May 2021 Volume 3

FORGING RESEARCH

4.5 Influence of strain rate and temperature on flow stress Many metals and steels display a behavior in which their flow stress increaseswith increasing strain rate, as shown inFigure13, but decreases with increasing working temperature (Figure 14). In addition, the influence of the strain rate on the flow stress increases with increasing working temperature. This is the main reason why many metals and steels are hot workedwhile being worked relatively slowly. Hot work is usually performed at above the recrystallization temperature (950°C-1050°C). One of the major benefits of working in this temperature range is the excellent workability it affords to steels owing to its finer grain sizes. In hot work, the tool speed is determined by the equipment’s load capacity resulting from the strain rate effect, which can have a direct impact on overall productivity. If a steel needs to be deformed by coldwork, it is extremely important to have the workpiece spheroidized annealed. Spheroidization is the most ductile and softest condition associated with the most stable microstructures that consist of spherical carbide particles uniformly dispersed in a ferrite matrix [7]. The separate carbide particles have little resistance to deformation. It is very common to see high tooling speeds in cold forming processes such as stamping because of the low strain rate sensitivity at room temperature.

4.6 The constitutive equations – the power laws Up to this point, although the derived relationships are theoretically sound and clear, these closed-form equations provide little practical use. This theoretical challenge has led to several empirical constitutive equations, or power laws, to express the relationship between the stresses and plastic strains. These empirical constitutive equations have made the computer calculation of plastic deformation possible. For example, the Johnson-Cook equation, a widely used empirical equation that addresses work hardening, strain rate and temperature in large plastic deformation, is expressed by Eq. (14) [6]: where is the flow stress, is the yield stress, are the effective plastic strains, are the effective plastic strain rates, is the initial plastic strain rate, K is the strength coefficient, n is the strain-hardening exponent, C is the strain-hardening coefficient, m is the strain rate sensitivity exponent, T is the transient temperature, T 0 is the room temperature, and T M is the material’s melting temperature. 4.7 Formability In metal forming, formability or workability is a major concern that requires a workpiece to be very ductile, at its softest and in need of only a small force to flow. The formability of a metal is a technical focal point that determines not only how complex the shape forms can be, but also if the workpiece can be formed free of any defects or fractures. Formability is affected by three factors: the selected material, the design philosophies and the process conditions. The material factors affecting formability are: · Flow characteristics (Sections 4.2 to 4.6) and ductility. ·Carbon content and alloys. The carbon content and the kinds of alloys in the material significantly influence its formability. With increasing carbon content, the material will have increasing resistance to deformation owing to the larger volume percent of hard and brittle carbides. Alloys effectively increase the equivalent carbon content. Some of them are strong carbide formers. Both of these characteristics will likely make deformation more difficult. · Microstructure and grain size (Section 4.5). The design factors affecting formability are: ·Machining stock allowance in bulk forming. In general, themore complex the shape of the workpiece is and the higher the carbon and alloy content are, the more machining stock allowance should be given to ensure cleanup of the formed surfaces. ·Curvatures, fillets and draft angles. Assigning proper curvatures, fillets and draft angles to the sharp corners and edges in the forming tools and dies are the most effective means of preventing the occurrence of common forming defects such as underfill, dead-metal zones and localized fractures. (14)

Figure 13. Influence of strain rate on flow stress [6]

Figure 14. Influence of working temperature on flow stress [6]

FIA MAGAZINE | MAY 2021 73