Introduction

POWDER METALLURGY is a near-net shape process capable of producing complex parts with little or no need for secondary operations, such as machining, joining, or assembly. However, the inability to produce certain geometrical figures (e.g., transverse holes, undercuts, and threads) frequently necessitates some machining, particularly drilling. Because of the growing use of P/M materials to increase productivity and reduce costs, the study of the machinability of these materials has become increasingly important, especially because powder metals are generally considered to have poor machinability in comparison with wrought or cast metals (Ref 1). In addition, there is a trend toward the design of complex P/M parts that call for machining, in contrast to the past practice to minimize machining (Ref 2).

The machinability of P/M components is dependent upon a number of factors, including workpiece and tool material properties, cutting conditions, machine and cutting tool parameters. Some P/M material and production process parameters (such as particle size, part geometry, porosity, and compaction and sintering methods) also influence machinability. In general terms, however, machining of P/M materials can be optimized or improved by:

• Taking advantage of the good machining properties of high-density parts (above 92% of theoretical density)

• Machining in the presintered condition

• Using additives that do not require sacrifices in powder processing or part properties

• Following the suggestions of published guidelines for material and machining methods

• Running machinability tests on P/M materials

These factors are briefly discussed in this article, along with more detailed discussions of the machining guidelines that improve the machinability of sintered steels. Additives and microstructural factors that enhance machining are discussed in more detail in the article "Machinability of Powder Metallurgy Steels" in this Volume.

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