Effect of beam interaction time on laser alloying with pulsed Nd–YAG laser

Abstract

Surface alloying of aluminium with nickel was carried out using a pulsed Nd–YAG laser. The effect of beam interaction time on laser alloying of aluminium with pulsed Nd–YAG laser has been studied. It was found that the beam interaction time of a pulsed laser has a significant effect on microstructure and properties of alloyed layers. The results indicated that with changes in the beam diameter, higher thickness of alloyed layer and higher microhardness are both obtained at a lower effective interaction time. When travel speed changes, the same conditions are obtained at a higher effective interaction time.     

Laser-beam and photon-assisted processed materials and their microstructures

Laser processing is a relatively new technique for modifying the near-surface region of materials without altering the in-bulk characteristics. A single laser can perform several functions by manipulating processing conditions such as laser power, beam diameter, and traverse speed. Lasers have shown attractive applications, such as cutting, welding, glazing, alloying, and cladding. A laser glazing process has demonstrated an improvement in the microstructure of vacuum plasma-coated copper-based alloys containing cavities, unmelted particles, and segregation. Laser glazing has also been shown to restore the degraded microstructure of components and make them equivalent to, or better than, the original wrought alloy. The laser cladding concept was used to develop nickel-based alloys for high-temperature applications that exhibited higher thermal stability than the nickel-based Rene-95 alloy. Rapid melting and quenching occurred during the laser glazing, alloying, and cladding processes resulting in a fine-grained microstructure, metastable phases and extended solid solubility of alloying additions in the matrix.Photon-assisted processing of material is a relatively new technique being explored to synthesize new materials from various substrates (solid, liquid, and gas). This process is successfully used to fabricate high-quality thin films for electronic industries. Thin films of multicomponents can be deposited with stoichiometric composition. Diamond thin films have been synthesized from liquid hydrocarbon (Benzene, C₆H₆) by laser-liquid hydrocarbon-substrate interaction. A laser-assisted physical vapour deposition process was found to be very successful in depositing stoichiometric compositions of multilayered thin films such as superconducting YBa₂Cu₃O₇, ferroelectric Pb_0.52Zr_0.48TiO₃ and other coatings such as TiN and CoSi₂. This review reports some of the major advances in the understanding and engineering of new materials for electronic industries and high-temperature applications in the auto, aerospace, and turbine industries.     

Functionally gradient ceramic/metallic coatings for gas turbine components by high-energy beams for high-temperature applications

Failure of turbine blades generally results from high-temperature oxidation, corrosion, erosion, or combinations of these procedures at the tip, and the leading and trailing edges of a turbine blade. To overcome these limitations, functionally gradient ceramic/metallic coatings have been produced by high-energy beams for high-temperature applications in the aerospace and turbine industries to increase the life of turbine components. Thermal spray processes have long been used to apply high-temperature thermal barrier coatings to improve the life of turbine components. However, these processes have not met the increased demand by the aerospace and turbine industries to obtain higher engine temperatures and increased life enhancement as a result of the inhomogeneous microstructure, unmelted particles, voids, and poor bonding with the substrate. High-energy beams, i.e. electron beam-physical vapour deposition (EB-PVD), laser glazing, laser surface alloying, and laser surface cladding, have been explored to enhance the life of turbine components and overcome the limitations of the thermal spray processes. EB-PVD has overcome some of the disadvantages of the thermal spray processes and has increased the life of turbine components by a factor of two as a result of the columnar microstructure in the thermal barrier coating (TBC). Laser glazing has been used to produce metastable phases, amorphous material, and a fine-grained microstructure, resulting in improved surface properties such as fatigue, wear, and corrosion resistance at elevated temperatures without changing the composition of the surface material. Laser surface alloying and laser surface cladding have shown promising results in improving the chemical, physical, and mechanical properties of the substrate's surface. Metal-matrix composite coatings have also been produced by a laser technique which resulted in increased wear and oxidation-resistant properties. The advantages and disadvantages of thermal spray processes, EB-PVD, laser glazing, laser surface alloying, and laser surface cladding will be discussed. Microstructural evolution of thermal barrier coatings, recent advancements in functionally gradient coatings, laser grooving, and multilayered textured coatings will also be discussed.     

A comparative study of Inconel 625 laser cladding by wire and powder feedstock

It has been established that laser cladding technique is useful for enhancing surface performances, hence extending the life of many components in severe corrosive-wear environments. However, a comparative study of the surface performances of laser coatings made via powder and wire feeding systems has not been performed. Inconel 625 powder and wire were deposited on the AISI 304 substrate using similar processing parameters. The microstructure, together with the depth of substrate penetration and the degree of dilution of the deposited tracks, was examined using an optical microscope and a scanning electron microscope (equipped with an energy-dispersive spectrometer). The micro-hardness was measured using a Vickers hardness tester. At optimized parameters, laser tracks of Inconel 625 wire and powder had a strong metallurgical bond at the track–substrate interface and no crack and pore were deposited. Due to the higher laser beam infiltration and a larger depth of substrate penetration, higher substrate dilution was observed in the powder-fed tracks. The tracks comprise continuous ?-matrix and secondary compounds (rich in Nb and Mb). However, finer dendritic microstructure and higher number density of inter-dendritic precipitates were observed in a typical powder laser track compared with the corresponding wire laser track. The typical powder laser track demonstrated higher hardness (245 HV_0.3) compared with the corresponding wire laser track (224 HV_0.3).     

Stellite 21 coatings on AISI 410 martensitic stainless steel by gas tungsten arc welding

Abstract

Degradation of AISI 410 martensitic stainless steel, a typical alloy for many applications such as steam turbine blade, could impair its efficiency and lifetime. To overcome this problem, critical surfaces could be modified by weld cladding via gas tungsten arc welding technique. In the present research, a comparative study of Stellite 21 weld overlays deposited in three different thicknesses, i.e. dilutions, at various preheat and post-weld heat treatment temperatures on the surface of AISI 410 martensitic stainless steel, has been made. The surface of coatings has been examined to reveal their microstructures, phase characterisation and mechanical properties using XRD, microhardness tester and metallographic techniques. The results showed that the deposition of Stellite 21 coating on AISI 410 martensitic stainless steel improved its corrosion resistance. Moreover, the volumetric dilution had a considerable effect on the hardness, microstructure and electrochemical corrosion behaviour of Stellite 21 weld overlays.     

Laser surface alloying of 1045 AISI steel using Ni–CrB2 powder

Abstract

Laser surface alloying is a process whose purpose is to improve the surface properties by incorporating alloying elements into the surface. The advantages of using laser for surface treatment are: formation of a non-equilibrium or amorphous phase as well as homogenisation and refinement of the microstructure, all without affecting the substrate properties. Powder (50 wt-%Ni–50 wt-%CrB₂) was injected into a melt pool created by a CW–CO₂ laser on AISI1045 steel plates. In order to alloy the entire surface, the sample was scanned at scan speeds in the range of 600–6000 mm min^–1 and the laser power was in the range of 1750–2500 W. The powder feed rate was 1·6 g min^–1, the laser beam was 2 mm in diameter, with 60% overlap between successive laser paths. Metallographic cross-sections were made of the samples. For each sample the following properties were characterised: layer depth, microhardness (HV), layer microstructure and composition. It has been found that the scan speed and the laser power affect the depth of the melt pool, the microstructure, the hardness and the treated layer composition. The laser boronised surface exhibits better wear resistance than D2 tool steel hardened to 59 ± 1 HRC. This will be discussed based on numerical analysis of the laser/material interaction.     

Effect of re-melting on the cladding coating of Fe-based composite powder

The aim of this paper is to investigate the influence of laser re-melting on the geometry and overlapping pores of cladding coating. In this study, Fe-based alloy mixed with 5 wt.% Cr₃C₂ powder was used as cladding material to obtain high hardness and strength coating. However, this coating has overlapping pores and a rough surface. Therefore, re-melting process is explored systematically. The geometry, microhardness and microstructure of the coatings have been analyzed and compared subsequently. Besides, a 3D finite element model has been built to provide a thermal field analysis for laser re-melting. Finally, it is found that re-melting process is an effective method to improve the surface smoothness of cladding layer, which could reduce the process cycle and cost of secondary operations. More importantly, it could remove the overlapping pores easily.     

铝合金表面激光熔覆铜镍合金涂层的组织、硬度与耐蚀特性研究

为提升铝合金材料的耐蚀性能,探究铝合金与合金熔覆层间的结合机理,本文利用激光熔覆技术在5083铝合金表面制备了不同Ni含量的铜镍合金熔覆层,并利用扫描电子显微镜、X射线衍射分析、硬度测试与电化学性能测试技术,分析了不同Ni含量的铜镍合金熔覆层相组成与组织形貌、铜镍合金熔覆层与铝合金基体的界面组织形貌,绘制了铝元素扩散曲线,分析了海水腐蚀过程中铜镍合金熔覆层的极化曲线。实验结果表明：所制备的铜镍合金熔覆层形貌良好无缺陷,熔覆层由网络状枝晶组成。对合金熔覆层进行XRD分析发现熔覆层主要由AlNi₃与CuNi两相组成。结合SEM、EDS分析,发现合金熔覆层的网络状枝晶为富铝相,即AlNi₃,晶间相为CuNi相。在硬度测试中,由于AlNi₃硬质相的生成,熔覆层硬度得到了提升且随着铝的向上扩散呈现一定的规律,电化学检测结果表明,铜镍合金熔覆层具有比5083铝合金更高的自腐蚀电位和较小的自腐蚀电流密度,可以有效提升5083铝合金在海水环境中的耐蚀性。     

激光熔覆涂层与热喷涂涂层组织性能比较

分别采用激光熔覆与火焰喷涂两种技术在38CrMoAl基体表面制备NiCrBi WC(wt20%)合金涂层,比较其组织结构及其硬度分布,试验表明：激光熔覆涂层无缺陷,成品率高,组织细密均匀,晶粒细小,硬度高,对基体热影响小,与基体结合为成分缓慢过渡的冶金结合。     

Laser surface hardening of AISI 01 tool steel and its microstructure

Abstract

The effects of laser surface hardening on AISI 01 tool steel samples were studied by changing the laser operating parameter combinations and the initial steel microstructure. Both melted and solid state transformed regions were produced, and then studied using optical microscopy, analytical electron microscopy, X-ray diffraction, and measurements of micro hardness to investigate the hardening mechanisms and the development of compressive residual stresses. The results indicate that hardened case depths up to 0·6 mm can be obtained using a laser beam operated at a power of 500 W and a scan rate of 2·1 mm s^?1, but that different amounts of retained austenite and undissolved carbides are observed for different beam powers. Quenched and tempered AISI 01 steel samples, with initial hardness values in the range 30–40 HRC, are better suited for laser surface hardening compared with the samples with initial hardness of 48–50 HRC, because the formation of an over tempered region adjacent to the hardened zone can be avoided.

MST/901     

Microstructure and micro‐hardness of in situ synthesized TiC particles reinforced Fe‐based alloy composite coating by laser cladding

A Fe‐based composite coating reinforced by in situ synthesized TiC particles was fabricated on Cr12MoV steel by using 6 KW fiber laser cladding. A serial of experiment has been carried out with different laser power, scanning speed, and powder feed rate, from which TiC could be in situ synthesized only in certain realms laser cladding parameters. X‐ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscope and a hardness tester are used to test the microstructure, micro‐hardness and component distribution. The coating is mainly composed of alpha ;‐Fe, TiC and Fe₃C. TiC particles were commonly precipitated in three kinds of morphologies, such as quadrangle, cluster, and flower‐like shape. The grains were refined, and there were no cracks and few stomas. Defect‐free coating with metallurgical joint to the substrate was obtained. TiC distributed more concentratively in the upper layers than the middle and bottom layers. From the surface of cladding layer 0.8 mm the highest micro‐hardness was up to HV930, obviously higher than that of the substrate.     

Microstructural characterization of AISI 431 martensitic stainless steel laser-deposited coatings

High cooling rates during laser cladding of stainless steels may alter the microstructure and phase constitution of the claddings and consequently change their functional properties. In this research, solidification structures and solid state phase transformation products in single and multi layer AISI 431 martensitic stainless steel coatings deposited by laser cladding at different processing speeds are investigated by optical microscopy, Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), orientation imaging microscopy (OIM), ternary phase diagram, Schaeffler and TTT diagrams. The results of this study show how partitionless solidification and higher solidification rates alter the microstructure and phase constitution of martensitic stainless steel laser deposited coatings. In addition, it is shown that while different cladding speeds have no effect on austenite–martensite orientation relationship in the coatings, increasing the cladding speed has resulted in a reduction of hardness in deposited coatings which is in contrast to the common idea about obtaining higher hardness values at higher cladding speeds.     

Novel Fe70Zr10Ni6Al4Si6B4 thick metallic glass coating produced by laser cladding

Abstract

A novel multicomponent thick metallic glass coating has been synthesised by laser cladding. The maximum coating thickness was 1 mm. The clad cooling rate restrained the epitaxial growth of dendrites in the metallic glass coating. The metallic glass had high glass forming ability with a wide supercooled liquid region ranging from 59 to 70 K. The metallic glass coating also revealed high hardness and good corrosion resistance.     

Cr3C2 incorporation into an Inconel 625 laser cladded coating: Effects on matrix microstructure,mechanical properties and local scratch resistance

There is an increasing industrial demand for metal alloys with high wear resistance under severe operating conditions. Ni-based alloys, such as Inconel superalloys, are an excellent option for these applications; however, their use is limited by their high cost. Ni-based coatings deposited onto carbon steel substrates are being developed to achieve desired surface properties with reduced cost. Laser cladding deposition has emerged as an excellent method for processing Ni based coatings. In this work, microstructure, mechanical properties and local wear behaviour have been investigated in response to the addition of Cr₃C₂ ceramic particles into an Inconel 625 alloy deposited onto a ferritic steel substrate by laser cladding. Using this deposition technique, a homogeneous distribution of Cr₃C₂ particles was observed in the coating microstructure. The addition of ceramic particles to the starting powder resulted in the formation of hard precipitates in the coating microstructure. The partial dissolution of Cr₃C₂ particles during the laser cladding process increased the hardness of the Inconel 625 matrix. Depth sensing indentation and scratch tests were performed to study the local wear behaviour and scratch resistance of the cermet matrix compared with the conventional Inconel 625 alloy. Finally, the effect of Cr₃C₂ on mechanical properties was correlated with the observed microstructure modifications.     

On the Global Efficiency of the Laser Optical Excitation of a Fast Atomic Beam

Abstract

The interaction of a fast atomic beam and a laser beam that crosses at right angles has been considered. We have studied the competition between the Doppler effect, due to the angular divergence of the atomic beam and the effect of the laser light intensity distribution. For low laser power values, an optimum waist size can be determined. For higher laser power values, the conditions for a maximum global efficiency are outlined.     

An analysis of high efficiency stimulated Raman scattering in water with a frequency-doubled pulsed Nd:YAG source

Abstract

Unexpectedly high stimulated Raman scattering (SRS) has been seen with a frequency-doubled Nd:YAG pump laser (532 nm) in water. The pump beam was propagated, both focused and unfocused, through a 4m long tank. Threshold pump irradiance for SRS was measured to be 2 × 10¹³ Wm^?2 and Stokes transitions were identified at approximately 3230 and 3380 cm^?1. The maximum conversion efficiency to the Stokes frequencies seen was 13% of pump input, for a focused beam, 4% for unfocused. SRS from an unfocused beam was seen to arise from high irradiance filaments caused by self-focusing in the water. The term Raman Interaction Function (RIF) has been defined here as the irradiance of the pump beam integrated over the interaction length (distance over which the pump beam irradiance is above threshold). The RIF model was been designed to describe the relationship between pump laser parameters and the output at the dominant Stokes frequencies.     

激光熔覆添加碳化钨的镍基合金层的组织和硬度研究

为促进激光熔覆在金属材料表面改性中的进一步应用,利用光学显微镜、扫描电镜、显微硬度计和电子探针微区成分分析,研究了Q235钢基体上激光熔覆添加有WC的Ni基合金层的特性及其演变,讨论了熔覆层合金成分和显微硬度的变化规律,力求得到熔覆粉的最佳WC含量.结果表明,当添加WC低于30%(质量分数)时,激光熔覆不出现裂纹;而当WC质量分数增加到30%(质量分数)时,熔覆层平均硬度增加,出现裂纹.在熔覆层中WC完全熔化并溶解,凝固组织主要由枝晶和枝晶间共晶组成,熔覆层呈胞状或条带状快凝亚稳的两相组织.     

Effect of process parameters and niobium carbide addition on microstructure and wear resistance of laser cladding nickel-based alloy coatings

Based on the background of the engineering application of automobile mold repair and surface strengthening, the effects of process parameters on the formation and microstructure of laser cladding nickel(Ni)-based alloy coating were studied. The optimal parameters were: laser power 2000 W, powder feeding rate 15 g/min, scanning speed 4 mm/s. Under this process, the cladding layer and the substrate can exhibit good metallurgical bonding, and the cladding layer has fine crystal grains and a low dilution ratio. On this basis, different mass fractions of niobium carbide (NbC) powder were added to the nickel-based powder and laser cladding was carried out on the surface of die steel. The phase composition, microstructure, hardness and wear resistance of the coating were studied. The results show that with the increasing of niobium carbide addition, the hardness of the cladding layer decreases, and the wear loss of the cladding layer decreases first and then increases. When the niobium carbide addition reaches 6 wt.%, the wear loss of the cladding layer is the least, and the wear resistance is the best.     

Integrated laser based pre-tempering at laser welding of AISI 1045 steel by using 3D-scanner optics

Laser-based pre-heating of laser beam welding with a 3D scanning optics, applied to AISI 1045 steel, is studied. Laser beam welding of heat-treatable steel is challenging due to martensitic hardening in combination with defects. Pre-tempering aims the reduction of the cooling rates and martensitic microstructure within the weld seam. An oscillating defocused laser beam was guided over the surface for pre-heating by means of a 3D scanner optics. During pre-heating, the laser power, the scanning speed and the number of cycles were varied. Welding with 4000 W and 2 m/min with a focused laser beam was executed. Thus the resulting temperature profile behind the ongoing laser beam and cooling time T_8|5 between 800 °C–500 °C was significantly extended. Two parameter combinations (15 cycles|600 W|50 mm/s(²) and 10 cycles|800 W|50 mm/s²) succeeded in a microstructure of bainite and martensite. By extending the cooling time T_8|5 to 3.11 s(²) and 4.17 s². Thus, average hardness for laser based pre-tempering of 487 HV 0.5(²) and 455 HV 0.5² was achieved. As a reference, global pre-heating at 400 °C using a heating plate can reduce the average hardness of the weld zone from 729 HV 0.5 at room temperature to 304 HV 0.5 at a cooling time T_8|5 of 5.63 s.     

Effect of heat treatment on microstructures of flow formed C-250 maraging steel

Abstract

The technique of forward flow forming has been used to produce a long thin walled tube of C-250 maraging steel. Forward flow forming can conserve material, increase strength and reduce production process time. Since the hardness of solution treated C-250, using flow forming, can be increased by approximately 16.2%, flow formed tube must be given an additional aging treatment to increase hardness and strength. Direct aging treatment yields low elongation, which is not suited to design requirements. Using a homogenisation treatment, the microstructure of C-250 was of coarse grained lath type on aging and of needle type after solution treatment and aging. These treatments resulted in low tensile strength, and tensile fracture showed a tendency to brittleness. A 'solution + aging' treatment yielded greater strength and elongation.