Method of producing hollow bodies in aluminum-silicon alloys by powder-extrusion

This invention concerns production by extrusion of hollow cylindrical bodies starting with granulated alloys of aluminum containing silicon. It consists of preparing the composition of the alloy in a liquid form, producing granules by centrifugal pulverization or atomization, introducing the granulated material into an extrusion press to obtain the hollow profile by extrusion and extruding the granular material within to form a cylindrical body. This invention is applicable to form hollow bodies and particularly sleeves of motors of high content silicon aluminum alloy through which size and distribution of primary silicon is improved over traditional casting methods.

The invention relates to a method of producing hollow bodies in aluminum 
alloys containing silicon and having improved properties, particularly as 
regards friction properties, compared with hollow bodies produced from 
these alloys by prior-art methods. These hollow bodies are for example 
sleeves of internal combustion engine cylinders, the bodies of hydraulic 
jacks and, in a general way, any hollow product that has a constant or 
only slightly variable cross-section over its entire length and that 
requires good sliding properties. 
Such hollow bodies are usually produced by either of two techniques, 
namely: 
A CASTING TECHNIQUE: THIS METHOD IS USED FOR PRODUCING CAST-IRON AUTOMOBILE 
ENGINE SLEEVES, GENERALLY BY CENTRIFUGAL CASTING, AND ALUMINUM ALLOY 
ENGINE SLEEVES BY PRESSURE-CASTING; 
AN EXTRUSION TECHNIQUE: THIS METHOD IS SOMETIMES USED FOR PRODUCING THE 
SEMI-FINISHED PRODUCTS FROM WHICH ALUMINUM ALLOY PUMP BODIES ARE MADE, THE 
IMT-EXTRUSION OF CAST OR CUT DISCS BEING USED. 
When aluminum alloys are used for producing these hollow bodies and, more 
particularly when the products are the sleeves of internal combustion 
engines, the present tendency is to make use of alloys containing silicon, 
and, particularly, hypereutectic alloys, i.e., alloys having a silicon 
content averaging above 12%. This type of alloy is particularly suitable 
for these uses of two main reasons, namely: 
(1) The hypereutectic Al-Si alloys have a lower coefficient of expansion 
than the other aluminum alloys, and this is clearly of advantage when the 
parts in question move relatively to each other with a small controlled 
clearance between them, and when they develop heat during operation. 
(2) The presence of hard primary Si crystals in a softer aluminum matrix 
makes these alloys particularly suitable, with or even without subsequent 
surface treatment, for providing surfaces having micro-rugosities which 
favor the retention of lubricants. 
However, this eutectic composition is not precisely defined and, because of 
divergences from equilibrium, crystals of primary silicon always occur in 
alloys that are very close to being eutectic, such as A-S13 or A-S12 UN, 
and even in alloys of hypoeutectic composition such as A-S10 UG. 
A great difficulty in the manufacture of these parts in alloys containing 
very large amounts of silicon or having a hypereutectic structure consists 
in the fact that the crystals of primary Si should not be too large. The 
acceptable maximum size is generally 100 micrometers. However, this 
requirement is difficult to meet in castings, particularly if they are of 
fairly large dimensions. Also, the silicon crystals in extruded parts are 
only slightly broken up as compared with the initial cast billet, and the 
same difficulties still occur. 
The applicants have invented a process for preparation of hollow bodies of 
aluminum alloys containing primary silicon and particularly containing 
from 12 to 30% silicon and preferably from 15 to 20%, and also from 1 to 
5% copper, from 0.5 to 1.5% magnesium, and from 0.5 to 1.5% nickel. 
These hollow bodies have the following properties: 
the primary silicon is of a size less than 20 microns, whereas the 
previously used methods have led to these crystals having a size greater 
than 20 microns; 
their pososity is low and is not concentrated in certain zones which could 
be the cause of mechanical weakness or lack of tightness with respect to 
fluids under pressure such as is sometimes the case with pressure-cast 
products; 
their ductility is better than that of the conventional cast product; 
they have better friction properties than those of the prior art products; 
their performance as regards friction can be further improved in comparison 
with those of the products hitherto used for these purposes, by 
incorporating in the alloy compounds which promote resistance to wear or 
reduce the coefficient of friction; and 
they can be machined much more easily than the products of similar 
composition produced by the conventional methods.

The method of the invention consists of using granules of aluminum alloy 
obtained by pulverization, in extruding these granules to form hollow 
bodies and, finally, in machining the hollow bodies thus obtained. The 
complete system for producing these hollow bodies is therefore as follows: 
preparation of ingots of an alloy, for example an alloy of aluminum base 
containing between 15 and 20% siicon, between 1 and 5% copper, between 0.5 
and 1.5% magnesium, and also 0.5 and 1.5% nickel. 
remelting of the ingots and granulation of the molten metal thus obtained 
by any of the existing processes, for example, centrifugal pulverization, 
atomization or the rotating electrode method; the particle-size of the 
product thus produced being between 5 .mu.m and 2 mm. Depending upon the 
method of preparation used, the particle-size will vary as will the 
cooling rate of the particles, resulting in a varying size of the silicon 
particles. Thus, in the case of granules produced by centrifugal 
pulverization and having a particle-size of between 300 .mu.m and 2 mm, 
the size of the primary silicon particles will be between 2 .mu.m and 20 
.mu.m, whereas for particles formed by atomization and having a size less 
than 100 .mu.m, the size of the primary silicon particles will be less 
than 5 .mu.m; 
optional mixing of the granulated alloy materials thus obtained with 
granules of silicon carbide, tin or graphite; 
optional isostatic or mechanical compression of the granules; 
optional heating to extrusion temperature of the granules which may have 
been previously compressed; 
introduction of the granular material, compressed or otherwise, into the 
container of the extrusion press; 
extrusion of tubing forming the sleeves; this is a conventional extrusion 
operation for producing hollow bodies and can be carried out using either 
of the two usual methods well known to the expert in the field: 
bridge extrusion; the bridge, located upstream of the die in the path of 
movement of the metal, secures a mandrel within the die so that the bore 
of the tube is formed; 
extrusion with a plain die and a floating mandrel which advances with the 
extrusion pad; (it is then necessary to use a hollow slug of compressed 
granular material which has an axial hole formed therein in which the 
mandrel is accommodated during extrusion); 
optional dressing and sizing; 
optional stabilization heat-treatment; and 
removal of material from inside the tubes, and machining. 
It is important to point out that certain of the succession steps 
constituting the above-described system are optional: 
the mixing of the granulated alloy material with granules of silicon 
carbide, tin or graphite is for the purpose of imparting to the hollow 
bodies, subsequently formed by extrusion, special degrees of hardness 
(silicon carbide) or good sliding properties (tin or graphite); 
the precompression of the granular material is not essential either. This 
precompression may be carried out either col or hot with the possible use 
of varying negative pressure so as to facilitate the suppression of 
porosity in the extruded product. 
The hollow bodies produced in accordance with the above-described method 
have a certain number of notable properties. First, their friction 
characteristics are distinctly improved, compared with those of the known 
products. In the examples detailed below for illustrating the invention, 
the experimental method whereby this improvement can be shown is 
indicated. 
This improvement involves obtaining a particularly fine product structure. 
The size of the crystals of primary silicon is less than 20 microns and, 
by selecting the appropriate production method, can be kept below 5 
microns. With conventional casting methods, such as pressure casting or 
low-pressure casting, the size varies between 20 and 80 microns. 
In FIG. 1, the micrograph is of a sample from a hollow body in an alloy of 
the A-S17U4G type (containing approximately 17% of silicon, 4% of copper 
and 0.5% of magnesium), obtained by powder extrusion. Most of the silicon 
crystals (in black) have dimensions less than 20 .mu.m. 
In FIG. 2, the micrograph is of a sample taken from a hollow body made of 
the same alloy but obtained by low-pressure casting. The difference in the 
size of the crystals can be clearly seen. 
The improvement also involves the presence of fine, uniformly distributed 
pores promoting lubrication by creating zones to retain oil. In cast 
products the pores are distributed unevenly and may occur in very great 
numbers in localized zones. 
The improvement further involves the possible presence in the matrix of 
compounds such as silicon carbide, tin or graphite which improve 
resistance to wear or reduce the coefficient of friction. 
Secondly, parts obtained by the method of the invention have a remarkable 
wear behavior distinctly better than that of alloys of similar composition 
worked by conventional methods. This behavior is revealed in excellent 
chip formation, good surface and in particular, light tool-wear. This good 
behavior results from the absence of crystals of primary silicon of large 
size, the effect of which is very damaging in machining operations. 
In the third place, the product obtained has fine, well distributed pores. 
Thus, there are no areas of reduced mechanical strength or areas which can 
be penetrated by fluids under pressure such as occur in pressure-cast 
products. 
On the other hand, this product has distinctly greater plastic range, i.e., 
difference between tensile strength and yield strength, of 15 hbars and 
elongation of 5%, than that of cast products wherein elasticity is 
virtually non-existent as indicated by the elastic limit (in the order of 
0.5 hbar) and elongations of less than 1%. 
To summarize, the hollow bodies made by powder-extrusion are notable, from 
the metallurgical point of view, because of the size of the crystals of 
primary Si being less than 20 .mu.m, small, evenly distributed pores and 
the alignment of constituents that is characteristic of the special 
texture of all extruded products. Furthermore, their oxygen content, 
resulting from the surface oxidation of the granulated material, is 
between 100 ppm and 15000 ppm. 
Also, the method of the invention has a number of features which enable the 
production procedure and the finishing operations of these hollow bodies 
to be considerably simplified. The provision, by extrusion, of a product 
having dimensions very close to the final dimensions and processing a good 
surface condition is a considerable advantage over the casting methods 
which call for considerable machining to bring the product to the required 
dimensions and surface condition; the greater ease in machining the 
powder-extruded products, as compared with products obtained by 
impact-extrusion or pressure casting, enables machining to be carried out 
more economically and tool-wear to be reduced; and the use of either 
alloys having a composition and structure not obtainable by existing 
methods, or composite products consisting of the basic alloy and 
additions, such as silicon carbide, tin and graphite, makes it possible, 
in most cases where the products are used as sliding parts, to dispense 
with the surface treatments that have sometimes been necessary in the 
past. 
In certain cases however, it will be advantageous to carry out a chemical 
treatment of the surface following a polishing or grinding operation. The 
object of this treatment is to smooth out the crystals of primary silicon 
over which a part will rub when moving relatively to the hollow body. 
The following Examples serve to illustrate the invention and to make it 
more readily understood. 
EXAMPLE I 
Internal combustion engine sleeves were produced by the following 
succession of operations: 
(a) Preparation of an A-S17U4G alloy having the composition: 
Si = 16.80% 
Cu = 4.40% 
Mg = 0.55% 
Fe = 0.80% 
Al = remainder 
and refining of the primary Si by the addition of phosphorous in accordance 
with a known technique. 
(b) Production of the granulated material. 
The cast metal was brought to a temperature of approximately 850.degree. C; 
it was held at this temperature for 30 minutes and then pulverized by 
centrifuging. The size of the particles thus obtained was between 50 .mu.m 
and 2 mm. The structure of the particles thus obtained wss fine; the 
crystals of primary silicon were of a size varying between 2 .mu.m and 20 
.mu.m maximum. 
(c) Powder-extrusion of tubes to be used as sleeves; this operation was 
carried out in the following manner: 
The extrusion press was a conventional press equipped with bridge tools. 
Without having been heated or precompressed, the granulated material was 
introduced into the container of the extrusion press in a loose mass; the 
container and the tools were not lubricated but were heated to a 
temperature of approximately 450.degree. C; to prevent the granulated 
material from flowing through the die during charging of the container, an 
aluminum foil was placed in front of the die. The extrusion pad was then 
fitted at the inlet to the container; the ram was applied so as to compact 
the granulated material; the pressure applied to the ram was increased 
until it was sufficient to cause the metal to flow through the die after 
the granulated material had been completely compacted. This metal-flow 
sufficed to ensure compactness in the extruded product and cohesion 
between the particles of the initial material; this flow in fact enables 
the oxide layer on the surface of the particles to be broken and thus 
creates metallic surfaces, completely free from oxide, that could readily 
fuse together when brought into contact with each other. 
(d) Dressing of the tube by a conventional drawing operation. 
(e) Cutting of the tube into lengths corresponding to those of the sleeves. 
(f) Stabilization heat-treatment for several hours at a temperature of 
220.degree. - 250.degree. C (this temperature being higher than that to 
which the products are subjected when in use). 
(g) Machining of the sleeves to the final dimensions. 
The sleeves thus obtained had a very fine metallurgical structure similar 
to that illustrated in FIG. 1. 
The mechanical properties were measured by means of tensile tests carried 
out on test-pieces cut in the direction of extrusion (L) and in the 
direction transverse thereto (T). For comparison purposes, the mechanical 
properties of the same alloy, pressure-cast, and of cast-iron are given: 
______________________________________ 
B.L. El. % 
Direction 
hbars 5.65.sqroot.So 
______________________________________ 
A-S17U4G L 26.6 5.0 
powder-extruded 
T 25.2 3.7 
A-S17U4G 
pressure-cast 29.0 &lt; 1.0 
Cast-iron 20 to 40 &lt; 1.0 
______________________________________ 
B.L. = breaking load (in hectobars) 
El. = elongation measured on the basis of 5.65 .sqroot.So 
So = cross-section of test-piece 
It was observed that with a breaking load approximating very closely to 
that of A-S17U4G, pressure-cast, and of cast-iron, the elongation values 
recorded for extruded A-S17U4G are higher, which indicates a much reduced 
brittleness. 
Sliding behavior was determined by a simulation test carried out in the 
following manner. The slide test-piece took the form of two tangent discs 
as shown in FIG. 3 and FIG. 4 (shown in elevation on the right in FIG. 3 
and in side-view on the left in FIG. 4). The discs were caused to rotate 
so as to cause a 10% pure slip (in angular speed) between the two 
test-pieces in contact; oil at a constant pressure was introduced at the 
zone of contact, and during the test the following could be measured; 
the load P applied to the upper disc, 
the contact temperature, and 
the frictional torque. 
The test-pieces were annular discs, having a thickness of 10 mm and an 
inside diameter of 16 mm. 
The lower disc, in A-S12UN had an outside diameter of 65 mm and was used as 
a reference (numeral 1 in the drawing). 
The other disc was made of the test metal and had an outside diameter of 35 
mm (numeral 2 in the drawing). 
The sliding tests were carried out in two stages; first stage, seizing 
test; second stage, wear test. Each of these two tests started with a 
running-in period. 
Seizing Test 
After a running-in period during which the two samples were placed in 
contact with each other under a relatively low load and in which the discs 
were rotated at constant speed, this test consisted in periodically 
increasing the load until seizing occurred, this mainly manifesting itself 
during the test by a sudden increase in the contact temperature, and by an 
increase and, in particular, destabilization of the coefficient of 
friction. The load being applied at the moment when seizing occurred was 
called the "gripping load." 
Wear Test 
This test, was preceded by a running-in operation identical to that used in 
the seizing test, and it consisted in carrying out a sliding test using a 
constant load equal to 0.5 to 0.8 times the seizing load and applied for a 
period of 2 to 5 hours, and in measuring the loss in weight of the 
test-pieces during the course of the test. 
The results of these sliding tests are shown in the following table wherein 
the values recorded for the powder-extruded alloy, the pressure-cast 
alloy, the alloy cast under low pressure and cast-iron are compared: 
______________________________________ 
wear in mg 
seizing 
Coefficient Disc in 
load of friction 
Disc in A-S17U4G or 
daN at P = daN 
A-S12UN cast-iron 
______________________________________ 
A-S17U4G 
powder-extruded 
90 0.015 67 8 
A-S17U4G 
pressure-cast 
80 0.015 52 12 
A-S17U4G 
cast under low- 
30 0.045 -- -- 
pressure 
cast-iron 80 0.109 2 100 0.5 
______________________________________ 
This test showed that the behavior of powder-extruded A-S17U4G is 
comparable with that of the pressure-cast alloy A-S17U4G as regards the 
seizing loads, the coefficient of friction and the wear on the parts. On 
the other hand, the behavior of the product produced by powder-extrusion 
is markedly superior to that of the same alloy, cast under low pressure, 
which has an appreciably lower seizing load and a higher coefficient of 
friction than in the two other cases. The behavior is also considerably 
better than that of cast-iron which, for an identical seizing load, has a 
higher coefficient of friction and as regards which the wear of the 
contacting part in aluminum alloy occurs more rapidly. 
EXAMPLE II 
(a) Preparation of an A-S25U4G alloy having the composition: 
Si = 25% 
Cu = 4.3% 
Mg = 0.65% 
Fe = 0.8% 
Al = remainder 
and refining of the primary Si by addition of phosphorous in accordance 
with a known technique. 
(b) Production of the granulated material. 
The cast metal was raised to a temperature of approximately 900.degree. C 
and was held at this temperature for 30 minutes and then pulverized by 
atomization. The size of the particles thus obtained was between 5 .mu.m 
and 500 .mu.m. Only those particles having a size of less than 100 .mu.m 
were retained. The structure of the particles thus produced was fine; the 
crystals of primary silicon had a size of less than 5 .mu.m. 
(c) Cold compacting. 
The granulated material was compacted cold in a vertical press and under a 
pressure of 50 kg/mm.sup.2. 
(d) Extrusion of tubes for use as sleeves. 
This operation was carried out on a conventional press provided with 
bridge-type tools. The compacting slug was extruded without heating, as a 
conventional solid billet. 
(e) Dressing of the tube. 
This was done by a conventional drawing operation. 
(f) Cutting of the tube into lengths corresponding to the length of the 
sleeves. 
(g) Stabilization heat-treatment for several hours at 220.degree. - 
250.degree. C (which temperature is higher than that at which the sleeves 
are used), or solution heat-treatment, quenching and tempering. 
(h) Machining of the sleeves to the final dimensions. 
The metallurgical structure of the sleeves thus obtained was very fine, and 
the size of the silicon crystals was less than 5 .mu.m. It was also 
observed, after heat-treatment, that the pores were very fine and evenly 
distributed in the product. 
The mechanical properties, measured in the same way as in the previous 
Example, are shown in the following table: 
Sleeves in A-S25U4G produced by powder-extrusion: 
______________________________________ 
B.L. El. % 
Direction 
hbars 5.65.sqroot.So 
______________________________________ 
L 29 4 
Stabilized 
T 28 2.5 
In solution 
L 55 2 
quenched and 
tempered T 52 0.7 
______________________________________ 
It will be seen that the material exhibits high breaking loads associated 
with quite considerable elongations. 
Regarding the sliding properties, the same simulation tests were carried 
out as in Example I. 
The performances of this alloy were identical to those of A-S17U4G, shaped 
by powder-extrusion of low-pressure casting, as regards the seizing load 
and the coefficient of friction; on the other hand, wear resistance is 
appreciably increased; loss in weight per unit of time is reduced in a 
ratio of 1.5 to 1.0.