| Summary: | 博士 === 國立成功大學 === 材料科學(工程)學系 === 84 === Porosity formation in A206 alloy (Al-4.5Cu-0.3Mg-0.4Mn) and A356
alloy (Al-7Si-0.3Mg) was studied experimentally and
theoretically. Castings with the variation of geometry, riser
size , mold temperature, mold material and initial hydrogen
content, and low pressure castings with varied gauge pressure
(Pa) were tested in order to allow the measured porosity content
to be verified by the theoretical model of porosity formation.
A monogram with a plot of porosity content as the ordinate,
freezing ratio (FR) as the abscissa and volume ratio (VR) as the
third variable was established in order to determine the
porosity formation as a function of riser size and casting
geometry. Accordingly, the amount of porosity tends to decrease
with decreasing freezing ratio and with increasing volume ratio.
Increasing riser size and placement of taper decrease the amount
of porosity through increasing the volume ratio and decreasing
freezing ratio
The experimental results also indicate that the amount of
porosity depends not only on alloy physical constants but also
thermal parameters and initial hydrogen content; the porosity
content decreases with increasing thermal gradient (G), and with
decreasing solidus velocity (Vs), solidification time (tf) and
initial hydrogen content ([H]); both decreasing riser size and
lowering mold temperature increase the solidus velocity and
cause to degrade the feeding efficiency during the
solidification process. The solidifification feeding
efficiency of a thermal index, denoted as G0.4/Vs1.6 for A206
alloy and G0.38/Vs1.62 for A356 alloy, is employed to estimate
the value of local pressure drop within interdendritic liquid
during the solidification process and to predict the formation
of porosity in castings. The porosity content tends to decrease
with increasing the solidification feeding efficiency of thermal
index when the index is less than 7 K0.4s1.6/mm2 for A206 alloy
or 0.6 K0.38s1.62/mm2 for A356 alloy. B as the value of the
index is increased over approximately 7 K0.4s1.6/mm2 for A206
alloy or 0.6 K0.38s1.62/mm2 for A356 alloy, the porosity
content is found to be independent of this thermal index. Based
on the effect of the thermal index on the local pressure drop or
porosity formation, the feeding efficency during solidification
process in A356 alloy is about 13 times higher as compared with
A206 alloy castings. However, whatever the value of thermal
index is, an increase of
Furthermore, a pressure index (P*), determined from the
atmospheric and hydrostatic pressure and from the effect of
local pressure drop based on Darcy*s law and the pressure of
surface tension effect (Ps), is introduced. The P* index could
be expressed in terms of the thermal parameters G, Vs, Tf and
the alloy physical constants, and was proposed to evaluate the
formation of porosity in alloys; the porosity content is
inversely proportional to P*. Based on the experimental and
theoretical results, the porosy content (Vp%) can be simply
represented as Vp(vol%) = K(H)/P*, where K(H) is a contant
depending on the hydrogen content. An increase of hydrogen
content increases the value of K(H). For low pressure casting,
although the gauge pressure exerts an effect of reducing
porosity content meanwhile it also causes the negative effect of
reducing the degree of directional solidification. In the
application of low pressure casting, P* can be represented by
P*= Pa+ P+ Ps.
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