Laser Surface Melting Research Articles

The effect of laser surface melting on the retained austenite and wear properties of AISI D2 tool steel

In this study the effect of laser surface melting (LSM) on the amount of retained austenite, hardness and wear resistance of AISI D2 steel was investigated utilizing hardness test, X-ray diffraction (XRD), pin on disk wear test and scanning electron microscopy (SEM). In order to evaluate wear resistance, pin on disk tests at two different loads were carried out on surface treated samples including as received annealed and a quenched and tempered specimen. Surface treatment has been carried out in three different conditions; LSM, LSM with pre-heating at 300 °C and LSM with pre-cooling in liquid nitrogen which resulted in 64, 42% and 90% of retained austenite respectively, revealing that higher cooling rate led to a higher amount of retained austenite. The results showed that in spite of the presence of austenite as major phase in re-melted surface, the hardness reached to the values of up to 600 HV10 which is approximately two times of the as received steel having a lot of carbides dispersed in the ferrite matrix. In addition, at lower load, conventionally heat treated sample exhibited better wear resistance, whereas at higher load, the wear rate of laser melted specimen was the least as a result of the transformation of retained austenite into martensite. At lower load the dominant wear mechanism was abrasion while Adhesion was identified as the main wear mechanism at higher load.

Effect of simultaneous application of ultrasonic vibrations during laser surface melting on electrochemical properties of 2024 aluminum alloy

In the present investigation, effect of laser melting with and without simultaneous application of ultrasonic vibrations on electrochemical properties of 2024 aluminum alloy is investigated. The electrochemical behavior of the laser melted specimens was studied using open-circuit and potentiodynamic polarization measurements. Subsequently, investigation of the corrosion films was performed using scanning electron microscopy. It was observed that the laser melted specimens exhibited significant improvement in open circuit potential (both with and without simultaneous ultrasonic vibration application). The open circuit potential of the laser melted specimen without ultrasonic vibrations was observed to be more stable compared to the laser melted specimen with ultrasonic vibrations. It was observed that the corrosion mechanism undergoes a transition from pitting to uniform corrosion in the laser treated specimen, particularly in the laser melted specimen without ultrasonic vibrations. However, the extensive agitations due to ultrasonic vibrations in the melt pool appear to restrict Cu migration to grain boundaries that result in porous and relatively inefficient passive layer formation in laser processed samples with application of ultrasonic vibrations. This behavior was also observed in the potentiodynamic polarization studies that showed that the laser melted specimen without ultrasonic vibrations exhibited lower corrosion current and corrosion rate compared to the laser melted specimen with ultrasonic vibrations as well as the as received substrate. Copyright © 2017 VBRI Press.

Tuning strength-ductility combination on selective laser melted 316L stainless steel through gradient heterogeneous structure

We report a strategy of gradient heterogeneous structure (GHS) architecture to achieve a superior strength-ductility combination in additively manufactured (AM) 316L stainless steel prepared by selective laser melting (SLM) and ultrasonic severe surface rolling (USSR). The GHS is a microstructure gradient system, which is composed of a gradient structured outer layer and a hierarchically heterostructured interlayer. The gradient structured outer layer exhibits a variation in grain size, phases, dislocations, twins, and cellular structure with depth and has a large thickness of approximately 1000 µm. The average grain size is approximately 93.9 nm in the topmost surface layer and increases with increasing depth from the treated surface. The hierarchically heterostructured interlayer has the heterogeneous grain structure, cellular structure, high-density dislocations, and nanoprecipitates, with length scales spanning several orders of magnitude. Due to the high microstructural heterogeneity arising from the combination of two types of heterogeneous structures, the GHS strengthens the synergistic effect, and thus results 316L stainless steel with a superior strength and high ductility. Furthermore, the mechanical properties of the GHS 316L stainless steel can be tuned by tailoring the microstructure: the larger the volume fraction of the gradient structured outer layer is, the higher the strength, but the lower the ductility. Molecular dynamics simulations indicate that deformation twinning, in addition to dislocation slipping, is an important deformation mechanism for nanocrystalline 316L samples at a large strain and thus plays an important role in maintaining strain hardening and ductility. We also discuss the potential application of USSR to AM parts to improve their mechanical properties and surface integrity. Our work demonstrates the potential of the proposed highly heterogeneous microstructure design strategy to prepare high-performance AM parts.

Laser processing of high chromium white iron

Mechanical properties, such as wear-resistance and hardness, of laser welded and cladded high chromium white iron were investigated. The study involves the laser welding, cladding, and laser surface melt treatment by 3-kW Nd:YAG continuous wave high power laser. The laser welding of 2-mm thick high chromium white iron samples was laser welded in two combinations: iron-to-iron and iron-to-steel. Strong metallurgical bonding was witnessed between not only the iron samples but also iron and steel samples by laser welding. Three types of powder materials were used to laser clad the samples: metal-ceramic (compositionally close to INCO-702), stellite-21, and stellite-1 in order to estimate mechanical property changes and process ability of high chromium white iron. Even though the microstructure of metal-ceramic coating shows worse than stellite powder coatings it has more hardness and wear resistant property which were comparable to base iron. Hardness of metal-ceramic coating was slightly lower than base iron, yet the wear resistance was increased twofold.

Mechanical property and strengthening mechanism on DH36 marine steel after laser surface melting

In this paper, the mechanical properties and microstructure of various laser power surface melting (LSM) on DH36 steel were studied, the temperature gradient (G), cooling rate (η) and solidification (R) were simulated by modified Rosenthal solution and the strengthening mechanism for different laser power was also elucidated. The depth of the coatings exhibited a hike when increasing laser power (P), which is 1.24 mm, 1.41 mm, 1.70 mm and 2.47 mm, respectively. The G and η monotonously increased while the R was independent of laser power. The results showed the LSM process significantly improved the surface mechanical properties. The maximum yield strength and tensile strength were 495.7 MPa and 615.5 MPa. Microhardness of coatings decreased along the z direction with the maximum value is 448 Hv for 1000 W considering the appearance of acicular ferities (AF) in the coating. It indicated mainly adhesive wear for high laser power treatment with a shallow wear depth. Two kinds of strengthening mechanism were observed: second phase strengthening for 1000 W treatment and solid solution strengthening for 2500 W treatment. In the process of low laser power treatment, nanoscale cementies appeared intragranular resulted from the inhabited carbon diffusion. The coherent boundary of 022¯TiC∥022¯Cementite, [233]TiC∥[155]Cementite between TiC and nanoscale cementies reduced the nucleation barrier to promote the nucleation of AF. As for high laser power treatment, micro-alloy elements in DH36 steel traveled through oxidation reaction as large size oxide compounds in grain and on grain boundary.

EFFECT OF LASER POST-TREATMENT ON MICROSTRUCTURAL AND SLIDING WEAR BEHAVIOR OF HVOF-SPRAYED NiCrC AND NiCrSi COATINGS

In this study, NiCrC and NiCrSi coatings are deposited on the MDN 310 steel using High-Velocity Oxy-Fuel (HVOF) process. Laser Surface Melting (LSM) post-heat treatment is carried out on as-sprayed coatings using Laser Engineered Net Shaping (LENSTM) with a power of 300[Formula: see text]W. The characteristics of both coatings in terms of mechanical and metallurgical properties have been investigated. The thicknesses of the as-sprayed NiCrC and NiCrSi coatings are in the range of 170–200[Formula: see text][Formula: see text]m. Laser-treated NiCrC and NiCrSi coatings exhibit a thickness range of 162–185[Formula: see text][Formula: see text]m, respectively. The microstructure of laser-treated NiCrC-300W coating clearly shows a dendrite-like structure, whereas the laser-treated NiCrSi coating exhibits hard layer and columnar homogeneity. Microhardness of as-sprayed NiCrC coating is [Formula: see text] [Formula: see text] and that of NiCrSi coating is [Formula: see text] HV[Formula: see text]. Microhardness of laser-treated NiCrC coating is [Formula: see text] HV[Formula: see text] and that of NiCrSi coating is [Formula: see text] HV[Formula: see text]. Dry sliding wear tests are conducted at room temperature (RT) and 400∘C with 10-N and 20-N loads. The wear rates at 400∘C temperature of laser-treated NiCrC and NiCrSi coatings produced are slightly below [Formula: see text][Formula: see text]mm3/m and [Formula: see text][Formula: see text]mm3/m, respectively. Laser-treated coatings produced better dry sliding wear behavior compared with as-sprayed coatings owing to dense microstructure. Formation of SiC phase in NiCrSi coating imparts high wear and frictional resistance compared to the NiCrC coating.

Corrosion and cavitation erosion resistance enhancement of cast Ni–Al bronze by laser surface melting

Corrosion and cavitation erosion resistance enhancement of cast Ni–Al bronze by laser surface melting

Fabrication of flapping-wing micromechanism assembly using selective laser melting and aerodynamic performance measures

The development of a flapping wing microaerial vehicle mechanism with a high strength-to-weight ratio to withstand high flapping frequency is of significant interest in aerospace applications. The traditional manufacturing methods such as injection moulding and wire-cut electrical discharge machining suffer from high cost, labour intensiveness, and time-to-market. However, the present disruptive additive manufacturing technology is considered a viable replacement for manufacturing micromechanism components. Significantly to withstand high cyclic loads, metal-based high strength-to-weight ratio flapping wing microaerial vehicle components are the need of the hour. Hence, the present work focused on the fabrication of flapping wing microaerial vehicle micromechanism components using selective laser melting with AlSi10Mg alloy. The manufactured micromechanism components attained 99% of dimensional accuracy, and the total weight of the Evans mechanism assembly is 4 g. The scanning electron microscopy analysis revealed the laser melting surface characteristics of the Al alloy. The assembled mechanism is tested in static and dynamic environments to ensure structural rigidity. Aerodynamic forces are measured using a wind tunnel setup, and 7.5 lift and 1.2 N thrust forces are experienced that will be sufficient enough to carry a payload of 1 g camera on-board for surveillance missions. The study suggested that the metal additive manufacturing technology is a prominent solution to realize the micromechanism components effortlessly compared to conventional subtractive manufacturing.

Study of melt pool geometry and solidification microstructure during laser surface melting of Inconel 625 alloy

The present study aims to comprehend thermo-fluid conditions during laser surface melting (LSM) of Inconel 625 (IN625) alloy using experimental and numerical modelling approaches. Nine tracks were melted on an IN625 plate at different laser powers and scan speeds. Melt pool geometry and grain morphology were evaluated using microscopy techniques. A 3-D finite volume model based on heat conduction solidification equation (HCS model) was used to simulate LSM process. Further, HCS model was expanded to include effects of fluid dynamics (HCS-FD model). Both the numerical models were used to predict melt pool geometry, peak temperatures, temperature gradients and cooling rates. The error in predictions of melt pool geometry from the HCS-FD model was lower when compared to the HCS model. The velocity vectors show a strong surface tension driven flow which has resulted in narrow and deeper melt pools in agreement with the cross sectional images of the melted tracks. Further, solidification characteristics were interpreted to obtain inferences about grain size and morphology.

Laser surface melting of nickel-based alloy reduces nickel release in the primary cooling system of a nuclear power plant

The surface of Ni-based alloy 690 was treated using a sub-microsecond pulsed laser in order to reduce the amount of nickel released when the surface is exposed to the primary cooling system of pressurized water nuclear reactors. A 2D array of laser treatment parameter sets was investigated. The results on sample surfaces was characterized using interferometric microscopy, Scanning Electron Microscopy (SEM), Glow Discharge Optical Emission Spectrometry (GDOES) and X-ray Photoelectron Spectrometry (XPS). The treatment leading to a continuous and defect-free chromine surface having the minimum nickel content over about the first two nanometers and no subsurface chromium depletion was selected for the nickel release test. This selection criterion proved to be very efficient as the total amount of nickel released in a standard qualification test using a simulated primary coolant was reduced, compared to a non-treated surface, by a factor of 7 during the heating phase and by a factor of 3.7 on average over the whole month-long test.

Effect of Microstructural Bands on the Localized Corrosion of Laser Surface-Melted 316L Stainless Steel

The localized corrosion of laser surface melted (LSM) 316L stainless steel is investigated by a combination of potentiodynamic anodic polarization in 0.1 M HCl and microscopic investigation of the initiation and propagation of localized corrosion. The pitting potential of LSM 316L is significantly lower than the pitting potential of wrought 316L. The LSM microstructure is highly banded as a consequence of the high laser power density and high linear energy density. The bands are composed of zones of changing modes of solidification, cycling between very narrow regions of primary austenite solidification and very wide regions of primary ferrite solidification. Pits initiate in the outer edge of each band where the mode of solidification is primary austenite plane front solidification and primary austenite cellular solidification. The primary austenite regions have low chromium concentration (and possibly low molybdenum concentration), which explains their susceptibility to pitting corrosion. The ferrite is enriched in chromium, which explains the absence of pitting in the primary ferrite regions. The presence of the low chromium regions of primary austenite solidification explains the lower pitting resistance of LSM 316L relative to wrought 316L. The influence of banding on localized corrosion is applicable to other rapidly solidified processes such as additive manufacturing.

Coarsening of martensite with multiple generations of twins in laser additively manufactured Ti6Al4V

Generation of inherently complex thermokinetics and thermomechanical conditions during the laser powder bed fusion-based additive manufacturing (LPBF-AM) process makes it challenging to understand the evolution of internally twinned martensite structure in Ti6Al4V. In view of this, the present study employed the finite element method based multiphysics thermokinetic and thermomechanical model in complement with experimental observations via scanning electron microscopy and transmission electron microscopy to study this occurrence. In order to understand the transient effects of thermokinetics and thermomechanics during multi-laser track treatment of LPBF-AM on evolution of the morphological features of martensite phase and twins, separate experiments involving laser surface melting with single, double, and triple laser tracks and conventional heat treatment (solutionized above β transus temperature of 1323 K followed by quenching in water at 298 K) of wrought Ti6Al4V were conducted. The major fraction of martensite laths were thinner in the water-quenched Ti6Al4V sample compared to LPBF-AM Ti6Al4V. The gradual coarsening and distortion of martensite lath seemed to occur with successive thermal cycles of single, double, and triple laser track treatments. In addition multiple generations of crystallographic twins were detected in LPBF-Ti6Al4V, which were rarely detected in solutionized and water quenched wrought Ti6Al4V. The multiple generations of twins were also distinct features of the single laser track treated wrought Ti6Al4V. The evolution of these morphologically and crystallographically diverse martensite phase structures were correlated with the computationally predicted thermokinetic and thermomechanical conditions during in-layer multi-laser track and multi-layer thermal treatments experienced during LBPF-AM process in the present study.

Effect of Grain Refinement by Laser Surface Treatment on Cell Adhesion Behavior of AZ91D Magnesium Alloy

Abstract Laser surface melting (LSM) has been widely reported for corrosion protection to magnesium alloys. However, magnesium alloys are much better known as implant materials; thus, in this work, cell adhesion behavior of AZ91D magnesium alloy by LSM treatment was investigated. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), contact angle measurements, hydrogen evolution tests, and cell culture study were used to characterize the microstructure, composition, surface energy, biodegradability, cell adhesion behavior, and cytocompatibility, respectively. Results showed that the biodegradation rate decreased, the surface energy (42.9 ± 1.3 mJ/m2, N = 10) was higher than that of the as-received one (23.1 ± 1.2 mJ/m2, N = 10), and the average surface roughness value (Ra) increased from 0.16 ± 0.02 μm to 2.53 ± 0.03 μm after LSM treatment. The laser-surface-treated magnesium alloys presented good cytocompatibility, and the cell adhesion on the surface of magnesium alloy was improved because of the higher surface energy, lower degradation, and increased roughness.

Microstructural Evolution upon 10-wt% Mo Alloying and Laser Surface Melting of M2 High Speed Steels Hardfacings

The article aims to comprehend the microstructural changes, in Plasma Transfer Arc (PTA) deposited M2 high speed steel (HSS) hardfacings upon incorporation of 10 wt% Mo alloying during deposition followed by laser surface melting. PTA deposited hardfacings were produced over 4140 steel. Then Mo alloyed and unalloyed PTA deposits were subjected to laser surface melting (LSM) process. A comprehensive microstructural characterization for all the resultant structures was carried out. Optical metallography using appropriate etching reagents and SEM microscopy in conjunction with XRD techniques were employed to ascertain the matrix structure and carbides morphology. The PTA microstructure was close to equilibrium structure of M2 HSS containing mixture of ferrite/austenite/martensite along with MC, M2C and M6C type carbides. While the LSM of M2 HSS caused higher fraction of martensite and finer grains in the structure resulting in increment in hardness. 10-wt% Mo addition changes the carbides from MC and rod like M2C to fibrous M2C and fishbone like M6C carbides. The LSM of Mo alloyed M2 HSS PTA deposits led to an overall decrease in the fraction of M6C carbides and fibrous M2C carbides accompanied by a decrease in hardness.

Rapid Solidification and Surfaces Alloying Using Continuous CO<sub>2</sub> Laser to Generate Ni-Aluminide on Steel

Rapidly solidified thin micro-alloyed surface layers are generated by laser melting of plasma thermal sprayed steel surfaces. Samples of carbon steel are plasma sprayed with fine nickel and aluminum powders. Laser surface melting generated a thin localized molten pool of metal with steep horizontal thermal gradient. The latter triggered intense vortex formation in the molten pool which thoroughly mixed the nickel and aluminum powders within the molten pool in a fraction of a second. As the sample is moved away with a predefined velocity, the cold substrate quenched the melt pool, generating rapidly solidified micro-alloyed surfaces. A 2.5 kilowatts continuous carbon dioxide laser was used for surface melting; laser power was maintained at 800 watts while the samples were moved with respect to the laser beam at linear velocities in the range of 100-200 mm/min. The technique generated metallurgical bonded novel surfaces. Depth of the laser modified layer was achieved in the range of 0.2-0.4 mm. Refined microstructures of pre-austenite grain size in the range of 4±2 µm were generated. Micro-hardness measurements of the modified layer indicate an almost three times enhancement in the hardness values; the latter are, in general indicative of mechanical strength of the material. The shape of the solid/liquid interface of the advancing molten pool determines the orientation of the growing dendrites; at higher velocities of sample translation with respect to the laser beam, these are almost parallel to the sample surface. The orientation of the dendrites, the uniformity in surface alloying (within fraction of a second) and the resulting hardness values are explained with the help of the modeled shape of the liquid metal pool. The laser processed material proved to be a flexible technique to synthesize novel surfaces for surface sensitive applications.

Effect of Laser Surface Melting on Microstructure of Cold Sprayed Ni20Cr Coatings

Effect of Laser Surface Melting on Microstructure of Cold Sprayed Ni20Cr Coatings

Study on the surface layer formation of aluminum matrix composites and associated machinability in precision milling based on laser melting modification

• A novel material processing method is presented for improving the machinability of SiCp/Al composites. • The surface modification mechanism is not only the sedimentation of SiC particles, but also the formation of new materials. • The invalid modified zone will be formed when the content of the SiC reinforced particles reaches the critical value. SiCp/Al composites are extremely difficult to process due to excessive tool wear and poor surface integrity. This paper presents a novel material processing method combining laser surface melting modification and precision milling to improve the machinability and surface performance of this material. The surface layer laser modification experiments were carried out by controlling the parameters of the laser processing and the volume fractions of the reinforcements. The mechanisms of the laser surface modification at the optimized temperatures between 1397℃ and 2000℃ are revealed as follows: a) a portion of the SiC reinforced particles will react with the molten liquid Al matrix to form the needle-shaped Al 4 SiC 4 ternary carbide and faceted Si particles, together with Al and Al-Si eutectics, constitute the laser modified zone; and b) the remaining SiC reinforced particles sink into the bottom of the Al matrix molten pool to form the particles accumulation zone. Then, the precision milling experiments were conducted to investigate the differences in cutting force, tool wear, and surface integrity between the modified zone and the original zone. When the total depth of cut doesn’t exceed the modified zone depth, the surface roughness and tool wear can be improved by 90.36 % and 61.03 %, respectively.

Room and High Temperature Sliding Wear Characteristics of Laser Surface Melted Stellite 6 and Mo-Alloyed Stellite 6 Hardfacings

In this work, a post treatment of laser surface melting (LSM) has been employed on Stellite 6 and 10 wt.% Mo-alloyed Stellite 6 hardfacings deposited by plasma transferred arc (PTA) process. LSM process refined the microstructures of both hardfacings, while favoring a network-like complex carbide dominated microstructure in the Mo-alloyed version. With reference to the PTA Stellite 6 hardfacing, LSM process led to an increment in surface hardness albeit a subsequent reduction of wear loss at room temperature, where abrasive wear mechanism was dominant. At 500 °C, oxidative wear contributed to the progress of wear by favoring CoO and Co3O4 type tribo-oxides on the contact surfaces of the PTA and LSM’ed hardfacings, respectively. However, Co3O4 type tribo-oxides exhibited poor mechanical stability, than CoO, which led to easier removal from the contact surface and aggravated the wear loss by abrasive wear mechanism. In this respect, LSM’ed hardfacings exhibited higher wear loss than PTA Stellite 6 hardfacing at 500 °C, unlike room temperature.

Microstructure and surface texture driven improvement in in-vitro response of laser surface processed AZ31B magnesium alloy

The present work explored effects of laser surface melting on microstructure and surface topography evolution in AZ31B magnesium alloy. Thermokinetic effects experienced by the material during laser surface melting were simulated using a multiphysics finite element model. Microstructure and phase evolution were examined using scanning electron microscopy, X-ray diffraction, and electron back scatter diffraction. Surface topography was evaluated using white light interferometry. The interaction of surface melted samples with simulated body fluid was monitored by contact angle measurements and immersion studies up to 7 days. Laser surface melting led to formation of a refined microstructure with predominantly basal crystallographic texture. Concurrently, the amount of β phase (Mg17Al12) increased with an increase in the laser fluence. β phase preferentially decorated the cell boundaries. In terms of topography, the surface became progressively rougher with an increase in laser fluence. As a result, upon immersion in simulated body fluid, the laser surface melted samples showed an improved wettability, corrosion resistance, and precipitation of mineral having composition closer to the hydroxyapatite bone mineral compared to the untreated sample.

The effect of laser energy density on the microstructure, residual stress and phase composition of H13 steel treated by laser surface melting

The effect of laser energy density on the microstructure, residual stress and phase composition were investigated of H13 steel treated by laser surface melting. The microstructure in the molten zone (MZ) displays a hypoeutectic microstructure consisting of fine lath martensite, retained austenite, carbides; the hardened zone (HZ) consists of lath martensite, retained austenite and carbides; the heat-affected zone (HAZ) consists of ferrite, a few lath martensite, retained austenite, and many undissolved carbides. Increasing laser energy density, the microstructure of the MZ is coarsened, and the lath martensite decreases. Moreover, the minimum micro-strain (0.179) appears at the bottom of the MZ, and the maximum one appears at the top of the MZ (0.371). When the laser energy density is 110 J/mm2, the ferrite in the HZ completely transformed into austenite, but there is a few undissolved carbides. Residual compressive stress of the HZ increases with the increasing laser energy density. The maximum residual compressive stress of 1403.4 MPa can be achieved when the energy density is 120 J/mm2.