Introduction

POWDER METALLURGY is considered a near-net shape manufacturing process, where designers seek to eliminate or significantly reduce secondary machining operations through die design, tool movements, and core rods. This ability to improve material utilization by reducing the need for machining operations has been a key to the success of P/M. However, improved machinability is becoming more important in extending the applications of P/M (Ref 1).

Machining is conducted for several reasons. Powder metallurgy methods have excellent shape-making abilities, but conventional high-volume techniques cannot make holes, or other re-entrants, normal to the pressing/compaction direction. Thus, cross holes required for lubrication must often be drilled after sintering. Accurate surfaces required for location or assembly also must sometimes be turned or milled. In addition, end users are increasing their use of subassemblies. Hence, P/M fabricators can supply a complete system, such as an oil pump rather than a gear, which can require machining operations previously undertaken by the end user.

When a parts fabricator supplies machined components, machining operations such as drilling, tapping, or boring can be the rate-limiting factor in the operation of an assembly line. Thus improved machinability of P/M materials to remove bottlenecks can then offer substantial increases in efficiency to both parts fabricator and end user. In cases where P/M steels compete with cast irons, machinability may be the leading factor in material selection. Thus, there is considerable interest and activity in measurement and improvement of machinability of P/M steels (Ref 2).

The relatively poor machinability of P/M steels compared to competing wrought products is usually considered to originate from a combination of factors, which include porosity, microcleanliness, microstructure, and knowledge. Some understanding of these factors is necessary to distinguish how the machinability of P/M steels is inherently different from wrought steels. These topics are discussed in this article, with a focus on the conditions of the base metal and additives to improve machinability. Coverage of the machining process is addressed in the article "Machining of Powder Metallurgy Materials" in this Volume.

The differences in machinability between wrought and P/M steels can be partially explained by the presence of porosity and the differences in microstructure required to obtain similar performance levels. The machinability of P/M steels can be improved significantly by systematic use of free-machining agents, cutting tools, and investigation optimizing machining conditions. In some applications, infiltration or polymer impregnation to close porosity will improve machinability significantly. In applications where machinability is key to the success of a P/M part, evaluation of machinability and possible improvements should begin at an early stage in part development and include part producer, tool suppliers, and powder producers.

This article shows that the machinability of sintered P/M steels can be improved by the use of free-machining agents that enhance machinability and the use of improved cutting tools. It appears that the use of green machining processes offers a means to improve machinability significantly. With correct choice of machining agent, cutting conditions, and cutting tools, it is possible to produce P/M steels with machinability equivalent to that of wrought steels; however, the optimization required can be time consuming. Where machinability is critical to the application and use of a P/M part, it should be considered at an early stage in the design process in cooperation with powder producers and cutting tool suppliers.

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