Page 440 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

20 Chapter 16 Sheet-Metal Forming Processes and Equipment

used for rapid prototyping of sheet-metal parts because the lead times associated
with hard tooling are not necessary. The main drawbacks to incremental forming
include low production rates and limitations on materials that can be formed.

l6.l0 Superplastic Forming

The superplastic behavior of certain metals and alloys was described in Section 2.2.7,
where tensile elongations on the order of 2000% were obtained within certain
temperature ranges. Common examples of such materials are zinc-aluminum and
titanium alloys, which have very fine grains-typically less than 10 to 15 /,tm (see
Table 1.1). Superplastic alloys can be formed into complex shapes by superplastic
forming-a process that employs common metalworking techniques-as well as by
polymer-processing techniques (such as thermoforming, vacuum forming, and blow
molding, to be described in Chapter 19). The behavior of the material in superplastic
forming is similar to that of bubble gum or hot glass, which, when blown, expands
many times its original diameter before it bursts.
Superplastic alloys, particularly Zn-22Al and Ti-6Al-4\L also can be formed by
bulk-deformation processes, including closed-die forging, coining, hubbing, and ex-
trusion. Commonly used die materials in superplastic forming are low-alloy steels,
cast tool steels, ceramics, graphite, and plaster of paris. Selection depends on the
forming temperature and the strength of the superplastic alloy.
The very high ductility and relatively low strength of superplastic alloys offer
the following advantages:

° Complex shapes can be formed out of one piece, with fine detail, close toler-
ances, and elimination of secondary operations.
° Weight and material savings can be realized because of the good formability of
the materials.
° Little or no residual stresses develop in the formed parts.
° Because of the low strength of the material at forming temperatures, the tool-
ing can be made of materials that have lower strength than those in other
metalworking processes; hence, tooling costs are lower.

On the other hand, superplastic forming has the following limitations:

° The material must not be superplastic at service temperatures; otherwise the
part will undergo shape changes.
° Because of the high strain-rate sensitivity of the superplastic material, it must
be formed at sufficiently low strain rates, typically 1()`4 to 10`2/s. Forming
times range anywhere from a few seconds to several hours; thus, cycle times
are much longer than those of conventional forming processes. Consequently,
superplastic forming is a batch-forming process.

Diffusion Bonding/Superplastic Forming. Fabricating complex sheet-metal struc-
tures by combining diffusion bonding with superplastic forming (SPF/DB) is an im-
portant process, particularly in the aerospace industry. Typical structures made are
shown in Fig. 16.48, in which flat sheets are diffusion bonded (see Section 31.7) and
formed. In this process, selected locations of the sheets are first diffusion bonded
while the rest remains unbonded, using a layer of material (stop-off) to prevent
bonding. The structure is then expanded in a mold, typically by using pressurized