Page 480 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

60 Chapter 17 Powder-Metal Processing and Equipment
° The availability of a wide range of compositions makes it possible to obtain spe-
cial mechanical and physical properties, such as stiffness, vibration damping,
hardness, density, toughness, and specific electrical and magnetic properties.
Some of the newer highly alloyed superalloys can be manufactured into parts
only by PM processing.
° It offers the capability of impregnation and infiltration for specific applications.

The limitations of PM are:

° The high cost of metal powder, particularly those for powder-injection mold-
ing, compared with that of raw materials to be cast or wrought.
° The high cost of tooling and equipment for small production runs.
° Limitations on part size and shape complexity.
° Mechanical properties, such as strength and ductility, that generally are lower
than those obtained by forging. However, the properties of full-density PM parts
made by HIP or by additional forging operations can be as good as those made
by other processes.

l1.8 Economics of Powder Metallurgy

Because powder metallurgy can produce parts at net or near-net shape and thus
eliminate many secondary manufacturing and assembly operations, it increasingly
has become competitive with casting, forging, and machining. On the other hand,
the high initial cost of punches, dies, and equipment for PM processing means that
production volume must be sufficiently high to warrant this expenditure. Although
there are exceptions, the process generally is economical for quantities over 10,000
pieces.
As in other metalworking operations, the cost of dies and tooling in powder
metallurgy depends on the part shape complexity and the method of processing the
metal powders. Thus, tooling costs for processes such as hot isostatic pressing and
powder-injection molding are higher than the more conventional powder process-
ing. Because it is a near-net shape-manufacturing method, the cost of finishing oper-
ations in PM is low compared to other processes, such as casting and forging.
However, if there are certain features to the part (such as threaded holes, undercuts,
and transverse cavities and holes), then finishing costs will increase. Consequently,
following design guidelines in PM to minimize or avoid such additional operations
can be more important in this process than in others.
Equipment costs for conventional PM processing are somewhat similar to
those for bulk deformation processing of metals, such as forging. However, the cost
increases significantly when using methods such as HIP and PIM. Although the cost
of materials has increased significantly (see Table 6.1), it has actually improved the
economic viability of PM, since tooling and equipment costs are a smaller fraction
of the total cost of the part.
Labor costs are not as high in other processes, primarily because the individual
operations (such as powder blending, compaction, and sintering) are performed on
highly automated equipment. Thus, the skills required are not as high.
The near-net-shape capability of PM significantly reduces or eliminates scrap.
For example, weight comparisons of aircraft components produced by forging and
by PM processes are shown in Table 17.6. Note that the PM parts are subjected to
further machining processes; thus, the final parts weigh less than those made by
either of the two processes alone.