Formation And Evolution Of Bifilm Defects In Casting Processes

Bifilm is a casting metallurgical defect that is prevalent in castings but has been neglected for a long time.Compared with the secondary oxidation inclusions,the formation process of bifilm is more complex,and is not limited to a specific casting stage.This thesis improved the characterization methodology of bifilm and investigated the formation and evolution mechanism of bifilm.The fracture behavior of bifilm in castings was studied to clarify its role in casting failure.This is undoubtedly the perfection and supplement to the traditional theory of casting defects,and has great theoretical significance and practical application value for the development of high-quality castings.Different types of bifilms in castings were characterized at multi-dimensional and multi-scale.It was concluded that microscopic bifilm existed as homogeneous and lap film,while macroscopic bifilm existed as simply folded film.3D computed tomography（CT）and electrochemical stripping technology were adopted in this study to separate bifilm from the casting during 3D reconstruction.Casting metallurgical quality evaluation method based on bifilm was proposed as well and its physical significance is the sum of surface areas of bifilm per unit volume of casting.In addition,quantitative analysis of bifilm obtained by separation indicated that the bifilm was occasionally folded in casting.The composition and phase structure transition process of bifilm in different castings were systematically analyzed.In aluminum bronze,the thickness of bifilm from several nanometers to several hundred nanometers corresponds to the phase transition path of amorphous,crystalline and fully crystallized alumina.It was shown quantitatively that the Young’s modulus of amorphous alumina film was between1.878 and 10.768 GPa.In Al-Si alloy,film thickness and composition are not uniform:the film area dominated by magnesium oxide and the interface area dominated by aluminum oxide jointly constituted the bifilm defect.The bifilm in rare earth Mg alloy exhibited a uniform and complete film structure.High-resolution image,electron diffraction pattern and energy spectrum analysis proved that the composition of the oxide film was Y₂O₃,Zr was arranged in the core of the oxide film orderly,and the intermetallic compounds Mg₃Gd and Mg₂₄Y₅ were attached to the interface between the oxide film and the matrix.The formation and evolution of bifilm were analyzed and verified.Traditional defect formation theory emphasizes the physical nature of defects,but neglects the surface properties of them,which is exactly what bifilm theory focuses on:the existence of microcosmic/macroscopic surface areas.The bifilm formed in the filling stage show characteristics of non-equilibrium solidification.The main reasons lie in the thermodynamic stability at a critical thickness of amorphous bifilm and the difficulty of oxygen diffusion after defects are formed.In contrast,the composition and phase structure of bifilm formed during the solidification phase is stable.The numerical simulation results showed that both surface turbulence and internal disturbance lead to repeated folding,entanglement and dispersion of melt surface film,as well as the continuously developed entanglement of micro-sized bifilm.Due to the differences in shape factor and comprehensive density,micro-bifilm involved in liquid level motion in different ways in melt,such as floating,suspending,and sinking.The simultaneous junction at the flow front also causes microscopic bifilm.For the bifilm of large size,their formation mechanism includes confluence occurring when the liquid flow front is different from the direct contraction of solid surface film.The initial formation position was the final existing one,and it was mainly a relatively simple and independent structure that regularly distribute in casting.The relevant casting sections or fracture showed that bifilm body directly forms cracks,and the oxidation inclusion arises due to the involvement of bifilm and their shrinkage process,while the confluent bifilm are directly manifested as cold isolation in castings.During the solidification process of castings,bifilm can also be used as nucleation particles of pores and shrinkage pores,where the difficulty of nucleation and growth of defects can be reduced by expanding themselves.Based on optical observation,digital speckle analysis and cohesion model calculation,fracture behavior of bifilm（pore）in castings was analyzed.Bifilm in the casting are not only the crack initiation point but also facilitate crack propagation.The superposition of each bifilm influences region constituting the strain field distribution of tensile specimen.Micro-bifilm formed at different stages presented different deformation behaviors during solidification and different forms in casting and/or fracture.It is found that the main reason for this phenomenon is the mismatch between Young’s modulus of bifilm and matrix.The"young"bifilm possessed a wrinkled surface and adhered to substrate,while the"old"ones had a smooth surface and were separated from substrate.Simulation of cohesive force model showed that the initial cracking position of bifilm was at their tip with stress concentration,where liquid-solid contraction drives solidification.In addition,with increase of tensile load,the interface between defect and matrix cracked gradually,and both the cracking amplitude and the cracking location of the"old"bifilm were more than that of the"young"defect.However,the air gap of bifilm is still the main cracking position because the defect does not adhere inside.The bifilm in castings are not only the crack initiation but also the path of crack propagation.The superposition of each independent bifilm influence region affects the strain field distribution of tensile specimen.