|
1. Mix the metal powder or powders with a suitable lubricant. 2. Load the mixture into a die or mould and apply pressure. This gives what is called a compact which requires only to have sufficient cohesion to enable it to be handled safely and transferred to the next stage. Such compacts are referred to as green, meaning unsintered: hence the terms green density and green strength. 3. Heat the compact, usually in a protective atmosphere, at a temperature below the melting point of the main constituent so that the powder particles weld together and confer sufficient strength to the object for the intended use. This process is called sintering hence the term sintered parts. In certain cases a minor constituent becomes molten at the sintering temperature in which case the process is referred to as liquid phase sintering. The amount of liquid phase must be limited so that the part retains its shape. In certain special cases stages 2 and 3 are combined i.e. compaction is done at an elevated temperature such that sintering occurs during the process. This is termed hot pressing, or pressure sintering. In many cases the sintered part is subjected to additional processing - repressing, plating etc. and these will be dealt with in the appropriate sections below. In certain special cases, e.g. in the manufacture of filter elements from spherical bronze powder, no pressure is used, the powder being placed in a suitably shaped mould in which it is sintered. This process is known as loose powder sintering.
The object of mixing is to provide a homogeneous mixture and to incorporate the lubricant. Popular lubricants are stearic acid, stearin, metallic stearates, especially zinc stearate, and increasingly, other organic compounds of a waxy nature. The main function of the lubricant is to reduce the friction between the powder mass and the surfaces of the tools - die walls, core rods, etc. - along which the powder must slide during compaction, thus assisting the achievement of the desired uniformity of density from top to bottom of the compact. Of equal importance is the fact that the reduction of friction also makes it easier to eject the compact and so minimises the tendency to form cracks. It has been suggested that an additional function of the lubricant is to help the particles to slide over each other, but it seems doubtful whether this factor is of much significance: - good compacts can be obtained without any admixed lubricant, e.g. using die wall lubrication or isostatic pressing. Care in the selection of lubricant is necessary, since it may adversely affect both green and sintered strengths especially if any residue is left after the organic part has decomposed. Over-mixing should be avoided, since this increases the apparent density of the mix. Additionally, over-mixing usually further reduces the green strength of the subsequent compacts probably by completely coating the whole surface of the particles, thereby reducing the area of metal to metal contact on which the green strength depends. The flow properties also are impaired and good flow is essential for the next step i.e. loading the powder into the die. In the special case of cemented carbides, the mixing process is carried out in a ball mill, one of the objects being to coat the individual particles with the binder metal e.g. cobalt, but as the very fine powders involved do not flow, the mixture is subsequently granulated to form agglomerates.
The mixed powders are pressed to shape in a rigid steel or carbide die under pressures of 150-900 MPa. At this stage, the compacts maintain their shape by virtue of cold-welding of the powder grains within the mass. The compacts must be sufficiently strong to withstand ejection from the die and subsequent handling before sintering.
A critical operation in the process, since the final shape and mechanical properties are essentially determined by the level and uniformity of the as-pressed density. Powders under pressure do not behave as liquids, the pressure is not uniformly transmitted and very little lateral flow takes place within the die. The attainment of satisfactory densities therefore depends to a large degree on press tool design
Sintering is the means whereby the powder particles are welded together and a strong finished part produced
Even with the best control that is feasible in practice, there will inevitably be some variation in the dimensions of parts produced from a given material in a given die set.
Dimensional accuracy can be greatly improved by re-pressing the part after sintering. This operation is called sizing.
HIP is used as a post-sintering operation to eliminate flaws and microporosity in cemented carbides.
Forging is a comparatively recent technique in which a blank is hot re-pressed in a closed die which significantly changes the shape of the part, and at the same time can give almost complete density and hence mechanical properties approaching or even surpassing those of traditional wrought parts. Sinter forging is dealt with in more detail in a later section.
An alternative method of improving the strength of inherently porous sintered parts is to fill the surface connected pores with a liquid metal having a lower melting point. Pressure is not required:
The process is used quite extensively with ferrous parts using copper as infiltrant but to avoid erosion, an alloy of copper containing iron and manganese, is often used. Clearly if the molten copper is already saturated with iron its ability to erode the surface is lost.
Infiltration is also used to make composite electrical contact material such as tungsten/copper and molybdenum/silver; the lower melting point metal being melted in contact with an already sintered skeleton of W or Mo.
This term is used for a process analogous to infiltration except that the pores are filled with an organic as opposed to a metallic material.
Although many, perhaps the bulk of sintered structural parts are used in the as-sintered or sintered and sized condition, large quantities of iron-based parts, correctly steels, are supplied in the hardened and tempered conditions. Conventional hardening processes are used, but because of the porosity inherent in sintered parts, they should not be immersed in corrosive liquids - salt baths, water, or brine - since it is difficult to remove such materials from the pores. Heating should be in a gas atmosphere followed by oil-quenching. These restrictions may not apply to very high density parts 7.2 g/cc nor to parts that have been infiltrated.
Carburizing and carbonitriding of PM parts is extensively used, and again gaseous media are indicated.
A process peculiar to PM parts is steam-treatment which involves exposing the part at a temperature around 500°C to high pressure steam. This leads to the formation of a layer of magnetite (iron oxide) on all accessible surfaces and a number of desirable property changes result.
The treatment is not generally applicable to hardened and tempered parts because the exposure to the high temperature would undo the hardening.
Heating in air at a lower temperature (200-250°C) can also be used to provide a thin magnetite layer that gives some increase in corrosion resistance, but it is much less effective than steam treatment.
Sintered parts may be plated in much the same way as wrought or cast metals, and copper, nickel, cadmium, zinc, and chromium plating are all used.
Recent work has shown that it may be possible successfully to plate unimpregnated porous parts with nickel by electroless plating, which process will plate also the surfaces of any of the pores into which the solution penetrates.
A large percentage of hardmetal cutting tool inserts are now coated using chemical vapour deposition (CVD) or physical vapour deposition (PVD). The lower temperature PVD process also allows steels to be given a wear resistant layer of TiC, TiN, Al2O3 or a combination of these materials and sintered high speed steel tools are also now being coated.
Although a major attraction of PM parts is that they can be produced accurately to the required dimensions, there are limitations to the geometry that can be pressed in rigid dies, and subsequent machining, for example of transverse holes or re-entrants at an angle to the pressing direction is not uncommon.
The machinability can be improved by incorporating certain additions in the powder mix - e.g. lead, copper, graphite, sulphur or a metal sulphide such as manganese sulphide, and, as already indicated, by infiltration or resin impregnation. If these points are borne in mind all the traditional machining processes - turning, milling, drilling, tapping, grinding, etc. - can be done quite readily.
De-burring is done with sintered parts, and is used to remove any 'rag' on edges, resulting from the compacting operation or a machining step. Tumbling, sometimes in a liquid medium with an abrasive powder, is normally employed. At the same time, brushing and some hardening of the surface layers may occur.
Another class of wrought sintered material that is beginning to make an impact is based on particulate material - powder or chopped ribbon - that has been solidified and cooled at a very high rate such that metastable non-equilibrium microstructures result. They may be microcrystalline or amorphous. The process is applicable only to certain alloys, and one important feature is that the matrix metal can retain in solid solution a much higher than the equilibrium percentage of the alloying element. Providing that the densification and mechanical working is carried out at a temperature low enough to avoid destroying the non-equilibrium structure, remarkably enhanced mechanical properties can be achieved. A major development programme is underway with alloys of aluminium, titanium, and magnesium, the hope being that their use in aircraft structures will significantly reduce the weight and increase the payload.
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
