Shaping processes: attributes and origins 417

18.4 Shaping processes: attributes and origins 417

Tension caused

More uniform

by differential

cross-section

shrinkage

avoids problem

Figure 18.6 (a) Differential shrinkage causes internal stress, distortion and cavitation in thick

sections or at changes of section. (b) Good design keeps section thickness as uniform as possible, avoiding the problem.

temperature, with a step increase in viscosity, but for alloys this happens over a range of temperature, known as the ‘mushy zone’, in which the alloy is part liq- uid, part solid. The width of this zone can vary from a few degrees centigrade to several hundred—so metal flow in castings depends on alloy composition. In general, higher pressure die-casting and molding methods enable thinner sections to be made, but the equipment costs more and the faster, more turbu- lent flow can entrap more porosity and cause damage to the molds.

Upper limits to size and section in casting and molding are set by problems of shrinkage. The outer layer of a casting or molding cools and solidifies first, giving it a rigid skin. When the interior subsequently solidifies, the change in volume can distort the product or crack the skin, or cause internal cavitation. Problems of this sort are most severe where there are changes of section, since the constraint introduces tensile stresses that cause hot tearing—cracking caused by constrained thermal contraction. Different compositions have differ- ent susceptibilities to hot tearing—another example of coupling between mate- rial, process and design detail.

Much of the documentation for casting and molding processes concerns guidance on designing both the component shape and the mold geometry to achieve the desired cross-sections while avoiding defects. Even when the com- ponent shape is fixed, there is freedom to choose where the material inlets are built into the mold (the ‘runners’) and where the air and excess material will escape (the ‘risers’). Figure 18.6 shows an example of good practice in design- ing the cross-sectional shape of a polymer molding.

Powder methods for metals and ceramics too depend on flow. Filling a mold with powder uses free flow under gravity plus vibration to bed the powder down and achieve uniform filling. This may be followed by compression (‘cold compaction’). Once full of powder, the mold is heated to allow densification by sintering or, if the pressure is maintained, by hot isostatic pressing (HIPing).

Metal shaping by deformation—hot or cold rolling, forging or extrusion—also involves flow. Solid metals flow by plastic deformation or by creep—Chapter 13 described how the flow rate depends on stress and temperature. Much forming

418 Chapter 18 Heat, beat, stick and polish: manufacturing processes

Billet Tool (a)

p max ⫽ 2-3 σ y

p max ⫽ 5-10 σ y

Friction hill

Friction hill

τ =k Tool

Figure 18.7 The influence of friction and aspect ratio on open die forging. (a) Uniaxial

compression with very low friction. (b) With sticking friction the contact pressure rises in a ‘friction hill’ causing barrelling. (c) The greater the aspect ratio, the greater the pressure rise and the barrelling.

is done hot because the hot yield stress is lower than that at room temperature and work hardening, which drives the yield strength up during cold deforma- tion, is absent. The thinness that can be rolled, forged or extruded is limited by plastic flow in much the same way that the thinness in casting is limited by vis- cosity: the thinner the section, the greater the required roll-pressure or forging force.

Figure 18.7 illustrates the problems involved in forging or rolling very thin sec- tions. Friction changes the pressure distribution on the die and under the rolls. When they are well lubricated, as in (a), the loading is almost uniaxial and the material flows at its yield stress σ y . With friction, as in (b), the metal shears at the die interface and the pressure ramps up because the friction resists the lateral spreading, giving a ‘friction hill’. The area under the pressure distribution is the total forming load, so friction increases the load. The greater the aspect ratio of the section (width/thickness), the higher the maximum pressure needed to cause yield- ing, as in (c). This illustrates the fundamental limit of friction on section thick- ness—very thin sections simply stick to the tools and will not yield, even with very large pressures. Friction not only increases the load and limits the aspect ratio that can be formed, it also produces distortion in shape—‘barrelling’—shown in