The design phase is the first step in the production of a metal powder part. Here, the application is evaluated carefully to determine the optimum choice of material, die configuration, sintering conditions and, if necessary, any secondary operations needed to achieve specified properties.
The conditions under which the
part will operate (e.g. stress state, stress level, temperature) define the
combination of properties needed and consequently, the materials capable of
meeting those needs. One benefit of PM is its ability to tailor the
microstructure to provide specific properties and performance. This can be done
by alloying the base powder with additives in the form of elemental or
multi-element metallic materials and usually a dry lubricant to facilitate
extraction of the part from the die. Alloying can be performed in three ways:
prealloying - adding the alloying elements to the liquid metal prior to
atomization; diffusion bonding - adding alloy powders to the base powder,
followed by a thermal treatment to bond the additives to the base powder; and
premixing - making a physical mixture of the base and additive powders. Each
mode produces different physical and mechanical properties even though the
overall chemical composition of the powder alloy is the same.
Design of the tooling is another
important consideration in the design stage. Since most PM parts are produced
by pressing, the configuration of the die and punches is very important. In
many cases, complex shapes with multi-level surfaces can be formed in a single
pressing operation. Consequently, creative tool design is essential in the
manufacture of parts with high performance and quality, while maintaining the
economic advantage of PM.
After pressing, the parts are
subjected to one or more thermal treatments. Usually, sintering is the step
directly after pressing. Consideration must be given to the microstructure
formed in the sintering operation in terms of property levels and applications.
Major variables in sintering are temperature, time, and the sintering
atmosphere. In addition, parts can be quenched and tempered to achieve
specified levels of hardness, strength, and toughness after sintering. Other
secondary thermal treatments, such as steam treating and infiltration can be
used to enhance physical and mechanical properties.