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:
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 :
Therefore it is reasonable to think that the sintered product obtained in accordance with the present invention also permits to make maintenance less frequent.