Densification of oxide superconductors by hot isostatic pressing

Currently, consolidation of high Tc superconductor powders is done by sintering, which is not effective in the reduction of porosity. This work assesses the feasibility of hot isostatic pressing (HIP) to obtain fully dense bulk superconductor using HIP modeling and experimental verification. It is concluded that fully dense YBa₂Cu₃O₇ can be obtained in reasonable times at temperatures down to around 650 °C. The trade-offs between temperature, time, and pressure are examined as well as the effects of powder particle size, powder grain size, and trapped gas pressure. The model has. been verified by experiment under three conditions: 100 MPa HIP at 900 °C for 2 hours, 100 MPa HIP at 750 °C for 2 hours, and sintering at 950 °C for 16 hours. The additional advantages of HIPing oxide superconductors are also discussed.     

冷等静压法制备Mo-30Cu合金的组织与轧制性能

采用冷等静压法(cool isostatic pressing,CIP)制得大尺寸钼骨架,对骨架进行渗铜制备Mo-30Cu合金,并在350℃进行温轧,研究CIP压力及熔渗温度和熔渗时间对合金致密度的影响以及合金的轧制性能。结果表明:采用冷等静压法在120~180 MPa压力下可制备孔隙分布均匀,无分层等缺陷的钼骨架,熔渗后坯料的线收缩率随CIP压力增加而逐渐降低,最佳CIP压力为160 MPa;在一定范围内升高熔渗温度与延长保温时间均有助于提高合金致密度;冷等静压–溶渗法制备的高致密Mo-30Cu合金具有较好的温轧性能,有效提高了大尺寸试样的加工性能。CIP压力为160 MPa压制的骨架在1 350℃渗铜6 h后相对密度达到99%以上,合金的温轧变形量可达到65%。     

Mechanical properties of molybdenum disilicide based materials consolidated by hot isostatic pressing (HIP)

The influence of hot isostatic pressing (HIP) consolidation parameters on the mechanical properties of molybdenum disilicide (MoSi₂) reinforced with ductile and brittle reinforcements was studied. MoSi₂, MoSi₂-20 vol.% coarse and fine niobium powder and MoSi₂-20 vol.% silicon carbide whiskers consolidation by HIP at 1200–1400°C, 207 MPa, for 1 and 4 h were tested in compression for elevated temperature strength and creep resistance. Single-edge-notched specimens of the three materials were tested in a three-point bend configuration for fracture toughness. Mechanical properties were related with consolidation parameters and post-HIP microstructures.     

Analysis of structural parameters for compacted porous materials. I. Compaction by pressing

Convenient expressions are obtained that describe the structural parameters of monodispersed two- and three-dimensional powder systems, compacted by pressing. The procedure is based on a structural model for monodispersed powder materials and an approximation by which with a reduction in porosity Voronoi polyhedra are assumed to be unchanged but the volume of pores decreases as a result of a corresponding increase in particle diameter. This makes it possible to obtain good agreement with experimental results over a wide range of change in porosity. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(407), pp. 106–110, May–June, 1999.     

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.     

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.     

The densification of powders by power-law creep during hot isostatic pressing

Hot isostatic pressing (HIP) is potentially a cost-effective and efficient process for the manufacture of high quality metal components from powders. The densification of the powder during HIP proceeds in stages marked by changes in the geometry of the pores and by the dominance of different densification mechanisms. When the density is high and the pores are isolated and roughly spherical, the densification rate under a compressive load due to creep can be approximated by the densification rate of a sphere of creeping material containing a single, centered void. The densification rates predicted by such a model are significantly increased by small deviations of the load from purely hydrostatic compression. Thus, a careful account of the coupling between the hydrostatic and deviatoric stresses is important in the accurate modelling of the process and the design of an efficient HIP cycle; this can be achieved through the introduction of an approximate strain rate potential for the porous body.     

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.