Method of producing an aluminum-silicon casting

An aluminium-silicon alloy casting in which zones formed so as to have a particularly fine-grained structure are cooled by way of individual thin-walled mould wall portions, which are prevented from dissolving completely by the melt and are intermetallically bonded to the casting and are detachable from the finished casting only in a destructive manner.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for the production of a casting made of 
an aluminum-silicon alloy. 
In aluminum-silicon castings, it is frequently desired to bring about a 
particularly fine-grained structure, at least at individual specific 
regions thereof. The fine-grained structure produces in a workpiece of 
this material characteristics of low thermal expansion, high tensile 
strength, and high resistance to fatigue. These characteristics may be 
achieved by annealing the finished casting which effects coagulation of 
the grains. 
2. State of Prior Art 
In aluminum-silicon castings silicon inclusions are not present in the form 
of separate particles but are interconnected by a dendritic network. In 
the event of the structural zone being externally loaded or deformed, this 
results in high peak stresses in the silicon which, even if the 
deformations are only relatively small, cause the brittle silicon 
inclusions to break and thus internal notches to be introduced into the 
material. The destruction of these unsuitable networks has so far only 
been possible by extremely long coagulation annealing of the castings. 
Such a method is described, for example, in German Patent Specification 
No. 12 34 399. 
In order to obtain fine-grained structural zones in shaped castings, it is 
already known from British Patent Specification No. 1,337,731 to cool the 
wall areas of the casting in the molten state thereof by spraying water 
against the mould walls surrounding the casting. Such a measure causes the 
grain of the structure to become indeed smaller but only to a relatively 
narrowly limited extent. This is due to the fact that the solidifying melt 
shrinks in the mould, causing the contact between the mould wall and the 
casting to be lost and an air gap to be formed between the mould wall and 
the casting. This considerably reduces the heat transfer from the casting 
to the cooling medium, which has the effect that the heat required for the 
formation of a particularly fine-grained structure can no longer be 
removed from the casting to a sufficient extent. 
OBJECTS OF THE INVENTION 
It is the object of the invention to provide an improvement in this 
respect. It is of particular importance that a fine-grained structure 
should be brought about during casting, in specific zones of the casting, 
so that the length of the above-mentioned coagulation annealing, which is 
necessary for the attainment of specific properties in the material, is 
shortened as much as possible. A start is made from the known circumstance 
that extremely fine-grained structures require only extremely short 
coagulation annealing times. 
SUMMARY OF THE INVENTION 
According to the invention there is provided a method of casting an 
aluminum-silicon casting to achieve at least one localized fine-grain 
region comprising providing a mold to receive the molten metal, providing 
at least one thin sheet adjacent the region of the resulting casting where 
fine-grain is required, the inner surface of said sheet defining with the 
mold walls the shape of the resulting casting, the thickness of the sheet 
having been predetermined to ensure against complete disintegration of the 
latter due to the melt, pouring the metal into the mold and effecting a 
bond between the thin sheet and the metal of the melt, applying a cooling 
medium to the outer surface of said sheet, allowing solidification, and 
thereafter removing the sheet by breaking the intermetallic bond between 
the sheet and the casting. 
By means of the invention it is possible to bring about, in any desired 
individual areas in the mould wall, a fine-grained formation of the 
eutectic silicon whose linear grain diameter is less than 1.5 .mu.m. 
Casting zones provided with such structural areas only require coagulation 
annealing times of between 10 seconds and 1 hour at temperatures of 
between 480.degree. C. and 540.degree. C. The method according to the 
invention is particularly advantageous with respect to a subsequent 
coagulation of the fine-grained structural area. However, it is not 
confined only to the present application, but can be used wherever local 
portions of a shaped casting are to be rapidly and intensively cooled with 
the aid of a fluid cooling medium. 
Further according to the invention there is provided a method including the 
steps of effecting application of the cooling medium until solidification 
occurs in the region adjacent the thin sheet, reducing the flow of cooling 
medium to cause an increase in temperature due to the remaining melt, 
which increase permits coagulation within the solidified region adjacent 
the sheet, and thereafter controlling the temperature of the solidified 
region at the temperature at which coagulation occurs by adjustment of the 
flow of cooling medium. 
The advantage of this performance of the method resides in the fact that 
coagulation annealing, which has usually to be effected on the finished 
casting in an additional operation, is dispensed with or is replaced by 
the selective control of the cooling of the casting.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The casting 2 is to have a fine-grained structure in zone 1. The dimensions 
of the cylinderical casting 2 are a diameter of 120 mm and a length of 180 
mm. The depth of the fine-grained structural zone 1 is 20 mm. The end wall 
of the mold 4 is formed by a 0.2 mm thick tin sheet 3 along the region 
that is adjacent the fine-grained structural zone. The fastening of the 
tin-plate 3 is effected in that, when the mold is closed, it is located by 
means of a rim and is clamped in position by a hollow cylindrical jacket 
8. The composition of the molten material 5 introduced into the mold 4 is: 
12% Si, 1% Ni, 1% Cu, 1% Mg and less than 0.7% Fe, with the balance 
aluminum. 
The tin-plate 3 effects a metallic bond to the solidifying melt. If other 
metal sheets are used, it may be necessary to produce in advance this 
property for the formation of a metallic bond between the sheet and the 
solidifying melt by the application of an intermediate layer. 
The mold 4 is not cooled. 
The tin-plate 3, which covers a cooling area of 100 cm.sup.2 (unit area), 
is cooled for a period of 50 seconds by a stream of water 7 of 50 
liters/min. 
The finished casting is removed from the mould 4 together with the 
tin-plate 3 bonded thereto. The portion of the casting 1 which is located 
above the tin sheet 3 has at a depth of 20 mm a structure of a linear 
grain diameter of the eutectic Si of less than 1.5 .mu.m. Coagulation 
annealing of the casting 2 for 1 hour at 500.degree. C. causes the 
material in the fine-grained structural zone 1 to have the following 
characteristics: 
Ultimate breaking strength: 320 N/mm.sup.2 at 20.degree. C. 
Breaking elongation: 3.5% at 20.degree. C.; 30.2% at 300.degree. C. 
Fatigue strength: 140 N/mm.sup.2 at 20.degree. C. 
The metallic bond between the solidified melt 1 and the inserted sheet 3 as 
well as the flexibility of the sheet 3 are to ensure an optimum heat 
transfer from the melt 5 to the cooling medium 7. If the sheet 3 were 
lifted from the solidifying melt 1, an insulating gap would be formed, 
which would as a rule no longer allow the heat to be dissipated 
sufficiently quickly. 
While the casting 2 is being cast, the temperature in the zone which is 
located in the interior of the mold 4 above the sheet insert 3 and is to 
receive a fine-grained structure is measured with the aid of a conical 
sheathed thermocouple 6 at a depth of 10 mm above the sheet 3 and the 
cooling flow 7 is then controlled in accordance with the determined 
temperatures. When the solidifying temperature of 575.degree. C. falls by 
approximately 100.degree. C., cooling is stopped until the zone has been 
heated up again to approximately 540.degree. C. by the heat from the 
adjoining melt. Thereupon the cooling flow 7 is controlled according to 
the temperature determined in the interior so that the zone 1 to be 
coagulated is kept at the temperature of 540.degree. C. for a period of 15 
seconds. Subsequently, cooling is continued by means of the cooling stream 
7 until the final solidification of the casting 2. Once the coagulation of 
the fine casting portion 1 has been effected, the thermocouple 6 is 
withdrawn from the casting 2. It is also possible to control the cooling 
process by measuring the surface temperature or by time programming. 
After cooling the tin-plate is removed from the casting by destruction of 
the tin-plate sheet. 
The casting zone provided with the structure which has been coagulated in 
this manner according to the invention has the following characteristics: 
Ultimate breaking strength: 300 N/mm.sup.2 at 20.degree. C. 
Breaking elongation: 3.5% at 20.degree. C.