Laser Surface Treatment Research Articles

SELECTIVE SURFACE HARDENING OF GEAR MECHANISM SHAFT BY ROBOTIC LASER 3D SYSTEM

A robot-based laser 3D hardening method is proposed as a finishing operation to increase the wear resistance of the metal end-products. The laser hardening process of the surface layer of the products by changing its structure is one of the most effective methods of selective surface hardening. Thermal hardening of metals and alloys by laser radiation is based on local heating of a surface area under the influence of radiation and subsequent cooling of this surface area at a supercritical speed due to heat removal into the inner layers of the metal. The used robot-based laser 3D system (FANUC industrial robot and SCANLAB scanning optics) allows processing the surfaces of any complexity and geometry, including the shafts of the gear mechanism of the seed drill. It was found that the development and improvement of the technological process of manufacturing steel shafts of the gearbox of the seed drill is an urgent technological problem due to the rapid retirement of the shaft, which leads to the expenditure of time and money for its replacement. The most heavily loaded sections of the shaft were pre-estimated using the SolidWorks Simulation software package. A high-power TruDisk 8002 solid-state disk laser with a maximum laser power of 8 kW and a wavelength of laser radiation of 1030 nm is used for high-quality laser surface treatment of the AISI 1066 steel shaft of the gear mechanism. The laser surface heat treatment was carried out using a constant power strategy (continuous mode), varying the laser power in the range of 1.35–2.25 kW. Based on the Fe-Mn two-component state diagram, the critical points of the temperature of complete austenization of the studied steel were previously predicted taking into account the chemical composition of the material. Additionally, the energy density values of the laser beam with a diameter of 1 mm on the working surface was estimated. The results showed that the surface hardness of the shaft was about 2.5 times higher compared to the untreated surface. The working ranges of laser heat treatment parameters of the gearbox shaft were established to increase the hardening intensity (100–150%) of the responsible areas.

Selective laser crystallization and amorphization in polymer fibers

Textile finishing is a huge industry for modification of textile surface properties to align with the desired end use spanning medical, engineering, and apparel applications. Laser-induced polymer morphology modification has been studied by researchers, with evidence suggesting a correlation between laser fluence and crystallinity. However, a lack of data has resulted in the mechanism for change remaining unknown. This paper aims to identify the mechanism of initial Young’s modulus control in polyethylene terephthalate (PET) monofilament yarn and explain the relationship between Young’s modulus and the degree of crystallinity using Takayanagi’s model. PET monofilament yarns were treated using a CO2 infrared laser at a 10.6 μm wavelength at fluences up to 0.086 J/mm2. Young’s modulus data obtained from stress-strain curves at 0.2% strain and fraction crystallinities obtained from differential scanning calorimetry were compared with Takayanagi’s series model. Laser surface treatment within a fluence range up to 0.087 J/mm2 on PET resulted in stable Young’s modulus values. A 3.5% reduction in Young’s modulus of PET was seen with a 6.1% reduction in crystallinity, which was in good agreement with Takayanagi’s series model. Lasers offer rapid material processing capable of increasing or decreasing fractional crystallinity selectively along the fiber where desired. Selective amorphization increases chemical absorption, allowing increased surface finishing uptake at milder processing conditions, whereas increasing fractional crystallinity imparts fiber strength. The relationship between fiber crystallinity and Young’s modulus for PET can be described by Takayanagi’s model, allowing a prediction of material properties, which can be extended from fibers to all thin-film polymers.

Comparative study on the influence of initial deformation and temperature of thermally induced white etching layers on rail wheels

Detailed investigations of white etching layers (WELs) formed on rails and wheels are of utmost importance to improve railway safety. This calls for lab-generated WELs with well-comparable characteristics to conduct further testing such as in twin-disc experiments. In this work, laser surface treatments were applied on wheel steels to induce thermally induced WELs. To evaluate the influence of the initial deformation state of the sample, undeformed and severely plastically deformed ER7 wheel steels were selected. In this study, it is shown that surface temperatures of > 640 °C lead to thermally induced WELs, with differences based on the initial deformation state. The thermally induced WELs are compared to a field-WEL present on an ex-service railway wheel. Results indicate that the WEL induced by laser treatment on an initially deformed wheel material provides the best match with the field-WEL. The most distinct difference is a grain size gradient within the outermost 30 µm of the field-WEL caused by deformation after the formation in service. This study clearly shows that the variation in initial microstructures and laser process parameters leads to differences in microstructural characteristics of thermally induced WELs and must be considered when imitating and testing WELs in laboratories.

Analysis of microstructure, treatment pattern, and corrosion behaviour of AZ91D magnesium alloy processed by selective laser surface remelting

In this work, we show that selective laser surface remelting, apart from modifying the microstructure, can also act as a design method to treat the surface of AZ91D magnesium alloy with varying treatment patterns, thus providing a feasible approach to tailor the electrochemical behaviour and to improve its corrosion resistance. Two treatment patterns, spots and stripes, are designed, in which cases, the boundary ratio and geometrical complexity are analyzed and compared. Besides microstructure, Hydrogen evolution, and potentiodynamic polarization curves are studied. It is found that in the central treated area, corrosion is hindered by formation of the greatly refined β-Mg17Al12 continuous network, together with the uniform distribution of surface potential. As a comparison, in the boundary region, extensive micro/galvanic couples are detrimental to the corrosion resistance, due to the oriented dendritic microstructure. As a result, good corrosion resistance, 3–5 times higher than the as-received material, is obtained despite of the variation of treatment patterns and presence of extensive micro/galvanic couples in the boundary region, indicating that selective laser surface remelting could act as a feasible way to tailor the corrosion behaviour of AZ91D. It is suggested that this technique provides not only a metallurgical approach, and also a design one to improve the corrosion resistance of AZ91D, which may lead to numerous applications of both AZ91D and laser surface treatment.

Microstructural characteristics and tribological properties of the localized laser surface treatment of AISI 420 stainless steel

In this study, a localized laser surface treatment (LLST) of AISI 420 including transformation hardening and surface melting processes was utilized using a high-power diode laser (HPDL) under different beam travel speeds in order to obtain transformed hardened and surface melted samples. Microstructural characteristics of the laser processing areas were evaluated using field emission scanning electron microscopy (FESEM) equipped with energy dispersive spectroscopy (EDS). Moreover, the tribological properties of the as-received, transformed hardened and surface melted samples were investigated. Obtained tribological results indicated that the surface melted sample has a higher coefficient of friction (COF) and greater sliding wear rate compared with the transformed hardened sample due to the increment of surface roughness and formation of δ-ferrite precipitates respectively.

Progress of laser surface treatment on magnesium alloy.

Magnesium (Mg) metals have been widely used in various fields as one of the most promising lightweight structural materials. However, the low corrosion resistance and poor mechanical properties restrict its applications. Surface treatments are common approach to enhance the mechanical strength and corrosion resistance of Mg metals. Among them, laser surface treatment generates novel tissues and structures in situ on the sample surface, thereby improving properties of mechanical strength and corrosion resistance. We briefly describe the changes in surface organization that arise after laser treatment of Mg surfaces, as well as the creation of structures such as streaks, particles, holes, craters, etc., and provide an overview of the reasons for the alterations. The effect of laser processing on wettability, hardness, friction wear, degradation, biocompatibility and mechanical properties were reviewed. At last, the limitations and development trend of laser treatment on Mg metals research were further pointed out.

Femtosecond laser treatment promotes the surface bioactivity and bone ingrowth of Ti6Al4V bone scaffolds

In this study, a femtosecond laser with a wavelength of 800 nm was used to modify the surface of a titanium alloy bone scaffold created via selective laser melting (SLM). The outcomes demonstrated that the surface morphology of the bone scaffold after femtosecond laser treatment was micro-nano morphology. The hydrophobic structure of the scaffold was changed into a super-hydrophilic structure, improving the surface roughness, which was highly helpful for osteoblast adhesion and differentiation. The femtosecond laser surface treatment in vitro samples produced a thick layer of hydroxyapatite (HAP) with improved surface bioactivity. The effectiveness of osseointegration and interstitial growth of the specimens treated with the femtosecond laser surface were found to be better when bone scaffolds were implanted into the epiphysis of the tibia of rabbits. As a result, femtosecond laser therapy dramatically enhanced the surface activity of bone scaffolds and their capacity to integrate with the surrounding bone tissues, serving as a trustworthy benchmark for future biological scaffold research.

The Corrosive Improvement with High Temperature Laser Surface Treatment of AA7075

A recently developed AA7075 was subjected to solid solution, high temperature laser surface treatment and artificial aging. The microstructure evolution and precipitation behavior were examined, and their effects on corrosion behavior and corrosion cracking were analyzed. The results show that the coarse η phase disappears and many smaller η phases are formed after high temperature laser surface treatment. Most of the phases dissolved into Al matrix during solution, and fine η phases precipitated in laser treatment. The η′ phases were appeared in the high temperature laser surface treatment samples. It was concluded that reasonable laser scanning power owned the best corrosion resistance, lower intergranular corrosion tendency. Both lower laser scan power and higher scan power samples were susceptible to the intergranular corrosion, and the intergranular cracking was observed. In contrary, the reasonable sample which the laser power is 1000 W showed much better corrosion resistance due to the coarsening and separation of grain boundary precipitations.

Conductive Heat Transfer in Materials under Intense Heat Flows

The paper presents the solution of the spatial transient problem of the impact of a moving heat flux source induced by the laser radiation on the surface of a half-space using the superposition principle and the method of transient functions. The hyperbolic equation of transient thermal conductivity accounting for the relaxation time is used to model the laser heating process. It is assumed that the heat flux is distributed symmetrically with respect to the center of the heating spot. The combined numerical and analytical algorithm has been developed and implemented, which allows one to determine the temperature distribution both on the surface and on the depth of the half-space. In this case, the principle of superposition is used with the use of a special symmetric Gaussian distribution to describe the model of a source of high-intensity heat flux. The use of such a symmetric distribution made it possible to calculate the integrals over the spatial variables analytically. The results of the work could be used to estimate the contribution of the conductive component in the overall heat transfer of materials exposed to intense heat flows (laser surface treatment, laser additive technologies, streamlining and heating of materials by high-enthalpy gases, etc.).

Effects of laser surface modification on stainless steel 316L thin annular discs under radial and cartesian scans

Lasers direct use on metals allows treatment of localized surfaces, without affecting nearby material or the body of the piece. Depending on the expected result, laser surface treatments are classified into hardening, texturing, alloying, coating, and melting. Using the latter is possible to modify the microstructure, dissolve precipitates, and increase hardness and wear resistance, among other applications depending on the material and the parameters used. In this work, it is proposed to empirically study the Laser Surface Melting (LSM) technique on 316L stainless steel annular discs, to evaluate the influence of laser scanning pattern on the results of the laser treatment. The considered scanning patterns correspond to a Cartesian, the most used pattern in the LSM process, also known as “zigzag”, and a Radial pattern, previously proposed in Additive Manufacturing research, which moves the laser in a cylindrical way. The main contribution of this work is to introduce the variable of the scanning pattern in LSM studies, besides, generating a complete characterization of laser treatment on 316L stainless steel annular discs. The results show that, under the same conditions, the variation of the scanning pattern produces a meaningful change in the stress state of the specimen and its deformation after treatment. The main results of LSM treatment were an increase in crystallite size and a decrease in dislocation density, a state of compression, a slight hardening in the melted layer, and a decrease in toughness. This knowledge can be used to encourage new scanning patterns in this field.

탄소섬유 강화플라스틱의 레이저 표면처리 적용에 의한 이종 접합부의 접합강도 개선에 관한 연구

The adhesive bonding technology of carbon fiber reinforced plastics (CFRP) and aluminum alloys, is one of the lightweight joining technologies for automobiles. The strength and properties of the bonded joint, depend on the surface of the bonded part that the adhesive touches. Thus, proper surface treatment is one of the most important steps in the bonding process. The laser surface treatment of carbon fiber composites is a new form of green and environmental surface treatment technology, which can effectively clean coatings and pollutants on the surface of materials. It is also possible to improve the bonding shear strength, by changing the microstructure and roughness of the material surface through laser micro texture processing, to form a mechanically interlocked structure. In this study, a pulsed laser was used to treat the surface of CFRP. By changing the scanning line spacing during laser micro texturing, the effect of laser micro texturing on the surface morphology of CFRP and the strength of aluminum alloy bonded joints was investigated. Results show that in the laser micro texturing process, when the scanning line spacing was 0.3 mm, the maximum tensile shear strength was 14.5 MPa, approximately 200% higher than that without laser treatment.

The effect of ceramic surface structure modification method on the ignition and combustion behavior of non-metallized and metallized gel fuel particles exposed to conductive heating

The study explores the effect of roughness characteristics of ceramic surfaces processed by diamond abrasives (ground and polished) and laser radiation on the consistent patterns and characteristics of the ignition and combustion of non-metallized and metallized gel fuel (GF) particles based on oil-filled cryogels exposed to conductive heating. Ceramics were produced from SiC F1200 powder using spark plasma sintering. It was established that when combustible liquid droplets in a normal state, as well as metallized and non-metallized GF particles are ignited and burned on a ceramic surface, the ignition delay times can be controlled by the characteristics of the heating surface roughness. Laser surface treatment of ceramics was shown to be a high-potential technique to control the ignition delay times of combustible liquid droplets (±48 %), as well as metallized and non-metallized GF particles (±35 %). A reduction in the ignition delay times of liquid fuels, when using laser surface treatment of ceramics, calls for developed, multi-level roughness with Sdr more than 6.3 %. To decrease the ignition delay times of GF on a ceramic surface by using laser treatment, it is necessary to create a texture with roughness parameters Sdr, Sq, Sz close to the respective characteristics of a polished (molecularly-smooth) heating surface. Evidence was provided that laser surface treatment of ceramics is more promising than mechanical processing using diamond abrasives, which is the most common method currently used in mechanical engineering. This is explained by the fact that laser surface treatment of ceramics creates conditions for enhancing the dispersion of GF melt droplets when ignited, as well as reduces the burnout time by creating a layered multimodal texture of the heating surface, characterized by more developed roughness as compared with the texture resulting from the mechanical processing by means of diamond abrasives.

Development and Validation of a Calculation Routine for the Precise Determination of Pulse Overlap and Accumulated Fluence in Pulsed Laser Surface Treatment

In laser material processing, a variety of parameters like pulse fluence, total dose, step size, and pulse‐to‐pulse overlap are used to define and compare laser processes. Of these parameters, the pulse‐to‐pulse overlap can be the hardest to access as it is not implemented directly but instead depends on the spot diameter, its shape, and the respective scanning path that is used to cover the surface. This article shows that existing calculation routes overestimate the actual overlap by up to 21%. A novel calculation route is developed that greatly facilitates the determination of the pulse overlap and thereby the average number of laser pulses that interact with a given point on the surface. This approach makes it possible to achieve more reliable and comparable laser processes, which in return leads to better control of the procedure as the effect of individual parameters on a given output can be determined with greater precision.

Experimental studies of the possibility of laser processing as a cleaner method of achieving a surface with good adhesion

Abstract In manufacturing processes many technological operations are designed, in which the adhesive properties of the treated surface are very important. These are processes related to application of any coating on the surface, such as gluing, painting, varnishing and others. Durability of coatings depends on proper preparation of the surface to which they are going to be applied. Conventional methods, such as grinding, sandblasting with subsequent washing and degreasing, as well as galvanic treatment applied to e.g. aluminium alloys - require the use of not only specific equipment but also chemical substances. They often lead to a significant burden on the environment due to their harmful properties. In an experimental study, attention was drawn to the significant environmental aspects of such a technological process and work was carried out to demonstrate whether it is possible to eliminate toxic and hazardous substances and to create good adhesion conditions by laser processing. To this purpose, samples were made out of two representative materials: X6Cr17 steel and AW-2024 aluminium alloy, abrasive surface treatment or in a galvanic bath and then washed, degreased and dried. Laser surface treatment without the use of additional chemicals was proposed as an environmentally cleaner technology. Surface roughness and adhesion of the test polymer coating were measured for comparative evaluation of the applied treatment methods. Obtained results were discussed in terms of the possibility of eliminating harmful influences and implementing laser treatment as a cleaner technology in the production of components requiring coating.

Impact of Ultra-Short Pulsed Laser (USPL) Ablation Process on Separated Loss Coefficients of Grain Oriented Electrical Steels

The purpose of this article is to study the impact of surface laser treatments with ultra-short pulses (USPs) (femtosecond laser) on the magnetic properties of grain-oriented electrical steels (GOESs) using the two-temperature model for the ablation process and the magnetic loss separation model of Bertotti. We demonstrated that the hysteresis and excess loss coefficients behave differently depending on the type of laser treatment and its pulse duration [long pulse (LP), short pulses (SPs), and USP]. We also presented the adjusted models to estimate the impact of the USP on the sheet surface in terms of laser energetic quantities; more precisely, the groove depth, the plasma maximum temperature, and the peak surface wave pressure were estimated, relative to its nominal value. The latter physical impacts of laser pulses were then correlated with Bertotti’s loss coefficients: the static hysteresis loss coefficient and the excess loss coefficient. The laser process is not always able to reduce simultaneously both loss contributions. Thus, a compromise must be found to optimize the process. The variation of the flux density level as a function of the applied magnetic field was measured with a single sheet tester (SST) under a one-directional field parallel to the rolling direction. From these measurements, we deduced the whole power loss contributions. Results showed that an optimization of the laser’s parameters ensured an iron loss reduction at 50 Hz up to −30% for an induction below 0.5 T and a percentage close to −15% for an induction above 1.5 T.

Femtosecond Laser Treatment for Improving the Corrosion Resistance of Selective Laser Melted 17-4PH Stainless Steel.

Currently, laser surface treatment (LST) is considered the most promising method available within the industry. It delivers precise control over surface topography, morphology, wettability, and chemistry, making the technique suitable for regulating the corrosion behavior of alloys. In this paper, femtosecond laser texturing with different parameters and atmosphere environments was adopted to clarify the effect of surface treatment on the corrosion resistance of selective laser melted (SLM-ed) 17-4PH stainless steel (SS) in a NaCl solution. The experimental results show that, after the heat treatment, the corrosion resistance of the laser-treated samples was enhanced. With the further laser treatment in an argon atmosphere, the oxidation of nanostructural surfaces was avoided. The Cr, Cu, and other alloying elements precipitated on the laser-ablated surface were beneficial to the formation of a passivation film, leading to an improved corrosion resistance performance.

Microstructure distribution and bending fracture mechanism of 65Mn steel in the laser surface treatment

Laser surface treatment of 65Mn steel was conducted using a high-power diode laser. Based on the numerical simulation of the temperature field, the distribution of the microstructure and the peak temperature at various depths were analyzed. An empirical model was proposed to predict the hardened depth. The bending property and fracture mechanism were studied. The results indicated that the martensitic microstructure was coarser near the outer surface of the hardened layer owing to the higher peak temperature. The peak temperature decreased exponentially with increasing depth and increased linearly with laser power and the reciprocal of the square root of the scanning rate. The maximum hardness of the laser-hardened layer was approximately 950 HV, and the self-tempering effect decreased the maximum hardness. Because of the higher brittleness of the hardened layer and the concentration of deformation near the hardened layer, the bending properties deteriorated after laser surface treatment. However, the hardened layer increased the resistance to early bending deformation. Tempering at 150 °C slightly increased the bending strength. The bending fracture consists of three stages: first, tensile load induces the initiation of microcrack, and the crack propagates in an intergranular manner near the crack source; second, the crack propagates in a transgranular manner, and the fracture morphology exhibits quasi-cleavage; and finally, the fracture morphology changes to a ductile fracture in the substrate region.

Charge Transport Mechanisms of Black Diamond at Cryogenic Temperatures.

Black diamond is an emerging material for solar applications. The femtosecond laser surface treatment of pristine transparent diamond allows the solar absorptance to be increased to values greater than 90% from semi-transparency conditions. In addition, the defects introduced by fs-laser treatment strongly increase the diamond surface electrical conductivity and a very-low activation energy is observed at room temperature. In this work, the investigation of electronic transport mechanisms of a fs-laser nanotextured diamond surface is reported. The charge transport was studied down to cryogenic temperatures, in the 30–300 K range. The samples show an activation energy of a few tens of meV in the highest temperature interval and for T < 50 K, the activation energy diminishes to a few meV. Moreover, thanks to fast cycles of measurement, we noticed that the black-diamond samples also seem to show a behavior close to ferromagnetic materials, suggesting electron spin influence over the transport properties. The mentioned properties open a new perspective in designing novel diamond-based biosensors and a deep knowledge of the charge-carrier transport in black diamond becomes fundamental.

The effect of laser surface treatment of titanium samples on the adhesive strength of adhesive joints

The article considers the strength of adhesive joints of specimens made of OT-4 titanium alloys, the surface of which has been treated with laser radiation. The paper presents the results of the dependence of the macro- and microgeometry of the obtained surface, chemical composition, wetting angle and adhesion strength of adhesive joints under shear on laser processing modes. The purpose of this work is to determine the optimal parameters of laser processing for obtaining high strength adhesive joints. The results obtained can be used for gluing spacecraft structural elements. The studies were carried out on the adhesive pair «Titan – Titan». Depending on the processing mode, characteristic surface textures were identified, which were combined into groups «B», «K» and «C». High-energy modes of laser surface treatment of samples before gluing lead to a significant increase in the strength characteristics of the adhesive joint (up to 70 %) due to an increase in the area of the gluing surface and mechanical locking of the adhesive in the microrelief of the texture of the samples. In addition, the use of this method, in comparison with mechanical and chemical methods, significantly increases the productivity of surface treatment.

A comparative study on surface topography and microhardness of laser polished-hardened AISI D2 tool steel

Although a laser beam with a small diameter (< 1 mm) can significantly improve surface morphology, the polishing efficiency is very low, and the improvement of surface hardness is negligible. In this work, a novel laser polishing-hardening (LPH) method with integration and high efficiency for the treatment of AISI D2 tool steel using a large-size laser beam (Φ2.8 mm) was proposed, and the effects of laser hardening (LH), laser polishing (LP), and LPH treatments on the surface topography and microhardness were examined. The results show that the LH method had a negligible effect on the surface roughness of the treated sample, while the surface roughness Ra of LP and LPH specimens was reduced by 74.6% and 80.9%, respectively, indicating that the milled surface topography had been significantly improved, especially LPH was more effective in reducing the roughness. Besides, the polishing efficiency of LPH was 10 times that of the LP approach. In terms of hardness improvement, the near-surface microhardness of LH and LPH samples increased by 1.5 times and 1.3 times, respectively, and the effective hardened zone (EHZ) depth was 0.42 mm and 0.24 mm, respectively, demonstrating that these two laser processing methods had a beneficial effect on the cross-section microhardness of D2 tool steel, while the increase of LP on the microhardness was insignificant. The comprehensive analysis of the surface morphology and microhardness of the LPH specimen indicates that LPH was a feasible laser surface treatment method for D2 tool steel. On the premise of ensuring a high surface finish, the polishing efficiency can be remarkably improved, and the subsurface microhardness and EHZ depth of processed specimen can be also significantly increased, which provided a feasible idea for the application of laser surface treatment technology in industrial mold production.