Functional encapsulation of laser melted Inconel 718 by Arc-PVD and HVOF for post compacting by hot isostatic pressing

In this study, the Selective Laser Melting (SLM) technology was used to manufacture flat specimens from Inconel 718 powder. The SLM process parameters have a major impact on the microstructure as well as on the mechanical properties of the fabricated specimens. Despite using optimized processing parameters, defects like pores cannot be completely avoided. These pores act as stress raisers and lead to premature crack initiation under cyclic loading, eventually reducing the fatigue strength of the material. Hot Isostatic Pressing (HIP) offers the possibility to eliminate the porosity and thus to increase the fatigue performance of the material. HIP combines high pressure and high temperature to produce materials with superior properties. Unfortunately, open porosity, i.e. open pores on the surface, can prevent full densification. In the present work, SLM flat specimens were encapsulated by means of Cathodic Arc Deposition (Arc-PVD) and High Velocity Oxygen Fuel Spraying (HVOF) to seal open pores. For this purpose, different encapsulation materials were investigated with a focus on materials offering additional functions such as an improved high temperature corrosion resistance or applicability as a bond coat for thermal barrier coatings.     

Densification by interface-reaction controlled grain-boundary diffusion

The densification of a fine-grained, high-purity aluminum oxide powder under hot isostatic pressing (HIP) has been found to occur by interface-reaction controlled grain-boundary diffusion. We discuss geometries and dislocation mechanics for this process for both the initial and final stages of densification and develop constitutive equations for densification rate as a function of density, materials constants, and experimental parameters. The model is used to explain the results of several HIP experiments at pressures of 34–102 MPa and temperatures of 1273–1423 K. Sources of variation from sample to sample are discussed. An analysis is made of the sensitivity of the model to its adjustable parameters. Alternative explanations for the experimental data are discussed and found to be inadequate.     

Parametric studies of the hot isostatic pressing of rapidly solidified 304 stainless steel powder

The increasing importance of powder materials fabrication by use of hot isostatic pressing (HIP) has led to recent emphasis on analytical techniques for describing and understanding the process. Understanding of particle consolidation during the HIP process has been attempted through the modelling of densification behavior by considering the deformation of a representative particle due to forces transmitted through the particle contacts. However, the properties of HIPed material have not been thoroughly investigated in terms of their deformation maps and HIP parameters. Mechanical properties of a compact can be quite different depending on the location of various deformation map boundaries. Diffusional creep is involved not only in densification but also in bonding at particle contacts. HIP pressure increases mechanical contact and enhances density but not particle bonding per se. Discrepancies between experimental and calculated data points for shorter HIP times may have been affected by oxide film layer on the original powder.     

热等静压烧结温度对Mo–Na合金性能影响

采用热等静压烧结法制备Mo–Na合金，研究了热等静压烧结温度对Mo–Na合金显微组织、硬度、密度及Na质量分数的影响，分析了Mo–Na合金热等静压烧结的致密化过程。结果表明:采用热等静压烧结法制备的Mo–Na合金显微组织细小均匀，平均晶粒尺寸在10 μm以下。随着热等静压烧结温度的升高，相对密度及硬度随之升高，在1100 ℃时达到最大，分别为99.58%和HRA 54.50，热等静压过程中液相的形成对Mo–Na合金的致密化起到了重要作用。热等静压过程很好地避免了低熔点Na金属高温烧结过程中的挥发，在1100 ℃烧结后Na质量分数基本无变化。     

Shear deformation and compaction of nickel aluminide powders at elevated temperatures

Powder compacts of nickel aluminide were compressed under uniaxial load above 1373 K, using a method that allowed the material to move laterally. Lateral and axial displacements were measured by means of three LVDTs. The resulting data fully described the applied stress state and the strain state as a function of time. That allowed us to obtain simultaneous measurements of the time dependent, and density dependent shear and densification behavior of the powder compact. The shear rate was non-linear in stress suggesting a dislocation flow mechanism. A model for densification by power law creep was applied to the data. It greatly overestimated the measured densification rates. Interestingly it was found that it is difficult to densify the powder to a value more than about 0.80 (relative) using uniaxial compression. In further experiments the powder was hot-pressed in a constraint cavity, allowing large hydrostatic pressures to be applied to the specimen. Near theoretical densities were obtained, presumably because the hydrostatic pressure promoted the diffusional transport mechanism of densification. The hot-pressing data were combined with the sinter forging data to obtain the correlation between densification rate and applied pressure. The diffusional mechanism of densification gave a good quantitative explanation for the densification behavior. In a broader context, we think that powder consolidation techniques ought to be optimized with a view to both shear strain as well as hydrostatic pressure. The shear strain can promote microstructure refinement through dynamic recrystallization, while pressure provides a driving force for diffusional densification.     

Sensing and modeling of the hot isostatic pressing of copper pressing

A detailed experimental evaluation of mathematical models for densification during hot isostatic pressing (HIP) has been conducted using high purity copper powder as a model system. Using a new eddy current sensor, the density of cylindrical compacts has been measured in situ and compared with model predictions for the HIP process. Pressure shielding by the can has been found to influence the densification, and a simple plastic analysis of a thin-walled pressure vessel was used to account for its effects in the models. The existence of a low temperature creep mechanism during consolidation has been found and a formulation to account for its contribution to densification has been developed and implemented in the models. Other effects, believed to be associated with transient creep and the temperature dependence of power law creep parameters, have also been observed in the experiments and suggest the need for further model refinement.     

Densification of iron compacts with various initial porosities under hydrostatic pressure

This study evaluated the effects of hydrostatic pressures up to 1104 MPa on densification of porous iron containing 0.3–11.1% porosity. For the porosities studied, densification as a result of pressurization increased with hydrostatic pressure and initial porosity. The 0.3% porosity iron was the only one whose density did not increase with pressurization up to 1104MPa. Current deformation models of ductile porous materials based on Gurson's yield criterion and rigid-plastic FEM analysis predicted much faster densification with pressurization than observed for porosity contents of 6.2% or less. A reason proposed for this behavior is the omission in the models of an internal pressure which builds up in the pores during the compaction process. A modification is introduced to Gurson's model to take account of internal pressure effects. The modified model exhibited excellent agreement with the experimental observations and provides a constitutive relation for the simulation of compaction and forming processes of P/M parts.     

Densification of titanium powder during hot isostatic pressing

The densification of spherical and angular titanium powder during hot isostatic pressing (HIP) at 700 °C has been studied. The angular powder densifies in a manner similar to the spherical powder despite its low initial packing density. Good agreement is found between a model for the HIP of monosize spheres and the experimental data for both spherical and angular powder provided that the transition between the initial stage of densification (contacts between individual particles) and the final stage (compact is a homogeneous solid with isolated pores) occurs at a density of roughly 95 pct of theoretical. This is consistent with observations of the pore microstructure as a function of density. Formerly with the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931.     

Grain growth enhancement in alumina during hot isostatic pressing

HIP experiments carried out on pure alumina show that grain growth is enhanced both during and after the densification process. These results are compared with those published in the literature on several superplastic ceramics and metals, and a general phenomenological model for grain growth enhancement is proposed. This model is used to simulate grain size gradients generated during the HIP densification of a complex part. TEM examination of the samples shows that point defects and dislocation loops are responsible for grain growth enhancement.     

Effect of hydrostatic pressure on deformation,damage evolution,and fracture of iron with various initial porosities

This study evaluated the effects of superimposed hydrostatic pressure (138 to 1104 MPa) on densification and plastic flow behavior of porous iron (0.3 to 11.1 % porosity). Pressurization alone caused densification of the porous iron with the effect being most pronounced when the porosity was greater than 3.7% and the pressure above 276 MPa. For the porosities studied, densification as a result of pressurization increased with hydrostatic pressure and initial porosity. The 0.3% porosity iron was the only one whose density did not increase with pressurization or deformation under pressure. The effect of hydrostatic pressure on the flow stress of porous iron was small when densification resulting from pressurization was not a factor. The ductility was found to increase linearly with pressure and the effect of pressure on fracture strain increased with the initial porosity of the iron. Evaluation of the effect of hydrostatic pressure on development of porosity and growth during tensile deformation was limited to hydrostatic pressures of 138 and 276 MPa and iron compacts with initial porosities of 0.3, 1.5, and 3.7% because of the pressurization effects. It appeared that the porosity at fracture was similar in these compacts at both pressures but it was much larger than that observed at 0.1 MPa. The greater ductility of the iron compacts tested under hydrostatic pressure results from a decrease in the growth of pores with deformation and from a greater damage tolerance prior to fracture. As observed for porosity, the average maximum pore diameters at fracture for the compacts tested under pressure were similar but larger than those observed at 0.1 MPa. It appears that a general model of ductile fracture for porous materials cannot be based solely on a critical degree of dilation or on maximum pore extension as a fracture criterion.     

On the development of constitutive relations for metallic powders

This article describes the development of elastic and plastic constitutive relations as functions of relative density for partially consolidated —100 mesh aluminum powder. First, measurements of yield stress as a function of stress state and relative density are described. Measurements of the plastic strain increments associated with yielding in unconstrained compression tests and elastic properties, both as functions of relative density, are also described. The experimental results are combined with the associated flow rule to show that the yield surface is asymmetric with respect to hydrostatic tension and compression. Second, it is shown that the yield stress results can be represented by a two-part (capped Drucker-Prager) yield surface. The consoli-dation yield surface moves along the hydrostatic stress axis during densification, while the shear yield surface approaches the Mises yield surface. For the Al powder used in the present inves-tigation, superposition of shear stress on a hydrostatic stress state aids the densification process. However, the hydrostatic stress requirement was found to be reduced by only about 20 pct for relative densities below 0. 98. Formerly with the Department of Materials Science Formerly with the Department of Materials Science     

Densification of rapidly solidified titanium aluminide powders—I. Comparison of experiments to hiping models

Rapidly solidified γ-titanium aluminide powders were characterized and subjected to consolidation by hot isostatic pressing (HIP). Primary solidification of h.c.p.-α phase followed by formation of interdendritic γ-segregate prevails in both REP (Ti-50% Al-1.8% Nb) and RSR (Ti-48% Al-2.4% Nb-0.3% Ta) powders, except for a minor fraction of fine powders that form b.c.c.-β phase from the liquid. The evolution of these microstructures and their subsequent solid state transformations are in line with previous observations on binary titanium aluminide droplets solidified with and without supercooling. In the consolidation experiments, the influences of time, temperature and pressure on final densification were experimentally determined and compared with predicted HIP maps using best estimates of input data on material properties. Experimentally it was found that the densification rate is independent of average mean particle diameter. Best agreement between predictions and experiments were obtained for power-law creep as the dominant consolidation mechanism at high pressure, i.e. 182 MPa. Modifications of the model are suggested for consolidation during the pressurization.     

Mechanisms, models, and simulations of metal-coated fiber consolidation

Recent experimental studies of the hot isostatic consolidation of Ti-6Al-4V-coated SiC fibers contained in cylindrical canisters have revealed an unexpectedly high rate of creep densification. A creep consolidation model has been developed to analyze its origin. The initial stage of consolidation has been modeled using the results of contact analyses for perfectly plastic and power-law creeping cylinders that contain an elastic ceramic core. Final stage densification was modeled using a creep potential for a power-law material containing a dilute concentration of cusp-shaped voids with a shape factor similar to that observed in the experiments. Creep rates were microstructure sensitive and so the evolution of matrix grain size and the temperature dependence of the α/β-phase volume fractions were introduced into the model using micromechanics-based creep constitutive relationships for the matrix. To account for load shielding by the deformation resistant canister, the consolidation model was combined with an analysis of the creep collapse of a fully dense pressure vessel. The predicted densification rates were found to agree well with the experimental observations. The high densification rate observed in experiments was the result of the small initial grain size of the vapor-deposited matrix combined with retention of the cusp shape of the interfiber pores.     

制备工艺对氮化硅泡沫陶瓷组织和性能的影响

采用泡沫塑料浸渍法成形、常压烧结法制备出氮化硅骨架体积分数从15％－70％范围的泡沫陶瓷。无包套热等静压处理（HIP）可以高泡沫陶瓷筋的致密度,其原理为HIP压力促进了烧结残余α氮化硅向β氮化硅的转变,从而提高了氮化硅中原子手扩散活性。     

Improving the Fatigue Strength of Laser Powder Bed-Fused AISI M3:2 by Hot Isostatic Pressing

Laser additive manufacturing enables the one-step fabrication of complex parts. However, pores and carbide networks, which are not avoidable from the laser powder-bed fusion (LPBF) process, deteriorate the fatigue strength significantly. Hot isostatic pressing (HIP) with integrated heat treatment is a powerful post-treatment that densifies the material and modifies the microstructure. Herein, AISI M3:2 samples are produced by LPBF and then are either austenitized, quenched, and tempered in a HIP unit under pressure or are only hardened and tempered in a vacuum oven. The corresponding microstructure is analyzed by optical microscopy, scanning electron microscopy using energy-dispersive X-ray spectroscopy, and X-ray diffraction. The fatigue strength is determined by rotation bending tests. Fracture surfaces are observed under scanning electron microscopy for failure analysis. While both post-treatments lead to similar microstructure, the fatigue strength is significantly improved by the HIP process.     

Consolidation of molybdenum disilicide based materials by hot isostatic pressing (HIP): comparison with models

Monolithic molybdenum disilicide (MoSi₂) powder and MoSi₂ powders blended with ductile and brittle reinforcements were consolidated by hot isostatic pressing (HIP). The extent of densification of the consolidated samples as a function of temperature, pressure and time was determined by precision density measurements. HIP diagrams were constructed based on theoretical models. Material properties, required as input for the HIP map software, were compiled or extrapolated from published literature, experimentally determined and in some cases were estimated from the rule of mixtures. The model predictions were compared with experimental data and dominant mechanisms of densification were identified.     

热等静压(HIP)技术和设备的发展及应用

从热等静压设备的组成部分及安全保护设计方面简要介绍国外该设备的发展,并介绍国外热等静压设备的特点。在国内热等静压设备的发展方面,简要介绍研发和为国内外用户制造热等静压设备并成功投入使用的情况。在热等静压技术应用方面,主要介绍该技术在铸件致密化处理、粉末冶金、热等静压连接和复合以及在重要工程项目中的实际应用。     

Densifying and hardening of martensitic steel powders in HIP units providing high cooling rates

Hot isostatic pressing (HIP) units are worldwide used for the compaction of metal alloy powders. The cooling rate in a HIP unit is usually comparatively low. This lengthens cycle times and requires an additionally heat treatment for quenched and tempered steels. Novel cooling HIP concepts in HIP units feature high quenching rates. In this study, tool steels were investigated with respect to their time–temperature–transformation behaviour for different cooling parameters. The paper shows that encapsuled powdered tool steels can be compacted and hardened in the HIP unit. The examined steels exhibit a comparable or even a higher hardness and a finer microstructure. HIP units with high-quenching rates enable to compact and heat treat materials in one step.     

纯钼烧结规律研究

系统研究了在1 000～1 900℃温度范围采用H2气氛中频感应加热烧结纯钼坯过程中不同温度阶段O含量、微观形貌、孔隙、密度、抗弯强度的变化规律。结果表明:随烧结温度提高钼坯O含量逐渐降低,最终降至30mg/kg左右。钼坯密度随温度升高呈增大趋势,致密化的本质是微观形态烧结变化的结果。致密进程可分为4个阶段,各阶段微观晶粒及孔隙作用机制不一致。抗弯强度与致密进程紧密联系但两者随温度变化趋势有所不同。     

热等静压对铸件致密化及组织演变机理的影响研究

研究了铸件制品的热等静压致密化机理,分析了铸件制品热等静压的致密化模型,综合应用合金的蠕变理论和粉末冶金的烧结理论,利用现有的物理和数学模型对铸件在热等静压过程中的内部孔洞类缺陷闭合过程机理进行了阐述。确认了热等静压过程中起主要致密化作用的孔洞形变机制,为选择制定合适的热等静压工艺参数提供了理论参考。