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

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

Microstructure and corrosion behavior of the AISI 304 stainless steel after Nd:YAG pulsed laser surface melting

Different laser energy densities were utilized to treat AISI 304 stainless steel via Nd:YAG pulsed laser surface melting (LSM). The surface composition and microstructure of the stainless steel were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM). In particular, the corrosion behaviors of the stainless steel surface without and with LSM were evaluated by the electrochemical polarization measurement in 3.5 wt.% NaCl aqueous solution at room temperature. The results showed that the stainless steel surface without LSM suffered severe localized pitting under the testing conditions. A thin surface oxide protective layer was produced on the stainless steel surface with LSM, which considerably improved the corrosion resistance properties of the stainless steel. The height differences of the corrosion regions on the stainless steel surface with LSM were measured to establish more corrosion resistant region, using scanning confocal laser microscopy. The underlying corrosion mechanism of the stainless steel with LSM was revealed.     

Effect of laser surface melting on intergranular corrosion behaviour of aged austenitic and duplex stainless steels

In the present study, the effect of laser surface melting (LSM) on intergranular corrosion behaviour of aged austenitic stainless steels (UNS S30400, S31603, S32100 and S34700) and aged duplex stainless steels (UNS S31803 and S32950) were investigated. LSM of the aged stainless steels was carried out using a 2.5 kW CW Nd:YAG laser. The microstructure of the aged stainless steels after LSM depends on their compositions. After LSM, the aged austenitic stainless steels mainly contain austenite (γ) with some ferrite (δ) as the minor phase, but the carbide phases are completely eliminated. For the aged duplex stainless steels after LSM, δ becomes the major phase and the δ/γ phase balance is disturbed, whereas the sigma (σ) phase is eliminated. The degree of sensitization (DOS) and corrosion morphology of the aged stainless steels before and after LSM were determined by the double loop electrochemical potentiokinetic reactivation (DL-EPR) using a potentiostat and SEM observation, respectively. Desensitization of the aged stainless steels has been successfully achieved by LSM and their intergranular corrosion resistance is found to be significantly enhanced as reflected by the decrease in DOS due to dissolution of the carbides or σ phase, which reduced Cr depletion or the possibility of solute segregation at the grain or phase boundaries, despite the presence of δ and disturbance of δ/γ phase balance.     

Corrosion protection of AA7449-T7951 friction stir welds by laser surface melting with an Excimer laser

The corrosion protection afforded by laser surface melting (LSM) AA7449-T7951 friction stir welds was investigated. LSM produced melting of the constituent particles and formation of a homogeneous 3–5 μm thick layer. Electrochemical tests showed a reduction in cathodic reactivity after LSM. The breakdown potentials, however, did not change significantly, indicating an anodically reactive surface. In situ and ex situ observation after immersion in 0.1 M NaCl showed that LSM reduced the depth attack in the weld region (particularly the HAZ), affording sacrificial protection to the substrate. Delamination of the treatment can occur during corrosion propagation.     

锆合金表面改性研究进展

综述了目前锆合金表面改性方面研究的进展状况。主要对表面预膜、离子注入和激光表面处理等3种方法进行总结。其中表面预膜主要简介了高压釜预膜、表面镀膜和阳极氧化等3种方法的研究现状：离子注入方面介绍了Y，La，Nb等离子注入对锆合金的改性效果及其原理；激光表面处理方面介绍了激光表面熔覆和激光表面合金化2种方法在锆合金表面改性方面的研究进展。     

激光修饰在制备生物活性表面中的应用

回顾了金属基体表面羟基磷灰石（HA）或合金层的激光重熔（LSRM）、金属表面激光熔凝（LSM）以及脉冲激光沉积钙磷层（PLD）等激光表面处理技术,利用LSRM、LSM、PLD可以明显改善基体表面的耐蚀性和生物相容性。     

Influence of heat treatments and chemical composition on SCC susceptibility during repairing procedure of overlaying of Inconel 182 by laser surface melting

Abstract

A practical repairing technique using laser surface melting (LSM) was developed to remove the stress corrosion cracking (SCC) in overlaying of Inconel 182. Influence of microstructure of different heat treatments performed during repairing process on intergranular cracking/intergranular stress corrosion cracking (IGC/IGSCC) susceptibility was discussed. The intergranular precipitate was identified as M₂₃ C₆ by TEM. The microstructure with no intergranular precipitate and refiner sub-grain after LSM process shows excellent IGC/IGSCC resistance. The stress relief heat treatment induced severe microstructure of high IGC/IGSCC susceptibility, owing to the semicontinuous intergranular precipitation. The influence of Nb/C ratio on IGC/IGSCC susceptibility of three nickel based superalloys after LSM process was also investigated. For both of the Inconel 182 alloys with different Nb content, the microstructure after LSM process and following sensitisation treatment showed precipitation free grain boundary. The results of corrosion tests also indicated that the material with higher Nb/C ratio showed higher IGC/IGSCC resistance after LSM process and following sensitisation treatment.     

Double loop electrochemical potentiokinetic reactivation test of Nickel-base Alloy 600 surface-melted by a CO2 laser beam

Ni-base Alloy 600 has been widely used as a steam generator (S/G) tubing material in nuclear power plants because of its good mechanical and corrosion properties at high temperatures. However, degradations of S/G tubes due to intergranular attack (IGA) and intergranular stress corrosion cracking (IGSCC) during normal operation have been frequently reported. In particular, Alloy 600 can be very susceptible to IGA/IGSCC in some sulfur-bearing environments by sensitization. In this paper, the beneficial effects of laser surface melting (LSM) on intergranular corrosion of the sensitized Alloy 600 is presented from the results of the double loop electrochemical potentiokinetic reactivation (DL-EPR) test. The DL-EPR test was performed in de-aerated 0.01 M H₂SO₄+20 ppm KSCN at a scan rate of 0.5 m V/sec at room temperature. The degree of sensitization (DOS) of the sensitized Alloy 600 measured from the DL-EPR test was considerably reduced by LSM. The sensitized Alloy 600 after LSM also exhibited a relatively low DOS, compared with that of the sensitized but not laser treated Alloy 600. From the microscopic observation, it was found that the microstructural changes brought about by the LSM process, especially changes in the precipitation behavior of grain boundary Cr-rich carbides, caused the improvement of resistance to intergranular corrosion of the laser treated Alloy 600. The resistance to IGSCC of the laser treated Alloy 600 in sulfur-bearing environments was also discussed from the results of measured DOS and microstructural examination. This article based on a presentation made in the symposium “The 4th International Conference on Fracture and Strength of Solid”, held at POSTECH, Pohang, Korea, August 16–18 under the auspices of Far East and Ocean Fracture Society (FEOFS),et al.     

Relationship between microstructure and corrosion performance of AA2050-T8 aluminium alloy after excimer laser surface melting

Surface modification by excimer laser surface melting (LSM) has been performed with the aim to improve the corrosion resistance of the AA2050-T8 alloy. LSM produced melted surfaces, largely free of precipitates, with both microstructure and corrosion behaviour depending upon the number of laser pulses employed. Increased number of laser pulses resulted in thicker melted layers, but also in greater trapped porosity and formation of micro-cracks at the overlapping area. Nevertheless, the LSM-treated specimens exhibited enhanced corrosion resistance compared to the untreated alloy, which was associated with the formation of a relatively uniform melted layer and a diminished presence of precipitates.     

High power diode laser treatments for improving corrosion resistance of A380/SiCp aluminium composites

Laser surface melting (LSM), using a high power diode laser, was used to modify the electrochemical behaviour of A380/SiC/xxp aluminium composites in aerated 3.5 wt.% NaCl solution. Corrosion mechanism was determined by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), scanning Kelvin probe force microscopy (SKPFM) and low-angle X-ray diffraction (XRD). Laser-treated surface revealed a refined microstructure with homogeneous microdendrites, good matrix/reinforcement bonding and absence of cathodic intermetallic compounds. As a result, laser treatment increased the surface hardness and reduced localised corrosion attack.     

Effect of excimer laser surface melting on the microstructure and corrosion performance of the die cast AZ91D magnesium alloy

Excimer laser surface melting (LSM) of the die cast AZ91D alloy has been investigated in terms of microstructure and corrosion behaviour. Excimer LSM of the alloy resulted in a highly homogeneous and refined melted microstructure, which improved the corrosion resistance of the alloy. The latter was associated with the large dissolution of intermetallic phases and the enrichment of aluminium within the melted layer. An increased number of laser pulses resulted in thicker melted layers, but also in enhanced porosity and the formation of micro-cracks at the overlapping area. Both factors diminished the corrosion resistance of the laser-treated alloy.     

Elimination of intergranular corrosion susceptibility of cold-worked and sensitized AlSl 316 SS by laser surface melting

Susceptibility to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) due to sensitization is one of the major problems associated with austenitic stainless steels. Thermal exposures encountered during fabrication (welding, hot working, etc.) and elevated temperature service may lead to sensitization of components of austenitic stainless steels. Laser surface melting (LSM) is an in-situ method to increase the life of a sensitized component by modifying the surface microstructure without affecting the bulk properties. In this paper, the results obtained in the attempt to improve IGC resistance of coldworked and sensitized 316 SS by LSM are presented. Type 316 SS specimens cold worked to various degrees ranging from 5 to 25% reduction in thickness and sensitized to different degrees by exposing at 898 K for different durations were laser surface melted using continuous wave (cw) CO₂ laser. ASTM standard A262 practice A, optical metallography, and ASTM standard G108 were used to characterize the specimens before and after LSM. Influence of prior deformation on the desensitization behavior was evaluated for the laser melting conditions adopted during the investigation. Complete dissolution of M₂₃C₆ due to laser melting and suppression of re-precipitation due to rapid quenching result in a desensitized homogenous microstructure, which is immune to IGC. Under identical laser melting conditions, the extent of desensitization decreases with an increase in the degree of cold work, and hence, higher power levels and an extended interaction time must be adopted to homogenize the sensitized microstructure with prior cold work.     

Microstructure and corrosion behavior of plasma electrolytic oxidation coated magnesium alloy pre-treated by laser surface melting

An AZ91D magnesium alloy was treated using duplex techniques of laser surface melting (LSM) and plasma electrolytic oxidation (PEO). The microstructure, composition and corrosion behavior of the laser melted surface, PEO coatings, LSM–PEO duplex coatings as well as the as-received specimen were characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electrochemical corrosion tests, respectively. Especially, the effect of LSM pre-treatment on the microstructure, composition and corrosion resistance of the PEO coatings was investigated. Results showed that the corrosion resistance of AZ91D alloy was marginally improved by LSM due to the refinement of grains, redistribution of β-phase (Mg₁₇Al₁₂) and increase of Al on the surface. Both the PEO and duplex (LSM–PEO) coatings improved significantly the corrosion resistance of the AZ91D alloys, while the duplex (LSM–PEO) coating exhibited better corrosion resistance compared with the PEO coating.     

Microstructure and corrosion behavior of laser surface-melted high-speed steels

Three high-speed steels (HSSs) M2, ASP23, ASP30 were surface-melted by a CW 2.5-kW Nd:YAG laser. The microstructure of the laser surface-melted HSSs was investigated by optical microscopy, scanning electron microscopy and X-ray diffractometry, and the hardness profiles of the laser surface-melted layers were determined by a Vickers hardness tester. The corrosion behavior in 0.6 M NaCl and 0.5 M NaHCO₃ solutions at 25 °C was studied by potentiodynamic polarization technique. Metallographical as well as electrochemical corrosion studies illustrated the beneficial effects of laser surface melting (LSM) in refining the microstructure and in enhancing the corrosion resistance of the HSSs. The large carbide particles of annealed HSSs were dissolved after LSM and ultrafine dendrites of austenite and martensite with submicroscopic carbide precipitation were formed in the melt zones of the laser surface-melted HSSs. LSM of M2, ASP23 and ASP30 produced surface layers of hardness 615, 580 and 665 Hv, respectively. The hardness of the laser surface-melted ASP23 and ASP30 reached about 0.75 to 0.80 that of the conventionally hardened ones, while the hardness of laser-melted M2 was comparable to that of conventionally hardened M2. The corrosion resistance of all laser surface-melted HSSs in both solutions was significantly improved, as evidenced by a noble shift of the corrosion potential and a reduction in the corrosion current density. Among the HSSs, laser surface-melted ASP23 possessed the highest corrosion resistance in both solutions. The presence of cobalt in ASP30 has no beneficial effect on enhancing its corrosion resistance. The enhancement in the corrosion resistance of the laser surface-melted HSSs is attributable to the combined effects of dissociation and refinement of large carbides and the increase of the passivating alloying elements such as Cr, Mo and W in solid solution.     

Precipitation and Cr depletion profiles of Inconel 182 during heat treatments and laser surface melting

Thermodynamic and kinetic modeling were conducted to simulate Cr depletion profiles near grain boundaries in Inconel 182 during heat treatments and laser surface melting (LSM) using Thermo-Calc and DICTRA code. The effect of Nb addition was also considered in the modeling. Based on the good agreement with Cr concentration distributions during the heat treatments measured experimentally, Cr depletion profiles adjacent to grain boundaries during the heat treatments and the LSM process were modeled. The Cr depletion profiles were evaluated using the Cr depletion area below the critical Cr concentration for intergranular cracking/intergranular stress corrosion cracking (IGC/IGSCC) susceptibility (12 mass%). Compared with the result of the Streicher test, the calculated Cr depletion areas showed good agreement with IGC/IGSCC susceptibilities. The sample after stress relief (SR) treatment had the largest Cr depletion area and showed the poorest IGC/IGSCC resistance. Cr depletion showed some recovery during subsequent low temperature sensitization (LTS). The sample after the LSM process had the smallest Cr depletion area and showed the best IGC/IGSCC resistance.     

Roughing,semi-finishing and finishing of laser surface modified nickel bonded NbC and WC inserts for grey cast iron (GCI) face-milling

An attempt to improve the machining performance of NbC-Ni cutting inserts by rapid pulse electric current sintering (PECS), TiC and Mo₂C additions and laser surface modification (LSM) was done. Use of a nickel binder and additions TiC and Mo₂C to liquid phase sintered (LPS) NbC based samples led to comparable hardness (>13 GPa) and K_IC (~10 MPa.m^1/2) to LPS WC-Co/Ni samples. The laser surface modification (LSM) technique produced a ~2.5 μm thick self-carbide coating, increasing the surface hardness of all the samples. Laser surface modification was done to improve abrasion and attrition wear resistance. Face-milling of grade 17 grey cast iron (BS 1452/GG35) was conducted at 100–500 m/min cutting speeds (v_c) and 0.25–1.5 mm depths of cut (a_p). The insert wear was measured after every pass, and analyzed by annular dark field scanning transmission electron microscopy (ADF-STEM). During roughing, WC-Co based inserts had the lowest flank wear rate (FWR) values, with the WC-10Co (LPS) insert having a FWR of 10.15 μm/min after 20 min cutting time. However, during semi-finishing and finishing, NbC-4TiC-12Ni (PECS) and NbC-4Mo₂C-4TiC-12Ni (PECS) inserts had the lowest FWR values, showing up to six times longer tool life than the WC-Co (LPS) inserts based inserts and 12 times longer life than the WC-Ni based inserts. Generally, LSM improved the NbC inserts' tool life, reducing the FWR values in all NbC based inserts in all cutting tests.     

Effect of laser surface melting on friction and wear behavior of AM50 magnesium alloy

In the present study, an attempt has been made to enhance tribological properties of AM50 magnesium alloy by laser surface melting (LSM) with a 2 kW continuous wave CO₂ laser. The microstructure of the laser surface melted zone consists of fine columnar dendrites growing epitaxially from the liquid-solid interface. Microhardness of the melted zone was improved to 55-75 HV as compared to 40 HV of the substrate. The friction and wear behavior of the laser surface melted layer were investigated using a ball-on-flat apparatus under dry sliding condition. It was found that the friction coefficient curve of the laser surface melted layer was similar to that of substrate. They showed a lower initial friction coefficient about 0.18 that after the running-in period increased up to about 0.38. Furthermore, compared with the AM50 substrate, the wear volume of the laser surface melted layer was decreased by 42%, the wear resistance of the laser surface melted layer was improved.     

Quantitative topography measurements of rolled aluminium surfaces by atomic force microscopy and optical methods

We have compared the results of quantitative topography measurements on typical industrial, rolled or etched aluminium surfaces, using three different methods that can measure surface features in the 10-μm to sub-micrometre range. The methods used, atomic force microscopy (AFM), white-light interferometry, and confocal laser scanning microscopy (LSM), utilize very different mechanisms of image formation. We find good agreement between data measured by AFM and by white-light interferometry. The two techniques complement each other. AFM is not limited to reflecting samples, it has a better resolution than interferometry, and it can measure steeper slopes. White-light interferometry is faster and allows larger areas to be analysed, and data from neighbouring areas can be spliced more easily. When used on strongly reflecting, rolled or etched aluminium surfaces, LSM yields results that deviate strongly from those of the other two methods and from the true sample surface topography.     

Microstructure and Electrochemical Behavior of La0.8Sr0.2MnO3 Deposited by Solution Precursor Plasma Spraying

采用液料等离子喷涂方法(SPPS)制备固体氧化物燃料电池多孔La0.8Sr0.2MnO3(LSM)阴极。用SEM观察LSM的微结构,用XRD研究其相结构。考察了喷涂距离和热处理温度对LSM微结构的影响规律。结果表明,SPPSLSM在1050℃热处理2h后形成连续的具有微纳介孔结构的涂层,且LSM具有单一的钙钛矿结构。利用电化学交流阻抗谱方法研究了LSM极化行为。微结构对极化性能有显著影响,1000℃时,LSM在喷涂距离为60mm时具有最佳的电化学性能,阴极极化电阻约为0.3Ω·cm2。通过工艺的控制,SPPS可以实现SOFC阴极相和微结构的优化。     

Study on laser-surface melting to enhance intergranular corrosion resistance of SUS 304 weld

A laser-surface melting method was studied with the aim of increasing resistance to intergranular corrosion of welded SUS 304 stainless steel. An Nd:YAG laser beam at a laser beam size of about 1 mm and power of 170 Wm was employed and the laser power density was varied to optimize the depth of the laser melted layers. The microstructures of TIG welded and laser-surface melted regions were measured via optical, scanning electron, and transmission electron microscopy. In addition, a comparative evaluation of the intergranular corrosion properties of as-TIG welded and as-LSM surfaces of the SUS 304 weld was carried out using a doubleloop electrochemical potentiodynamic reactivation(DL-EPR) polarization method in a 1 L aqueous solution of 0.5 M H₂SO₄ and 0.01 M KSCN. According to the test results, the maximum melted depth was obtained at a beam scan rate of 600 mm/min and laser power density of 20 J/mm². The laser-surface melted (LSM) region was observed to have a very fine, homogenous, and cellular microstructure compared to that of the TIG welded region. Grain growth in the laser-surface melted region from the substrate occurred epitaxially. The absence of Cr depletion along the grain boundary in the LSM region, which would result in increased resistance to intergranular corrosion of the welded SUS 304, was confirmed using an energy dispersive X-ray spectroscope attached to the electron microscope.