Stock material or miscellaneous articles – All metal or with adjacent metals – Having metal particles
Patent
1995-03-20
1997-08-05
Jordan, Charles T.
Stock material or miscellaneous articles
All metal or with adjacent metals
Having metal particles
428550, 428566, 428567, 419 27, 419 47, B22F 326
Patent
active
056541066
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method for the manufacture of elongate tubular articles by powder metallurgy (PM) techniques and to a product produced thereby.
SUMMARY OF THE INVENTION
Articles having a generally elongate tubular form may be used in many diverse applications such as, for example, valve guides for engines and bearing bushes for sliding contact. The present invention will be illustrated by the particular problems associated with the manufacture of valve guides for internal combustion engines, but it is stressed that the method described hereinafter is equally applicable to the manufacture of many other articles having a generally elongate tubular form.
It is known to manufacture valve guides by PM techniques for the types of engine generally found in passenger car vehicles for example. Such guides are generally of relatively plain tubular form and have an axial length of less than 70 mm. Such valve guides are produced in very large numbers. PM valve guides are frequently manufactured from ferrous materials and may or may not be infiltrated with, for example, a copper-based alloy. Infiltration with such alloys can greatly improve both the machinability of the guide during manufacture and the wear-resistance in service.
Conventionally, larger valve guides for the types of engine used in generating sets, military vehicles, marine propulsion applications, larger commercial vehicles such as trucks and highway construction vehicles for example, have used valve guides machined from solid, cast materials. Valve guides used in these larger types of engine are often of relatively intricate design having machined features such as location flanges or grooves for example. With the advent of ever more stringent environmental regulations applying to the emissions from all engines and also due to the constant pressure to improve the performance of all components that go into an engine, it is being found that the conventional cast materials such as cast-iron and phosphor-bronze no longer have the wear resistance demanded by the higher loads and temperatures of modern higher performance engines. In addition to this, materials such as phosphor-bronze are very expensive.
PM manufacturing techniques allow the materials engineer to fine-tune material compositions and the metallurgical microstructure in a way that is denied to conventional ingot metallurgy, this is particularly so in the case of composite microstructures which are highly suited to sliding and bearing applications. Alloy compositions and microstructures may be produced which are impossible to produce by ingot metallurgy methods. However, the pressing of valve guides is limited to a maximum axial length of about 70 mm. This limitation is due to the height of the powder column which may be pressed and which is constrained by press dimensions, kinetics and most importantly by frictional energy losses at the pressing tool/pressed component interfaces and within the body of the compressed powder mass itself. The result of these losses is a variation in density between the axial ends of the pressed tube and the mid-point, assuming that double-ended pressing is employed. At about 70 mm axial length, the mid-point of the tube has a significantly lower density than the ends of the tube resulting in weakness. The variation in density between the ends and the mid-point increases as the length of the pressed tube increases, leading to the stated practical maximum of 70 mm axial length. This limitation on length has not permitted powder metallurgy valve guides to enter the field of larger engines significantly.
In the case of a non-infiltrated valve guide, the lower density at the mid-point produces an area of weakness which makes the pressed blank (called a "green" blank) susceptible to damage by cracking, chipping-or fracture during handling prior to sintering. In the case of an infiltrated guide, the above disadvantages still occur, but there is the additional disadvantage that the lower density, weaker centre region which
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Brownlie Helen Ann
Purnell Charles Grant
Brico Engineering Limited
Carroll Chrisman D.
Jordan Charles T.
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