In the last two decades, processing maps have been developed on a wide variety of materials including metals and alloys, metal matrix composites, and aluminides, and applied to optimizing hot workability of materials and for process design in bulk metal working. Processing maps consist of a superimposition of efficiency of power dissipation and the instability maps, the former revealing the "safe" domain for processing and the latter setting the limits for avoiding undesirable microstructures. The dynamic materials model, which forms the basis for processing maps, is discussed in relation to other materials models. The application of dynamical systems principles to understanding of deterministic chaos in the system will help in achieving a greater degree of microstructural control during processing. The patterns in the hot working behavior as revealed by the processing maps of several classes of alloys relevant to technology are reviewed briefly. Processing maps have also been applied to analyze several industrial problems including process optimization, product property control, and defect avoidance, and a few examples are listed. With the processing maps reaching a matured stage as an effective tool for optimizing materials workability, expert systems and artificial neural network models are being developed to aid and prompt novice engineers to design and optimize metal processing without the immediate availability of a domain expert, and the directions of research in this area are outlined.

Processing Maps: A Status Report

The hot deformation behavior of IN 718 superalloy has been characterized in the temperature range 900–1100°C and strain rate range 0.001–1.0 s−1 using compression tests on process annealed material, with a view to obtain a correlation between grain size and the process parameters. At a strain rate of 0.001 s−1, the material exhibits dynamic recrystallization (DRx) at 975°C and superplasticity at 1100°C, while metadynamic recrystallization (MDRx) occurs in the temperature range 950–1100°C and strain rate range 0.01–1.0 s−1. Unlike in the DRx domain, the grain size (d) variation in the MDRx regime could not be correlated with the standard Zener–Hollomon (Z) parameter due to strong thermal effects during cooling after hot deformation. However, it follows an equation of the type d=cexp(−Q/RT), where c, p and R are constants, Q the activation energy for MDRx and T the temperature. The value of p is very low (0.028) and the apparent activation energy is about 275 kJ mole−1, which is very close to that for self-diffusion in pure nickel. The data obtained from several investigators are in agreement with this equation. Such an equation combines the mild dynamic effect in MDRx with a stronger post-deformation cooling effect and may be used for predicting the grain size of IN 718 hot forged in the MDRx regime.

Microstructural modeling of metadynamic recrystallization in hot working of IN 718 superalloy

The hot deformation behavior of extra-low interstitial (ELI) grade Ti–6Al–4V alloy with Widmanstatten preform microstructure over wide temperature (750–1100oC) and strain rate ranges (0.001–100 s−1) has been studied with the help of processing maps. In the lower temperature and strain rate regime (850–950°C and 0.001–0.1 s−1), globularization of the lamellar structure occurs while at higher temperatures (980–1100oC) the β phase exhibits large-grained superplasticity. The tensile ductility reaches peak values under conditions corresponding to these two processes. A dip in ductility occurs at the β transus and is attributed to a possible nucleation of voids within prior β grains. At lower temperatures and strain rates below about 0.1 s−1, cracking at the prior β grain boundaries occurs under mixed mode conditions. At strain rates higher than 1 s−1 and temperatures lower than about 950oC, the material exhibits a wide regime of flow instabilities. On the basis of these results, a temperature–strain rate window for hot working this material without microstructural defects is identified.

Hot deformation and microstructural damage mechanisms in extra-low interstitial (ELI) grade Ti–6Al–4V

The hot deformation behaviour of Mg–3Al alloy has been studied using the processing-map technique. Compression tests were conducted in the temperature range 250–550 °C and strain rate range 3 × 10 −4 to 10 2  s −1 and the flow stress data obtained from the tests were used to develop the processing map. The various domains in the map corresponding to different dissipative characteristics have been identified as follows: (i) grain boundary sliding (GBS) domain accommodated by slip controlled by grain boundary diffusion at slow strain-rates ( −3  s −1 ) in the temperature range from 350 to 450 °C, (ii) two different dynamic recrystallization (DRX) domains with a peak efficiency of 42% at 550 °C/10 −1  s −1 and 425 °C/10 2  s −1 governed by stress-assisted cross-slip and thermally activated climb as the respective rate controlling mechanisms and (iii) dynamic recovery (DRV) domain below 300 °C in the intermediate strain rate range from 3 × 10 −2 to 3 × 10 −1  s −1 . The regimes of flow instability have also been delineated in the processing map using an instability criterion. Adiabatic shear banding at higher strain rates (>10 1  s −1 ) and solute drag by substitutional Al atoms at intermediate strain rates (3 × 10 −2 to 3 × 10 −1  s −1 ) in the temperature range (350–450 °C) are responsible for flow instability. The relevance of these mechanisms with reference to hot working practice of the material has been indicated. The processing maps of Mg–3Al alloy and as-cast Mg have been compared qualitatively to elucidate the effect of alloying with aluminum on the deformation behaviour of magnesium.

Hot deformation behaviour of Mg–3Al alloy—A study using processing map

The hot deformation behavior of electrolytic touch pitch (ETP) copper has been studied in the temperature range 300–950 °C and strain rate range 0.001–100 s −1 using isothermal hot compression tests. Processing maps are developed at different plastic strains with a view to characterize the dynamics of hot deformation while the standard kinetic analysis has been applied to evaluate the rate controlling mechanisms. The processing maps have exhibited three deterministic domains representing dynamic recrystallization (DRX) of ETP copper and occurring in the following temperature and strain rate ranges: (1) 400–600 °C and 0.001–0.01 s −1 , (2) 650–950 °C and 0.001–3 s −1 and (3) 700–950 °C and 10–100 s −1 . While all these domains merge with increasing plastic strain, the first domain has a lower efficiency of power dissipation, the second domain appears early in deformation (strain of 0.1) and the third domain occurs at larger strains (>0.2). The results on processing maps have correlated well with those obtained from kinetic analysis and the apparent activation energy values estimated in these DRX domains are 159, 198 and 91 kJ/mol, respectively, which suggest that dislocation core diffusion, lattice self-diffusion and grain boundary self-diffusion are the rate controlling mechanisms. In all the three DRX domains, the average grain diameter varies linearly with the Zener–Hollomon parameter.

Yellapregeda Prasad V.R.K.

Papers

Hot Working Guide: A Compendium of Processing Maps