Effect of re-scanning on tribological characterization of laser surface alloyed layers

Laser surface alloying (LSA) is increasingly recognized as a powerful surface modification tool for enhancing the wear resistance of engineering components. Experimental investigations have been carried out to examine the influence of re-scanning on tribological behavior in laser surface alloying of Al with Ni. Dry sliding wear tests have been conducted using a reciprocating machine under different normal loads of 10, 15, and 20 N and at a constant sliding speed of 0.1 m/s. Re-scanning was found to substantially affect the laser-alloyed layer, including its phase constitution. Re-scanning leads to higher hardness compared to a single-time scan; however, the specific wear rate following re-scanning is lower. These observations are rationalized in terms of microstructure and the wear mechanism.     

Development of wear resistant composite surface on mild steel by laser surface alloying with silicon and reactive melting

The present study concerns laser surface alloying with silicon of mild steel substrate using a high-power continuous wave CO₂ laser with an objective to improve wear resistance. The effect of surface remelting using nitrogen as shrouding environment (with and without graphite coating) on microhardness and wear resistance has also been evaluated. Laser surface alloying leads to formation of a defect free microstructure consisting of iron silicides in laser surface alloyed mild steel with silicon and a combination of silicides and nitrides when remelted in nitrogen. Carbon deposition prior to remelting leads to presence of a few martensite in the microstructure. A significant improvement in microhardness is achieved by laser surface alloying and remelting to a maximum of 800 VHN when silicon alloyed surface is melted using nitrogen shroud with carbon coating. A detailed wear study (against diamond) showed that a significant improvement in wear resistance is obtained with a maximum improvement when remelted in nitrogen atmosphere followed by carbon coating.     

Corrosion behaviour of a steel surface laser alloyed with chromium borides

The corrosion behaviour of a carbon steel surface laser alloyed with chromium borides was investigated using electrochemical and surface analytical methods. Chromium and boron were unevenly distributed in the laser-alloyed layer. The distribution of chromium controlled the anodic dissolution of the alloy. Iron was dissolved preferentially from the surface and a loose corrosion product layer enriched in chromium was formed during active dissolution in 0.1m H₂SO₄. The chromium boride inclusions on the surface promoted hydrogen evolution and controlled cathodic behaviour. In neutral and basic solutions, the corrosion behaviour of the laser-alloyed surface was very close to that of the matrix steel. The effects of the alloying elements on corrosion behaviour are discussed.     

Influence of laser-mechanical treatment on surface topography,erosive wear and contact stiffness

This paper presents the study of surface topography, erosive wear and contact stiffness after the new hybrid method has been applied for the modification of steel surface layer. A combination of laser alloying and the burnishing process, both performed in laser set up, was proposed in order to reduce surface roughness being formed during laser treatment, and to improve contact stiffness. The experiments were conducted on stainless steel, alloyed with Stellite 6, and simultaneously burnished in hot and cold conditions in one operation. The alloying process was performed with continuous laser CO₂ at different parameters. The influence of hybrid treatment parameters on surface topography was examined. This hybrid treatment reconstructed the surface topography and caused an increase in surface smoothness compared to laser alloying. More than a threefold decrease in the average level of roughness, S_a, due to the burnishing process was stated. A good correlation between the parameters of hybrid treatment and roughness was demonstrated. The study shows that laser-mechanical treatment improves erosive wear and contact stiffness compared to laser alloyed and thermal sprayed Stellite 6 layers.     

Development of microstructure in laser surface alloying of steel with chromium

Laser surface alloying (LSA) was used to formin situ Fe-Cr-C alloys on AISI 1018 steel substrates. Chromium powders of different particle sizes were mixed together to obtain optimum packing density of the powder deposited on the substrate. The surface was then melted using a 2kW CW carbon dioxide laser. The processing conditions were related to solute (chromium) content, microstructural refinement of the laser alloyed zone and the heat affected zone (HAZ). The microstructure of the laser surface alloyed region was investigated by optical, scanning and transmission electron microscopy, and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying element. This process produced a fine distribution of complex type of carbide precipitate in the martensite-ferrite matrix because of the high cooling rate. An alloy of this composition does not show any retained phase. The nature of alloying and chemical diffusion profile as a function of intertrack separation distance affects the final content of alloying element in the surface layer.     

Laser surface alloying of case hardening steel with tungsten carbide and carbon

Abstract

The laser surface alloying process was used to introduce two different alloying materials, tungsten carbide (WC/Co) and carbon, into the molten surface of a case hardening steel (16MnCrS5), to improve its hardness and wear resistance. The chemical composition and the resulting microstructure in the alloyed layers were of particular interest in this investigation, because the strengthening mechanism was strongly dependent upon the type and amount of the alloy material. For laser alloying with carbon the increase in hardness and wear resistance was based on the martensitic transformation in the composition range concerned. For alloying with tungsten carbide it was necessary to consider two different strengthening mechanisms, namely, martensitic transformation and precipitation of carbides. In both cases the grain refinement in the laser affected zone had an additional effect. Resistance to dry abrasive sliding wear was measured using a conventional pin-on-disc wear testing machine. For both alloy materials the wear rate was substantially lower than that of a substrate that had been laser remelted without alloying additions.

MST/1556     

Influence of laser surface alloying with chromium and nickel on corrosion resistance of type 304L stainless steel

Abstract

The influence of laser surface alloying (LSA) with Cr and Cr + Ni on the corrosion behaviour of type 304L stainless steel (SS) was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) in chloride (0·5M NaCl) and acidic (1 N H₂SO₄) media. Surface alloying was carried out by laser cladding type 304L SS substrate with premixed powders of AISI type 316L SS and the desired alloying elements. The results indicated that Cr surface alloyed specimen exhibited a duplex (γ + α) microstructure with Cr content of ～24 wt-%, whereas Cr + Ni surface alloyed specimen was associated with austenitic microstructure with Cr and Ni contents of ～22 wt-% each. The potentiodynamic polarisation results in chloride solution indicated that LSA with Cr + Ni considerably enhanced the pitting corrosion resistance compared with LSA with Cr alone. In acidic media, such beneficial effects were not observed. Electrochemical impedance spectroscopy results showed an increase in semicircle arc for both chloride and acidic media for both Cr and Cr + Ni clad samples indicating improvement in the oxide film stability compared with untreated specimen. The polarisation resistance was higher and capacitance values of the laser clad specimen were lower than those in the untreated specimen. The microstructural changes and compositional variations produced by LSA are correlated to the corrosion behaviour.     

Microstructure and wear resistance enhancement of cast steel rolls by laser surface alloying NiCr–Cr3C2

The laser surface alloying technique was used to form wear resistant layers on 70MnV cast steel rolls with NiCr–Cr₃C₂ powders. The objective was to investigate the effects of the scanning speed on microstructure, phases, microhardness and wear resistance. Results indicate that the alloyed layers had dense, pore and crack free and homogeneous structures, as well as a metallurgical bonding with the substrates. With the increase of scanning speed, volume of retained austenite in the alloyed layer increased, microhardness and wear resistance increased and the microstructure refined. Wear results indicated that the wear resistance of the alloyed layer was enhanced by 7.8 times compared with that of the cast steel substrate. The improvement in wear resistance was attributed to the combined results of the grain refining effect, the solution strengthening effect, the tough γ-Fe matrix of the layer, the distribution of the hard Cr₇C₃, Fe₃C and martensite phases, and the good bonding between these hard phases and the matrix.     

Microstructural study and wear behavior of ductile iron surface alloyed by Inconel 617

In this research, microstructure and wear behavior of Ni-based alloy is discussed in detail. Using tungsten inert gas welding process, coating of nearly 1–2 mm thickness was deposited on ductile iron. Optical and scanning electron microscopy, as well as X-ray diffraction analysis and electron probe microanalysis were used to characterize the microstructure of the surface alloyed layer. Micro-hardness and wear resistance of the alloyed layer was also studied. Results showed that the microstructure of the alloyed layer consisted of M₂₃C₆ carbides embedded in Ni-rich solid solution dendrites. The partial melted zone (PMZ) had eutectic ledeburit plus martensite microstructure, while the heat affected zone (HAZ) had only a martensite structure. It was also noticed that hardness and wear resistance of the alloyed layer was considerably higher than that of the substrate. Improvement of wear resistance is attributed to the solution strengthening effect of alloying elements and also the presence of hard carbides such as M₂₃C₆. Based on worn surface analysis, the dominant wear mechanisms of alloyed layer were found to be oxidation and delamination.     

High-temperature oxidation resistance improvement of titanium using laser surface alloying

Commercially pure titanium substrate was coated with aluminium by flame spraying or arc spraying techniques and was irradiated with a laser to form a laser-alloyed zone (LAZ). The microstructure of the LAZ was investigated. High-temperature oxidation tests were carried out in air at different high temperatures and the characteristics of the oxide layer were investigated using Auger electron spectroscopy. The aqueous corrosion tests were carried out in 3.5% NaCl solution at room temperature. The laser-processed material showed considerable improvement in the high-temperature oxidation-resistant property while the aqueous corrosion-resistant property of the LAZ was found to remain almost unaffected.     

Effect of silicon content on the microstructure and properties of Fe–Cr–C hardfacing alloys

In this study, the surface of St52 steel was alloyed with preplaced powders 55Fe39Cr6C, 49Fe39Cr6C6Si, and 45Fe39Cr6C10Si using a tungsten-inert gas as the heat source. Following surface alloying, conventional characterization techniques, such as optical microscopy, scanning electron microscopy, and X-ray diffraction were employed to study the microstructure of the alloyed surface. Microhardness measurements were performed across the alloyed zone. Room-temperature dry sliding wear tests were used to compare the coatings in terms of their tribological behavior. It was found that the as-deposited coatings contained higher volume fractions of carbides (Cr₇C₃). The presence of 6%Si in the preplaced powders caused an increase in microhardness and wear resistance.     

Friction and wear of ZrO2-TiO2 surface-alloyed Al2O3 ceramics in unlubricated sliding contact

Slightly and highly porous Al₂O₃ ceramics were surface remelted and alloyed by adding ZrO₂ and TiO₂ using infrared CO₂ laser radiation. The resulting composite layers of thickness of about 200 m?m contained about 31 vol.% of ZrO₂- and TiO₂-rich phases which were homogeneously distributed at the grain boundaries of the alumina matrix. Microstructures and worn surfaces were analysed by electron microscopy and EDX analysis. Tribological tests were carried out unlubricated using conditions of abrasive wear and oscillating slinging wear, respectively. The results showed that the average grain size and hardness of the ceramics were reduced due to alloying. Despite decreasing hardness the wear resistance was substantially increased. Friction and wear of the untreated ceramics depended strongly on the amount of porosity which was removed or substantially reduced by laser treatment. Surface alloying of Al₂O₃ ceramics can offer an effective process for producing components showing very different surface and bulk properties and particularly improved tribological behaviour. For the alloying elements and experimental conditions used, the improvement was more pronounced in wear resistance than in friction coefficient.     

In situ formation of TiC particulate composite layer on cast iron by laser alloying of thermal sprayed titanium coating

Commercial flake graphite cast iron substrate was coated with titanium powder by low pressure plasma spraying and was irradiated with a CO₂ laser to produce the wear resistant composite layer. The macro and microstructural changes of an alloyed layer with the traveling speeds of laser beam, the precipitate morphology of TiC particulate and the hardness profile of the alloyed layer was examined. From the results, it was possible to composite TiC particulate on the surface layer by direct reaction between carbon existed in the cast iron matrix and titanium with thermal sprayed coating by remelting and alloying them using laser irradiation. The cooling rate of the laser remelted cast iron substrate without a titanium coating was about 1 × 10⁴ K/s to 1 × 10⁵ K/s in the order under the condition of this study. The microstructure of the alloyed layer consisted of three zones; the TiC particulate precipitate zone (MHV 400–500), the mixed zone of TiC particulate + ledeburite (MHV 650–900) and the ledeburite zone (MHV 500–700). TiC particulates were precipitated as a typical dendritic morphology. The secondary TiC dendrite arms were grown to a polygonized shape and were necking. Then the separated arms became cubic crystal of TiC at the slowly solidified zone. In the rapidly solidified zone near the fusion boundary, however the fine granular TiC particulates were grouped like grapes.     

Schwingungsverschleiß an TiB2-ZrO2 und TiC-ZrO2 oberflächenlegierter Al2O3-Keramik unter Variation der Luftfeuchte

Oscillating sliding wear of TiB₂-ZrO₂ and TiC-ZrO₂ surface alloyed Al₂O₃ ceramic at different humidity A commerical alumina ceramic was surface-alloyed by adding TiB₂ and TiC in addition to ZrO₂ using infrared CO₂ laser radiation. Aside from the type of hard particles their volume fraction was varied between 6 and 31%. The average thickness of the alloyed surface layer was about 150 ¨?m. Tribological tests were carried out unlubricated under oscillating sliding contact against alumina counterbodies at relative humidities of 3 and 50% as well as in distilled water. Ceramographic studies showed that multiphase structures containing hard particles of TiC or TiB₂ embedded in the Al₂O₃ matrix were obtained by alloying. In addition Al-Zr-Ti-O complex phases were analyzed by using X-ray diffraction technique. Compared to the commercially available alumina ceramic A123, laser alloying reduced the friction coefficient and improved substantially the wear resistance under the applied conditions of tribological testing. It was found that tribological behaviour was strongly influenced by environmental humidity in addition to the effect of the type and volume fraction of the phases produced by alloying. The unalloyed alumina ceramic depended more sensitively on humidity than the alloyed ceramic.     

钛合金激光表面处理研究

本文采用5KWTEACO_2激光器,对 Ti—6Al—4V 钛合金进行表面合金化处理,实验结果表明,经激光辐照后,材料表层及次表层组织结构产生变化,与基体组织大不相同,其硬度有很大提高,由 Hv250提高到 Hv800～900。磨损试验表明,经激光处理的试件,其耐磨性提高2～3倍。扫描电镜分析表明:处理区域由精细枝晶区,等轴细晶区和马氏体区组成,表层硬度的提高及耐磨性的改善与表层组织形态、晶粒细化及硬质点的弥散分布有关。     

High-temperature thermal barrier coating formation by laser alloying of CP-Ti with pre-placed Ni-SiC coating

High-temperature thermal barrier coating was created on CP-Ti using a pre-placed Ni-SiC layer by laser alloying technique. The coating was developed using 80% Ni + 20% SiC, 50% Ni + 50% SiC and 60% Ni + 40% SiC, and the latter two compositions are found to be efficient in producing a uniform layer. The 100% SiC pre-placement was also used. A flaw-less coating of 0.4–0.6 mm thickness was produced at a lower power density of 1.3 to 1.9 × 10⁵ W cm^–2. Very high power density of 2.5–3.0 × 10⁵ W cm^–2 is inefficient to produce uniform coating. The laser alloyed coating consists of dendrites and intermetallic precipitates. The degree of dendrite population depends upon the coating composition and laser processing conditions. The coating hardness was 600–1200 HV, which is three to six times higher than the base titanium. Uniform hardness was obtained for the coatings produced at a laser power density of 1.3 × 10⁵ W cm^–2. The titanium silicide (TiNiSi, Ti₅Si₃, TiSi) and nickelide (NiTi₂) phases formed on the laser-alloyed coating surface was confirmed by X-ray analysis. These intermetallic phases can improve high-temperature properties of titanium and its alloys. The effect of laser power density and coating composition on the alloying depth alloying width, hardness and microstructure are discussed. The present work investigated the microstructure evolution, hardness and compound phases by means of optical and scanning electron microscopy, Vickers hardness testing, EDXRD and SIMS analysis. A 5 kW CW CO₂ laser was used for laser alloying experiments.     

Laser surface coating of Mo–WC metal matrix composite on Ti6Al4V alloy

Laser surface alloying of Mo, WC and Mo–WC powders on the surface of Ti6Al4V alloys using a 2 kW Nd-YAG laser was performed. The dilution effect upon the microstructure, microhardness and wear resistance of the surface metal matrix composite (MMC) coating was investigated. With a constant thickness of pre-placed powder, the dilution levels of the alloyed layers were found to increase with the incident laser power. The fabricated surface MMC layer was metallurgically bonded to the Ti6Al4V substrate. The microhardness of the fabricated surface layer was found to be inversely proportional to the dilution level. The EDAX and XRD spectra results show that new intermetallic compounds and alloy phases were formed in the MMC layer. With the existence of Mo content in the pre-placed powder, the β-phase of Ti in the MMC coating can be retained at the quenching process. With increasing weight percentage content of WC particles in the Mo–WC pre-pasted powder, the microhardness and sliding wear resistance of the laser surface coating were increased by 87% and 150 times, respectively, as compared with the Ti6Al4V alloy. The surface friction of the laser-fabricated MMC coatings was also decreased as compared with the worn Ti6Al4V substrate.     

温度对40Cr13钢等离子表面渗铌层组织的影响

为提高马氏体不锈钢的耐蚀和耐磨性能,选择40Cr13不锈钢为基材、纯铌板为靶材,采用双辉等离子表面冶金技术在不锈钢表面制备合金化层.用SEM、GDOES、XRD等方法分析渗铌温度对铌合金层组织、成分、相组成、表面形貌及硬度的影响,并对渗层形成机制及表面硬化机理进行了研究.结果表明:在900~1 000℃形成的铌合金层组织均匀致密,合金层主要由Nb2C、Nb C、Fe2Nb、Cr2Nb及铌组成;合金层表面粗糙度随渗铌温度的提高而增加;合金层厚度随渗铌温度改变发生不同变化规律,950℃渗铌形成的渗层约13μm,900和1 000℃渗铌后合金层厚度均为7μm左右;不同温度渗铌后试样的表面硬度与基体相比均有较大幅度的提高,1 000℃渗铌后试样表面硬度高达约985 HV0.025,900℃渗铌后约758 HV0.025,而950℃渗铌后表面硬度最低,约698 HV0.025.     

Y_2O_3含量对38CrMoAl钢表面激光合金化WC/Ni金属陶瓷组织与性能的影响

采用激光合金化技术,在38CrMoAl钢表面制备不同Y_2O_3含量的WC/Ni合金化层。利用X射线衍射仪(XRD)、扫描电镜(SEM)、电子探针(EPMA)、显微硬度计和摩擦磨损试验机,系统研究合金化层的相组成、显微组织、显微硬度及摩擦磨损性能随Y_2O_3含量的变化规律。结果表明:不同Y_2O_3含量的合金化层皆是由γ-(Fe,Ni)、基体马氏体、M3C及WC相组成,其中纳米WC颗粒主要分布在合金化层上部的枝晶间,而微米WC颗粒则分布于合金化层底部边缘区,且在颗粒边缘形成有明显的外延生长层。随着Y_2O_3含量的增加,具有亚共晶形貌特征的凝固组织逐渐细化,γ-(Fe,Ni)和M3C数量增多,基体马氏体数量略有减少。但当Y_2O_3含量(质量分数,下同)超过1.0%时,凝固组织开始有所粗化。随Y_2O_3含量增加,合金化层硬度呈先增后降、摩擦因数和磨损失重呈先减后增的变化趋势。当Y_2O_3含量为1.0%时,合金化层硬度(781HV0.2)最高,为基体的2.4倍;摩擦因数和磨损失重最小,分别为基体的17%和8.9%。     

20Cr_（2）Ni_（4）W钢钴粉涂层激光合金化的试验研究

本文采用２ｋＷＣＯ_２激光器对２０Ｃｒ_（２）Ｎｉ_（４）Ｗ钢进行了激光表面钴合金化的试验研究。用光学显微镜、扫描电镜、电子探针、Ｘ射线衍射、透射电镜、显微硬度计等手段分析了合金化区域的成分、组织和性能。结果表明，激光表面钴合金化可获得含钴均匀的合金化层，合金化层的高温硬度和热疲劳性能均有提高，在热作工具钢表面性能优化领域有着良好的应用前景。