Laser-melted Layer Research Articles

Surface microstructure and corrosion characterization of AZ31 magnesium alloys fabricated by laser surface-modification

This study aimed to improve the corrosion resistance of Mg alloys, and to this end, AZ31 Mg alloy was subjected to laser surface re-melting using a fiber laser. Subsequently, a comparative analysis was conducted on the microstructures of the re-melted surfaces under various laser power settings, and the influence of laser surface re-melting on corrosion characteristics was investigated through a potentiodynamic polarization test. The test results show that laser surface re-melting produced a distinct reduction in the crystallite size of magnesium, ranging from 11.23–15.80 μm within the laser-melted layers. The maximum melted layer thickness of 820 μm was achieved with a laser power of 2100 W. The process of laser surface re-melting notably heightened the hardness, particularly as the power increased, which was attributed to swift solidification. The most noteworthy hardness measurement of 92.1 HV0.02 was recorded at 2100 W. Thus, the study shows that laser surface re-melting significantly enhances the corrosion resistance of Mg alloys, primarily owing to the formation of a corrosion product. The smaller grain size on the surface contributes to the formation of a dense product film.

Optimization of a metallographic methodology for the study of laser-melted layers of austenitic stainless steel

Optimization of a metallographic methodology for the study of laser-melted layers of austenitic stainless steel

Influence of laser parameters on corrosion resistance of laser melting layer on C45E4 steel surface

The key to further improve the corrosion resistance of the laser melting layer on steel surface is to select appropriate laser parameters. In this paper, a single factor laser melting experiment on C45E4 steel surface by using a pulsed laser beam at a wavelength of 1064 nm and electrochemical analysis were conducted to explore the influences of laser parameters including single pulse energy density, spot overlap ratio and number of laser scans on the corrosion resistance of the laser melting layers. The optimal laser parameter combination was obtained: single-pulse energy density of 3.82 J·cm−2, spot overlap rate of 80 % and 4 number of laser scans. The thickness of the optimal laser melting layer prepared with the optimal laser parameters was only 10.85 μm, and it had a maximum self-corrosion potential of 1.031 V, a minimum corrosion current of 3.451 × 10−9 A·cm−2 and a maximum charge transfer resistance of 1541.2 Ω·cm−2. During the salt spray corrosion test, the corrosion rate of the optimal laser melting layer was only one third of that of the traditional hot alkaline blackened layer with a thickness of 12.04 μm. The results showed that the corrosion resistance of the optimal laser melted layer was significantly improved compared to that of the conventional thermally alkaline blackened layer. The main reason was that the determination of the optimal single-pulse energy density, spot overlap ratio and number of laser scans in turn, made the optimal laser melting layer had a stable Fe3O4-FeO corrosion resistance structure, effectively reduced the surface roughness, microcrack density and oxidation leakage point, and prevented the C45E4 steel surface from being over-oxidized.

Fabrication of Mesh Patterns Using a Selective Laser-Melting Process

The selective laser-melting (SLM) process can be applied to the additive building of complex metal parts using melting metal powder with laser scanning. A metal mesh is a common type of metal screen consisting of parallel rows and intersecting columns. It is widely used in the agricultural, industrial, transportation, and machine protection sectors. This study investigated the fabrication of parts containing a mesh pattern from the SLM of AISI 304 stainless steel powder. The formation of a mesh pattern has a strong potential to increase the functionality and cost-effectiveness of the SLM process. To fabricate a single-layered thin mesh pattern, laser layering has been conducted on a copper base plate. The high thermal conductivity of copper allows heat to pass through it quickly, and prevents the adhesion of a thin laser-melted layer. The effects of the process conditions such as the laser scan speed and scanning path on the size and dimensional accuracy of the fabricated mesh patterns were characterized. As the analysis results indicate, a part with a mesh pattern was successfully obtained, and the application of the proposed method was shown to be feasible with a high degree of reliability.

Control of Porosity in Parts Produced by a Direct Laser Melting Process

Recent advances in direct laser melting (DLM) have demonstrated its great potential for manufacturing three-dimensional porous metal parts. Various combinations of powder layering and processing parameters can be set to adjust the porous properties of the final parts. This study presents the effects of powder morphologies and process parameters on porosity formation during DLM. Four types of Fe-powders composed of spherical or non-spherical particles with different sizes were experimentally investigated. Furthermore, the laser processing parameters, such as laser energy density, laser focus, and line spacing, which have a significant effect on the results, were characterized. In the case of a mixed powder composed of spherical and non-spherical powders, the packing density decreases as the non-spherical powder size increases. The porosity of the laser-melted layer increases with the degree of size misfit between the non-spherical and spherical powders. Decreased laser absorption and enlargement of the powder-depleted zone as a result of decreased packing density increases the porosity during DLM. The overall results show that the porosity of DLM parts could be actively controlled by adjusting the process parameters and powder morphologies.

Functionally graded properties in directed-energy-deposition titanium parts

The directed energy deposition (DED) process can directly build full-density, high-performance, complex metal parts by melting metal powder with laser scanning. Functionally graded properties are characterized by gradual property variations over the volume according to the designed pattern. This study investigated the functionally graded properties obtained from DED of titanium powder. The formation of titanium nitride in the laser-melted layer has a strong potential to increase the hardness of the titanium alloy. Thus, the hardness of the deposited part was controlled by changing the concentration of nitrogen in the shielding gas. Furthermore, the effect of laser surface remelting on the deposited layer was investigated to control the surface qualities of the part, such as the roughness, hardness, and surface colour. From the analysis, a functionally graded part having variable hardness was obtained on a trial basis, and the proposed method is shown to be feasible with a high degree of reliability.

Influence of diode laser surface melting on microstructure and corrosion resistance of 7075 aluminium alloy

High power diode laser (HPDL) has been used for surface melting of 7075-T651 aluminium alloy in order to induce changes in the microstructure and corrosion performance. Laser surface melting was carried out using 4 kW power with the scan rate of 10 mm/s in the nitrogen atmosphere. The microstructural study shown that within the resolidified laser-melted layer, most of the coarse constituent particles and the original grain boundaries of the wrought structure were removed. The grain boundaries of the resolidified structure were observed to be free from precipitates. The corrosion studies of the alloy were determined by potentiodynamic polarisation measurements and electrochemical impedance spectroscopy measurements in 3.5% sodium chloride solution. It was observed that the corrosion rate was reduced by four times, when compared with the untreated sample. The superior corrosion resistance was attributed due to absence of detrimental Al-Cu-Fe constituent particles from the surface of laser-melted layer.

Investigation of the electro-chemical corrosion of laser-melted layers of stainless steel in artificial saliva

Austenitic stainless steel is one of the materials commonly used for manufacturing of orthodontic appliances and brackets. In this study, samples of AISI 321 (EN X6CrNiTi 18-10) steel in initial condition and after laser surface melting were investigated in artificial saliva with pH 5.6. Two tests were conducted: measurement of open circuit potentials (free potentials) Ef until reaching steady state potentials Ess and potentiodynamic anodic polarisation. The samples’ surface was characterised by SEM, and EDX analysis was also done. Small amount of pits were observed on the surface of all samples – untreated and laser-melted layers as well. The steady state potential of the base metal was about +238 mV, while that of the laser-melted layers was approximately 50 mV lower. The pitting potentials Epit of the laser-treated samples were lower than those of the untreated stainless steel (+802 mV). It was established that the laser-melted surface layers of stainless steel are more susceptible to pitting corrosion in artificial saliva with higher acidity than those of the base metal.

Microstructure of surface layer of T1 and D2 steels after laser melting

The aim of this study was to investigate the in-depth structure transformations of the laser-melted layers of the steels mostly used in tool production – T1 and D2 steels. The experiments of laser surface melting of T1 and D2 steels were done with continuous wave CO2 laser (wavelength λ = 10.6 μm). The sample’s microstructure was investigated, and the EDX analysis was made by SEM JEOL JMS-35C. Our results show that the microstructural changes of the laser-melted layer of T1 steel were determined by the process of directed crystallisation from the bottom layer to the surface. There is considerable inhomogeneity, followed by the predominantly ongoing process of chemical liquation during crystallisation. Cellular structure, consisting high-carbon martensite and significant amount of retained austenite, prevails in the melted pool bottom. Austenite–carbide eutectic with possible presence of lamellar martensite is situated along the dendrites’ boundaries (in inter-dendrite spaces). The austenite–carbide quasieutecticum with carbide-type M6C possesses ‘skeletal’ morphology in these regions. In the laser melting of D2 steel, the phase transformations in the transition zone (liquid–solid state) were defined by running of reverse eutectic reaction at the places of overheated and partially melted carbide phases. During cooling of these regions, the inhomogeneous melt goes through changes that involve precipitation of oversaturated with carbon and chrome austenite and disperse microquasieutectic with carbides of the M7C3 type.

Improving corrosion resistance of AZ91D magnesium alloy by laser surface melting and micro-arc oxidation

Laser surface melting (LSM) pretreatment and micro-arc oxidation (MAO) were performed to the AZ91D magnesium alloy to improve the corrosion resistance. The microstructure, surface morphologies, phase constituents and electrochemical corrosion behavior were investigated by optical microscope (OM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS) measurements respectively. The results showed that the AZ91D alloy obtained a continuous and uniform laser-melted layer with a thickness of 1 mm and refined microstructure. The MAO coating on laser-melted AZ91D magnesium alloy had refined micro-pores with uniform size than that on the original magnesium substrate without laser melting pretreatment, and correspondingly, the lower corrosion current density and larger MAO coating electrochemical impedance value were found on the AZ91D magnesium alloy with LSM and MAO treatment. It implies that the LSM-MAO coating had more effect on improving the corrosion resistance compared with the singular MAO coating.

Influence of pre-heating on the surface modification of powder-metallurgy processed cold-work tool steel during laser surface melting

In this study we determine the optimal parameters for surface modification using the laser surface melting of powder-metallurgy processed, vanadium-rich, cold-work tool steel. A combination of steel pre-heating, laser surface melting and a subsequent heat treatment creates a hardened and morphologically modified surface of the selected high-alloy tool steel. The pre-heating of the steel prior to the laser surface melting ensures a crack- and pore-free modified surface. Using a pre-heating temperature of 350°C, the extremely fine microstructure, which typically evolves during the laser-melting, became slightly coarser and the volume fraction of retained austenite was reduced. In the laser-melted layer the highest values of microhardness were achieved in the specimens where a subsequent heat treatment at 550°C was applied. The performed thermodynamic calculations were able to provide a very valuable assessment of the liquidus temperature and, especially, a prediction of the chemical composition as well as the precipitation and dissolution sequence for the carbides.

Nd-YAG Laser Surface Melting of Aluminum Alloy 6013 for Improving Stress Corrosion Cracking Resistance

Laser surface melting (LSM) of aluminum alloy 6013 was conducted using a high power Nd-YAG laser under nitrogen gas with the aim of improving the stress corrosion cracking (SCC) resistance of the alloy. The SCC behavior was studied in a 3.5% NaCl solution using a slow strain rate test (SSRT). A laser-melted layer in the order of 500µm thick was produced, which consists of fine dendritic and cellular structures with some Al-Si-Cu-Fe-Mn phase particles formed at interdentritic boundaries. The results of the SSRT test showed that the susceptibility of the alloy to SCC, in terms of total elongation to failure, was decreased after the laser surface modification. This was considered to be attributed to the presence of the laser-formed AlN film, which acted as a barrier to the ingress of the corrosive Cl- into the material and enhanced the pitting resistance of the material. An examination of the fractured surface indicated that the crack propagation path, in the early stage of SCC, was along the tortuous dendrite and cell boundaries. This caused the crack path deviated from the growing direction and resulted in a longer crack length covered before failure thus increasing the overall crack propagation resistance.

Excimer laser treatment of ZE41 magnesium alloy for corrosion resistance and microhardness improvement

A laser surface melting treatment (LSMT) was performed on a ZE41 Mg-alloy using an excimer KrF laser. The laser-melted layer depth depends on the laser scan speed. The morphology and the microstructure of the laser-melted surface were characterized, thanks to the scanning electron microscopy (SEM). The melted Mg-alloy presented a homogenous distribution of the alloying elements in the magnesium matrix. The laser surface melting treatment increased the microhardness of the ZE41 Mg-alloy and improved its corrosion resistance.

Effect of laser melting on plasma-sprayed aluminum oxide coatings reinforced with carbon nanotubes

The effect of laser melting on the microstructure and mechanical properties of plasma-sprayed aluminum oxide composite coating reinforced with 4 wt% multi-walled carbon nanotubes (CNTs) is reported. Laser-melted layer consists of dense, coarse columnar microstructure which is significantly different from plasma-sprayed coating that consists of splats and porosity. CNTs retained their original cylindrical graphitic structure after undergoing laser irradiation. Three dimensional heat flow model has been developed to estimate temperature variation in the laser-melted composite layer. Laser-melted layers show an increase in the microhardness at the expanse of degradation of fracture toughness. Nanoindentation study indicates an increase in the elastic modulus and yield strength of the laser-melted layer which is attributed to dense microstructure with absence of weak-bonding splats and porosity.

Efficient laser polishing of silica micro-optic components

We report a study of the basic characteristics of laser polishing of fused silica with a protocol that is particularly suitable for surface smoothing of micro-optic elements fabricated by a laser ablation process. We describe a new, to our knowledge, approach based on scanning a highly controlled small size laser beam and melting areas of tens to hundreds of micrometers of glass using a computer-controlled raster scan process, which does not require beam shaping, substrate preheating, or special atmospheres. Special test samples of silica substrates with prescribed spatial frequency content were polished using a range of irradiation conditions with the beam from a well-controlled CO2 laser operating at a wavelength of 10.59 microm. An analysis is presented of the laser-generated reduction in surface roughness in terms of measurements of the spatial frequency characteristics, and the results are compared with the predictions of a simple model of surface-tension-driven mass flow within the laser-melted layer. This technique is shown to be capable of smoothing silica surfaces with approximately 1 microm scale roughness down to levels < 1 nm with no net effect on the as-machined net surface shape, at realistic production rates without a preheating stage, and with noncritical residual stresses.

Stress corrosion cracking behavior of Nd:YAG laser-treated aluminum alloy 7075

Abstract Nd-YAG laser surface treatment was conducted on 7075-T651 aluminum alloy with the aim of improving the stress corrosion cracking resistance of the alloy. Laser surface treatment was performed under two different gas environments, air and nitrogen. After the laser treatment, coarse constituent particles were removed and fine cellular/dendritic structures had formed. In addition, for the N 2 -treated specimen, an AlN phase was detected. The results of the stress corrosion test showed that after 30 days of immersion, the untreated specimen had been severely attacked by corrosion, with intergranular cracks having formed along the planar grain boundaries of the specimen. For the air-treated specimen, some relatively long stress corrosion cracks and a small number of relatively large corrosion pits were found. The cracks mainly followed the interdendritic boundaries; the fusion boundary was found to be acting as an arrestor to corrosion attacks. In contrast, only few short stress corrosion cracks appeared in the N 2 -treated specimen, indicating an improvement in corrosion initiation resistance. The superior corrosion resistance was attributed to the formation of the AlN phase in the surface of the laser-melted layer, which is an electrical insulator. The electrochemical impedance measurements taken during the stress corrosion test showed that the film resistance of the laser-treated specimens was always higher than that of the untreated specimen, with the N 2 -treated specimen showing the highest resistance.

Effect of high power diode laser surface melting on wear resistance of magnesium alloys

In the present work, an attempt has been made to improve the surface hardness and wear properties of magnesium alloys AZ31 and AZ61 through solid solution hardening and refinement of microstructures using a 1.5 kW high power diode laser (HPDL) as a heat generating source. One millimetre thick uniform overlapping melt tracks were produced on Mg alloy samples. Laser-melted samples were subjected to a two-body abrasive wear test using a modified pin-on-disc set up. The hardness and wear resistance of laser-melted samples were found far better than the as-received Mg alloy substrates. Scanning electron microscopy was used to examine microstructure of the laser-melted layers and the worn surface morphology. Fine microstructures were observed with an average grain size of less than 5 μm.

Microstructure analysis of magnesium alloy melted by laser irradiation

The effects of laser surface melting (LSM) on microstructure of magnesium alloy containing Al8.57%, Zn 0.68%, Mn0.15%, Ce0.52% were investigated. In the present work, a pulsed Nd:YAG laser was used to melt and rapidly solidify the surface of the magnesium alloy with the objective of changing microstructure and improving the corrosion resistance. The results indicate that laser-melted layer contains the finer dendrites and behaviors good resistance corrosion compared with the untreated layer. Furthermore, the absorption coefficient of the magnesium alloy has been estimated according to the numeral simulation of the thermal conditions. The formation process of fine microstructure in melted layers was investigated based on the experimental observation and the theoretical analysis. Some simulation results such as the re-solidification velocities are obtained. The phase constitutions of the melted layers determined by X-ray diffraction were β-Mg 17Al 12 and α-Mg as well as some phases unidentified.

Effect of excimer laser surface melting on intergranular corrosion cracking of the aluminum–lithium alloy 8090

Excimer laser surface melting of the aluminum alloy 8090 has been conducted with the aim of improving the intergranular corrosion resistance of the alloy. Both the results of the electrochemical and the intergranular corrosion cracking immersion tests show that the laser treatment significantly increases the intergranular corrosion resistance of the alloy, especially when the material is treated in nitrogen gas. The improvement in intergranular corrosion resistance is considered to be primarily due to the elimination of grain boundary Cu enriched precipitates in the laser-melted layer, which has effectively prohibited intergranular corrosion cracking. Besides, the reduction of Cu and Fe enriched constituent particles increases the pitting corrosion resistance of the alloy. As a result, a reduction in corrosion current density of one order of magnitude has been obtained for the laser-treated material. Moreover, a strong passivation has been obtained for the N2-treated specimen. This is attributed to the presence of the chemically stable AlN phase in the laser-melted layer. The analysis of the electrochemical impedance data shows that the polarization resistance (Rp) of the laser-treated specimens is at least two orders of magnitude higher than that of the untreated specimen; while the double layer capacitance (Cdl) is about one order of magnitude lower.

Instrumented indentation probing of laser surface-refined cast Al alloy

The surface layer of A319 alloy was refined by selective remelting using laser energy. Instrumented indentation technique was used to measure hardness (H) and elastic modulus (E) of the laser-melted layer. Berkovich tip was used to indent the material for 100-nm, 200-nm, 500-nm, and 1000-nm depths. The H and the E were found to be 1.22 GPa and 78.2 GPa, respectively, for 1000-nm indentation depths. The variances associated with H and E were minimal, whereas, the same for substrate possessed significant scattering. Also, H and E increased with decreasing depth of indentation. Closer examination suggested that when the heterogeneity in the material was in the scale of indentation depth, significant scattering took place and the hard phase Si influenced the average hardness. However, the influence of indentation depth on elastic modulus was not statistically significant.