Patent Publication Number: US-2021170478-A1

Title: Powder Metal Alloy Composition for Sintered Powder Metal Insert for Aluminum Casting

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of the filing date of U.S. Provisional patent Application No. 62/592,814 entitled “Powder Metal Alloy Composition for Sintered Powder Metal Insert for Aluminum Casting” filed on Nov. 30, 2017, which is hereby incorporated by reference for all purposes as if set forth in its entirety herein. 
    
    
     STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     FIELD OF THE INVENTION 
     This disclosure relates to an improved ferrous powder metal alloy composition for forming a sintered powder metal insert for casting into an aluminum casting. 
     BACKGROUND 
     In a piston engine, camshafts are used to control the valves and crankshafts are connected to the pistons through connecting rods in order to turn linear motion into rotary motion that can then be usefully employed (e.g., driving the motion of the vehicle). Such shafts are held and supported by bearings that enable the rotation of the shafts during engine operation. During engine operation, these bearings and the shafts attached to them are subjected to fairly significant engine loads and therefore need to be adequately structurally supported. 
     While there is a need for structural support of the bearings that support the engine shafts, there are also engine design considerations (e.g., reduction in weight for improved fuel economy, reductions in material cost) that have led many modern engine blocks to be cast from aluminum or its alloys, when possible, rather than iron or its alloys. While utilizing aluminum helps to reduce weight, using an aluminum-based rather than an iron-based engine block often requires further supporting the regions receiving the bearings. 
     For cast aluminum engine blocks, it is often the case that supportive inserts, often made from cast iron, are cast into the aluminum engine block. These inserts provide increased strength and stiffness in the regions of bearing support when the cam shaft is bolted and engine loads are applied. As one particular example, for main bearing inserts (“MBI”), the insert might be formed from cast iron and then an aluminum alloy (typically an A380 aluminum alloy) is cast around the cast iron insert. During casting, the insert is mechanically locked in place with the cast material solidifying around portions of the geometry of the insert to retain the insert in position. 
     SUMMARY 
     Disclosed herein is an improved powder metal composition for forming bearing inserts which can be cast into aluminum or aluminum alloys in the production of engines. While conventional cast iron inserts rely on mechanical interlocking in which the aluminum or aluminum alloy is cast around geometric features of the insert, the disclosed ferrous sintered powder metal bearing insert has a microstructure in which copper is preferentially higher at the surface of the sintered powder metal insert. This increased amount of copper at the surface of the sintered powder metal insert enables the creation of a metallurgical bond upon contact with the aluminum casting material. To achieve increased copper at the surface of the insert to enable the formation of such a metallurgical bond during casting, the powder metal formulation of the ferrous insert can be carefully engineered such that a copper gradient develops during the fabrication of the powder metal insert that can be further exploited during casting. 
     According to one aspect, a powder metal alloy composition is disclosed for production of a sintered powder metal insert for casting into an aluminum casting. The powder metal alloy composition includes an iron powder metal base, copper added to the iron powder metal base such that the copper is 3.5 weight percent or more of the powder metal alloy composition, and carbon in an amount of 0.1 to 1.0 weight percent of the powder metal alloy composition. It is contemplated that the copper may be supplied as pre-alloyed copper in the iron powder metal base and/or an admixed copper-containing powder metal. Upon compacting and sintering the powder metal alloy composition to form the sintered powder metal insert, the sintered powder metal insert has a copper gradient that provides a higher concentration of copper on the surface of the sintered powder metal insert than in a center of the grains of the sintered powder metal insert. 
     In some forms of the composition, the admixed copper-containing powder metal may be elemental copper powder metal. This admixed copper-containing powder metal may also be non-ferrous (meaning that it could not be possibly characterized as pre-alloyed in the context of the ferrous composition). 
     In some forms of the composition, the copper present in the powder metal alloy composition may be partially pre-alloyed in the iron powder metal base and may also be partially in the admixed copper-containing powder metal that is admixed with the iron powder metal base. In such forms, the copper pre-alloyed in the iron powder metal base may be in an amount of 3 weight percent of the powder metal alloy composition and the copper in the admixed copper-containing powder metal may be in excess of 0.5 weight percent of the powder metal alloy composition. In still other forms, it is generally contemplated that the amount of copper pre-alloyed in the iron powder metal base may be 1, 2, or 3 weight percent of the powder metal alloy composition (with the remainder of the copper being supplied by the admixed copper-containing powder). 
     In some forms of the composition, the copper present in the powder metal alloy composition may include at least 3.5 weight percent copper that is provided in the form of the admixed copper-containing powder metal. It may be the case that all of copper added to the iron powder metal base is from the admixed copper-containing powder metal and there is no pre-alloyed copper in the iron powder metal base. 
     In some forms of the composition, the iron powder metal base may include, at least in part, the carbon therein. 
     According to another aspect, a sintered powder metal insert for casting into an aluminum casting is disclosed. The sintered powder metal insert includes a sintered body comprising any powder metal alloy composition described herein in which a plurality of particles of the powder metal alloy composition are sintered together to form the sintered body. The sintered body has a copper gradient which results in a higher concentration of copper on a surface of the sintered powder metal insert than in a center of the grains of the sintered powder metal insert. This gradient and increased amount of copper at the surface of the insert facilitate formation of a metallurgical bond with an aluminum casting material of the aluminum casting upon casting of the sintered powder metal insert into the aluminum casting material. 
     In some forms of the insert, the sintered body may include a pair of opposing ends on lateral sides thereof, a bearing-receiving surface positioned on a side of the body between the pair of opposing ends in which the bearing-receiving surface is adapted for reception of a bearing therein, and a pair of bolt holes extending through the body in an axial direction perpendicular to an axis of the bearing-receiving surface. 
     In some forms of the insert, a microstructure of the sintered body may include grains of ferrite, pearlite, and discrete MnS. 
     In some forms of the insert, a microstructure of the sintered body includes free copper. In those forms, at least some of the free copper in the microstructure may result from adjacent iron grains reaching their solubility limit of copper. 
     According to another aspect, a component is disclosed including the sintered powder metal insert as recited above and herein in which the sintered powder metal insert has been cast into the aluminum casting material and a metallurgical bond has been formed between the aluminum casting material and the copper in the sintered powder metal insert. 
     In some forms of the component, a microstructure of a region of the metallurgical bond between the copper at the surface of the sintered powder metal insert and the aluminum casting material may include a and el phases of Al—Cu. 
     In some forms of the component, the metallurgical bond may an interfacial layer comprising both aluminum and copper atoms intermixed with one another or diffused or disperse into one another. 
     According to another aspect, a method of forming a sintered powder metal insert and of forming a casting utilizing the insert are disclosed. The powder metal alloy composition disclosed above and herein is compacted and sintered to form a sintered powder metal insert (in which the powder metal alloy composition contains an iron powder metal base, copper added to the iron powder metal base in the form of a pre-alloyed copper in the iron powder metal base and/or an admixed copper-containing powder metal) such that the copper is 3.5 weight percent or more of the powder metal alloy composition, and carbon in an amount of 0.1 to 1.0 weight percent of the powder metal alloy composition. The sintered powder metal insert has a copper gradient that provides a higher concentration of copper on the surface of the sintered powder metal insert than in a center of the grains of the sintered powder metal insert. 
     In some forms of the method, the method further includes casting the sintered powder metal insert into an aluminum casting material to form an aluminum casting with the sintered powder metal insert cast therein in which copper at the surface of the powder metal insert forms a metallurgical bond with the aluminum casting material. 
     These and still other advantages of the invention will be apparent from the detailed description and drawings. What follows is merely a description of some preferred embodiments of the present invention. To assess the full scope of the invention the claims should be looked to as these preferred embodiments are not intended to be the only embodiments within the scope of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a cast component with a sintered powder metal insert cast therein. 
         FIG. 2A-2C  are a microstructure of a 3 weight percent copper ferrous microstructures in which the pre-alloyed amounts of copper are 1 weight percent, 2 weight percent, and 3 weight percent in  FIGS. 2A, 2B, and 2C , respectively, with any remaining copper to reach 3 weight percent copper being in the form of admixed copper. 
         FIG. 2D  illustrates some small amounts of copper in the 2 weight percent admixed sample with 1 weight percent copper pre-alloyed. 
         FIG. 3A  is an exemplary micrograph of a sample illustrating a pathway along which copper concentration is measured in which the pathway traverses multiple grains. 
         FIG. 3B  illustrates a copper mapping line scan for 3 weight percent pre-alloyed copper. 
         FIG. 3C  illustrates a copper mapping line scan for 3 weight percent admixed copper. 
         FIG. 4  is an aluminum-copper phase diagram. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein is a ferrous powder metal composition that provides a metallurgical bond when the powder metal of the composition is compacted and sintered to provide an insert that is subsequently cast in an aluminum or an aluminum alloy. By adding copper to the ferrous powder metal in a sufficient amount and in a way to produce a microstructure having a copper gradient with high amounts of copper—or even free copper—at the powder surface of the compacted and sintered part, the resultant insert can be cast into an aluminum-based material and react with and form a metallurgical bond therewith. Thus, in contrast to conventional inserts, which may rely solely on mechanical bonding (e.g., macroscopic geometric engagement of the cast material solidifying around the insert), an even stronger metallurgical bond can be formed through this altering of chemistry. 
       FIG. 1  illustrates a side view an exemplary component  100  that may be formed by casting an insert  102  into an aluminum material  104 . As illustrated, this component  100  can be one half of a bearing support for a bearing of a camshaft or crankshaft in an engine. As such, it has a bearing-receiving surface  106  in the form of a radially-inward facing semi-cylindrical surface that can be mated with a component having the other half of the radially-inward facing semi-cylindrical surface to capture a bearing therebetween in which the bearing might support a camshaft or crankshaft. The insert  102  also has a generally planar mating surface  108  that is positioned to contact the other component and two threaded holes  110  or bolt holes that extend in a direction perpendicular to the central axis of the bearing-receiving surface  106 . When the mating surfaces of this component  100  and the mating component are placed together, these threaded holes  110  may be used to help secure the components together with bolts or other fasteners. 
     It is also noted that the insert  102  has an outer periphery  112  which, when the aluminum material  104  is cast around it, has a geometric shape that helps to maintain the mechanically secure the insert  104  in the aluminum material. 
     An insert such as insert  102  can be formed from a powder metal by taking a powder metal composition (typically, involving a plurality of individual particles and some amount of wax/lubricant), compacting the powder metal composition to form a preform having the rough geometric shape of the insert (typically, under uniaxial compression in a tool and die set), and then sintering the preform such that the individual particles neck into one another fuse together while reducing porosity. The compacting and sintering might occur separate and sequentially or might occur simultaneous with one another (e.g., as part of a powder forging step). During sintering as the preform is sintered to form the insert  102 , the part densifies during the sintering process, with pores decreasing in size and closing off as the process continues. After the part is sintered, additional finishing or machining steps may occur such as, for example, the drilling or tapping of the holes  110  or the precise finishing of the dimensions of the bearing receiving surface  106 . 
     To form the component  100  illustrated, the insert  102  is put in a form and an aluminum material is cast around the insert  102 , typically an aluminum alloy (e.g., A380 which has copper between 3-4 wt %, magnesium of 0.1 wt %, maximum iron of 1.3 wt %, maximum tin of 0.35 wt %, maximum nickel of 0.5 wt %, zinc of 3 wt %, manganese of 0.5 wt %, silicon of 7.5-9.5 wt %, other metallic not exceeding 0.5 wt % total, with aluminum being the balance). While A380 is mentioned above because it is a well-known die casting aluminum alloy, A380 is representative but in no way limiting of the aluminum casting materials the might be used. 
     Notably, to enable reaction metallurgical bonding between a sintered powder metal insert  102  and the aluminum material  104  during casting, the powder metal alloy composition used to form insert  102  can be modified. It has been found that by the addition of appreciable amounts of copper to the ferrous powder metal composition used to make the insert, a copper gradient can develop, which leads to copper-rich surfaces of the insert  102  (with copper content either being high in the ferrous grains or copper being present as free copper), which can promote the formation of a reaction resulting in a metallurgical bond between the insert  102  and the aluminum material  104  with this metallurgical bond promoting an even stronger connection than the conventional mechanical bond that occurs during casting. 
     In order to promote the metallurgical bond between the insert  102  and the aluminum material  104 , the copper in the ferrous powder metal is increased to an amount at or above 3.5 weight percent. This 3.5 weight percent copper exceeds the solid solubility limit of copper in grains of iron, and results in a copper gradient occurring, with higher copper amounts developing at the surface of the sintered powder metal particles and perhaps even the presence of free copper. 
     Accordingly, the powder metal alloy composition for production of a sintered powder metal insert for casting into an aluminum casting can include a plurality of powder metal particles including an iron powder metal base (i.e., grains of an iron powder metal which possibly including some or all of the copper and/or carbon), copper (pre-alloyed with the base powder and/or admixed) in an amount of 3.5 weight percent or more of the composition, and carbon in an amount of 0.1 to 1.0 weight percent of the composition to add additional strength. While the carbon need not necessarily be found as part of the iron powder metal base, it is contemplated that in most cases, the carbon will be present in the iron powder metal base. Upon compacting and sintering the powder metal alloy composition to form the sintered powder metal insert  102 , the sintered powder metal insert  102  has a copper gradient that provides a higher concentration of copper on the surface of the sintered powder metal insert  102  than in a center of the grains of the sintered powder metal insert. 
     It is contemplated that the copper can be added to the iron powder metal base in the form of pre-alloyed copper in the iron powder metal base and/or in the form of an admixed copper-containing powder metal (which may simply be an elemental copper powder metal admixed with the iron powder metal base). Thus, in some forms, the copper present in the powder metal alloy composition can be partially pre-alloyed in the iron powder metal base and can be partially in the admixed copper-containing powder metal that is admixed with the iron powder metal base. In one form, 100% of the added copper to reach the 3.5 weight percent threshold may be from admixed copper-containing powder metal with no pre-alloyed copper at all being present in the iron powder metal base. It is contemplated that admixing copper at least some amount of the copper may be advantageous to developing a copper rich powder surface because admixing copper—as opposed to pre-alloying copper—can result in an increased copper gradient (as will be evident from the results in  FIGS. 3B and 3C  below). 
     For example, in some forms, the copper pre-alloyed in the iron powder metal base may be in an amount of 3 weight percent of the powder metal alloy composition and the copper in the admixed copper-containing powder metal may be in excess of 0.5 weight percent of the powder metal alloy composition. Likewise, it is contemplated that copper could be pre-alloyed in the iron powder metal based in amounts of, for example, 1, 2, and 3 weight percent of the powder metal alloy composition, with additional amounts of admixed copper being present to reach the 3.5 weight percent overall threshold (e.g., for 1 weight percent pre-alloyed, there would be at least 2.5 weight percent admixed) 
     Referring now to  FIGS. 2A-2C , microstructures of 1 weight percent pre-alloyed copper/2 weight percent admixed copper; 2 weight percent pre-alloyed copper/1 weight percent admixed copper; and 3 weight percent pre-alloyed copper after production forging trials. Again, for the sake of clarity, the pre-alloyed copper is pre-alloyed in an iron or ferrous powder metal. It can be seen that all of the 3 percent copper samples had ferrite, pearlite, and discrete MnS phases. 
     With reference to  FIG. 2D , free copper is again present in the microstructure. 
     Turning now to  FIGS. 3A-3C , copper mapping was performed on some of the prepared samples to illustrate the copper distribution and gradient across the grains.  FIG. 3A  illustrates an example path for a line scan (which was performed using a JEOL JSM-6460LV SEM with EDX). This line scan was performed over a distance sufficient to cross multiple prior particles to show the relative uniformity. Sampling was performed at 7 micron increments. 
       FIG. 3B  shows the copper mapping results for a 3 weight percent pre-alloyed sample, while  FIG. 3C  shows the copper mapping results for a 3 weight percent admixed copper sample. The relative homogeneity of the distribution of the copper in the pre-alloyed sample can be seen, as can be the significant peaks and troughs in the admixed copper sample (which represent copper rich surfaces of grains and copper-free regions of the center of the grains, respectively). 
     It is noted that admixing copper to an iron powder metal base already having 3 weight percent copper pre-alloyed, would result in a combined copper profile with high copper at the surfaces. This is because the solubility limit of copper in iron is around 4 weight percent and the admixed copper would not be able to appreciably diffuse into the pre-alloyed iron grains. 
     Finally, turning to  FIG. 4 , an aluminum-copper phase diagram is shown. During casting, when the molten aluminum (or aluminum alloy) comes into contact with copper, some amount of the copper on the surface will dissolve into the aluminum. Given the rate of casting and cooling, it is contemplated that not all copper will dissolve into the aluminum liquid. This will create some localized regions of the aluminum liquid with low copper percentages. As the liquid Al—Cu melt cools, the α-phase will form and, depending on kinetics and dissolved copper composition, most likely, some amount of the α-phase plus θ-phase will form. This creates a localized region between the aluminum casting material and the copper, along which a metallurgical bond is created. 
     It should be appreciated that various other modifications and variations to the preferred embodiments can be made within the spirit and scope of the invention. Therefore, the invention should not be limited to the described embodiments. To ascertain the full scope of the invention, the following claims should be referenced.