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Review Article

# Open-Cell Metallic Porous Materials Obtained Through Space Holders—Part II: Structure and Properties. A Review

[+] Author and Article Information
Lenko Stanev

Institute of Metal Science,
Equipment and Technology,
Center of Hydro- and Aerodynamics,
67 Shipchenski Prohod,
Sofia 1574, Bulgaria
e-mail: stanev@ims.bas.bg

Mihail Kolev

Institute of Metal Science, Equipment
and Technology,
Center of Hydro- and Aerodynamics,
67 Shipchenski Prohod,
Sofia 1574, Bulgaria
e-mail: mihail1kolev@gmail.com

Boris Drenchev

Institute of Electrochemistry and Energy Systems,
Academic G. Bonchev Street, Block 10,
Sofia 1113, Bulgaria
e-mail: bdrenchev@abv.bg

Ludmil Drenchev

Institute of Metal Science, Equipment
and Technology,
Center of Hydro- and Aerodynamics,
67 Shipchenski Prohod,
Sofia 1574, Bulgaria
e-mail: ljudmil.d@ims.bas.bg

1Corresponding author.

Manuscript received March 30, 2016; final manuscript received August 5, 2016; published online November 21, 2016. Editor: Y. Lawrence Yao.

J. Manuf. Sci. Eng 139(5), 050802 (Nov 21, 2016) (31 pages) Paper No: MANU-16-1193; doi: 10.1115/1.4034440 History: Received March 30, 2016; Revised August 05, 2016

## Abstract

This work presents an overview of structural characteristics and basic mechanical properties of the open-cell metallic foams obtained by different space-holder methods, which are discussed in Part I of the same review. The presentation is arranged with respect to foam material, and the structure and properties are compared for different space holders and production techniques. In order to have more clear information for the structures obtained and their relation with production techniques, many images are provided and discussed. Compressive behavior of the foams is shown, and stress–strain curves are analyzed with respect to the energy absorption characteristics. The analysis are made on the basis of different porosities and processing parameters. Some applications of the open-cell metallic foams are discussed in the end of the article.

## Figures

Fig. 1

Microstructure of Ti-foam with relative density 0.26 (a), relative density 0.35 (b), higher magnification microstructure (c), after Ref. [1]. (Reprinted with permission from Elsevier.)

Fig. 2

Three-dimensional porous structure of sintered Ti measured by a computer tomography, after Ref. [2]. (Reprinted with permission from Elsevier.)

Fig. 3

XRD pattern of sintered Ti foam, after Ref. [1]. (Reprinted with permission from Elsevier.)

Fig. 4

Compressive stress–strain curves of Ti foams with different relative densities at a strain rate of 0.01, after Ref. [1]. (Reprinted with permission from Elsevier.)

Fig. 5

Variation of plateau stress as a function of relative density, after Ref. [1]. (Reprinted with permission from Elsevier.)

Fig. 6

Strain–stress curves for Ti materials: (a) grade 2, grade 4, and sintered Ti without macroporosity due to space-holder application, (b) experimental and calculated results of compression tests for Ti grade 2, grade 4, and sintered Ti porous material (relative density RD = 0.6) with macroporosity produced by space holder, and (c) enlarged graph, after Ref. [2]. (Reprinted with permission from Elsevier.)

Fig. 7

SEM micrographs of Ti foam after different stages of its production: (a) pore morphology (dark regions) in the matrix after preheating, (b) the acicular porous region having microporosity, and (c) a typical acicular porous region with an original larger pore, after Ref. [4]. (Reprinted with permission from Elsevier.)

Fig. 8

Micropore size distribution in the sintered Ti foam with porosity 46%, after Ref. [4]. (Reprinted with permission from Elsevier.)

Fig. 9

True stress–strain curve for the sintered Ti foam with porosity 46%, after Ref. [4]. (Reprinted with permission from Elsevier.)

Fig. 10

Structural characteristics of porous Ti scaffolds obtained by various initial Mg contents of 50, 60, and 70 vol. %, after Ref. [6]. (Reprinted with permission from Elsevier.)

Fig. 11

Compressive stress–strain (the compression direction coincides with the compacting one) curves of the porous Ti scaffolds produced with initial Mg contents of 50, 60, and 70 vol. %, after Ref. [6]. (Reprinted with permission from Elsevier.)

Fig. 12

Two types of pores in sintered Ti foam made with sugar space holder: (a) micro-CT cross section and (b) SEM micrograph, showing two types of pores, after Ref. [8]. (Reprinted with permission from Elsevier.)

Fig. 13

Stress–strain curves for Ti foams with spherical pores and designed porosity of 50%, 60%, and 70% made from (a) 325 mesh Ti and (b) 100 mesh Ti, after Ref. [7]. (Reprinted with permission from Elsevier.)

Fig. 14

Compressive stress–strain curve of Ti foam with a different relative density (RD). Here, (sp) and (an) denote spherical particles and angular particles, respectively, after Ref. [9]. (Reprinted with permission from Elsevier.)

Fig. 15

The pore structural morphology of porous titanium with 50 wt.% space holder of different sizes: (a) −50/+60mesh, (b) 40/+50mesh, (c) −30/+40mesh, and (d) −20/+30mesh, after Ref. [10]. (Reprinted with permission from Elsevier.)

Fig. 16

Compressive strength–strain curves of the porous titanium obtained with (a) 50 wt.% space holder and sintered at different temperatures for 2 h and (b) space-holder contents in the range from 40 to 70 wt.% and sintered at temperature 1100 °C for 2 h, after Ref. [10]. (Reprinted with permission from Elsevier.)

Fig. 17

Pore morphology of fabricated Ti foams applying starch space holder: (a) 79% porosity and (b) 73% porosity. The macro- and micropores can be observed, after Ref. [11]. (Reprinted with permission from Elsevier.)

Fig. 18

Actual compressive stress–strain curves for three Ti samples (64%, 73%, and 79% porosity) sintered at 1200 °C for 3 h. Three typical regions are well defined, after Ref. [11]. (Reprinted with permission from Elsevier.)

Fig. 19

Compression stress–strain curves of foams with 60% porosity obtained via unidirectional freeze casting (two orientations of load with respect to freezing direction) and via water evaporation, after Ref. [12]. (Reprinted with permission from Elsevier.)

Fig. 20

Structure of ordered porosity Ti foam obtained by reverse freezing casting with camphene space holder: (a) macrostructure of sintered porous Ti and (b) typical SEM image of microstructure of sintered Ti strut, after Ref. [13]. (Reprinted with permission from Elsevier.)

Fig. 21

Compressive behavior of Ti foam obtained by reverse freezing casting with camphene space holder: (a) engineering stress–strain curves of porous titanium tested in compression parallel to the direction of pore alignment for various porosities and (b) compressive strength of the samples as a function of the porosity tested either parallel or normal to the direction of pore alignment, after Ref. [13]. (Reprinted with permission from Elsevier.)

Fig. 22

Mechanical properties of porous Ti–7.5Mo fabricated under various ball milling and sintering conditions: (a) relative density, (b) compressive strengths, and (c) elastic modulus, after Ref. [16]. (Reprinted with permission from Elsevier.)

Fig. 23

Stress–strain curves at room temperature of porous TiNi alloys before and after aging: (a) sample with a porosity of 38% in as-processed and aged conditions and (b) effect of 20 cycles of prestraining on maximum compression stress and fracture strain of a 51% porous sample, after Ref. [20]. (Reprinted with permission from Elsevier.)

Fig. 24

SEM images of Al-based foams with different pore sizes obtained by NaCl spacer: (a) pure Al matrix and 20 μm average pore size, after Ref. [25], (b) pure Al matrix and 400 μm average pore size, after Ref. [25], and (c) AlSi10Mg matrix and 670 μm average pore size and 60% porosity, after Ref. [44]. (Reprinted with permission from Elsevier.)

Fig. 25

TEM images of thin foils from 26 μm pore size foams obtained by leaching in (a) distilled water and (b) the chromate conversion solution, after Ref. [25]. (Reprinted with permission from Elsevier.)

Fig. 26

Compression curves of aluminum foam: (a) 400 μm pore diameter and porosity ε = 82%, (b) 75 μm pore diameter and porosity ε varying between 71% and 75%, and (c) 26 μm pore diameter and relative porosity ε = 78%, after Ref. [25]. (Reprinted with permission from Elsevier.)

Fig. 27

The compressive response of pure aluminum open-cell foams (salt spacer) of various densities demonstrating the strain-hardening character of the foams, after Ref. [24]. (Reprinted with permission from Elsevier.)

Fig. 28

Tensile creep of aluminum open-cell foam: (a) typical tensile creep curve obtained at 250 °C and 0.4 MPa and (b) corresponding curve of strain rate as a function of time, showing the well-defined steady-state regime, after Ref. [27]. (Reprinted with permission from Elsevier.)

Fig. 29

Frequency distribution of pore size for Al-foam (sugar spacer) specimen sintered at 680 °C, having 60% porosity and mean pore size of 0.75 mm, after Ref. [29]. (Reprinted with permission from Elsevier.)

Fig. 30

SEM micrographs of cell walls of Al foams sintered at (a) 600 °C; (b) 680 °C; (c) 750 °C; and (d) Al matrix before sintering, after Ref. [31] and Al matrix before sintering, after Ref. [30]. (Reprinted with permission from Elsevier.)

Fig. 31

Compressive stress–strain curves for sintered Al foams obtained via sugar spacer: (a) various porosities and sintering temperature 680 °C; (b) porosities of about 61%, mean pore sizes of 0.35 mm, 0.75 mm, and 1.3 mm, and sintering temperature 680 °C; and (c) porosity 60%, pore size 750 μm, and different sintering temperatures, after Ref. [29]. (Reprinted with permission from Elsevier.)

Fig. 32

Compressive stress–strain curves of sintered Al foams obtained by carbamide spacer: (a) foams produced by spherical (1) and angular (2) shape carbamide particles, cell size 1–1.5 mm, and 65% porosity; (b) effect of cell size on compressive mechanical properties at 65% porosity: 1, 3.5–5 mm; 2, 2.5–3 mm; 3, 2–2.5 mm; 4, 1.5–2 mm; and 5, 1–1.5 mm; and (c) effect of different relative densities at cell size 3.5–5 mm: 1, pure aluminum; 2, 0.47; 3, 0.41; 4, 0.37; 5, 0.32; 6, 0.27; 7, 0.23; 8, 0.20; and 9, 0.16, after Ref. [34]. (Reprinted with permission from Elsevier.)

Fig. 33

Tensile creep of Al–5% Mg open-cell 400 μm pore size foam: (a) typical curves obtained at 350 °C and 450 °C and relative foam density, Vm, 0.114, 0.134, and 0.138 and (b) corresponding curves of strain rate as a function of strain for relative density of 0.115 tested at 350 °C and at various applied stress values, after Ref. [26]. (Reprinted with permission from Elsevier.)

Fig. 34

Compressive properties of AlSi10Mg alloy and open-cell foams: (a) stress–strain curves of nominally nonporous matrix material, (b) stress–strain curves of foam with 0.61 and 0.71 porosity, (c) energy absorption capacity, and (d) energy absorption efficiency, after Ref. [44]. (Reprinted with permission from Elsevier.)

Fig. 35

Energy absorption characteristics of open calls Al foams: (a) energy absorbed per volume versus relative density of foam and (b) energy absorbed capacity versus strain in different density foams, after Refs. [25] and [34]. (Reprinted with permission from Elsevier.)

Fig. 36

Morphologies of porous Cu specimen with 71.3% porosity obtained at compressive pressure 300 MPa and sintering temperature 940 °C: (a) morphology of sample for compressive stress–strain test, (b) surface morphology of sample, and (c) SEM micrograph of pores in the matrix, after Ref. [48]. (Reprinted with permission from Elsevier.)

Fig. 37

Effect of porosity on the compressive curves (strain rate of 10−2 s−1) of porous Cu specimens fabricated by replication of NaCl spacer with an average pore size 0.75 mm obtained at densification pressure 300 MPa and sintering temperature 940 °C, after Ref. [51]. (Reprinted with permission from Elsevier.)

Fig. 38

Effect of strain rate on the compressive strain–stress curves of porous Cu specimens, after Ref. [51]. (Reprinted with permission from Elsevier.)

Fig. 39

Mechanical properties of open-cell copper foams obtained by friction powder compaction technique: (a) compressive stress–strain curves for foam with porosities 63.3%,72.8%, and 79.6% and (b) relationship between compression stress and energy absorption per unit volume, after Ref.[53]. (Reprinted with permission from Elsevier.)

Fig. 40

Scanning electron microscopy images of pure copper powder: (a) as-received and (b) mechanically activated, after Ref. [54]. (Reprinted with permission from Elsevier.)

Fig. 41

Stress–strain curves of foams produced from as-received (dashed) and mechanically activated (continuous) Cu powder in two different densities (AC7050, 65.7% porosity; C7050, 68.7% porosity; AC8050, 77.2% porosity; and C8050, 78.7% porosity), after Ref. [54]. (Reprinted with permission from Elsevier.)

Fig. 42

Energy absorption properties of the porous Cu specimens fabricated by replication of NaCl spacer as a function of strain εf: (a) effect of porosity on the energy absorption capacity and (b) effect of porosity on the energy absorption efficiency, after Ref. [51]. (Reprinted with permission from Elsevier.)

Fig. 43

Transformation of the sample morphology during preparation process of Fe–10 wt. % Al sample after (a) compacting with resin and NaCl powder, (b) leaching process, and (c) after sintering process, after Ref. [58]. (Reprinted with permission from Elsevier.)

Fig. 44

Morphologies of open-cell Fe–10 wt. % Al foam with 55% NaCl spacer, after Ref. [59]. (Reprinted with permission from Elsevier.)

Fig. 45

Effect of the Al content on the oxidation resistance Ki, after Ref. [58]. (Reprinted with permission from Elsevier.)

Fig. 46

The effect of sintering temperature and time on sintering density, after Ref. [58]. (Reprinted with permission from Elsevier.)

Fig. 47

Characteristics of Mg foam obtained by NaCl-flour spacer: (a) SEM image of Mg foam with porosity 64%, (b) stress–strain curves of two Mg foam samples machined along two perpendicular directions, and (c) effect of porosity on stress–strain curves of Mg foams, after Ref. [60]. (Reprinted with permission from Elsevier.)

Fig. 48

Characteristics of Mg foam with 51% porosity obtained by titanium wire spacer: (a) macroscopic view of as-prepared porous specimen, (b) SEM micrographs of a channel, and (c) stress–strain curves of as-prepared specimens with the same porous structure, after Ref. [25]. (Reprinted with permission from Elsevier.)

Fig. 49

Compressive properties of ZA22 open-cell foams obtained via infiltration of NaCl preform under pressure of 6 MPa. Nominal strain rate of 2.2 × 10−3 s−1: (a) stress–strain curves for different relative densities ρ*/ρS, (b) energy absorption capacity for different relative densities ρ*/ρS, and (c) energy absorption efficiency at strain 55% (E55) and strain 70% (E70), after Ref. [60]. (Reprinted with permission from Elsevier.)

Fig. 50

Micrographs of Ag–28 wt. % Cu open-cell foam produced by melt infiltration of MgSO4 preform: (a) optical image of the metal structure in the foam. The white arrow depicts the presence of primary silver dendrites. (b) SEM image of the foam surface showing the fine lamellar structure of the eutectic for the foam samples rapidly cooled and solidified on a copper chill (the emerging dots are dendrite tips), after Ref. [33]. (Reprinted with permission from Elsevier.)

Fig. 51

Compressive mechanical properties of Ag–28 wt. % Cu open-cell foam: (a) true stress–strain curves (bold line relates to 48% porosity and fine lamellae and thin line relates to 43% porosity and coarse lamellae) as well as pure open-cell copper foam and (b) Young's modulus versus relative density, after Ref. [61]. (Reprinted with permission from Elsevier.)

Fig. 52

Some compressive mechanical properties of open-cell Al–Al2O3 composite foams sintered at 580 °C with various volume fractions of Al2O3 (Vp) and porosity (Vc): (a) stress–strain curves, (b) effect of the volume fraction of Al2O3 on the Young's modulus of the foams with different porosities, (c) effect of the volume fraction of Al2O3 on the plateau stress, and (d) effect of the volume fraction of Al2O3 on the energy absorption capacity W, after Ref. [35]. (Reprinted with permission from Elsevier.)

Fig. 53

Microstructure of metallic walls of pores after final sintering for FeAl porous graded foam with discrete gradient, after Ref. [37]. (Reprinted with permission from Elsevier.)

Fig. 54

Compressive stress/strain curves of FeAl sample with (a) discrete gradient of pore size and (b) quasi-continuous gradient in pore size, after Ref. [37]. (Reprinted with permission from Elsevier.)

Fig. 55

SEM images of NiAl porous intermetallic obtained through self-propagating high-temperature synthesis: (a) intermetallic with porosity of 67.39% and average pore size of 0.9–2.0 mm and (b) micropores on cell walls, after Ref. [43]. (Reprinted with permission from Elsevier.)

Fig. 56

Nominal stress–strain curves of porous NiAl intermetallics (pore size in the range of 0.6–0.9 mm) with different relative densities, after Ref. [43]. (Reprinted with permission from Elsevier.)

Fig. 57

Cross sections of the samples made with 40 vol. % and 60 vol. % NaCl space holder: macroscopic cross section (top row), pores formed due to space holder (middle row), and pores formed after Ti–Al reaction (lower row), after Ref. [62]. (Reprinted with permission from Elsevier.)

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