Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/02—Alloys based on aluminium with silicon as the next major constituent

Definitions

• the invention relates to aluminum alloys for parts subjected to high thermal and mechanical stresses, in particular the pistons of internal combustion engines, and more particularly turbo-charged gasoline or diesel engines.
• aluminum alloys with a high silicon content (10 to 24%) are usually used to facilitate their moldability and to give them good wear resistance.
• their composition systematically includes an addition of magnesium and copper hardeners, soluble at high temperature, but poorly soluble at room temperature. This double addition gives the alloy, by a complete heat treatment in the T6 or T7 state or a simple T5 state, good mechanical strength at room temperature.
• the typical magnesium contents are between 0.3 and 1.5% and those in copper between 0.3 and 5%.
• patent FR 2690927 in the name of the applicant, filed in 1992 describes creep resistant aluminum alloys containing from 4 to 23% of silicon, at least one of magnesium elements (0.1 - 1%) , copper (0.3-4.5%) and nickel (0.2-3%), and 0.1 to 0.2% titanium, 0.1 to 0.2% zirconium and 0.2 to 0.4% vanadium.
• An improvement in the creep resistance at 300 ° C was observed without significant loss of elongation measured at 250 ° C.
• Another patent, filed by Toyota in 1996 discloses an alloy for application to a piston of an internal combustion engine, composed of 13 to 25% of silicon and particularly charged with copper and magnesium (8 to 22% copper, 0.2 to 1.5% magnesium) with additionally 0.3 to 1.5% iron, 0.1 to 2.5% manganese, 0.01 to 1.5% titanium, 1 to 5% nickel, 0.002 to 0.4% phosphorus, other impurities , aluminum balance.
• a patent application also filed by Toyota, but in 1998 JP2000 / 054053 discloses a method of manufacturing alloy for piston leading to a maximum grain size of 200 microns, and a composition also loaded both copper and aluminum. 'magnesium (3.0 to 10.0% copper and 0.7 to 1.3% magnesium with 1.5 to 3.0% nickel, 1.0% iron at most, 0.5 to 1.0% manganese, 0.05 to 0.3% vanadium and 12 to 14% of silicon).
• NASH has more recently filed several patents also on alloy compositions with high silicon content, copper and magnesium also containing titanium, zirconium, vanadium ....
• the application US 2001/0010242 A1 relates to a process for manufacturing a part molded from an alloy of composition: Si: 14.0-25.0%, Cu: 5.5-
• Another route is the selection of low magnesium alloys associated with relatively moderate levels of copper.
• the object of the present invention is to further improve the mechanical strength and the creep resistance, in the temperature range 230-380 ° C., of parts subjected locally to such temperatures, in particular the pistons of internal combustion engines, including turbo-charged and direct injection engines.
• the subject of the invention is an aluminum alloy for a piece having to have a high mechanical resistance when hot, of composition (% by weight):
• FIG. 1 shows a section of a piston with the piston head at 1, the piston skirt at 2, the piston bowl at 3, the bowl edge at 4, and the spindle boss at 5.
• FIG. 2 represents the percentage of pistons having a crack of more than 1.5 mm at the edge of the bowl (in the ordinate), as a function of the number N of thermal cycling of 380 to 150 ° C. (in the abscissa) during the simulator tests. the thermal fatigue stress at the edge of the bowl.
• FIG. 2 represents the percentage of pistons having a crack of more than 1.5 mm at the edge of the bowl (in the ordinate), as a function of the number N of thermal cycling of 380 to 150 ° C. (in the abscissa) during the simulator tests. the thermal fatigue stress at the edge of the bowl.
• FIG. 3 shows in ordinate the limit pressure, in bars (10 5 N / m 2 ), deduced from the simulation of mechanical stress stressing of the pin bosses (in English "pin boss pulsator") on pistons cast with the various alloys indicated on the abscissa, these being maintained at a temperature of 250 ° C. for 20 h.
• Figure 4 shows the mechanical tensile characteristics at room temperature, obtained on dissecting specimens in pistons poured with the various alloys indicated on the abscissa, on the ordinate and in N / mm 2 on the left for the yield strength, in% and in the right for the elongations at break.
• the invention is based on the finding made by the applicant that it is possible to obtain heat-resistance properties, especially between 230 ° C. and 430 ° C., which are significantly improved over existing alloys by combining, in an alloy Al-Si type molding, 4 to 9% copper content and low magnesium content.
• the alloys according to the invention have, as their main characteristics, a relatively moderate silicon content of 8 to 13%, associated with a high copper content (4 to 9%), a relatively high nickel content (1.5 to 4% ) and a very limited zirconium content. More specifically, the alloys according to the invention have the following composition: Si: 8 to 13%, Fe ⁇ 1.0%, Cu: 4 to 9%, Mg: ⁇ 0.5%, Mn ⁇ 1%, Zn ⁇ 1, 0%, Ni: 1 to 4%, Ti ⁇ 0.3%, P: 0.001 to 0.05%, V ⁇ 0.3%, Zr ⁇ 0.05%.
• composition compared to those described in the prior art, in particular in the case of application to pistons, provides a solidification structure of the eutectic intermetallic phases AlCuNi, A13Ni, ... little segregated and good homogeneity throughout the room and, with the conventional addition of phosphorus, a hypereutectic structure of primary silicon of slightly elongated shape and distributed regularly.
• this type of composition also significantly improves the resistance to hot mechanical fatigue, which is necessary in particular in the regions of the axis bosses, by virtue of obtaining, during the heat treatment, a microstructure comprising phases metastable copper type ⁇ '- ⁇ "derived from the precipitation system Al 2 Cu.
• phase phases are more stable at moderate and high temperatures (180 to 300 ° C.) than the ⁇ ' ⁇ "binary phases based on Mg 2 Si and the quaternary phases ⁇ ' ⁇ " AlCuMgSi, which are formed during the incipient income. high amounts of magnesium and limited copper contents.
• the copper content is kept below 1%, which means that they may be alloys of first or second fusion; this limit can be lowered below 0.3% (first melting), and preferably below 0.2% when a high elongation at break is desired.
• the titanium content is maintained between 0.03 and 0.3%, which is quite usual for this type of alloy.
• compositions chosen according to the invention is indeed the possibility of obtaining ⁇ -Al dendritic structures that are varied and controllable according to the zones of the piston.
• an equiaxed dendritic structure refined at the bottom of the piston favors the resistance to hot mechanical fatigue of this zone.
• the alloys according to the invention make it possible, by adapting the local dendritic structure, to optimize the resistance of the piston both in thermal fatigue and mechanical fatigue (bowl edge and axis boss).
• the zirconium content is kept below 0.05%, since this element refines the grain and prevents obtaining a columnar or herbaceous structure precisely sought in hot areas.
• the alloy contains nickel at a high level of 1 to 4%, which contributes to the heat resistance. It may also comprise vanadium at a content of less than 0.30%, and preferably between 0.04 and 0.20%. At a content of more than 0.1%, manganese has a positive effect on the mechanical strength between 230 ° C and 43O 0 C, but this effect then caps, which explains its limitation to 1% maximum.
• the limitation of the magnesium content to less than 0.5%, preferably 0.4% and even more advantageously 0.3% has the advantage of substantially reducing the oxidation of the alloy, this facilitating 1 Obtaining particularly sound parts (few inclusions of oxides, shrinkage, gassing porosity), resulting in a much better consistency of performance in thermal fatigue and mechanical fatigue
• the limitation of the magnesium content to less than 0.1% confers additional advantages: -It allows obtaining at the top of the piston, subjected to high loads of thermal fatigue at high temperature (350 ° C-430 ° C), a structure comprising a large fraction of eutectic phases Al-Cu and Al-Ni-Cu without magnesium, particularly favorable to good mechanical behavior at high temperature. With the choice of a high copper content according to the invention, it has no negative impact on the level of curing at room temperature, nor on the fatigue strength, thermal and mechanical.
• the alloys with a very low magnesium content have a solidus temperature and a temperature of burn above 507 ° C. They can therefore be heat treated in the T6 or T7 state with a dissolution temperature of between 515 and 525 ° C. depending on the copper content, and this without any particular precaution, that is to say without necessity. a slow rise in temperature or an intermediate stage, while the alloys of the same type with more than 0.5% magnesium form a quaternary eutectic invariant with the risk of a burn at 508 ° C.
• the possibility of carrying out a heat treatment at more than 515 ° C has several advantages: it is possible to obtain a further homogenization of the copper phases, a better globulization of the silicon phases and a more complete precipitation of the titanium and vanadium peritectic phases. .
• the alloys according to the invention are suitable for the usual molding processes, in particular gravity mold casting and low pressure shell molding, but also sand casting, squeeze casting (particularly in the case of insertion of composites) and lost-foam molding. They may be suitable for other forming processes such as, but not limited to, stamping, forging, "casting-molding" (Cobapress®) or molding in the semi-solid state.
• the heat treatment of the T7 type may comprise dissolution typically from 15 minutes to 10 hours at a temperature of between 515 and 525 ° C., quenching preferably with cold water or softened quenching, and over-tempering. 0.5 to 10 h at a temperature between 220 and 28O 0 C. If, for particular reasons, it is desired to avoid too much globulization of the eutectic silicon network, it is preferable to carry out the dissolution at a lower temperature, between 490 and 515 ° C.
• the alloys according to the invention and in particular in the case of application to pistons of automobile or aircraft engines, have excellent mechanical resistance to heat and creep resistance greater than that of the alloys of the prior art in the temperature range 230-430 ° C.
• the alloys according to the invention can obviously contain impurities at the level encountered either in first fusion (alloys resulting from electrolysis without recycling), second melting (alloys resulting from recycling), or at a purity level. intermediate between the two previous ones.
• compositions have been used to produce, by molding according to the process
• the temperature is generally set at 50 ° C.
• a calculation makes it possible to determine the maximum combustion pressure at which the bosses can withstand. It is the latter which appears on the ordinate in FIG. 3, in this case for a hold of 20 h at 250 ° C.
• the pistons were previously subjected to a heat treatment involving solution with a plateau of 60 minutes at 500 ° C., quenching and tempering for 5 hours at 230 ° C.
• Figure 4 shows the tensile test results at room temperature on dissection specimens.
• the alloy C according to the invention has characteristics of the same level as those of the standard alloy A reference.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Pistons, Piston Rings, And Cylinders (AREA)
• Pressure Welding/Diffusion-Bonding (AREA)

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• 2004
  • 2004-11-26 FR FR0412579A patent/FR2878534B1/fr not_active Expired - Fee Related
• 2005
  • 2005-11-23 EP EP05820753A patent/EP1815036A2/de not_active Withdrawn
  • 2005-11-23 WO PCT/FR2005/002909 patent/WO2006056686A2/fr active Application Filing

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