had finer precipitates. A route involving aging at this lower temperature and LSM is proposed for use when a graded material combining a less rigid surface with the original core stiffness and strength is required, as in orthopedic implants.

Study of Optical Properties of Surface Layers Produced by Laser Surface Melting and Laser Surface Nitriding of Titanium Alloy.

This study measured optical properties, such as specular, diffuse, and total reflection for 808 nm wavelength, characteristic for high power diode lasers radiation, from the surface of titanium alloy Ti6Al4V at delivery conditions, polished, and oxidized. Moreover, the optical properties of surface layers produced by high power direct diode laser (HPDDL) melting and nitriding were determined. Additionally, a methodology for determining the value of absorption for 808 nm wavelength of the HPDDL radiation on the surface of a melt pool during laser surface melting and nitriding of titanium alloy was proposed. The results show that the distinct differences in absorption affect the heat transfer, thermal conditions of laser heating and thereby the penetration depth during laser melting and nitriding of the titanium alloy.

The effect of surface treatment on the corrosion behavior of a pure Ta sheet in an equimolar NaCl–KCl melt at 850°C in air. Part 1: Laser surface melting

AbstractThe corrosion behavior of pure Ta and laser surface melting–treated Ta sheets was investigated in an equimolar NaCl–KCl melt at 850°C in air. The results show that the predominant factors involved in the consumption of materials were reactions that form sodium‐containing oxides and Ta2O5. High corrosion stress led to slip of the untreated Ta surface grains, further resulting in preferential corrosion along slip bands. The slip of surface grains substantially deteriorated the bonding between the corrosion scale and matrix, thus leading to rapid degradation of the corrosion scale. The melt layer consisted of a thin Ta2O5 film, and the inner dendrite layer demonstrated self‐healing characteristics during corrosion and presented a better hot corrosion resistance than the untreated Ta owing to the excellent bonding at the scale and melt layer interface.

Influence of laser surface melting on the properties of MB26 and AZ80 magnesium alloys

In the present investigation, laser surface melting (LSM) was performed on MB26 and AZ80 magnesium alloys and their surface properties were evaluated. The finite element method (FEM) was used to calculate the resulting temperature during the laser surface melting process. The melted layer average thickness values were about 296 and 190 μm for MB26 and AZ80 alloys, respectively and the variation in the thickness is mainly attributed to the absorption in laser energy. The average grain size of the LSMed MB26 alloy was decreased about 14.5 times and AZ80 had very finer grains than the MB26 along with the fine distribution of second phases. Further, the X-ray diffraction results revealed that the second phase intensities were decreased in both the alloys due to their dispersion. The laser absorption efficiency of MB26 was higher due to the higher plasma attachment during LSM process, thereby has higher melt layer thickness compared to AZ80. The refined microstructure of the melted layer resulted in an increase of micro-hardness up to 110 Hv. Potentiodynamic polarization test results revealed that the corrosion current density (icorr) values of the LSMed MB26 and AZ80 alloys were decreased about 1.8 and 2.5 times, respectively compared to the as-received alloys. A variation in solidification rates of the melt pool due to the alloying elements were attributed to an improvement in the surface and electrochemical properties.

Surface modification of aluminum alloys with carbon nanotubes by laser surface melting

Carbon nanotubes (CNTs) could be an excellent reinforcement for metal matrix composites, specifically for composites with aluminum or aluminum alloy matrix. Surface modification to improve hardness and other material properties has been performed by laser surface melting (LSM) process, where the laser beam melts the substrate together with alloying elements or reinforcing phase additives. In this work, Multiwalled CNTs (MWCNTs) were mixed by the electrostatic adsorption process with aluminum powder and the resulting mixed powder was laser melted on the surface of a 6061-aluminum alloy substrate. As a result, a modified substrate surface has been obtained from the Al/MWCNT – substrate co-melting, dilution and re-solidification processes. This modified layer was obtained by different LSM parameters and were characterized by Optical (OM) and Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), Field Emission Gun Scanning Electron Microscopy (FEG-SEM), X-Ray Diffraction (XRD) and Raman Spectroscopy (RS). The resulting mechanical behavior was evaluated by Vickers microhardness tests. The results showed that the presence of MWCNT in the aluminum powder improves the laser energy absorption, leading to the formation of a deeper modified layer, with segregation of Si particles homogeneously dispersed and improving the hardness. The formation of Al4C3 was not observed, evidencing that the MWCNTs did not react with the molten Al matrix during the LSM process.

An experimental investigation of surface integrity in selective laser melting of Inconel 625

Inconel 625 is a Ni-based superalloy widely used in nuclear and aerospace applications because of its high strength and corrosion resistance. The current paper investigates selective laser melting of Inconel 625 using a wide range of process parameters: four laser powers, five scan speeds, and three hatch spacings. Cube coupons are produced and their end properties are measured, specifically, relative density, surface roughness, microhardness, and surface residual stresses. Process maps are constructed to correlate processing parameters to the part surface integrity, and the possible underlying causes are highlighted. Experimental results show that highly dense parts can be produced using selective laser melting and low average surface roughness can be obtained in both the scan and hatch directions. Hatch spacing is the most prominent factor to attain relative densities above 99% while high laser powers are key to lower the average surface roughness. Microhardness and microstructure examination are done for a subset of the process parameters and found to not significantly change with laser power. Surface residual stresses are measured on the top surface in the scan and hatch directions and process maps are developed. There is no particular parameter that solely affects surface residual stresses, despite several cases where the increase in laser power reduced the surface residual stresses. 90% of measurements showed surface tensile residual stresses except for a few cases that resulted in surface compressive residual stress.

Effect of Thermal Fatigue on Microstructure and Mechanical Properties of H13 Tool Steel Processed by Selective Laser Surface Melting

Selective laser surface melting, which brings together the bionic theory and the laser process, is an effective way to enhance the thermal fatigue behavior of materials. In this study, in order to examine the relationship between the mechanical properties and thermal fatigue behavior of materials processed by selective laser surface melting, the tensile properties at room temperature and elevated temperature of treated specimens and untreated specimens after different numbers of thermal fatigue cycles were investigated and compared. Moreover, the microstructure evolution and the microhardness of the laser-affected zone were investigated after different numbers of thermal fatigue cycles. The results show that microhardness of the laser-melted zone gradually decreases with an increasing number of thermal fatigue cycles; the number of thermal fatigue cycles has little effect on the grain size in the laser-melted zone, and the percentage of low-angle grain boundaries decreases with an increasing number of thermal fatigue cycles. The strength of specimens gradually decreases, whereas the fracture elongation gradually increases with an increasing number of thermal fatigue cycles at room temperature and elevated temperature. In addition, the stress distribution on the specimen surface during tensile test was investigated using the finite element method, and the results indicate that the stress transfer exists between the laser-affected zone and the untreated zone.

The effect of laser surface melting on grain refinement of phase separated Cu-Cr alloy

Grain refinement and homogenization of Cr phase were achieved by laser surface melting (LSM) method, and the properties of Cu-Cr alloy were significantly improved. In this study, LSM of Cu-50Cr alloy (wt.%) was conducted with a high power density (106–107 W/cm2) laser beam, the microstructure and the properties of melt layer were investigated. The size of Cr phase was effectively refined from hundreds of micron scale to several micron scale, and the average size of Cr particles decreased to a few hundred nanometers. High cooling rate effectively inhibited coarsening effect on the Cr particles during liquid phase separation. Spherical Cr particles were dispersed in the melt layer with a thickness of 165 ± 20 μm. Microhardness was obviously enhanced and the maximum hardness was 232HV, which was 2.8 times that of the substrate. Arc duration of the LSM treated contacts increased up to 18%. The withstanding voltage of the fixed and the moving contact increased to 28.7% and 35.4%, respectively. The results show that LSM is an effective method to refine the microstructure of Cu-Cr alloy, and it is a promising modification method for electrical Cu-Cr vacuum contacts.

Grain Refinement of Hypereutectic Immiscible Cu-50Cr Alloy during Rapid Melting and Solidification Induced by High Power Density Laser Beams

The microstructure of hypereutectic immiscible Cu-50Cr alloy (wt%) was effectively refined and homogenized by a high power density Nd:YAG continuous laser beam (power density 104–105 MW/m2). The average grain size of Cr was effectively refined from ~100 μm to ~1 μm, and some of the Cr particles even decreased to a few hundred nanometers. The high cooling rate (7.29 × 106 K/s) effectively inhibited the coarsening effect on Cr particles during liquid phase separation (LPS). The spherical Cr particles were preferably dispersed in the melt layer, and the thickness of the layer was up to ~150 μm. The refinement and dispersion of the Cr phase contributed to improving the interruption capability of the Cu-Cr contacts. Compared with the untreated samples, the arc duration and the withstanding voltage of the laser surface melting (LSM) treated contacts with refined microstructure increased to 21% and 33%, respectively. The results demonstrated that the LSM method was an effective approach to optimize the microstructure of Cu-Cr alloy, which made it a promising modification method for Cu-Cr vacuum contact applications.

Additively manufactured calcium phosphate reinforced CoCrMo alloy: Bio-tribological and biocompatibility evaluation for load-bearing implants

Cobalt-chromium-molybdenum (CoCrMo) alloys are widely used in load-bearing implants; specifically, in hip, knee, and spinal applications due to their excellent wear resistance. However, due to in vivo corrosion and mechanically assisted corrosion, metal ion release occurs and accounts for poor biocompatibility. Therefore, a significant interest to improve upon CoCrMo alloy exists. In the present work we hypothesize that calcium phosphate (CaP) will behave as a solid lubricant in CoCrMo alloy under tribological testing, thereby minimizing wear and metal ion release concerns associated with CoCrMo alloy. CoCrMo-CaP composite coatings were processed using laser engineered net shaping (LENS™) system. After LENS™ processing, CoCrMo alloy was subjected to laser surface melting (LSM) using the same LENS™ set-up. Samples were investigated for microstructural features, phase identification, and biocompatibility. It was found that LSM treated CoCrMo improved wear resistance by 5 times. CoCrMo-CaP composites displayed the formation of a phosphorus-based tribofilm. In vitro cell-material interactions study showed no cytotoxic effect. Sprague-Dawley rat and rabbit in vivo study displayed increased osteoid formation for CoCrMo-CaP composites, up to 2 wt.% CaP. Our results show that careful surface modification treatments can simultaneously improve wear resistance and in vivo biocompatibility of CoCrMo alloy, which can correlate to a reduction of metal ion release in vivo.