Modeling of dynamic material behavior in hot deformation: Forging of Ti-6242
01 Oct 1984-Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science (Springer)-Vol. 15, Iss: 10, pp 1883-1892
TL;DR: In this article, a new method of modeling material behavior which accounts for the dynamic metallurgical processes occurring during hot deformation is presented, which considers the workpiece as a dissipator of power in the total processing system and evaluates the dissipated power co-contentJ = ∫o σ e ⋅dσ from the constitutive equation relating the strain rate (e) to the flow stress (σ).
Abstract: A new method of modeling material behavior which accounts for the dynamic metallurgical processes occurring during hot deformation is presented. The approach in this method is to consider the workpiece as a dissipator of power in the total processing system and to evaluate the dissipated power co-contentJ = ∫o σ e ⋅dσ from the constitutive equation relating the strain rate (e) to the flow stress (σ). The optimum processing conditions of temperature and strain rate are those corresponding to the maximum or peak inJ. It is shown thatJ is related to the strain-rate sensitivity (m) of the material and reaches a maximum value(J max) whenm = 1. The efficiency of the power dissipation(J/J max) through metallurgical processes is shown to be an index of the dynamic behavior of the material and is useful in obtaining a unique combination of temperature and strain rate for processing and also in delineating the regions of internal fracture. In this method of modeling, noa priori knowledge or evaluation of the atomistic mechanisms is required, and the method is effective even when more than one dissipation process occurs, which is particularly advantageous in the hot processing of commercial alloys having complex microstructures. This method has been applied to modeling of the behavior of Ti-6242 during hot forging. The behavior of α+ β andβ preform microstructures has been exam-ined, and the results show that the optimum condition for hot forging of these preforms is obtained at 927 °C (1200 K) and a strain rate of 1CT•3 s•1. Variations in the efficiency of dissipation with temperature and strain rate are correlated with the dynamic microstructural changes occurring in the material.
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15 Mar 1998-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the results obtained on the characterization of hot working behavior of titanium and its alloys using the approach of processing maps are described, and they show that the preform microstructure has a significant influence on the processing maps and hence careful process design has to be adopted for successful forging and microstructural control.
Abstract: In recent years, processing maps are being used to design hot working schedules for making near-net shapes in a wide variety of materials. In this paper, the results obtained on the characterization of hot working behavior of titanium and its alloys using the approach of processing maps are described. In commercial purity α titanium, dynamic recrystallization (DRX) domain occurs at 775°C and 0.001 s−1 with an efficiency of power dissipation [2m/(m+1) where m is the strain rate sensitivity of flow stress] of 43%. The DRX domain shifts to higher strain rates when the interstitial impurity content is lowered. In the near-α and α-β alloys like IMI 685, Ti–6Al–4V, the preform microstructure has a significant influence on the processing maps. For example, in the transformed β (Widmanstatten) preform microstructures, these alloys exhibit a domain of spheroidization at lower temperature and a domain of β superplasticity at higher temperatures, both occurring at slow strain rates. These domains merge at the β transus because of the occurrence of damage processes which lower the tensile ductility. On the other hand, processing maps on alloys with equiaxed preform microstructure exhibit a clear superplasticity domain in the α-β range and the β phase undergoes DRX with a power dissipation efficiency of ≈45–55%. Titanium materials in general, exhibit wide flow instability regimes due to adiabatic shear bands formation at higher strain rates and hence careful process design has to be adopted for successful forging and microstructural control.
463 citations
31 May 2000-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, a processing map has been developed on the basis of flow stress data as a function of temperature, strain rate and strain, which is used for obtaining processing windows and achieving microstructural control during hot working.
Abstract: The hot deformation behavior of Ti–6Al–4V with an equiaxed α–β preform microstructure is modeled in the temperature range 750–1100°C and strain rate range 0.0003–100 s−1, for obtaining processing windows and achieving microstructural control during hot working. For this purpose, a processing map has been developed on the basis of flow stress data as a function of temperature, strain rate and strain. The map exhibited two domains: (i) the domain in the α–β phase field is identified to represent fine-grained superplasticity and the peak efficiency of power dissipation occurred at about 825°C/0.0003 s−1. At this temperature, the hot ductility exhibited a sharp peak indicating that the superplasticity process is very sensitive to temperature. The α grain size increased exponentially with increase in temperature in this domain and the variation is similar to the increase in the β volume fraction in this alloy. At the temperature of peak ductility, the volume fraction of β is about 20%, suggesting that sliding of α–α interfaces is primarily responsible for superplasticity while the β phase present at the grain boundary triple junctions restricts grain growth. The apparent activation energy estimated in the α–β superplasticity domain is about 330 kJ mol−1, which is much higher than that for self diffusion in α-titanium. (ii) In the β phase field, the alloy exhibits dynamic recrystallization and the variation of grain size with temperature and strain rate could be correlated with the Zener–Hollomon parameter. The apparent activation energy in this domain is estimated to be 210 kJ mol−1, which is close to that for self diffusion in β. At temperatures around the transus, a ductility peak with unusually high ductility has been observed, which has been attributed to the occurrence of transient superplasticity of β in view of its fine grain size. The material exhibited flow instabilities at strain rates higher than about 1 s−1 and these are manifested as adiabatic shear bands in the α–β regime.
383 citations
03 Jan 2014-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the authors investigated the hot compressive deformation behaviors of a typical Ni-based superalloy over wide ranges of forming temperature and strain rate and developed processing maps to optimize the hot working processing.
Abstract: The hot compressive deformation behaviors of a typical Ni-based superalloy are investigated over wide ranges of forming temperature and strain rate. Based on the experimental data, the efficiencies of power dissipation and instability parameters are evaluated and processing maps are developed to optimize the hot working processing. The microstructures of the studied Ni-based superalloy are analyzed to correlate with the processing maps. It can be found that the flow stress is sensitive to the forming temperature and strain rate. With the increase of forming temperature or the decrease of strain rate, the flow stress significantly decreases. The changes of instability domains may be related to the adiabatic shear bands and the evolution of δ phase(Ni 3 Nb) during the hot formation. Three optimum hot deformation domains for different forming processes (ingot cogging, conventional die forging and isothermal die forging) are identified, which are validated by the microstructural features and adiabatic shear bands. The optimum window for the ingot cogging processing is identified as the temperature range of 1010–1040 °C and strain rate range of 0.1–1 s −1 . The temperature range of 980–1040 °C and strain rate range of 0.01–0.1 s −1 can be selected for the conventional die forging. Additionally, the optimum hot working domain for the isothermal die forging is 1010–1040 °C and near/below 0.001 s −1 .
221 citations
TL;DR: In this paper, the high-temperature flow behavior of 7075 aluminum alloy was studied by hot compressive tests and the efficiencies of power dissipation and instability parameter were evaluated.
217 citations
15 Oct 1998-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, a simple instability condition based on the Ziegler's continuum principles is developed for delineating the regions of unstable metal flow during hot deformation, which can be used for any type of the flow stress versus strain rate curve.
Abstract: A simple instability condition based on the Ziegler’s continuum principles as applied to large plastic flow, is developed for delineating the regions of unstable metal flow during hot deformation. It can be used for any type of the flow stress versus strain rate curve. This criterion has been validated using the flow stress data of IN 718 with microstructural observations. The optimum hot working conditions for the superalloy IN 718 are suggested based on the instability map.
200 citations
References
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397 citations
TL;DR: In this article, a fracture initiation map is developed which should be useful in fast forming operations at strain rates greater than about 10-3 s-1 at elevated temperatures, and two types of cavitation mechanisms, one pertaining to cavity formation at second phase particles, as in ductile fracture, and the other pertaining to wedge type microcracking at grain boundaries, are considered.
Abstract: A fracture initiation map is developed which should be useful in fast forming operations at strain rates greater than about 10-3 s-1 at elevated temperatures. Two types of cavitation mechanisms, one pertaining to cavity formation at second phase particles, as in ductile fracture, and the other pertaining to wedge type microcracking at grain boundaries, are considered. In addition, dynamic recrystallization and adiabatic heating effects are considered. When these concepts are applied to aluminum, it is shown that there may be an intermediate region in the strain rate and temperature field in which neither of these mechanisms should operate and within which the material would, therefore, be safe from fracture.
313 citations
TL;DR: In this article, a method of discretizing the die boundary conditions is considered for the analysis of metal forming processes by the rigid viscoplastic finite element method, and solutions of the spike forging process are obtained by using the method.
179 citations
TL;DR: In this paper, the stable and unstable plastic flow of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) has been investigated at temperatures from 816 to 1010 °C (1500 to 1850 °F) and at strain rates from 0.001 to 10 s-1 in order to establish its hot forging characteristics.
Abstract: The stable and unstable plastic flow of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) has been investigated at temperatures from 816 to 1010 °C (1500 to 1850 °F) and at strain rates from 0.001 to 10 s-1 in order to establish its hot forging characteristics. In hot, isothermal compression, Ti-6242 with an equiaxed a structure deforms stably and has a flow stress which decreases with straining due to adiabatic heating. With a transformed-β microstructure, unstable flow in hot compression is observed and concluded to arise from large degrees of flow softening caused by microstructural modification during deformation and, to a small extent, by adiabatic heating. Both microstructures have a sharp dependence of flow stress on temperature. Using the concepts of thermally-activated processes, it was shown analytically that this dependence is related to the large strain-rate sensitivity of the flow stress exhibited by the alloy. From lateral sidepressing results, the large dependence of flow stress on temperature was surmised to be a major factor leading to the shear bands occurring in nonisothermal forging of the alloy. Shear bands were also observed in isothermal forging. A model was developed to define the effect of material properties such as flow softening rate and strain-rate sensitivity on shear band development and was applied successfully to predict the occurrence of shear bands in isothermal forging.
149 citations