Underwater and water-assisted laser processing: Part 1—general features, steam cleaning and shock processing

Water is always present in laser processing in air: as vapor, condensate or adsorbate. Water is the working environment in underwater processing—but it can also be added on purpose to gain better results: to avoid redeposition of debris, to cool the material, to increase plasma pressure or to conduct light. Water can also act as a chemical reagent. The first part of the article will review the advantages and disadvantages of laser processing in the presence of water, light transmission by water, and the two most mature methods of water-assisted laser processing: steam cleaning and shock processing.     

Effect of laser shot peening on precipitation hardened aluminum alloy 6061-T6 using low energy laser

Mechanical properties of engineering material can be improved by introducing compressive residual stress on the material surface and refinement of their microstructure. Variety of mechanical process such as shot peening, water jet peening, ultrasonic peening, laser shot peening were developed in the last decades on this contrast. Among these, lasers shot peening emerged as a novel industrial treatment to improve the crack resistance of turbine blades and the stress corrosion cracking (SCC) of austenic stainless steel in power plants. In this study we successfully performed laser shot peening on precipitation hardened aluminum alloy 6061-T6 with low energy (300 mJ, 1064 nm) Nd:YAG laser using different pulse densities of 22 pulses/mm² and 32 pulses/mm². Residual stress evaluation based on X-ray diffraction sin² ψ method indicates a maximum of 190% percentage increase on surface compressive stress. Depth profile of micro-hardness shows the impact of laser generated shock wave up to 1.2 mm from the surface. Apart from that, the crystalline size and micro-strain on the laser shot peened surfaces have been investigated and compared with the unpeened surface using X-ray diffraction in conjunction with line broadening analysis through the Williamson-Hall plot.     

Real time NDE of laser shock processing with time-of-flight of laser induced plasma shock wave in air by acoustic emission sensor

Acoustic emission sensor is used to research the time-of-flight of the shock wave induced by laser-plasma in air for real time nondestructive evaluation (NDE) of laser shock processing. The time-of-flight of the shock wave propagating from the source to the sensor declines nonlinearly and similarly at the different distances for different laser energies. The velocity of the shock wave at the distance of 30 mm increases faster than that of the distance of 35 mm. The relationship between the laser energy and the distance is almost linearly when the signal with distortion is measured by acoustic emission sensor. Finally, Taylor solution is used to analyze the experimental results, and the empirical formula between the energy of the shock wave and the laser energy is established, which will provide a theoretical basis for real time NDE of laser shock processing.     

强激光辐照下纯铝的力学响应和层裂的实验测量与分析

 采用速度干涉（VISAR）测试技术,对强激光辐照下纯铝的动态力学响应和层裂特性进行了实验测量和分析。样品厚度分别为200 μm 和485 μm,激光脉冲的半高宽约为10 ns,功率密度变化范围为10¹⁰~10¹¹ W·cm^-2。实测了样品自由面速度波形,反映了强激光加载作用下材料损伤演化过程以及损伤对材料动态响应的影响。计算得到了冲击波强度(2.0~13.4 GPa) 和不同拉伸应变率下铝的层裂强度(1.6~2.3 GPa)。在所采用的实验条件和1维近似下,激光辐照产生的冲击波强度与激光功率密度之间成线性关系。最后讨论了层裂强度与拉伸应变率之间的关系,显示层裂强度随着拉伸应变率的增加而增大。     

放大自发辐射对激光冲击处理7050铝合金的影响

利用高功率钕玻璃激光器对7050铝合金表面进行了冲击处理,保持泵浦功率不变,用压电传感器和示波器测试了激光冲击产生的压力波形,用X射线应力测量仪测量了激光冲击后铝合金表面的残余应力,并用非接触式光学轮廓仪分析了激光冲击后铝合金的表面形貌,研究了放大自发辐射强度对激光冲击处理7050铝合金冲击压力、残余应力和粗糙度影响的微观机理。研究结果表明:在泵浦功率不变前提下,放大自发辐射强度的增大降低了激光冲击波压力,铝合金表面残余压应力也随之降低,同时铝合金表面粗糙度增大。     

放大自发辐射对激光冲击处理7050铝合金的影响

 利用高功率钕玻璃激光器对7050铝合金表面进行了冲击处理,保持泵浦功率不变,用压电传感器和示波器测试了激光冲击产生的压力波形,用X射线应力测量仪测量了激光冲击后铝合金表面的残余应力,并用非接触式光学轮廓仪分析了激光冲击后铝合金的表面形貌,研究了放大自发辐射强度对激光冲击处理7050铝合金冲击压力、残余应力和粗糙度影响的微观机理。研究结果表明:在泵浦功率不变前提下,放大自发辐射强度的增大降低了激光冲击波压力,铝合金表面残余压应力也随之降低,同时铝合金表面粗糙度增大。     

Dissimilar autogenous full penetration welding of superalloy K418 and 42CrMo steel by a high power CW Nd:YAG laser

Experiments of autogenous laser full penetration welding between dissimilar cast Ni-based superalloy K418 and alloy steel 42CrMo flat plates with 3.5 mm thickness were conducted using a 3 kW continuous wave (CW) Nd:YAG laser. The influences of laser welding velocity, flow rate of side-blow shielding gas, defocusing distance were investigated. Microstructure of the welded seam was characterized by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Mechanical properties of the welded seam were evaluated by microhardness and tensile strength testing. Results show that high quality full penetration laser-welded joint can be obtained by optimizing the welding velocity, flow rate of shielding gas and defocusing distance. The laser-welded seam have non-equilibrium solidified microstructures consisting of γ-FeCr_0.29Ni_0.16C_0.06 austenite solid solution dendrites as the dominant and very small amount of super-fine dispersed Ni₃Al γ′ phase and Laves particles as well as MC needle-like carbides distributed in the interdendritic regions. Although the microhardness of the laser-welded seam was lower than that of the base metal, the strength of the joint was equal to that of the base metal and the fracture mechanism showed fine ductility.     

Microstructure and properties of 6FeNiCoSiCrAlTi high-entropy alloy coating prepared by laser cladding

The content of each constituent element in the newly developed high-entropy alloys (HEAs) is always restricted in equimolar or near-equimolar ratio in order to avoid the formation of complex brittle phases during the solidification process. In this study, a 6FeNiCoSiCrAlTi high-entropy alloy coating with simple BCC solid solution phase has been prepared by laser cladding on a low carbon steel substrate. The microstructure, hardness and magnetic properties have been investigated. The experimental results show that the tendency of component segregation in the conventional solidification microstructure of multi-component alloy is effectively relieved. The microstructure of the coating is mainly composed of equiaxed polygonal grains, discontinuous interdendritic segregation and nano-precipitates. EBSD observation confirms that the polygonal grains and interdendritic segregation have similar BCC structure with lots of low angle grain boundaries at the interface. The microhardness of the coating reaches 780 HV_0.5, which is much higher than most of the HEAs prepared by other methods. In addition, the coating shows excellent soft magnetic properties.     

Effects of laser shock processing on 00Cr12 mechanical properties in the temperature range from 25 °C to 600 °C

Laser shock processing was performed on 00Cr12 standard tensile specimens to reveal its effect on fatigue properties. Mechanical properties of the specimens were tested at the temperatures of 25 °C, 400 °C, 500 °C and 600 °C respectively. The correlations between the fatigue times and the axial strain at different temperatures were explored. The results indicate that the anti-fatigue life of material is enhanced greatly at the room temperature after laser shock processing in which residual compressive stress is mechanically produced into the surface. The yield strength and the elasticity coefficient of 00Cr12 specimens are enhanced greatly after laser shock processing; the cycle times are obviously longer at the elevated temperature, and the laser-shocked samples exhibit lower plastic strain amplitudes compared with the non-treated ones.     

Improvement of abrasion resistance in artificial seawater and corrosion resistance in NaCl solution of 7075 aluminum alloy processed by laser shock peening

As 7075 aluminum alloy is widely used in a humid environment, in order to enhance its abrasion resistance and electrochemical corrosion resistance, the paper studied the effect of laser shock peening on abrasion resistance in artificial seawater and corrosion resistance in 3.5% NaCl solution of 7075 aluminum alloy. Result shows that when specimens were treated once and twice with 7.17 GW/cm² the abrasion loss would be reduced by 43.75% and 46.09% compare to untreated respectively, and the corrosion rate of 7075 aluminum alloy could be reduced as much as 50.32% by LSP treatment with 7.17 GW/cm². What’s more, the effects on the microhardness, microstructure and residual stress with different LSP impacts and power density were investigated to find out strengthening mechanism of laser shock peening, which were observed and measured by microhardness tester, optical microscope and X-ray diffraction (XRD) residual stress tester. In the entire laboratory tests, it is considered that LSP is a practical option to improve abrasion resistance in seawater and corrosion resistance of 7075 aluminum alloy.     

Morphologies,microstructures, and mechanical properties of samples produced using laser metal deposition with 316 L stainless steel wire

Laser metal deposition (LMD) with a filler has been demonstrated to be an effective method for additive manufacturing because of its high material deposition efficiency, improved surface quality, reduced material wastage, and cleaner process environment without metal dust pollution. In this study, single beads and samples with ten layers were successfully deposited on a 316 L stainless steel surface under optimized conditions using a 4000 W continuous wave fibre laser and an arc welding machine. The results showed that satisfactory layered samples with a large deposition height and smooth side surface could be achieved under appropriate parameters. The uniform structures had fine cellular and network austenite grains with good metallurgical bonding between layers, showing an austenite solidification mode. Precipitated ferrite at the grain boundaries showed a subgrain structure with fine uniform grain size. A higher microhardness (205–226 HV) was detected in the middle of the deposition area, while the tensile strength of the 50 layer sample reached 669 MPa. In addition, ductile fracturing was proven by the emergence of obvious dimples at the fracture surface.     

Numerical modeling of laser shock peening with femtosecond laser pulses and comparisons to experiments

A physics-based model has been developed for laser shock peening (LSP) with femtosecond (fs) laser pulses (fs-LSP), which has never been reported in literature to the authors’ best knowledge. The model is tested by comparing simulations with measured plume/shock wave front transient propagations and the LSP-induced hardness enhancement layer thickness. Reasonably good agreements have been obtained. The model shows that fs-LSP can produce much higher pressure than LSP with nanosecond (ns) laser pulses (ns-LSP), and it can also generate very large compressive residual stress in the workpiece near-surface layer with a thickness up to ∼100 μm. The developed model provides a powerful guiding tool for the fundamental study and the practical applications of fs-LSP. This study, together with the recently reported work by Nakano et al. [Journal of Laser Micro/Nanoengineering 4(1) (2009) 35-38], has confirmed the feasibility of fs-LSP on both theoretical and experimental sides.     

Microstructure and selected properties of hot-work tool steel with PVD coatings after laser surface treatment

The paper presents the effect of HPD laser treatment on the microstructure and selected properties of the PVD CrN, (Ti,Al) and Ti(C,N) coatings deposited onto hot-work tool steel substrates. The microstructure and surface topography of the investigated samples are characteristic of the diversified morphology connected with the applied laser beam power. Employment of laser beam at 0.7 kW power to the laser treatment of samples with Ti(C,N) coatings causes clear coating adhesion growth because of the diffusive processes induced by heat release. Because of the higher value of the (Ti,Al)N absorption coefficient one can state that the observed substrate materials change and finally coatings destruction in case of those samples is the most noticeable. The moderate effect of the laser beam treatment of the hot-work tool steel with the PVD coating was observed for CrN coatings. However, for laser beam power above 0.5 kW differences in the thermal expansion coefficients of the substrate materials and coatings generate coating crackings.     

Laser Shock Processing of 6061-T6 Al alloy with 1064 nm and 532 nm wavelengths

Laser Shock Processing (LSP) has been proposed as a competitive alternative technology to classical treatments for improving fatigue and wear resistance of metals. We present a configuration and results in the LSP concept for metal surface treatments in underwater laser irradiation at 532 nm and 1064 nm. The purpose of the work is to compare the effect of both wavelengths on the same material. A convergent lens is used to deliver 1.2 J/pulse (1064 nm) and 0.9 J/pulse (532 nm) in a 8 ns laser FWHM pulse produced by 10 Hz Q-switched Nd:YAG laser with spots of a 1.5 mm in diameter moving forward along the work piece. A LSP configuration with experimental results using a pulse density of 2500 pulses/cm² and 5000 pulses/cm² in 6061-T6 aluminum samples are presented. High level compressive residual stresses are produced using both wavelengths. It has been shown that surface residual stress level is comparable to that achieved by conventional shot peening, but with greater depths. This method can be applied to surface treatment of final metal products.     

Microstructure and corrosion behavior of austenitic stainless steel treated with laser

Surface modification of AISI316 stainless steel by laser melting was investigated experimentally using 2 and 4 kW laser power emitted from a continuous wave CO₂ laser at different specimen scanning speeds ranged from 300 to 1500 mm/min. Also, an investigation is reported of the introduction of carbon into the same material by means of laser surface alloying, which involves pre-coating the specimen surfaces with graphite powder followed by laser melting. The aim of these treatments is to enhance corrosion resistance by the rapid solidification associated with laser melting and also to increase surface hardness without affecting the bulk properties by increasing the carbon concentration near the surface. Different metallurgical techniques such as optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize the microstructure of the treated zone. The microstructures of the laser melted zones exhibited a dendritic morphology with a very fine scale with a slight increase in hardness from 200 to 230 Hv. However, the laser alloyed samples with carbon showed microstructure consisting of γ dendrite surrounded by a network of eutectic structures (γ+carbide). A significant increase in hardness from 200 to 500 Hv is obtained. Corrosion resistance was improved after laser melting, especially in the samples processed at high laser power (4 kW). There was shift in I_corr and E_corr toward more noble values and a lower passive current density than that of the untreated materials. These improvements in corrosion resistance were attributed to the fine and homogeneous dendritic structure, which was found throughout the melted zones. The corrosion resistance of the carburized sample was lower than the laser melted sample.     

Microstructure and properties of high chrome steel roller after laser surface melting

Laser surface melting of high chrome steels was achieved by a 5 kW continuous wave CO₂ laser. The microstructure of the laser surface-melted steels was investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffractometry, and the hardness profiles were determined by a Vickers hardness tester. The corrosion behavior in 3.5% NaCl solution was studied by electrochemical corrosion equipment. The large carbides of high chrome steels are completely dissolved and ultrafine dendrites of austenite with submicroscopic M₂₃C₆ carbides precipitation are formed in the melted zone. The austenite in the melted zone has a high tempering stability. The corrosion resistance of the laser surface-melted steels is significantly improved due to the dissolution of carbides and the increase of the alloying elements in the solid solution as well as the large amount of austenite.     

激光冲击对不锈钢焊接接头残余应力场的影响

针对不锈钢焊接接头存在残余应力且分布不均匀、容易发生应力腐蚀的问题,采用激光冲击强化对其进行处理,探究激光功率密度和冲击次数对表面残余应力状态的优化作用,并通过应力腐蚀试验验证优化效果。结果表明：随着功率密度增加,表面残余应力明显下降,但下降幅度逐渐减小,功率密度4.24 GW/cm2与2.83 GW/cm2冲击产生的残余应力相差不大,熔合区还存在残余拉应力,说明高功率密度不足以消除表面残余拉应力;随着冲击次数增加,残余拉应力显著降低,2.83 GW/cm2冲击3次之后,残余拉应力完全消除,局部最高应力梯度从54.7 MPa/mm下降到11.7 MPa/mm,获得了高数值、分布均匀的残余压应力层。激光冲击强化后,焊接试样的应力腐蚀断裂时间提高了33.48%,激光冲击强化产生的残余压应力是其应力腐蚀抗性提高的重要原因。     

Study on the solidification microstructure in AZ91D Mg alloy after laser surface melting

Laser surface melting (LSM) is known to enhance the wear and corrosion resistance of Mg alloys, but its effect on microstructural evolution of Mg alloys is not well understood. An effort has been made to study the effect of rapid solidification following LSM on the microstructural evolution of AZ91D Mg alloy. The results of X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy indicated that the solidification microstructure in the laser-melted zone was mainly cellular/dendrite structure of primarily α-Mg phase and continuous network of β-Mg₁₇Al₁₂ phase. Numerical prediction of the laser-melted zone suggested that cooling rates increased strongly from the bottom to the top surface in the irradiated regions. An attempt has been made to correlate dendrite cell sizes of the solidification microstructure with the cooling rates in the laser-treated AZ91D Mg alloy.