Method of reducing the pore density in a casting

A casting (e.g. of aluminum or aluminum alloy) is made at least approximately free from pores by being subjected, in heated state, to isostatic compression in a liquid pressure medium. A rapid pressing operation of under one minute can be achieved if the casting is placed in a container, the container and the casting are heated to the required compression temperature before they are thereafter placed in a press chamber of a fast-acting press. When liquid pressure medium is fed to the press chamber and the necessary pressure is applied thereto, the pressure medium flows into the interior of the container via a number of channels in its wall and is simultaneously heated to the necessary temperature, prior to its contacting the casting.

TECHNICAL FIELD 
The present invention relates to a method of making a casting (e.g. of 
aluminum or aluminum alloy) at least approximately free from pores by 
subjecting the casting, in a heated state, to isostatic pressure. 
DISCUSSION OF PRIOR ART 
Aluminum castings are currently manufactured mainly by two methods, namely, 
by die casting or chill casting. With both methods a porous casting 
results, the pores weakening the casting. Among other things, the fatigue 
strength of the casting is reduced by the pores. It is known that porous 
castings can be densified by subjecting the castings to a hot isostatic 
compression. In the known method, the casting is placed in the press 
chamber of a press of autoclave type, whereafter the casting is heated 
within the press chamber to the necessary temperature for pressure 
treatment and is then subjected to the necessary pressure in the press 
chamber, usually via a gaseous pressure medium. Such an isostatic pressing 
operation is a relatively slow process. 
The present invention is based on the realization that the treatment time 
for densifying a casting by isostatic compression can be drastically 
reduced by the simultaneous use of a liquid pressure medium, with its 
inherent low compressibility, and a press with a rapid pressure-increasing 
capacity such as a piston press, provided that the casting can be brought 
to the temperature necessary for rapid densifying without the liquid 
pressure medium having to be heated, in its entirety, up to this 
temperature. 
SUMMARY OF THE INVENTION 
According to the present invention, the casting is heated while located 
within a special container before the container with its casting, is 
located in the press chamber of the fast-acting press and the liquid 
pressure medium is supplied to the press chamber. In at least one of the 
walls of the container, a number of through-channels are provided, through 
which the liquid pressure medium has to pass in order to contact the 
casting. In its passage through the channels, the pressure medium is 
heated by the container wall(s) to the necessary temperature, so that the 
casting is not subjected to any significant temperature reduction on being 
contacted by the liquid pressure medium. The container is thus utilized as 
a heat reservoir. Using the method of this invention, only the relatively 
small volume of pressure medium that passes through the channels needs to 
be heated to the elevated temperature required in order not to jeopardize 
the densification of the casting. This results in the process becoming 
fast. The fact that the rest of the pressure medium does not need to be 
subjected to the same degree of heating, is an advantage for reasons other 
than the speeding-up of this process. Among other things, thermal 
decomposition changes in the pressure medium are reduced. 
The method according to the present invention is particularly suitable for 
densification of light weight metal and light weight metal alloys. 
The isostatic compression is suitably carried out at a pressure of at least 
100 MPa and preferably at a pressure in the range 100 to 1000 MPa. A 
pressure in excess of 300 MPa is particularly preferred. The casting and 
the container are suitably heated to a temperature which lies above 
300.degree. C. but below the solidus temperature of the casting material 
in question. For pure aluminum the maximum temperature is 659.degree. C. 
and for pure magnesium 651.degree. C. For most aluminum and magnesium 
alloys a temperature in the range 370.degree. to 550.degree. C. is 
suitable. The invention is applicable to the densification of castings of 
all conventional aluminum and magnesium alloys, which are used for 
castings. Such aluminum alloys contain at least 85 percent by weight Al as 
well as one or more additional elements which form a eutectic with the 
aluminum, normally Si, Cu and Mg. Examples of such alloys are an alloy 
containing 7 percent by weight Si and 0.37 percent by weight Mg, the 
balance being Al; an alloy containing 4.5 percent by weight Cu, 1.5 
percent by weight Mg and 2 percent by weight Ni, the balance being Al, and 
an alloy containing 9 percent by weight Si, 0.5 percent by weight Mg and 
1.8 percent by weight Cu, the balance being Al. And such magnesium alloys 
contain at least 85 percent by weight Mg as well as one or more additional 
elements which form a eutectic with the aluminum, normally, Zn, Zr, Al, Mn 
and Th. Examples of such alloys are an alloy containing 4.6 percent by 
weight Zn an 0.7 percent by weight Zr, the balance being Mg; an alloy 
containing 10 percent by weight Al and 0.1 percent by weight Mn, the 
balance being Al; an alloy containing 6 percent by weight Al, 0.15 percent 
by weight Mn and 3 percent by weight Zn, the balance being Mg, and an 
alloy containing 3.3 percent by weight Th and 0.7 percent by weight Zr, 
the balance being Mg. 
The liquid pressure medium may advantageously consist of a vegetable or 
animal oil or of a mineral oil. Such pressure media also function as 
lubricant. It would be possible, per se, to use other liquid pressure 
media. Among oils, those which have good thermal stability and for which 
the fire risk is small are particularly preferred. Especially preferred is 
castor oil, but also palm oil and colza oil may be used to advantage. 
The free volume in the container, available for the liquid pressure medium, 
between the casting and the inner walls of the container is normally 
considerably smaller than the volume of the material making up the 
container and suitably constitutes at most 30% and preferably at most 20% 
of the volume of said material. 
The free volume available for the liquid pressure medium between the 
casting and the inner walls of the container is suitably considerably 
smaller than the volume of pressure medium in the piston press. By taking 
steps to make the volume in the container, which is available for the 
pressure medium, small in relation to the volume of the material making up 
the container and in relation to the volume of pressure medium in the 
piston press, a rapid heating of the pressure medium which comes into 
contact with the casting is made possible, whereas the remainder of the 
pressure medium in the press need not be subjected to a heating which may, 
in course of time, become detrimental. Part of the material within the 
container may consist of separate filling bodies which are arranged 
between the casting and the actual container walls. The material in the 
separate filling bodies is then included with the material making up the 
container when calculating the total volume of material making up the 
container. When filling bodies are used, they are suitably of the same 
material as the material from which the container walls are made. The 
container is preferably made of a metallic material with a higher melting 
point than that of the casting, for example copper, steel or cast iron 
when densifying castings of light weight metals and light weight metal 
alloys. 
Suitably, any wall of the container which contains the channels is formed 
with a greater thickness than the other walls thereof. 
Desirably, the channels in the container wall(s) are arranged to be longer 
than the thickness of the wall in which they are located.

DESCRIPTION OF PREFERRED EMBODIMENT 
A chilled casting 10 of an aluminum alloy containing 7 percent by weight 
Si, 0.37 percent by weight Mg, the balance being Al (Al-Si7Mg), is placed 
in a steel container 11. The volume of the space 12 left between the 
internal walls of the container 11 and the casting 10 constitutes about 
10% of the volume of the steel making up the container 11. In one wall 11a 
of the container, a plurality of channels 13 for pressure medium are 
provided. These channels 13 each have a diameter of about 4 mm. The wall 
11a, in which the channels are arranged, has a greater thickness than the 
outer walls of the container 11 in order for the pressure medium to be 
heated sufficiently before it contacts the casting 10. The container 11, 
with its casting 10, is heated up to a temperature of about 500.degree. C. 
and is then placed on support means 14 in a piston press 15. 
The piston press 15 comprises a cylinder 16, which is provided with a 
wire-wound reinforcing mantle 16a, a bottom plate 17, which is in 
liquid-tight sealing engagement with the cylinder 16, and a movable piston 
18. The integers 16, 17, 18 and 19 define a press chamber 20 that 
surrounds the container 11. Between the cylinder 16 and the piston 18, an 
annular seal 19 is provided. The piston press 15 is placed in a hydraulic 
press (not shown), in which there is a cylinder with a piston for applying 
a force, in the direction of the arrow A, on the piston 18. 
After the container 11 with its casting 10 has been heated and placed in 
the press chamber 20 in the piston press, a liquid pressure medium 21, in 
the exemplified case consisting of castor oil, is supplied to the press 
chamber and a pressure of about 400 MPa is quickly generated in the press 
chamber by means of the piston 18. The castor oil, which is supplied at 
room temperature or heated somewhat, passes, via the channels 13 in the 
container 11, into the free space 12 in the container available for the 
pressure medium. In passing through the channels 13, the castor oil is 
heated to a temperature close to 500.degree. C. As soon as the pressure 
medium completely surrounds the casting 10, the casting is subjected to an 
isostatic pressure, removing the porosity thereof and rendering the 
casting at least approximately free of pores. The process time for the 
treatment of the casting in the piston press can be made to be less than 1 
minute. 
The channels 13 in the container wall 11a can be extended, for example by 
being formed with a zigzag configuration as shown at 13a in FIG. 2, or 
otherwise by being shaped so that the direction of flow of the pressure 
medium is changed one or more times in its flow through the wall 11a. 
The volume within the container 11 is partly occupied by the casting 10 and 
one or more filling bodies (only one of which is shown at 22) so that the 
free space 12 is less than 30% of the combined volume of the container 11 
and the filling bodies 22. 
Various modifications of the exemplified process are clearly possible and 
are embraced by the spirit and scope of the invention as set out in the 
following claims.