Papers by Keyword: Hot Deformation

Abstract: In this work, two approaches for determining critical stress and strain for initiation of dynamic recrystallization (DRX) of the AISI 4340 steel were presented.The first one applied a polynomial function to represent relationship between work hardening rate and flow stress. Secondly, Cingara constitutive model were employed. To investigate hot deformation behavior of the steel, compression tests were performed at different temperatures between 850 °C and 1150 °C and strain rates between 0.01s^-1and 10 s^-1. Obviously, both methods provided different values of critical stress and critical strain.Accuracy of the first method depended on fluctuations of the fitted strain hardening curve. On the other hand, results of the Cingara model was primarily related to the described flow curves up to their peak points. It could be noticed that the DRX occured during hot deformation of the examined steel started when the normalized critical stress and strain reached the values of 0.735 and 0.324, respectively.

Abstract: The article studies on sections microstructure of 430ferritic stainless steel after tension, the tensile temperatures are the 1073K, 1173K, 1223K, 1273K, 1323K and 1423K. The transverse sections (vertical tensile direction) of fractured specimens microstructure of 430ferritic stainless steel were observed and compared with those of longitudinal sections (parallel tensile direction). Moreover, we compare microstructure of transverse section specimens with the salt water-cooled condition and air-cooled condition. The optical micrograph of fractured tensile specimens of 430stainless steel after cooling to room temperature indicated that the volume fraction of the martensite is gradually increased and then declined from 1073K to 1423K. At 1223K, the martensite content is highest. At 1423K, martensite is sharply reduced and disappeared, the microstructure of 430ferritic stainless steel is almost all of ferrite and grain boundary obviously observed. Due to tensile deformation, the morphology of martensite is massive in the transverse section specimens. Whereas, the strip-type morphology of martensite was observed in the longitudinal section specimens. The cooling rate impact on the microstructure was also discussed.

Abstract: The high strength and low cost Ti-Fe based alloy was produced by double vacuum induction melting method followed by hot deformation. The microstructure has been investigated by Optical Microscopy, Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM). The microstructure of as-forged alloy is composed of α and β phase without the precipitation of TiFe intermetallic compound. The Ti-Fe-Al alloys show good comprehensive mechanical properties, demonstrating ultimate tensile strength of 1100MPa and elongation above10%. The results indicate the Fe is a good candidate for solution strengthening and simultaneously increasing ductility in titanium alloys. Effect of the Fe and Al elements on the microstructure and mechanical properties have been discussed.

Abstract: The compression tests of solution treatment ZL109 alloy have been performed in the compression temperature range from 250°C to 450°C and the strain rate range from 0.0005s^-1 to 0.5s^-1. A processing map has been developed on the basis of flow stress data obtained as a function of temperature and strain rate, which revealed two domains of hot working for the alloy: one is situated at temperature between 270°C and 340°C with strain rate between 0.05s^-1 and 0.5s^-1, the other is situated at the temperature between 380°C and 450°C with strain rate between 0.0005s^-1 and 0.004s^-1. Combining with the processing map, the optimum parameters of hot working for ZL109 alloy are that 300°C/0.5s^-1 and 450°C/0.0005s^-1, respectively. Microstructure observations indicated that DRX occurred in both these domains. The instable zones, i. e., adiabatic shear bands formation, wedge cracking, were also identified in the processing map and microstructural examination was performed for validation.

Abstract: Magnesium alloy EZ10 was deformed in tension at temperatures from room temperature up to 400 °C with an initial strain rate of 2.7x10^-3 s^-1. Deformation tests showed a rapid decrease of the tensile yield strength at temperatures higher than 300 °C. Microstructure of the deformed samples was analysed with light microscope. Fracture mechanisms were estimated using scanning electron microscopy.

Abstract: The dynamic recrystallization (DRX) behavior of GH80A superalloy was investigated by isothermal compression tests on a Gleeble1500 thermomechanical simulator. True stress-strain curves and deformed specimens were obtained at the temperature range of 1273-1473K and the strain rate range of 0.01-5s^-1. Experimental results show that the stress-strain curves at low strain rate display a typical DRX characteristic. By regression analysis of experimental results, Materials constant n, activation energy Q and Zener-Hollomon (Z) parameter were determined, and the critical strain model and austenite grain size model for dynamic recrystallization were established as a function of deformation temperature and strain rate. The dynamic recrystallization kinetic model for GH80A was established on the basis of the Avrami equation.

Abstract: In the present study, compression tests were performed at a strain rate of 0.001 to 0.1 sˉ1 and in the range of 600°C to 900°C to investigate the high temperature deformation behavior and flow stress model of commercially pure titanium after severe plastic deformation (SPD). It was found that the effects of temperature and strain rate are significant in dictating the steady state flow stress levels. Flow accompanied by thermal softening was observed due to a combination of dynamic recovery and recrystallization for deformation at or above 600°C. Furthermore, microstructural evolutions of the as processed and hot deformed material were investigated. Based on constitutive equations, the flow stress was modeled for this light ultra-fine grained (UFG) material. The validity of the model was demonstrated with satisfactory agreement in light of the experimental mechanical behavior.

Abstract: The dynamic recrystallization behavior of TC21 alloy during hot compression deformation was investigated at 870~990 °C and strain rate of 0.001~10 s^-1 on a Gleeble-3500 thermo-simulation machine. The results show that dynamic recovery and dynamic recrystallization occurs during hot deformation. As the deformation temperature increases and strain rate decreases, the softening caused by dynamic recrystallization is more obvious. According to the relevance of flow stress, strain rate and deformation temperature, the dynamic recrystallization activation energy is obtained. The constitutive equation and dynamic recrystallization kinetics motel are set up through analyzing and calculating the data of thermo-simulation.

Abstract: The behavior evolvement of Mg-7.22Gd-4.84Y-1.26Nd-0.58Zr (EW75) magnesium alloy during the hot deformation process was discussed. The flow stress behavior of magnesium alloy over the strain rate range 0.002s^-1 to 2s^-1and the temperature range 623K to 773K had been researched on Gleeble-1500D hot simulator under the maximum deformation degree 60%. A mathematical model was established to predict the stress-strain curves of this alloy during deformation. The experimental results showed that the stress-strain curves were obviously affected by the strain rates and deformation temperatures. The mathematical model could predict the stress-strain curves when the strain rates were under 0.2^-1, but there was significant error in some of stress-strain curves when the strain-rate was 2^-1 by the reason of deformation temperature rising.

Abstract: In the present work we develop a physically based model of strength evolution during hot deformation of Al-Mg-Si alloys. The goal is to predict a change of material strength taking into account the impact of microchemistry, i.e. the influence of solutes and precipitates on strengthening and softening mechanisms. The material strengthening is considered in the present work in terms of solid solution strengthening (the Labusch-Naborro model), work hardening (the advanced one-parameter Kocks model), as well as precipitation strengthening due to the stress contribution of non-deformable particles, i.e. dispersoids (the Orowan by-pass). The material softening is described by dynamic recovery through thermal activation of dislocation climb. For the precipitation kinetics the computational thermodynamics code MatCalc (Materials Calculator) was used. The model was validated by comparison with experimental data of compression tests of the 6xxx series aluminium alloys and a reasonable agreement of the simulated and measured flow stress curves was found.