A Novel Approach to The Fabrication of Aluminium Foam Through Pressure Assisted High-Frequency Induction Heated Sinterin2 Dissolution Process (P ASDP)

It was known that it is difficult' to form a metallurgical bonding in a packed
aluminium (AI) powder during solid state sintering due to the encapsulation of AI by a
thermo chemically stable Ah03 film which hinders the mass transport mechanism by
the diffusion process. The disruption of this oxide film is indeed crucial for achieving
strong metallurgical bonding during solid state sintering of AI powder. This is largely
due to the ability of this oxide film to serve as a hard shell which prevents the AI
particle from establishing a direct contact with adjacent AI particles. This work
discusses the improvement made by pressure assisted high-frequency induction
heated sintering dissolution process (P ASDP) as opposed to the solid state sintering
dissolution process (SSSDP) which was used in producing a much improved open
celled AI foam. In the preparation of P ASDP AI foam specimens, the AI powder and
the sodium chloride (NaCI) acting as filler material were dry-mixed together in order
to prepare homogenously mixture. The blended mixture was then subjected to
pressure assisted sintering in which a pressure beyond atmospheric level is externally
applied to the specimen during high-frequency induction heated sintering. The
embedded filler material was then dissolved in order to leave behind an open-celled
AI with the same chemical composition as that of the original AI powder. The final
material is highly porous and has an interconnected porosity network. The structure of
the resulting material has three levels of porosity (i.e. main cells, windows and
microporosity). The cell morphology and the size of AI foam closely matt:h those of
the filler materials used. The X-ray diffraction analysis shows that as the content of
NaCI is increased to the volume fraction of 60%, no compound of NaCI presents in
the foam. The improvement of this P ASDP technique leads to an aluminium foam
possessing a high dense of cell wall which can be attained in a much shorter time and
possesses superior mechanical properties. The influence of processing parameters on
the physical and mechanical of the fabricated foams were studied. The compressibility
behaviour of sodium chloride I aluminium (NaCI! AI) specimen with different amount
of filler material content was investigated. The Heckel equation was used to assume
pressure effect at elevated temperature on the density of the specimen. Due to lower
plastic deformation capacity, the specimen with higher content of fillet material exhibited lower compressibility. The relationship was established between the
compaction pressure and the density of specimens. Williamson-Hall plot method was
also applied to determine the crystallite size and lattice strain of the consolidated
samples at various pressures. As can be observed, the crystallite size tends to decrease
with increasing NaCI content. Results indicate that the compressive properties (i.e.
compressive strength, modulus, and stress at densification) increase with compaction
pressure at elevated temperature, sintering temperature and sintering time.
Considering all contributing factors in forming AI foam using P ASDP method,
statistical evaluation utilising the Taguchi's Design of Experiments were carried out
extensively to predict the optimum processing parameters in this work. The
processing parameters were analysed based on the Taguchi's signal to noise ratio
(SIN) and analysis of variance (ANOV A) which is a major improvement from the
conventional Full Factorial Design of Experiment statistical method. Results show
that the rnost notable factor influencing the fabrication of AI foam was the
compaction at elevated temperature, followed by temperature, time and heating rate of
the process. The viability of electrical conductivity as a tool for evaluating variations
in morphology of P ASDP AI foam and the influence of processing parameters:
volume fraction of leaching agent, compaction pressure at elevated temperature,
sintering temperature, sintering time and heating rate on the electrical conductivity of
the final foams were investigated. It was found that all the chosen factors have
significant effects on the physical and mechanical properties of the foam and AI
volume fraction is the most important parameter affecting the relative electrical
conductivity and relative density of the resultant foam. The findings also suggests that
the relationship between relative electrical conductivity and density of the specimen
does not follow the mathematical description given by Skorohod and Solanin, instead
the power law seems to be a good characterization of the relationship between
experimental data of electrical conductivity and density of the resultant foams. It was
observed that the addition of silicon carbide, which was synthesised at lower reaction
temperature using structured alteration local silica sand from the carbothermal
nitridation process, improved the physical and mechanical properties of the foam.