Densification behavior of aluminum alloy powder under cold compaction

Densification behavior of aluminum alloy (Al6061) powder was investigated under cold compaction. Experimental data were obtained under triaxial compression with various loading conditions. A special form of the Cap model was proposed from experimental data of Al6061 powder under triaxial compression. The proposed yield function and several other yield functions in the literature were implemented into a finite element program (ABAQUS) to compare with experimental data for densification behavior of Al6061 powder under cold isostatic pressing and die compaction. The agreement between finite element calculations from the proposed yield function and experimental data is very good under cold isostatic pressing and die compaction.     

A densification model for mixed metal powder under cold compaction

Densification behavior of mixed copper and tool steel powder under cold compaction was investigated. By mixing the yield functions proposed by Fleck et al. and by Gurson for pure powder in terms of volume fractions of Cu powder and the fraction of contact, a new mixed yield function was employed for densification of powder composites under cold compaction. The constitutive equations were implemented into a finite element program (ABAQUS) to compare with experimental data and with results from the model of Kim et al. for densification of mixed powder under cold isostatic pressing and die compaction. Finite element calculations by using the yield functions mixed by the fraction of contact agreed better than those by volume fractions of Cu powder with experimental data.     

Effect of friction between powder and a mandrel on densification of iron powder during cold isostatic pressing

The effects of friction between the powder and the mandrel on densification behavior of metal powder were investigated under cold isostatic pressing. The friction coefficients between the powder and the mandrels with different surface roughness were determined from the relationship between the compaction pressure and the ejection pressure of the mandrel from powder compacts. The elastoplastic constitutive equations based on the yield function of Shima and Oyane were implemented into a finite element program (ABAQUS) to simulate compaction responses of metal powders during cold isostatic pressing. Finite element results were compared with experimental data for pure iron powder under cold isostatic pressing.     

Synthesis of silicon carbide ceramics from a polysilastyrene at low temperatures

The conventional route for preparation of silicon carbide ceramics is by the use of pressureless sintering, hot pressing, or hot isostatic pressing of silicon carbide starting powders. High sintering temperatures (2073–2473 K) and the addition of sintering additives are normally used to enhance densification. These sintering additives, however, form second phases at grain boundaries which impair the mechanical properties of the material, particularly at high temperatures. It is therefore desirable that new processing routes are developed that overcome these difficulties. A proposed route is to use a polymeric pressure which can provide a Silicon carbide matrix as binding agent for silicon carbide powders, thus making the requirement for high temperatures and sintering additives unnecessary. This paper reports observations of the direct transformation of a polymeric precursor into amorphous Si–C, and crystalline SiC at low temperatures, and the use of this precursor as a binder for the production of SiC powder/ex-precursor SiC composites.     

Densification of mixed metal powder at high temperature

Densification behaviors of mixed metal powder under high temperature were investigated. Experimental data of mixed copper and tool steel powder with various volume fractions of Cu powder were obtained under hot isostatic pressing and hot pressing. By mixing the creep potentials of McMeeking and co-workers and of Abouaf and co-workers originally for pure powder, the mixed creep potentials with various volume fractions of Cu powder were employed in the constitutive models. The constitutive equations were implemented into a finite element program (ABAQUS) to compare with experimental data for densification of mixed powder under hot isostatic pressing and hot pressing. Finite element calculations by using the creep potentials of Abouaf and co-workers agreed reasonably well with experimental data, however, those by the model of McMeeking and co-workers underestimate experimental data as observed in the case of pure metal powders.     

Densification behavior of tungsten-fiber-reinforced copper powder compacts under hot isostatic pressing

Densification behavior of tungsten-fiber-reinforced copper powder compacts under hot isostatic pressing was investigated. Hot isostatic pressing was carried out for a bundle of copper-coated tungsten fibers in copper powder. Due to tungsten-fibers and copper coating layers, the densification rate of a tungsten-fiber-reinforced copper powder compact was slower than that of pure copper powder. The constitutive equations by McMeeking and co-workers and by Abouaf and co-workers were implemented into a finite element program (ABAQUS) to analyze densification behavior of tungsten-fiber-reinforced copper powder compacts under hot isostatic pressing. Finite element calculations were compared with experimental results for the variation of relative density with time for copper powder compacts during hot isostatic pressing. Density distributions in copper powder compacts were also investigated by comparing experimental results with finite element calculations.     

Densification of alumina components via indirect selective laser sintering combined with isostatic pressing

To improve the final density of ceramic parts via indirect selective laser sintering (SLS), cold/hot isostatic pressing (CIP/HIP) technologies were introduced into the process. The proposed approach in the present study combined spray drying with mechanical mixing by which we prepared a kind of compound powder consisting of polyvinyl alcohol (PVA, 1.5 wt%), epoxy resin E06 (8 wt%), and alumina so as to get a good fluidity for SLS. At the first step, SLS parts reached the highest relative density of about 32 % when the laser energy density was 0.094 J/mm², which facilitated the next operation and improvement of final density. Then, a soft polymer canning was prepared for CIP around the surface of SLS alumina ceramic parts using pre-vulcanized natural rubber latex RTV-2, gelation and film. Following that, we experimented on different CIP maximum pressure which had different effects on densification of SLS alumina parts, the whole process in CIP was divided into three stages of I, II, and III. Based on thermal gravity curve of epoxy resin E06, ignoring the impacts of the only 1.5 wt% PVA on degreasing, green bodies were degreased and furnace-sintered. Finally, the relative density of alumina parts reached 95.94 % after HIP process. Field emission scanning electron microscopy was used to analyze the densification evolvement in each stage of process and the fracture mechanism. The study showed a positive and practical approach to manufacture ceramic matrix and ceramic components with complex shape by indirect SLS technology.     

A study of pressureless microwave sintering,microwave-assisted hot press sintering and conventional hot pressing on properties of aluminium/alumina nanocomposite

Bulk Al/4wt-%Al₂O₃ nanocomposites were prepared by consolidating nanocomposite powders using pressureless microwave sintering, microwave-assisted hot press sintering and conventional hot pressing techniques. Microstructural observations revealed that the microwave- assisted hot press sintering at different sintering temperatures of 400°C and 500°C resulted in more densification and smaller grain size for Al/Al₂O₃ nanocomposite as compared with the conventional hot pressing. Moreover, the application of pressure in microwave sintering process led to more densification and grain growth. Mechanical properties resulting from microhardness and nanoindentation tests were also compared between three-method processed samples. It was found that the microwave-assisted hot-pressed sample exhibited higher hardness and elastic modulus in comparison with microwave-sintered and conventional hot-pressed samples. The improvement in the mechanical properties can be ascribed to lower porosity of microwave-assisted hot-pressed sample.     

Near-net-shape forming of 316L stainless steel powder under hot isostatic pressing

Near-net-shape forming of 316L stainless steel powder is investigated under hot isostatic pressing (HIPing). A stainless steel powder compact and an insert were encapsulated by a stainless steel container and hot isostatically pressed to produce an axisymmetric near-net-shape part. To simulate densification and deformation of a powder compact in the container during HIPing, the constitutive model of Abouaf et al., and that of McMeeking and co-workers were implemented into a finite element analysis. The thickness effect of the container on densification was also studied for the axisymmetric part during HIPing. Densification of a three-dimensional asymmetric part during HIPing was also investigated by comparing finite element calculations with experimental data by Eisen et al.     

Calculation of a constitutive potential for isostatic powder compaction

A macroscopic constitutive potential has been developed for the deformation of a powder compact of cylindrical particles during pressure sintering. The derivation is based on finite-element simulations of the densification process that proceeds under the synergistic action of power-law creep deformation in the particles, evolution of the nonlinearly developing contact area between the particles, and interparticle and pore free-surface diffusional mass transport. Solution to this initial/boundary-value problem as deformation proceeds with time provides all necessary information for the calculation of the constitutive potential. The associated constitutive law predicts the densification rate of the powder compact at a given temperature and pressure in terms of material parameters, such as creep constants and diffusion coefficients, and reflects the role played in the densification process by various micromechanical features at the microscale such as the pore surface curvature. The model predictions are compared with the existing analytical models for plane strain densification and experimental data from sintering of copper wires by grain boundary and curvature-driven pore surface diffusion.     

Near-net-shape forming of alumina powder under hot pressing and hot isostatic pressing

Densification and deformation of alumina powder under hot pressing and hot isostatic pressing were investigated. Finite element calculations were performed by implementing constitutive equations for grain growth, power law creep and diffusional creep in the user defined subroutine CREEP of ABAQUS. An alumina compact of valve head shape was produced under hot pressing and its forming process was predicted by the finite element calculation. Densification behavior of an alumina powder compact encapsulated by a stainless steel container was also investigated under hot isostatic pressing. Inhomogeneous deformations of an alumina powder compact due to the shield effect of a container during hot isostatic pressing were observed experimentally and predicted by the finite element analysis.     

Genetic algorithm-based numerical optimization of powder compaction process with temperature-dependent cap plasticity model

In this paper, a shape optimization technique is presented for the cold and hot isostatic pressing of metal powders based on the genetic algorithm (GA) approach. The GA technique is used to obtain the desired optimal compacted component by changing the boundaries of component and verifying the prescribed constraints. The coupled thermomechanical analysis of hot isostatic pressing is employed for metal powders during densification process. The numerical modeling of hot powder compaction simulation is performed based on the large deformation formulation, temperature-dependent cap plasticity model, and frictional contact algorithm. The modified cap plasticity takes the temperature effects into the numerical simulation of highly nonlinear behavior of metal powder. Finally, numerical examples are analyzed to demonstrate the feasibility of proposed optimization algorithm for designing powder components in the cold- and hot-forming processes of powder compaction.     

Simulation of microscopic shrinkage behaviour in sintering of powder compact

A method for simulating microscopic shrinkage behaviour of powder particles in sintering of a compact is proposed on the basis of the granular element method. In this method, the powder particles are modelled as many circular elements undergoing viscoplastic deformation due to surface tension during the sintering, and the microscopic shrinkage is calculated by equilibrating forces acting on the elements. The variation in shape of necks between the elements during the sintering is taken into consideration. Plane-strain shrinkage in sintering is calculated under regular and irregular dispositions of powder particles. In the regular disposition of powders having the same diameter, the obtained shrinkage behaviour is compared with the experimental one using glass rods and the calculated one by the viscoplastic finite-element simulation respectively. It is shown from the simulation of irregular disposition that the densification due to the sintering is accelerated by mixing powders having different diameters.     

Abrasive wear in ceramic laminated composites

Laminated ceramic structures in the system Al₂O₃/Al₂O₃ + 3Y-TZP (A/AZ) were prepared using a tape casting technique in order to obtain ceramic layers with different compositions and thicknesses. Piezo-spectroscopy was used to evaluate the residual stresses arisen from a calibrated mismatch in thermal expansion coefficients of the layers during the sintering process of the composite. The dependence of the residual stresses in the A and AZ layers on their thickness ratio was established. A microscale ball cratering method was used to investigate the influence that the surface compressive stress can play on the abrasive wear resistance of the composite structures. The results were compared with those obtained with an unstressed reference material prepared either by lamination of pure alumina green-sheets or by cold isostatic pressing of alumina powder. The experimental results have shown that the abrasive wear resistance is higher for samples with compressive residual stresses within the surface regions.     

Tribological properties of pressureless sintered advanced alumina matrix ceramic materials improved by Al–Ti–B and diopside

Al–Ti–B master alloys and diopside were incorporated into alumina matrix and advanced alumina matrix ceramic materials were fabricated by pressureless sintering technology. The mechanical properties of this new composite as well as its wear behaviours, coupled with carbon steel ring in unlubricated conditions at room temperature, were investigated systemically. SEM technology was adopted to observe the worn surfaces of specimens and wear mechanisms were simultaneously discussed. Analysis of the experimental data and observations on the worn surfaces revealed that the improvement in the wear resistance of the composites might be attributed mainly to the strong toughening effect due to the introduction of Al–Ti–B master alloys and diopside in the alumina matrix.     

Processing and properties of Al2O3/SiC nanocomposites

Alumina/SiC nanocomposites were produced by mechanical mixture of commercial powders. The preparation steps involved the vigorous mixing of the powders and drying under conditions where the homogeneous mixture was kept stable. Pressureless sintering of die-pressed powders achieved reasonable densities (～97% theoretical density) for 2·5wt% of SiC on sintering at 2073 K. Higher SiC contents strongly reduced the sintered density. The use of a more reactive alumina (finer matrix powder) gave similar results. Hot pressing at 1973 K/1 h/25 MPa produced high-density materials for SiC contents as high as 20 wt%. Transmission and scanning electron microscopy analysis showed that the SiC particles were well distributed and were situated both inside the grains and on the grain boundaries of the alumina matrix. The SiC strongly inhibited grain growth in the matrix in keeping with the Zener model. The bend strength increased as the SiC content increased, a result partly explained by the grain size refinement. The strength improvement of 20% over monolithic was explained in terms of the change to an intergranular fracture mode.     

Creep densification of copper powder compact

Densification of metal powder under high temperature processing was investigated. Experimental data were obtained for copper powder under hot isostatic pressing, hot pressing and uniaxial creep compression. Theoretical calculations from the constitutive models by McMeeking and co-workers were compared with the experimental data. The agreements between experimental data and theoretical calculations are reasonably good when hydrostatic stress is dominant, but not as good when deviatoric stress increases.     

Influence of carbon content on workability behavior in the formation of sintered plain carbon steel preforms

Complete experimental investigation on the workability behavior of sintered plain carbon steel cylindrical preforms with carbon contents of 0%, 0.35%, 0.75%, and 1.1%, under cold upsetting, has been studied in order to understand the influence of carbon content on the workability process. The abovementioned powder metallurgy sintered preforms with constant initial theoretical density of 84% and aspect ratio of 0.4 were prepared using a suitable die set assembly on a 1 MN capacity hydraulic press and sintered for 90 min at 1,200°C. Each sintered preform was cold upset under nil/no frictional constraint. Under triaxial stress state condition, densification, axial stress, hoop stress, hydrostatic stress, effective stress, and formability stress index against axial strain relationship were established and presented in this work. Further, attained density is considered to establish formability stress index and various stress ratio parameters’ behavior.     

Mathematical modeling of sintering during powder forming processes

This paper describes a study of densification induced by local capillary forces during compaction of powder based materials. A coupled sinter-compaction model with an internal state parameter was proposed. An internal state parameter was assumed as the sintering stresses on contact areas between powder particles. The mechanical model describing the plastic deformation during the P/M forging of a preform is based on the plasticity theory of porous metals. The numerical investigation of P/M forming processes is based on the rigid-plastic finite element model. A finite element program taking into account the sintering effect during P/M forming is created. A numerical example is considered.     

Densification behavior of copper powder during the coupled multi-physics fields-activated microforming

External electric field-activated sintering techniques have been widely investigated and applied for the forming of large-sized components. These techniques are, however, rarely utilized for the manufacture of miniature and microsized components. In this paper, a novel, coupled forming, and sintering method is reported, which has been used for the fabrication of microcomponents, wherein the loose powder is loaded directly into the die, followed by simultaneous electrical forming and electric sintering (named coupled multi-physics-fields activation). In the study, the gears with the module of 0.2 and the pitch diameter of 1.6 mm were formed from copper powder. The coupled multi-fields activations were enabled using a Gleeble-1500D thermal simulation machine. Sintered samples with a relative density of 97.20 % have been fabricated at a sintering temperature of 700 °C, heating rate of 50 °C/s, forming pressure of 100 MPa, while these parameters were applied cyclically. The study showed that the axial reduction of the samples increased rapidly with the increase of temperature during the sintering, while the external pressure was maintained. Based on the experimental observations, it can be concluded that the deformation of the particles resulted in an increase in, and then subsequent disappearance of, the interface areas among the particles, which feature plays a key role in the densification of the copper powder.