Page 476 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 17 Powder-Metal Processing and Equipment

TABLE l1.5

Mechanical Property Comparisons for Ti-6AL-4V Titanium Alloy
Ultimate
Relative Yield tensile Reduction
density strength strength Elongation of area
Process (‘*) (%) (MPa) (MPa) (%) (%)
Cast 100 840 930 7 15
Cast and forged 100 875 965 14 40
Blended elemental (P-l-S) 98 786 875 8 14
Blended elemental (HIP) > 99 805 875 9 17
26
Prealloyed (HIP) 100 880 975 14 -
Electron-beam melting 100 910 970 16
P-l-S = pressed and sintered, HIP = hot isostatically pressed.
Source: Courtesy of R.M. German and Stratasys, Inc.

l7.5 Secondary and Finishing Operations

In order to further improve the properties of sintered PM products or to impart spe-
cial characteristics, several additional operations may be carried out after sintering:

Coinin and sizin are com actin o erations erformed under hi h ressure in
g g P s P P _g P
presses. The purposes of these operations are to impart dimensional accuracy
to the sintered P art and to im rove its stren th and surface finish b further
g
P
Y
densification.
Preformed and sintered alloy-powder compacts subsequently may be cold or
hot forged to the desired final shapes and sometimes by impact forging. These
products have a good surface finish, good dimensional tolerances, and a uni-
form and fine grain size. The superior properties obtained make this technology
particularly suitable for such applications as highly stressed automotive and jet-
engine components.
Powder-metal parts may be subjected to other finishing operations, such as
° Machining: for producing various geometric features by milling, drilling,
and tapping (to produce threaded holes).
° Grinding: for improved dimensional accuracy and surface finish.
° Plating: for improved appearance and resistance to Wear and corrosion.
° Heat treating: for improved hardness and strength.
The inherent porosity of PM components can be utilized by impregnating
them with a fluid. Bearings and bushings that are lubricated internally with up
to 30% oil by volume are made by immersing the sintered bearing in heated
oil. These bearings have a continuous supply of lubricant (due to capillary ac-
tion) during their service lives (also referred to as permanently lubricated).
Universal joints also are made by means of grease-impregnated PM tech-
niques, thus no longer requiring traditional grease fittings.
Infiltration is a process whereby a slug of a lower-melting-point metal is placed
in contact with the sintered part. The assembly is then heated to a temperature
sufficiently high to melt the slug. The molten metal infiltrates the pores by cap-
illary action and produces a relatively pore-free part having good density and
strength. The most common application is the infiltration of iron-based com-
pacts by copper or bronze. The advantages of infiltration are that the hardness