The Microstructure Evolution Of Solidification Process In The Tungsten Inert Gas Welding Pool Of Aluminium-copper Alloy: A Phase-field Study

Aluminum alloy is widely used in the industrial production,but different kinds of defects,such as pores,inclusions and cracks,appear during the welding process,which affect the mechanical properties of weld joints.It could provide theoretical basis for process optimization to investigate the solidification behavior in the molten pool and study the relationship between welding parameters and morphological evolution dynamics.In this paper,the transient conditions of welding process and phase-field?PF?model are developed to reproduce the dendrites evolution during solidification process of aluminum-copper alloy in the argon tungsten-arc welding pool.Firstly,the macroscopic heat transfer model and the quantitative PF model were developed to reproduce the dendrite evolution duing the actural welding process.The macroscopic model of interface without deflection and with deflection are developed respectively to reflect the different characteristics of interface in the early stage and late stage.The transient PF model of single crystal and poly crystal were developed respectively.Then the corresponding welding tests were carried out to verify the model.The initial instability of interface will appear first during solidification process in the molten pool.The effect of anisotropy strength and crystalline preferred orientation on the initial instability dynamics was investigated in this paper.The results show that the influence of surface tension anisotropy on the initial plane instability depends only on the-?₄cos?4?₀?term,the incubation time of instability and initial average wavelength are positively related with the-?₄cos?4?₀?.The surface tension anisotropy does not affect the solute diffusion process,but changes the stability of interface to influence solidification.The welding parameters influence the solidification process by changing the thermal gradient and growth rate.The planar crystal begins to transform into cellular crystal after the initial instability.The influence of the crystalline preferred orientation of base metal on the morphological evolution was investigated.The results show that the growth angle?is similar to the misorientation angle?₀ in the early stage,while the growth direction of dendrite deflected to the thermal gradient direction with the solidification time.At the late stage with the increase of misorientation angle?₀,the interface morphology of the crystals changes from dendrites to titled dendrites and seaweed crystals.In the late stage,the influence of deflection angle on the morphological evolution of solidification structure in the molten pool should be considered.The modified model with the interface deflection was developed and the evolution of solidification structure based on the modified model was studied.The results via the deflected model show that the planar crystal turn to the cellular crystal from the tail of the molten pool,that is to say,the morphology evolution is not periodic.Compared with the model without deflection,the incubation time is earlier,the cell crystal spacing is larger and the degree of solute segregation is lower.The growth angle?is smaller than that of previous at the same time.The comparisons with the experimental results indicate that the deflected model could reflect the curvature evolution of the primary dendrite better than the previous model.Finally,the morphological evolution of solidification structures in the entire molten pool was carried out.The effects of welding parameters and grain size of base metal on the evolution were studied.The results show that the final microstructures are dorminated by the epitaxial growth and competitive growth process.The welding parameters dorminate the evolution dynamics by changing the thermal gradient and crystalline preferred orientations.The axises of columnar crystals are curved under the relatively low welding speed,while there will be some axialite along with the the axis of welding beam.With the welding speed increase,the axialite start to vanish and the axises of columnar crystals turn to be flat.At the same time,the dendritic structure will be finer with the increase of welding speed.The average size of the columnar crystals increases with the increase grain size of base metal,while the morphology of dendrites is dominated by the welding parameters.Based on the theoretical analysis and phase field model results,the quantitative data were obtained to reflect the evolution of the characteristic parameters of the interface tip with time.Based on the results,the morphological evolution mechanism of solidification structures was investigated.These research results lay the foundation for subsequent research.