Development of a Processing Map for Use in Warm-Forming and Hot-Forming Processes
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THE yield and the product quality in a high temperature forming process depends to a large extent on the ductility of the material. It is also true that the metal often becomes rate-sensitive during high temperature deformation; this requires forming to be carried out within a certain "window" in the strain rate and temperature field. Metallurgical phenomena are so complex that one would always have to rely on experience to successfully operate a process, but it has now become possible to define these "windows" on a mechanistic basis, at least for simple materials such as aluminum, which can serve as a guideline for the control of process variables.* * W r a y 24 has thought about ~ vs T maps to categorize failure mechanisms several years ago. The attempt in this paper is to consider the problem more quantitatively.
This paper attempts to build such a process map on the basis of cavity nucleation mechanisms, dynamic recrystallization and adiabatic heating effects. We limit ourselves to fairly fast processes, typically at strain rates higher than about 10 -3 s-~ and a temperature field which extends from about 0.4 T m to 0.8 Tin--equivalent to 373 to 750 ~ in aluminum. The map is constructed on the basis of nucleation thresholds. As such as it represents a conservative account of the fracture strain, since fracture requires nucleation, growth and linking of cavitation damage. The growth strains have not been included in the map, because doing so would have required the specification of the strain state and the stress state in addition to strain rate and temperature. In certain instances, when the integrity of the forged component is important, it may in fact be more sensible to design the process in terms of initiation of microcracks rather than fracture, since microcracks may cause degradation of properties such as fatigue.
DAMAGE MECHANISMS A. Cavities at Particles of a Second Phase The influence of hard particles on low temperature ductility of metals has been rigorously studied by Palmer and Smith ~ and Edelsen and Baldwin? Cavities initiate because particles do not deform themselves, which forces the matrix around the particles to deform more than average, which in turn produces more work hardening and, therefore, a higher stress near the particles. When the stress gets large enough, the interface may separate or the particle itself may crack, whichever occurs first. A picture from Palmer and Smith ~ and an accompanying schematic is shown in Fig. 1. The result is that the ductility is strongly dependent on the size and the density of the second phase particles. If a material containing hard particles is deformed at an elevated temperature then the rate of work-hardening will be reduced by recovery. In addition, diffusion will transport matter from regions of compression around the particles to regions of tension, thus relieving the stress concentration. The relaxation time for the diffusional mechanism can be theoretically calculated. Other creep processes will serve to reduce this relaxation time further, bu
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