Method for obtaining composite cast cylinder heads

A process is disclosed for the production of cast cylinder heads made of aluminium alloys from at least two different "liquid" alloys. The liquid alloys at the time of casting may contain solid particles of varied size and shape so as to produce composites with a metal matrix after solidifying. The process for moulding composite cylinder heads includes a number of successive layers consisting of at least two different alloys and consists in casting each alloy layer in the cavity of a mould via a feed system with a waiting time between the end of casting of one layer and the beginning of the second layer, so that the first layer contains between 50 and 100% of solid fraction in its lower part and 0 to 80% of solid fraction in the upper part, the interface region, when the second alloy is introduced.

BACKGROUND OF THE INVENTION 
The invention relates to the production of cast cylinder heads made of 
aluminium alloys comprising at least two different alloys. The liquid 
alloys may comprise solid particles at the time of casting of varied size 
and shape so as to produce composites with a metal matrix after 
solidifying. 
This technique makes it possible to optimise the choice of the materials 
according to the main functions required in the different parts of the 
cylinder heads. By way of illustration there may be mentioned the 
requirement of a maximum tolerance to damage by heat in the vicinity of 
the combustion chamber, especially in the regions between the valve seats. 
On the other hand, in the cold part of the cylinder head, especially the 
securing posts, the critical property is mechanical strength, so as to 
endow the cylinder head with maximum stiffness and the best possible 
aptitude to clamping, with a minimum weight of the finished component. 
At present, however, there is no manufacturing technique permitting the 
problem specified above to be solved in a satisfactory and economically 
viable manner. 
In fact, it is possible, to be sure, to investigate materials exhibiting 
both a high mechanical strength and good heat resistance. However, 
experience shows that materials of this type are costly. For example, 
according to the manufacturers' estimates, metal matrix composites 
reinforced with silicon carbide particles of the Duralcan type cost 2 to 3 
times more than conventional casting alloys, and this rules out their use 
for the whole of the cylinder head. 
In general, the use of high-performance materials must be restricted to a 
local application in the regions where they are indispensable, this being 
due to their cost. 
Furthermore, so far as we are aware, there is no technique in existence 
enabling such materials to be inserted into a cylinder head. The insertion 
of aluminium alloys or of metal matrix composites (for example the AlFe 
AlFeCe alloys obtained by powder metallurgy, followed by bonding, high 
heat-performance alloys obtained by a process of the Osprey type, metal 
matrix composites resulting from the impregnation of preforms, for example 
by liquid forging--Squeeze Casting--etc) placed in the solid state in the 
cylinder head at the time of casting comes up against the difficulty of 
successful metallurgical bonding between the material of the cylinder head 
and that of the insert(s). 
Finally, another route which is at present developed for locally 
reinforcing the material of a cylinder head consists of impregnation when 
preforms are being cast (especially with alumina or silicon carbide or 
reinforcements consisting of long fibres). However, technology of this 
type introduces high manufacturing overcosts when compared with the usual 
techniques of gravity and/or low pressure casting, especially because of 
the need to produce a partial vacuum and then to apply overpressures of 
several Pa which make it necessary to cover the sand cores with a 
protective film so that they themselves are not impregnated with liquid 
metal. 
SUMMARY OF THE INVENTION 
Applicant has therefore investigated and developed production techniques 
permitting different alloys to be cast in a cylinder head, and especially 
alloys with a high tolerance to damage on the combustion chamber side and 
alloys of low cost of manufacture and high mechanical strength in the 
remainder of the component. 
The component according to the invention consists of successive, adjoining 
and substantially horizontal layers. 
More precisely, it has become apparent that it is necessary for each layer 
i.sub.n-1 (n.gtoreq.2) to meet the following conditions at the time of the 
casting of the subsequent layer i.sub.n. 
lower face of layer i.sub.n-1 : 50 to 100% of solid fraction 
upper face of layer i.sub.n-1 : 0 to 80% of solid fraction and preferably: 
lower face of layer i.sub.n-1 : 70 to 100% of solid fraction 
upper face of layer i.sub.n-1 : 10 to 40% of solid fraction 
These conditions can be obtained by adjustment of the method of cooling of 
the cast metal, aiming at a maximum heat extraction via the base of each 
layer and waiting for the time needed for the above conditions to be 
established. 
In practice, it is a matter of defining the waiting time, t.sub.w, between 
the end of casting of each layer (i.sub.n-1) and the beginning of the 
layer i.sub.n (n.gtoreq. 2), as a function of the conditions of cooling of 
the cast component. 
For obvious production efficiency reasons the aim is to make t.sub.w, as 
small as possible by consequently sizing the system for cooling the layer 
i.sub.n-1. The cooling of the cast component is generally ensured by a 
metal sole plate carrying a heat transfer fluid such as water. 
The solid fractions are determined beforehand experimentally by thermal 
analysis, for example by placing at least two thermocouples in each layer 
(i.sub.n-1), one in the region close to the interface with the next layer 
and the other in the neighbourhood of the base of the layer. 
The solid fractions are determined from these thermal analyses by the use 
of equilibrium diagrams of the east metal which is generally assumed to be 
similar to an Al-based binary alloy. The principle of the calculation is 
given in the Appendix. 
The feed systems will be adapted so that the casting of each layer i.sub.n 
(n.ltoreq.2) does not create any unacceptable erosion of layer i.sub.n-1 
and so that the layers are as uniform as possible. This adjustment is 
within the scope of a person skilled in the art, for example by virtue of 
the optimisation of feed channels or by the use of metal or ceramic 
filters placed in the feed system, in order to regulate its flow rate. It 
is necessary, in fact, to obtain one or more substantially planar and 
uniform interface(s) between the layers, which can be checked, for 
example, by micrography, macrography or scanning microscopy on 
cross-section(s) perpendicular to the interface. 
The feed systems may be unsymmetrical, but they will preferably be made 
symmetrical to make it easier to obtain layers having uniform thicknesses. 
Finally, it is possible to provide the mould cavity with inert protection 
by an inert gas (CO.sub.2, argon, nitrogen, and the like) in order to 
reduce to a minimum the oxide layer which is naturally formed at the 
surface of the liquid metal during the casting, and hence to promote 
metallurgical bonding between the layers. 
When the mould is filled under these conditions, cylinder heads are 
obtained which exhibit successive layers of different alloys with a 
metallurgical bond of high quality without oxide defects (see FIGS. 5 and 
6), in accordance with the specifications of the motor vehicle 
manufacturers. 
In the case of twin-alloy cylinder heads, a layer of material intended to 
give heat resistance is formed which typically has a thickness of 15 to 25 
mm on the combustion chamber side, the remainder consisting of the second 
alloy. 
According to the invention the process for obtaining a bi- (or 
multi-)metallic cylinder head is therefore carried out by successively 
casting in the cavity of a mould which is either metallic or made of sand, 
or mixed, two (or more) distinct aluminium alloys with one or more 
interface region(s) which is (are) as thin as possible, consisting of a 
mixture of the cast alloys and without any trace of oxide skins. 
To do this, the alloys are introduced into the mould cavity by independent 
feed systems. The level of each layer is obtained by metering its 
quantity, for example by volume. 
In order to avoid an excessively large mixing region of the two different 
and successive alloy layers, it is advisable to allow the alloy of layer 
i-1 (i.gtoreq.2) to cool so that it is pasty at the time of arrival of the 
liquid metal intended to form layer i. 
The production of a multialloy cylinder head can take place by a gravity, 
low pressure, casting technique, by liquid forging (squeeze casting) or 
any other industrial foundry technique suitable for the production of 
cylinder heads.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
EXAMPLE 1 
Twin-alloy cylinder head: AS7G-AS5U3G (FIG.2) 
The mould is made up of a metal sole plate (1) made of cuprochrome 
(approximate composition 60% Cu, 40% Cr) 100 mm in thickness and of sand 
blocks (2). This sole plate comprises a cooling circuit (3) in which water 
circulates so as to maintain its temperature between 80.degree. and 
100.degree. C. 
The mould is provided with two feed systems (4) and (5), vents, water and 
oil circulation circuit cores, entry and escape pipes and the usual 
runners (not shown). 
The coring process is the Pepset process in the case of the blocks (2), the 
cores of the oil circulation circuits and the entry and escape pipes, and 
Ashland in the case of the water circulation circuit cores. 
The first metal, AS7GO,3 (according to French Standard NF A 57702) is cast 
at a temperature of 710.degree. C. (target temperature) via the feed (4) 
over a height of 20 mm corresponding to the thickness of the table of the 
cylinder head (volumetric metering). The feed system (5) is calculated so 
that the delivery of AS7GO,3 lasts approximately 15 s with a speed or flow 
rate of approximately 6.5 /min at the gates (6). As soon as the casting of 
the first alloy is finished, the second alloy, an AS5U3G (Standard 57702) 
is introduced at a temperature of 720.degree. C. via the feed system (5) 
at a speed or flow rate of 30 /min at the gates so that the horizontal 
component of the speed of this alloy is approx. 0.5 m/s so as to fill the 
remainder of the mould without eroding the first metal. 
The calculation of the solid fractions in the first alloy (AS7GO3) at the 
time of the arrival of the second metal with the aid of the recording of 
the temperature of the first alloy, of the Al--Si diagram and of the 
application of the rule of levers by applying the method given in the 
Appendix gives the following results: 
lower part 10 (in contact with the sole plate): 82%; 
upper part 11 (in the interface region): 18%. 
EXAMPLE 2 
Twin-alloy cylinder head--Duralcan F3A--AS5U3G 
Duralcan F3A, consisting of AS7GO,3+15% of SiC particles is employed as 
first alloy and is cast under the same conditions as the AS7G of example 
No. 1. The SiC particles do not modify the thermal analysis of the alloy 
and the method of calculating the solidified fractions for normal 
aluminium alloys is applicable. Nevertheless, the casting temperature of 
Duralcan is increased by 20.degree. C. so as to obtain the same fluidity 
as that of the pure base alloy, and therefore the same filling speeds, 
APPENDIX 
Method of calculation of the solidified fractions in the case of 
hypoeutectic alloys of Al--Si type (general case of the foundry alloys for 
cylinder heads). 
The following are defined from the equilibrium diagram of FIG. 8 for an 
alloy of Al--Si type of overall composition Co: 
T the temperature of the alloy 
T1 temperature of onset of solidification 
T2 temperature of end of solidification (here coinciding with the eutectic 
plateau temperature) 
C1 concentration of addition element in the metal solidified first 
C2 concentration of addition element in the metal solidified last, before 
transformation of the eutectic liquid. 
CM, average composition, solidified before the eutectic transformation is 
assumed to be similar to: 
EQU CM=(C1+C2)/2 
C3 eutectic concentration 
The usual rule of levers is then applied to determine the solidified 
fraction at each stage of the solidification preceding the isothermal (or 
eutectic) transformation. 
Let fso be the solidified fraction obtained Just before the solidification 
of the eutectic (T=T2): 
EQU fso=(C3-Co)/(C3-CM) 
The fraction, fs, solidified between T1 and T2 can be calculated either by 
this same rule of levers at each temperature or by the following formula, 
which is quicker, if the solidus and the liquidus of the alloy is assumed 
to be similar to two straight lines between T1 and T2 (a hypothesis which 
is wholly acceptable within the scope of the use of this patent 
application): 
##EQU1## 
The fraction solidified during an isothermal, in particular eutectic, 
transformation plateau can be estimated from thermal analysis by virtue of 
a thermocouple placed in the layer being considered, assuming that the 
solidified fraction varies linearly with time during the isothermal 
transformation. 
In the case of a transformation of binary type (FIG. No. 7) it can 
therefore be written, with a very good approximation, that the total 
solidified fraction Fs is equal to: 
EQU Fs=fso+(1-fso)(t-to)/(t1-to) (to.ltoreq.t.ltoreq.t1) 
since the alloy is completely solid at time t1.