2 µm - eTheses Repository - University of Birmingham
of 0.023 m and nil saturation behind that point was found with Darcy´s model, demonstrating its inaccuracy. Having validated the Preform 1D code for Saffil fibre preforms (FA24), focus turned to AOPC20. Based on the preform parameters listed in Table 5.1, the modelling results were compared to the experimental results of local saturation after CP infiltration at different pressures, as shown in Figure 5.12. As shown in Figure 5.11 b) the melt did not reach the end of the FA 24 preform and therefore the end drag phenomenon could be omitted which is similar to the experimental infiltration results of AOPC20 at a constant pressure of 0.8 MPa shown in Figure 4.39. In contrast, at 1.2 MPa, the melt fronts contacted each other and therefore influenced each other. In order to allow for this, the centre of the preform was set as an impermeable boundary in the Preform 1D code which led to a time dependent increase in saturation in front of it, as shown by the results at different infiltration times of 2, 4 and 16 s in Figure 5.12. At an early stage, a negative gradient toward higher x-values was found, whereas a homogenous saturation profile was reached at an extended pressurisation (16 s). For applied pressures of 0.8 and 1.2 MPa at 16s, the calculated Sloc were homogenous at 0.23 and 0.61 respectively. At constant pressure, the respective saturation at the ingate (x = 0 m) was similar for different infiltration times. As, by definition, the ingate pressure is constant in the constant pressure mode, similar pressure and hence similar saturations were calculated for all infiltration times at the ingate. This behaviour is expressed by the saturation function in Equation 29 where all preform parameters were assumed to be constant during the entire infiltration process apart from the applied pressure Pappl. 231
Local Saturation saturation S () loc 0.8 0.6 0.4 0.2 p appl = 1.2 MPa experiment p appl = 0.8 MPa experiment α= 3.25x10 -6 / t = 2 s 16 s 0.0 0.00 0.01 0.02 0.03 Position x x / m(m) Figure 5.12 Experimental Sloc of MMC after infiltration of AOPC20 in CP mode at 0.8 and 1.2 MPa and modelling results after different infiltration periods (dashed lines) and variation of α for the applied pressure of 1.2 MPa (red line). With only minor deviations between x = 0.007 and 0.015 m, at 0.8 MPa, the negative slope toward the centre observed in the experiments could be best reproduced at an infiltration time of 2 s. However, in the experiments the pressure was kept constant for more than 60 s. It is therefore assumed that metal flow stopped 2 s after pressurization and therefore prior to pressure release. This has to be attributed to solidification of the metal at the die walls as these were held below the solidus of the alloy. The experimental results for a Pappl of 1.2 MPa showed a steep decrease in saturation which started from 0.73 and reached a minimum of 0.25 at x = 0.028 m. After that point, a steep increase could be observed. A symmetric saturation was assumed in CP infiltration but the centreline was shifted by 0.002 m toward the half of the investigated sample, which may be a 232 16 s result of marginal differences in permeability of the initial half of the preform. 4 s 2 s 4 s 2 s
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
(a) AOPC20 (b) AGPC15 2 µm (c) TOP
At higher magnification, Figure 4.2
intrusions started at 4 µm and end
As shown in Figure 4.27, the pore s
An overview of the specific values
1.71 to 1.98·10 6 m²/m³. The sim
logarithmic compression behaviour,
The volumetric stiffness Eiso of th
Figure 4.37 shows that the TOPC20 p
unhindered through the gap between
intrusions and the other areas were
4.8.1 Unreinforced matrix propertie
die, Tmelt,die , could not be recor
pressure was recorded as a function
the linear fits for AOPC20, TOPC20
4.8.6 Non destructive testing of MM
X-Y Y-Z Figure 4.51 Virtual cross-s
The metal filling the intragranular
the ceramic particles was not visib
etween the dark grey ceramic phases
The windows, one of which is marked
potential interfacial reactions, th
In order to determine the effect of
Infiltration depth L² L² (mm²) /
4.8.12 Microstructure of MMCs with
minor fraction of suboxides with hi
4.9. High pressure die casting infi
In the Y-Z plane section in Figure
4.9.2 Compression of preforms The c
Relative preform compression c pr (
decrease depended on the tooling us
Bending stress σ (MPa) / MPa 500 4