GB2300198A - Aluminium alloy - Google Patents

Abstract

An alloy having the composition in wt % Si 10.5-12.5 Cu 2.8-4.0 Mg 1.1-1.6 Ni 1.0-1.8 Fe 0.3 to 1.0 Mn up to 0.5 B, V, Zn, Cr, Ti up to 0.2 each, 0.5 total Si modifier Functional concentration Balance Al with incidental impurities up to 0.05 each, 0.15 total, is extruded and forged to make a piston for a motor vehicle engine.

Description

ALUMINIUM ALLOY This specification describes a new aluminium alloy composition having good high temperature stability, good wear resistance, strength and stiffness, and a low coefficient of thermal expansion. The alloy is particularly suitable for the manufacture of forged pistons for automobile engines. Other uses are also envisaged.

Pistons in automobiles endure very demanding conditions.

They must be as light as possible, since this reduces the moving mass of the engine. Aluminium has obvious advantages here. Pistons operate at temperatures of 150 - 300"C where most aluminium alloys soften or become unstable. High wear resistance is required, especially in modern engines using unleaded fuel. Design considerations require high strength, high stiffness and low coefficient of thermal expansion. Existing aluminium alloys are used for pistons, but there is a need for alloy having higher properties that is capable of being formed by casting to shape or by forging extruded bar.

Alloys such as AA261 8A (AA is Aluminum Association Inc Register) have good high temperature thermal stability, but are not sufficiently wear resistant. AA 4032 is highly wear resistant (see US Patent 5,123,973) but lacks adequate high temperature strength.

According to the present invention there is provided an alloy having the composition in wt % Si 10.5-12.5 preferably 11.0-12.0 Cu 2.8-4.0 preferably 3.0- 3.6 Mg 1.1-1.6 preferably 1.2-1.4 Ni 1.0-1.8 preferably 1.1-1.5 Fe 0.3 to 1.0 preferably 0.3 to 0.6 Mn up to 0.5 preferably up to 0.25 B, V, Zn, Cr, Ti up to 0.2 each, 0.5 total Si modifier functional concentration Balance Al with incidental impurities up to 0.05 each, 0.15 total The silicon contributes to the wear resistance and low coefficient of thermal expansion of the alloy. Copper contributes to alloy hardness, both initially and after heat treatment. Magnesium contributes to hardness and machinability. Nickel improves machinability, lowers the coefficient of thermal expansion and increases the initial hardness.Iron is present in these aluminium alloys1 but its concentration needs to be controlled to avoid the risk of forming large eutectic needles on casting which cannot be removed by homogenisation. Precipitates containing Cu, Mg, Fe, Si, Mn and Ni all contribute to the high temperature strength of the alloy. These may form during homogenisation and/or during solution treating or ageing.

These alloys are preferably fabricated into workpieces, for example pistons for motor vehicle engines, by the following processing route. Alternatively, it is envisaged that the alloys can be fabricated by casting, e.g. by pressure die casting.

To a melt of the required composition is added an Si modifier such as As, Sb, Na, Ca or preferably Sr. Sr (or other modifier) may be added to the melt by rod injection into the launder. On casting, this strontium (or other modifier) causes the silicon phase to form as spheroids (i.e. particles having three orthogonal dimensions of roughly comparable size) rather than plates. The amount of Sr required for this purpose is typically 0.01-0.1% e.g. 0.01 - 0.07%; of Na, 0.004 - 0.012%; of Ca, 0.01 - 0.07%; of Sb or As, 0.05 - 0.15%.

The treated melt is cast, e.g. by direct chill casting. The resulting billet, in which an Si phase is present as spheroids rather than plates, is homogenised. Homogenisation is preferred for castings and necessary before extrusion. Homogenisation is at 450 - 515"C, preferably 465 - 475"C for a time to precipitate Fe, Ni, Mn, Cr, e.g. 0.5 - 12 hours preferably about 4 hours.

The resulting billet is hot extruded. The extruded sections may be quenched or slowly cooled and are cut into lengths for forging at 400 - 470 C.

The forged workpieces are solution heat treated, e.g. at 480 520"C for at least one hour, and cooled quickly. The workpieces are finally aged by heating at 140 - 200"C for a period of at least one hour.

EXAMPLE 1 Two 12" diameter billets were made by the D.C. Casting process to the compositions in Table 1.

Before casting the alloys were modified by the addition of Sr (Strontium) at a rate of 4 kgfton of 10% Sr alloy key, and degassed in the holder for 30 minutes with Argon only.

Both alloys were homogenised for 4 hours at 475"C j 5"C and air cooled.

Alloy 2 containing the high levels of iron and manganese was found to have large intermetallic phases that could not be removed by homogenisation. These phases took the form of large eutectic needles some 500 pm long and most probably of the form FeAISis, Fe2SiAl9, (FeMn)3Si2Als, FeNiAlg. The presence of these coarse needles rendered the alloy brittle and therefore this composition was changed, to that of alloy 1. Microstructural evaluation of alloy 1 revealed the absence of these needles and a uniformly modified Si phase.

Table 1

Si Fe Cu Ni Mg Cr Zn Zr Mn Alloyl 11.41 0.43 3.43 1.18 1.29 0.01 0.05 0.01 0.03 Alloy2 10.5 1.4 2.8 1.08 1.1 0.40 EXAMPLE 2 Alloy 1 was extruded to a 100 mm dia bar using the following conditions: Billet temp 460"C, Extrusion speed 2 - 3 m/min, air cool to room temperature. The bar was cut in to 100 mm lengths forged into pistons for car engines. Forging was carried out at 460"C, air cooled.

The pistons were solution treated for 2 hours at 505, 510 or 515"C and aged under the conditions given in Table 2. Hardness was measured using a Vickers hardness test with a 10 kg load.

Table 2

SHT AGE 505"C 51 00C 51 5"C 6 Hours165"C 130.3 136 146 8 Hours 165"C 133.5 138 146 9.67 Hours 165"C 133 138 145 12 Hours 165 C 136.6 137 148.3 Note: When solution treated at 520 C for 2 hours water quenched and aged by heating for 6 hours at 175 C the hardness fell to 110 indicating overheating.

Tensile properties are given in Table 3. Longitudinal samples were taken parallel to the original extrusion direction which now lies along the length of the piston. Transverse samples were taken tangential to the circumference.

The pistons were solution treated at 505 C for 2 hours and quenched into water at 80 C before ageing under the conditions shown in Table 3.

Table 3

UTS MPa Proof MPa Elong % Trans Long Trans Long Trans Long 12 Hours 378 385 316 311 2.6 2.92 1600C 16 Hours 374 386 305 312 2.3 3.5 1600C 6 Hours 387/390 3931390 333/323 318/329 2.412.7 3.0/3.3 175 C

Claims (7)

1. CLAIMS 1. An alloy having the composition in wt % Si 10.5-12.5 Cu 2.8-4.0 Mg 1.1-1.6 Ni 1.0-1.8 Fe 0.3 to 1.0 Mn up to 0.5 B, V, Zn, Cr, Ti up to 0.2 each, 0.5 total Si modifier Functional concentration Balance Al with incidental impurities up to 0.05 each, 0.15 total.
2. 2. An alloy as claimed in claim 1 wherein the proportions are Si 11.0-12.0 Cu
3. 3.0 - 3.6 Mg 1.2-1.4 Ni 1.1-1.5 Fe 0.3toO.6 Mn up to 0.25 3. A cast billet of the alloy claimed in claim 1 or claim 2, wherein an Si phase is present as spheroids rather than plates.
4. 4. An extruded section of the alloy of any one of claims 1 to 3.
5. 5. A workpiece forged from the extruded section claimed in claim 4.
6. 6. A workpiece as claimed in claim 5 which is a piston for a motor vehicle engine.
7. 7. A workpiece as claimed in claim 6 having the characteristics Vickers hardness (10 kg load) at least 130 Ultimate tensile strength in both longitudinal and transverse direction, at least 350MPa.

   Elongation at break in both longitudinal and transverse directions, at least 2.0%.