3-D finite element modeling of laser cladding by powder injection: effects of laser pulse shaping on the process

This paper introduces a 3-D transient finite element model of laser cladding by powder injection to investigate the effects of laser pulse shaping on the process. The proposed model can predict the clad geometry as a function of time and process parameters including laser pulse shaping, travel velocity, laser pulse energy, powder jet geometry, and material properties. In the proposed strategy, the interaction between powder and melt pool is assumed to be decoupled and as a result, the melt pool boundary is first obtained in the absence of powder spray. Once the melt pool boundary is obtained, it is assumed that a layer of coating material is deposited on the intersection of the melt pool and powder stream in the absence of the laser beam in which its thickness is calculated based on the powder feedrate and elapsed time. The new melt pool boundary is then calculated by thermal analysis of the deposited powder layer, substrate and laser heat flux. The process is simulated for different laser pulse frequencies and energies. The results are presented and compared with experimental data. The quality of clad bead for different parameter sets is experimentally evaluated and shown as a function of effective powder deposition density and effective energy density. The comparisons show excellent agreement between the modeling and experimental results for cases in which a high quality clad bead is expected.     

On the delamination and crack formation in a thin wall fabricated using laser solid freeform fabrication process: An experimental–numerical investigation

Parts fabricated using laser solid freeform fabrication (LSFF) are subject to thermal stresses due to the layer-by-layer material deposition and the temperature distribution characteristic throughout the process domain. The thermal stress patterns and intensity contribute significantly to potential delamination and crack formation. In this paper, the temperature distribution and stress field induced during the multilayer LSFF process, and their correlation with delamination and crack formation are studied. This is performed by a numerical and experimental investigation in the fabrication of a thin wall of 304L stainless steel. For time-dependent predictions on the locations of maximum temperatures and thermal stresses and their patterns, a three-dimensional (3D) transient finite element model is employed to simulate the process, including the geometry of the deposited materials as well as coupled temperature and stress distributions across the process domain. The experimental results are used to verify the numerical results as well as to investigate the correlation between the numerical results and micro-crack formations across the fabricated parts. The experiments are conducted with the same process parameters used in the numerical analyses using a 1 kW Nd:YAG pulsed laser. The trend of numerical and experimental results reveals that by preheating the substrate prior to the fabrication process, it is possible to substantially reduce the micro-cracks formed across the part. To demonstrate the feasibility of preheating on the reduction of micro-cracks, several simulations and experiments are performed in which a crack-free result is obtained when the substrate is preheated to 800 K. For this case, 22% reduction in thermal stresses is obtained throughout the process domain.     

Solute transport and composition profile during direct metal deposition with coaxial powder injection

Direct metal deposition (DMD) with coaxial powder injection allows fabrication of three-dimensional geometry with rapidly solidified microstructure. During DMD, addition of powder leads to the interaction between laser and powder, and also the redistribution of solute. The concentration distribution of the alloying element is very important for mechanical properties of the deposited clad material. The evolution of concentration distribution of carbon and chromium in the molten pool is simulated using a self-consistent three-dimensional model, based on the solution of the equations of mass, momentum, energy conservation and solute transport in the molten pool. The experimental and calculated molten pool geometry is compared for model validation purposes.     

Determination of geometrical factors in Layerwise Laser Melting using optical process monitoring

Layerwise Laser Melting (LLM) is a layerwise production technique enabling the production of complex metallic parts. In the process a thin layer of powder is first deposited on a base plate. With the energy of a scanning laser beam this layer is melted at selected places, according to a predefined scanning pattern. After scanning, a new layer of powder is deposited on top of the previous layer and selectively melted. This sequence of depositing and scanning is repeated until the complete part is built. The local geometry surrounding the melt pool has a large influence on the processing behavior. For process control issues, this influence must be known and quantified, in order to determine a priori optimal processing conditions and to interpret measured melt pool radiation. In order to study the melt pool behavior, optical process monitoring of LLM has been applied using a high speed near-infrared CMOS camera and a large area silicon photodiode sensor. Data processing rates up to 10 kHz and real-time process monitoring are achieved using image and signal processing on a Field Programmable Gate Array (FPGA). Several case studies will be presented showing that the geometric influencing factors can be studied and quantified by analyzing the melt pool sensor output.     

Porosity formation and gas bubble retention in laser metal deposition

One of the inherent problems associated with laser metal deposition using gas-assisted powder transfer is the formation of porosity, which can be detrimental to the mechanical properties of the bulk material. In this work, a comprehensive investigation of porosity is carried out using gas atomised Inconel 718 powder. In the analysis, a clear distinction is made between two types of porosity; namely lack of fusion and gas porosity. The results show that the two types of porosity are attributed by different factors. The gas porosity, which is more difficult to eliminate than the lack of fusion, can be as high as 0.7%. The study shows that the gas porosity is dependent on the process parameters and the melt pool dynamics. The flotation of entrapped gas bubbles was analysed, showing that in a stationary melt pool the gas would be retained by Marangoni-driven flow. The overall Marangoni-driven flow of the melt pool is in the order of five times higher than the flotation effect, and this is the reason why the melt pool geometry would tend to dominate the flow direction of the gas bubbles. Through optimisation, the gas porosity can be reduced to 0.037%.     

Impact of localized surface preheating on the microstructure and crack formation in laser direct deposition of Stellite 1 on AISI 4340 steel

Crack formation in laser cladding of the hardfacing alloy Stellite 1 on AISI-SAE 4340 steel was prevented through locally preheating the substrate prior to the deposition process. Numerical analysis showed that the preheating process helps developing a relatively steadier melt temperature as well as decreasing the cooling rates and consequently the thermal stresses during the subsequent deposition process. Microstructural analysis revealed a thicker cross-section with smoother surface profile, more uniform surface hardness and even distribution of a dendritic morphology in the preheated sample. This confirmed the presence of a well-developed melt pool with a homogeneous composition at solidification. The microstructure of non-preheated sample was, however, considerably non-uniform consisting of macro-scale colonies of dendritic and lamellar (eutectic) structures. The experimental observations, as implied through the numerical results, showed that the preheated sample, in general, reveals more uniform structure and properties making it less prone to cracking during the deposition process.     

Effects of powder concentration distribution on fabrication of thin-wall parts in coaxial laser cladding

In this paper, model of effects of powder concentration distribution on fabrication of thin-wall parts in coaxial laser cladding was developed. There exists relationship between powder concentration distribution and power density distribution, which affects fabrication of thin-wall parts in coaxial laser cladding. Changes in powder concentration distribution lead to changes in wall thickness and wall growing rate. Fluctuation of powder feed rate deteriorates the growing wall in laser cladding. Deviation of the powder flow stream makes the powder concentration distribution, the thermal flux density and consequently the molten pool not symmetrical against the x-axis, resulting in irregular upper faces of the formed wall. This was verified by the results of experiment.     

Study on cross-section clad profile in coaxial single-pass cladding with a low-power laser

In this paper, a model of cross-section clad profile on the substrate in coaxial single-pass cladding with a low-power laser was studied. The static model of powder mass concentration distribution at cold-stream conditions was defined as a Gaussian function. In coaxial single-pass cladding with a low-power laser, since the influence of surface tension, gravity and gas flow on the clad bead could be neglected, the cross-section profile of the clad bead deposited by a low-power laser on the substrate was dominated by the powder concentration at each point on the pool and the time when the material was liquid at this point. The height of each point on the cross-section clad profile was defined as a definite integration of a Gaussian function from the moment at which the melt pool was just arriving at the point to the moment at which the point left the melt pool. In the presented experiment, powder of Steel 63 (at 0.63 wt% C) was deposited on a substrate of Steel 20 (at 0.20 wt% C) at the laser power of 135 W. The experimental results testified the model.     

New parameter correlations for deep penetration welding with high energy focused beams

As a result of the unification of two models previously described by the author, a three-dimensional treatment of the workpiece temperature distribution, which takes into consideration the solid to liquid phase change of the processed material, is presented for deep penetration welding (d.p.w.) with high energy focused beans.The new model improves the accuracy of weld geometry prediction by better correlating the shape, width and depth of the cross-section of the resolidified weld with beam characteristics, material thermophysical properties, workpiece velocity and preheating temperature.     

Investigation of temperature and stress fields in laser cladded coatings

Temporal and spatial distributions of temperature and strain-stress have been modelled and investigated experimentally for the laser cladding process. The model corresponded to experimental conditions where the multilayer protective coatings were prepared by direct laser cladding of stellite SF6 powder on X10Cr13 chromium steel by means of a 1.2 kW CO₂ laser. For calculations the effect of base preheating, temperature dependent material properties, and also influence of time-break between cladding of the consecutive layers were taken into account. The calculated temperature fields indicated good bonding of the substrate and coating, which was in agreement with the micro-analytical test results. A decrease of the number of microcracks in the coating with an increase of substrate preheating temperature was concluded from stress calculations and confirmed in the experiment. Moreover, an increase of the cracking susceptibility with an increase of the time delay between cladding of the consecutive layers was evidenced by modelling. The best technological results were obtained for the case of single-layer coatings prepared on a preheated substrate and for higher coating thickness required the processing of consecutive layers with a possibly short time delay is advisable due to effective usage of laser beam energy for preheating and lower temperature gradients.     

Investigation on the direct laser metallic powder deposition process via temperature measurement

The direct laser metallic powder deposition process was investigated with the aid of a radiant thermometer by building thin walls. The measured infrared (IR) temperature signal showed good correlation with the deposition process and the quality of the deposited samples. The influence of the powder particle size and the z-increment on the quality of the deposited samples and the IR-temperature signal was examined. It was found that the particle size of the powders shows no significant influence on the measured IR-temperature signal and the deposition process. However, both the deposition process and the measured temperature signal depended strongly on the z-increment. The variation of the melt pool temperature and cooling rate resulted in an inhomogeneous dimension accuracy, microstructure and hardness of the deposited sample. An abnormal deposition process can be recognized by the IR-temperature signal.     

Analysis of hybrid Nd:Yag laser-MAG arc welding processes

In the hybrid laser-arc welding process, a laser beam and an electric arc are coupled in order to combine the advantages of both processes: high welding speed, low thermal load and high depth penetration thanks to the laser; less demanding on joint preparation/fit-up, typical of arc welding. Thus the hybrid laser-MIG/MAG (Metal Inert or Active Gas) arc welding has very interesting properties: the improvement of productivity results in higher welding speeds, thicker welded materials, joint fit-up allowance, better stability of molten pool and improvement of joint metallurgical quality. The understanding of the main relevant involved physical processes are therefore necessary if one wants for example elaborate adequate simulations of this process. Also, for an efficient use of this process, it is necessary to precisely understand the complex physical phenomena that govern this welding technique. This paper investigates the analysis of the effect of the main operating parameters for the laser alone, MAG alone and hybrid Laser/MAG welding processes. The use of a high speed video camera allows us to precisely characterize the melt pool 3D geometry such as the measurements of its depression and its length and the phenomena occurring inside the melt pool through keyhole-melt pool-droplet interaction. These experimental results will form a database that is used for the validation of a three-dimensional thermal model of the hybrid welding process for a rather wide range of operating parameters where the 3-D geometry of the melt pool is taken into account.     

A finer modeling approach for numerically predicting single track geometry in two dimensions during Laser Rapid Manufacturing

We simulated the temperature distribution and single track geometry in Laser Rapid Manufacturing (LRM) of metal structures using two dimensional FEM with a finer modeling approach. This approach involved the calculation of excessive enthalpies above melting point for all nodal points in the process domain and using those for the computation of local track height at every node along the track width on the substrate. Laser power, laser beam size with its spatial profile, scan speed, powder feed rate and powder stream diameter with its flow distribution were taken as user-defined input parameters to simulate the single track geometry during LRM. The simulated results were experimentally verified. The percentage errors in simulated and corresponding experimental track heights along the normalized track width were calculated and compared with those of other existing models. With our modeling approach, the localized errors in predicting track geometries were found to be the least. This model is capable of dynamically predicting the temperatures and track geometry at various user-defined input parameters.     

Residual stress of physical vapor-deposited polycrystalline multilayers

An extended one-dimensional stress model for the deposition of multilayer films is built based on the existing stress model by considering the influence of deposition conditions. Both thermal stress and intrinsic stress are considered to constitute the final residual stress in the model. The deposition process conditions such as deposition temperature, oxygen pressure, and film growth rate are correlated to the full stress model to analyze the final residual stress distribution, and thus the deformation of the deposited multilayer system under different process conditions. Also, the model is numerically realized with in-house built code. A deposition of Ag-Cu multilayer system is simulated with the as-built extended stress model, and the final residual stresses under different deposition conditions are discussed with part of the results compared with experiment from other literature.     

Heat transfer modelling and stability analysis of selective laser melting

The process of direct manufacturing by selective laser melting basically consists of laser beam scanning over a thin powder layer deposited on a dense substrate. Complete remelting of the powder in the scanned zone and its good adhesion to the substrate ensure obtaining functional parts with improved mechanical properties. Experiments with single-line scanning indicate, that an interval of scanning velocities exists where the remelted tracks are uniform. The tracks become broken if the scanning velocity is outside this interval. This is extremely undesirable and referred to as the “balling” effect. A numerical model of coupled radiation and heat transfer is proposed to analyse the observed instability. The “balling” effect at high scanning velocities (above ∼20 cm/s for the present conditions) can be explained by the Plateau-Rayleigh capillary instability of the melt pool. Two factors stabilize the process with decreasing the scanning velocity: reducing the length-to-width ratio of the melt pool and increasing the width of its contact with the substrate.     

Optical focus control system for laser welding and direct casting

A non-intrusive optical sensor system has been developed for focus control of laser welding. This detects the light generated by the process through the laser delivery optics, and exploits the chromatic aberrations of these optics to detect any laser focal error at the workpiece. This system works for a wide range of materials and welding parameters, and example results are presented. The sensor has also been applied to laser ‘direct casting’, a process in which 3-D structures are built by flowing metal powder into a focused laser beam. In this case, melt pool temperature is also important, and so additional optics are incorporated into the sensor to provide a pyrometric temperature measurement which is used to control the laser power.     

Process monitoring of powder pre-paste laser surface alloying

Instead of the continuous powder delivery method using a powder feeder for thick layer laser cladding, pre-pasting of the alloying powder on the substrate is a widely used method to supply the coating powders into the melt pool for LSA. A method to monitor the process of laser surface alloying based on the infrared emission from the melt pool using infrared photodiodes was developed. The technique is solely aimed at the process of laser surface alloying using pre-paste metal powder on the substrate surface prior to laser melting. This monitoring technique is able to distinguish the existence or the absence of the pre-paste powder and the consistency of the laser surface alloying process. The technique is of low cost and is simple to implement into the process.     

Theoretical and experimental investigation into laser melting of steel samples

Among the available laser applications, laser melting has turned out to be a powerful technique for the production of mechanically improved surfaces. To enhance the understanding of the laser melting process investigations into modeling of the heating mechanism initiating the laser melting are necessary. In the present study, a mathematical modeling of the laser melting process is introduced and power require ments for the laser melting are predicted as functions of workpiece properties and velocity. Maximum melt width is predicted analytically and compared with the experimental results. In this regard, an experiment is conducted to melt the mild steel samples with a cw CO₂ laser at different power settings and workpiece velocities. It is found that the melt variables predicted from theory are in agreement with the experimental results.     

Experimental study of temperature and clamping force during Nd:YAG laser butt welding

During welding, a high quality clamping device not only holds workpieces firmly together, but should also take the thermal strain of the welding heat without undermining the strength of the weld joint, inducing any excessive distortions, misalignment of workpieces or reducing the weld joint strength. This paper studies the clamping force during laser butt welding of steel workpieces. The clamping force and welding temperature for a butt welded joint during laser welding are measured simultaneously. The preset clamping force is varied during welding for different thicknesses of workpieces and weld joint strengths. The thermal expansion, cooling contraction, and workpiece width reduction during welding induce variations in the preset clamping force and consequently change the weld joint strength. Our study also reveals that there is an optimal preset clamping force that improves the weld joint strength significantly and the welding temperature during steady welding process remains unchanged for any preset clamping force.     

Research on Raman-scattering and Fabrication of Multilayer Thin Film with Different Structures and Components Based on Pt/Ti/Si3N4/SiO2/Si Substrate

选用三水醋酸铅、乙酰基丙酮酸锆、四异丙氧基钛、乙酰丙酮作初始材料,用同样的方法分别制备了锆钛酸铅(PZT)和钛酸铅(PT)两种固体前驱物. 采用改良型的溶胶-凝胶工艺技术,分别在不同的Pt-Ti-Si₃N₄-SiO₂-Si基底上,按照不同的组合方式,制备了三种多层薄膜：PZT、PT/PZT-PZT/PT、PT/PZT/-/PZT/PT. 较详细地讨论了薄膜制备的工艺技术,发现当凝胶通过烧结和干燥后变成固态物质时,薄膜内部存在着较大的残余应力,当薄膜在600 ºC下退火时其内部残余应力可以被减小. 通过拉曼