The microstructural kinetics of <i>β</i> grain growth in the <i>β</i> field of a Ti-6Al-4V alloy was studied by a series of controlled heat treatments at constant temperature rates. Heating r...The microstructural kinetics of <i>β</i> grain growth in the <i>β</i> field of a Ti-6Al-4V alloy was studied by a series of controlled heat treatments at constant temperature rates. Heating rates of 5<span style="white-space:nowrap;">°</span>C/s, 50<span style="white-space:nowrap;">°</span>C/s and 500<span style="white-space:nowrap;">°</span>C/s were considered, stopping at different peak temperatures. The thickness evolution of martensitic needles and lamellar <i>α</i> laths, formed on cooling, was also investigated, by soaking the material above its <i>β</i>-transus temperature and cooling down at 5<span style="white-space:nowrap;">°</span>C/s, 50<span style="white-space:nowrap;">°</span>C/s, 100<span style="white-space:nowrap;">°</span>C/s and 300<span style="white-space:nowrap;">°</span>C/s till ambient temperature. Quantitative microstructural analyses were used to measure the particle dimensions. The <i>β</i> grain growth kinetics was reasonably well described by a modified Avrami equation. The thickness of <i>α</i> lamellae was a function of the cooling rate and the <i>β</i> grain dimension in which they nucleated. The martensite needle thickness was shown to be a function of the cooling rate to which the material was subjected.展开更多
Finite element (FE) modelling methods were implemented to perform a weakly-coupled weld simulation activity on a series of simple plate welds, to determine the effects of altering the frequency of saving the thermal t...Finite element (FE) modelling methods were implemented to perform a weakly-coupled weld simulation activity on a series of simple plate welds, to determine the effects of altering the frequency of saving the thermal time-step result upon the mechanical results. By definition, the thermal results will be unaffected, but the mechanical results are calculated from the saved thermal results, hence can be changed when the frequency of saving thermal time-steps is altered. By default, most weakly-coupled thermal-mechanical solvers will save every single thermal time-step, for accuracy. Results indicated that during the welding operation, the thermal time-steps could be reduced to saving 1-in-every-2 thermal time-steps with minimal loss in mechanical accuracy. However, during the cooling operation, every time-step was required to be saved. Whilst this seems almost counter-intuitive that the time-step during the cooling operation is in some way more critical than during welding, it must be stated that the FE software employed for this exercise has a setting allowing the time-steps to become progressively large during cooling, when thermal gradients are much lower and as such both thermal and mechanical calculations are easier to converge.展开更多
The velocity–versus-time rundown curves from two experimental Ti-6Al-4V inertia friction welds were analysed and differentiated several times, to produce rotational acceleration, jerk, jounce (or snap), crackle and p...The velocity–versus-time rundown curves from two experimental Ti-6Al-4V inertia friction welds were analysed and differentiated several times, to produce rotational acceleration, jerk, jounce (or snap), crackle and pop versus-times curves for each weld. Titanium alloys and their mechanical properties are known to be highly sensitive to strain rate as the material is deformed, though nothing has ever been considered in terms of the higher-order time-derivatives of position. These curves have been studied and analysed further, for a more complete understanding of the derivative trends. Rotational acceleration and jerk traces both display behavior patterns across the two welds as the part rotates under action from the flywheel. The rotational snap also displays a pattern in this derivative during the final approximately 0.5 s of welding, as the energy dissipates. Evidence of a distinct oscillatory pattern in the rotational crackle and pop terms was noted for one weld when differentiating over a larger time-base, though could not be replicated in the 2<sup><span style="font-family:Verdana;">nd</span></sup><span style="font-family:Verdana;"> weld. The </span><span style="font-family:Verdana;">higher derivative curves allow distinction of different process regimes, indi</span><span style="font-family:Verdana;">cat</span><span style="font-family:Verdana;">ing that inertial energy mostly influences the time-base of dynamically</span><span style="font-family:Verdana;"> steady-</span><span style="font-family:Verdana;">state phase. Qualitative differences between initial energies are evident in</span><span style="font-family:Verdana;"> higher derivatives.</span>展开更多
文摘The microstructural kinetics of <i>β</i> grain growth in the <i>β</i> field of a Ti-6Al-4V alloy was studied by a series of controlled heat treatments at constant temperature rates. Heating rates of 5<span style="white-space:nowrap;">°</span>C/s, 50<span style="white-space:nowrap;">°</span>C/s and 500<span style="white-space:nowrap;">°</span>C/s were considered, stopping at different peak temperatures. The thickness evolution of martensitic needles and lamellar <i>α</i> laths, formed on cooling, was also investigated, by soaking the material above its <i>β</i>-transus temperature and cooling down at 5<span style="white-space:nowrap;">°</span>C/s, 50<span style="white-space:nowrap;">°</span>C/s, 100<span style="white-space:nowrap;">°</span>C/s and 300<span style="white-space:nowrap;">°</span>C/s till ambient temperature. Quantitative microstructural analyses were used to measure the particle dimensions. The <i>β</i> grain growth kinetics was reasonably well described by a modified Avrami equation. The thickness of <i>α</i> lamellae was a function of the cooling rate and the <i>β</i> grain dimension in which they nucleated. The martensite needle thickness was shown to be a function of the cooling rate to which the material was subjected.
文摘Finite element (FE) modelling methods were implemented to perform a weakly-coupled weld simulation activity on a series of simple plate welds, to determine the effects of altering the frequency of saving the thermal time-step result upon the mechanical results. By definition, the thermal results will be unaffected, but the mechanical results are calculated from the saved thermal results, hence can be changed when the frequency of saving thermal time-steps is altered. By default, most weakly-coupled thermal-mechanical solvers will save every single thermal time-step, for accuracy. Results indicated that during the welding operation, the thermal time-steps could be reduced to saving 1-in-every-2 thermal time-steps with minimal loss in mechanical accuracy. However, during the cooling operation, every time-step was required to be saved. Whilst this seems almost counter-intuitive that the time-step during the cooling operation is in some way more critical than during welding, it must be stated that the FE software employed for this exercise has a setting allowing the time-steps to become progressively large during cooling, when thermal gradients are much lower and as such both thermal and mechanical calculations are easier to converge.
文摘The velocity–versus-time rundown curves from two experimental Ti-6Al-4V inertia friction welds were analysed and differentiated several times, to produce rotational acceleration, jerk, jounce (or snap), crackle and pop versus-times curves for each weld. Titanium alloys and their mechanical properties are known to be highly sensitive to strain rate as the material is deformed, though nothing has ever been considered in terms of the higher-order time-derivatives of position. These curves have been studied and analysed further, for a more complete understanding of the derivative trends. Rotational acceleration and jerk traces both display behavior patterns across the two welds as the part rotates under action from the flywheel. The rotational snap also displays a pattern in this derivative during the final approximately 0.5 s of welding, as the energy dissipates. Evidence of a distinct oscillatory pattern in the rotational crackle and pop terms was noted for one weld when differentiating over a larger time-base, though could not be replicated in the 2<sup><span style="font-family:Verdana;">nd</span></sup><span style="font-family:Verdana;"> weld. The </span><span style="font-family:Verdana;">higher derivative curves allow distinction of different process regimes, indi</span><span style="font-family:Verdana;">cat</span><span style="font-family:Verdana;">ing that inertial energy mostly influences the time-base of dynamically</span><span style="font-family:Verdana;"> steady-</span><span style="font-family:Verdana;">state phase. Qualitative differences between initial energies are evident in</span><span style="font-family:Verdana;"> higher derivatives.</span>
