After the component is sintered, additional changes in size, shape, surface finish, or heat treatment are possible. The material is essentially in the annealed condition after sintering and there is no significant difference in secondary operations from any other manufacturing route; the response is the same as with wrought materials.
Common secondary operation steps include the following:
- Coining or cold deformation – A sintered component is forced to conform to a rigid mandrel or substrate to straighten or ensure desired flatness or dimensions. This allows for proper sizing of features with a reduced spread in dimensions.
- Hot deformation – The sintered component is heated and deformed by a rapid forging stoke to ensure proper size and density. For a steel, the sintered strength jumps from 500 MPa to 720 MPa after hot deformation.
- Machining – All common machining operations can applied to sintered MIM components
– to add threads, undercuts, grooves, ultra-tight tolerances, or special features difficult or expensive to place in the tooling.
- Heat treatment – Sintering leaves the material in an annealed condition. For low carbon ferrous alloys, this is of little consequence, but for high carbon levels it is possible to adjust hardness and other properties via heat treatment. In some cases it is possible to incorporate the heat treatment into the sintering cooling cycle, but more often it is performed as a second step. Precipitation hardened stainless steels require cycles of heat and hold to properly optimize the mechanical properties.
- Hot Isostatic Press (HIP) – MIM parts are typically 98 – 99% dense after sintering. If full density is necessary the parts can be HIP treated. The components are loaded into a sealed chamber, a gas is introduced (most commonly argon) and the chamber is heated. The heat creates pressure that compresses the parts and collapses residual internal porosity.
- Surface carburization – Carbon is important to attaining high strengths in steels. A high surface hardness is attained with carbon surface additions using a heating cycle with an atmosphere containing methane. Surface carburization cycles result in some loss of dimensional precision, so trade-off is required between surface hardness due to the addition of carbon (carburization) and dimensional accuracy.
- Joining – Like other metallic components, MIM components are joined by welding, brazing, or even adhesive techniques. For the most part, MIM materials behave the same as standard metals. Laser welding proves very effective with MIM stainless steels.
- Surface treatments – Surface treatments such as polishing, coating, painting, cleaning, anodizing, plating, sealing, and laser glazing are all applied to MIM components. Surface hardening treatments sustain a tough core with a hard surface. Electroplating is used for either improved aesthetics or corrosion resistance. Nickel electroplate is a favorite on instrumentation, firearm, or magnetic components.
A key point is that after sintering, there is no behavior difference for a MIM component versus any of the other manufacturing routes. Consequently, no special handling procedures or steps are required in the secondary operations for MIM parts.