Aluminium alloy

Description

[0001] The present invention relates to an aluminium-silicon alloy. The alloy has use in the manufacture of pistons, in particular for use in internal combustion engines.

[0002] A satisfactory piston material must meet many differing requirements. In use, pistons are exposed to both static and dynamic stresses, while operating in bulk temperatures from subzero to up to 400°C. These stresses will also differ in different regions of the piston, for example a combustion bowl in a piston will be subject to different thermal and mechanical stresses than piston pin bosses. The piston must also have low thermal expansion, and possess good bearing characteristics with marginal lubrication over the noted range of temperatures. Also, the piston material must lend itself to being formed into a piston, for example by casting with subsequent working.

[0003] Known casting alloys for piston manufacture include those disclosed in US 3 765 877. Among the alloys disclosed therein is an aluminium based alloy including silicon from 7 to 20 percent, copper from 3.5 to 6 percent, up to 2.5 percent nickel, from 0.1 to 0.6 percent magnesium, and from 0.1 to 1.0 percent silver with the balance being aluminium and unavoidable impurities.

[0004] The present invention has as an advantage improved boss strength and also improved high temperature strength in the region of the piston crown.

[0005] The alloy of the present invention is selected from a group of aluminium alloys with each alloy component element being present in weight percent as follows:

Si = 10.5 - 13.5

Cu = 2.0 - 4.0

Ni = 0.5 - 2.0

Mg = 0.8 - 1.5

Ag = 0.2 - 0.6

Co = 0.2 - 0.6

P at least 20 ppm,

and balance Al and unavoidable impurities.

[0006] The Nickel content in this alloy is believed to lead to the formation of thermally stable intermetallics. This in turn leads to high temperature strength for the alloy. More than 2 wt% Nickel leads to the formation of large NI rich intermetallics, particularly for the low cooling rates associated with large piston castings, which are seriously detrimental to the high temperature fatigue strength of the alloy.

[0007] The Cobalt content is chosen to allow the formation of a large number of small intermetallics. This is believed to improve the mechanical properties of the alloy at 350°C. In addition, the presence of the Cobalt in the Aluminium alloy at a level in excess of 0.2 wt% is believed to reduce the diffusivity of the Copper in Aluminium, thereby slowing the overaging mechanism of the alloy. This has particular importance when considering the operation of a piston pin boss operating at around 200°C. However, the presence of the Cobalt is believed also to lead to an increase in fatigue strength of the alloy at 350°C. This is of particular importance when considering the operation of a combustion bowl of a piston which is typically subject to such temperatures.

[0008] The Silver is believed to give improved boss strength to a piston manufactured from this alloy. In particular, up to 0.6 weight percent Silver increases the fatigue strength of the alloy at 200°C. The thermal conductivity of the alloy at 350°C is raised without adversely affecting the expansion coefficient. The addition of Silver in this range does not appear to cause castability problems.

[0009] Zinc, Lead and Tin may each also be present in amounts up to 0.1 wt%.

[0010] The alloy may additionally comprise at least one of the following component elements

up to 0.2 wt% Ti

up to 0.2 wt% Zr

up to 0.2 wt% V.

[0011] The Titanium, Zirconium and/or Vanadium are each believed to act as grain refining additions in the alloy.

[0012] Preferably, the Copper content is at least 2.5 wt %, and may be less than 3.5 wt %.

[0013] The present invention will now be described, by way of example only, with reference to the following Illustrative Examples.

EXAMPLE 1

[0014] The use of a specific alloy composition in manufacture of forged pistons has proven to be particularly advantageous. The metal alloy compositions of a first alloy according to the present invention with the component elements being indicated in weight percent are:

Si = 10.5 - 13.5

Cu = 2.0 - 4.0

Ni = 0.5 - 2.0

Mg = 0.8 - 1.5

Ag = 0.2 - 0.6

Co = 0.2 - 0.6

P at least 20 ppm,

and balance Al and unavoidable impurities.

EXAMPLE 2

[0015] A second alloy according to the invention has a similar composition to the first alloy save that the nickel is present from 0.5 to 1.5 wt%.

EXAMPLE 3

[0016] A third alloy according to the present invention has a similar composition to the first and second alloys, but may additionally comprise at least one of the following component elements:

up to 0.1 wt% zinc,

up to 0.1 wt% lead,

up to 0.1 wt% tin.

EXAMPLE 4

[0017] A fourth alloy according to the present invention has a similar composition to that of Example 3, but may additionally comprise at least one of the following component elements:

up to 0.1 wt% titanium;

up to 0.1 wt% zirconium;

up to 0.1 wt% vanadium.

EXAMPLE 5

[0018] A fifth alloy according to the present invention with the component elements being indicated in weight percent are

Si = 10.5 - 11.5,

Cu = 2.75 - 3.25,

Ni = 0.8 - 1.2,

Mg = 0.9 - 1.2,

Ag = 0.45 - 0.55,

Co = 0.25 - 0.35,

Ti = 0.14 - 0.19

Pb = up to 0.1

Sn = up to 0.1, the total amount of Pb & Sn not exceeding 0.1

between 50 - 100 ppm P,

and balance Al and unavoidable impurities.

EXAMPLE 6

[0019] A sixth alloy according to the present invention has a similar composition to fifth alloy according to the present invention, but may additionally comprise at least one of the following component elements:

up to 0.35 wt% iron;

up to 0.15 wt% manganese;

up to 0.1 wt% zinc;

up to 0.03 wt% chrome.

[0020] The alloys of the present invention, in addition to their use in the manufacture of forged pistons, may be used in the manufacture of gravity die cast pistons.

Claims

1. An aluminium alloy in which the component elements are indicated in weight percent comprising:

Si = 10.5 - 13.5

Cu = 2.0 - 4.0

Ni = 0.5 - 2.0

Mg = 0.8 - 1.5

Ag = 0.2 - 0.6

Co = 0.2 - 0.6

P at least 20 ppm,

and balance Al and unavoidable impurities.

2. An alloy according to claim 1, characterised in that the element Nickel is present in the range 0.5 to 1.5 wt%.

3. An alloy according to claim 1 or claim 2, characterised in that the alloy may additionally comprise at least one of the following component elements:

up to 0.1 wt% Zn

up to 0.1 wt% Pb

up to 0.1 wt% Sn.

4. An alloy according to any previous claim, characterised in that the alloy may additionally comprise at least one of the following component elements

up to 0.2 wt% Ti

up to 0.2 wt% Zr

up to 0.2 wt% V.

5. An alloy according to claim 4, characterised in that the component elements in weight percent comprise:

Si = 10.5 - 11.5,

Cu = 2.75 - 3.25,

Ni = 0.8 - 1.2,

Mg = 0.9 - 1.2,

Ag = 0.45 - 0.55,

Co = 0.25 - 0.35,

Ti = 0.14 - 0.19

Pb = up to 0.1

Sn = up to 0.1, the total amount of Pb & Sn not exceeding 0.1

P between 50 - 100 ppm,

and balance Al and unavoidable impurities.

6. An alloy according to claim 5, characterised in that the alloy may additionally comprise at least one of the following component elements:

up to 0.35 wt% Fe

up to 0.15 wt% Mn

up to 0.1 wt% Zn

up to 0.03 wt% Cr

and up to 10 ppm each of Ca, Na, Sr and/or Li.

7. A piston manufactured from an alloy according to any previous claim.

8. A piston according to claim 7, characterised in that the piston is manufactured by forging.

9. A piston according to claim 7, characterised in that the piston is manufactured by gravity die casting.