Complementary manufacturing operations
Applications of complementary operations on a powdered metal component allows to improve one or more characteristics not directly achievable from the basic process.
Sizing is a re-pressing the sintered part inside a rigid mould which has the symmetrical opposite shape of the part. Sizing can have several purposes:
- To increase the dimensional radial precision, reaching up to IT 5, with a densification degree lower than 4%.
- To create relief geometrical details not feasible in the compacted part, or to improve roughness.
- To densify, by appling a densification degree is 7-10% (also known as Coining). Densities up to 7.6 g/cm3 are possible.
Impregnation consists of filling pores with a chemical product:
- Oil impregnation. Oil contained in the pores can act as lubricant of a bushing-shaft interface, hence self-lubrication is provided.
- Plastic or resin impregnation. It provides sealing, and it is quite common as preparation of the part before coating/plating.
- Copper impregnation, called Infiltration. It consists of sintering the part together with a pressed copper part on it. Copper melts during sintering, and becomes infiltrated inside the pores by capillarity. This technique is used to increase the toughness and mechanical srenght of low alloy powdered metal steels.
Machining of powdered metal components is needed when a shape or tolerance is not reachable directly by pressing. All conventional machining operations are possible, ie., turning, milling, drilling, threading, grinding, lapping, reaming, polishing, etc.
Deburring isused to remove burrs either in bulk (tumbling, shot blasting), or on a unit basis (brushing, polishing, electrolytic deburring). Bulk deburring is often used to provide an homogeneous surface aspect, or a very low roughness.
Cleaning operations reduce or eliminate the amount of pollutants, solid or liquid, that a part may contain. There are many techniques, depending on material, type of pollutant, and required specifications.
Steam treatment is a thermal cycle of controlled oxidation of a steel, carried out in continuous furnaces with water steam. A magnetite layer is formed on the part surface and porosity. It increases compression strength, seals porosity, and improves the environmental corrosion resistance.
A heat treatment is a thermal cycle that modifies the material properties. They are typically applied to increase hardness and strength of the component. The main surface or core heat treatments are: quenching, case hardening, and carbonitriding. Induction hardening is used to increase hardness in a local area of the part. Other possible treatments are ion nitriding, oxynitrocarburizing, or annealing.
A particular case are the sinter-hardened components, which are made of a special type of steel that becomes hardened in the sintering furnace during cooling.
COATING / PLATING
It is a material deposition on the component surface, which modifies the surface properties without changing the base metal chemical composition. Coatings fulfil functions under fatigue, wear, friction, or corrosion protection.
Powder metal parts accept almost all conventional coatings, like zinc plating, chromium plating, nickel plating, phosphating, metallisation, PTFE, and other special coatings.
Powder metal parts are susceptible to be joined to other sintered parts or to components made from other technologies. Typical joining techniques are welding, brazing, sinter-bonding, fitting, sticking, riveting, over-moulding, etc.