Structure and Properties of Open-Cell 316L Stainless Steel Foams ...

MetFoam’2007, 5th International Conference on Porous Metals and Metallic Foams, Sept. 5-7, 2007, Montreal, Canada

Structure and Properties of Open-Cell 316L Stainless Steel Foams Produced by a Powder Metallurgy-Based Process Maxime Gauthier Industrial Materials Institute National Research Council Canada 75 de Mortagne Blvd., Boucherville, Canada ABSTRACT This paper presents the structure and properties of stainless steel foams produced using a powder metallurgy approach. Stainless steel powders, a polymeric binder and a chemical foaming agent were dry blended together. The resulting powder was molded and heat-treated to foam, debind and sinter the particles. During the foaming step, the binder melts and flows around the particles. Once the binder is melted, the foaming agent starts to decompose and generates a gas which expands the material. After the foaming step, the material is a polymer foam charged with the stainless steel particles. The material is then treated at higher temperatures to decompose the polymer and finally to consolidate the metallic particles by sintering. The density, porosity and morphology of the stainless steel foams were studied at different stages of the production process. The mechanical properties of foams were also evaluated in compression after sintering. The resulting material has open and closed pores, and could be used in structural applications.

INTRODUCTION Metallic foams are lightweight materials which have attracted an increasing level of attention from the scientific community in recent years. This is due to their many interesting properties like thermal conductivity, high working temperature, low density, high specific surface area, high specific mechanical properties, high impact energy absorption, electrical conductivity and acoustic absorption. Porous (or cellular) metals can be produced using a series of different processes which have been detailed in several reviews [1,2]. Of the different type of porous metals, aluminum foams have been studied intensively. However there is a need for lightweight materials which combine

both high performance and low cost, especially for the automotive industry. Iron-based foams are good candidate materials for such a field of application. As relatively little work has been done on iron-based metal foams to this day, 316L stainless steels foams were produced using the IMINRC patented process [3] and characterized.

EXPERIMENTAL Water-atomized 316L stainless steel powder (Grade PF20R, d50 = 11.67 μm, Atmix Corporation) was mixed with powders of a polymeric binder, a foaming agent and a cross-linking agent. Three different powder formulations were prepared with increasing proportions of metal powder (formulations A, B and C). These were poured in steel cans (along with three 12.7 mm (½”)-diameter steel balls per can) and blended for 30 minutes using a Turbula mixer. The homogeneous formulations were then foamed in cylindrical aluminum molds for two hours at 210°C in air. The resulting stainless-steel-charged polymer “prefoams” were then pyrolysed for 8 h at 450°C in a repurified argon atmosphere (~6x10-9 ppm O2) obtained by passing Ultra-High Purity argon gas (4 ppm O2) though a Centorr Furnaces/Vacuum Industries Model 2A Gas Purifier. Finally, the pyrolysed foams were sintered for 1h at either 1200 or 1350°C under a 100% H2 atmosphere. Closed-porosity contents were determined using an AccuPyc 1330 Helium Gas Pycnometer (Micromeritics). SEM observations (Hitachi S-4700) were used to characterize the morphology of the stainless steel foams. Uniaxial compression testing was performed to evaluate the mechanical properties of the foams. Cylinders 23 mm in diameter and 10 mm in height were used for the compression tests, which were performed on a MTS 100 kN testing machine using a crosshead speed of 1.25 mm/min.


RESULTS

80 Total Open Closed

70

Density (g/cm3)

4

3

Prefoams 1200°C 1350°C

60 Porosity (%)

Figure 1 presents the densities of as-foamed precursors (or Prefoams) and of stainless steel foams sintered at 1200 and 1350°C. It can be seen that the prefoams have densities in the range of about 1.55 to 2.0 g/cm3 and that their density increases with the amount of metal particles in the initial formulation (i.e. from formulation A to C). After removal of the organic material from the prefoams (pyrolysis step) and sintering at either 1200 or 1350°C, the density of the foams increases. This indicates that, even though material was removed from the foams, substantial densification occurred during the sintering step. Once again, the density is proportional to the amount of 316L particles in the powder formulation.

50 40 30 20 10 0 A

B

2b) Figure 2. Porosity Level of Sintered Foams Sintered at: a) 1200°C and b) 1350°C

MORPHOLOGY

2

1

0 A

B

C

Figure 1. Density of Prefoams and Sintered Foams

The typical morphology of the prefoams and stainless steel foams can be observed in Figures 4 and 5. Figure 4a presents the structure of a typical prefoam (Formulation A). The pore shape and size distribution are seen to be heterogeneous. In figure 4b, the homogeneous distribution of stainless steel particles (white) in the organic binder (dark grey) in a broken strut section can readily be observed. It also appears that the concentration of metal particles on the inner surfaces of the pores is similar than in the core of the struts (upper left corner of Fig.4b).

The porosity levels of the sintered foams are shown in Figure 2. These graphs present the total porosity of the sintered materials, but also indicate the amount of closed and open porosity of the 316L foams as determined by gas pycnometry. It can be seen that the total porosity decreases with SS content and with sintering temperature, and also that the proportion of closed porosity over total porosity also increases in the same way. 80 Total Open Closed

70

Porosity (%)

60 50 40

4a) 30 20 10 0 A

C

B

2a)

C


4b) Figure 4. Morphology of Prefoams (Formulation A): a) General View (Secondary Electrons); b) Detail of Strut Structure (Backscattered Electrons).

5c) Figure 5. Morphology of Foams (Formulation A) Sintered at: a) 1200°C (General View,); b) 1200°C (Detailed view); c) 1350°C (Detailed view). After pyrolysis and sintering, the foam’s general morphology is preserved (Figure 5). It can be seen (Fig. 5a) that the foams feature large primary cells pierced with a few holes or “windows”, while the cell walls are thick and contain smaller pores. The improvement of the bonds between the SS particles with higher sintering temperature can easily be observed by comparing Figures 5b and 5c. MECHANICAL PROPERTIES

5a)

5b)

Typical compression curves for the sintered foams are given in Figures 6a and b. The tests were interrupted at ~200-225MPa as this was the upper stress limit of the testing machine. The curves present a steady increase in stress as the strain increases during the three stages (nonzero stress rate at second stage, i.e. no plateau). The mechanical behavior of the foams is clearly related to density, which itself is determined by both formulation and sintering temperature. The compressive properties of the foams are shown in Figure 7. The compressive modulus (Fig. 7a) is seen to be roughly within 1.4 and 2.4 GPa for the present range of SS foam densities. The drawn curve represents the scaling law for Ec (see equation 1) as given by Ashby et al [1]. The correlation is not very good, especially for the higher densities (formulation C, 1350°C). The compressive yield strength (Fig.7b), which ranges from 18 to 72 MPa, correlates much better with the corresponding scaling law (equation 2). The strain at densification (Fig.7c) is seen to decrease with foam density and the corresponding scaling equation adequately fits the data (equation 3). The energy absorbed at densification is within ~20-38 MJ/m3. However, it may have been underestimated as the tests were interrupted before reaching the near vertical densification stage of the compression curves (especially


250 200 σ (MPa)

A

B

C

150 100

60 Densification Strain (%)

for the higher density foams); this probably resulted in lower apparent densification stresses and strains.

50

55

45 40 35 30 0.25

0 0

10

20

30

40

50

60

A-1200°C B-1200°C C-1200°C A-1350°C B-1350°C C-1350°C

50

0.3

0.35

0.4

0.45

0.5

ρ/ρ s

70

7c)

ε (%)

40 3

Energy Absorbed (MJ/m )

6a) 250 200 σ (MPa)

A

B

C

150 100 50 0 0

10

20

30

40

50

60

70

35 A-1200°C B-1200°C C-1200°C A-1350°C B-1350°C C-1350°C

30 25 20 15 0.25

0.3

Compressive Modulus (GPa)

2.6

0.45

0.5

7d) Figure 7. Compressive Properties of 316L SS Foams vs Relative Density: a) Modulus; b) Strength; c) Densification Strain; d) Energy Absorbed at Densification;

2.4 A-1200°C B-1200°C C-1200°C A-1350°C B-1350°C C-1350°C

2.2 2 1.8 1.6 1.4 1.2 0.25

0.3

0.35

0.4

0.45

0.5

7a) 80 70 A-1200°C B-1200°C C-1200°C A-1350°C B-1350°C C-1350°C

60 50 40 30 20 10 0.25

[

Ec = 0.08 × Ec ,s ( ρ / ρ s ) 2

(1)

σ y ,c = 1.1× σ s ( ρ / ρ s )3 / 2

(2)

]

ε D = 0.9 × 0.891 − 1.169(ρ / ρ s ) + 0.417(ρ / ρ s )3 (3) CONCLUSION

ρ/ρ s

Compressive Strength (MPa)

0.4 ρ/ρ s

ε (%)

6b) Figure 6. Compression Curves for 316L SS Foams Sintered at: a) 1200°C and b) 1350°C

0.35

0.3

0.35

0.4 ρ/ρ s

7b)

0.45

0.5

316L stainless steel foams were produced using the IMINRC, powder metallurgy-based foam production process. The heterogeneous foams feature a mix of open and closed pores depending on processing conditions. The mechanical properties were related to foam density and generally followed the scaling laws for metal foams. REFERENCES [1] M.F. Ashby, A. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson, H.N.G. Wadley. Metal Foams: A Design Guide, Butterworth Heinemann, 2000, 251 p. [2] P.S. Liu, K.M. Liang, J. Mater. Sci., 36, 5059 (2001). [3] L. P.Lefebvre, Y. Thomas, Method of Making Open Cell Material, US Patent 6,660,224 B2, Dec. 9, 2003.