The effect of localized dynamic surface preheating in laser cladding of Stellite 1

In laser cladding, high cooling rates create outcomes with superior mechanical and metallurgical properties. However, this characteristic along with the additive nature of the process significantly contributes to the formation of thermal stresses which are the main cause of any potential delamination and crack formation across the deposited layers. This drawback is more prominent for additive materials such as Stellite 1 which are by nature crack-sensitive during the hardfacing process. In this work, parallel to the experimental investigation, a numerical model is used to study the temperature distributions and thermal stresses throughout the deposition of Stellite 1 for hardfacing application. To manage the thermal stresses, the effect of preheating the substrate in a localized dynamic fashion is investigated. The numerical and experimental analyses are conducted by the deposition of Stellite 1 powder on the substrate of AISI-SAE 4340 alloy steel using a 1.1 kW fiber laser. Experimental results confirm that by preheating the substrate a crack-free coating layer of Stellite 1 well-bonded to the substrate with a uniform dendritic structure, well-distributed throughout the deposited layer, can be obtained contrary to non-uniform structures formed in the coating of the non-preheated substrate with several cracks.     

Rapid tooling by laser powder deposition: Process simulation using finite element analysis

Laser powder deposition (LPD) is a rapid manufacturing process, whereby near-net-shape components are fabricated by the successive overlapping of layers of laser melted and resolidified material. As new layers of material are deposited, heat is conducted away from recently resolidified material, through the previously deposited layers, inducing cyclic thermal fluctuations in the part as it is built up. These thermal cycles can activate a variety of metallurgical phenomena, such as solid-state transformations, leading to a progressive modification of the material’s microstructure and properties. Since the thermal history of the material in the deposited part will differ from point to point and depends on the deposition parameters and build-up strategy, the finished part may present complex distributions of microstructure and properties. In order to achieve the best properties, the deposition process must be optimized and, given its complexity, this optimization can only be effectively done using mathematical simulation methods. In this paper a thermo-kinetic LPD model coupling finite element heat transfer calculations with transformation kinetics and quantitative property–structure relationships is presented. This model was applied to the study of the influence of substrate size and idle time between the deposition of consecutive layers on the microstructure and hardness of a ten-layer AISI 420 steel wall built by LPD. The results show that the thermal history and, hence, the microstructure and properties of the final part, depend significantly on these parameters.     

Problems in laser repair cladding a surface AISI D2 heat-treated tool steel

The aim of the present work is to establish the relationship between laser cladding (LC) process parameters (power, process speed, and powder feed rate) and AISI D2 tool steel metallurgical transformations, with the objective of optimizing the processing conditions during real reparation. It has been deposited H13 tool steel powder on some steel substrates with different initial metallurgical status (annealed or tempered) using a coaxial LC system. The microstructure of the laser clad layer and substrate heat-affected zone (HAZ) was characterized by optical microscopy, scanning electron microscopy, and electron backscattered diffraction. Results show that the process parameters (power, process speed, feed rate, etc.) determine the dimensions of the clad layer and are related to the microstructure formation. Although it is simple to obtain geometrically acceptable clads (with the right shape and dimensions) in many cases some harmful effects occur, such as carbide dilution and non-equilibrium phase formation, which modify the mechanical properties of the coating. Specifically, the presence of retained austenite in the substrate–coating interface is directly related to the cooling rate and implies a hardness diminution that must be avoided. It has been verified that initial metallurgical state of the substrate has a big influence in the final result of the deposition. Tempered substrates imply higher laser absorption and heat accumulation than the ones in annealed condition. This produces a bigger HAZ. For this reason, it is necessary to optimize the process conditions for each repair in order to improve the working behaviour of the component.     

The influence of combined laser parameters on in-situ formed TiC morphology during laser cladding

In this paper, the effect of laser cladding process parameters on TiC morphology is studied. Results show that laser parameters play a crucial role in morphology of TiC. Dendritic or spherical TiC particles with different distribution are observed depending on applied laser parameters. Two combined parameters, effective energy and powder deposition density, are used in order to study the effect of laser process parameters on TiC morphology.A series of experiments are conducted in constant laser power and scan speed, constant effective energy and constant powder deposition density in order to study the TiC morphologies. Results show that both combined parameters and laser parameters should be considered in order to interpret the results. Laser parameters have crucial role in establishing the TiC morphologies by means of temperature and chemical composition.Hardness results of the clad zone depend on morphologies and distributions of TiC particles in the clad.     

铝合金激光熔化沉积应力场及力学性能分析

采用激光熔化沉积工艺对6 mm厚的ZL114A铝板中的通槽进行了修复试验. 试验中采用AlSi10Mg粉末作为沉积过程的填充材料. 为了选择合适的激光扫描方法,通过数值模拟的方式,探究了不同扫描路径下产生的残余应力情况. 模拟结果显示,相比于自一侧向另一侧平行扫描,自下向上逐层扫描方式更有助于减小残余应力,残余应力分布也更均匀. 采用自下向上的逐层扫描方式实现了铝板通槽的激光熔化沉积的工艺试验,并探索了缺陷及热输入对试件力学性能的影响. 结果表明,通过工艺试验得到的沉积试件,最优性能的试件抗拉强度为268 MPa,达到母材的89%. 试件断裂并未沿着沉积区与母材的界面处断裂,而是在沉积区内部层与层界面间的搭接区域断裂.     

旁轴送粉式激光扫描熔覆工艺研究

以Q235钢为基板,采用IPGYLS-4000型光纤激光器以及旁轴送粉器,搭建旁轴同步送粉式激光扫描熔覆工艺试验平台,在基板上进行熔覆试验。研究了激光功率、扫描速度和扫描宽度对熔覆层成形尺寸和对熔覆层组织的影响,结果表明:随着激光功率增大,铁基合金粉末熔化量提高,单层金属熔覆层的余高增加;扫描速度对熔覆层熔宽和余高的影响均较大,随着扫描速度的降低,金属熔覆层熔宽余高均增加,裂纹数量增多。约束应力是导致熔覆层出现裂纹的主要原因,通过优选工艺参数可以获得工艺良好无裂纹熔覆层,为下一步研究激光扫描多层熔覆无(小)变形焊接技术提供理论和技术依据。     

激光熔覆数值模拟过程中的热源模型

综述了几种可用于激光熔覆过程中温度场和应力场数值模拟的热源计算模型,并对其进行分析比较.指出采用有限元法,利用计算软件来实现熔覆过程中热源的添加是目前常用的优选方法.在此基础上考虑到粉末、基体、光源三者的相互作用提出了新的激光熔覆热源计算模型.关于激光熔覆热源模型的探讨为建立更加准确的激光熔覆计算模型,深入研究熔覆过程中的应力场与变形场提供帮助.     

Modelling of energy attenuation due to powder flow-laser beam interaction during laser cladding process

Laser cladding is becoming an emergent process with high interest for industries dedicated to high added value parts production. Laser cladding introduces new manufacturing concepts such as direct manufacturing of parts, avoiding in this way the excessive waste of material in the form of chips, inherent to traditional machining processes. This process is based on the use of a source of high energy density, such as laser beams, to generate a melt pool on a substrate where a filler material is injected. Thus, when studying the process it is necessary to know the effective energy that reaches the base material. This energy does not correspond to the one provided by the laser beam, considering that the laser beam has to go through a cloud of injected powder before it reaches the substrate. In this region there is an interaction between the laser beam and the filler material in which a significant amount of energy is absorbed by the powder. As a result, attenuation values can reach up to high percentages of the initial energy value.This paper presents a model based on the shadow created by powder particles on the substrate, with capabilities of estimating the attenuation suffered by the beam and characterizing the density of energy that reaches the surface of the substrate. The model starts from the initial energy density of the laser and the powder concentrations obtained from a CFD model, which has been experimentally validated. In addition, three different approaches have been introduced for the model solution. Initially a constant powder particle size and a perfectly cylindrical laser beam are considered. Subsequently, an experimentally measured particle size distribution and the divergence of the beam are introduced in order to accurately adjust the model and the real process. The attenuation model has been experimentally adjusted and validated. The error average is below 10% of measured values.     

激光熔覆在AlN陶瓷表面制备铜基金属覆层缺陷分析及控制

通过脉冲式YAG激光器在AlN陶瓷表面制备铜基金属覆层,分析熔覆层的缺陷,并研究如何能控制熔覆层缺陷的发生,熔覆试样的缺陷主要表现为陶瓷基板炸裂、熔覆层成形不完整、熔覆层微观裂纹和气孔.结果表明,通过调节激光熔覆的热输入可以保证陶瓷基板的完整性并且熔覆层成形良好;通过焊前预热和焊后缓冷的工艺可以降低熔覆层微观裂纹和气孔的形成几率.通过优化激光熔覆工艺参数和工艺方法,可以形成良好的熔覆层,并且AlN陶瓷基板和铜基金属覆层之间形成过渡层,形成良好的冶金结合.     

激光熔覆数值模拟过程中的热源模型

综述了几种可用于激光熔覆过程中温度场和应力场数值模拟的热源计算模型，并对其进行分析比较。指出采用有限元法，利用计算软件来实现熔覆过程中热源的添加是目前常用的优选方法。在此基础上考虑到粉末、基体、光源三者的相互作用提出了新的激光熔覆热源计算模型。关于激光熔覆热源模型的探讨为建立更加准确的激光熔覆计算模型，深入研究熔覆过程中的应力场与变形场提供帮助。     

Influence of process parameters on microstructure of reactive plasma cladding TiC-Fe-Cr coating

A certain amount of Ti was added to the plasma cladding Fe-Cr-C coating in the early stage in order to improve the quality and properties of the coating. Ti-Fe-Cr-C composite powder was prepared by precursor carbonization-composition process. In situ synthesized TiC-Fe-Cr coatings were fabricated on substrate of Q235 steel by plasma cladding process with the composite powder. Microstructures of the coatings with different process parameters, including cladding current, cladding speed, number of overlapping cladding layers, were analyzed by scanning electron microscope. The results show that the structure of the TiC-Fe-Cr coating is greatly affected by the cladding current, the cladding speed and the overlapping cladding process. In this test, when the cladding current of 300 A and the cladding process parameter of the cladding speed of 50 mm/min are clad with three layers, a well-formed and well-structured TiC-Fe-Cr coating can be obtained in this test. TiC-Fe-Cr coating has good wear resistance and good load characteristics under dry sliding wear test conditions.     

Direct laser fabrication of thin-walled metal parts under open-loop control

Direct laser fabrication (DLF) is an advanced manufacturing technology, which can build full density metal parts directly from CAD files without using any modules or tools. The investigation on the fabrication of thin-walled parts of nickel alloy using open-loop DLF process is introduced in this paper. The experimental setup consisted of a CO₂ laser, a 3-axis CNC table, a coaxial powder nozzle and a powder recycler. The 3D-CAD file of a thin-walled metal part was converted into the STL file format and imported into software HUST-RP to generate ‘pseudo-random’ scanning paths of laser beam. The influence of process parameters on the build height of thin-walled metal parts was studied by 1–10 layered single-bead stacks of nickel alloy. The result shows that the interference factors which affect the build height of thin-walled metal parts occur randomly during the process. For open-loop DLF process, thin-walled metal parts can achieve much better shape quality if the process parameters are suitable. Multilayer single-bead walls were built up with different scanning velocity to obtain the optimal process parameters of thin-walled parts of nickel alloy. It shows that thin walls of nickel alloy with uniform height can be built up layer by layer in a certain range of specific energy. However, it is difficult to control the build height of complex thin-walled metal parts in an accurate manner just using optimal parameters. A special coaxial powder nozzle was designed in this paper. In a certain range, the deposition thickness of the nozzle is nearly linearly increased with increase in the standoff distance between the powder focusing point of the nozzle and the deposition substrate. By means of the nozzle, a novel method to control the build height of thin-walled metal parts using open-loop DLF process was introduced. The difference in build height of a thin-walled part can be compensated automatically in one or several layers during the process. It is proved that the build height of a thin-walled metal part can be accurately controlled in theory using the nozzle. A complex single-bead part of nickel alloy whose geometry was designed to be the well-known Chinese ‘FU’ was fabricated and explained in this paper. The result shows that the shape quality of the sample is quite good, and actual build height of the sample is 53.54 mm while the designed value is 54 mm.     

Sliding wear resistance of TiCp reinforced titanium composite coating produced by laser cladding

Titanium metal matrix composite coatings (MMC) are considered to be important candidates for high wear resistance applications. Laser cladding (LC) by coaxial powder feeding is an advanced coating manufacturing process, which involves laser processing fine powders into components directly from computer aided design (CAD) model.In this study, the LC process was employed to fabricate TiC particle reinforced Ti6Al4V MMC coatings on Ti6Al4V hot rolled samples.The experimental results show that during LC process, TiC particles are partially dissolved into melted Ti-base alloy and precipitated in the form of TiC dendrites during cooling.Dry sliding wear properties of these MMC layers have been compared with substrate materials wear. The observed wear mechanisms are summarized and related to detailed microstructural observations. The layers have been found to show improved tribological properties connected with the TiC_p addition and the LC process parameters.     

An experimentally based thermo-kinetic hardening model for high power direct diode laser cladding

High power direct diode laser (HPDDL) based cladding is found to be an economical process for repairing or building valued components and tools that are used in the automotive, aerospace, nuclear and defense industries. In this study, a 2-kW HPDDL of 808 nm in wavelength, rectangular-shaped laser spot of 12 mm × 1 mm with uniform distribution (top-hat) of laser power is used to carry out the experiments. An off-axis powder injection system is used to deposit tool steel H13 on the AISI 4140 steel substrate. A number of experiments are carried out by changing the laser power and scanning speeds while keeping a constant powder feed rate to produce different sizes of clad. An experimentally based finite element (FE) thermal model is developed to predict the cross-sectional temperature history of the cladding process. The temperature-dependent material properties and phase change kinetics are taken into account in this model. As-used experimental boundary conditions are adopted in this model. The acquired temperature history from the FE model is used to predict the temperature gradient, rates of heating and cooling cycles, and the solidification of the clad to the substrate. The FE thermal model results are coupled with thermo-kinetic (TK) equations to predict the hardness of the clad to the substrate. Metallurgical characterization and hardness measurements are performed to quantify the effect of processing parameters on the variation of clad geometry, microstructure, and the change of hardness of the clad to the substrate. The results show that a good metallurgically bonded clad of hardness uniformity is achieved.     

Direct laser cladding of Co on Ti–6Al–4V with a compositionally graded interface

In the present study, attempts have been made to fabricate Co layers on the surface of Ti–6Al–4V substrate with a compositionally graded interface by direct laser cladding. Laser processing is carried out by pre-placing the powder (or powder blends) on the substrate, and melting it using a high power continuous wave CO₂ laser with Ar as shrouding gas. A compositionally graded interface is developed by applying powder blends of Ti to Co at a ratio of 90:10 near to Ti–6Al–4V substrate to 10:90 prior to development of Co layer. A defect-free microstructure is developed with the presence of Ti₂Co and TiCo and Co₂Ti at the interface. The volume fraction of individual phase was found to vary with the depth from the Co-clad zone. A significant improvement in microhardness is achieved at the interfacial region. Uniform corrosion resistance increases along the graded interface, but the pitting corrosion resistance is marginally deteriorated. Direct laser clad layer possesses a better biocompatibility than that of as-received Ti–6Al–4V sample.     

Experimental Research on 32Cr3Mo1V Substrate AntiMolten Aluminum Corrosion by Laser Treatment

Two types of layers were prepared by means of laser surface alloying process using High-power Transverse Flow CO2 laser beam with different additions,which were pure Cr powder and Co alloy powder on the 32Cr3Mo1V substrate.The unique characters of laser treatments,such as microstructure,distribution of elements,and micro-hardness were studies in thorough;furthermore,the abilities anti-molten aluminum’s corroding of the layers were tested in 700 centigrade molten aluminum.The results indicate that both laser treated layers can enforce the ability anti-corroding of the molten aluminum on 32Cr3Mo1V substrate,while the laser alloying layer with Cr powder shows the most advantage of anti-aluminum corrosion character,i.e.the layer does not be corroded at all in the corrosion experiment.And the diffused thickness of aluminum in the layers by laser alloying process with Co powder is half below that of in the 32Cr3Mo1V substrate.     

Phase-field simulation of solidification morphology in laser powder deposition of Ti-Nb alloys

A phase-field model of alloy solidification is coupled to a new heat transfer finite element model of the laser powder deposition process. The robustness and accuracy of the coupled model is validated by studying spacing evolution under the directional solidification conditions in laser powder deposition of Ti-Nb alloys. Experimental Ti-Nb samples reveal the microstructure on a longitudinal section with significant change in the size of the dendrites across the sample. Quantitative phase-field simulations of directional solidification under local steady-state conditions extracted from the results of the finite element thermal model confirmed this behavior. Specifically, the phase-field simulations agree with the results of the analytical model of Hunt in predicting a minimum spacing value, which is due to the mutual effects of the increasing temperature gradient and decreasing solidification velocity towards the bottom of the microstructure. This work demonstrates the potential of coupling the phase-field method to complex heat transfer conditions necessary to simulate topologically complex microstructure morphologies present in laser powder deposition and other industrially relevant casting conditions.     

基于PCA与RSM-DE算法的激光熔覆参数多目标优化

为获得激光熔覆Inconel 718粉末在Q690高强钢板上的最优熔覆工艺参数,设计响应曲面法中的BBD(Box-Benhnken Design)试验设计模型.构建输入变量(激光功率、扫描速度、送粉速率)与响应值(稀释率、热影响区深度、显微硬度)之间的数学模型,通过主成分分析法建立熔覆层综合评价指标,利用差分进化算法进行寻优,确定最优工艺参数.采用最优工艺参数进行试验验证,对其最优工艺参数下试件的宏观形貌与组织形态进行观察与分析,并与优选出的试件进行响应值比较.结果表明,最优工艺参数为激光功率1 800 W、扫描速度28 mm/s、送粉速率1.9 r/min,该参数下获得的热影响区深度为294μm,稀释率为14.2%,显微硬度为276.6 HV0.5.最优工艺参数下的试件热影响区深度减小了6.8%,稀释率降低了24.7%,显微硬度增大了21.7%,且最优试件中的组织形态为较小的树枝晶与少量的胞状晶.     

Effects of the cladding parameters on the deposition efficiency in pulsed Nd:YAG laser cladding

The purpose of the present study is to analyze the effects of the cladding parameters on the deposition efficiency in cladding of Co alloy powder by a low power pulsed Nd:YAG laser, and to optimize the cladding parameters for maximizing the deposition efficiency. Experiments were designed, conducted and analyzed by the Taguchi experimental method using a L18 orthogonal array. It was found that the powder feed position, powder feed angle, powder feed rate and travel speed had significant effects on the deposition efficiency, but the shielding gas type, laser pulse shape and focal position had nearly no effects. The optimal cladding conditions in the experimental range were obtained at 0 mm of the powder feed position, 50° of the powder feed angle, 0.62 g/s of the powder feed rate and 6.7 mm/s of the travel speed. From the confirmation experiment, the average deposition efficiency of 12.3% was achieved at the optimized cladding conditions as statistically predicted.     

Study on process optimization of WC-Co50 cermet composite coating by laser cladding

In order to optimize the process of tungsten carbide (WC)-reinforced Co50 cermet composite coating by laser cladding, Co-based coatings with 40 wt% WC were deposited on the surface of cone bit 15MnNi4Mo steel by 4 kW fiber laser. A single-factor experiment was designed to study the variation of the geometrical size, dilution rate and hardness of cladding layers with the change of various factors. Then, an orthogonal experiment was designed to study the optimal parameters for the laser cladding process by taking the hardness and dilution rate of the coatings as comprehensive indexes. Based on the results of the above experiments, the mathematical model of the relationship between the geometrical size of the cladding layers with the process parameters was established by regression analysis. In addition, the three-dimensional structure and microstructure of the coatings were analyzed by optical microscopy (OM), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The results revealed that with the increase of the laser power, the width of the cladding layer, the depth of the molten pool and the dilution rate gradually increased, while the coating height remained basically unchanged. Additionally, with the increase of the scanning speed, the coating height and the molten pool depth were relatively greatly reduced, while the coating width decreased little. Furthermore, with the increase of the powder feeding rate, the width of the cladding layer, the molten pool depth and the dilution rate gradually decreased, while the coating height gradually increased. The optimal process parameters are as follows: laser power of 2.4 kW, scanning speed of 7 mm/s, and powder feeding rate of 0.5 g/s. The mathematical model established by regression analysis fitted the width of the cladding layer best, and the minimum relative error was only 0.023%. The microstructure showed that metallurgical bonding was achieved between the coatings and substrates. Also, the coatings were compact and free of defects such as cracks and pores.