Aluminium alloy containing silicon for use as pistons in automobiles

Abstract: An improved aluminium - silicon alloy for use in the manufacture of pistons is disclosed. The improved alloy is of the following composition in which the component elements are 10.5 to 13.5 silicon 2.0 to less than 4.0 copper 0.8 to 1.5 magnesium 0.5 to 2.0 nickel 0.3 to 0.9 cobalt at least 20 ppm phosphorous and either (i) 0.05 to 0.2 titanium; or (ii)at least one of the following up to 0.2 zirconium up to 0.2 vanadium; in either case with the balance Aluminium and unavoidable impurities.

Description: The present invention relates to a aluminium-silicon alloy. The alloy has use in the manufacture of pistons, in particular for pistons for use in internal combustion engines. 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 sub-zero 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. Known casting alloys for piston manufacture include those disclosed in Japanese Patent Application JP1108339-A. Among the alloys disclosed therein is an aluminium based alloy including silicon from 9 to 12 percent, copper from 0.5 to 2.5 percent, magnesium from 0.8 to 2.0 percent, cobalt from 0.5 to 3 percent, nickel from 1 to 3 percent, iron from 0.3 to 1.0 percent, manganese from 0.1 to 1.0 percent and titanium from 0.01 to 0.15 percent with the balance being aluminium and unavoidable impurities. This alloy is said to provide high strength from 150 to 250 °C. Another alloy is disclosed in DE 44 04 420 which may have application as an alloy for use in the manufacture of pistons for an automotive vehicle. The alloy comprises from 8.0 to 10.9 percent silicon, from 0.8 to 2.0 percent magnesium, from 4.0 to 5.9 percent copper, from 1.0 to 3.0 percent nickel, from 0.2 to 0.4 percent manganese, less than 0.5 percent iron and at least one element from the group including antimony, zirconium, titanium, strontium, cobalt, chrome and vanadium whereby at least one element is present in an amount greater than 0.8 percent and the sum of the elements in this group is no more than 0.8 percent, with the balance of the alloy being aluminium and unavoidable impurities. The present invention has as an advantage that it provides good strength over the range of temperatures in which a piston made from the alloy operates. 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: 10.5 to 13.5 silicon 2.0 to less than 4.0 copper 0.8 to 1.5 magnesium 0.5 to 2.0 nickel 0.3 to 0.9 cobalt at least 20 ppm phosphorous and either (i) 0.05 to 0.2 titanium; or (ii) at least one of the following up to 0.2 zirconium up to 0.2 vanadium; in either case with the balance Aluminium and unavoidable impurities. Preferably Zinc, Lead and Tin may also be present up to 0.15 weight percent. More preferably, the total amount of Lead and Tin may not exceed 0.15 weight percent. The Copper allows age hardening of the alloy. An increase the amount of Copper beyond the limit stated reduces the high temperature fatigue strength of the alloy due to the generation of larger Copper-Nickel rich intermetallic crystals. An increase in the Copper level can also lead to shrinkage porosity problems in larger castings. The Magnesium is present to contribute to the strength of the alloy. However, increasing the amount of Magnesium will lead to larger intermetallics in the as cast condition, but after aging these will be removed. The Magnesium is added at a level where it provides good strengthening after aging through the formation of Mg 2 Si. Higher Magnesium levels lead to greater oxidation losses during the casting process, and thus a greater tendency for the cast metal to contain oxide defects. The Nickel contributes to the high temperature strength of the alloy by the formation of thermally stable intermetallic crystals. However, exceeding the limits stated will reduce the high temperature fatigue strength of the alloy due to the precipitation of coarse intermetallic crystals. This tendancy is made worse by the low cooling rates associated with the casting of large pistons. 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 the levels stated 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. The Titanium or Zirconium and/or Vanadium are each present as a grain refining addition. Preferably, the alloy may at the expense of aluminium optionally include at least one of the following up to 0.5 iron up to 0.25 manganese up to 0.05 chrome, and up to 15 ppm each of calcium, sodium, strontium and lithium. The present invention will now be described, by way of example only, with reference to the following Illustrative Examples. Example 1 The use of a specific alloy composition in the manufacture of forged pistons has proven to be particularly advantageous. The metal alloy compositions of this alloy with the component elements being indicated in weight percent are as follows: 10.5 to 13.5 silicon 2.0 to less than 4.0 copper 0.8 to 1.5 magnesium 0.5 to 2.0 nickel 0.3 to 0.9 cobalt 0.05 to 0.2 titanium at least 20 ppm phosphorous with the balance Aluminium and unavoidable impurities. Example 2 Another alloy within the scope of the present invention also found to have utilty in the manufacture of pistons has the following composition with the component elements being indicated in weight percent as follows: 10.5 to 13.5 silicon 2.0 to less than 4.0 copper 0.8 to 1.5 magnesium 0.5 to 2.0 nickel 0.3 to 0.9 cobalt at least 20 ppm phosphorous; and at least one of the following up to 0.2 zirconium up to 0.2 vanadium; with the balance Aluminium and unavoidable impurities. Example 3 Another alloy within the scope of the present invention found to have utilty in the manufacture of pistons has the following composition with the component elements being indicated in weight percent as follows: 10.5 to 11.5 silicon 2.5 to 3.5 copper 0.8 to 1.5 magnesium 0.5 to 1.5 nickel 0.3 to 0.7 cobalt 0.05 to 0.20 titanium up to 0.2 zirconium up to 0.2 vanadium up to 0.50 iron up to 0.25 manganese up to 0.05 chrome up to 0.15 zinc up to 0.15 lead up to 0.15 tin; the total of lead and tin not to exceed 0.15 at least 20 ppm phosphorous up to 15 ppm each of calcium, sodium, strontium and lithium with the balance Aluminium and unavoidable impurities 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: An aluminium alloy in which the component elements are present in weight percent as follows: 10.5 to 13.5 silicon 2.0 to less than 4.0 copper 0.8 to 1.5 magnesium 0.5 to 2.0 nickel 0.3 to 0.9 cobalt at least 20 ppm phosphorous and either (i) 0.05 to 0.2 titanium; or (ii) at least one of the following up to 0.2 zirconium up to 0.2 vanadium; in either case with the balance Aluminium and unavoidable impurities. An alloy according to claim 1, characterised in that Zinc, Lead and Tin may also be present up to 0.15 wt% as unavoidable impurities. An alloy according to claim 2, characterised in that the total amount of Lead and Tin does not exceed 0.15 wt%. An alloy according to any of claims 1 to 3, characterised in that the alloy, at the expense of aluminium, may optionally include at least one of the following up to 0.5 iron up to 0.25 manganese up to 0.05 chrome, and up to 15 ppm each of calcium, sodium, strontium and lithium. A piston manufactured from an alloy according to any previous claim. A piston according to claim 5, characterised in that the piston is manufactured by forging. A piston according to claim 5, characterised in that the piston is manufactured by gravity die casting.

Inventors: BARNES SIMON, GAZZARD SIMON THOMAS, PHILBY JONATHAN DAVID, BARNES, SIMON, GAZZARD, SIMON THOMAS, PHILBY, JONATHAN DAVID
Applicants: FEDERAL MOGUL BRADFORD LTD, FEDERAL-MOGUL BRADFORD LIMITED
Application year: 1998
Main classification: C22C21
IPC codes: C22C21, F02F7, F02F3