DESCRIPTION

The invention relates to a compressor impeller for a turbocharger, particularly in a diesel engine, according to the preamble of claim 1, to an exhaust gas turbocharger with a compressor impeller, according to the preamble of claim 7, and also to a method for producing the compressor impeller according to the invention, according to the preamble of claim 12.

Exhaust gas turbochargers are systems for increasing the power of piston engines. In an exhaust gas turbocharger, the energy of the exhaust gases is utilized in order to increase the power. The power increase results from a rise in the mixture throughput per working stroke.

A turbocharger consists essentially of an exhaust gas turbine with a shaft and a compressor impeller, the compressor arranged in the intake tract of the engine being connected to the shaft, and the blade wheels located in the casing of the exhaust gas turbine and in the compressor impeller rotating.

Exhaust gas turbochargers are known which allow multistage, that is to say at least two-stage, supercharging, so that even more power can be generated from the exhaust gas. Such multistage exhaust gas turbochargers have a special set-up which comprises a regulating member for highly dynamic cyclic stresses, which comprises, for example, a flap dish, a lever or a spindle.

The compressor impeller in the exhaust gas turbocharger has to satisfy extremely stringent material requirements. The material from which the compressor impeller is formed must be heat-resistant, that is to say still afforo suff_cie^t strength even at h_gh temperatures of at least up to abo _^ t 280 ⁰ C. Furthermore, the material must be resistant to intercrystallme corrosion and stress crack formation m the acid medium, and, moreover, it should have high material resistance, along with a low stress cycle coefficient. Furthermore, the ductility of the material should De sufficiently high so tnat, in the event of overload, the parts can experience plastic deformation and do not break, the result of this being that a sudden release of energy and damage resulting from this are possible.

An ex ^h aust gas t _^ rbocharger with a double-flow exhaust gas inlet duct is known from TE 10 200^ 018 61^ Al.

The object of the present m\ent_on, then, was to provide a compressor impeller for a turbocharger, according to the preamble of claim 1, and a turoocharger accorαing to the preanble of clam I ₁ which has improved heat resistance and ^" temperature resistance and is distinguished by a good resistance to corrosion and stress crack formation in acid media. Furthermore, the material should have optimal ductility εtno an improved oscillation fatigue strength performance. A wear-resistant and permanently stable compressor impeller is consequently to be capable of being produced. Furthermore, the object was to provide a method for producing the compressor impeller according to the i ⁿ vention, a material being produced which is distinguished by the abovementioneα advantageous properties.

The oojects are achieved by means of t^e features of claims 1, 7 ana 13. Owing to the design according to the invention of the compressor impeller for a turbocharger, consisting of an aluminum-based alloy with dendritic precipitation phases, which contains dispersions of the elements lanthanum and zirconium, what is achieved is that the material which ultimately provides the compressor impeller in the exhaust gas turbocharger is distinguished by particularly good heat resistance, temperature resistance and stability. The stability and temperature resistance of the material according to the invention are achieved particularly in that the dendritic precipitation phases form a high ramification in the material due to the interaction of the intermetallic phεtses. The high ramification of the intermetallic phases in the microstructure of the material is critical for the supporting action of the microstructure and therefore also for resistance to lattice slip, with the result that the material becomes consolidated and is resistant to both static and cyclic mechanical load. Owing to the specific material combination for the compressor impeller according to the invention, moreover, the adhesive and cohesive forces in the material matrix are increased. It was found that a combination of the elements lanthanum and zirconium in the aluminum-based alloys is essential for the stability of the material. The combination of these elements in the alloy leads to a very high stability of the material, since a particularly good microstructure is formed by means of the dendritic ramifications.

It was found, furthermore, that lanthanum in an aluminum-based alloy gives rise to a markedly strength- enhancing action both at room temperature and at component temperatures of up to 280 ⁰ C. It is presumed that thermally stable Al^La phases are formed, which also exert a positive action with respect to the creep resistance of the material. Lanthanum consequently increases the heat resistance εtnd also resistance to - A -

mateπal fat _^ g _^ e in tne case of ._ow stress cyι_le coefficients ano, consequently, t e stabil_ty cf tne material accord_ng to the inventio ⁿ for ere compressor impeller .

The element zirconium also forms phases m the alloy, to oe precise Al ₅ Zr ohases, with the result that a cross-linking structure, what is know ¹ " as a c^a-.ning characteristic, _^ s achie/ed in tne microstructure . It was fou^d that precipitation phases are formed within the alloy material oue to the comomation of aluminum ana zirco^i _^ rr. The rrecna^ical properties of t~e material are thereby significa ⁿ tly improved. A material thus formed or a component formed from j _^ st such a material is distinguished by very good temperature resistance ana heat resistance up to z80°C, is stable to corrosion ana _^ s insensitive to stress ciack formation .

However, it is exactly the comoinat^on of tne elements zirconium and lanthanum m the aluminum-bas^α alloy which _s essential for provid.ng naterial for the compressor impeller according to t~e irvention whic", αue to the formation of fine strong ramifications of the intermetallic phases, has very good material resistance m the case of a low stress cycle coefficient and exhibits high heat resistance up to 280 ⁰ C and, furthermore, a αuctility sufficient for s _^ c ¹ " a material. Without being involved in theory, it is presumed that the combinatio ⁿ of the elements lanthanum and zirccnium m the aluminum-based a_loy affords a~ extremely stable highly heat-resistant material and, consequently, component exactly because the Al La phases cr Al _^ Zr phases which fcrm m the microstructure structures vrh÷c" encroach one upon tre other and w ⁿ ich, because of their structural similarity to one another, give rise to an improved bonα within the material. Furthermore, it was shown that the compressor impeller according to the invention, produced from t^e material just described, is resistant to corrosion a^d stress crack formation even in acid media. An acid media m the co^ext of the invention io m this case to be understood as meaning a medium which has a pH value of aoout 3.5 to 6 and, m part_cular, of about 4 to 5.!T. Owing to condensation water and chlorides from the surroundings of the engine space, "aciα" conditio ⁿ s also prevail in t^e exhaust gas turoocharger . The material accoromg to the invention is resistant to this a^d therefore also resistant to mtercrystalli^e corrosion. The tendency to stress crac< formation under tensile stress is thereoy markedly reduced.

Owing to these outstandi ⁿ g material properties, the compressor impeller according to t^e invention for an exhaust gas turbocharger is also suitable part_cαlarly for two-stage turbochargers and nost particularly for those which are used in motor trucks and here, m particular, for highly cyclic bus applications w ^h ich have a high running performance of up to 1.6 ir_llion running kilometers m the fielo.

The subclaims contain advantageous developme ⁿ ts of t^e invention .

In a preferred embodiment, the aluminum-based alloy contains the elements lanthanum a^d zircori _^ m wit ¹ " a content of 0.08 to 1.0 by weight and more preferably cf 0.2 to 0.5 o by weight for lantha ⁿ um and 0.35 to 8 by weight and more preferably 1 to 5 by weight for zirconium m relation to the overall weignt of t^e alloy. Zirconium is essential for the occurre ⁿ ce and Distribution of the precipitation phases m zre alummum-Dased alloy. As already stated, al _^ mm _^ m m this case forms with the zirconium Al Zr phases y ⁿ ich are precipitated separεttely and bring ≤tbout an adαitional stabilization of the material a±so with respect to the heat resistεtnce and osci_lat_on fatigue strength of the alloy and, consequently, of the compressor impeller according to t^e invention. Tnese important material properties come into particularly sharp foc _^ s when the quantitative fraction of zirconium is in a range of 1 to 5 oy weight ii relat_or to the overall weight of the alloy. A hig^ contact of zirconium, in other words a content of more than 8c by weight in relation to the alloy, leaαs, m turn, to a lowering of the heat resistance of the material, alo^g with a lower long-time rupture strength.

Additions of lεtnthεtnum to the alloy increase the strength of the material a ^d consequently of the component. As already stated, lanthanum too forms with aluminum Al ₃ La phases which are precipitateo separately and which, moreover, nave a positive influence on the creep resistance of the material. At the same time, with the introduction of these phases, the "eat resistance of the material rises, and tne resistance to material fatigue _n the case of a low stress cycle coefficient is improλ/ed. This is t ^h e case particularly when lanthanum is employed in a quantitative fraction of 0.08 to 1.0 Ov weight and particularly preferably in a qua ⁿ titative fraction of 0.2 to 0.5 o by weight m relation to the overall weight of the aluminum-baseα alloy. As already stated, it is exactly tne combination of lanthanum a^d zirconium in the alumi ⁿ um-based alloy which is important for achieving the staoility parameters of the material which were described above. A cortent of more than 1° by weight of lanthanum m the al_oy _eads to a reduction m the heat resistance and, furt ^h ermore, reduces the long-time rupture strength.

The pr.ysica- ana rrecr.a^ica- properties of the material ana consequently of the compressor impeller can be optimized even further . _^ n a preferred embodiment, the compressor impeller according to the invention is distinguished m that the aluminum-based alloy co ^Λ tai ⁿ s further elements, such as iron and/or manganese and/or vanadium and/or nickel and/or niobium and/or scandium. The property profile of each element is basically knovir to a person skilled in the art.

For exarap.e, an admixture of the elements manganese and iron to an alloy contributes to increasing the heat resistance of the material due to the formation of dispersoid phases on account of their chaining properties. It is particularly advantageous for this purpose if the iron content is m a rεtnge of 1 to 15 _t by weight ano preferably 3 to HO by weight m relation to tne overall weig ^h t of tne alloy, and t ^κ e manganese content is 1 to 12 ³ by weight and preferab_y 3 to 9.5c by weigit in relat_on to the o\era_l weight of the alloy .

The addition of vanadium to an alloy usually leads to an improvement m t ^H e grain distribution of the mtermetallic composition. Vanadium at the same time has a consolidating action on the dispersoid precipitations here in the overall composite alloy structure. Vanadium increases the ad^esi\e and cohesive forces l" the mcrostruct _^ re of the material and therefore stabilizes the structure produced. Preferably, vanadium is used in a content of 0.5 to 8°. by weight m relatio ⁿ to the overall weight of the alloy and particularly preferably m a content cf 1.5 to € Joy weight m relation to tre overall weignt of the a_loy. In this concentration range, vanadium orings about a particularly pronounced improvement m adhesion and cohesion m the alloy structure ano therefore contributes considerably to optimizing tne stability of the compressor impeller according to the invention. The element nickel is an element v^ich likewise exerts an εtppreciable influence on the heat res_sta^ce of the compressor impeller according to tre invention. It _s exactly t^e combination of aluminum and nickel whicn i" this case decisively i ⁿ creases t ^κ e aαhesive and cohesive forces of ere material, particularly also _^ n the acid medium, that is to say in a pH raige of 3.5 to 6 and preferably of 4 to 5.5. Preferably, nickel is used with a content of 1 to 14 _o by weig ^h t in re.ation to the overall weight of t^e alloy and particularly preferaoly with a content of 3 to ICo by weight m relation to the overall ^" weight or the alloy. The properties improving the stability in the acid pH range come into particularly sharp focus here.

Niojoium is an element, the use of whicn m alumi^um- baseα al_oys is u ⁿ usual. It was found, then, that niobium, like zirconium, optimizes tie occurience and distribution of the precipitation phases m the alloy. As a result, the mechanical properties both at roon temperature and in a temperature range of up to 28C°C are i ⁿ fluenced positively. Ti^e materia- for t^e compressor mpeller according to the invention is therefore distinguished by a heat resistance of up to 280 ⁰ C. Furthermore, the creep resistance and t^e oscillation fatigue strength of the alloy are also improved oy a multiple. Preferably, niobium is used with a content of 0.5 to 8 _O by weight ana particularly preferably with a content of 1.5 to 6°. bv weight in relation to the overall weight of the alloy. In th_i_s concentration range, niobium contributes particularly highly to the ircrease m the heat resistance ana oscillation fatigue strength of the compressor impeller according to the invention.

Scandium has a similar action on alloy materials to lanthanum. It contributes to increasing t^e strength of the material. It was found that scandium, too, forms special phases with aluminum, to be precise Al _^ Sc precipitation harαemng phases wh_ch l_kew_se have a positive effect on the creep resistance or creep strength of the material. Moreover, the heat resistance is a_so increased, and, furt ^h ermore, also ^he resistance to material fatigue in the case of low stress cycle coefficients. The preferred actions come into particular focus wήe" scandium is used m a quantitative fraction of 0.05 to 1.0 Oy weight and preferaoly m a quantitative fract_on of 0.1 to 0.4 by weight i^ relation to the overall weight of the alummum-Dased alloy.

In a particularly advantageous emoodiment, a combination of the elements _^ arthanum and scandium is used m the aluminum-based alloy, t ^κ e aluirmuir-based alloy containing the elements lanthanum and scandium i" a total fraction of 0.13 to 2.C by we_ght and preferaoly of 0.3 to 0.9 by weight ir relation to the o\erall weight of the alloy. It is precisely t^en that the material is distinguis ^h ed by particularly "igh heat resistance and resistance to material fatigue i" the case of low stress cycle coetficients .

In a further advantageous embodiment, tπe compressor impeller accord_ng to the invention is αistmguisheα by an alummum-based alloy which contains, furthermore, the following elements or components with the following quantitative fractions, the quantitative fractions in each case relating to t ^κ e overall weight of the alloy: Fe: 1 to 15 by we_ght, Mn: 1 to 12 by weight, Kb: 0.5 to 8 o by weight, V: 0.5 to 8 by weig ^h t, Ni: 1 to !&%, by weight, Zr: 0.35 to 8° by weight, tne sum of La a^d Sc: 0.13 to 2.0° by weight, and .Al. It is exactly the comomation of these elements _^ n the quantitative tractions given which leads to a material which, when processed i^to a compressor impeller for an exhaust gas turoocharger, gives this particularly high stεtoility - IO -

with respect to corrosion and, furthermore, is distinguished by very good heat resistance and resistance to material fatigue in the case of a low stress cycle coefficient. The compressor impeller exhibits improved creep strength and an excellent oscillation fatigue strength performance. The abovementioned properties come into particular focus when the compressor impeller according to the invention contains the following elements in the quantitative fractions given: Fe: 3 to 11% by weight, Mn: 3 to 9.5% by weight, Nb: 1.5 to 6% by weight, V: 1.5 to 6% by weight, Ni: 3 to 1OO by weight, Zr: 1 to 5° by weight, the sum of La and Sc: 0.3 to 0.9% by weight, in each case in relation to the overall weight of the alloy, and Al.

A compressor impeller consisting of an aluminum-based alloy containing the abovementioned elements is distinguished by particularly good properties.

Thus, a material produced according to the last- mentioned specific compositions has the following properties :

To determine the stability of the material, the following tests were conducted: - I l -

• outdoor exposure tests

• tensile tests under heat up to 300 ⁰ C

• creep strength up to 300 °C

• changing climate in an acid medium: 150 hours at pH 4 to 5.5

• LCF test: 2 000 000 cycles at 220 ⁰ C, amplitude: 170 MPa.

The oscillation fatigue strength tests were conducted exclusively under single-stage axial pulsating load with force regulation (R=O) . The ambient medium was air. The material was investigated (material temperature) in a temperature interval from room temperεtture (that is to say, about 20 ⁰ C) up to 220 ⁰ C.

A most particularly preferred alloy is obtained from the elements listed below: Fe: 3 to 5.3 ¹ O by weight, Mn: 3.2 to 5.3% by weight, Nb: 1.8 to 3.2% by weight, V: 1.5 to 3.3% by weight, Ni: 3.7 to 5.8% by weight, Zr: 1 to 3.1% by weight, La: 0.1 to 0.4% by weight and Sc: 0.05 to 0.3% by weight, in each case in relation to the overall weight of the alloy, and Al.

This aluminum-based alloy according to the invention exhibited the best results in the above tests.

According to the invention, the aluminum-based alloy, on which the compressor impeller according to the invention for a turbocharger is based, can be produced by means of suitable methods and, in particular, by means of a spray-compacted method still to be carried out. The respective materials are weldable by means of conventional WIG plasma methods and also EB methods. Heat treatment takes place by solution annealing at about 640 ⁰ C for 2 hours and with subsequent air cooling. Precipitation hardening takes place at about 250 ⁰ C for 2 hours, with air cooling, in a box furnace. The processing of the aluminum-oased alloy takes place according to conventional processes by the fusion of the basic alloy, PS spraying, precompaction (densal HIP treatment) , extrusion, forming and further machining (for example, milling) .

Claim 7 defines as ai independently handlable art_cle an exhaust gas turboc^arger whic ¹ ^, as already αescπbeα, comprises a compressor impeller which consists of an alummum-baseα alloy with dendr_tic precipitation phases and which contains dispersions of the elements lanthanum and zirconium.

The compressor impeller according to the ..nvertior ._s produced by means of a special methoo which comprises the following steps:

lntroαuction of a f _^ sed material consisting of an aluminum-based alloy as claimed m one of claims 1 to 6 into a special crucible, transfer of the fused material into an admission vessel with the following "atomizer", and fine spraying of t ^h e fused material under a protective gas atmosphere on a rotating disk.

Only X^ this way is an alloy naterial obtained which, after further machining, provides a compressor impeller which is distinguished, by extremely good resistance to corrosion and stress crack formation, has a heat resistance αp to 280 ⁰ C, a^d exhibits a markedly reduced material fatigue frequency m the case of a low stress cycle coefficient and a very good oscillation fatigue strength performance. It was found that, oy virtue of the method acccrαing to t^e indention, a very fme- grained, even oispersoid powder formation can be processed m the run-up. In this case, various grai^ sizes of the mαiviαual elements can be defined, and this can additionally bring about a highly lmproveα - 15 -

creep stre ⁿ gth ana a goυα oscillation fatigue performance .

Figure 1 shows a partial illustration of an embodime ⁿ t of the turborharger 1 according to r ^κ e :nve ⁿ t:o" v ^h ich does not need to be described m any more αetail with regard to the compressor, the compressor casiig, the compressor shaft, the bearing casing and the bearing arrangement and also a_l other conventional parts. The exhaust gas inlet duct cannot oe see" here. The exhaust gas inlet ouct is provided ^• with a double-flow bypass duct 4 which Branches off rrom t^e exhaust gas inlet αuct and which leads to an exhaust gas outlet 5 of the turome casing 2. The bypass duct 4 nas a regiuatirg flap 6 for opening and closing.

LIST OF REFERENCE SYMBOLS

Turbocharger Turbine casing Bypass duct Exhaust gas outlet Regulating flap/wastegate flap