Processing map for hot working of powder

The constitutive flow behavior of a metal matrix composite (MMC) with 2124 aluminum containing 20 vol pct silicon carbide particulates under hot-working conditions in the temperature range of 300 °C to 550 °C and strain-rate range of 0.001 to 1 s^-1 has been studied using hot compression testing. Processing maps depicting the variation of the efficiency of power dissipation given by [2m/(m + 1)] (wherem is the strain-rate sensitivity of flow stress) with temperature and strain rate have been established for the MMC as well as for the matrix material. The maps have been interpreted on the basis of the Dynamic Materials Model (DMM). [3] The MMC exhibited a domain of superplasticity in the temperature range of 450 °C to 550 °C and at strain rates less than 0.1 s^-1. At 500 °C and 1 s^-1 strain rate, the MMC undergoes dynamic recrystallization (DRX), resulting in a reconstitution of microstructure. In comparison with the map for the matrix material, the DRX domain occurred at a strain rate higher by three orders of magnitude. At temperatures lower than 400 °C, the MMC exhibited dynamic recovery, while at 550 °C and 1 s^-1, cracking occurred at the prior particle boundaries (representing surfaces of the initial powder particles). The optimum temperature and strain-rate combination for billet conditioning of the MMC is 500 °C and 1 s^-1, while secondary metalworking may be done in the super- plasticity domain. The MMC undergoes microstructural instability at temperatures lower than 400 °C and strain rates higher than 0.1 s^-1.     

Flow Behavior and Hot Workability of Nb-15Si-22Ti-5Cr-3Al-2.5Hf Alloy

Constitutive models for flow behaviors of an arc-melted Nb-15Si-22Ti-5Cr-3Al-2.5Hf alloy at temperatures of 1350 °C to 1500 °C and strain rates of 0.001 to 0.1 s⁻¹ have been successfully established during work hardening and dynamic softening stages, respectively, and relatively small average absolute relative errors of the predicted flow stresses are reached (7.7 pct for the work hardening stage and 5.7 pct for the dynamic softening stage). The hot processing map has also been established successfully for this Nb-Si-based ultrahigh temperature alloy. The favorable conditions for hot working of this alloy have been determined as 1350 °C to 1380 °C/0.001 to 0.003 s⁻¹ and 1380 °C to 1440 °C/0.001 to 0.01 s⁻¹. The deformed microstructures under different conditions have been explored and the mechanisms for flow instability of this alloy have been revealed. Flow instability at relatively low temperatures and high strain rates (1350 °C and 1410 °C, 0.1 s⁻¹) is mainly derived from the cracking of Nb₅Si₃ and the debonding of Nbss/Nb₅Si₃ interfaces, while flow instability at higher temperatures (1500 °C) should primarily result from the development of cracks and holes within the Nbss phase because of excessive strain accumulation.

     

Processing map for controlling microstructure in hot working of hot isostatically pressed powder metallurgy NIMONIC AP-1 superalloy

The hot deformation behavior of hot isostatically pressed (HIP) NIMONIC AP-1 superalloy is characterized using processing maps in the temperature range 950 °C to 1200 °C and strain rate range 0.001 to 100 s^•1. The dynamic materials model has been used for developing the pro-cessing maps which show the variation of the efficiency of power dissipation given by [2m/ (m + 1)] with temperature and strain rate, withm being the strain rate sensitivity of flow stress. The processing map revealed a domain of dynamic recrystallization with a peak efficiency of 40 pct at 1125 °C and 0.3 s^•1, and these are the optimum parameters for hot working. The microstructure developed under these conditions is free from prior particle boundary (PPB) de-fects, cracks, or localized shear bands. At 100 s^•1 and 1200 °C, the material exhibits inter-crystalline cracking, while at 0.001 s^•1, the material shows wedge cracks at 1200 °C and PPB cracking at 1000 °C. Also at strain rates higher than 10 s^•1, adiabatic shear bands occur; the limiting conditions for this flow instability are accurately predicted by a continuum criterion based on the principles of irreversible thermodynamics of large plastic flow.     

Characterization of hot deformation behavior of brasses using processing maps: Part I. α Brass

The constitutive flow behavior of α brass in the temperature range of 500°C to 850°C and strain rate range of 0.001 to 100 s⁻¹ has been characterized with the help of a power dissipation map generated on the basis of the principles of the Dynamic Materials Model. The map revealed a domain of dynamic recrystallization in the temperature range of 750°C to 850°C and in the strain rate range of 0.001 to 1 s⁻¹, with a maximum efficiency of power dissipation of about 54 pct. The optimum hot working conditions are 850°C and 0.001 s⁻¹, and these match with those generally employed in industrial practice. In the temperature range of 550°C to 750°C and strain rates lower than 0.01 s⁻¹, the efficiency of power dissipation decreases with decreasing strain rate, with its minimum at 650°C. In this regime, solute drag effects similar to dynamic strain aging occur to impair the hot workability. The material undergoes microstructural instabilities at temperatures of 500°C to 650°C and at strain rates of 10 to 100 s⁻¹, as predicted by the continuum instability criterion. The manifestations of the instabilities have been observed to be adiabatic shear bands.     

Microstructural Evolution During the Hot Deformation of Ti-55Ni (at. pct) Intermetallic Alloy

The hot deformation behavior of Ti-55Ni (at. pct) alloy was studied using compression testing at 1173 K (900 °C) to 1323 K (1050 °C) and at the strain rates of 0.001 to 0.35 s⁻¹. The microstructure evolution was characterized using optical and scanning electron microscopy (SEM). The influences of hot-working parameters on the flow stress and microstructural features of this alloy were then analyzed. The results indicate that, depending on the temperature and strain rate, the dynamic recrystallization (DRX) is the dominate mechanism. Besides, the particle-stimulated nucleation (PSN) mechanism could partially recrystallize the structure. The PSN phenomenon is of significant importance for the Ti-55Ni (at. pct) that suffers from insufficient workability because of its high content of intermetallic phases. It is of interest that the discontinuous yielding phenomenon has been observed when the specimens were deformed at 1173 K (900 °C). Finally, the optimum parameters for hot working of Ti-55Ni (at. pct) alloy are determined as well.     

Characterization of hot deformation behavior of brasses using processing maps: Part II. β Brass and α-β brass

The hot deformation behaviors of β brass in the temperature range of 550°C to 800°C and α-β brass in the temperature range of 450°C to 800°C have been characterized in the strain rate range of 0.001 to 100 s⁻¹ using processing maps developed on the basis of the Dynamic Materials Model. The map for β brass revealed a domain of superplasticity in the entire temperature range and at strain rates lower than 1 s⁻¹, with a maximum efficiency of power dissipation of about 68 pct. The temperature variation of the efficiency of power dissipation in the domain is similar to that of the diffusion coefficient for zinc in β brass, confirming that the diffusion-accommodated flow controls the superplasticity. The material undergoes microstructural instability in the form of adiabatic shear bands and strain markings at temperatures lower than 700°C and at strain rates higher than 10 s⁻¹. The map for α-β brass revealed a wide domain for processing in the temperature range of 550°C to 800°C and at strain rates lower than 1 s⁻¹, with a maximum efficiency of 54 pct occurring at about 750°C and 0.001 s⁻¹. In the domain, the α phase undergoes dynamic recrystallization and controls the hot deformation of the alloy, while the β phase deforms superplastically. At strain rates greater than 1 s⁻¹, α-β brass exhibits microstructural instabilities manifested as flow rotations at lower temperatures and localized shear bands at higher temperatures.     

Optimization of hot workability in stainless

The hot working behavior of 304L stainless steel is characterized using processing maps developed on the basis of the Dynamic Materials Model and hot compression data in the tem- perature range of 700 °C to 1200 °C and strain-rate range of 0.001 to 100 s♪-1. The material exhibits a dynamic recrystallization (DRX) domain in the temperature range of 1000 °C to 1200 °C and strain-rate range of 0.01 to 5 s_-1. Optimum hot workability occurs at 1150 °C and 0.1 s_-1, which corresponds to a peak efficiency of 33 pct in the DRX domain. Finer grain sizes are obtained at the lower end of the DRX domain (1000 °C and 0.1 s^-1). The material exhibits a dynamic recovery domain in the temperature range of 750 °C to 950 °C and at 0.001 s"1. Flow instabilities occur in the entire region above the dynamic recovery and recrystallization domains. Flow localization occurs in the regions of instability at temperatures lower than 1000 °C, and ferrite formation is responsible for the instability at higher temperatures.     

Dynamic recrystallization during hot deformation of aluminum: A study using processing maps

The hot deformation behavior of aluminum of different purities has been studied in the temperature range of 250 °C to 600 °C and strain-rate range of 10 3 to 102 s’1. On the basis of the flow stress data, the strain-rate sensitivity (m) of the material is evaluated and used for establishing power dissipation maps following the Dynamic Materials Model. These maps depict the variation of the efficiency of power dissipation [η = 2m/(m +1)] with temperature and strain rate. A domain of dynamic recrystallization (DRX) could be identified in these maps. While the strain rate at which the efficiency peak occurred in this domain is 10_-3 s⁻¹ the DRX temperature is purity dependent and is 375 °C for 99.999 pct Al, 450 °C for 99.995 pct Al, 550 °C for 99.94 pct Al, and 600 °C for 99.5 pct Al. The maximum efficiency of power dissipation for DRX in aluminum is about 55 pct. The sigmoidal increase of grain size with temperature in the DRX domain and the decrease in the DRX temperature with increase in the purity of aluminum are very similar to that observed in static recrystallization, although DRX occurred at much higher temperatures.     

Microstructural Evolution and Flow Analysis during Hot Working of a Fe-Ni-Cr Superaustenitic Stainless Steel

Hot compression tests were conducted in a temperature range of 1173 K to 1323 K (900 °C to 1050 °C) and strain rates of 0.001 seconds⁻¹ to 1 second⁻¹ to investigate the hot deformation behavior of the austenitic stainless steel type 1.4563. The results showed that hot deformation at low temperatures, i.e., 1173 K to 1223 K (900 °C to 950°C), and at low and medium strain rates, i.e., 0.001 seconds⁻¹ to 0.1 seconds⁻¹, results in the dynamic formation of worm-like precipitates on existing grain boundaries. This in turn led to the restriction or even inhibition of dynamic recrystallization. However, at higher temperatures and strain rates when either the time frame for dynamic precipitation was too short or the driving force was low, dynamic recrystallization occurred readily. Furthermore, at low strain rates and high temperatures, there was no sign of particles, but the interactions between solute atoms and mobile dislocations made the flow curves serrated. The strain rate sensitivity was determined and found to change from 0.1 to 0.16 for a temperature increase from 1173 K to 1323 K (900 °C to 1050 °C). The variations of mean flow stress with strain rate and temperature were analyzed. The calculated apparent activation energy for the material was approximately 406 kJ/mol. The hyperbolic sine function correlated the Zener-Hollomon parameter and flow stress successfully at intermediate stress levels. However, at low levels of flow stress a power-law equation and at high stresses an exponential equation well fitted the experimental data.     

Hot deformation mechanisms in a powder metallurgy nickel-base superalloy IN 625

The hot working behavior of the nickel-base superalloy IN 625 produced by hot extrusion of a powder metallurgy (P/M) compact has been studied by compression testing in the temperature range 900 °C to 1200 °C and true strain rate range 0.001 to 100 s⁻¹. At strain rates less than about 0.1 s⁻¹, the stress-strain curves exhibited near steady-state behavior, while at higher strain rates, the flow stress reached a peak before flow softening occurred. The processing maps developed on the basis of the temperature and strain rate and strain dependence of the flow stress exhibited three domains. (1) The first domain occurs at lower strain rates (<0.01 s⁻¹) and temperatures higher than about 1050 °C. The peak efficiency and the temperature at which it occurs have increased with strain. The microstructure of the specimen deformed in this domain exhibited extensive wedge cracking. (2) The second domain occurs in the intermediate range of strain rates (0.01 to 0.1 s⁻¹) and temperatures lower than 1050 °C, and in this domain, microstructural observations indicated dynamic recrystallization (DRX) of γ containing δ precipitates and carbide particles resulting in a fine-grained structure. (3) The third domain occurs at higher strain rates (> 10 s⁻¹) and tempe ratures above 1050 °C, with a peak efficiency of about 42 pct occurring at 1150 °C and 100 s⁻¹. Microstructural observations in this domain revealed features such as irregular grain boundaries and grain interiors nearly free from annealing twins, which are typical of DRX of homogeneous γ phase. The instability map revealed that flow instability occurs at strain rates above 1 s⁻¹ and temperatures below 1050 °C, and this is manifested as intense adiabatic shear bands. These results suggest that bulk metal working of this material may be carried out in the high strain rate domain where DRX of homogeneous γ occurs. On the other hand, for achieving a fine-grained product, finishing operations may be done in the intermediate strain rate domain. The wedge cracking domain and the regime of instability must be totally avoided for achieving defectfree products.     

Plastic Flow Properties and Microstructural Evolution in an Ultrafine-Grained Al-Mg-Si Alloy at Elevated Temperatures

An AA6082 alloy was subjected to eight passes of equal channel angular pressing at 100 °C, resulting in an ultrafine grain size of 0.2 to 0.4 μm. The tensile deformation behavior of the material was studied over the temperature range of 100 °C to 350 °C and strain rate range of 10⁻⁴ to 10⁻¹ s⁻¹. The evolution of microstructure under tensile deformation was investigated by analyzing both the deformation relief on the specimen surface and the dislocation structure. While extensive microshear banding was found at the lower temperatures of 100 °C to 150 °C, deformation at higher temperatures was characterized by cooperative grain boundary sliding and the development of a bimodal microstructure. Dislocation glide was identified as the main deformation mechanism within coarse grains, whereas no dislocation activity was apparent in the ultrafine grains.     

Mechanical Behavior and Microstructural Change of a High Nitrogen CrMn Austenitic Stainless Steel during Hot Deformation

The hot deformation behavior of a high nitrogen CrMn austenitic stainless steel in the temperature range 1173 to 1473 K (900 to 1200 °C) and strain rate range 0.01 to 10 s⁻¹ was investigated using optical microscopy, stress-strain curve analysis, processing maps, etc. The results showed that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate in 18Mn18Cr0.5N steel. The dynamic recrystallization (DRX) grain size decreased with increasing Z value; however, deformation heating has an effect on the DRX grain size under high strain rate conditions. In the processing maps, flow instability was observed at higher strain rate regions (1 to 10 s⁻¹) and manifested as flow localization near the grain boundary. Early in the deformation, the flow instability region was at higher temperatures, and then the extent of this unstable region decreased with increasing strain and was restricted to lower temperatures. The hot deformation equation as well as the quantitative dependence of the critical stress for DRX and DRX grain size on Z value was obtained.     

Flow Curve Analysis of 17-4 PH Stainless Steel under Hot Compression Test

The hot compression behavior of a 17-4 PH stainless steel (AISI 630) has been investigated at temperatures of 950 °C to 1150 °C and strain rates of 10⁻³ to 10 s⁻¹. Glass powder in the Rastegaev reservoirs of the specimen was used as a lubricant material. A step-by-step procedure for data analysis in the hot compression test was given. The work hardening rate analysis was performed to reveal if dynamic recrystallization (DRX) occurred. Many samples exhibited typical DRX stress-strain curves with a single peak stress followed by a gradual fall toward the steady-state stress. At low Zener–Hollomon (Z) parameter, this material showed a new DRX flow behavior, which was called multiple transient steady state (MTSS). At high Z, as a result of adiabatic deformation heating, a drop in flow stress was observed. The general constitutive equations were used to determine the hot working constants of this material. Moreover, after a critical discussion, the deformation activation energy of 17-4 PH stainless steel was determined as 337 kJ/mol.     

Enhanced Densification of Carbonyl Iron Powder Compacts by the Retardation of Exaggerated Grain Growth through the Use of High Heating Rates

An investigation of the effect of heating rates on the densification behavior of carbonyl iron powder compacts, particularly on the exaggerated grain growth during the α-γ phase transformation, was carried out in this study. Compacts heated at 1200 °C/min and then sintered for 90 minutes at 1200 °C attained 7.14 g/cm³, while those heated at 10 °C/min reached only 6.61 g/cm³. Dilatometer curves using heating rates of 2 °C/min, 5 °C/min, 10 °C/min, 30 °C/min, and 90 °C/min demonstrate that 90 °C/min yields the highest sintered density. The microstructure analysis shows that high heating rates inhibit exaggerated grain growth during the phase transformation by keeping the interparticle neck size small and pinning the grain boundaries. This explanation is supported by the calculation that shows that the energy barrier preventing the grain boundary from breaking away from the neck is reduced hyperbolically as the neck size and the amount of shrinkage increase. The high heating rate, however, shows little beneficial effect for materials that have no allotropic phase transformation or have less drastic grain growth during heating, such as nickel and copper. Thus, bypassing the low temperatures to suppress the surface diffusion mechanism, which does not contribute to densification, is ruled out as the main reason for the enhanced densification of carbonyl iron powders.     

Deformation Behavior of Powder Metallurgy Connecting Rod Preform During Hot Forging Based on Hot Compression and Finite Element Method Simulation

Powder-forged connecting rod with a complex geometry shape always has a problem with nonuniform density distribution. Moreover, the physical property of preform plays a critical role for optimizing the connecting rod quality. The flow behavior of a Fe-3Cu-0.5C (wt pct) alloy with a relative density of 0.8 manufactured by powder metallurgy (P/M, Fe-Cu-C) was studied using isothermal compression tests. The material constitutive equation, power dissipation (η) maps, and hot processing maps of the P/M Fe-Cu-C alloy were established. Then, the hot forging process of the connecting rod preforms was simulated using the material constitutive model based on finite element method simulation. The calculated results agree well with the experimental ones. The results show that the flow stress increases with decreasing temperature and increasing strain rate. The activation energy of the P/M Fe-Cu-C alloy with a relative density of 0.8 is 188.42 kJ/mol. The optimum temperature at the strain of 0.4 for good hot workability of sintered Fe-Cu-C alloy ranges from 1333 K to 1380 K (1060 °C to 1107 °C). The relative density of the hot-forged connecting rod at the central part changed significantly compared with that at the big end and that at the small end. These present theoretical and experimental investigations can provide a methodology for accurately predicting the densification behavior of the P/M connecting rod preform during hot forging, and they help to optimize the processing parameters.

     

Dynamic Recrystallization in Sintered Cobalt during High-Temperature Deformation

The constitutive flow behavior of sintered cobalt in the temperature range 873 to 1473 K (600 to 1200 °C) and at strain rates from 0.001 to 10 s⁻¹ has been studied using constant true strain rate hot compression tests. On the basis of these data, a processing map has been generated that depicts the variation of strain rate sensitivity with temperature and strain rate. The processing map reveals a domain of dynamic recrystallization (DRX) with an optimum condition of processing at 1273 K (1000 °C) and at 10⁻³ s⁻¹. When deformed within the domain, the stress-strain curves exhibit a single peak followed by flow softening, leading to steady-state behavior. In addition to this, a recently developed approach based on flow curve analysis is used to study the DRX kinetics, which is found to follow an Avrami-type relation.     

Hot workability analysis and processing parameters optimisation for 20CrMnTiH steel by combining processing map with microstructure

The processing map of 20CrMnTiH steel is developed by using the dynamic material model according to the hot compression experiments, performed on a Gleeble-3500 thermal simulator at the temperature range of 850–1150°C and the strain rate of 0.01–1?s^?1. Hot workability characteristics of 20CrMnTiH steel are analysed based on the developed processing map. The safe deformation regions with higher power dissipation efficiency η exhibit the dynamic recrystallisation (DRX) mechanism and show fine and homogeneous microstructure. The unstable regions with negative instability coefficient ξ occur at both lower temperature with all strain rates and at high temperature with high strain rate at the strain of 0.2. The area of instability gradually decreases with the increasing strain and only appears at lower temperature and higher strain rate when the strain is above 0.2. The unstable regions indicate the flow localisation by microstructure analysis. Combining with the developed processing map with DRX behaviour, the optimal values of hot processing parameters for 20CrMnTiH steel are obtained to achieve good hot workability and small grains sizes at the process parameters ranged at 1036–1070°C/0.1–1?s^?1 and 918–985°C/0.01–0.014?s^?1.