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Figure 4.68 Optical micrograph taken from the centre of FATOIM. There were no major differences in the ceramic phase when the MgO preform infiltrated with alloy IM, Figure 4.69, was compared to infiltration with IS (Figure 4.55 b). Therefore reactions between the reinforcement and the alloy were not evident. Light grey precipitates were found in the alloy areas of the MMC similar to those visible in the unreinforced alloy IM, Figure 4.44 a). The binary phase diagram in Figure 2.2 would suggest the precipitates in the alloy IM to be the intermetallic Al3Mg2. Figure 4.69 Optical micrograph of magnesia-reinforced MMC infiltrated with the aluminium-magnesium alloy IM (MOPC20IM). 167
4.9. High pressure die casting infiltration As well as squeeze cast infiltration, some preforms were infiltrated using high pressure die casting. Two different infiltration modes were used which were characterised by significantly different metal flow velocities at the ingate: the indirect squeeze casting (ISQC) and the high pressure die casting (HPDC) mode. For each mode, four velocities were selected using a natural logarithmic velocity graduation shown in Table 3.8. The resulting MMCs were characterised in terms of their homogeneity and the relative compression of the preforms. 4.9.1 Homogeneity of MMC The homogeneity of the MMCs was characterised using 3D X-ray computed tomography. In the resulting virtual cuts, the light areas were the MMC, the grey areas the non-reinforced metal alloy and the black areas were either porosity or the atmosphere surrounding the sample. Prior to the 3D analysis, approximately 2 mm were milled off the castings along the y-axis until the surface of the MMC was reached. As a result, no alloy coverage is visible on top of the MMC in the XZ-plane in the virtual cuts. Each figure shows the virtual cuts of the central layer in the MMC in all three cartesian coordinates, as defined in Figure 3.11. The virtual cut in Figure 4.70 shows the internal structure of the AODY30 preform infiltrated in the ISQC mode using a velocity v0 of 0.72 m/s. The parameter v0 is defined by Equation 24. In each plane, significant fractions of porosity and multiple cracking were detected in the MMC. There was also porosity in the unreinforced alloy on top of the MMC. Examination of the X-Y plane reveals the porosity in the MMC was predominantly concentrated in a rectangular area 6 mm from the edges of the preform. The porosity was subdivided into round pores with a diameter of up to 1.5 mm and pore agglomerations exhibiting a cellular arrangement. The cracking of the preform was most obvious in this plane. 168
Pressure Infiltration Behaviour and
ABSTRACT In the pressure infiltrati
CONTENTS 1. INTRODUCTION 1 2. LITER
4.8.3 Evaluation of infiltration be
Symbol Meaning γRv surface energy
Symbol Meaning TYS tensile yield st
these materials are the detrimental
2. LITERATURE REVIEW 2.1. Materials
changes in the oxide film chemistry
or inside the bulk fluid only. Inte
that are most effective in decreasi
initiation stress of 25 %. Further,
Beffort (36) suggested that even th
einforcement interface and reinforc
It is interesting to note that, for
20 Table 2.1 Compilation of the mec
General models to predict fracture
with values observed by others for
The work of adhesion characterises
and vapour, is difficult to evaluat
system Al-Al2O3 is 10 -49 Pa at 700
In the Al-Cu system, although the p
The heat of reaction ΔGr may be es
al. (100) who found non-wetting beh
capillary or threshold pressure has
using constant gas pressure. Infilt
The superficial velocity v0 in the
The permeability K can be expressed
2.4. Preform fabrication Composites
According to Kniewallner (51) even
2.4.3. Foamed preforms Another inte
structure. This is shown schematica
2.5.1. Gas pressure infiltration (G
MMCs infiltrated with an Al-9Mg or
layer oxide films. The Weber number
Long et al. (50) suggested that v0
3. EXPERIMENTAL PROCEDURE The influ
sintered at 1550°C, which represen
using a AVT-Horn (Aalen, Germany) m
squares fit function within the MAP
areas, SsBET ,of the powders were m
with dimensions of 65 mm x 46 mm x
The preform sintering process was o
in the evaporation of mercury at lo
The compressive strength, σc , of
as the measured mean value 0.23. Th
for 90 s to ensure complete solidif
ottom punch surface. The temperatur
A graphic presentation of the relat
detected. This operation took appro
modulus Edyn of the unreinforced al
calculated using the methods outlin
Positive volume changes were predic
Figure 4.5 Droplet formation of the
with the metal alloy IM: examples a
As shown in Figure 4.9, apart from
4.3.2 Powder specific surface area
The particles of TO and MO were dis
oom temperature and 270°C, with a
obtain usable products when they we
strengths, whereas with 10 and 20 w
strength showed no significant diff
Relative change in dimension s x, s
Compared to Hg, the Al melt may con
preforms with IM, Figure 4.67. For
preform compression, cpr , increase
Specific Specific permeability Perm
Permeability (m²) / m² 1x10 -12 1
As the predominant fluid flow was a
In the CP mode, the Preform 1D code
Local Saturation saturation S () lo
listed in Table 5.1 and 5.3 were us
6. CONCLUSIONS 1. An aqueous proces
anged between 112 and 131° for the
8. REFERENCES 1. Altenpohl, D.: Alu
43. Davis, L.C. and Allison, J.E. :
85. Gennes, P.G. : “Wetting: Stat
127. Corbin, S.F., Lee, J. and Qiao
171. Gmelin, L. : Handbook of Inorg
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