Simulation and Experimental Study of Dynamical Recrystallization Kinetics of TB8 Titanium Alloys

The dynamic recrystallization (DRX) behavior in the hot working of TB8 titanium alloy was studied by using the experiment and finite element simulation (FEM) method. The results showed that the DRX behavior of TB8 titanium alloys was drastically affected by the hot processing parameters. The rising deformation temperature and reducing strain rate led to an augmentation in the grain size (dDRX) and volume fraction (XDRX) of DRX grains. In view of the true stress–strain curves gained from the experiment, the dDRX and XDRX models of DRX grains were constructed. Based on the developed models for DRX of TB8 titanium alloy, the isothermal forging process of the cylindrical samples was simulated by the DEFORM-3D software. The distributions of the effective strain and XDRX for DRX were analyzed. A comparison of the dDRX and XDRX of DRX grains in the central regions of the samples between the experimental and FEM results was performed. A good correlation between the experimental and simulation results was obtained, indicating that the established FEM model presented good prediction capabilities.

FE Analysis of Dynamical Recrystallization during the Seamless Tube Extrusion of Semicontinuous Casting Magnesium Alloy and Experimental Verification

In this paper, empirical dynamic recrystallization (DRX) models for the semicontinuous AZ31 magnesium alloy were established based on the stress–strain curves and microstructure observations generated from hot compression tests. The DRX evolution during the seamless tube extrusion of the alloy was studied by numerical and experimental methods. The extruding temperature and the ram speed were two important parameters during the extrusion. With the increase of the two parameters, the volume fraction of DRX and the recrystallized grain size were observably increased. The predicted DRX fraction and grain size were in an excellent agreement with the experimental results.

Physical Nature of Structure and Properties Degradation of Rail Surface after Long Term Operation

By methods of optical, scanning and transmission electron diffraction microscopy and microhardness and tribology parameters measurement the changes regularities of structure-phase states, defect substructure of rails surface after the long term operation (passed tonnage of gross weight 500 and 1000 mln. tons) were established. It is shown that the wear rate increases in 3 and 3.4 times after passed tonnage of gross weight 500 and 1000 mln. tons, accordingly, and the friction coefficient decreases in 1.4 and 1.1 times. The cementite plates are destroying absolutely and cementite particles of around form with the sizes 10-50 nm are forming after passed tonnage 500 mln tons. The appearance of dynamical recrystallization initial stages is marked after the passed tonnage 1000 mln tons. It is shown that the operation of steel rails is accompanied by full fractures in surface layers with lamellar pearlite grains and the formation of ferrite–carbide mixtures with nanosized particles. The deformation of steel increases the densities of scalar and excess dislocations, the curvature–torsion values of the crystal lattice, and the amplitudes of internal stress fields. The possible mechanisms of established regularities are discussed. It is noted that two competitive processes can take place during rails long term operation: 1. Process of cutting of cementite particles followed by their carrying out into the volume of ferrite grains or plates (in the structure of pearlite). 2. Process of cutting, the subsequent dissolution of cementite particles, transition of carbon atoms to dislocations (into Cottrell atmospheres), transition of carbon atoms by dislocations into volume of ferrite grains or plates followed by repeat formation of nanosize cementite particles.

The Effect of Aluminum Clad on the Electrochemical Properties of the Similar Weld of Friction-Stir Welding of 2524

Microstructures, static weight loss and potentiodynamic polarization curves were carried out on the Friction-stir welds of Al clad 2524 alloy with the thickness of 1.35 mm. The results indicated that the microstructures of the Nugget had been remarkably reshaped. A dynamical recrystallization took place on the zone of the weld nugget and an equiaxed grain structure formed. The static weight loss and potentiodynamic polarization curves results indicated that the resistance of corrosion of the base material (with aluminum clad) was superior to that of its weld, but the anti-corrosive ability of the weld was superior to that of the base material without aluminum clad.

Effect of Deformation Condition on the Mechanical Behavior of AZ80 Magnesium Alloy

With the deformation temperature between 250°C and 450°C as well as the strain rate between 0.01s-1 and 5s-1, the hot compression tests of AZ80 magnesium alloy were performed on Gleeble-3800 thermal simulation testing machine, so as to seek out the responses of mechanical behavior of AZ80 magnesium alloy under different deformation conditions. The results indicated that AZ80 magnesium alloy shows dynamical recrystallization when hot compessed, the recrystallization is prone to happen and the stress peak decreases with the temperature increased, and the critical strain to produce the transformation of recrystallization augments with the strain rate increased.

A Mathmatical Approach for Modeling Real Hot Forming Process Using Physical Simulation Results

Recently, physical simulation has played a more and more important role in modeling hot forming process. However, difficulty still existed in simulating real hot forming process using physical simulation results directly for obvious difference in deformation history between physical simulation condition and real hot forming process. In this work, difference between physical simulation and real hot forming process was discussed and a mathmatical approach was proposed to model real hot forming process using physical simulation results. The main consideration of the method was to put physical simulation results into differential forms in order to take count in the contribution of deformation history (temperature and strain rate) at each incremental step. For the application of the approach, modeling of material flow stress, dynamical recrystallization including critical condition and recrystallziaton fraction, damage evolution and fracture criteria during real hot forming process were presented as examples, although experimental support was still needed for validation and further application.

Microstructure Evolution of Mg-Y-Nd-Zr Alloy during High Temperature Deformation

The variation of the flow stress and the microstructural evolution during hot deformation at different test temperatures and strain-rates are studied. During hot deformation, dynamical recrystallization did not occur completely; after annealing at 523K, microstructural changes due to static recrystallization and small grain sizes of several micrometers and fine nanometer-precipitates can be attained. The stress exponent (n) and the activation energy (Q) for high temperature deformation have been evaluated and the deformation mechanisms at different temperature have also been discussed.