Laser Surface Remelting Process Research Articles

Enhanced corrosion and wear resistances in a precipitate-hardened magnesium alloy by laser surface remelting

The corrosion and wear behaviors of materials are usually sensitive to the microstructural features. In the current work, a novel strategy using laser surface remelting (LSR) to modify the surface microstructure of a precipitate-hardened magnesium alloy was proposed to synergistically enhance the corrosion and wear resistances. The dedicated microstructural observations indicated that the surface average grain size was remarkably refined (only ∼2.8 μm) after LSR treatment. Meanwhile, the secondary phases in the modified surface region no longer existed randomly within the grains, but preferred to be regularly located along the grain boundaries with a network pattern. In addition, the surface hardness of the specimens subjected to LSR treatment was also improved. Based on those experimental observations, the possible mechanisms responsible for the synergy of the corrosion and wear performances were proposed and discussed. The current results would open a possibility for obtaining magnesium alloys with high corrosion and wear resistances synergistically, and this strategy may also be applicable to other materials. • The microstructural evolution of AZ80 magnesium alloy has been investigated before and after laser surface remelting process. • The corrosion and wear behavior of the as-received and LSR specimens were compared. • The LSR specimens had better corrosion and wear resistance than AR specimens. • The possible mechanisms responsible for the synergy of the corrosion and wear performances were proposed and discussed.

A study on developing process-structure–property relationship with molten pool thermal history during laser surface remelting of Inconel 718

In the area of laser material processing, laser surface remelting has been found to be an effective method of improving surface properties such as hardness, wear, and corrosion resistance. However, the scale of improvement depends on the evolving microstructure and phases, which depend on the cooling rates. Therefore, in the present study, laser surface remelting of Inconel 718 was carried out and a process-structure–property relationship with respect to the cooling rates has been developed. During the laser surface remelting process, the molten pool thermal history i.e. cooling rate, molten pool lifetime, and solidification shelf time is monitored and estimated using an IR pyrometer. The evolution of microstructure is later correlated with these parameters. With an increase in scan speed, the cooling rate is found to increase resulting in transformation of microstructure from equiaxed grains to columnar epitaxial growth. Based on the results obtained, a process map is proposed to establish a particular type of microstructure with respect to the cooling rate. Further, the effect of cooling rate and microstructure on the surface hardness and specific wear rate has also been investigated. Both surface hardness and specific wear rate got reduced with decreasing cooling rate at a slower scan speed due to grain coarsening and an increase in elemental segregation or Laves phase formation.

Grain refinement and high thermal stability in laser surface remelted Mg-4Y-3Nd-1.5Al alloy

Aluminum is a low-cost alloying element. Adding 1.5 Al (wt.%) to Mg-4Y-3Nd (Mg-RE) alloy leads to a significant grain refinement in the laser surface remelting (LSR) process. The in-situ precipitated Al2RE particles and interdendritic eutectic Al2RE phase formed during the previous solidification turn into a new class of nuclei, and refine the grain structure. The rapid solidification results in the formation of nanoscale Al-RE intermetallic compounds within alloy that ensure excellent thermal stability in grain structure with only 1.85% hardness loss after heat treatment at 545 °C for 30 h by Zener pinning mechanism.

Experimental characterisation of laser surface remelting via acoustic emission wavelet decomposition

This paper presents a novel technique of monitoring laser surface remelting (LSR) via measurements of process generated acoustic emission (AE). The LSR conditions include a variable laser power level and an ambient air or argon 5.0 inert gas atmosphere. The microstructure and microhardness of the remelted surface layer are evaluated. The recorded AE signal datasets are analysed via the wavelet decomposition technique. The resulting energy and energy proportions of approximation and detail coefficients are calculated. The decomposed AE signal features are evaluated in correlation with the LSR process conditions. A supervised machine learning cubic support vector machine classifier type is used for the classification of laser pulses. The experimental results show an overall 98% classification accuracy into the corresponding LSR laser power level and atmosphere conditions, confirming the utility of monitoring the LSR process and the resulting material surface layer characteristics via the AE wavelet decomposition technique.

Microstructural evolution and hardness of as-cast Be-Al-Sc-Zr alloy processed by laser surface remelting

As-cast beryllium-aluminum (Be-Al) alloy exhibits a coarse microstructure with pore defects due to a large solidification interval, greatly limiting its mechanical properties. In this research, the relationship between laser surface remelting process and microstructure and hardness of as-cast Be-Al-Sc-Zr alloy was established. The experimental results demonstrated that a pore-free refined microstructure of remelted layer was obtained by controlling the parameter of effective laser energy input. The microstructure of as-cast Be-Al-Sc-Zr alloy consisted of equiaxed grains with Al phase forming a continuous frame wrapping Be phase, which was significantly refined in the remelted zone (from 25 μm to 2 μm). The Vickers hardness in the remelted zone (approximately 210 HV) was approximately 3 times that of as-cast Be-Al-Sc-Zr alloy. Analysis of the Vickers hardness and the Be phase size showed a good agreement with a Hall-Petch equation. In addition, transmission electron microscopy (TEM), auger electron spectroscopy (AES) and X-ray diffraction (XRD) analysis evidenced that Sc and Zr elements formed a single blocky phase Be13(Scx,Zr1-x), which was also greatly refined from 8 μm to 1.5 μm in the remelted zone. The results obtained in this study indicate that the laser surface remelting allowed refining the microstructure and further strengthening the Vickers hardness of Be-Al-Sc-Zr alloy.

Application of optimized laser surface re-melting process on selective laser melted 316L stainless steel inclined parts

Absract Lower surface quality of selective laser melting (SLM) manufactured parts remains to be a key shortcoming particularly for high performance functional components. In this paper, the authors utilized Box–Behnken methodology to explore the effect of laser surface re-melting process parameters. The process parameters are:laser power, laser exposure time, laser point distance, and shell layer thickness. The experiments were conducted using Renishaw AM-250 machine. SLM manufactured parts with inclination of 45˚ up-skin were treated with a given surface roughness using laser surface re-melting (LSR). The optimization of process parameters was conducted using response surface methodology and the validation tests was carried out utilizing the determined input parameters. The results verified the effectiveness of the integrated approach and the proposed statistical model. The outcomes of this study demonstrated that selective laser melting process followed by the laser surface re-melting process is very likely to become a fast and economic integrated method for improving the inclined surface quality of SLM manufactured parts.

Influence of cooling rate on microstructure of an eutectic Al-Ni alloy

The microstructure of an eutectic Al-6,3wt.%Ni alloy solidified under transient conditions were analysed in as-cast condition and after a laser surface remelting process. Considering that the laser treatment reaches a small portion of sample surface and provide higher cooling rates (Ṫ) than conventional cast processes, Ṫ had to be calculated with the aid of a computational program. Therefore, microstructure of both as-cast and treated samples were compared. The microstructure of the remelted zone showed cell-like structures, which achieved very refined scale (< 10 μm).

Effect of heat treatment and laser surface remelting on AlSi10Mg alloy fabricated by selective laser melting

The selective laser melting (SLM) of aluminium alloys is of interest to researchers because of these alloys’ potential applications in the aerospace and automotive domains. Post-processing is generally required in order to enhance the mechanical properties of devices that involve moving parts, where surface mechanical properties are significant factors. This paper describes a preliminary study that was conducted to investigate the effect of post-processing on the microstructure and mechanical properties of SLM fabricated AlSi10Mg alloy, with an emphasis on the laser surface remelting (LSR) process. The experimental results demonstrate that the heat treatment degraded tensile strength while improving ductility by achieving grain growth and residual stress release. The yield strength obtained in the experiment was reduced from 200 to 100 MPa, whereas the elongation increased from 6 to 22%. The LSR process was found to contribute to an improvement in surface finish. The surface roughness indicator Ra was determined to be 0.93 μm in the LSR post-processed sample, compared to a fairly high value of 19.3 μm in the as-fabricated samples. The LSR process also enhanced the microhardness by refining the microstructure; the Si eutectic dendritic structure that formed was found to be finer than that of the as-fabricated samples. Compared to the as-fabricated samples, the LSR process contributed to a 19.5% increase in microhardness. The findings suggest that the microstructure and mechanical properties of SLM-fabricated AlSi10Mg parts could be tailored by suitable post-processing such as heat treatment and LSR. The significance of this research is its proposal of a novel technique to enhance surface hardness using LSR, which is a significant step towards the combination of SLM and LSR processes to manufacture customised aluminium components for the automotive and aerospace sectors.

Laser surface remelting of Ti and its alloys for improving surface biocompatibility of orthopaedic implants

Titanium and its alloys are widely popular for manufacturing of orthopaedic implants. Surface modification of those implants is carried out to improve the biocompatibility. Laser can be used for surface modification to enhance surface biocompatibilty through different routes, surface cladding and alloying being the most common. Laser surface remelting process, being a simpler process with less number of process parameters involved, has potential advantages in industrial environments. This paper discusses the process fundamental of laser surface remelting, and explores the effects of different process parameters on microstructural, topographical, tribological and bio-interfacial aspects of biocompatibility of Ti alloys, as found from existing literature. The limitations associated with laser surface remelting processes are also identified.

ω-assisted α phase and hardness of Ti-5Al-5Mo-5V-1Cr-1Fe during low temperature isothermal heat treatment after laser surface remelting

Abstract The near-β titanium alloy Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) was treated by laser surface remelting (LSR) with a series of holding times at 500 °C in order to study the growth behavior of ω-assisted α phase and its effect on mechanical properties. Transmission electron microscope and hardness test was used to characterize different phases and surface property of different samples. Results show that only a small quantity of athermal ω phases precipitate from β matrix at the remelted zone and there are incomplete collapsed {111}β lattice planes at the β/ω boundaries because Ti-55511 has low electron/atom ratio and high Al contents. So the athermal ω phase precipitated during LSR process has little effect on increasing surface hardness of Ti-55511. ω-Assisted nano-scaled α lamellas can be formed by the connection of some initial α phases at the early stage of low temperature isothermal heat treatment. And all the ω phase dissolved after 500°C/10min. Further increasing of holding times will cause volume fractions increase of the nano-scaled α lamellas, resulting in gradually improving hardness of the alloy at the remelted zone.

Theoretical and experimental study on surface roughness of 316L stainless steel metal parts obtained through selective laser melting

Purpose The purpose of this paper is to provide a theoretical foundation for improving the selective laser melting (SLM) surface roughness. To improve the part’s surface quality during SLM process, the upper surface roughness of SLM parts was theoretically studied and the influencing factors were analyzed through experiments. Design/methodology/approach The characteristics of single track were first investigated, and based on the analysis of single track, theoretical value of the upper surface roughness would be calculated. Two groups of cubic sample were fabricated to validate SLM parts’ surface roughness, the Ra and relative density of all the cubic parts was measured, and the difference between theoretical calculation and experiment results was studied. Then, the effect of laser energy density on surface roughness was studied. At last, the SLM part’s surface was improved by laser re-melting method. At the end of this paper, the curved surface roughness was discussed briefly. Findings The SLM upper surface roughness is affected by the width of track, scan space and the thickness of powder layer. Measured surface roughness Ra value was about 50 per cent greater than the theoretical value. The laser energy density has a great influence on the SLM fabrication quality. Different laser energy density corresponds to different fabricating characteristics. This study divided the SLM fabrication into not completely melting zone, balling zone in low energy density, successfully fabricating zone and excessive melting zone. The laser surface re-melting (LSR) process can improve the surface roughness of SLM parts greatly without considering the fabricating time and stress accumulation. Originality/value The upper surface roughness of SLM parts was theoretically studied, and the influencing factors were analyzed together; also, the LSR process was proven to be effective to improve the surface quality. This study provides a theoretical foundation to improve the surface quality of SLM parts to promote the popularization and application of metal additive manufacturing technology.

Experimental and numerical analyses of laser remelted Sn–0.7 wt%Cu solder surfaces

Numerous studies on the effects of solidification thermal parameters, microstructure and reliability of Sn–Cu solder alloys can be found in the literature, however only a very limited number of investigations deal with the application of laser surface remelting (LSR) in soldering processes. One of the key advantages of the LSR rapid solidification conditions relies on the reduction in size and the more homogeneous distribution of intermetallic particles. This study aims to analyze the effect of LSR process parameters on microstructural changes and hardness evolution in the remelted region of a Sn–0.7 wt%Cu alloy. Optical and SEM microstructures were used for determination of geometrical and dimensional aspects of the remelted pool profile such as depth (d) and width (w) as a function of the laser beam speed (Vb). Both dimensions are shown to vary linearly with Vb. Hardness varied noticeably with Vb (from 11.5 to 19.5 HV for Vb varying in the range 2.4–9.0 m/min) caused by the fineness of the eutectic mixture and the corresponding more homogeneous distribution of intermetallic Cu6Sn5 particles. The theoretical predictions provided by a heat transfer numerical model are shown to represent the dimensions of the laser-remelted Sn–0.7 wt%Cu alloy pool. A growth law is proposed relating the microstructural interphase spacing to the cooling rate, encompassing both directional solidification and LSR.

Thermal fatigue resistance of H13 die steel repaired by partial laser surface remelting process

In order to study the thermal fatigue resistance of H13 die steel repaired by a partial laser surface remelting process, a strengthening latticed morphology was designed and processed using a pulsed Nd:YAG laser by imitating the structures of certain animals and plants. The ultrafine microstructure within the strengthening unit is composed of martensite, austenite and finer carbides. The average microhardness of the unit is much higher than that of the original surface, even after thermal fatigue cycles. The results show that both percentage increase in crack length and crack density are reduced owing to the blocking effect of strengthening units. Partial laser surface remelting with the smallest spacing is the most effective for repairing cracks and improving the overall thermal fatigue resistance. A manufacturing verification test demonstrates the effectiveness of the partial laser remelting process in repairing thermal fatigue cracks of a casting die and improving its thermal fatigue resistance.

Influence of laser surface treated on the characterization and corrosion behavior of Al–Fe aerospace alloys

In this research laser surface remelting without protective coating with a 2kW Yb-fiber laser (IPG YLR-2000S) was applied in the Al–1.5wt.%Fe alloy in order to investigate the layer treated with different techniques of superficial characterization, thereby, the technique of optical microscopy, atomic force microscopy and low-angle X-ray diffraction were used. The present work mainly focuses on the corrosion study by diverse techniques in aggressive environment of the laser-treated area and the substrate material was carried out, thereby, at open circuit potential testing, the results have shown a displacement to more anodic values in the corrosion potential for the laser-treated specimen when compared to the untreated specimen; in potentiodynamic polarization tests have shown that as a result of the laser treatment, the corrosion current can be reduced by as much as ten times, and a passive region was obtained, which served as an effective barrier for reducing anodic dissolution and finally, the result in cyclic polarization curves of the untreated sample there was a greater area of the hysteresis loop, implying that it is more susceptible to corrosion. This study was complemented by other techniques mentioned above in order to elucidate this study. Laser surface remelting process has definitely modified the surface film, which results in higher corrosion resistance, a large range of passivation and a lower area of the hysteresis loop.

Crack Growth Resistance of Laser Surface Remelted Nodular Cast Iron

Abstract The laser surface remelting (LSR) process is applied to modify the surface properties of nodular cast iron and is suitable for manufacture of various machine and tool parts. Usually, studies of surface remelted layers include microstructural and microchemical analyses, measurements of residual stresses, and various wear and corrosion tests. This paper will represent microstructural and microhardness analyses of the thin surface layer after laser remelting of nodular cast iron 500-7. Particular attention is paid to fatigue crack initiation and propagation tests. Crack initiation and growth behavior was assessed under bending test conditions. The comparison is made between as-received nodular cast iron 500-7 and the same material, where one of the surfaces is altered with the LSR process. As the crack propagation is perpendicular to the laser remelted layer, the specimens with a thin laser surface remelted layer exhibited higher resistance to fatigue crack growth in the low stress intensity factor range ΔKth than as-received nodular cast iron specimens. Also, ΔKIC is higher in the case of the laser remelted surface.

Laser remelting of Al–1.5 wt%Fe alloy surfaces: Numerical and experimental analyses

A 3D heat transfer mathematical model based on the finite element method is applied to the laser surface remelting (LSR) process with a view to simulating temperature fields and melt pool dimensions. The theoretical predictions furnished by the model are validated against LSR experimental results from tests carried out in the present study with Al–1.5wt%Fe alloy samples. The work also encompasses an analysis of microstructural and microhardness variations throughout the resulting treated and untreated zones. A remarkable effect of the LSR treatment on the mechanical and corrosion resistance of the treated samples is shown.

The Laser Surface Remelting of Austenitic Stainless Steel

The laser surface remelting (LSR) process was successfully applied to restore localized corrosion resistance in sensitized stainless steel and also as a useful method to improve passivity of some martensitic stainless steels. The LSR process can be successfully applied to repair cracks and defects at the surface of highly thermo-mechanically loaded parts of stainless steel. The purpose of presented study was to evaluate the microstructure and properties of laser remelted surface of stainless steels. The wrought austenitic stainless steel and sintered in vacuum 316L type were studied. The laser treatment was performed with the use of high power diode laser (HPDL) and the influence of beam power of 0.7-2.1kW on the properties of the surface layer was evaluated. The geometrical characteristics and x-ray analysis of weld bead were studied as well as microhardness, surface roughness and corrosion resistance were measured. The increase of laser beam power of LSR resulted in the increase of hardness of sintered stainless steel due to the reduction of porosity and formation of fine dendritic and cellular-dendritic microstructure. The corrosion resistance of remelted surface increased for sintered materials, when remelted at 2.1kW. The wrought stainless steel revealed impairment of pitting corrosion when remelted at lower beam power rate.

Numerical Simulation and Experimental Analysis of Laser Surface Remelting of AISI 304 Stainless Steel Samples

The aim of this work is to develop a heat transfer mathematical model based on the finite difference method in order to simulate temperature fields in the laser surface remelting process. Convective heat transfer in the remelted pool is taken into account by using the effective thermal conductivity approach. Experiments of laser surface remelting of AISI 304 austenitic stainless steel samples were carried out in the present investigation, and numerical simulations were applied for the CO2 laser machine operating parameters. The work also encompasses the analysis of microstructural and microhardness variations throughout the resulting treated and unmolten zones. This study permits to conclude that numerical simulation is a useful tool in setting the laser operating parameters, enabling pre-programming of the extent of the treated area.

Numerical and experimental analysis of rapidly solidified laser remelted Al 5wt pct Ni surfaces

The aim of this work was to develop a heat transfer mathematical model based on the finite difference method in order to simulate temperature fields in the laser surface remelting process. Convective heat transfer in the remelted pool was taken into account by using the effective thermal conductivity approach. Theoretical predictions furnished by previous models from the literature were used for validation of numerical simulations performed with the proposed model. Experiments of laser surface remelting of Al-5 wt pct Ni samples were carried out in the present investigation, and numerical simulations were applied for the laser machine operating parameters. The work also encompasses an analysis of microstructural and microhardness variations throughout the resulting treated and untreated zones.

Numerical Simulation and Experimental Analysis of Laser Surface Remelting of AISI 420 Stainless Steel Samples

The aim of this work is to develop a heat transfer mathematical model based on the finite difference method in order to simulate temperature fields in the laser surface remelting process. Convective heat transfer in the remelted pool is taken into account by using the effective thermal conductivity approach. Experiments of laser surface remelting of AISI 420 stainless steel samples were carried out in the present investigation, and numerical simulations were applied for the laser machine operating parameters. The work also encompasses the analysis of microstructural and microhardness variations throughout the resulting treated and unmolten zones.