Papers by Keyword: Dynamic Recrystallization (DRX)

Abstract: The compression behavior and mechanical properties of the Mg-13Gd-4Y-2Zn-0.5Zr (wt.%) alloy filled with intragranular long-period stacking ordered (LPSO) phases at different temperatures were investigated. The results showed that the higher the compression temperature, the smaller the plastic strain that the grains withstand. The grains changed from equiaxed to flat strips when compressed at 350°C, and the morphology of the grains did not change at 450°C. Due to the existence of DRX grains, compression at 450 °C didn’t cause large-angle kink, but the kink angle at 350°C was very large. DRX grains only appeared at the grain boundaries and around the intergranular LPSO phase at the beginning of compression, and only appear at the kink bands (KBs) after the lamellar LPSO phases begin to kink. DRX grains gradually increased with the KBs increasing.

Abstract: In order to determine the deformation temperature of next pass, the hot compression tests were performed by Gleeble-3800 at different temperature form 380 to 420 °C. The microstructure and texture evolution of repetitive upsetting-extruded (RUEed) Mg-Gd-Y-Zn-Zr alloy during hot compression were studied by electron backscattering diffraction (EBSD) analysis. The results showed that the dynamic recrystallization (DRX) occured during the hot compression processing from the strain-stress flow curves. When the temperature increased to 420 °C, the average grain size reduced to 6.64 μm, and the volume fraction of DRXed grains increased to 81.5%. All the compressed alloys exhibited a typical compression texture, the maximum texture intensity of {0001} plane gradually decreased with increasing temperature. When the compression temperature was up to 420°C, the the maximum texture intensity of {0001} plane was 3.207 due to the effect of DRXed grains. Finally, 420°C is chosen as the next deformation of next pass because of the more precipitation and DRXed grains.

Abstract: To determine the impact of dynamic recrystallization on frictional behaviour in the tool-chip interface, a specific friction test called the Warm and Hot Upsetting Sliding Test (WHUST) is implemented. This friction bench simulates tests with contact pressure, sliding velocity, contactor and specimen temperatures similar to industrial ones. Several tests are performed on specimens at different sliding speed, penetrations and work-piece temperatures to reach different dynamically recrystallized states. A numerical model of this test using Arbitrary Lagrangian Eulerian (ALE) method is implemented. Thanks to a specific rheological model, we are able to predict the evolution of the volume fraction of recrystallized grains.

Abstract: The effect of forging process on microstructure and performance of V-Ti, V-Nb-Ti non-quenched and tempered microalloying steel was investigated in this study. By comparing the characteristics between V-Ti and V-Nb-Ti microalloyed non-quenched and tempered steels by the forging process,the machinsm of micro-alloy technology and forging process for dynamic recrystallization and microstructure refinement was investigated. It revealed that addition of Nb in micro-alloying steel can increase significantly non-recrystallization temperature during forging process, and enhance the V (C, N) precipitation strengthening effect of V-Ti microalloyed non-quenched and tempered steels. V-Nb-Ti microalloying technology and controlling forging temperature in non-recrystallization region, can refine grain size, promote proeutectoid ferrite emerging and pearlite morphology alienating. By controlling final forging temperature in non-recrystallization region, the strength and toughness of V-Nb-Ti micro-alloy non-quenched and tempered steels can be improved.

Abstract: The Friction Stir Welding (FSW) differs from the conventional welding process inter alia that the joint properties are not symmetric to the axis of joint line. Basically, it follows from the principle of the method because a rotary tool is used for welding. This asymmetry is due to created temperature field during welding, and the pressure force distribution along the thickness, so the investigation of these effects is important to create good quality welds. Experiments were performed on FSW joints made of non-heat treatable (5754-H22) and heat treatable (6082-T6) aluminium alloys, because there are clear difference between the reacts to these effect and show different changing on the microstructure. A special FSW tool was developed for the experiments and lowest inhomogeneity. Dynamic recrystallization and dynamic recovery can occur during welding so these were investigated by Gleeble material simulating system. During the experiments the grain sizes and grain shapes were also investigated.

Abstract: Two regimes, equilibrium and non-equilibrium interaction of shock wave and inner structure of solid are studied. The theoretical analysis of the regimes is carried out by using the concept of the meso-macro momentum exchange. As a test material for the experiments, D16 Al alloy is taken, firstly because of its initial heterogeneity in equilibrium regime of dynamic straining and, secondly, due to increasing heterogeneity in non-equilibrium regime. Shock tests of D16 Al alloy within impact velocity range of 85÷450 m/s evidence that maximum dynamic strength is realized under conditions: (i) equilibrium regime of meso-macro momentum exchange, (ii) velocity defect equals to mean velocity variation. In non-equilibrium regime, the shock-induced dynamic recrystallization occurs, which is investigated with the metallography and X-ray analysis.

Abstract: In order to verify influence of particle velocity non-uniformity on dynamic recrystallization (DRX), shock tests of D16 Al alloy were conducted under uniaxial strain conditions within strain-rate range of 10⁵ ÷ 10⁷ s^-1. The particle velocity non-uniformity arises due to both initial heterogeneity and non-linearity of shock-wave process. Apart from the nature of DRX mechanism, migrational or rotational, the particle velocity non-uniformity facilitates growth of local strain, strain rate and temperature. To adjust a duration of the particle velocity non-uniformity degree, two limiting situations is provided by means of single and double shock loading (reloading). The experimental technique used allows to register both mean particle velocity profile and particle velocity variation which is the quantitative characteristic of velocity non-uniformity. Shock tests of D16 Al alloy in single and reloading regimes [ show that dynamic recrystallization takes place only in the second case. The reloading regime initiates a ten-fold increase in duration of the particle velocity non-uniformity stage, which is sufficient for fulfillment of the well-known DRX conditions: γ 3, dγ/dt 10⁴ s¹, T 0.4T_m. The regions of DRX with equal-axis grains of ~1 μm in diameter are revealed with the metallography and X-ray analysis.

Abstract: The high strain rate deformation behavior of as-annealed and as-cold rolled pure titanium was inspected by Split Hopkinson Pressure Bar (SHPB). The effect of deformation structure on adiabatic shear behavior in pure titanium was analyzed from the aspect of dynamic mechanical response and microstructural evolution. It was found that the strong {0001} basal texture was formed in as-cold rolled pure titanium. There were Geometrically Necessary Boundaries (GNBs) with spacing of 0.6μm and Incidental Dislocation Boundaries (IDBs) with size of 80nm in one grain. The enhancement of adiabatic shear sensitivity in as-cold rolled titanium was attributed to the deformation induced dislocation boundaries. The core of adiabatic shear band (ASB) was full of fine equiaxed grains with average size of 0.4μm, which was induced by dynamic recrystallization.

Abstract: In this research, one solid state processing technique, friction stir processing, is applied to modify the AZ61 magnesium alloy billet. The FSP modified AZ61 alloy could be refined to 3-8 μm via the dynamic recrystallization during processing. The AZ61 magnesium alloy billet with 75μm grain size could be refined to about 7.5μm by four-pass friction stir processing. The hardness of the stirred zone could increase to around 70-80 after friction stir processing, and after a further compressive strain of about 3% could raise the hardness to 81. The ductility of the weld direction specimens of the modified alloy could have a 235% elongation at 300°C and 1x10^-4 s^-1. The grain boundary sliding (GBS) might be the dominant deformation mechanism during superplastic deformation.

Abstract: In constant strain rate tests, the occurrence of dynamic recrystallization (DRX) is traditionally identified from the presence of stress peaks in flow curves. However, not all materials display well-defined peaks when tested under these conditions. It is shown that the onset of DRX can also be identified from the inflection point on the strain hardening rate (θ=dσ/dε)versus flow stress (σ) curve. In this paper, the hot compression curves can be described by an equation that fits the experimental θ-σ data from zero to the peak stress. An appropriate third order equation was fitted to the strain hardening data. The results show that the critical stress at initiation σc=-B/3A where A and B are coefficients of the third order equation. It is evident that this value depends on the deformation conditions. The stress–strain curve was then normalized with respect to the peak stress, leading to a normalized value of the critical stress (u_c) equal to uc=σc/σp=-B'/3A'. Here A'and B'are coefficients of the normalized third order equation. This value is constant and independent of the deformation conditions.