production, properties and fracture of ...powder metallurgy progress, vol.2 (2002), no 4 251...

Powder Metallurgy Progress, Vol.2 (2002), No 4 251

PRODUCTION, PROPERTIES AND FRACTURE OF BRONZE BASED CELLULAR MATERIAL

S. Strobl, E. Dudrová

Abstract Cellular materials offer an attractive potential for the production of lightweight components. Here, the manufacturing of sintered bodies from bronze hollow spheres is described. The process starts with fabrication of hollow copper particles by cementation of Cu on iron particles. The still fragile Cu shells are consolidated by coating with Tin and subsequent gravity sintering. The resulting specimens exhibit a closed cell bronze structure with open interparticle porosity. The apparent density may range between 1.6 and 3.4 g.cm-3, and can be controlled by variation of particle size, wall thickness and sintering temperature. Static compression tests were performed on sintered bodies, and others were fractured in bending. The principal mode of compressive deformation occurred by pushing the hollow spheres close together and eliminating the free space, with a slight plastic deformation of the spheres themselves. Only at higher degrees of deformation were the hollow spheres crashed. Particle connections and fracture surfaces of Cu-10wt%Sn bronze samples are characterised by scanning electron microscopy (SEM). Keywords: powder metallurgy, bronze cellular structure, production, density, static compression, fracture

INTRODUCTION Porous lightweight materials have received increasing interest due to the fact that

natural cellular structures such as wood or bone [1] offer very attractive strength-to-weight ratios. In particular, lightweight Al foams, which in principle have been known for decades [2], are manufactured by the consolidation of Al-TiH2 powder mixes with foaming above the melting point [3]. The materials thus generated are characterised by fairly thin walls between the cells, and by density and cell size gradients from the surfaces to the interior.

More regularly structured cellular materials might be accessible by using already hollow powder particles [4, 5, 6, 7] for lightweight structures, with possibly thicker and better-defined walls. The problem here is main the manufacturing and processing of the particles. Thin-walled hollow particles of different metals have been produced [e.g. 5, 7] which are, however, difficult to obtain in small diameters. The thin shell also makes them mechanically weak.

Thick-walled, and thus stronger, hollow particles might be of interest for less extremely lightweight but mechanically stronger components. It has been found that, for moderate density PM structural parts which contain almost exclusively interconnected porosity, the mechanical strength is almost exclusively determined by the geometry of the sintering contacts. With increasing porosity, the cross section of the contacts decreases

Susunne Strobl, Institute for Chemical Technology of Inorganic Materials, Vienna University of Technology, Austria Eva Dudrová, Institute of Materials Research, Slovak Academy of Sciences, Košice, Slovak Republic

Powder Metallurgy Progress, Vol.2 (2002), No 4 252 much faster than the relative density [8], and higher porosity results in a lower strength-to-weight ratio since the particle cores do not contribute to strength, but are only dead weight.

This unsatisfactory behaviour might be remedied by making the powder particles themselves lighter, without changing the geometry of the sintering contacts. Within this work, the manufacturing of hollow spheres for model investigations, the structure, density, compression behaviour and fracture surface of gravity sintered specimens made from such powders, are described.

PRODUCTION OF BRONZE BASED HOLLOW SPHERES AND PARTS As starting materials, spherical Fe powders were produced from iron oxide

obtained in pickling acid recovering plants, following the fluidised bed line. This oxide was reduced in flowing hydrogen for 6 hrs at 800°C, to result in similarly spherical iron powder. After screening, the fraction between 710-500 μm was used. Coating with copper was done by pouring the powder into a CuSO4-solution, the Cu content of which was adjusted so as to result in the desired Cu content of the coated powder (taking into account the amount of Fe dissolved). The suspension was stirred carefully until a complete replacement of Fe by Cu took place. The deposited copper layers are apparently sufficiently porous to enable diffusion of the Fe ions from the Cu-Fe interface into the solution (Fig.1). Since the shell is highly porous (which is necessary to remove the Fe2+ generated) however, and the particles are very weak and difficult to handle, a strengthening treatment was required. This was done by annealing at 800°C in a H2 atmosphere, which causes reduction of oxides and also some sintering within the shell (although the morphology is not significantly changed), resulting in mechanically more reliable particles that could be handled more safely (Fig.2).

Fig.1. Cu hollow sphere after cementation, SEM. Fig.2. annealed - 800°C, 1h, H2 (cross section).

Sintering activation with liquid phase and smoothing of the rough surfaces – which cause too much friction between the particles to give a reasonable packing density of the spheres - were expected to be obtained by introducing tin to the system [9]. Tin bronze of the type Cu-10% Sn is commonly used e.g. for gravity sintering of filters.

The addition of tin as fine powder with the subsequent sintering was not successful since even distribution cannot be attained by admixing. Therefore, here a coating process was selected as well. After heat treatment (Fig.5, the 2nd step), the Cu hollow spheres were tin coated in an aqueous solution (Fig.5, the 3rd step). It showed that, in a single step process, maximum Sn contents of 5 wt% could be introduced (Fig.3), the Sn layer apparently acting as a “barrier” against further deposition. If more Sn is to be added, an intermediate anneal between 600 - 700°C in H2 has to be done to dissolve the deposited Sn;

Powder Metallurgy Progress, Vol.2 (2002), No 4 253 afterwards, a second deposition run is possible by which Sn contents >10% can be attained (Fig.5, the 2nd and 3rd step must be repeated).

Fig.3. Annealed Cu hollow sphere coated with 5% Sn (cross section).

Fig.4a. Cu-10%Sn, 800°C, 3 hrs, H2 (cross

section). Fig.4b. Cu-10%Sn, 800°C, 3 hrs, H2 (sintered

bodies).

Fig.5. Flow chart for manufacturing bronze cellular materials.

CementationCementation

AnnealingAnnealing

Sn-CoatingSn-Coating

GravityGravitySinteringSintering

11stst. step. step

22ndnd. step. step

33ndnd. step. step

44thth . step. step

If the Sn coated powders are tapped into moulds and gravity sintered, cellular structures with more or less sufficiently strong sintering contacts can be attained according to the sintering temperature (Fig.4a,b).

The procedures described above are shown in the process scheme (Fig.5).


DENSITIES OF BRONZE CELLULAR STRUCTURES To optimise the sintering condition for Cu10%Sn-bronze, the hollow spheres were

tapped into moulds and gravity sintered in a pusher furnace for 1 hour in flowing hydrogen. The sintering temperature was varied and the densities of the specimens were determined by buoyancy measurement (Table 1). The results are shown in Fig.6: the density increases with higher sintering temperature, but there is a remarkable increase in density observed above 820°C.

Tab.1. Density of sintered Cu10%Sn hollow spheres, sintered 1hr at varying temperatures in H2 (average values of 4 samples).

ST [°C] 770 790 800 810 820 830 840 850

Density [g/cm³] 1.63 1.68 1.69 1.75 1.90 2.31 2.89 3.37

Density (relative) 0.186 0.192 0.193 0.200 0.217 0.264 0.330 0.385

Porosity % 81.4 80.8 80.7 80.0 78.3 73.6 67.0 61.5

Sintering temperature / Density

1

2

3

4

760 780 800 820 840 860

Sintering temperature [°C]

Sint

erin

g de

nsity

[g/c

m³]

Fig.6. Density of sintered Cu10%Sn-bronze hollow spheres as a function of the sintering

temperature.

STATIC COMPRESSION The basic feature of cellular materials is their high porosity, which minimises the

load-bearing cross section [10]. The energy absorbing ability of cellular metals can be estimated from the macro- and micro-strain response in compression. When a cellular metallic body is compressed, the stress-strain curve exhibits a characteristic behaviour that can be described as follows [11, 12, 13]:

In general, the stress-strain curves can be divided into 3 distinct regions: 1. A quasi-elastic linear increase of the stress-strain curve, which is controlled by cell-

wall bending and cell-face stretching. 2. A deformation “plateau” characterised by a small or vanishing slope of the stress-strain

curve. This region is associated with cell collapse. 3. A region of rapidly increasing stress, where the cell walls touch each other and the

material is densified.


Depending on the properties of the material the samples are made of, there are several modes of cell collapse, like elastic buckling, yielding, or brittle fracture of the cell walls. The detailed shape of the plateau region depends on the particular mode of cell collapse. For nonlinear elastic and plastic materials, the plateau is quite smooth and the stress rises more or less with the strain. In the case of brittle materials, the stress-strain curve in the plateau region is rather rough while the average stress is almost constant, giving a long, flat plateau.

The character of the plateau region depends also on whether the cells are closed or open. In the case of closed cells, the membrane stresses generated in the cell faces result in a stress-strain curve where the stress increases with the strain increase. In cellular materials with open cells, the collapse occurs at almost a constant load, resulting in a long flat plateau. In both cases, the plateau region is longer when the density of the cellular metallic body is lower.

As described before, cylindrical samples of bronze hollow spheres were sintered by varying the temperature, which resulted in different densities (see Table 1, Fig.6). Static compression tests were carried out using a Zwick testing machine at a cross-head speed of 1mm/min (i.e. 0.016 mm/sec).

For each temperature, resp. density, three specimens were compressed. Fig.7 shows the stress-strain curves.

Fig.7a. Compressive stress-strain curves of bronze hollow spheres sintered at different

temperatures.

Fig.7b. Detail of 7a, sintering temperature in the range of 850-810°C.

Fig.7c. Detail of 7a, sintering temperature in the range of 800-770°C.


Two groups of curves can be distinguished: 1. Samples which were sintered between 850-810°C show a typical shape of the stress-

strain curve as described above, with 3 different stress levels. At the quasi-elastic region, the open interparticle porosity is reduced and the distance between the single spheres decreases. The following so-called plateau is quite smooth and the stress rises more or less with the strain, depending on the density of the sintered body. In this state, the cell collapse starts and the spheres are deformed. In the last stage the stress rises rapidly, the spheres are damaged, the specimens densify and are pressed into solid billets.

2. The other group of samples, sintered at 800°C and below, have a different type of stress-strain curve (Fig.7c). At the beginning, stress and strain are increasing and there is no plateau region, in the last stage the stress decreases. The sintering contacts seem to be so weak that the samples disintegrate to single spheres, and are completely destroyed.

From the shape of the stress-strain curves, esp. the plateau region, it can be concluded that the material is ductile and the structure has a closed-cell character. The plateau is longer, when the density of the samples is lower (Fig.7a, b).

MORPHOLOGY OF SINTERING CONTACTS AND FRACTURE SURFACES The following Figures 8, 9, 10 and 11, show the morphology of particle surfaces

and connections in specimens sintered at 770 (comparable to 790), 810 (comparable to 800), 820 and 830°C.

Fig.8a, b. Sintering contacts of bronze hollow spheres (1h, H2, 770°C).



Fig.10a, b. Sintering contacts of bronze hollow spheres (1h, H2, 820°C)

At lower sintering temperatures (Fig.8), the surfaces of the hollow spheres are rough, showing a large number of surface bulbs (“asperities”) and small voids. Between the individual spheres are many regions like large cavities, which also contribute to the high porosity level of the samples. After sintering at 770°C, the contacts between the individual hollow spheres are relatively small, punctual and sporadically located (Fig.8). So, the cohesion is weak, rupture strength is too low, and sintering temperature is undervalued. Fig.8a shows that the sintering contacts of the hollow particles are formed by a set of small bonds that have developed between the surface bulbs. A detailed view on Fig.8b shows such a bond with a dimension of ~10 µm.

When the sintering temperature is raised (Fig.9 and 10), the diffusion activity of the system increases, what results is the formation of further small bonds. The extent of the connections between the particles increases (Fig.9a and 10a). These connections are equally not compact, as they are not after sintering at 770°C. They are formed by a system of small bonds with pores between them (bridge porosity) [14]. The quantity of the surface voids is reduced, the whole structures seem to be more compact and the densities are growing. Higher sintering temperature results in rounding up the surface of the small bonds. As a result of surface diffusion, the particle surfaces relief smoothes out. These processes are connected with reducing the distance of particle centres. It results in the diminishing of cavities between the hollow particles. Higher sintered bronze spheres exhibit more compact structures and are generating mechanically stronger components, which required higher forces to be fractured in bending and are completely deformed.

Sintering at temperatures above 820°C resulted in well developed and sufficiently strong connections and in a decreasing of the distance between the centres of the hollow particles (Fig.11). This resulted in a significant density increase (Fig.6). Due to the high plasticity of the material, the loading in bending caused deformation of the hollow spheres with intense plastic strain on the microvolumes of the connections. The samples are bent without fracture.

It can be shown that, by varying sintering temperature, the morphology and pore structure is changed. The pores between the spheres, as well as on the surfaces, are reduced with increasing temperature, and this is in accordance with the measured densities in Tab.1.

After sintering at different temperatures, the samples were fractured in bending. Cylindrical bodies, which were produced at lower sintering temperatures (between 770 and 800°C), broke easily, but the higher temperature, the higher the force to destroy the specimens. Samples sintered above 820°C couldn´t be broken – they were completely deformed.



Fig 12a, b. Fracture surface of bronze hollow spheres (1h, H2, 770°C).

Fig.13a, b. Fracture surface of bronze hollow spheres (1h, H2, 810°C).

Fig.12a shows the fracture surface of particle connections between the hollow

spheres after sintering at 770°C. The facets which were formed by the failure of the small connections have a ductile, mostly linear character.


Locally, small ductile dimple facets occur as well. The dimples are initiated by pores, or occasionally by small secondary particles (Fig.12b). The facet morphology shows that the connection failure occurred after intensive plastic deformation localised in the bond microvolumes.

Fig.14a, b. Fracture surfaces of bronze hollow spheres (1h, H2, 820°C).

Fig.14c, d. Fracture surfaces of bronze hollow spheres (1h, H2, 820°C).

The Figures13 and 14 show the fracture details of particle connections formed during sintering at 810 and 820°C. The facets have the typical dimple morphology with well developed slip systems (Fig.14c, d). Some of the dimples were initiated by small pores in the connections (e.g. Fig.14b).

CONCLUSIONS • Hollow and near-spherical bronze based powder particles were prepared by

cementation of Cu on spherical Fe powders. The Cu shells (after Fe dissolution) were annealed and coated with Sn. From these hollow particles, bronze specimens with closed cellular structure were produced by gravity sintering at temperatures between 770–850°C.

• The density of Cu-10%Sn cellular materials slightly increases with higher sintering temperatures, but there is a remarkable increase observed above 820°C.

• Compression tests were performed with cylindrical bodies. Those sintered in the range of 850-810°C have typical stress-strain curves with 3 distinct regions.


• The plateau region is smooth with a varying increase of stress depending on the density of the specimens, and the longer it is, the lower the density of the cellular metal body.

• The sintered bronze hollow spheres have a closed-cell structure with an open interparticle porosity.

• Samples sintered at 800°C and below develop only weak sintering contacts, and under compression forces, they disintegrate to single spheres. They show atypical stress-strain curves.

• Sintering at temperatures up to ~810°C results in the formation of particle connections formed by a set of small bonds developed between the small surface bulbs of the hollow spheres.

• Sintering at too high temperature results in an unwelcome densification, at least partially destroying the cellular structure.

• The sintering specimens were fractured in bending. At low sintering temperatures, the fracture surfaces are dominantly formed by line-shaped facets and small dimple facets. Local plastic deformation occurred. Samples sintered above 820°C could not be broken – they were completely bent without fracture.

Acknowledgement This work was financially supported by the Austrian Fonds zur Förderung der

wissenschaftlichen Forschung (Project T52) and the Aktion Österreich – Slowakei / Wissenschaft und Erziehungskooperation (Project 34s17).

The authors would like to thank Mrs. Věra Madáčová (SAS Košice), Mr. A. Haschemi (TU-Wien) for assistance in preparing the SEM photography and Mr. W. Prohaska (TU-Wien) for his support during the compression tests.

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