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A physically-based material model has been developed to describe the austenite formation in a manganese-boron steel during heating in hot stamping processes. The equations were formulated based on three austenite formation mechanisms: nucleation, growth and impingement. It is able to characterize the phase transformation process under both non-isothermal and isothermal conditions, where the effects of heating rate and soaking temperature on the austenite formation have been rationalised. Heat treatment tests of the manganese-boron steel were performed on a Gleeble 3800 subjected to various heating conditions (heating rate: 1–25 K/s, soaking temperature: 1023–1273 K). The dimensional changes of specimens associated with the phase transformation, which was measured using a high resolution dilatometer, has been quantitatively related to the volume fraction of austenite formation. The experimental data were used to calibrate and validate the equations. Good agreement between the experimental and predicted results has been obtained. Further analysis has been made to illustrate the significance of the model in applications. [ABSTRACT FROM AUTHOR]

The formation of austenite in manganese-boron steels during selective heat treatment has great significance in the application of innovative hot stamping processes. Heat treatment tests were designed according to the thermal cycle of industrial heating and hot stamping processes and were conducted on a Gleeble 3800 thermomechanical testing system. Specimens were subjected to non-isothermal (heating rates: 1 K/s–25 K/s) and isothermal (soaking temperatures: 1023 K–1173K) temperature profiles. A high-resolution dilatometer was employed to detect the dimensional change of the specimens associated with austenitization. The dilatometric measurement was quantitatively related to the volume fraction of austenite. By analysing the evolution curves of austenite fraction, the effects of heating rate and temperature on the progress of austenite formation under both non-isothermal and isothermal conditions were investigated and characterised, improving the current understanding of the mechanisms that control austenite formation in manganese-boron steels. [ABSTRACT FROM AUTHOR]

Powder metallurgy (PM) processing of nickel-based superalloys has been used for a wide range of near net-shape fine grained products. In this paper a novel forming process, i.e. direct forging of unconsolidated powder superalloys is proposed. In this process, encapsulated and vacuumed powder particles are heated up to a forming temperature and forged directly at high speed to the final shape, by using a high forming load. Experiments of direct powder forging have been conducted on an upsetting tool-set. Microstructure, relative density and hardness of the formed specimen have been investigated. A finite element model of the direct powder forging process has been established in DEFORM and validated by the comparisons of experimental with simulation results of load variation with stroke as well as relative density distribution. The stress state and the relative density variation have been obtained from FE simulation. The correlation between the stress and consolidation condition has been rationalised. The developed FE model can provide a guide to design the geometry and thickness of preform for direct powder forging. [ABSTRACT FROM AUTHOR]

Abstract: During hot/warm metal forming processes, interfacial heat transfer between work-piece and dies has an important effect on the temperature distribution, microstructure evolution and mechanical properties of formed parts. An efficient numerical model has been developed for the determination of interfacial heat transfer coefficient (IHTC) at different forming and contact conditions. Based on FE heat transfer analysis for a typical tester, a 1D heat transfer model is proposed. A numerical procedure has been developed and implemented for determining the surface temperatures of work-piece and die, the contact heat flux, and the IHTC. The numerical model has been validated by the comparison of experimental, numerical and finite element results. For gas turbine blade hot forging process, the effects of pressure, glaze thickness and surface roughness on the IHTC of Ti-6Al-4V work-piece and H13 steel die were investigated. The application results indicated that this numerical method can provide a reliable means of predicting the IHTC in hot metal forming conditions. [Copyright &y& Elsevier]

Abstract: Aluminium alloy 2024 (AA2024) is extensively used as a structural material in the aircraft industry because of its good combination of strength and fatigue resistance. However, complex shaped components, particularly those made from sheet, are extremely difficult to form by traditional cold forming due to its low ductility at room temperature. A possible solution of this problem is to form sheet workpieces at elevated temperature. The aim of the work described in this paper is to determine the relationship between formability and temperature for AA2024 by conducting a series of tensile tests at elevated temperatures ranging from 350 to 493°C. Ductility of AA2024 was found to increase gradually with increasing temperature up to 450°C, followed by a sharp decrease with further increase in temperature. So-called cup tests confirmed that the formability of AA2024 is very high at a temperature of about 450°C. Fracture surfaces and longitudinal sections of formed samples were examined by scanning electron microscope. It was found that fracture occurred in three different modes depending upon the temperature, and the sharp decrease in ductility when the temperature exceeds 450°C was caused by softening of grain boundaries by solute enrichment (at higher heating rates liquation may be involved) and softening of the matrix around inclusion particles. [Copyright &y& Elsevier]