Phase Field Research On Transient Dendrite Growth Mechanism During The Solidification Of Al-Cu Alloy Welding Pool

The grain size and component distribution formed during the microstructure evolution of the solidification of the welding pool directly affect the mechanical property of the weld. However, the solidification of the welding pool is a highly non-linear, multi-field coupling and hyperthermal transient process, its observation can not be achieved by traditional experiment methods, but the numerical simulation could. Although, there are lots of the simulation researches on the microstructure evolution of the weld, most of them remained on exploration stage, and focused on the morphology or single process of dendrite growth, too many assumptions caused the result to lack fidelity. There is no series of achievements published in the research field on quantitatively and rational studying on the dendrite growth during the solidification of the welding pool under transient conditions. Therefore, in this work, the PF model, a advanced and quantitative computer simulation technology, is used to investigate the linear instability dynamics, transient evolution of the primary dendrite spacing and sidebranching dynamics during the solidification of the welding pool.The quantitative PF model used in the directional solidification under steady-state conditions is modified by way of changing the constant control parameters to variable ones. The modified model has an enormous application range, including the dendrite growth under all kinds of non-steady-state conditions. The traditional initial conditions where the dendrite seeds are preset in a planar interface are instead by reasonable ones, i.e. the solidification interface begins with epitaxial growth of a planar interface where the undercooling equals zero. Thus, the simulation process could consider the integrality and the effect of history dependent of the solidification of the welding pool. In addition, the transient conditions of the welding pool are developed, and then a series of coupled equations which contain the shape parameters of the pool, welding parameters, solidification time and the position of the molten pool etc.. The equations should be solved by the computer and coupled to the PF model.Based on the Warren-Longer theory, a analytic model is developed to investigate in linear instability dynamics under transient conditions. By using this analytic model, how the interface microstructure generates from the background infinitesimal fluctuations and the early instability stage of a planar interface can be predicted. The crystal growth in the linear growth stage of the solidification of the welding pool of aluminum alloy is investigated by analytic calculation, PF simulation and experimental technique. The research results show the analytic calculation results of crossover time and initial wavelength are in good agreement with the PF simulation ones and the experiment ones. Moreover, the impacts of welding parameters on the interface morphology and the characteristics of the unstable interface are also studied at the end of the linear growth stage.The primary dendrite spacing in the microstructure is very important to the mechanical property of the weld, thus its transient evolution is mainly investigated in this work. The dendrite growth enters a competition growth stage after the linear growth stage, and few surviving cellulars change to primary dendrites. The influences of welding parameters on the primary dendrite spacing of the Al-Cu alloy weld are studied and it shows that the welding power and welding velocity have positive and negative correlations with the primary dendrite spacing, respectively. Furthermore, The influences of the history dependent of the dendrite growth on the primary dendrite spacing are also investigated. Overall, the primary dendrite spacing is not only related to the temperature gradient and pulling speed and theirs transient history, but also to the linear growth results, such as the interface morphology, initial spacing and the concentration distribution.Compare with the primary dendrite growth, the sidebranch growth, especially in the welding pool is on smaller growth space and higher growth frequency, therefore, it needs higher-demanding for PF model to simulate the sidebranch growth. Firstly, the capacity of PF model to reproduce sidebranch growth is validated, and the results show that PF model could investigate quantitatively the sidebranching dynamics in more microcosmic scale. Then this PF simulation procedures are used to simulate the sidebranching dynamics during solidification of the welding pool of the Al-Cu alloy near the region of fusion line. It is worth noting that, for a moment, the sidebranching pacing under the transient conditions of the welding pool is larger than that obtained in the directional solidification under the steady-state conditions whose values equal to that of the transient conditions at this moment. According to the noise amplification theory, its fundamental reason is that the tip velocity in the former growth is smaller that that in the latter growth, the initial disturbance with larger wavelength in a wave packet localized initially near the tip contains an infinite range of frequencies is amplified selectively in the former sidebranch growth, that makes the sidebranching spacing of the former growth is larger than that of the latter growth.The transient dendrite growth mechanism during the whole solidification of Al-Cu alloy welding pool is investigated quantitatively by the PF model. This study enriches the solidification theory of welding pool, and lays the foundation for the investigation of the microstrss and generation mechanism of weld defects, and for instructing the project application.