New approach for modelling strain induced precipitation of Nb(C,N) in HSLA steels during multipass hot deformation in austenite

A new model for strain induced precipitation of Nb(C,N) is developed from the existing model for single pass hot deformation. This new model can be extended to multipass deformation to explain the microstructural evolution during the hot deformation of Nb supersaturated high strength low alloy (HSLA) steels. The key feature of this model is the microband geometry employed, which leads to determination of the local solute concentration at microbands, and hence the potential for carbonitride precipitation on the microbands. The model also validates the need for concurrent growth and coarsening processes, even at the early stages of precipitation. The evolution of the precipitate radius, number density and volume fraction are compared with the experimental results obtained from thin foil TEM micrographs on Fe-30 wt-%Ni alloys ( that are austenitic at room temperature and are similar to HSLA steels in deformation behaviour) subjected to deformation by plane strain compression. The model predictions are in good agreement with experimental results

Microstructure modelling of hot deformation of Al–1%Mg alloy

This study presents the application of the finite elementmethod and intelligent systems techniques to the prediction of microstructural mapping for aluminium alloys. Here, the material within each finite element is defined using a hybrid model. The hybrid model is based on neuro-fuzzy and physically based components and it has been combined with the finite element technique. The model simulates the evolution of the internal state variables (i.e. dislocation density, subgrain size and subgrain boundary misorientation) and their effect on the recrystallisation behaviour of the stock. This paper presents the theory behind the model development, the integration between the numerical techniques, and the application of the technique to a hot rolling operation using aluminium, 1 wt% magnesium alloy. Furthermore, experimental data from plane strain compression (PSC) tests and rolling are used to validate the modelling outcome. The results show that the recrystallisation kinetics agree well with the experimental results for different annealing times. This hybrid approach has proved to be more accurate than conventional methods using empirical equations

Hot deformation behavior and processing maps of diamond/Cu composites

The hot deformation behaviors of 50 vol pct uncoated and Cr-coated diamond/Cu composites were investigated using hot isothermal compression tests under the temperature and strain rate ranging from 1073 K to 1273 K (800 C to 1000 C) and from 0.001 to 5 s1, respectively. Dynamic recrystallization was determined to be the primary restoration mechanism during deformation. The Cr3C2 coating enhanced the interfacial bonding and resulted in a larger flow stress for the Cr-coated diamond/Cu composites. Moreover, the enhanced interfacial affinity led to a higher activation energy for the Cr-coated diamond/Cu composites (238 kJ/mol) than for their uncoated counterparts (205 kJ/mol). The strain-rate-dependent constitutive equations of the diamond/Cu composites were derived based on the Arrhenius model, and a high correlation (R = 0.99) was observed between the calculated flow stresses and experimental data. With the help of processing maps, hot extrusions were realized at 1123 K/0.01 s1 and 1153 K/0.01 s1 (850 C/0.01 s1 and 880 C/0.01 s1) for the uncoated and coated diamond/Cu composites, respectively. The combination of interface optimization and hot extrusion led to increases of the density and thermal conductivity, thereby providing a promising route for the fabrication of diamond/Cu composites

STATIC RECRYSTALLIZATION AFTER HOT WORKING OF Al-Li ALLOYS

Small slabs of DC cast and homogenised 8090 and 8091 have been hot rolled in an experimental mill at temperatures in the range 300-500°C and subsequently solution treated at 550°C for times up to 48 hrs. Increase in rolling temperature leads to slower recrystallization, but the effect diminishes at long solution treatment times when the rolling temperature is above 400°C. Recrystallized grains are nearly equiaxed after rolling at temperatures below 400°C but are elongated after rolling at higher temperatures. After high temperature rolling, 8091 recrystallizes more rapidly to give less elongated grains than 8090. Fine particles appear to play a critical role in determining the recrystallization behaviour

Semi-solid processing of tool steel

Semi-solid processing (SSP) of alloys has been used commercially since the early eighties for the manufacture of components in the automobile and aerospace industries. The thrust of the early development has been in aluminium and copper alloys. The nature of the process makes it an ideal candidate for the manufacture of components in alloys which are either difficult or virtually impossible to forge by conventional means. Alloys such as tool steels, stellites and superalloys, offer the possibility of lucrative niche for commercial exploitation. The work described in this paper is the result of three years of development in the semi-solid processing of high melting point alloys. The development of the appropriate microstructures for thixoforming M2 tool steel (both through Spray Formed and Recrystallisation and Partial Melting (RAP) routes) is examined and discussed. The thixoforming of M2 tool steel in non-metallic dies is described in some detail, together with the way process parameters appear to affect the structure and properties of the final product. The resulting microstructures are closely examined and presented with values of mechanical properties in the as forged and heat treated conditions

Process optimisation in the semi-solid forming of hypereutectic Al/Si MMCs

A novel processing route for the fabrication of Al/high silicon MMCs is presented. The silicon size is much finer than can be achieved by casting, yet the materials can still be formed into a near-net shape. Initial properties of the MMCs are presented, and methods under investigation to optimise processing and improve properties are discussed