ALLOY

FIELD OF THE INVENTION

The present invention relates to an aluminium (Al) containing iron (Fe) based intermetallic alloy with improved mechanical properties. More particularly, the present invention relates to a Silicon-Molybdenum-lron-Aluminium (SiMoFeAI) based intermetallic alloy with improved creep-rupture life, good oxidation and corrosion resistance, used for automotive applications, in particular manufacturing of turbine housing for turbochargers.

BACKGROUND OF THE INVENTION A turbocharger is basically a centrifugal compressor driven by a turbine with the help of hot exhaust gases or emission gases from the vehicle's engine. The compressor in the turbocharger draws the air from the atmosphere and compresses it, this pre-compressed air is then allowed to enter the engine during suction stroke, which gets further compressed during compression stroke. As a result of these actions, high pressure air gets mixed with the fuel inside the engine and helps in effective burning of fuel thereby increasing the engine efficiency for the same displacement of engine eliminating the need of a larger displacement engine. During such conditions, turbocharger housing which accommodates turbine wheel with high rotational speed as a result of exhaust gas, are subjected to elevated operating temperatures. Due to such elevated operating temperature, the thermal load on the parts such as exhaust manifold and turbine housing increases, leading to various problems including thermal deformation of the housing material. In particular the turbine housing, which frequently operates between low and high temperature conditions, is subjected to thermal expansion and shrinkage causing surface oxidation wrinkles and subsequently develops into a penetration crack. Several classes of heat resistant alloys such as cast irons, stainless steels, and Ni-base superalloys have been considered for high temperature automotive applications more particularly for exhaust systems. These types of alloys contain various alloying elements, such as nickel, chromium, cobalt, boron, zirconium, niobium, tantalum and tungsten, to improve the high temperature strength, ductility and corrosion resistance properties of the alloy. The most heat-resistant alloys utilized in the industry are nickel-based alloys. D5S is one such high Ni based alloy which has been used for an operating temperature between 850 °C - 930 °C. Due to its high Ni content (34 - 37 wt. %), the material cost is high. This limits their application in the automotive industry. Cast iron alloys having high mechanical strength is limited in its application by having a relatively low ductility, resulting in more brittle components. Alternative solution for this requirement of high strength material at elevated temperature with good thermal fatigue and oxidation resistance, is the development of cast irons with high silicon and high molybdenum content, known as SiMo alloys. These SiMo alloys exhibit improved high temperature strength, thermal fatigue and oxidation resistance but the maximum operating temperature restricted to 850 °C. Hence it would be desirable to develop a cost effective material with improved mechanical strength for higher operating temperature.

Currently, various austenitic heat-resistant casting steel are used for automotive application, in particular turbine housing applications such as GX40CrNiSi22-10 (1.4826), GX40CrNiSi25-

12 (1.4837), GX40CrNiSi25-20 (1.4848), GX40CrNiSiNb38-19 (1.4849) with Chromium (Cr) and Nickel (Ni) as major alloying elements with small amount of Niobium (Nb). Weight % of alloying elements of Cr, Ni is ranging between: 18.00 - 27.00 and 9.00 - 39.00 respectively.

Though presence of Ni content is less in some of the aforementioned grades, still their application is limited because of its high processing and machining cost. Hence, reducing the material cost and developing an economical material is very important from the industrial point of view.

Possible promising solution for the above reported problems are aluminides. Among various aluminides, ferritic base alloys with high aluminum content called as Iron-Aluminides (FeAI) are one such intermetallic compounds with high aluminum content in the iron matrix. It has received much attention since 1930s when their superior oxidation was first noticed. Further, this class of intermetallic alloy has advantages of high temperature structural applications over the Ni-based superalloys which includes low cost of production, high operating temperatures without phase transformation up to 1300 °C (FeAI phase with B2 structure), superior corrosion, oxidation and sulfidation resistance and low density. Due to aforesaid qualities, it make them special candidate in industrial applications. But at the same time, it suffers with limited ductility and inferior elevated temperature properties. Therefore there exists a need in the art to develop a cost effective alloy with improved mechanical properties for high temperature applications. WO2016084021 discloses a silicon molybdenum aluminium ferritic iron alloy that includes, in addition to greater than about 50.0 wt % iron, the following amounts: from about 2.0 to about 4.0 wt % aluminum and from about 2.0 to about 4.3 wt % silicon, with the proviso that an equivalent silicon amount be greater than about 4.7 wt %. A ratio of the amount of silicon to the amount of aluminum is from about 1.1 :0.9 to about 0.9:1.1. The iron alloy further includes greater than about 1.5 wt % carbon, with the proviso that an equivalent carbon amount be less than about 5.2 wt %, and from about 0.2 to about 1.1 wt % molybdenum. This ferritic alloy exhibits oxidation resistance at elevated temperatures and improved ductility and are used for casting applications, such as turbine and turbocharger housings, exhaust manifolds, and combustion chambers. WO2015086893A1 provides Fe-based ferro-aluminide intermetallic composition for use in a prechamber component of a piston engine, comprises at least 55 percent by weight of iron, 10 to 40 percent by weight of aluminum, and at least one of the substances selected from the group of B, C, Cr, Zr, Nb, Ti; the total amount of the substances selected from the group not exceeding 5 percent by weight. It's also describes the method for manufacturing a prechamber component comprises a step of manufacturing a prechamber component by a powder metallurgical process from the said Fe-based composition.

US20080118355 details about the method for manufacturing turbine blades for turbo- engines, which are capable of withstanding high thermal stress and able to maintain an adequate mechanical strength even at raised operating temperatures. The turbine blades comprises a heat-insulating layer of a metallic open-cell foam which is integrally connected by sintering to the surface of a core element formed from titanium aluminide; and the outer contour of the turbine blade is formed with at least one shell element made of a nickel-base alloy, also integrally connected by sintering to the open-cell foam which forms the heat- insulating layer formed with an aluminum content which is in the range between 20 and 75% by weight and additional alloy elements which are selected from chromium, niobium, molybdenum, manganese, copper, silicon and bismuth.

US2010074796 discloses an aluminium alloy of type AIMgSi with good creep strength at elevated temperatures for the production of castings subject to high thermal and mechanical stresses, comprises of: 0.1 to 1 % w/w manganese, max. 1 % w/w iron, max. 3% w/w copper, max. 2% w/w nickel, max. 0.5% w/w chromium, max. 0.6% w/w cobalt, max. 0.2% w/w zinc, max. 0.2% w/w titanium, max. 0.5% w/w zirconium, max. 0.008% w/w beryllium, max. 0.5% w/w vanadium, as well as rest being aluminium with further elements and manufacturing- related impurities of individually max. 0.05% w/w and max. 0.2% w/w in total. The alloy is suitable in particular for the production of cylinder crankcases by the pressure die casting method.

US6033623 discloses a powder metallurgical process of preparing iron aluminide useful as electrical resistance heating elements having improved room temperature ductility, electrical resistivity, cyclic fatigue resistance, high temperature oxidation resistance, low and high temperature strength, and/or resistance to high temperature sagging. The iron aluminide has an entirely ferritic microstructure which is free of austenite and can include, in weight %, 20 to 32% Al, and optional additions such as less than less than or equal to 1 % Cr, greater than or equal to 05% Zr or ZrC , less than or equal to 2% Ti, less than or equal to 2% Mo, less than or equal to 1 % Zr, less than or equal to 1 % C, less than or equal to 0.1 % B, less than or equal to 30% oxide dispersoid and/or electrically insulating or electrically conductive covalent ceramic particles, less than or equal to 1 % rare earth metal, less than or equal to 1 % oxygen, and/or less than or equal to 3% Cu. The process includes forming a mixture of aluminum powder and iron powder, shaping the mixture into an article such as by cold rolling the mixture into a sheet, and sintering the article at a temperature sufficient to react the iron and aluminum powders and form iron aluminide. The sintering can be followed by hot or cold rolling to reduce porosity created during the sintering step and optional annealing steps in a vacuum or inert atmosphere.

All of the above cited prior-art documents deals with improving the mechanical properties of the alloy such as corrosion resistance, high temperature strength, hydrogen embrittlement, ductility etc., but only minor attention has been given to the creep-rupture life of the alloys used for casting applications in particular, manufacturing of turbine housing applications. The present invention eliminates the aforesaid problem by providing an aluminium based SiMoFeAI intermetallic alloy with improved creep-rupture life for casting applications, in particular manufacturing of turbine housing parts. The present invention eliminates the need of expensive Ni-based alloys such as D5S which were conventionally used in automotive sector for turbocharger applications. Further the present invention provides a cost effective intermetallic alloy with improved mechanical properties for high temperature applications.

OBJECTIVE OF THE INVENTION

The main objective of the present invention is to provide an aluminium (Al) containing iron (Fe) based intermetallic alloy with improved mechanical properties, for automotive applications.

Another objective of the present invention is to eliminate the need of expensive Ni based alloys and reduce the material cost in the manufacturing industry by providing a cost effective aluminium based SiMoFeAI intermetallic alloy with improved creep-rupture life, good oxidation and corrosion resistance, for automotive and other high temperature applications.

It is still another objective of the present invention is to provide a cost effective aluminium based SiMoFeAI intermetallic alloy with improved creep-rupture life, good oxidation and corrosion resistance for the manufacture of turbine housing parts.

SUMMARY The present invention provides an aluminium based SiMoFeAI intermetallic alloy, having silicon, molybdenum and aluminium has its major constituents. The said aluminium based SiMoFeAI alloy comprises a composition that includes, in addition to greater than about 50% by weight iron; aluminium, ranging from about 20 weight % to about 27 weight %, preferably 23 % by weight to about 25 % by weight; silicon, ranging from about 0.5 weight % to about 4 weight %, preferably 1.5 % by weight to about 2.5 % by weight; and molybdenum, ranging from about 0.1 weight % to about 1 weight %, preferably 0.2 % by weight to about 0.5 % by weight. The present invention may further contain carbon ranging from about 1 weight % to about 5 weight %, preferably 2 % by weight to about 3 % by weight; manganese ranging from about 0.05 weight % to about 0.8 weight %, preferably 0.2 % by weight to about 0.5 % by weight; and niobium ranging from about 0.1 weight % to about 1.5 weight %, preferably 0.4 % by weight to about 0.8 % by weight. Additionally the aluminium based SiMoFeAI intermetallic alloy of the present invention further comprises of less than about 0.5 weight %of boron, preferably less than 0.2 % by weight of boron; less than about 0.5 weight % of zirconium, preferably less than 0.1 % by weight of zirconium; and less than about 0.5 weight % of titanium, preferably less than 0.1 % by weight of titanium. The aluminium based SiMoFeAI intermetallic alloy of the present invention has an improved creep rupture life, which is 25 times higher than the conventional D5S alloy. Further, the present invention eliminates the use of expensive Ni based alloys by providing a cost effective aluminium based SiMoFeAI intermetallic alloy with improved creep-rupture life, high oxidation and corrosion resistance, and machinability for the use in casting applications.

DETAILED DESCRIPTION OF THE INVENTION:

The present invention as embodied by "Cast Silicon-Molybdenum-lron-Aluminium (SiMoFeAI) based intermetallic alloy", succinctly fulfils the above-mentioned need(s) in the art. The present invention has objective(s) arising as a result of the above-mentioned need(s), said objective(s) being enumerated below. In as much as the objective(s) of the present invention are enumerated, it will be obvious to a person skilled in the art that, the enumerated objective(s) are not exhaustive of the present invention in its entirety, and are enclosed solely for the purpose of illustration. Further, the present invention encloses within its scope and purview, any structural alternative(s) and/or any functional equivalent(s) even though, such structural alternative(s) and/or any functional equivalent(s) are not mentioned explicitly herein or elsewhere, in the present disclosure. The present invention therefore encompasses also, any improvisation(s)/modification(s) applied to the structural alternative(s)/functional alternative(s) within its scope and purview. The present invention may be embodied in other specific form(s) without departing from the spirit or essential attributes thereof.

Throughout this specification, the use of the word "comprise" and variations such as "comprises" and "comprising" may imply the inclusion of an element or elements not specifically recited. The present invention provides an aluminium based SiMoFeAI intermetallic alloy with improved mechanical properties for casting applications, comprising a composition that includes, in addition to greater than about 50% by weight iron; aluminium at levels, independently, from about 20 % by weight to about 27 % by weight, preferably 23 % by weight to about 25 % by weight; silicon at levels, independently, from about 0.5 % by weight to about 4 % by weight, preferably 1.5 % by weight to about 2.5 % by weight; and molybdenum at levels, independently, from about 0.1 % by weight to about 1 % by weight, preferably 0.2 % by weight to about 0.5 % by weight.

Alloy design must be targeted to achieve specific microstructural features for various applications. The properties of an alloy are closely linked to its composition and processing parameters. Aluminium is a ferrite stabilizer. Generally, higher aluminium content increases oxidation resistance especially at high temperatures and decreases the density of the material. Further increasing aluminium content results in an increased volume fraction of the eutectic aluminium content in the microstructure of the material, which in turn improves the yield strength and creep strength of the alloy. Molybdenum is a carbide forming element. Addition of molybdenum increases high temperature creep-rupture strength of the alloy by forming very fine Molybdenum Carbides (MoC) near the grain boundaries. Further addition of molybdenum also increases the overall mechanical properties of the alloy including increased tensile strength, hardness, hardenability, and high temperature fatigue resistance of the alloy composition. Silicon a deoxidizer, helps to improve the oxidation resistance and it also resist the absorption of nitrogen at high temperature to avoid nitrogen embrittlement. Silicon when used in conjunction with other alloying elements increases the toughness and hardness of the alloy composition.

The aluminium based SiMoFeAI intermetallic alloy of the present invention may further contain carbon at levels, from about 1 % by weight to about 5 % by weight, preferably 2 % by weight to about 3 % by weight; manganese at levels from about 0.05 % by weight to about 0.8 % by weight, preferably 0.2 % by weight to about 0.5 % by weight; and niobium at levels from about 0.1 % by weight to about 1.5 % by weight, preferably 0.4 % by weight to about 0.8 % by weight. Carbon is a graphite and carbide forming element. It improves strength and hardness of the alloy. As carbon level increases the alloy becomes stronger but less ductile. Hence optimum carbon level increases the hardness and wear resistance of the material. Manganese is added to improve strength, toughness, hardenability and oxidation resistance of the material. Manganese also helps to improve weldability and corrosion resistance of the material. Addition of Niobium improves high temperature creep strength and thermal fatigue resistance by forming very fine niobium carbides in the matrix.

In the preferred embodiment of the present invention, the alloy composition further comprises of: less than about 0.5 % by weight of boron, preferably less than 0.2 % by weight of boron; less than about 0.5 % by weight of zirconium, preferably less than 0.1 % by weight of zirconium; and less than about 0.5 % by weight of titanium, preferably less than 0.1 % by weight of titanium.

Titanium is a carbide forming element helps to improve the alloy strength. Boron is an interstitial element and tends to concentrate at the grain boundaries. It helps to prevent crack formation in the casting and also in grain refinement. By adding small quantity of Boron creep-rupture life can be improved. Zirconium is a strong carbide forming element which helps to improve strength-ductility and in combination with boron improves hot tear properties. By adding small amount of rare earth element such as Cerium with Magnesium helps in formation of favourable graphite structure. The alloy composition of the present invention is referred as SiMoFeAI intermetallic alloy, due to the presence of silicon, molybdenum and aluminium content. The SiMoFeAI intermetallic alloy of the present invention has an improved creep rupture life, which is approximately 25 times higher than the conventional D5S alloy. Further, the SiMoFeAI alloy of present invention eliminates the use of expensive Ni based alloys by providing a cost effective aluminium based SiMoFeAI intermetallic iron aluminides with improved mechanical properties such as high creep-rupture life, high oxidation resistance, high corrosion resistance and reasonable machinability for use in casting applications.

The alloy composition of the present invention finds application in various casting applications including but not limited to automotive exhaust components including exhaust manifolds and catalytic converter cans, turbocharger components including turbine housings and centre housings, and engine components such as blocks, heads, cylinder liners, cam shafts, crank shafts, dampers, bed plates, valve train components, pistons, piston inserts, bearing caps, and pump housings. The composition of the present invention also finds application in transmission components including cases, carriers, housings, barring caps, fly wheels, and pump housings etc. Other applications will be appreciated by those having ordinary skill in the art.

EXAMPLE 1

Creep-rupture test was conducted at 800 °C and 850 °C with stress of 30 MPa for both D5S and aluminium based SiMoFeAI alloy of the present invention. At 800 °C and 850 °C, the creep-rupture life of D5S is 4.5 hrs and 1 hr respectively. Whereas creep-rupture life of aluminium based SiMoFeAI alloy at 800 °C and 850 °C was 130 hrs and 28 hrs respectively, which is approximately 25 times more than that of D5S alloy. Room temperature tensile properties of the aluminium based SiMoFeAI alloy falls between 165 - 255 MPa with < 1 % ductility. Detailed characterizations of the aluminium based SiMoFeAI alloy was carried out with the following techniques to understand the structure-property relationship: Phase transformation studies using TG-DSC, X-ray diffraction studies. Comprehensive microstructural analysis was carried to understand the structure-property relationships using Scanning Electron Microscopy (SEM). It will be apparent to a person skilled in the art that the above description is for illustrative purposes only and should not be considered as limiting. Various modifications, additions, alterations, and improvements without deviating from the spirit and the scope of the invention may be made by a person skilled in the art.