T

Step 4

Snap Back Forming Male Tooling

Fig. 2.22. SPFMethodsforReducingNon-UniformThin-Out1'

in Fig. 2.23. Back pressures of 100-500psi are normally sufficient to suppress cavitation.

Gas pressure is an effective pressure medium for SPF for several reasons: (1) it permits the application of a controlled uniform pressure; (2) it avoids the local stress concentrations that are inevitable in conventional forming where a tool contacts the sheet; and (3) it requires relatively low pressures (< 1000psi). Forming parameters (time, temperature, and pressure) have traditionally been determined empirically by trial and error; however, there are now a number of finite element programs that greatly aid in reducing the development time. The disadvantages of SPF are that the process is rather slow and the equipment and tooling can be expensive. For example, a part undergoing 100% strain at

Pi = Forming Pressure P2 = Back Pressure

Fig. 2.23. Back Pressure Forming to Suppress Cavitation

Pi = Forming Pressure P2 = Back Pressure

Fig. 2.23. Back Pressure Forming to Suppress Cavitation

0.000 1/s would require almost 3 h at temperature, including the time required for heat-up and cool-down.

For titanium alloys, SPF can be combined with diffusion bonding (SPF/DB) to form one piece unitized structure. Titanium is very amendable to DB because the thin protective oxide layer (TiO2) dissolves into the titanium above 1150° F leaving a clean surface. However, the aluminum oxide (Al2O3) on aluminum does not dissolve and must either be removed, or ruptured, to promote DB. Although DB of aluminum alloys has successfully been demonstrated in the laboratory, SPF/DB of aluminum alloys is not yet a commercial process.

2.8 Casting

Due to their lower properties and higher variability than wrought product forms, aluminum castings are not used for primary structural applications. But for lightly loaded secondary structures, castings can offer significant cost savings by reducing part count and the associated assembly cost. Aluminum casting alloys have different compositions than the wrought alloys, i.e. they are tailored to increase the fluidity of the molten metal, be resistant to hot tearing during solidification, and reproduce the details of the mold shape.

The chemical compositions of two casting alloys used in aerospace are given in Table 2.11. Note that since A357.0 contains an appreciable amount of silicon, it is easier to cast than A201.0. On the other hand, since A201.0 contains copper and silver, it is capable of higher strength levels through precipitation hardening. The alloy A357.0 can also be hardened by the precipitation of Mg2Si, but the strength is not as high as for the copper and silver containing alloy A201.0.

Silicon is by far the most important alloying addition for aluminum castings. It greatly improves the fluidity of molten aluminum, especially when the amount approaches the eutectic. It is used in amounts up to about 25% in some commercial alloys. Alloys that contain 12% are referred to as eutectic alloys, those

Table 2.11 Chemical Composition of Aerospace Cast Aluminum Alloys7

Alloy

Si

Fe

Cu

Mn

Mg

Ti

Be

Ag

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