EP0447701B1

EP0447701B1 - Reinforced heat resisting member and production method

Info

Publication number: EP0447701B1
Application number: EP90302967A
Authority: EP
Inventors: Yoshihiro Suzuki
Original assignee: Izumi Industries Ltd
Current assignee: Mahle Engine Components Japan Corp
Priority date: 1988-11-14
Filing date: 1990-03-20
Publication date: 1995-08-02
Anticipated expiration: 2010-03-20
Also published as: EP0447701A1; JPH02133534A; DE69021369D1; DE69021369T2

Description

This invention relates generally to a heat resisting aluminium alloy member reinforced locally by inorganic fibers, and particularly but not exclusively is applicable to piston heads, cylinder heads and the like of internal combustion engines.
In general, if a repetition of thermal loads is locally exerted on a member of a structure, cracks are initiated in the member due to repetition of local compression stresses at a higher temperature area and local tension stresses at a cooler temperature of the area, so that the life of the member is shortened.
For example, in an internal combustion engine, to deal with the repetition of thermal loads which is exerted on an aluminium alloy piston head, or the space between valves of an aluminium alloy cylinder head or the like, it has been proposed already to reinforce locally such portions by a metal matrix composite which contains fibrous inorganic reinforcing material, such as a SiC-whisker or a silicon nitride whisker, so as to prolong the life of the aluminium alloy piston etc (for example, see Japanese Laid Open Patent No 62-233456).
However, the thermal expansion coefficient of the reinforced portion is very small compared with that of the non-reinforced body portion, so that the difference of thermal expansion coefficients at the interface between the reinforced portion and the non-reinforced body portion of the member will cause a high stress at the interface at higher temperatures and finally cracks are initiated in the interface under the repetitive thermal loads.
One effective means of avoiding such damage is to enlarge the reinforced portion to keep the interface away from the hottest zone, so that the interface is not exposed to such high temperatures, but consequently the amount of the expensive inorganic fiber to make the reinforced portion increases and as a result the cost of the heat resisting member will be raised.
Accordingly, the present invention is concerned with providing a heat resisting aluminium alloy member with a local metal matrix composite on which cyclic thermal load can be applied, wherein the likelihood of cracks arising under repetition of heat cycles is reduced, and the manufacturing cost is reduced.
In accordance with the present invention there is provided a heat resisting member, comprising: an unreinforced body portion of a first aluminium alloy, and a reinforcing portion of a metal matrix composite of a second material reinforced with inorganic fibres, the second material being aluminium or a different aluminum alloy consisting of:
(a) aluminium; (b) Si, Cu, Ni and Mg each at less than 1% by weight; (c) Fe and Mn as impurities each at less than 0.5% by weight; and (d) other impurities at less than 0.3% by weight.
Patent Applications EP-A-0 170 396 and EP-A-0 106 108 disclose many examples of metal matrix composites including composites in which the metal is aluminium, but neither is concerned with the thermal characteristics of the metal matrix composites or the problem with which the present invention is concerned.
In preferred embodiment of this invention, a volumetric ratio of the inorganic fibers of the metal matrix composite lies within a range of 5 to 25%.
In a piston of a preferred embodiment of the invention, the reinforced portion has a coefficient of thermal expansion of 19.5 x 10⁻⁶/°C and the body portion of JIS:AC8A alloy has a coefficient of 22.3 x 10⁻⁶/°C.
Preferably the reinforcing portion is welded to the body portion.
A specific embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which:

Figures 1 to 6 are explanatory drawings of a process to make a piston according to an embodiment of this invention;
Figure 7 is a graphical representation for a volumetric ratio - tensile strength relationship of the reinforced portion of the piston of Figure 6;
Figures 8 to 11 are explanatory drawings of a process to make a conventional piston; and
Figure 12 is a graphical representation of heat cycle - number of cracks relationship for the three kinds of pistons.

According to several test results to increase the heat shock resistance of metal matrix composites, alloying elements which are added to an aluminium alloy matrix to increase its strength exert rather an unfavorable influence upon crack initiation caused by cyclic thermal shocks, and inorganic fibers in the metal matrix composite produce a very good effect on the crack prevention. That is, when the alloying elements, such as Si, Cu, Ni, Mg and the like, exist each at less than 1%, the elongation, at high temperatures, of the aluminium alloy is very improved. Further, it produces a good effect on the crack prevention if the aluminium alloy contains Fe and Mn, which exist as impurities, each at less than 0.5%, and other impurities at less than 0.3%.
A metallic fiber, a carbon fiber, an alumina fiber, a boric alumina fiber or an alumina-silica fiber can be used as the fibrous inorganic material, and whiskers such as SiC, silicon nitride or boric alumina produce a better effect on the crack prevention. Further, the volumetric ratio of the inorganic fiber is preferably selected within a range of 5 to 25%, because the heat resistant property is hardly improved if the volumetric ratio is less than 5%, and if the volumetric ratio is more than 25%, the thermal expansion coefficient of the metal matrix composite becomes too small, compared with that of the body member, so that cracks are easily initiated in an interface between the metal matrix composite and the body member aluminium alloy due to great difference of the coefficients of expansion between them.
A metal matrix composite was made of an inorganic fiber whose volumetric ratio was selected within the range of 5 to 25%, and an aluminium base metal which contains Si, Cu, Ni and Mg each at less than 1%; Fe and Mn each at less than 0.5%; and impurities at less than 0.3%, and thereafter, was welded to the body portion of a heat resisting member by electron beam welding, friction welding or the like so as to obtain the partially reinforced heat resisting member. Thus, it is easy to make the body portion of the heat resisting member of complicated shape.
Referring to the drawings and table, and initially to Figure 1, a preform 1 was made of SiC-whisker (manufactured by "Tokai-Carbon" Co Ltd and identified by "β-type whisker") so as to have a volumetric ratio V_f of 15%, and is set in a metal mold 2. Then, molten pure aluminium of 99.7% was poured into the metal mold 2 as shown in Figure 2, and a pressure of 800kgf/cm² was applied on the molten aluminium to squeeze the melt into the fine cavities of the whisker preform (Figure 3) to produce the metal matrix composite. The composite was machined to the form 3 in Figure 4. Shown in Figure 7 is a relationship between the volumetric ratio V_f of the reinforcing fiber in the metal matrix composite and the tensile strength of the metal matrix composite.
A piston body 4 to be reinforced by the metal matrix composite 3 was made of aluminium alloy (JIS:AC8A) by gravity casting, and in the piston body 4, a tapered portion 4b is provided on the outlet of the combustion chamber 4a as shown in Figure 5 to fit the metal matrix composite 3 therein. The metal matrix composite 3 was welded to the piston body 4 by electron beam welding (Figure 6).
A piston to be compared with the above piston was made by a conventional process. That is, a preform 11 was made of Sic-whisker (the same as that described above) so as to have a volumetric ratio V_f of 15%, and was set in a metal mold 12 as shown in Figure 8. Then, molten aluminium alloy (JIS:AC8A) was poured into the metal mold 12 (Figure 9), and after the metal mold 12 was closed up tight as shown in Figure 10, the melt was squeezed into fine cavities of the whisker preform under a pressure of 800kgf/cm² to form local metal matrix composite on a piston head. Thereafter, the piston shown in Figure 11 was machined from the casting.
A thermal shock test was conducted to compare the piston of this invention with the conventional piston. The piston was exposed to alternate temperatures of 400 and 150°C, with a cycle period of 12 seconds.
As shown in Figure 12, no crack was found in the piston of this invention even after repetition of 6000 heat cycles, but in the conventional piston and in a piston made of aluminium alloy AC8A only, cracks were found after repetition of 3000 cycles and 1000 cycles, respectively. Further, many cracks were initiated at the interface between the piston body and the outer periphery of the reinforced portion of the conventional piston, but in the piston of this invention, no crack was found at the above interface. It is noted that the lengths of the outer periphery and the inner periphery are 60mm and 50mm, respectively.
According to Table 1, the coefficient of expansion of the piston body is nearer to that of the reinforced portion of the piston of this invention than to that of the reinforced portion of the conventional piston. This seems to be a reason why the piston of this invention shows no crack at the interface between the reinforced portion and the body portion.
Having described an illustrative embodiment of this invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to that precise embodiment, and that various changes and modifications may be effected therein by one skilled in the art.
In the embodiment, the invention is applied to a piston of an internal combustion engine, but it is widely applicable to such members which are exposed to a cyclic local thermal load as to be locally exposed to the repetition of heat cycles. Further, in the embodiment, the composite material 3 is fixed to the piston body 4 by electron beam welding, but it can be fixed also by friction welding.
The matrix alloy of the reinforced portion contains only a small amount of alloying elements, which are added normally to aluminium alloy members but have negative effects on thermal shock resistance, in order to attain the best thermal shock resistance of the metal matrix composite which contains inorganic fibrous material as a reinforcing material. As silicon which reduces the thermal expansion coefficient of aluminium alloys is not included to a significant extent in the reinforced portion of the heat resisting member, the thermal expansion coefficient of the reinforced portion increases, resulting in a smaller difference of the coefficients of expansion between the body portion and the reinforced portion of the heat resisting member, so that no crack is initiated in the interface between the body portion and the reinforced portion of the member.

Moreover, the composite material and the body portion of the heat resisting member are made separately, so that the body portion can be molded by gravity casting. Therefore, it is easy to reduce the manufacturing cost of the member.

Table 1

Coefficient of Expansion (x 10⁻⁶/C°) within a Range of 20 to 300°C
Piston body, common to the two kinds of tested pistons, of aluminum alloy AC8A	22.3
Reinforced portion of the conventional piston	16.1
Reinforced portion of the piston of this invention	19.5

Claims

A heat resisting member, comprising: an unreinforced body portion (4) of a first aluminium alloy, and a reinforcing portion (3) of a metal matrix composite of a second material reinforced with inorganic fibers, the second material being aluminium or a different aluminum alloy consisting of: (a) aluminium; (b) Si, Cu, Ni and Mg each at less than 1% by weight; (c) Fe and Mn as impurities each at less than 0.5% by weight; and (d) other impurities at less than 0.3% by weight.
A heat resisting member according to claim 1, wherein a volumetric ratio of the inorganic fibers in the metal matrix composite lies within a range of 5 to 25%.
A member as claimed in claim 1 or 2, wherein the reinforcing portion is welded to the body portion.
A heat resisting member as claimed in any preceding claim, in the form of a piston or a cylinder head of an internal combustion engine.