Patent Description:
Turbochargers for gasoline and diesel internal combustion engines are devices known in the art that are used for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust gas-driven turbine wheel to spin within the housing. The exhaust gas-driven turbine wheel is mounted onto one end of a shaft that is common to a radial air compressor mounted onto an opposite end of the shaft and housed in a compressor housing. Thus, rotary action of the turbine wheel also causes the air compressor to spin within a compressor housing of the turbocharger that is separate from the turbine housing. The spinning action of the air compressor causes intake air to enter the compressor housing and be pressurized or boosted a desired amount before it is mixed with fuel and combusted within the engine combustion chamber.

In recent years, there has been increasing pressure in the form of governmental legislation to reduce internal combustion engine emissions, such as NOx and particulate matter (PM). Oxides of nitrogen (NOx) may be formed when temperatures in the combustion chamber are about <NUM>° F or hotter. At these elevated temperatures, the nitrogen and oxygen in the combustion chamber may chemically combine to form nitrous oxides.

Exhaust gas recirculation (EGR) is a method that has been used to reduce the level of NOx in exhaust gases. In EGR systems, some of the exhaust gases that would otherwise be discharged into environment are recirculated into the intake stream. The recirculated exhaust gases have already combusted and have a significantly lower oxygen content, so they do not burn again when they are recirculated. The exhaust gases may displace some of the normal intake charge. As a result, the combustion process may be cooler by several hundred degrees so that NOx formation may be reduced.

The use of EGR, however, results in an increased amount of water that is condensed out of the recirculated exhaust gasses. The amount of water that is condensed may depend, for example, on temperature, humidity, and operating speed of the engine. If condensed water droplets impact the spinning compressor wheel, an erosive effect may be observed over time. As a result, the components may prematurely fail.

To overcome this problem, some turbocharger manufacturers have developed compressor stage components, such as the compressor wheel, made of titanium alloy. However, the use of titanium may not be desirable for several reasons. First, titanium is substantially more expensive than aluminum and is more difficult to work with, thus increasing the costs of producing the turbocharger unit. Second, titanium is heavier than aluminum and thus increases the rotational inertia of the compressor wheel. As a result, the turbocharger may be less responsive than an otherwise equivalent unit employing an aluminum wheel.

Accordingly, it would be desirable to provide turbocharger compressor wheels that are able to withstand the erosive effects of water droplets, without requiring the use of heavier and more expensive materials such as titanium. Furthermore, it would be desirable to provide such turbocharger compressor wheels that are able to be manufactured easily with existing technologies that do not result in significant additional manufacturing complexity or expense. Furthermore, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the inventive subject matter. Documents cited during prosecution include <CIT>; <CIT>; and <CIT>.

Turbocharger compressor wheels having a bi-layered coating of an electroless nickel-phosphorous base layer and a hard chrome top layer, and methods for manufacturing the same, are disclosed herein.

In an exemplary embodiment, a compressor wheel for a turbocharger includes a hub portion defining a rotational axis and a plurality of blades extending radially outward from the hub portion. Each blade of the plurality of blades includes a leading edge, the leading edges of each blade of the plurality of blades forming an inducer portion of the compressor wheel. Each blade of the plurality of blades further includes a trailing edge, the trailing edges of each blade of the plurality of blades forming an exducer portion of the compressor wheel. The inducer portion is positioned longitudinally forward from the exducer portion along a rotational axis with respect to a flow of air along the compressor wheel. The hub portion and the plurality of blades include a substrate metal. The substrate metal of the hub portion and the plurality of blades is coated directly thereon a first coating layer including electroless nickel-phosphorous. The first coating layer is coated directly thereon a second coating layer including hard chrome. The second coating layer has a thickness that is greatest at the inducer portion, with the thickness of the second coating layer decreasing rearward towards the exducer portion such that the thickness of the second coating layer is about zero microns at or longitudinally forward of the trailing edges of each blade of the plurality of blades.

In another exemplary embodiment, a method for manufacturing a bi-layer coated compressor wheel for a turbocharger includes the step of providing or obtaining a substrate compressor wheel. The substrate compressor wheel includes a hub portion defining a rotational axis and a plurality of blades extending radially outward from the hub portion. Each blade of the plurality of blades includes a leading edge, the leading edges of each blade of the plurality of blades forming an inducer portion of the compressor wheel. Each blade of the plurality of blades further includes a trailing edge, the trailing edges of each blade of the plurality of blades forming an exducer portion of the compressor wheel. The inducer portion is positioned longitudinally forward from the exducer portion along a rotational axis with respect to a flow of air along the compressor wheel. The hub portion and the plurality of blades include a substrate metal. The method further includes the step of forming on the substrate metal of the hub portion and the plurality of blades a first coating layer including electroless nickel-phosphorous. Forming the first coating layer includes immersing the substrate compressor wheel in an electroless nickel-phosphorous plating bath including nickel cations and phosphorous oxide anions. Still further, the method includes the step of forming on the first coating layer a second coating layer including hard chrome. The second coating layer has a thickness that is greatest at the inducer portion, the thickness of the second coating layer decreasing rearward towards the exducer portion such that the thickness of the second coating layer is about zero microns at or longitudinally forward of the trailing edges of each blade of the plurality of blades. Forming the second coating layer includes immersing the compressor wheel coated with the first coating layer in a chromium plating bath including an oxide of chromium and an acid of sulfur, and applying an electric current using an anode and a cathode, with the compressor wheel coated with the first coating layer functioning as the cathode. The inducer portion is oriented facing the anode in the chromium plating bath.

The inventive subject matter will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein:.

Unless specifically stated or obvious from context, as used herein, the term "about" is understood as within a range of normal tolerance in the art, for example within <NUM> standard deviations of the mean. "About" can be understood as within <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>% of the stated value. "About" can alternatively be understood as implying the exact value stated. Unless otherwise clear from the context, all numerical values provided herein are modified by the term "about.

The present disclosure is generally directed to turbocharger compressor wheels having a bi-layered coating of an electroless nickel-phosphorous base layer and a hard chrome top layer, and methods for manufacturing the same. In particular, the present disclosure addresses the aforementioned erosion problem with the use of electroless nickel-phosphorus as a base layer followed by a hard chrome top layer. The purpose of the electroless nickel-phosphorous layer as a base layer is to minimize the difference in hardness between the relatively hard chrome top layer and the relatively soft aluminum substrate. That is, a hard chrome top layer disposed directly on the soft aluminum substrate could potentially fail in service due to the poor combination of mechanical strength.

The present disclosure utilizes a relatively high phosphorus content (for example, greater than or equal to about <NUM> wt-%) electroless nickel-phosphorous coating as the base coating. The combination of a relatively high phosphorus content and controlled process parameters ensures a compressive residual stress in the coating that will help to reduce failures of the compressor wheel due to fatigue. The aforesaid base layer of electroless nickel-phosphorous covers the entire compressor wheel, with the exception of several functional regions for reasons of manufacture/assembly. The functional regions not requiring the coating are masked during the process.

To provide additional hardness, the aforesaid hard chrome layer is employed, which has a hardness greater than about <NUM> HV, or greater than about <NUM> HV. In accordance with the present disclosure, the hard chrome layer is provided only on inducer/leading edge region of compressor wheel, using electrochemical deposition techniques. This selective provision is achieved by positioning the compressor wheel in an electrochemical cell in a manner such that the leading edge faces the anodic surface. The ionic flow in the electrolyte is thus focused towards the leading edges. The chrome deposition process is performed such that the thickness of the chrome coating is greatest at the leading edges (for example, a thickness from about <NUM> to about <NUM> microns) and then gradually decreases (in a tapering manner) towards the back-disc of the compressor wheel. Erosion from water droplets has been found to be greatest at the leading edges. At the fillet root, which is the area of maximum stress during operation, the thickness is reduced to about <NUM> microns, such that the effect of tensile stresses caused by the additional of the hard chrome coating layer is effectively eliminated at those locations.

With reference now to <FIG>, illustrated is a turbocharger <NUM> in accordance with the present disclosure having a radial turbine and that includes a turbocharger housing and a rotor configured to rotate within the turbocharger housing along an axis of rotor rotation <NUM> on thrust bearings and two sets of journal bearings (one for each respective rotor wheel), or alternatively, other similarly supportive bearings. The turbocharger housing includes a turbine housing <NUM>, a compressor housing <NUM>, and a bearing housing <NUM> (i.e., a center housing that contains the bearings) that connects the turbine housing <NUM> to the compressor housing <NUM>. The rotor includes a turbine wheel <NUM> located substantially within the turbine housing <NUM>, a compressor wheel <NUM> located substantially within the compressor housing <NUM>, and a shaft <NUM> extending along the axis of rotor rotation <NUM>, through the bearing housing <NUM>, to connect the turbine wheel <NUM> to the compressor wheel <NUM>.

The turbine housing <NUM> and turbine wheel <NUM> form a turbine configured to circumferentially receive a high-pressure and high-temperature exhaust gas stream <NUM> from an engine, e.g., from an exhaust manifold <NUM> of an internal combustion engine <NUM>. The turbine wheel <NUM> (and thus the rotor) is driven in rotation around the axis of rotor rotation <NUM> by the high-pressure and high-temperature exhaust gas stream <NUM>, which becomes a lower-pressure and lower-temperature exhaust gas stream <NUM> and is axially released into an exhaust system (not shown).

The compressor housing <NUM> and compressor wheel <NUM> form a compressor stage. The compressor wheel <NUM>, being driven in rotation by the exhaust-gas driven turbine wheel <NUM>, is configured to compress axially received input air (e.g., ambient air <NUM>, or already-pressurized air from a previous-stage in a multi-stage compressor) into a pressurized air stream <NUM> that is ejected circumferentially from the compressor. Due to the compression process, the pressurized air stream is characterized by an increased temperature over that of the input air.

Optionally, the pressurized air stream may be channeled through a convectively cooled charge air cooler <NUM> configured to dissipate heat from the pressurized air stream <NUM>, increasing its density. The resulting cooled and pressurized output air stream <NUM> is channeled into an intake manifold <NUM> on the internal combustion engine, or alternatively, into a subsequent-stage, in-series compressor. The operation of the system is controlled by an engine control unit (ECU) <NUM> that connects to the remainder of the system via communication connections <NUM>.

With further reference now to <FIG>, the compressor wheel <NUM> is a radial compressor wheel that includes a hub <NUM> and a plurality of blades, including a plurality of main blades <NUM> and optionally a plurality of splitter blades <NUM>. The blades have a backward curvature (i.e., a back swept angle wherein the wheel exit blade angle is backward swept circumferentially relative to a radial line and the leading edges of the blades lead the trailing edges of the blades when the hub is rotated to compress air) rather than being configured to extend in a purely radial blade configuration. Each main blade <NUM> has a leading edge <NUM> that defines the beginning of an inducer (i.e., an intake area for the combined set of main blades, extending through the circular paths of roughly the upstream ⅓ of the main blades), and a trailing edge <NUM> that defines the end of an exducer (i.e., a typically annular output area for the combined set of main blades, extending through the circular paths of roughly the downstream ⅓ of the main blades). Alternative embodiments may include compressor wheels without splitter blades (i.e., with main blades only).

The compressor housing <NUM> and compressor wheel <NUM> form a compression-air passageway, serially including an intake duct <NUM> leading axially into the inducer, an impeller passage leading from the inducer through the exducer and substantially conforming to the space through which the main blades rotate, a diffuser <NUM> leading radially outward from the exducer, and a volute <NUM> extending around the diffuser. The volute forms a scroll shape and leads to an outlet port through which the pressurized air stream is ejected circumferentially (i.e., normal to the radius of the scroll at the exit) as the pressurized air stream <NUM> that passes to the (optional) charge air cooler and intake manifold. As is typical in automotive applications for a single stage turbo charging system, the intake duct is fed a stream of filtered external air from an intake passage in fluid communication with the external atmosphere. Each portion of the compression-air passageway is serially in fluid communication with the next. Alternative embodiments may include other types of turbo charging systems, such as two-stage turbochargers configured such that the air compressed by a first stage is used as the intake air of a second stage.

A hub edge <NUM> of each main blade <NUM> connects to the hub <NUM> on a hub wall <NUM> that extends along one side of an impeller passage from the upstream edge of the inducer to the outermost portion <NUM> of the hub that delimits the compression air passageway, which typically is substantially at the outer radial limit of the hub edge of the main blade (i.e., the hub edge of the main blade extends substantially to an outer radial limit of the hub wall). The hub edge of each main blade defines a three-dimensional curve along which the main blade connects to the hub at the hub wall. This may be curved both because of the axial-to-radial curvature of the hub wall and because of the backward curvature of the main blades. Opposite the hub edge of each main blade is a shroud edge <NUM>, which also forms a curve, and which substantially conforms to a shroud wall <NUM> of the compressor housing <NUM>.

The intake duct <NUM> of this embodiment defines a cylindrical shroud-side inlet wall portion <NUM> extending axially to the inducer, the shroud-side inlet wall portion being integral with, the extension of, and smoothly transitioned to (i.e., extending at the same axial-to-radial angle and aligned with) the shroud wall <NUM> at the upstream end of the impeller passage. In some embodiments the hub wall <NUM> may be configured such that the hub-side of the impeller passageway at the upstream end of the impeller passageway is substantially cylindrical, and parallel to the wheel axis of rotation, but in the other embodiments it may be at least slightly angled from the axis of rotation. The hub <NUM> defines a hub-side inlet wall portion <NUM> extending to the inducer, the hub-side inlet wall portion being integral with, the extension of, and smoothly transitioning to the hub wall <NUM>.

The diffuser <NUM> defines a hub-side diffuser wall portion <NUM> (that may or may not be planar and normal to the axis of rotation <NUM>) around the outer radial limit of the hub wall, and a shroud-side diffuser wall portion <NUM> that is integral with, and the extension of, the shroud wall <NUM> through the diffuser. The hub <NUM> is configured such that the hub-side of the impeller passageway at the outer radial limit of the hub wall is smoothly transitioned to (i.e., extending at the same axial-to-radial angle, and aligned with) the hub-side diffuser wall portion (which also may or may not be planar and normal to the axis of rotation). Likewise, the shroud-side diffuser wall portion smoothly transitions from (i.e., it extends at the same axial-to-radial angle and is aligned with) the shroud wall. Embodiments may have various configurations, e.g., wherein the hub-side of the impeller passageway at the outer radial limit of the hub wall is or is not planar and is or is not substantially normal to the wheel axis of rotation.

The compressor wheel <NUM> may be formed from aluminum (or an aluminum alloy) as the substrate. The compressor wheel <NUM> is provided with a first (base) coating layer on and overlying the substrate including electroless nickel-phosphorous. The phosphorous content of the first coating layer may be greater than or equal to about <NUM> wt. -%, for example from about <NUM> wt. -% to about <NUM> wt. -%, such as from about <NUM> wt. -% to about <NUM> wt. -%, or about <NUM> wt. -% to about <NUM> wt. The thickness of the first coating layer may be from about <NUM> microns to about <NUM> microns, for example about <NUM> microns to about <NUM> microns.

The compressor wheel <NUM> is provided with a second (top) coating layer <NUM> on and overlying portions of the first coating layer including hard chrome. The second coating layer has a thickness over the first coating layer that varies gradually (in a tapering manner) across the forward-facing surfaces of the compressor wheel <NUM> (including the main blades <NUM>, the splitter blades <NUM> if present, the hub wall <NUM>, and the hub-side inlet wall portion <NUM>, for example). In an embodiment, the thickness of the second coating layer is greatest at the leading edges <NUM> and the shroud edges <NUM> adjacent to the leading edges <NUM>. This greatest thickness may be from about <NUM> microns to about <NUM> microns, such as about <NUM> microns to about <NUM> microns. The thickness of the coating layer gradually decreases from the leading edges <NUM> rearwardly (along the axis of rotation) in the direction of the trailing edges <NUM>. In embodiments, the thickness of the second coating <NUM> at the trailing edges is <NUM> or about <NUM> microns. In such embodiments, it is not necessary that the thickness reach <NUM> microns exactly at the trailing edges <NUM> as it decreases from the leading edges. Rather, the thickness of the second coating may reach <NUM> microns at any percentage of the overall distance rearwardly from the leading edges <NUM> to the trailing edges <NUM>, for example from about <NUM>% to about <NUM>%, or about <NUM>% to about <NUM>%. The decreasing thickness in the rearward direction is illustrated in <FIG> as the shading indicating layer <NUM> decreasing in density rearwardly.

The compressor wheel <NUM> may be manufactured in accordance with a method <NUM> as illustrated in the flowchart shown in <FIG>. The method <NUM> includes a step <NUM> of manufacturing or providing a compressor wheel made of aluminum (or alloy thereof) in the configuration shown in <FIG>, with the exception of the coating layers. The compressor wheel <NUM> may be manufactured using conventional manufacturing processes, such as casting and/or machining, or the like.

The method <NUM> continues with a step <NUM> of depositing forming a first (base) electroless nickel-phosphorous layer onto the compressor wheel (substrate). Electroless nickel-phosphorus plating is a chemical process that deposits an even layer of nickel-phosphorus alloy on the surface of the compressor wheel substrate. The process involves dipping the substrate in a water solution containing a nickel salt and a phosphorus-containing reducing agent, for example a hypophosphite salt. The concentration of the phosphorous-containing reducing agent is selected so as to achieve a phosphorous amount in the first layer greater than or equal to about <NUM> wt. -%, as described above. The reduction of the metal cations in solution to metallic form is achieved by purely chemical means, through an autocatalytic reaction. Before plating, the surface of the substrate may be cleaned. Cleaning may be achieved by a series of chemical baths, including non-polar solvents to remove oils and greases, as well as acids and alkalis to remove oxides, insoluble organics, and other surface contaminants. Further, functional portions of the substrate, as described above, may be optionally masked. Ingredients of the electroless nickel plating bath include a source of nickel cations Ni<NUM>+, for example nickel sulfate and a suitable reducing agent, such as hypophosphite H<NUM>PO<NUM>-. The plating bath may further include complexing agents, such as carboxylic acids or amines; stabilizers, such as lead salts or sulfur compounds; buffers; surfactants; and accelerators. The plating process is controlled with temperature and time to achieve a desired uniform thickness of about <NUM> to about <NUM> microns, as described above. Once Ni-P plating is complete, the substrate, now having the first layer plated thereon, may be rinsed to remove any residues from the plating process, and the masking (if any) may be removed.

Method <NUM> continues with a step <NUM> of electrochemically plating a second, hard chrome layer over the first Ni-P layer. Chrome plating provides a hard chrome layer, for example, greater than about <NUM> HV, or greater than about <NUM> HV, over the Ni-P layer. The chrome plating process may include an initial step of placing the Ni-P coated substrate in an activation bath, which may include chromic acid. The activation bath removes any scale that may have formed. Thereafter, the activated substrate is placed into a chromium bath, which may include a mixture of chromium trioxide (CrO<NUM>) and sulfuric acid (H<NUM>SO<NUM>), the ratio of which may vary between about <NUM>:<NUM> and about <NUM>:<NUM>, based on the desired process parameters. The temperature of the chromium bath may be from about <NUM> to about <NUM> during the plating process. As noted above, the desired configuration of the second layer is to provide a thickness over the first coating layer that varies gradually (in a tapering manner) across the forward-facing surfaces of the compressor wheel <NUM> such that the thickness of the second coating layer is greatest at the leading edges <NUM> and the shroud edges <NUM> adjacent to the leading edges <NUM>, and the thickness of the coating layer gradually decreases from the leading edges <NUM> rearwardly (along the axis of rotation) in the direction of the trailing edges <NUM>. This gradual reduction in thickness across the compressor wheel is accomplished by positioning the compressor wheel in the chromium bath (electrochemical cell) in a manner such that the leading edges <NUM> (and the hub-side inlet wall portion <NUM>) face the anodic surface of the electrochemical cell. In this manner, the ionic flow in the electrolyte is thus focused towards the leading edges <NUM>. The plating process is controlled with temperature and time to achieve a desired greatest thickness of about <NUM> to about <NUM> microns, as described above. Once hard chrome plating is complete, the substrate, now having the second layer plated thereon, may be rinsed to remove any residues from the plating process.

The method <NUM> concludes with step <NUM>, which may optionally include performing various finishing process, such as final cleaning, polishing, machining, heat treatment at temperatures of up to about <NUM> (for example from about <NUM> to about <NUM> or about <NUM> to about <NUM>) for time period of about <NUM> hour to about <NUM> hours, such as about <NUM> hours to about <NUM> hours, and others as conventionally known in the art. The result is a compressor wheel <NUM> in accordance with that described above in connection with <FIG>.

Accordingly, the present disclosure has provided turbocharger compressor wheels having a bi-layered coating of an electroless nickel-phosphorous base layer and a hard chrome top layer, and methods for manufacturing the same. The present disclosure has addressed the aforementioned erosion problem with the use of electroless nickel-phosphorus as a base layer followed by a hard chrome top layer, located in greatest thickness near the leading edges of the main blades. As such, the turbocharger compressor wheels of the present disclosure are able to withstand the erosive effects of water droplets, without requiring the use of heavier and more expensive materials such as titanium. Moreover, the turbocharger compressor wheels disclosed herein are able to be manufactured easily with existing technologies, such as chemical deposition, that do not result in significant additional manufacturing complexity or expense.

Claim 1:
A compressor wheel for a turbocharger (<NUM>), comprising:
a hub portion defining a rotational axis; and
a plurality of blades (<NUM>, <NUM>) extending radially outward from the hub portion, wherein each blade of the plurality of blades comprises a leading edge, the leading edges of each blade of the plurality of blades forming an inducer portion of a compressor wheel (<NUM>), wherein each blade of the plurality of blades comprises a trailing edge, the trailing edges of each blade of the plurality of blades forming an exducer portion of the compressor wheel, the inducer portion being positioned longitudinally forward from the exducer portion along a rotational axis with respect to a flow of air along the compressor wheel,
wherein the hub portion and the plurality of blades comprise a substrate metal,
wherein the substrate metal of the hub portion and the plurality of blades has coated directly thereon a first coating layer comprising electroless nickel-phosphorous,
characterised in that the first coating layer has coated directly thereon a second coating layer (<NUM>) comprising hard chrome, and
wherein the second coating layer has a thickness that is greatest at the inducer portion, the thickness of the second coating layer decreasing rearward towards the exducer portion such that the thickness of the second coating layer is about zero microns at or longitudinally forward of the trailing edges of each blade of the plurality of blades.