Metal foams for structural applications: design and manufacturing
MASSIMILIANO BARLETTA§*, STEFANO GUARINO§ and ROBERTO MONTANARI§, VINCENZO
TAGLIAFERRI§.
§ University of Rome “Tor Vergata” Department of Mechanical Engineering, Via del Politecnico, 1 – 00133
Rome (I)
Correspondence
*Corresponding author. Email: barletta@mail.mec.uniroma2.it
Abstract
In this paper, the design and manufacture of metal foams, by using the powders compact melting method
(PCMM) , is investigated. Experimental tests were performed to study the influence of several process
parameters, that is, compaction pressure, foaming time, temperature and amount of foamable precursor
material, on the kinetics of foaming process. Because of the large number of experimental factors involved in
metal foams manufacturing it is very difficult to obtain products with repeatable characteristics. For this
reason an experimental approach based upon DOE techniques was employed to reduce the trials need for
individuating the best process windows. Hence, in such operative ranges, further experimental tests were
carried out to trace the full trends of foaming efficiency according to leading parameters, thereby laying the
basis to support manufacturers on how to deal with the operative troubles and process settings.
Keywords: Aluminum, Foaming, Process Parameters
1 Introduction
Aluminium foams are a recent class of materials that for their unique characteristics have been stimulating
great interest in several technological domains. They combine part of characteristics of a bulk metal with the
structural advantages of foams. In the field of metal foams, aluminium foams have been creating particular
interest due to their light weight structure and good physical, chemical and mechanical properties as, which
make them suitable for a range of industrial applications (Hong-Wei Song et al. 2005, K. Boomsma et al.,
2003, Banhart J. 2001, J. Baumeister et al. 1997). Specifically, an aluminium foam possesses high specific
stiffness, it has good energy-absorbing properties (A. Czekanski et al. 2005, C. Motz and R. Pippan 2001)
making it good for crash-protection and packaging and it has attractive heat-transfer properties (C.Y. Zhao et
al. 2004, K. Boomsma et al. 2003), hence allowing its employment to cool electronic equipment and as heat
exchangers in engines (open cells structures).
In the light of growing industrial interest towards metal foaming as manufacturing technique, different
foaming processes have been developed and reported in literature (Maxime Gauthier et al. 2004, Banhart
2001, Duarte and Banhart 2000, Ma L. et al. 1999, Kathuria 2003). Each method can be used just with
selected metals, if a porous material with monitored density and cell size is desired. Unfortunately, as few
efforts were already done to study the correlations between process parameters and the characteristics of the
end-product, a full knowledge of foaming process and its implications in manufacturing still misses, thereby
limiting, at large extent, the employment of such materials in large classes of industrial application
(Baumeister et al. 1997., Yang and Nakae 2000). Besides, several drawbacks like the low foaming process
repeatability, its unsuitability in manufacturing of complex shapes and high associated running and plant costs
constitute an additional limit to the ultimate development of such technology.
The PCMM process consists of mixing and, then, compacting, metal powder and a powdered blowing agent in
a closed crucible to achieve a dense semi-finished product, namely, foamable precursor material, by hot
pressing, extrusion, powder rolling or other methods. Once achieved, the foamable precursor material is
subsequently expanded by heating it up its melting point (Banhart 2001). The heating process takes the metal
into a semi-liquid viscous state and, concurrently, makes the blowing agent decompose, thus releasing gas and
creating a highly porous structure. Such foaming process was found to be particularly successful upon low
density and not susceptible to oxidation alloys like the aluminium. Besides, being the foaming process
developed in a well-confined mould, the end-product is better finished, hence limiting the need for further
machining or reprocessing.
Under the new push exerted by the alternative foaming technology, several analysis have been developed to
thoroughly study the physical, thermo-mechanical and chemical properties of foamed structures as well as the
leading phenomena of foaming kinetics for potential applications (Banhart 2001, Baumeister et al. 1997,
Yang and Nakae 2000, Belkessam and Fritsching 2003). However, a systematic approach to the influence of
leading process parameters on foaming efficiency, the analyses to reduce the number of trials to start up the
manufacturing of new foamed components and the codification of experimental data in process map useful
both for process control and automation still miss. In this context, our work is focused to investigate the
closed cell metal foam process from metal powder. Experimental tests, based upon design of experiment,
DOE, were scheduled and performed with the aim of understanding the influence of the process parameters
(pressure, foaming temperature, treatment time and amount of powdered blowing agent) on the foaming
efficiency. In particular, a Taguchi’s reduced experimental plan was first developed to identify the best
operative windows. Next, based upon the first experimental findings, further experimental tests were
performed and, consequently, 3D process maps of foaming efficiency according to operative parameters were
traced. Accordingly, from the development of the systematic experimental analysis, new elements in setting
operative parameters and in choosing best process condition arose, thereby laying the foundation for the
definition of a first instrument to support producers on how to deal with the design and manufacturing of
metal foamed components.
2 Experimental apparatus and procedure
2.1 Experimental apparatus
Three different types of powder were employed to prepare the foamable precursors:
 Powder of aluminium AlSi7 (i.e., 7% wt as Si) as basic metal,
 Titanium hydride TiH2 as foaming agent,
The first phase of the foaming process was the preparation of foamable precursors. For this purpose, all the
powders were first accurately weighted by using a digital scale Sartorius Model BP 211-D with a resolution of
0.01 mg and then inserted in proper amounts into an electrical tumbler. In such way, they were thoroughly
mixed for one hour so as to reach a uniform distribution of the basic constituents in the mixture. After that, the
compaction of the mixture to a dense semi-finished product (foamable precursor material) was operated by
cold pressing the powders into a stainless steel cylindrical mould, 10 mm as diameter, by using a
counteracting stainless steel piston. As press, a static test machine, MTS Alliance RT/50, with a maximum
applicable load of 50 kN was employed. In such way, the evolution of the load according to the piston
displacement into the mould during the compaction process could be recorded. The precursor obtained during
the pressing process was measured by using a Mitutoyo digital palmer (± 1 mm, as accuracy), inserted into a
stainless steel crucible, 200 mm in length and 10 mm as diameter, and then baked in an electrical oven (1200
°C as maximum temperature) under different temperature-time process conditions. During the baking, the
titanium hydride was supposed to rapidly decompose at about 450°C (Guarino and Tagliaferri 2004), that is,
below the aluminium alloy melting point (~ 600°C). Next, the further increase in temperature was expected to
cause the expansion of the gas and the contemporary reduction of the metal resistance to the growth of the gas
bubbles by plastic deformation and semi-solid slide. At last, when the baking process elapsed, the foamed
precursor was extracted from the crucible and, once more, measured by using the digital palmer so as to asses
the foaming process efficiency. However, the whole experimental apparatus and a sketch of all the phases
involved during the foaming process are reported in Figure 1.
2.2 Experimental procedure
The first part of experimental tests was focused in determining the best amount of titanium hydride for
foaming process. The behaviour of different amounts of titanium hydride in the range of 0.2% to 0.8 % were
examined in terms of foaming efficiency , which is defined as ratio of the end volume of the foamed sample
to the starting volume of its precursor. A standard test condition of 400 MPa as compaction pressure, 800 °C
as foaming temperature and 8 min as foaming time was employed. Then, based upon first cues coming from
the previous tests, a first reduced Taguchi’s design was employed to study the influence on foaming efficiency
of compaction pressure, foaming time and temperature. In such case, powders mixture with a fixed weight
composition (0.5 % wt as TiH2), found to be the better for foaming process, was used. However, Table 1
summarizes the experimental plan developed. In Taguchi’s reduced experimental plan three process
parameters were considered: compaction pressure p varying in the range of 300 to 500 MPa, foaming
temperature T varying in the range of 700 to 900 °C, and the foaming time t varying in the range of 5 to 15
minutes. All the tests reported in Table 1 were repeated four times to assure process repeatability and
reproducibility. An ANOM was developed to evaluate the influence of operative parameters on foaming
efficiency in the range investigated. Experimental data were reported in terms of average with variability
expressed in terms of standard deviations.
Based upon experimental findings deducted from the development of Taguchi’s experimental campaign, the
best process conditions for each experimental factor were inferred. Accordingly, a full factorial campaign
aimed at tracing the 3D experimental maps of foaming efficiency according to operative parameters was
developed. For this purpose, firstly, the compaction pressure was fixed at 400 MPa, being found to be the best
choice for foaming efficiency, and foaming time and temperature varied according to the experimental levels
reported in Table 1 for a total amount of 25 tests, four times repeated. Lastly, foaming time and foaming
temperature were, respectively, kept fixed at their best operative values, 8 min and 800 °C, concurrently
varying the other two experimental factors according to operative levels reported in Table 1. Accordingly, a
total amount of 50 tests, 25 for foaming temperature and 25 for foaming time, four times repeated were
carried out. All the foaming efficiencies were collected as average of the repeated tests, with standard
deviations accounting for process variability.
3 Results and discussion
3.1 The influence of titanium hydride on foaming efficiency
Figure 1 reports the trend of foaming efficiency according to percentage of titanium hydride. As can be seen,
the amount of titanium hydride percentage was found to generate the best foaming efficiency was worth about
0.5%. Besides, a significant growth in foaming efficiency could be expected if, starting from titanium hydride
percentage values as low as 0.2%, progressively higher amounts of foaming agent were used. Nevertheless, if
higher titanium hydride amounts than 0.5 % were employed, a subsequent worsening of foaming efficiency
occurred. This peculiar behaviour can be attributed to the size distribution of rising bubble inside the foam
structure. When low amount (< 0,4-0,45 %) of titanium hydride was employed, a foamed structure
characterized by just a few of small bubbles arose. This was due to the lack of foaming agent in the foaming
precursor. Going towards higher amounts (0.45 – 0,45 %), an uniform distribution of a higher number of
small bubbles all over the foam structure was observed (Figure 3) and was found to cause the better foaming
efficiency. In fact, if further amounts of foaming agent were added to the mixture, the small bubbles coalesced
in a lower number of big bubbles (Figure 3), thereby decreasing the foaming efficiency. This decreasing in
foaming efficiency was inferred to be caused both from the peculiar bubble distribution and to the collapse
phenomena, which could affect such kind of foamed structures, characterized by large bubble with a weak
structure due to the very low ratio between bubble exterior surface and bubble volume.
3.2 The analysis of process parameters: Taguchi’s reduced experimental design
Experimental tests planned according to Taguchi’s reduced experimental findings allowed studying the
influence of the process parameters on the evolution of the foam component expressed in terms of foaming
efficiency according to foaming temperature, foaming time and compaction pressure. The results of ANOM
are reported in Figure 4. As can be seen, three different trends characterized the experimental factors in the
range investigated. As regard foaming time, it can be noted that value of 8 min could allow the best foaming
efficiency. This trend can be attributed to the peculiar behaviour of foamed structure during the foaming
process. During the first minutes of the foaming process (< 5 min), the titanium hydride start decomposing in
according with indications provided in literature (Duarte and Banhart 2000). However, to properly activated
the foaming process, temperature as large as 600 °C, close to the aluminium alloy melting point, must be
waited for. This phenomenon was found to happen in the range of 5 to 8 min, where the largest values of
foaming efficiency could be found. In fact, at such a temperature, the endurance of the foaming mixture to the
growth of the bubble was very low, being in a sort of semi-liquid state and, consequently, the material was
submitted to large plastic deformation, to the development of the gas bubbles and to the a concurrent
remarkable increase of the foaming efficiency. Nonetheless, if the foaming mixture was submitted to too
longer foaming time, such bubbles kept on growing until a collapse of the entire or part of foamed structure
occurred, thereby decreasing the end-volume of the foamed structure and worsening the foaming efficiency.
This phenomenon was found to characterize the foaming process, when foaming time higher than 8 min were
employed.
Examining the second diagram in Figure 4, compaction pressure was found out to guarantee the best foaming
efficiency starting from 400 MPa. This value was in accordance with several experimental findings reported
in literature. However, values of compaction pressure of, at least, 400 MPa, was found to cause the minimum
compaction of foaming mixture need for assuring enough fast heat transfer phenomena during the foaming
process so as to allow the foaming process to properly occur. If lower value of compaction pressure (less than
400 MPa) were employed, the compaction process was too scant to produce an enough continuous structure to
permit to the heat to diffuse rapidly during the heating phase of the foaming process, hence inhibiting the
production of the foamed structure. To the contrary, even if compaction pressure higher than 400 MPa were
used, no further improvement to the foaming process can be conferred. In fact, for compaction pressure higher
than 400 MPa, just slight differences in relative density of foaming mixture occurred (significantly less than
1%) and, so, no significant improvement to heat transfer phenomena and, subsequently, to the foaming
process can be expected. At last, if the foaming efficiency according to foaming temperature is considered,
similar considerations to those performed for foaming time can be repeated. Going towards higher values of
foaming temperature meant to significantly increase the kinetics of foaming process. Therefore, if low values
of foaming temperature were used, higher foaming efficiency could not be expected as a result of the
incomplete development of the bubble inside the foaming mixture. To the contrary, if too high values were
employed, a remarkable risk of a collapse of the foamed structure arose, with consequent lowering of the
foaming efficiency. However, as the third diagram reported in Figure 4 shows, ANOM was not able to
accurately take into account the kinetics of foaming, being all results averaged on the other experimental
factors. As can be seen, the third diagram went flat for foaming temperature higher than 800 °C, hence
suggesting the need for further experimental tests.
3.3 The built-up of 3D process maps
As described in previous section, in the light of experimental findings coming from the execution of
Taguchi’s reduced experimental design, further tests were performed aimed at tracing 3D process maps of
foaming efficiency according to foaming time and temperature as well as compaction pressure.
The results of experimental analysis, carried out as scheduled in the experimental procedure, are reported in
Figure 5-6. Figure 5 reports the foaming efficiency according to foaming time and temperature with different
compaction pressure of the precursors. As can be seen, in agreement with previous experimental indications,
an increase of compaction pressures over 400 MPa does not influence the foaming efficiency. In fact, as said,
to such pressure value, the precursor reaches a relative density not so far from the unit values. This allows to
have a precursor with thermal properties similar to the bulk material. In such way, the thermal exchange
during the treatment in oven results optimal and no further improvement can be expected if higher compaction
pressures are used.
Figure 6 reports the influence of time-temperature relation upon the foaming efficiency. This time seems very
clear how the best results can be achieved both using lower foaming time joined with higher foaming
temperature or higher foaming time with lower foaming temperature. This is the results of foaming kinetics.
At lower temperature, the foaming process was found to be very slow, hence requiring maximum foaming
time to occur. To the contrary, if higher foaming temperature were chosen, the foaming phenomena happened
very fast. In such case, very low foaming time had to be set in order to avoid the detrimental effect of a too
high foaming temperature with consequent collapse of the foamed structure connected to the development of
too large gas bubbles in the foamed structure. Therefore, as the green zone on the top of the map reported in
Figure 6 states, if lower foaming temperature, close to 700 °C, were used during experimental trials, foaming
time as large as 15 min allowed obtaining the best foaming efficiency (close to a factor of 5). Vice versa, if
foaming temperature as large as 900 °C were used during experimental trials, foaming time as low as 5 min
allowed achieving the best foaming efficiency.
4 Conclusion
In this paper, the influence of operative parameters upon manufacturing of metal foam was studied. In
particular, an attempt to associate the mechanisms of foaming process based upon the decomposition of
foaming agent with gas release in semi-solid state to foaming efficiency according to operative parameters
was operated. First, the influence of foaming agent amount on foaming efficiency was studied, stating that a
value of 0.5 % allowed attaining the best foaming efficiency with best distribution of small gas bubbles all
over the foamed structure. Besides, lower values of foaming agent should be avoided in order to stay away
from a too massive slowing down of the foaming kinetics, hence producing a too scant distribution of gas
bubbles all over the foamed structure. Lastly, higher value of foaming agent should be avoided in order to not
cause the collapse of the foamed structure due to the too rapid development of few but big gas bubbles during
the decomposition of the foaming agent in the foaming mixture.
Once set the best composition for foaming mixture, the foaming process parameters, that is, foaming time and
temperature as well as compaction pressure of foaming precursor were investigated. From the development of
the Taguchi’s reduced experimental trials, foaming was found to be strongly governed by compaction
pressure and foaming time-temperature relation. Optimal choices for all operative parameters were proposed:
400 MPa for compaction pressure, 8 min for foaming time and 800 °C for foaming temperature.
Next, a full factorial experimental campaign allowed tracing several process maps, first useful support
instrument to metal foam manufacturers. Several further experimental results arose. Specifically, if higher
foaming efficiencies were sought, compaction pressure of, at least, 400 MPa had to be employed in order to
assure higher relative density (close to 1). Subsequently, enough fast heat transfer phenomena inside the
foaming mixture (like the analogous bulk material) during the heating process were activated, thereby
promoting the proper reactive phenomena and allowing the correct development of the foamed structure.
Besides, the kinetics of foaming was found to be severely influenced by both foaming time and temperature.
In particular, lower foaming time coupled with higher foaming temperature or higher foaming time coupled
with lower foaming temperature should be used to attain the best foaming efficiency. In fact, if too low
foaming time and temperature were employed, scant foaming efficiency occurred as a result of the poor
foaming kinetics. To the contrary, if too high foaming time and temperature were used, poor foaming
efficiency occurred as a result of the too fast foaming kinetics with consequent development of too bug gas
bubbles, hence causing the collapse of part or all of the foamed structure.
Acknowledgments
The authors would like to acknowledge Mr. Daniele Ceccarelli for his stimulating insights and useful
suggestions during the development of experimental apparatus.
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Figure Caption
Figure 1 – Foaming experimental apparatus
Figure 2 – Foaming efficiency vs. TiH2 weight percentage
Figure 3 – Bubble size according TiH2 weight percentage
Figure 4 – ANOM on foaming efficiency
Figure 5 – 3D process maps of foaming efficiency according to: (a) compaction pressure and foaming time; (b) compaction pressure
and foaming temperature
Figure 6 – 3D process maps of foaming efficiency according to foaming time and temperature
Table Caption
Table 1 – Table of experiments
Table 1 – Table of experiments
Experimental Factors
Experimental Compaction Foaming time,
Levels
pressure,
min
MPa
I
300
5
II
350
8
III
400
10
IV
450
12
V
500
15
Foaming
temperature,
°C
700
750
800
850
900
Figures
Figure 1 – Foaming experimental apparatus
Figure 2 – Foaming efficiency vs. TiH2 weight percentage.[5]
Figure 3 – Bubble size according TiH2 weight percentage
Figure 4 – ANOM on foaming efficiency
Figure 5 – 3D process maps of foaming efficiency according to: (a) compaction pressure and foaming time; (b) compaction pressure
and foaming temperature
Figure 6 – 3D process maps of foaming efficiency according to foaming time and temperature