Lost-foam casting of aluminum under pressure

An organic foam pattern of the article to be cast is immersed in a dry sand mold containing no binder, the mold is filled with molten aluminum or aluminum alloy which replaces the foam and gradually solidifies, and an increasing isostatic gas pressure is simultaneously applied to the mold and to the aluminum or aluminum alloy after filling of the mold is complete. The isostatic gas pressure rises to a maximum value higher than 1.5 MPa and up to 10 MPa, and results in cast articles having improved mechanical characteristics and, in particular, better resistance to fatigue.

The present invention relates to a process for the lost-foam casting, under 
pressure, of metal articles, in particular of aluminum and alloys thereof. 
It is known to a person skilled in the art, mainly from the teaching of 
U.S. Pat. No. 3,157,924, that patterns of polystyrene foam which are 
immersed in a mold formed from dry sand containing no binder can be used 
for casting. In such a process, the metal to be cast, which has previously 
been melted, is brought into contact with the pattern by means of channels 
traversing the sand and is gradually substituted for said pattern by 
burning it and transforming it into vapour which escapes between the 
grains of sand. 
This method has proven to be attractive on an industrial scale because it 
avoids the preliminary manufacture, by compacting and agglomeration of 
powdered refractory materials, of rigid molds connected in a fairly 
complicated manner via channels to cores, and allows simply recovery of 
the castings and easy recycling of the casting materials. However this 
method is handicapped by two factors: 
the relative slowness of solidification which promotes the formation of 
gassing pin-holes 
the relative weakness of the thermal gradients which can cause 
micro-shrinkage if the outline of the part complicates feeding thereof. 
With aim of overcoming such drawbacks, a lost-foam casting process has been 
developed which forms subject of the patent application published in 
France under No. 2606 688 and which corresponds to U.S. application Ser. 
No. 116,213, filed Nov. 3, 1987, now abandoned. 
This application teaches that, after having filled the mold with the molten 
metal, that is to say when the pattern has been destroyed completely by 
the metal and the vapour given off by the foam has been evacuated, an 
isostatic gas pressure is exerted on the assembly of mold and metal, 
preferably before the metal begins solidifying. This pressure is applied 
with values which increase over time so as to avoid the phenomenon of 
metal penetration and so that the maximum value is attained in less than 
15 seconds. 
Under these conditions, the castings obtained have increased density which 
is manifested by an improvement in the mechanical characteristics, in 
particular with regard to the strength. 
However, it is disclosed in this application that it was preferable to 
employ a maximum pressure of between 0.5 and 1.5 MPa and that it was 
unnecessary to exceed the latter limit. 
In fact, it was noted after more advanced research that if the pressure 
were further increased, not only the mechanical characteristics such as 
the breaking strength Rm, the yield stress LE and the elongation A but 
also the resistance to fatigue F were improved. 
Hence the present invention, which involves a process for the lost-foam 
casting, under pressure, of metal articles, in particular of aluminum and 
alloys thereof, in which an organic foam pattern of the article to be cast 
is immersed into a mold, the walls of which are defined by a bath of dry 
sand containing no binder, the mold is filled with the molten metal which 
is substituted for the foam and gradually solidifies, and an increasing 
isostatic gas pressure is applied simultaneously to the mold, and to the 
metal at the earliest on completion of filling, characterised in that the 
pressure exerted rises to a value of between 1.5 and 10 MPa. 
Therefore, the invention involves employing pressures of between 1.5 and 10 
MPa and preferably between 5 and 10 MPa. 
As in French patent application No. 2 606 688, the pressure can be exerted 
by means of a sealed box in which the mold is placed, said box being 
equipped with one or more nozzles conveniently distributed over its wall 
and connected to a source of gas under pressure. 
In the selected pressure range, it has been found that the phenomena 
produced during the application of pressure were quite different from 
those according to the prior art. 
In fact, between 0.5 and 1.5 MPa, the pressure serves mainly to accelerate 
the flow of molten metal between the dendrites of the solidifying metal 
and the effect stops when the solid network has reached a certain stage of 
development. In particular, this is how the low pressures enable the 
feeder effectively to prevent the phenomenon of shrinkage marks due to the 
contraction of the solidfying metal. 
On the other hand, the flowing effect of the molten metal, which is 
preponderant at the beginning of solidification, is gradually replaced by 
an effect of hot deformation of the already solidified metal network, 
under pressures higher than 1.5 MPa and, in particular, higher than 5 MPa, 
this phenomenon becoming dominant and then exclusive when the 
solidification rate reaches values of about 50 to 70%, depending on the 
type of alloy cast. The application of high pressures therefore produces a 
type of isostatic forging which affects the entire surface of the casting. 
The accompanying FIG. 1 is a micrograph of an A-S7G03 alloy cast according 
to the invention under a pressure of 7 MPa then heat treated. This 
micrograph shows the plastic deformation imposed on the dendritic network 
which has the effect of filling up the pores, and illustrates well the 
forging effect to which the metal is subjected in this process. 
Under these conditions, it is found that the mechanical characteristics of 
the articles are significantly improved and, in particular, the resistance 
to fatigue. Pressures higher than 10 MPa only produce insignificant 
improvements. 
This new pressure range is preferably applied before the quantity of 
solidified metal reaches 40% by weight so that the liquid flow can be 
acted upon. 
It is also preferable for the maximum pressure applied to be attained 
before the quantity of solidified metal exceeds 90% so as to benefit fully 
from the effect of deformation. 
As in French patent application No. 2 606 688, it is preferable for the 
pressure to be applied by a gradual increase, in particular at the 
beginning of solidification, to prevent "metal penetration", a phenomenon 
resulting from a transitory imbalance between the pressure exerted 
directly on the metal and the pressure exerted on the metal by means of 
the sand bath. In fact, the bath causes a relatively great loss of charge 
in the transmission of pressure resulting, in the region of the metal 
which is in contact with the sand, in a tendency for this pressure to push 
the metal through the grains of sand and to deform the casting.

The invention can be illustrated by the following embodiment: hollow 
cylindrical bodies having an external diameter of 45 mm and a wall 
thickness of 4 mm and comprising adjacent ribs and bosses of 
20.times.20.times.80 mm were cast by the earlier process and by the 
process according to the invention, that is to say an isostatic gas 
pressure corresponding to atmospheric pressure, to 1 MPa, to 5 MPa and to 
10 MPa respectively was applied to the interior of the chamber containing 
the mold just before the beginning of solidification. 
These bodies were produced from two types of alloys having high mechanical 
characteristics: 
A-S7G03 having a composition in per cent by weight of Fe 0.20; Si 6.5-7.5; 
Cu 0.10; Zn 0.10; Mg 0.25-0.40; Mn 0.10; Ni 0.05; Pb 0.05; Sn 0.05; Ti 
0.05-0.20; remainder Al; 
A-U5GT having a composition: Fe 0.35; Si 0.20; Cu 4.20-5.00; Zn 0.10; Mg 
0.15-0.35; Mn 0.10; Ni 0.05; Pb 0.05; Sn 0.05; Ti 0.05-0.30; remainder Al. 
The mechanical tests carried out on said bodies after standard Y23 heat 
treatments in the case of A-S7G03 and Y24 heat treatments in the case of 
A-U5GT enabled the following characteristics to be measured as a function 
of the pressures applied: 
In the A-S7G03, the quality index Q in MPa which corresponds to the formula 
Q=R+150 log A, wherein R is the strength in MPa and A is the elongation in 
% on both the thick and thin regions of the articles. 
In A-U5GT, the yield stresses LE in MPa, the strength R in MPa and the 
elongation A in %, also in both the thick and thin regions. 
Furthermore, the resistance to fatigue F was measured in MPa for each of 
the alloys and each of the pressures applied from torsion tests on a 
sample machined at 10.sup.7 cycles by the staircase method. F applies both 
to the thick and thin regions because it does not depend on the rate of 
solidification but on the porosity and consequently on the pressure 
applied. 
The results are given in the following table. 
TABLE 
__________________________________________________________________________ 
A-S7GO3 
Thick 
Thin A-U5Gt 
region 
region Thick region 
Thin region 
Q Q F LE R A LE R A F 
__________________________________________________________________________ 
Solidification under 
240 325 40 235 
340 
8 
260 
355 
7 
90 
atmospheric P 
Solidification under 
335 420 65 240 
365 
8 
260 
405 
11 
120 
1 MPa 
Solidification under 
410 460 85 250 
400 
12 
260 
410 
15 
130 
5 MPa 
Solidification under 
440 490 100 
260 
420 
15 
260 
420 
18 
140 
10 MPa 
__________________________________________________________________________ 
An improvement in all the characteristics measured and, in particular, 
increased resistance to fatigue are observed.