Page 474 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 17 Powder-Metal Processing and Equipment

. Neck formation
“
1.
1.
; Neck formation
4, + by diffusion by vapor-phase
2. P panides bonded
materlal transport
2.

Distance between
+ Sggggisgintsgiicles no shrinkage (center
+
distances constant)
3. bonded 3.
(21) (D)

FIGURE l1.20 Schematic illustration of two mechanisms for sintering metal powders:
(a) solid-state material transport; and (b) vapor-phase material transport. R = particle
radius, 1' = neck radius, and p = neck-profile radius.

the other, the particle will melt and (because of surface tension) surround the parti-
cle that has not melted. An example of this mechanism, known as liquid-phase sin-
tering, is cobalt in tungsten-carbide tools and dies (see Section 22.4). Stronger and
denser parts can be obtained in this way. In spark sintering (an experimental
process), loose metal powders are placed in a graphite mold, heated by electric cur-
rent, subjected to a high-energy discharge, and compacted-all in one step. Another
technique under development is microwave sintering, which reduces sintering times
and thereby prevents grain growth, which can adversely affect strength.

Mechanical Properties. Depending on temperature, time, and the processing his-
tory, different structures and porosities can be obtained in a sintered compact and
thus affect its properties. Porosity cannot be eliminated completely because (a) voids
remain after compaction and (b) gases evolve during sintering. Porosity may consist
either of a network of interconnected pores or of closed holes. Generally, if the den-
sity of the material is less than 80% of its theoretical density, the pores are intercon-
nected. Although porosity reduces the strength of the PM product, it is an important
characteristic for making metal filters and bearings and to allow for infiltration with
liquid lubricants by surface tension.
Typical mechanical properties for several sintered PM alloys are given in Table
17.3. The differences in mechanical properties of wrought versus PM metals are
given in Table 17.4. To further evaluate the differences between the properties of
PM, wrought, and cast metals and alloys, compare these tables with the ones given
in Parts I and II.
The effects of various manufacturing processes on the mechanical properties
of a titanium alloy are shown in Table 17.5. Note that hot-isostatic-pressed (HIP)
titanium has properties that are similar to those for cast and forged titanium. It
should be remembered, however, that unless they are precision forged, forgings gen-
erally require some additional machining or finishing processes that a PM compo-
nent may not require.