Process to manufacture a sintered metallic product and product thus obtained

Abstract: The process essentially consists in carrying out, successively, the following main operations : - to manufacture a sintered substratum ( 12 ) which consists of a first metallic material with pores occupied by a second metallic material ( 10 ) . - to lay at least one part of the sintered substratum (12) with a predetermined amount of a third pulverized "noble" metallic material ( 13 ); and - to sinter the predetermined amount of the third material until a required degree of porosity is obtained.

Description: The present invention relates to a process to manufacture a sintered metallic product and to the product thus obtained. Particularly, but not exclusively, the aforesaid invention consists of a lifter head for an internal combustion engine. Preferably, lifter heads consist of a first base material layer which, in use, is joined to a respective shank by means of a conventional operation ( welding, brazing, mechanical junction, etc. ) and of a second layer, bound up with the first layer, which consists of a material so-called "noble" owing to its particular resistance to the abrasion resulting from the percussion, rolling and slip stresses owing to the cam which, in use, acts upon it. Processes ( plasma spray, transferred arc plasma, laser alligation, etc. ) able to lay a base metallic material substratum with a layer of "noble" metallic material are known. Although widely used, these processes are not without drawbacks. Actually the "noble" material laid onto the metallic substratum is prevented from undergoing the thermal contraction during the temperature decreasing phase. Thereof it is subject to internal tensile stresses and consequently susceptible of cracking. It is known from the document GB-A-544 903 a process of manufacturing a continuous strip of sintered materials, including the steps of diffusing into a porous matrix a soft metal powder. The strip is firstly rolled by pressure rollers and then heated in a furnace to cause the metal powder to melt and to flow into the pores of the matrix. The molded article is a porous strip of soft metal. It is also known form the document WO-A-83/04382 a process of making a wear resistant ferrous part, provided with wear surfaces formed of a diluted aqueous slurry of wear resistant particles deposited into the pores of the part, which are finally dried. The object of the present invention is to conceive a process and to manufacture a product able to obviate the aforesaid drawbacks of the known processes and products. Said object is attained according to the present invention by a process to manufacture a sintered metallic product having at least a portion subject to abrasion stresses, comprising the following operations: providing a porous matrix by compacting premixed iron-based powders as to form a housing corresponding to said portion and laterally delimited by a projecting part; filling said housing with a layer of metallic material capable of diffusing into said porous matrix at the sintering temperature of said porous matrix; sintering said porous matrix and said layer at said temperature until the material of said layer is completely diffused into the pores of said porous matrix, thus obtaining a substratum substantially deprived of porosity; depositing in said housing a predetermined amount of a powder metallic material resistant to abrasion stresses; compacting said amount of powder metallic material till obtaining therein a predetermined degree of porosity; and sintering said amount of powder metallic material so compacted to form a filling in said housing adhering to said substratum. Said object is also attained according to the present invention by a sintered metallic product having at least a portion subject to abrasion stresses, comprising a substratum having a housing in correspondence with said portion, and a filling located in said housing, characterized in that said substratum is formed of a porous matrix made of compacted premixed iron-based powders sintered together with a second metallic material as to cause the pores of said porous matrix to be filled with said second metallic material, said housing being laterally delimited by a projecting part, said filling being made of a predetermined amount of powder metallic material resistant to abrasion stresses, which is compressed and sintered in said housing. Further embodiments are disclosed in the dependent claims. The present invention will be more apparent from the description of a preferred embodiment thereof given hereinafter by way of non-limiting example with reference to the accompanying drawings, in which figures 1 to 8 illustrate the successive phases of the process, and figure 8 particularly illustrates a section of an example of a product manufactured in accordance with the process of the present invention. In figure 1 the reference numeral 1 indicates a mould provided with a cavity filled with a first metallic material 3, and the reference numeral 5 indicates a countermould presenting an annular recess 6 around it. According to the present invention, the material 3 essentially consists of a mixture of preliminarily premixed iron-based powders. Figure 2 shows how the countermould 5 cooperates with the mould 1 to compact the metallic material 3 in order to obtain ( figure 3 ) a matrix 7 with a concave housing 8 laterally delimited by an annular projecting part 9. Figure 4 shows how the housing 8 is filled with a second metallic material 10 which, in accordance with the present invention, consists of compacted copper or of a mixture of copper-based powders. In figure 5, the matrix 7, which consists of the first material 3 and contains the second material 10 in its own housing 8, is subjected to sintering, preferably at a temperature of about 1120°C and in the presence of hydrogen, until the second material ( copper or copper-base powders ) diffuses into the first material ( compacted mixture of iron-base powders ) and completely impregnates it, that is to say, substantially occupying all the pore that the matrix 7 presented at the end of the compacting illustrated in figure 2 . In this way, we obtain a substratum 12 which essentially consists of an iron matrix ( first material ), the pores of which are completely saturated with copper ( second material ) . Actually, as it is well known in the field of sintered materials, the fusion ( or rather the diffusion and then the fusion ) of the copper powder into the iron matrix enables to obtain a substratum substantially with no porosity whatsoever ; moreover the copper which is in the pores of the iron matrix favours the adhesion of subsequent fillings, according to the description hereinafter . In figures 6, the housing 8 of the substratum 12 is filled with a third "noble" metallic material 13, and particularly able to resist abrasion stresses such as percussion, rolling, slip, etc. Then the material 13 ( see figure 7 ) is tamped and/or compacted by means of the mould 1 and a countermould 14 and then sintered . In particular, during this latter sintering phase the mould 1 as well, as the countermould 14, are kept at a temperature substantially identical to the sintering temperature of the material 13 . In this way, we obtain a product 15 ( figure 8 ) without residual tensile stresses and therefore not susceptible of cracking . Moreover, thanks to the presence of the copper which was impregnated the pores of the matrix 7, all the metallic material 13 stays within the housing 8 so as to fix a filling 16 which substantially consists of "noble" material only . Preferably, the sintering of the "noble" material 13 is carried out so as to control the porosity of the filling 16, in order to be able to impregnate it later with lubricating oil ( according to a conventional method ) and therefore to make it self-lubricating . The advantages of the process and the product provided in accordance with the present invention are therefore apparent from an examination of their characteristics . In case the product 15 should be used as a lifter head for an internal combustion engine, it would fit all the best service requirements, such as : a good abrasion resistance of the filling 16, which entirely consists of self-lubricating noble material ; actually this noble material could not migrate into the substratum 12 during the respective sintering , the pores of the matrix 7 being impregnated by the second metallic material . a strong substratum 12 which can be conveniently joined to a respective support shank; and no predisposition whatsoever to cracking, thanks to the gradual and simultaneous cooling of the substratum 12 and the filling 16. Therefore it is reasonable to think that the sintered product obtained in accordance with the present invention also permits to make maintenance less frequent.

Claims: A process to manufacture a sintered metallic product having at least a portion subject to abrasion stresses, comprising the following operations: providing a porous matrix (7) by compacting premixed iron-based powders (3) as to form a housing (8) corresponding to said portion and laterally delimited by a projecting part (9); filling said housing (8) with a layer of metallic material (10) capable of diffusing into said porous matrix (7) at the sintering temperature of said porous matrix (7); sintering said porous matrix (7) and said layer (10) at said temperature until the material of said layer (10) is completely diffused into the pores of said porous matrix (7), thus obtaining a Substratum (12) substantially deprived of porosity; depositing in said housing (8) a predetermined amount of a powder metallic material (13) resistant to abrasion stresses; compacting said amount of powder metallic material (13) till obtaining therein a predetermined degree of porosity; and sintering said amount of powder metallic material (13) so compacted to form a filling (16) in said housing (8) adhering to said substratum (12). A process as claimed in claim 1, characterized in that said layer of metallic material (13) is in form of compacted copper. A process as claimed in claim 1, characterized in that said layer of metallic material (13) is in form of a mixture of copper-based powders. A process as claimed in claim 2 or 3, characterized in that the sintering of said porous matrix (7) and said layer of metallic material (10) is effected at a temperature of about 1120°C and in presence of hydrogen. A process as claimed in any previous claim, characterized in that said powder metallic material (13) is compacted in a support body (1, 14) including a mold (1) and a countermold (14), the sintering of said filling (16) being carried out by substantially keeping said support body (1, 14) at a temperature identical to the temperature of said filling (16) to provide, after the sintering, the same decrease of temperature in said substratum (12) and in said filling (16). A process as claimed in any previous claim, characterized by providing, after the sintering of said filling (16), an impregnating of lubricating oil into the pores of said filling (16). A sintered metallic product having at least a portion subject to abrasion stresses, comprising a substratum (12) having a housing (8) in correspondence with said portion, and a filling (16) located in said housing (8), characterized in that said substratum (12) is formed of a porous matrix (7) made of compacted premixed iron-based powders (3) sintered together with a second metallic material (10) as to cause the pores of said porous matrix (7) to be filled with said second metallic material (10), said housing (8) being laterally delimited by a projecting part (9), said filling (16) being made of a predetermined amount of powder metallic material (13) resistant to abrasion stresses, which is compressed and sintered in said housing (8). A product as claimed in claim 7, characterized in that said second metallic material (10) is formed of compacted copper or a mixture of copper-based powders. A product as claimed in claim 7 or 8, characterized in that said filling (16) is compacted as to provide a predetermined degree of porosity and after being sintered, is impregnated with lubricating oil. A product as claimed in any claim from 7 to 9, characterized it is designed to form a lifter head of an internal combustion engine.

Inventors: ANTONA PAOLO LUCA, TARICCO STEFANO, ANTONA, PAOLO LUCA, TARICCO, STEFANO
Applicants: FIAT AUTO SPA, FIAT AUTO S.P.A.
Application year: 1990
Main classification: B22F3
IPC codes: B22F3, B22F7, F01L1