Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Post sintering operation
Patent
1982-06-10
1983-12-20
Hunt, Brooks H.
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Post sintering operation
419 36, 419 53, B22F 312
Patent
active
044217170
DESCRIPTION:
BRIEF SUMMARY
RELATED APPLICATIONS
A corresponding PCT application, PCT/US82/00789, has been filed.
BACKGROUND OF THE INVENTION AND PRIOR ART STATEMENT
Rocker arms, valve guide sleeves, and other high wear parts for an engine have typically been made of solid material and in the case of rocker arms have usually been castings comprised of cast iron. These iron parts have required costly post-treatment, such as furnace heating, to improve wear characteristics. In the hope of reducing cost and weight, other materials have been substituted for the solid castings, such as sheet metal stampings. The substituted materials have shown poor wear along with low strength and low durability. When aluminum was substituted for the iron castings, special precautions had to be made to control the quantity of alloying ingredients, such as silicon, and the addition of lubricating substances, such as tin and lead, in order to achieve a reasonable degree of wearability (see U.S. Pat. No. 4,147,074).
In recent years vehicle manufacturers have turned to powder metal parts in the hope of reducing the processing costs, but have not been able to achieve high wear resistance in such parts. They have thus relegated powder metal to applications of low to moderate wear. For powdered parts to be utilized in critical applications, such as in valve seats, the powdered metal was made of a high alloy content material and the particles were agglomerated in conventional fashion (see U.S. Pat. No. 4,062,678 and 4,035,159). Each of these patents fail to exhibit superior surface wear resistance as demanded by critical engine applications today.
In the past, coatings have been applied to powdered metal parts to improve surface conditions, but in each instance the process involved multiple sintering steps with interposed coating steps, the coatings not being of a particularly low friction characteristic and typically were of a smearable type characterized by the group including copper, tin, zinc and lead (see U.S. Pat. No. 3,684,498 and 2,299,192). Techniques used in related arts to apply coatings to powdered metal parts have included plasma spraying of a melted alloy to obtain adherence by impact and high temperature bonding. Such spraying has usually been carried out on solid finished parts to obtain proper bonding. Plasma spraying suffers from high cost and the necessity for a post grinding operation. Another technique casts-in-place an equivalent to a coating about a desired part. Unfortunately, such cast-in-place coatings have not necessarily maintained successful adherence under service and have suffered from delamination. Moreover, such coatings undergo different shrinkage than the main body of the casting.
SUMMARY OF THE INVENTION
The invention is a method of making a wear resistant, ferrous based part by: (a) molding a uniform mixture of ferrous based powder and binder material into a compacted shape; (b) heating said compacted shape to remove said binder material and to partially sinter said mixture to a strength of about 1000-8000 psi, while maintaining a porosity of 20-40% at least along the outer region of said part; (c) depositing a fluid suspension of wear resistant powder particles onto at least a selected surface zone of said partially sintered shape, said particles coating said zone and permeating the surface region of said zone by absorption as permitted by said porosity; and (d) heating said coated shape to remove said fluid, to bond said particles to said shape, and to fuse said powder and particles together to define said part.
Preferably, step (a) may be carried out with one or more of the following features: by mixing the ferrous powder and binder material in a ratio of 70/30% by weight, mechanically mixing the powder and a thermoplastic binder for 15-20 minutes at a temperature of 300.degree.-350.degree. F., by forming the mixture into pellets, constituting the ferrous powder of SAE 52100 steel, controlling the particle size of the mixture to have at least a 35% volume fraction which is -325 mesh and, more particularly, controlli
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Brookes Anne
Ford Motor Company
Hunt Brooks H.
Johnson Olin B.
Malleck Joseph W.
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