The rapid growth of powder metal is primarily due to its net-shape part-making capability. In the past, the primary PM process (compaction, sintering, and – where applicable – sizing) was sufficient for most PM applications. However, dimensional precision has tended to steadily increase. A recent survey by the PM industry indicates that about 30% of all PM parts require some machining and, of that 30%, about 80% require enough machining to warrant addressing the issue of material machinability. Obviously, by incorporating machining issues during the design cycle, material selection can be improved and processing can be designed to optimize post-sinter machining.
The machinability of PM steels does not equal that of wrought steels of similar composition, due primarily to the inherent PM properties of interconnected porosity and heterogeneous microstructure. The presence of porosity significantly changes the cutting and drilling process. The most significant effect in cutting is the interruption of force on the cutting tool when a pore is intercepted. The action of small impacts on the cutting-edge increases tool failure when compared to a constant-force cutting action. Porosity also:
- Results in an increase in temperature in the cutting zone because it reduces the thermal conductivity of the material, accelerating tool wear.
- Allows absorption of cutting oil, thereby minimizing boundary lubrication.
- Contributes to the inclusion level in PM by providing areas for oxides to form during sinter and heat treat operations.
- Can allow carbides and divorced pearlite to form (thereby reducing machinability) if the sintering process is not rigidly controlled.
Microstructure and composition also influence machinability in a similar manner to wrought steels. Microstructures of PM steels are more heterogeneous compared to those of wrought steels. Copper melts and goes into solution during the sintering cycle. Nickel, however, due to its higher melting point, forms a nickel-rich iron phase that has different machining characteristics compared to the pearlite matrix of the steel.
The machinability of PM steels can be enhanced by the use of additives such as manganese sulfide and boron nitride, and by such measures as:
- Machining between a pre-sinter and full-sinter operation.
- Applying a post-sinter anneal to initiate spheroidization.
- Modifying the cooling rate from the sinter temperature.
- Warm compaction to a high level of green density and green strength, allowing parts to be machined prior to sintering.
- Resin impregnation after sintering.
In an attempt to relate machinability to mechanical properties, a series of tests were run to correlate drilling force and number of holes drilled to hardness. The study confirmed a known relationship which exists in wrought steels: machinability increases as hardness decreases. The objective of the study was to develop a machinability rating for the various PM compositions used in the study. (The resulting machinability values can be obtained by contacting me at the number shown below.)
Some conclusions can be drawn from the ratings. Machinability:
- Increases with a decrease in carbon content
- Is reduced by alloy additions of copper and nickel
- Decreases with increases in apparent hardness over HRB 70
- Is improved by MnS additions (0-0.5% optimum)
- Is not significantly affected by density, although increasing porosity increases temperature and reduces drill life
- Is improved most significantly by resin impregnation
- And, the machinability rating of a PM material changes with the type of machining