Droplet velocity and thermal state from hot gas atomization of steel melt: Impact on the quality of the spray-formed tubular deposit

The velocity and thermal behavior (temperature, enthalpy, solid fraction) of atomized droplets in a metal spray play the most important role in the spray forming process. These properties mainly determine the materials yield and the final product quality (e.g., porosity, microstructure) of the as-sprayed materials. Changing the gas temperature in the atomization process directly influences these droplet properties in the spray. To understand the droplet behavior in the spray at various atomization gas temperatures (i.e., room temperature RT 293 K, 573 K, 873 K), numerical simulations using computational fluid dynamics (CFD) techniques have been performed and validated by experiments. A series of atomization runs (powder production and spray-forming with AISI 52100 steel) has been conducted at different atomization gas temperatures and pressures with a close-coupled atomizer (CCA). The in-situ temperature detection of the deposit surface (pyrometer) and in the substrate (thermocouples) has been performed to observe the effect of particle properties on the deposit. The result shows that hot gas atomization provides smaller droplets with faster velocity in the spray, affecting the droplet impact and deformation time in the deposition zone. A higher solid fraction of the smaller droplets by hot gas atomization also reduces the deposit surface temperature. Increasing the substrate diameter further decreases the deposit surface temperature without compromising the deposit quality (i.e., porosity) and also refines the grain size. Pre-heating of the substrate up to 573 K results in lower porosity in the vicinity of the substrate.     

Spray forming of high density sheets

Porosity is one of the most important quality criteria of spray‐formed materials in the as‐sprayed condition. Typically, spray‐formed sheets have a porous rim close to the substrate and depending on the spray conditions cold or hot porosity may also be present in the core of the deposit. This porosity has to be removed or minimized to make further processing steps such as rolling, forging or extrusion possible. In this paper, the influence of both substrate temperature and deposit surface temperature on porosity in spray‐formed sheets is studied. For this purpose spray forming experiments (sheet size 1000 mm × 250 mm) were carried out using three different materials: aluminium‐bronze, tin‐bronze and a nitriding steel. For the copper‐base alloys preheated steel‐substrates with different temperatures were moved through a scanning spray cone. In the case of steel a ceramic substrate at room temperature was used. In addition to the variation of the substrate temperature, the gas to metal mass flow ratio (GMR) was varied to achieve different deposit surface temperatures. During the run the surface temperature in the deposition zone was measured using a scanning, multi‐wavelength pyrometer. Samples of the deposits were polished and rasterized by light microscopy. The local porosity was characterized by digital image analysis. The influence of the substrate temperature and the GMR on the porosity in the vicinity of the substrate is evaluated and discussed in detail. The impact of the deposit surface temperature on the porosity was analyzed and is discussed as well. It was found that the deposit surface temperature has a strong impact on porosity for spray‐formed sheets. Finally, experimental results were used to develop a new approach to predict the porosity in spray‐formed sheets. The results clearly show the dependence on material properties. This approach can be used to identify process parameters to generate high density sheets in the future.     

Numerical simulation of coating growth and pore formation in rapid plasma spray tooling

Rapid plasma spray tooling (RPST) is a process that can quickly make molds from rapid prototyping or nature patterns without limitation of pattern's size or material. In this paper, the process of coating growth and pore formation in RPST has been analyzed by numerical simulation. The objective of this work was to determine the porosity in plasma sprayed coatings and verify the developed computer model, which might serve for future thermal residual stress studies of plasma sprayed coatings. The analysis was divided into two steps: particle flattening and coating growth. In the analysis, a ballistic model was used for modeling the in-flight powder particles. The method allows for the calculation of off-normal spray angle, which is common in plasma spraying of engineering components. Also, a set of rules for coating growth as well as pore formation in the coating has been proposed. Based on these works, a computer program was developed to calculate the effects of process parameters, such as gun scanning velocity, spray angle, etc., on the porosity of the coating. Finally, an experiment was carried out to verify the effects of spray parameters on the porosity. The results agree with the prediction of the model.     

Materials characterization and mechanical properties of graded tool steels processed by a new co‐spray forming technique

In order to meet the requirements of micro cold forming tools, a new co‐spray forming process has been applied to produce graded materials from two different tool steels in this study. The two steel melts were atomized and co‐sprayed simultaneously onto a flat substrate, resulting in a flat graded deposit when the two sprays were overlapped. To eliminate porosity and break up carbide network, the graded deposits were further hot rolled. The resultant graded tool steels were investigated with respect to porosity, element distribution, microstructure, hardness, strength, and toughness. The degree of overlapping of the two sprays determined the concentration gradient of the chemical elements in the deposits. The overlapping of the spray cones also contributed to low porosity in the gradient zone of the deposits. The porosity in the graded deposits could be essentially eliminated by means of hot rolling. The carbides and grain structures of the hot rolled tool steels were fine and homogeneous. By means of combining different tool steels in a single deposit, different microstructures and properties were combined.     

The formation of hollow spherical ceramic oxide particles in a d.c. plasma

Investigations have been carried out to determine the conditions that lead to the production of spherical hollow ceramic oxide particles during melting in a d.c. plasma jet. Reports in the literature indicated that such ceramic particles were formed by plasma spraying spray dried agglomerates, but precise details of the conditions necessary for their formation were not stated. In this study it is shown that for hollow particles to be formed several conditions had to be met. Spherical spray dried agglomerates had to be used as starting materials, the material being sprayed had to melt over a narrow temperature range and the size of the particles had to exceed a certain diameter. Experiments, using yttria, showed that the relative size of the pore was dependent on particle diameter, and it has been proposed that the major controlling factor that influences this dependence is the escape of gas trapped in the spray dried agglomerate during melting rather than surface tension or undercooling which were shown to produce only minor effects. In addition, the results also showed that the nature of porosity within the hollow particles as well as the surface morphology was dependent on the material being sprayed.     

Modeling the shape of flat deposits during spray forming using a scanning gas atomizer

A three‐dimensional shape model, tracing the coordinates of the moving surface of a growing flat deposit spray‐formed using a scanning gas atomizer, has been developed in this study. Mass flux distributions in the spray cones generated by the scanning atomizer under a typical spray forming condition have been revealed for the shape modeling. Geometrical evolution of flat deposits in spray forming has been investigated based on analysis of the scanning mechanism of the atomizer. The influence of processing conditions on the shape and dimensions of flat deposits have been simulated and discussed. Finally, the shape modeling has been validated by experimental investigations.     

结构材料喷射成形技术与雾化沉积高温合金

喷射成形是利用快速凝固方法直接制备金属材料坯料或半成品的先进材料制造技术 ,喷射沉积高温结构材料的冶金性能好、生产效率高、成本低 ,因而在近几年得到了迅速发展 .本项研究的主要目的是要通过喷射成形工艺参数的调整、最大限度地直接减少喷射成形坯中的孔隙度 ,进而得到优质坯料 .利用优化的雾化喷射沉积技术制备了多种高温合金沉积坯 ,沉积坯整体致密、晶粒细小、组织均匀、无宏观偏析、含气量低、力学性能提高 .还简要地比较了喷射成形高温合金与用常规铸锭冶金工艺和粉末冶金工艺制备高温合金的异同 ;总结了航空材料研究院喷射成形高温材料近年来的研究状况 ,包括专用高温材料喷射成形装置和技术及其应用 .     

High-velocity oxygen fuel spray coatings for reclamation

These coatings are produced by a spray of molten powder in a system which has recently been developed utilizing a high-velocity fuel mixture of oxygen-propylene or oxygen-hydrogen. These gases are ejected at high speed from a nozzle and then ignited externally to form a flame which envelops the spray material. A high-velocity flame of compressed air carries the molten powder to a specially prepared surface where it solidifies to form a very dense coating of high bond strength and low porosity. The composition of the coating can be varied and depends upon that of the powder, the most commonly used for reclamation being based on either iron or nickel or tungsten carbide. During coating, the substrate temperature is held below 423 K so the process is considered ‘cold’. The spray coatings have good adhesion with low porosity. The high hardness and cost competetiveness means that they are beginning to replace electrodeposited nickel and chromium which have long been used for the repair of engineering components.     

Simulation Using Realistic Spray Cooling for the Continuous Casting of Multi—component Steel

A three-dimensional heat transfer model for continuous steel slab casting has been developed with realistic spray cooling patterns and a coupled microsegregation solidification model that calculates the solidification path for multi-component steels.Temperature and composition dependent properties are implemented in a database for 15 chemical species.Considerable effort is made to accurately model the spray cooling heat transfer.Each spray nozzle position and distribution is considered, including variations of the spray patterns with flow rate, and spray overlay .Nozzle type, layout, nozzle-to-slab distance, and spray span and flux are variable.Natural convection,thermal radiation and contact cooling of individual rolls are computed. The present model provides more comprehensive information and realistic slab surface temperatures than results from a model using the “averaged“ treatment of boundary comditions .Cooling operating conditions and parameters of individual spray nozzles can be analzed to optimize muzzle spray distribution ,improve product quality,and troubleshoot issues such as nozzle clogging that may arise during production.One spray cooling correlation is used for the entire machine,achieving as good or better agreement with surface temperatre measurements than was found previously for the model using an “averaged“ treatment of boundary conditions and using three machine-segment-dependent correlations.     

Influence of the HVOF spraying process on the microstructure and corrosion behaviour of Ni-20%Cr coatings

The microstructure and the aqueous corrosion resistance of coatings produced by High Velocity Oxy-Fuel (HVOF) spraying techniques has been investigated. Two types of spraying processes have been employed i.e., Topgun HVOF using propylene gas and Met-Jet II HVOF with kerosene liquid fuel together with two forms of Ni-20%Cr powders i.e., water and inert gas atomised. The oxide, porosity and the amount of melted material in the coatings were characterised using scanning electron microscopy (SEM) and X-ray diffraction (XRD), whilst the corrosion resistance of the coatings and the ability to protect the underlying mild steel substrate was evaluated by use of a salt spray chamber and potentiodynamic tests.MetJet II produced coatings from gas-atomised powder with a lower oxide content, a reduction in porosity and less melted material, as the residence time of particles in the combusted gas stream was shortened. Water atomised powder formed a higher volume fractions of unmelted material and porosity when compared with gas-atomised powder coatings. This was encouraged by the presence of a thin oxide layer, which formed during the production of the water-atomised powder. The orientation of oxides and pores in the coatings had a major effect on their aqueous corrosion behaviour. Better protection for the underlying steel substrate (>3000 h exposure in a salt spray test) was obtained with the coating produced from the gas-atomised powder with the MetJet II system, which had the lowest porosity/oxide content running perpendicular to the substrate surface. The major factor in preventing attack on the mild steel substrate is the amount of interconnecting porosity which allows the corrodant to percolate through the coating.     

A study on coatability of marble dust on metal substrates

Marble dust is a hazardous construction/industrial waste generated during the extraction, cutting and polishing of marble-producing rocks. The present work explores the potential of marble dust to be used as a coating material on metal substrates. For this, the high-velocity oxy-fuel (HVOF) spraying route is adopted to deposit marble dust coatings on four different metallic substrates: mild steel, inconel, aluminum, and copper. Liquefied petroleum gas is used as the fuel and nitrogen as the carrier gas while the coating deposition is performed by varying the spray distance over a range from 50 mm to 250 mm. The coating microstructure is studied using a scanning electron microscope (SEM) and the developed phases are identified using an x-ray diffractometer. The developed coatings are characterized in terms of deposition efficiency, coating thickness and adhesion strength. It is found that the coatings exhibit fairly good interfacial adhesion and thickness values that varied quite significantly with the spray distance. Maximum deposition efficiency of about 68 % is obtained for the copper substrate when the deposition is performed at a spray distance of 100 mm. This experimental investigation thus shows that despite being a waste, marble dust is eminently coatable on several metallic substrates.     

Simulation of a spray scrubber performance with Eulerian/Lagrangian approach in the aerosol removing process

In this study, a mathematical model has been developed to simulate the performance of a spray scrubber in an industrial ammonium nitrate plant. The model is based on the Lagrangian approach for the droplets movement and particle source in cell (PSI-CELL) model for calculating the droplet concentration distribution. Consequently, unlike former research, the emphasis is on the droplet dynamic behavior. In the current study, for approaching a realistic model, a droplet size distribution rather than average diameter, and also liquid film formation rather than uniform and constant droplet flow rate has been applied. Also, the Eulerian method has been used for the calculation of the particles removal efficiency and energy balance has been applied on the gas to estimate the droplet size distribution. In the experimental section, the concentration of particles and their size distribution in both inlet and outlet gas of the studied scrubber has been measured for the validation of the predicted particles collection efficiency. In addition, the temperature of the gas at inlet, outlet and in the middle of the tower has been measured for the confirmation of the predicted droplet size distribution in the tower. A good consistency between the model and data has been observed. After the model is validated, it is used to investigate the various variable profiles such as liquid film, total projected surface area of the droplets, velocity profile of the droplets and some of the other parameters in the spray scrubbers.     

Microstructural features of spray-formed AZ31 magnesium alloy

Abstract

Mg alloy AZ31 was spray-formed using an indigenously developed spray atomisation and deposition unit under protective atmosphere and various processing parameters were optimised. The microstructural features of the bell shaped AZ31 spray-formed deposit were characterised using optical microscope, scanning electron microscope/energy dispersive spectrometer, X-ray diffraction and high resolution transmission electron microscope. It was observed that the microstructural features are critically dependent on location in the spray-formed deposits. Under optimised processing conditions, the central region of the bell shaped deposit exhibited minimal porosity and a uniform fine grained equiaxed microstructure with fine Mg₁₇Al₁₂ intermetallics preferably located at the grain boundaries. However, the peripheral regions of the spray-formed deposit indicate higher porosity with distinct microstructural characteristics different from those in the central region. These microstructural features, observed at different locations in the spray-formed deposit, have been analysed and their evolution is discussed in the light of variations in thermal and solidification conditions of the droplets in flight, during impingement as well as those of the deposition surface.     

喷射成形高合金化材料沉积坯内部疏松成因分析与改善

本文主要研究了大尺寸喷射成形高合金化材料高温合金沉积坯内部疏松成因及其改善工艺。研究认为,高温合金沉积坯中最后凝固部位液相金属补缩不足和断裂应变低是产生疏松的必要条件,而沉积坯的局部收缩变形产生的热应力是造成疏松的重要原因。通过有限元计算,提出一种热控喷射成形工艺,并得到试验验证。该工艺有效减少了坯体内部的疏松等冶金缺陷,改善了沉积坯的质量。     

Microstructural features induced by spray forming of a ternary Pb-Sn-Sb alloy

An alloy containing Pb-12% Sn-12% Sb with small addition of copper and arsenic was spray deposited employing two different atomization gas pressure and nozzle to substrate distances. The temperature of the spray-deposit was measured during deposition at a distance of 2 and 10 mm above the substrate-deposit interface. Thermal profile data indicated small variation in temperature with time during deposition stage whereas during post deposition stage an exponential decrease in temperature was recorded. Second phase particle size along the thickness of the deposit varied from 4 to 8 μm compared to 70 to 80 μm size of these particles in the as cast alloy. Maximum porosity occurred in the section of the deposit near the contact surface of the substrate and also in its peripheral regions. X-ray diffraction analysis exhibited the formation of additional Cu₂Sb phase in the spray-deposit and CuSn and Cu_3.3Sb phases in atomized powders compared to that of the as cast alloy. The microstructural evolution during spray deposition of this alloy is discussed.     

Application of Taguchi Method to the Optimization of Detonation Spraying Process

The present study deals with an application of the Taguchi method to the optimization of a detonation spray process for alumina coatings. Coating experiments were conducted using the TaguchUfractional factorial (L₈) design parametric study to optimize spray process parameters. The Taguchi design evaluated the effects of four detonation spray process parameters: acetylene to oxygen ratio, carrier gas flow rate, frequency of detonations and spray distance. The coating qualities evaluated were surface roughness, porosity, microhardness, and abrasion mass loss. The influence of process parameters on the as-sprayed coating qualities is discussed. The results of the study indicate that the higher fuel ratio and lower spray distance will result in higher hardness, lower porosity and lower abrasion mass loss. The Taguchi analysis employed in the present investigation led to optimized process parameters for the most abrasive wear resistant alumina coatings.     

On the role of particle state and deposition procedure on mechanical,tribological and dielectric response of high velocity oxy-fuel sprayed alumina coatings

It is well known that the high velocity oxy-fuel based thermal spray process impart high density and reduced porosity in coatings compared to those produced by other ambient thermal spray processes. The benefits of HVOF have largely remained in the domain of metals and cermets and limited investigations have been carried out in ceramic coatings. The ability to produce high density ceramic coatings (e.g. alumina) offers potential in high performance applications in the field of wear, corrosion resistance and dielectric coatings. However, due to extreme operational limits of the HVOF process, the fundamentals of process–structure–property relationships are not fully understood.In this paper, we report an integrated approach to establish processing–microstructure–property correlations in order to optimize coatings for such applications. This approach involves diagnostic studies, microstructure development and its resultant influence on properties of high velocity oxy-fuel (HVOF) sprayed alumina coatings. The diagnostic studies were aimed to investigate the effects of fuel gas/oxygen ratio and amount of total gas flow on the particle temperature and velocity. Furthermore, splats and coatings were deposited to investigate the relationship between diagnostic data, melting behavior and droplet substrate interactions. Such a comprehensive study, coupled with property measurements of the coatings, demonstrates critical operational variables among deposition procedure, coating microstructure and the deposit properties.     

Herstellung HVOF gespritzter,feinststrukturierter Cermetschichten unter Verwendung von feinen WC‐12Co Pulvern

Manufacturing of HVOF sprayed, finest structured cermet coatings using fine WC‐12Co powders The continuous increase in productivity and performance of modern sheet metal forming processes combined with the employment of novel, high strength materials cause high wear on tool systems. Coating technologies like thermal spraying provide a high potential to functionalize and to protect the surface of forming tools. However, it has to be ensured that the high shape and dimensional accuracy of the tool contour is preserved after the application of a wear protective coating. This aim cannot be achieved using currently applied, thermally sprayed coating systems with conventional, coarse grained microstructure. To solve this problem, novel finest structured coatings have been developed in this study by thermal spraying of fine WC‐12Co powders using the HVOF technique. For this purpose the influence of varying HVOF combustion gas compositions on the spray process as well as on the corresponding coating properties has been investigated. Next to a high surface quality the focus was placed on achieving coatings with high hardness and corresponding high wear resistance, low porosity as well as a good adhesive strength on the substrate material.     

Numerical simulation on gas-droplet flow characteristics and spray evaporation process in CFB-FGD tower

Nozzle arrangement in the nozzle spray system has a significant impact on the gas-droplet flow characteristics and the temperature distribution within the circulating fluidized bed flue gas desulphurization (CFB-FGD) tower, which is critical to the SO₂ removal efficiency. The effects of spray direction, nozzle number and nozzle spray angle on gas-droplet distribution and temperature distribution inside the FGD tower are investigated with numerical simulation based on a Eulerian-Lagrangian mathematical model. An optimal nozzle arrangement scheme is proposed to improve the contact between gas and water droplets and the flue gas temperature distribution. Results show that upward spray direction is beneficial to the interaction between water droplets, improving gas-droplet flow characteristics and spray evaporation process, and water droplets number trapped by tower wall could be reduced in the water droplets evaporation. With the increase in nozzle number, it is conducive to the contact between flue gas and water droplets to increase the evaporation efficiency of water droplets, as well as the uniformity of temperature distribution inside the tower. With nozzle spray angle increases from 30° to 120°, flue gas velocity decreases, water droplets number trapped by the tower wall increases. The temperature distribution at different cross-section is the most uniform when the nozzle spray angle is 60°.     

Transport of sputtered atoms investigated by Monte Carlo method

Increasing attention has been paid to the sputtering process as a tool to deposit films and to the study of the interaction between the film properties and the deposition parameters. It is obvious that the energy and direction of these particles arriving at the substrate is in close relation with the transport process from the target to the substrate. This work deals with the computer simulation of the sputtered Ag atoms trajectories through the background gas in a diode-sputtering configuration. For that, we have developed a numerical model to simulate the transport process. We followed the three-dimensional trajectory of each sputtered atom separately and calculated the scattering angle and the energy loss if a collision took place. A statistical method, Monte Carlo simulations is used. The model predicts the flux of Ag atoms arriving at the substrate, their energies and angular distribution. The dependence of the deposition rates of Ag atoms on the gas pressure and the distance between target to substrate were investigated.