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dc.contributor.advisorLi, Yanjun
dc.contributor.advisorMathiesen, Ragnvald
dc.contributor.authorWang, Shubo
dc.date.accessioned2020-06-11T13:40:30Z
dc.date.available2020-06-11T13:40:30Z
dc.date.issued2020
dc.identifier.isbn978-82-326-4721-7
dc.identifier.issn1503-8181
dc.identifier.urihttps://hdl.handle.net/11250/2657739
dc.description.abstractAluminium (Al), and titanium (Ti) alloys are two commonly used light metal materials in the aerospace and automotive industry due to their high strength-to-weight ratio. Various heat treatments are applied to improve the formability or the mechanical properties of materials by microstructure optimization. An application of external electric current on metals and alloys affect their plasticity, solidification behavior, fatigue life and precipitation, and thus has been considered as an effective way to manipulate the microstructures and properties of metals. In this work, a comprehensive experimental study was carried out to understand the effect of electric current on the recrystallization behavior of cold rolled Al alloys and phase transformation of Ti alloys. Electric current flash heating (ECFH) was conducted on different types of Al alloys and Ti alloys. In the Al-5wt% Cu alloy, it is found that a high density of θ-Al2Cu precipitates precipitate out during heat treatment. This concurrent precipitation yields a strong Zener drag effect for recrystallization. A fully recrystallized grain structure of the alloy is obtained by ECFH for 4.5 s to a peak temperature of 407 °C. However, only partial recrystallization can be achieved by salt bath isothermal annealing at the same peak temperature for the same time. The results suggest that the recrystallization kinetics is greatly accelerated by ECFH. In a simple binary Al-6Mg alloy without concurrent precipitation, a full recrystallization can be achieved within several seconds for moderate current density ECFH and hundreds of milliseconds for high current density ECFH, which is much shorter than that of salt bath isothermal annealing. ECFH leads to rapid polygonization and formation of fine equiaxed grains at low peak temperature. Such a microstructure change facilitates the nucleation of recrystallization. The accelerating effect of ECFH on recrystallization is also found in commercial Al alloys where random texture components are preferred. In a cold rolled AA6016 alloy, the results have demonstrated that ECFH can significantly reduce the temperature and time that are necessary to achieve a fully recrystallized grain structure with finer and more equiaxed grains, as well as a more random recrystallization texture. A high current density continuous alternating current has been used to heat treat a commercial purity (CP) titanium. It is shown that after ECFH followed by water cooling, a fine martensitic lamellar shaped α is obtained. As a result, a 40% improvement in strength has been achieved. Grain boundary misorientation angle distribution and crystallographic orientation analysis demonstrates that the β g α transformation follows the Burgers orientation relationship. Synchrotron X-ray diffraction characterization shows that the α g β transformation during ECFH can finish within 0.088 s, which is through a diffusionless shear reverse martensite transformation. In a α+β two phase Ti-6Al-4V (Ti64) alloy, it shows that a diffusionless reverse martensitic hcp α g bcc β transformation can also be achieved during ECFH. The β g α transformation happens via martensite transformation during water cooling, resulting in an ultrafine α’ martensitic lath structure. During air cooling, the phase transformation is through a diffusional phase transformation, resulting in much coarser α lath structure. The morphology changes of α phase influenced by cooling rate leads to reduced yield strength (YS) and ultimate tensile strength of the Ti64 by air cooling, but improved YS and UTS by water cooling. The work presented could be of great significance to understand the influence of current on solid state phase transformation in metals. It also provides us with a both energy and time efficient alternative heat treatment strategy to control the microstructures of alloys by careful controlling of the parameters, e.g. the electric current density, heating time (peak temperature) and coolingen_US
dc.language.isoengen_US
dc.publisherNTNUen_US
dc.relation.ispartofseriesDoctoral theses at NTNU;2020:186
dc.relation.haspartPaper 1: Wang, Shubo; Jia, Hailong; Li, Yanjun. Accelerated recrystallization by electric current flash heating in cold-rolled Al-5Cu alloy under the influence of concurrent precipitation. Journal of Alloys and Compounds 2019 ;Volum 811 https://doi.org/10.1016/j.jallcom.2019.151891en_US
dc.relation.haspartPaper 2: Shubo Wang, Hailong Jia, Håkon W. Ånes, Feng Qian, Yanjun Li. Revealing the grain structure and texture evolution during recrystallization induced by electric current flash heatingen_US
dc.relation.haspartPaper 3: Shubo Wang, Hailong Jia, Yanjun Li. Achieving fine recrystallized structure in AA6016 alloy by electric current flash heatingen_US
dc.relation.haspartPaper 4: Shubo Wang, Hailong Jia, Eva. A. Mørtsell, Ragnvald H. Mathiesen, Yanjun Li. Phase transformation in CP Ti subjected to electric current flash heatingen_US
dc.relation.haspartPaper 5: Shubo Wang, Eva. A. Mørtsell, Hailong Jia, Ragnvald H. Mathiesen, Yanjun Li. Accelerated phase transformation induced by electric current flash heating in Ti-6Al-4V alloyen_US
dc.titleRecrystallization and phase transformation of light metals induced by electric current flash heatingen_US
dc.typeDoctoral thesisen_US
dc.subject.nsiVDP::Technology: 500::Materials science and engineering: 520en_US


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