Development of a theoretical process map for laser cladding using two-dimensional conduction heat transfer model

In blown powder laser cladding process, the powder travels across the laser path, gets heated up by absorbing laser energy, and finally melts on the substrate under the intense laser beam; as the substrate moves away this melt pool solidifies to form a continuous built-up layer. In the present study a two-dimensional conduction heat transfer equation has been solved using finite volume method to develop a theoretical process map for laser cladding. The developed process map indicates a range of scanning speed and powder feed rate for the feasibility of the process; the lower limit is dictated by the maximum melt pool temperature, and the higher limit by poor bonding due to lack of melting of the substrate (i.e. low dilution). Parametric regions for thick and thin cladding with low dilution can be decided from the process map. It is found that the process range expands with the increase in total absorbed power as well as power directly absorbed by the powder. Correlations for maximum melt pool temperature and dilution are presented. A process map for identifying the form and scale of the microstructure in the solidified layer is also presented.     

智能手机交互界面设计的认知基础

目的在智能手机逐渐成为人们生活必需品的当下,根据用户的认知特点提出关于智能手机交互界面设计的建议,使智能手机更符合用户的认知习惯,提高交互效率。方法从感觉、知觉、注意及记忆四方面出发分析个体与智能手机进行交互时所涉及的认知过程。根据颜色视觉、视觉适应等感觉特点提高用户对界面信息的感觉,根据用户的知觉的加工及知觉的特性改善用户对界面信息的知觉,根据选择性注意、注意的分配特点及注意的特性促进用户对有效信息的注意,利用记忆的存储及提取的特点增加个体对界面信息的记忆及记忆的使用。结论结合用户的认知特点提出字体、图标、背景等界面因素的设计要点优化用户与智能手机交互界面的交互过程。     

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.     

Effect of three-dimensional melt pool convection on process characteristics during laser cladding

A three-dimensional heat transfer model is developed to simulate the cladding process that include the different physical phenomena such as heat transfer, phase changes, addition of powder particles and fluid flow due to Marangoni–Rayleigh–Benard convection. It is found that the Rayleigh–Benard convection is insignificant and Marangoni–Benard convection is dominant for the studied cases. By varying the scanning speed and Marangoni number the melt pool size and strength of convection are changed and its influence on clad built-up geometry, dilution level, maximum and average melt pool temperatures and the form and scale of the microstructure of the solidified clad track has been studied.     

钛合金表面激光熔覆TiC/NiCrBSi涂层温度场有限元模拟

为在钛合金表面获得优质激光熔覆涂层,用有限元方法研究了激光熔覆工艺对熔池温度场分布和凝固后熔覆层组织的影响,考虑相变潜热、辐射对流散热以及温度对热物理性能的影响等因素,建立三维有限元模型模拟了Ti6Al4V合金表面激光熔覆TiC/NiCrBSi复合涂层过程中的温度场,并结合熔覆过程的温度场分布,对涂层的形貌、结合区、基...     

工艺参数对304不锈钢表面激光熔覆Ni基合金涂层的组织、耐磨性及耐腐蚀性的影响

研究了不同工艺参数对304不锈钢表面激光熔覆Ni基合金后熔覆层微观组织及硬度、耐磨、耐蚀性能的影响,并寻求最佳激光工艺参数,以期获得冶金结合较好,耐磨、耐蚀性能良好的熔覆层。根据组织与性能的综合分析可知,最优激光工艺参数为激光功率2.5kW、扫描速度4mm/s、送粉速率300mg/s。利用优化工艺参数熔覆后的熔覆层宏观形貌平整、光滑,熔覆层宽度为14.36mm,高度为1.612mm,熔池深度为0.248mm,稀释率为13.33,硬度较高,平均显微硬度为646.4HV,并且耐磨损性能较好,磨损量较低。此外,熔覆层的耐腐蚀性能也较好,自腐蚀电位为-286.77mV。在一定的激光工艺参数下,组织从结合区至熔覆层表层依次为平面晶、胞状晶、柱状晶、树枝晶、等轴晶。激光功率、扫描速度、送粉速率不同,熔覆层中组织粗细变化呈现一定的规律性:随着激光功率的增大,组织由细小逐渐变的粗大;随着扫描速度的增大,组织先变细小,然后变粗大;随着送粉速率的增大,组织逐渐变细小。合金的耐磨性与耐蚀性不仅与组织大小有关,而且与组织物相组成密切相关。     

Thermal behavior and densification mechanism during selective laser melting of copper matrix composites: Simulation and experiments

Simulation of temperature distribution and densification process of selective laser melting (SLM) WC/Cu composite powder system has been performed, using a finite volume method (FVM). The transition from powder to solid, the surface tension induced by temperature gradient, and the movement of laser beam power with a Gaussian energy distribution are taken into account in the physical model. The effect of the applied linear energy density (LED) on the temperature distribution, melt pool dimensions, behaviors of gaseous bubbles and resultant densification activity has been investigated. It shows that the temperature distribution is asymmetric with respect to the laser beam scanning area. The center of the melt pool does not locate at the center of the laser beam but slightly shifts towards the side of the decreasing X-axis. The dimensions of the melt pool are in sizes of hundreds of micrometers and increase with the applied LED. For an optimized LED of 17.5 kJ/m, an enhanced efficiency of gas removal from the melt pool is realized, and the maximum relative density of laser processed powder reaches 96%. As the applied LED surpasses 20 kJ/m, Marangoni flow tends to retain the entrapped gas bubbles. The flow pattern has a tendency to deposit the gas bubbles at the melt pool bottom or to agglomerate gas bubbles by the rotating flow in the melt pool, resulting in a higher porosity in laser processed powder. The relative density and corresponding pore size and morphology are experimentally acquired, which are in a good agreement with the results predicted by simulation.     

45钢表面激光熔覆CoCrNi中熵合金涂层组织与性能

为探究CoCrNi中熵合金在激光熔覆领域中的应用,以CoCrNi合金粉末作为熔覆粉末,在45钢表面采用同轴送粉法制备合金涂层。利用扫描电镜、X射线衍射仪、显微硬度仪、摩擦磨损实验机和电化学工作站等设备研究了熔覆层微观组织、硬度、耐磨性和耐腐蚀性能。结果表明：熔覆层成形良好,组织均匀致密,组成相主要为FCC单相固溶体;熔池与基体交界处为平面晶,底部靠近中心为柱状晶,顶部分别为胞状晶和等轴晶,3种元素在熔覆层深度方向上的比例几乎相同;熔覆层平均硬度为250HV,摩擦系数、磨损量较基体分别降低了11.7%和36.7%;自腐蚀电流密度略有降低,CoCrNi熔覆层的钝化区域为-150到1 100 mV,表明熔覆层显著提高45钢的耐腐蚀性能。     

激光熔覆玻璃涂层

本文研究在Ａ３钢基材上激光熔覆玻璃涂层的可行性，激光熔覆玻璃涂层的过程是玻璃熔体在基材表面上湿润、铺展，然后冷凝的过程。玻璃熔体在基材表面上铺展，不仅与它们之间的浸润有关，而且与玻璃熔体的流动性有关。玻璃熔体的流动性在熔覆过程中随时间变化；激光处理参数的不同会改变熔体的随时间变化过程。合理选择激光参数能熔覆形成均匀的、光滑的、粘附的、无裂纹的玻璃涂层。在激光熔覆过程中，会在玻璃熔体中出现结晶现象，     

不同工艺参数对高硬度铁粉等离子弧增材熔覆的影响

目的优化马氏体不锈钢等离子弧增材熔覆工艺。方法利用等离子热源在300M表面对新型高硬度铁粉的增材熔覆工艺进行研究。基于三因素三水平熔覆工艺的正交试验,研究在大温度梯度熔覆环境下,等离子弧电流、焊枪移动速度及送粉速率对单道金属熔池的非平衡凝固组织的影响机制,综合考虑单道焊道宏观形貌、成形尺寸及稀释率,确定增材熔敷的电流、焊枪移动速度及送粉速率;在此基础上,基于多道单层及单道多层熔覆的宏观形貌及拉伸性能,确定多道多层增材熔覆的搭接率及熔覆工艺。结果在电流为140～180 A、移动速度为20～30 cm/min和送粉速率为20～30 r/min的工艺窗口内,电流对焊道的稀释率有较大影响,送粉速率次之;送粉速率对高宽比的影响最大,扫描速度次之。结论当电流大小为140 A,送粉速率为30 r/min,扫描速率为30 cm/min时,单道单层焊道的稀释率及宽高比最小,热影响区晶粒尺寸较小,叠加率为40%时,熔覆层表面平整度较高。     

基于PSO–BP–GA混合算法的激光熔覆工艺多目标优化

目的 为了提高镍基高温合金熔覆涂层的综合质量,提出了一种基于PSO–BP–GA混合算法的激光熔覆工艺优化方法。方法 选取工艺参数（激光功率、扫描速度、送粉速率）为优化变量、熔覆层质量（稀释率、显微硬度、热影响区深度）为优化目标,根据正交试验结果建立PSO–BP神经网络预测模型,采用线性加权法和层次分析法建立熔覆层质量的综合评价体系,结合GA算法探寻综合质量最优的工艺参数组合。结果 PSO–BP神经网络模型预测值与试验值之间的相对误差不超过6%,最优工艺参数组合如下：激光功率为2 158 W、扫描速度为10.4 mm/s、送粉速率为2.9 r/min,其熔覆层稀释率降低了70.4%、显微硬度增大了25.4%、热影响区深度减少了41.8%。结论 该算法为制备出高性能镍基高温合金熔覆涂层提供了一定的参考与借鉴。     

Directed energy deposition (DED) additive manufacturing: Physical characteristics,defects, challenges and applications

Directed energy deposition (DED) is a branch of additive manufacturing (AM) processes in which a feedstock material in the form of powder or wire is delivered to a substrate on which an energy source such as laser beam, electron beam, or plasma/electric arc is simultaneously focused, thus forming a small melt pool and continuously depositing material, layer by layer. DED has several unique advantages compared to other AM processes, such as site-specific deposition and repair, alloy design, and three-dimensional printing of complex shapes. Herein, recent advances as well as the main aspects governing laser-material interactions during the DED process, melt pool thermal behavior, advanced in situ monitoring, and interaction mechanisms are critically reviewed. The most critical processing variables and their influence on the deposited material properties, along with defect formation mechanisms and characterization techniques, are also identified and discussed. An overview of high-end applications, current challenges associated with DED processing, and a critical outlook of the technology are presented.     

Laser Surface Nitriding of Ti-6A1-4V Titanium Alloy

Titanium and its alloys are known for their high specific strength as well as fatigue and corrosion resistance. However, they suffer from poor wear and friction resistance, limiting their use in tribological applications. Nitriding of these materials may be carried out favourably to harden them and thus to improve both wear and friction resistance. The laser nitriding process involves using the intense energy of the laser to melt the surface in a nitrogen comprising atmosphere. This results in creation of a very hard layer consisting of dendritic structures of nitride. But the non-uniformity of the melt pool and cracks in the nitrided layers have been generally observed. Our study deals with the results of Ti-6Al-4V laser surface nitriding and with the effects of a sample preheating on the cracks generation.     

The influences of processing parameters on forming characterizations during laser rapid forming

Laser rapid forming experiments were carried out with 316L stainless steel and nickel-base alloy to investigate the influences of the processing parameters on forming characterizations systematically. It is found that the height of a single cladding layer was very important for fabrication accuracy and forming stability of components of laser rapid forming because it was affected by almost all processing parameters and was quite difficult to precisely control. For the system with lateral powder feeding pattern, the powder injection point was the most important factor to the height control of single layer. The variation of the width of single clad, which was mainly affected by laser power, spot diameter and scanning velocity, was similar to that in laser surface melting. The surface quality was another important characterization for laser rapid forming and was remarkably affected by oxidation and the powder adhesion. In order to improve the surface quality, the flow flux of shielding gas should be 10 l min⁻¹ and the powder stream cannot be injected to the tail part of the melt pool. Based on the investigation, some metal components were fabricated.     

Hardfacing characteristics of S42000 stainless steel powder with added silicon nitride using a CO2 laser

This study examined hardfaced cladding of S42000 stainless steel powder with added silicon nitride Si₃N₄ on a medium carbon steel using a 1200-W CO₂ laser. Experimental results indicated that a well-proportioned cladding layer was obtained with an overlap of 50% at a traverse speed of 5 mm/s and a powder feed rate of 2 g/min. The degree of dilution, depth/width (D/W) ratio and microhardness of the cladding layer was increased by adding up to 5 wt.% silicon nitride. Retained austenite and wear decreased with increasing amounts of silicon nitride addition. The silicon nitride decomposed in the cladding layer during the laser treatment.     

Influence of processing conditions on geometrical features of laser claddings obtained by powder injection

The influence of processing conditions on the geometrical aspects of laser claddings is studied. Attention is drawn to the following laser-processing parameters: laser-power interaction time, surface of the irradiated zone, powder-feed rate. Different Ni-Cr-Co-Fe base powders are deposited on an austenitic stainless steel. It is observed that the mass of powder participating in the clad layer — which is proportional to the effective cross-sectionS of the layer — increases linearly with irradiance and interaction time. However, threshold values are detected; they are due to the existence of a minimum energy value required to melt the powder particles. The influence of the powder-feed rate, F, is more complicated. For low values,S is directly proportional toF, while beyond a critical value,F ^*, the powder injected in the laser beam acts as a screen, the opacity of which increases due to the multiple scattering phenomenon; energy absorption is more and more efficient and process efficiency is enhanced. However, this increase is limited to a maximum value, since the energy delivered by the laser source is also limited and since cladding requires melting of the particles. A simple explanation is proposed, based on mass- and energy-transfer governing equations.     

激光多层熔覆纳米陶瓷层工艺参数优化

为了深入了解激光多层熔覆工艺与涂层性能之间的关系,采用压片预置式激光多层熔覆工艺制备了纳米Al2O3-13%TiO2(质量分数)陶瓷层;通过3因素3水平正交试验分析了激光熔覆熔池闭环控制温度、超声振动频率及保温箱预热温度对涂层结合强度的影响,并对激光熔覆工艺参数进行了优化;通过扫描电镜(SEM)和结合强度测试研究了最优工艺下所得涂层的形貌和性能。结果表明:影响涂层结合强度的因素主次顺序依次为熔池闭环控制温度、保温箱预热温度、超声振动频率;激光多层熔覆纳米Al2O3-13%TiO2涂层最佳工艺参数为熔池闭环控制温度2 500℃,超声振动频率50 kHz,保温箱预热温度400℃;优化工艺熔覆的涂层各层之间无明显界面,涂层内部致密、连续,基本无孔隙及贯穿性大裂纹,涂层结合强度明显提高,达66.3 MPa。     

Characterisation and formation of boundary in laser cladding AZ91D with Al + Al2O3

Abstract

The laser surface cladding of AZ91D magnesium alloy with Al + Al₂O₃ powders was investigated. The growth morphology of the boundary zone between the laser surface cladding layer and AZ91D substrate, and the distribution of Al₂O₃ particles in the laser surface cladding layer, were observed by optical microscope and scanning electron microscope. Furthermore, the element line and mapping scans analyses on the boundary zone were carried out with energy dispersive spectrum. The phases in the laser cladding layer and the substrate of AZ91D magnesium alloy were identified by X-ray diffraction. The results showed that the distribution of the Al₂O₃ particles was homogeneous in the laser surface cladding layer and the growth morphology of the boundary zone was in a unique parallel branching feature. The formation of the parallel branching dendrite is mainly related to the temperature gradient, the ultimate rate of dendrite growth and the rate of pool solidification on the growing fronts.     

Finite element simulation of the temperature and stress fields in single layers built without-support in selective laser melting

Overhanging and floating layers which are introduced during the build in selective laser melting (SLM) process are usually associated with high temperature gradients and thermal stresses. As there is no underlying solid material, less heat is dissipated to the powder bed and the melted layer is free to deform resulting undesired effects such as shrinkage and crack. This study uses three-dimensional finite element simulation to investigate the temperature and stress fields in single 316L stainless steel layers built on the powder bed without support in SLM. A non-linear transient model based on sequentially coupled thermo-mechanical field analysis code was developed in ANSYS parametric design language (APDL). It is found that the predicted length of the melt pool increases at higher scan speed while both width and depth of the melt pool decreases. The cyclic melting and cooling rates in the scanned tracks result high VonMises stresses in the consolidated tracks of the layer.     

Dual Coating Laser Cladding of NiCrBSi and Inconel 718

This paper presents an alternative approach to obtaining crack- and pore-free NiCrBSiFeCuMoC hard coatings on a low alloy steel substrate through coaxial laser cladding, by using Inconel 718 as a buffer layer between the hard coating and the base material. The presence of the buffer layer reduces the overall cracking susceptibility of the hardfacing material by reducing the compressive stresses developed during the cladding process and ensuring a more uniform heat distribution gradient at the surface of the material than the base metal alone, which provides an additional hardness and wear coefficient increase of 7% and improves the corrosion resistance of the obtained materials by 20%, in comparison with the reference sample obtained without an intermediate layer, by using the same operational parameters and minimizes elemental dilution with the substrate. Our method could prove useful in increasing the quality and life cycle of expensive high-performance hard-coated materials, especially those working under demanding operational and environmental conditions.