WO2000071765A1

WO2000071765A1 - Aluminum-base alloy for cylinder heads

Info

Publication number: WO2000071765A1
Application number: PCT/BR2000/000056
Authority: WO
Inventors: Fernando Barata De Paula Pinto; Antonio Silvio Carmezini; Eduardo Celso Fonseca; Ricardo Fuoco
Original assignee: Ford Motor Company Brasil Ltda.; Instituto De Pesquisas Tecnológicas Do Estado De São Paulo S.A. - Ipt
Priority date: 1999-05-19
Filing date: 2000-05-19
Publication date: 2000-11-30
Also published as: DE60001577D1; DE60001577T2; EP1190107B1; EP1190107A1; AU4529400A; BR9901553A

Abstract

Aluminum-base alloy, for the production of engine cylinder heads without heat treatment, containing as alloying elements: silicon, in contents varying between 5.0 % and 10.0 %, by weight; copper, in contents varying between 4.0 % and 6.0 %, by weight; magnesium, in contents varying between 0.6 % and 1.0 %, by weight; iron, with a content of less than 0.7 %, by weight; manganese, with a content of less than 0.3 %, by weight; nickel, with a content of less than 0.1 %, by weight; zinc, with a content of less than 1.0 %, by weight; titanium, with a content of less than 0.2 %, by weight; strontium, with a content of less than 0.02 %, by weight; and tin, with a content of less than 0.20 %, by weight; the balance, making up 100 % of the weight, consisting of aluminum.

Description

ALUMINIUM-BASE ALLOY FOR CYLINDER HEADS

Automotive engine cylinder heads are usually manufactured by casting process in alloys of the Aluminum-Silicon-Copper class. When the mechanical load imposed on the cylinder heads is extreme, the use of heat treatments becomes necessary as a means of improving the qualities of the alloys.

The present invention concerns an alloy of the Aluminum- Silicon-Copper class improved for the production of the said cylinder heads, presenting better mechanical properties than those obtained in the alloys traditionally used, thus doing away with the need for heat treatment. General Observations about the Invention

Automotive manufacturers traditionally make use of two alloys for the casting of cylinder heads: Al-6%Si-3.5%Cu (a variant of the AA 3 19 alloy) or Al-8.5%Si-3%Cu (a variant of the AA 380 alloy), as described in detail in Table I.

According to former practice, cylinder heads were used in their rough casting state for engines of an older design. In regard to the engines currently used by the automotive industry, however, the properties to be achieved require the use of solution and precipitation heat treatments.

The alloys used for the manufacture of cylinder heads with or without heat treatment are basically the same, differing only in the magnesium contents, lower than 0.20% for non-treated alloys, and lying between 0.30% and 0.50% for treated alloys. This increase in the magnesium content helps the response to the heat treatment, so that increases of hardness and mechanical resistance are obtained.

Table II shows the minimal mechanical property values obtained in these alloys when they are cast in sand molds in the rough casting state and after the solution and precipitation heat treatment (T6).

The results show that the heat treatment raises the limits of resistance, the limit of flow and the hardness of alloys. The heat treatment stage takes a long time and is costly, however, having a significant impact on the component's final cost.

An alternative serving to improve the mechanical properties of the pieces without raising the cost of the final product (without the use of heat treatment) would be to increase the speed of cooling during the solidification of the aluminum alloys. Since most cylinder heads are produced by casting in metal molds, containing some cores of sand in order to form the internal cavities, an increase in the speed of cooling might be obtained by means of the forced cooling of the metal molds. It is important to point out that this procedure improves the properties only of the regions that are in contact with the cooled metal molds, and, even so, its action is limited only to the superficial layer of the piece.

The minimal mechanical properties obtained in surfaces that are in contact with water-cooled metal molds are shown in Table III.

The results in Table III show that the improvements obtained by an increase of the cooling speed are more limited than those obtained from heat treatment. In addition, the procedure of increasing the cooling speed is not applicable to cylinder heads with a complex external geometry, in which the great majority of surfaces is solidified by the contact of the metal with the sand cores, which present a low cooling speed.

The present invention concerns an alloy that is modified in relation to those that are traditionally used in the production of cylinder heads, and presents, in the rough casting stage, mechanical properties similar to those obtained in pieces after the heat treatment.

Basically, the same alloys described in Table I are used, except that they have a higher content of copper (from between 3.0% and 4.0%o to between 4.0% and 6.0%>), and of magnesium (from a maximum 0.5% to between 0.6% and 1.0%).

The principle of the hardening of the alloy owing to the larger copper and magnesium content applies to the low- and high-silicon alloys (the A and B alloys described in Table I). Fundamentals of the Invention

One the whole, the microstructures of the alloys traditionally used in the production of cylinder heads are constituted by the α phase (dendrites), the +Si eutectic, copper-rich eutectics, and intermetallic phases rich in iron, of the Al₅FeSi and Al,₅(Fe,Mn)₃Si₂ kinds.

The alloy that is the subject matter of this invention is described in Table IV.

The present invention can be illustrated by the enclosed figures:

Figures la and lb present typical microstructures of an automotive cylinder head cast in the alloy described in Table IV (with a content of 4.5% of copper and 0.7% of magnesium respectively). Figures 2a and 2b provide a detailed view of the microstructures seen in Figure 1 , showing the eutectics rich in copper and magnesium respectively of the alloy described in Table IV. Figure 3 shows the Brinell hardness figures obtained with test specimens cast in sand molds in accordance with the copper content of the alloys described in Table IV, without magnesium.

Figure 4 shows the Brinell hardness figures obtained with test specimens cast in sand molds in accordance with the magnesium content of the alloys described in Table IV, without copper.

Figure 5 shows the Brinell hardness figures obtained with test specimens cast in sand molds in accordance with a magnesium content ranging from 0.4% to 0.9% of the alloys described in Table IV, with a 4.5% copper content.

Figure 6 shows the differential thermal analysis curve of the traditional B alloy (Table I) (DTA) and its respective derivative (DDTA- hatched line).

Figure 7 shows the differential thermal analysis curves of two specimens of the modified alloy (Table IV) with 4.5% of copper, by weight, and 0.7% of magnesium, by weight (DTA-1 and DTA-2).

Figure 8 shows the reproduction of one of the differential thermal analysis curves of the special modified alloy (Table IV), in which the four solidification stages (described in Table V) have been market out. Figure 9 shows the Brinell hardness evolution curves in alloy test specimens described in Table IV, with a copper weight of 4.2% and a magnesium weight of 0.7%, relative to the three room temperatures (5°C, 22°C and 35°C). For all cases the process of precipitation is more intense in the first 24 hours. Figures la and l b show the general appearance of the microstructure of alloy B with a copper content of 4.5%, by weight, and a magnesium content of 0.7%, by weight, respectively, and Figures 2a and 2b show the detailed appearance of the eutectic that is rich in copper and magnesium respectively.

Based on these two microstructures, one may describe at least two possible hardening mechanisms that come into play with the increase of the copper and magnesium contents in the traditional alloys, as follows:

(i) Hardening by the precipitation of second phases (by means of an increase of the number of phases of high hardness, such as secondary eutectics rich in copper and magnesium);

(ii) Hardening by the natural precipitation of phases rich in copper and magnesium, consistent with the phase α matrix;

Hardening by the precipitation of secondary phases -

Jonason' ", studying the thermal fatigue of aluminum cylinder heads, mentions that the greater volumetric fraction of the hard phases would result in an increase of the alloy's final hardness (and consequent reduction of the elongation and resistance to thermal fatigue).

The aim is, by means of an increase of the copper and magnesium contents, to increase the volumetric fraction of hard phases without impairing the casting characteristics of the alloys.

Colwell and Kissling^<2) studied the addition of magnesium from 0% to 0.6% in aluminum alloys, having observed results showing increasing mechanical resistance.

Hardening by the natural precipitation of coherent phases - Trela^(j) mentions the existence of Al-Zn alloys developed in the 1950s which present a natural hardening, i.e., self aging, without the need for heat treatments. These alloys contain roughly 7% to 8% zinc, by weight; 0.4% magnesium, by weight; and 0.5% to 0.8% copper, by weight. With this composition, the principal difficulty in its use would be the low fusibility. Analogously, Wiss e Sanders'⁴' observed the same phenomenon in the Al-Cu class of alloys (containing roughly 2.6% Cu, by weight, and 0.20% Mg, by weight).

Although no references to this sort of phenomenon have been

5 found in the Al-Si class of alloys, it seems likely that the combination of high contents of zinc+magnesium and/or copper+magnesium in Al-Si alloys may present this same hardening mechanism at room temperature that would result in an increase in the hardness and mechanical resistance of the alloys.

Figure 3 shows the evolution of hardness figures in test

10 specimens cast in sand molds in respect of the alloys described in Table IV, without the presence of magnesium, when the copper content was raised. The figures presented show hardness results right after the casting (on the outset of the natural precipitation), and after 5 days (at the end of the natural precipitation).

1 5 Figure 4 presents the evolution of the hardness figures in sand-mold-cast test specimens in respect of the alloys described in Table IV without the presence of copper, when the content of magnesium was increased. The figures presented show hardness results right after the casting

(on the outset of the natural precipitation), and after 5 days (at the end of the

20 natural precipitation).

The results of Figures 3 and 4 show that both copper and magnesium present a hardening effect on aluminum alloys. In both cases one notes a tendency toward some hardening by natural precipitation. The hardness levels reached, however, show that neither copper nor magnesium o would, on their own, be capable of improving the mechanical properties of the alloys at the desirable rate.

To demonstrate the hardening potential of copper in conjunction with magnesium, Figure 5 presents the evolution of the hardness values with the rising content of magnesium in respect of the alloys described in Table IV with a 4.5% copper content. The figures presented show hardness results right after the casting, and after 2 days.

Summary of the Invention This invention concerns an aluminum-base alloy, the principal elements of which are silicon, copper and magnesium. The limits of chemical composition established for this alloy are described in detail in Table IV.

The main application of this alloy is that of the manufacture of automotive engine cylinder heads in the rough casting state (without heat treatments).

By using the copper and magnesium contents within the limits given in Table IV (copper content between 4.0% and 6.0% and magnesium content between 0.6% and 1.0%), the mechanical properties after casting will be slightly better than those usually found in typical casting alloys. After natural aging lasting at least one day, however, an increase of the hardness and mechanical resistance levels sets in, the final figures drawing close to those obtained in heat-treated casting alloys.

Detailed Description of the Invention

Solidification Characteristics of the Alloy Figure 6 presents a typical differential thermal analysis curve of the traditional B alloy (Table I) and its derived curve (shown in the hatched line).

In the differential thermal analysis curves such as the one in

Figure 6, the solidification reactions are evidenced by increases in the differences of temperature between the sample and the standard, that is, since the figures are negative, the reactions correspond to the slumps in the DTA curve (from the points of maximum values of the DTA to the troughs). The derivative curve can also be used to determine the regions of occurrence of reactions, being represented by values of the derivative above zero (peaks).

Figure 7 presents the differential thermal analysis curves of two samples of the alloy described in Table IV with a copper content of 4.2% and a magnesium content of 0.75%). The curves obtained with the two specimens are very similar, showing the reproducibility of the results.

A comparison between the curves of Figures 6 and 7 shows that the traditional B alloy and the modified B alloy present virtually the same solidification procedure. The beginning and final solidification temperatures of the traditional B alloy are respectively 581°C and 490°C, and of the modified B alloy respectively 588°C and 490°C. The solidification intervals are also similar, being respectively 94°C e 91°C.

The results show, furthermore, that the solidification of the alloys occurs over four distinct stages, as marked out in Figure 8 in respect of the modified B alloy. The occurrence of several stages characterizes a solidification mode of the pasty kind.

Table V shows the principal reactions that occur during the solidification of the alloy described in Table IV.

Owing to the similarity between the curves of Figures 6 and 7, it is likely that the alloys of the traditional B (Table I) and the modified

(Table IV) kinds present the same solidification reactions, for the same products, differing only in the volumetric fractions of the phases that occur at each stage of the solidification.

Table VI presents the volumetric fractions that occur at each one of the solidification stages of the alloys of the traditional B and of the modified kinds with a copper content of 4.5% and a magnesium content of 0.7%.

Kinetics of the alloy's natural precipitation

The B type alloy modified with greater contents of copper and magnesium, as described in Table IV, presents an increase of hardness in accordance with the time elapsed after the casting. This is probably the result of a natural precipitation phenomenon, as observed in other aluminum alloys, such as the 2024 ⁽⁴⁾. This phenomenon is usually explained as being the consequence of the metastability of the maintenance of certain alloying elements in a solid solution in the aluminum in the post-casting state.

Right after the solidification of the piece, a part of the alloying elements, such as those of copper and magnesium, is found to be dissolved in the α phase. During the cooling until room temperature is reached, the solubility of these elements in the α phase diminishes, favoring the precipitation of rich phases in these alloy elements. But, since this cooling is relatively quick, the time available for the diffusion of those elements in the solid state is insufficient for the entire precipitation of the phases, so that this occurs during the first hours after the casting, even at room temperature, and as a consequence of this, there is an increased hardening of the alloy. This phenomenon is known as precipitation or natural aging^<4).

Since the natural precipitation is a thermally-activated phenomenon (as it involves the diffusion of the alloying elements), its kinetics vary with the room temperature.

Figure 9 show the curves of hardness evolution resulting from the natural precipitation in respect of three room temperatures applied. The results are very similar, showing that temperature variations between 5°C and 35°C effect only a very small change in the kinetics of natural precipitation and in the level of hardness reached at the end of the precipitation process.

The results of Figure 9 show, furthermore, that the precipitation process is more intense during the first 24 hours after the casting. After this period, there is only a slight hardness increase. The results of this series show that the natural aging process is responsible for raising the hardness by roughly 10 points on the Brinell scale of the B type alloy with larger contents of copper and magnesium. Typical mechanical properties of the alloy

Table VII presents the typical mechanical results obtained in the production of cylinder heads with the modified B-type alloy, with copper contents of roughly 4.5% and magnesium contents of roughly 0.7%o. The results in respect of the limits of resistance were obtained on the basis of tensile tests of test specimens cast according to the ASTM B 108 standard. The hardness results were obtained by means of direct measurements of the cylinder head faces, on two surfaces: in contact with a metallic mold and in contact with sand cores.

Bibliographical References

1. JONASON, P. - "Thermal Fatigue of Cylinder Head Alloys." Trans. AFS, 1992, pages 601-7.

2. COLWELL, D.L. & KISSLING, R.J. - "Die and Permanent Mold Casting

Aluminum Alloy Minor Elements." Trans. AFS, 1961 , pages 610-5.

3. TRELA, E. - "Aluminum Casting Alloys and Properties." Trans. AFS, 1964, pages 840-9. 4. WYSS, R.K. & SANDERS Jr., R.E. - "Microstructure-Properties Relationship in a 2XXX Aluminum Alloy with Mg Addition." Metallurgical Transactions A, vol. A, October 1988, pages 2523-30.

Claims

1. Aluminum-base alloy for the production of engine cylinder heads without heat treatment, characterized by the fact that it contains as alloying elements: silicon, in contents varying between 5.0%> and 10.0%o, by weight; copper, in contents varying between 4.0% and 6.0%>, by weight; magnesium, in contents varying between 0.6% and 1.0%, by weight; iron, with a content of less than 0.7%, by weight; manganese, with a content of less than 0.3%, by weight; nickel, with a content of less than 0.1 %>, by weight; zinc, with a content of less than 1.0%, by weight; titanium, with a content of less than 0.2%>, by weight; strontium, with a content of less than 0.02%), by weight; and tin, with a content of less than 0.20%>, by weight; the balance, making up 100%) of the weight, consisting of aluminum.

2. Aluminum alloy in accordance with Claim 1 , characterized by the fact that it contains, in addition, sodium, with a content of less than

0.02%, by weight.

3. Aluminum alloy in accordance with Claim 1 , characterized by the fact that it contains, in addition, traces of impurities originating in the ore from which the alloy is made.