Modeling of porosity during spray forming: Part I. Effects of processing parameters

In this article, a porosity model is developed based on particle packing theory, fluid mechanics, and particle thermal and dynamic behavior during spray forming. According to this model, the amount of porosity in as-deposited materials can be estimated on the basis of the average fraction of solid in the incident spray and the solid particle packing density. A porosity coefficient Φ is introduced. By using this model, the effects of deposition distance, atomization gas pressure, melt flow rate, and melt superheat on porosity are investigated. The amount of porosity demonstrates distinct V-shaped variations with the processing parameters. Finally, the optimal processing parameters for achieving low porosity are discussed on the basis of the calculated results.     

Characterization of spray atomization of 3003 aluminum alloy during linear spray atomization and deposition

Linear spray atomization and deposition is an attractive technique to produce near-net-shape deposits, such as aluminum sheet and strip. In the present study, phase Doppler interferometry (PDI) was used in a backscatter mode to characterize, in situ, the droplet size and velocity distributions during linear spray atomization and deposition of a 3003 aluminum alloy. The PDI measurements were obtained along axes corresponding to the direction parallel to the nozzle slit and to the direction perpendicular to the slit. The PDI results delineate the temporal and spatial distribution of the droplet size and velocity during the metal spray. Both point and “line” measurements were obtained and are reported. The line measurements resulted from the integration of measurement made along a line scan obtained in real time (i.e., not ensemble averaged). Postrun analysis of the droplet size distribution using laser diffraction and sieving techniques is also reported. The PDI point measurements revealed that droplet size and velocity distribution were relatively invariant with time. The line measurements of droplet velocity showed that the droplet axial velocity exhibits a bimodal behavior, which becomes more apparent with increasing atomizing gas pressure, a result of droplet recirculation inside the spray chamber. In addition, the peak in the droplet axial velocity distribution increased as atomizing gas pressure increased. The line characterization also showed that the droplet size distribution became more homogeneous with increasing gas pressure, and that the distribution characteristic diameters of droplets decreased consistently with increasing gas pressure. Postrun characterization of the droplet size distribution of the entire metal spray using diffraction and sieving methods indicated that the mass (volume) median diameter D ₅₀ and the Sauter mean diameter (SMD) D ₃₂ decreased with increasing gas pressure in a manner consistent with PDI results.     

Structure and properties of a rapidly solidified Al-Li-Mn-Zr alloy for high-temperature applications: Part II. spray atomization and deposition processing

A new Al-Li alloy containing 2.3 wt pct Li, 6.5 wt pct Mn, and 0.65 wt pet Zr for high-temperature applications has been processed by a rapid solidification (RS) technique (as compacts by spray atomization and deposition) and then thermomechanically treated by hot extrusion. As-received and thermomechanically treated deposits were characterized by X-ray diffraction and scanning electron microscopy (SEM). Phase analyses in the as-processed materials revealed the presence of two Mn phases (Al₄Mn and Al₆Mn), one Zr phase (Al₃Zr), two Li phases (the stable AlLi and the metastable Al₃Li), and the aAl solid solution with high excess in Mn solubility (up to close the nominal composition in the as-atomized powders). As-deposited and extruded pieces were given heating treatments at 430 °C and 530 °C. A two-step aging treatment was practiced, to check for the optimal (for tensile properties) aging procedure, which was found to be the following: solutioning at 430 °C for 1 hour and water quenching + a first-step aging at 120 °C for 12 hours + a second-step aging at 175 °C for 15 hours. The mechanical properties, at room and elevated temperatures, of the hot extruded deposits are compared, following the optimal solutioning and aging treatments. The room-temperature (RT) strength of the proposed alloy is distinctly better for the as-deposited specimens (highest yield strength, 320 MPa) than for the as-atomized (highest yield strength, 215 MPa), though less than 65 pct of the RT strength is conserved at 250 °C. Ultimate strengths are quite comparable (in the 420 to 470 MPa range). Ductilities at RTs are in the low 1.5 to 2.5 pct range and show no improvement over other Al-Li alloys.     

The Effect of ceramic reinforcements during spray atomization and codeposition of metal matrix composites: Part II. Solid-state cooling effects

In the present study, the mechanisms governing the evolution of microstructure during spray atomization and codeposition of metal matrix composites (MMCs) were investigated, with particular emphasis on the effects of the ceramic phase on the resulting microstructure during solidstate cooling. The grain size refinement that is commonly observed when a distribution of ceramic particulates is coinjected into a metallic spray during spray atomization and deposition processing was rationalized in terms of three distinct effects: (a) solidification effects, (b) heattransfer effects, and (c) solid-state cooling effects. The solidification and heat-transfer effects were discussed in Part I. [1] Regarding solid-state cooling effects, the present results show that the presence of a dispersion of ceramic particulates in the aluminum matrix effectively reduces the rate of grain growth during solid-state cooling. Experimental support to this suggestion was provided by the results of an investigation on the changes in grain size following isochronal thermal anneals.     

气雾化制备微细球形钴铬钼钨合金粉末及其SLM成形性能

利用自行研制的防返风超音速气雾化设备制备钴铬钼钨合金粉末,对粉末的形貌、粒度与粒度分布以及显微组织等进行分析,并研究其激光选区熔化成形件的显微组织、硬度和拉伸性能。结果表明,气雾化制备的Co Cr Mo W合金粉末主要为球形,部分有卫星颗粒,粉末组织由胞状晶和树枝晶组成。激光选区熔化成形的成形件表面熔道搭接良好,表面粗糙度为11.0μm,相对密度达到98.7%,组织为γ马氏体和ε马氏体;抗拉强度为1 283 MPa,屈服强度为852 MPa,伸长率为7.9%,显微硬度HV达到398.8;拉伸断口呈现准解理断裂特征。     

Deformation and fracture of a particle-reinforced aluminum alloy composite: Part II. Modeling

In this article, the tensile and fracture properties of a discontinuously reinforced aluminum (DRA) alloy composite are modeled to determine the influence of constituent parameters on material behavior. Comparison of the elastic-modulus calculations to the experimental data suggest that the angular particles are more effective in load transfer than spherical particles, and that a unit cylinder geometry is a good representation of the particles under elastic conditions. This same geometry is used in the finite element-based elastic-plastic model of Bao et al., and reasonably good agreement is obtained between the experimental and predicted yield strengths. A fracture-mechanics model is proposed for predicting the elongation to failure. The model assumes the existence of particle cracks, and criticality is based on the strain required for matrix rupture between cracked particles. The damage criterion of Cockcroft and Latham is utilized, and model predictions are compared to data from different investigations. It is shown that the volume fraction of particles and the work-hardening coefficient of the matrix have a strong influence on the strain to failure. Fracture toughness modeling one again exposes the limitations of existing zero-degree crack-propagation models, such as that of Hahn and Rosenfield, which predict increased toughness with yield strength rather than a decrease, which is observed experimentally. A shear-failure model along a 45-deg direction is proposed for the higher-strength conditions, where concentrated slip bands were observed. The model exhibits the inverse toughness dependence on strength and better correlation to peak-aged (PA) data, but shows poorer agreement with underaged (UA) data. Thus, a transition from zero-degree propagation to 45-deg propagation with increasing strength is suggested. A simplified method for extracting particle stresses is illustrated and is used to estimate a Weibull modulus of 4.9 and a Weibull strength of 2450 MPa for the SiC particles of an average diameter of 10 μm. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Deformation and fracture of a particle-reinforced aluminum alloy composite: Part II. Modeling

In this article, the tensile and fracture properties of a discontinuously reinforced aluminum (DRA) alloy composite are modeled to determine the influence of constituent parameters on material behavior. Comparison of the elastic-modulus calculations to the experimental data suggest that the angular particles are more effective in load transfer than spherical particles, and that a unit cylinder geometry is a good representation of the particles under elastic conditions. This same geometry is used in the finite element-based elastic-plastic model of Bao et al., and reasonably good agreement is obtained between the experimental and predicted yield strengths. A fracture-mechanics model is proposed for predicting the elongation to failure. The model assumes the existence of particle cracks, and criticality is based on the strain required for matrix rupture between cracked particles. The damage criterion of Cockcroft and Latham is utilized, and model predictions are compared to data from different investigations. It is shown that the volume fraction of particles and the work-hardening coefficient of the matrix have a strong influence on the strain to failure. Fracture toughness modeling once again exposes the limitations of existing zero-degree crack-propagation models, such as that of Hahn and Rosenfield, which predict increased toughness with yield strength rather than a decrease, which is observed experimentally. A shear-failure model along a 45-deg direction is proposed for the higher-strength conditions, where concentrated slip bands were observed. The model exhibits the inverse toughness dependence on strength and better correlation to peak-aged (PA) data, but shows poorer agreement with underaged (UA) data. Thus, a transition from zero-degree propagation to 45-deg propagation with increasing strength is suggested. A simplified method for extracting particle stresses is illustrated and is used to estimate a Weibull modulus of 4.9 and a Weibull strength of 2450 MPa for the SiC particles of an average diameter of 10 μm. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Modeling of the incorporation of ceramic participates in metallic droplets during spray atomization and coinjection

In the present investigation, a theoretical model was developed to study the penetration behavior of ceramic particulates into metallic droplets during spray atomization and coinjection. In formulating the penetration problem, a force balance approach was adopted that considers the variations of both surface-tension resistance and fluid drag during the penetration processes. Using this model, the factors that affect the penetration behavior of ceramic particulates into Al droplets were systematically discussed. These include size, morphology, and density of ceramic paniculate; wetting angle between ceramic and liquid Al; and fraction of solid contained in the semiliquid droplets. It was found that the critical velocity required for penetration increased with increasing wetting angle and fraction of solid but decreased with increasing particulate density. The penetration ability of various ceramic particulates was compared. It was found that the penetration ability of ceramic particulates that are normally encountered in Albased metal matrix composites (MMCs) decreases in the following sequence: TiB₂, Al₂O₃, SiC, and graphite.     

喷射成形时雾化压力与静压头对雾化熔滴尺寸影响的理论分析

本文建立了静压头、雾化压力与雾化熔滴尺寸的模型关系。雾化压力一定，雾化熔滴质量中值直径随静压头的减小而减小；静压头一定。雾化熔滴尺寸随着雾化压力的增加而减小。通过模型，调节雾化压力以适应静压头变化引起的金属液体质量流率变化，以期得到具有合适尺寸的雾化熔滴，优化沉积坯组织。     

喷射成形技术在高合金工模具钢中的应用

采用喷射成形技术制备了几种高合金工模具钢,并与其它工艺生产的同成分钢种作了比较。喷射成形工模具俐具有均匀细化的等轴品组织,经过热加工和球化退火处理,可以获得与粉末冶金工模具钢相似的球化组织和硬度值,从而为顺利地进行后续淬、回火热处理创造条件。利用喷射成形技术制备高合金工模具钢具有明显优势,它既能有效解决成分偏析、组织不均匀性、网状碳化物等问题,又能明显简化生产工艺,降低生产成本和能源消耗等,具有明显的潜在市场竞争力。     

气体雾化Al-Zn-Mg-Cu铝合金粉末的形貌及组织性能研究

利用氮气雾化法制备了Al-Zn-Mg-Cu合金粉末,通过扫描电镜、光学显微镜和X射线衍射仪对粉末的形貌及组织特征进行了研究;检测了粉末热挤压法制备的合金棒材的力学性能,并对其断口进行了分析。结果表明:随着粉末颗粒尺寸减小,颗粒形状由以长条形为主转变为以近球形为主。同时,显微组织中的枝晶臂间距减小,晶粒细化效果明显;合金以α-Al相为主,还有少量η-MgZn2平衡相存在。随粉末颗粒尺寸减小基体过饱和度增加,基体相和MgZn2相衍射峰宽化;粉末粒度减小,挤压合金力学性能提高;挤压合金拉伸断口属于韧性断裂。     

Hydrodynamics of gas stirred melts: Part II. Axisymmetric flows

A predictive model of gas stirred melt is presented. Based on the differential approach and following a review of previous models, the importance of natural convection or buoyancy driven phenomena is underscored. Predicted flow patterns are shown to be consistent with laboratory and pilot scale experiments, and with the macroscopic plume model in Part I of this paper.     

Numerical results of temperature distribution and solidification behaviour during spray forming

Based on a grid transformation method, the temperature and solidification behaviour in the substrate and deposit for the spray forming process of metals has been simulated. Numerical results will be compared with experimental data of temperature measurements in the deposit and substrate. Some limitations of this model will be outlined.     

Numerical simulation of temperature distribution and solidification behaviour during spray forming

A mathematical, two-dimensional model for the analysis of the temperature and solidification behaviour in the substrate and deposit for the spray forming process of metals has been developed. Due to the irregular shape of the growing deposit, which changes with spraying time, a grid transformation method was successfully developed for the calculation of the temperature distribution and solidification front in the growing deposit. The influence of process parameters, i.e. thermal conditions of the metal spray at impact, heat transfer coefficient at the surface of the deposit and geometrical parameters, are qualitatively and quantitatively discussed.     

The Effect of ceramic reinforcements during spray atomization and codeposition of metal matrix composites: part i. heat transfer

The effects of ceramic participates on microstructure during spray atomization and codeposition of metal matrix composites (MMCs) were investigated, with particular emphasis on the transfer of thermal energy from the atomized matrix to the ceramic particulates. A thermal energy model, based on fundamental heat-transfer considerations, was formulated, and the numerical results were compared to the microstructural findings. In this model, the transfer of thermal energy from the atomized metal to the ceramic phase was computed for two separate stages: (a) atomization and (b) deposition. The numerical results obtained using SiC particulates in an aluminum matrix show that 10 pct of the enthalpy of the atomized spray is transferred to the ceramic particulates during atomization, whereas 10 pct of the thermal energy available after deposition will be consumed in the process of equilibrating the temperature of the particulates to that of the matrix. The enhanced heat transfer achieved as a result of the presence of ceramic reinforcement was correlated with the grain sizes of various unreinforced and reinforced spraydeposited MMCs.     

Production of Nd-Fe-B alloy powders using high-pressure gas atomization and their hard magnetic properties

Fine spherical Nd−Fe−B powders with a tetragonal Nd₂Fe₁₄B phase have been produced by high pressure argon or helium atomization. The average size defined by 50 pct cumulative weight fraction is as small as about 25 μm. The Curie temperature of the powders is about 580 K and the intrinsic coercivity (iH _c ) of the bonded products made from the powders increases with decreasing particle size and reaches about 0.581 MA/m for the powder below 25 μm diameter. TheiH _c value increases with an increase in the cooling rate by helium atomization as well as with an increase in neodymium and boron content to 18 at. pct Nd and 12 at. pct B with the highestiH _c value reaching 0.716 MA/m. Annealing in the range of 773 to 1073 K gives rise to a further increase ofiH _c to 1.035 MA/m. The highiH _c value is promising for practical use as isotropic bonded powder magnets. The increase of magnetization for the bonded powder magnets takes place rapidly and the behavior is similar to that for sintered Nd−Fe−B magnets, in goods contrast to a sluggish increase of magnetization for the bonded Nd−Fe−B powder magnets made from the comminuted powders of melt-spun ribbon. From the magnetization behavior, it was presumed that the generation of the large intrinsic coercivity is due to the difficulty of the nucleation of reverse domain rather than the pinning of domain walls.     

Slag-metal reactions during welding: Part II. Theory

A kinetic model is developed to describe the transfer of alloying elements between the slag and the metal during flux-shielded welding. The model accounts for changes in alloy recovery based on the geometry of the resulting weld bead. It also distinguishes compositional differences between single-pass and multiple-pass weld beads. It is further shown that the final weld metal oxygen content is directly related to the weld solidification time as well as the type of flux used.     

Structure and properties of a rapidly solidified Al-Li-Mn-Zr Alloy for high-temperature applications: Part I. inert gas atomization processing

A new Al-Li alloy containing 2.3 wt pct Li, 6.5 wt pct Mn, and 0.65 wt pet Zr, for high-temperature applications, has been processed by a rapid solidification (RS) technique (as powders by inert gas atomization) and then thermomechanically treated by hot isostatic pressing (hipping) and hot extrusion. As-received and thermomechanically treated powders (of various size fractions) were characterized by X-ray diffraction and scanning and transmission electron microscopy (SEM and TEM, respectively). Phase analyses in the as-processed materials revealed the presence of two Mn phases (Al₄Mn and Al₆Mn), one Zr phase (Al₃Zr), two Li phases (the stable AlLi and the metastable Al₃Li), and the αAl solid solution with high excess in Mn solubility (up to close the nominal composition in the as-atomized powders). Extruded pieces were solutionized at 370 °C and 530 °C for various soaking times (2 to 24 hours). A variety of aging treatments was practiced to check for the optimal (for tensile properties) aging procedure, which was found to be the following: solutioning at 370 °C for 2 hours and water quenching + 1 pct mechanical stretching + one step aging at 120 °C for 3 hours. The mechanical properties, at room and elevated temperatures, of the “hipped” and hot extruded powders are compared following the optimal solutioning and aging treatments. The results indicate that Mn is indeed a favorable alloying element for rapidly solidified Al-Li alloys to retain about 85 to 95 pct of the room-temperature tensile properties even at 250 °C, though room-temperature strength is not satisfactory in itself. However, specific moduli are by 20 to 25 pet higher than those of the 2024 series duralumin-type alloys. Ductilities at room temperatures are in the low 1 to 2.5 pct range and show no improvement over other Al-Li alloys.     

Modeling of mechanical alloying: Part II. Development of computational modeling programs

Computational modeling programs incorporating the physics of powder deformation, fragmentation, and coalescence occurring during mechanical alloying (MA) are developed. The programs utilize the equations developed in part I of this series; equations predicting the extent of powder deformation during an effective impact in MA and those specifying criteria for powder particle fragmentation and coalescence. Two programs have been developed for these purposes. One, MAPI, considers the behavior of a single species with the option of adding dispersoids. The other, MAP2, considers two ductile species being welded to form a third, composite species. Applications of the programs to previous experimental data, and for the purpose of identifying the effect of material and process variables on alloying behavior, are provided in the article following this one. Formerly Graduate Student, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903 Formerly Professor, Department of Materials Science and Engineering, University of Virginia