Characterization And Numerical Simulation On The Microstructure Of Laser Additive Manufactured TC4 Alloy

TC4 alloy is a typical ?+? duplex phase titanium alloy.Its specific strength in a wide temperature range is almost the highest in all alloys,so it is extensively used in aerospace industry as a type of structural materials.However,the high cost restricts its wider application in other industries.Recently,laser additive manufacturing attracts great attention because of the lower cost in complex prototype.Compared to traditional manufacturing processes,the drastically change in temperature during laser additive manufacturing processes will generate unique microstructure,which has an important effect on final performance.The effects of thermal cycles on the microstructure of TC4 alloy were investigated by applying different thermal cycles on samples made of bulk alloy through instrument in which temperature can be accurately controlled.Results show that different microstructural morphologies of transformed ? appear under distinct cooling rate;primary? or acicular ?'appears during primary thermal cycle while secondary ?grows beside primary a or a plates grow beside acicular ?' during secondary thermal cycle.The microstructure of laser additive manufactured sample was characterized through OM,SEM and EBSD.Experiment results reveal that in LENS(Laser Engineered Net Shaping)sample there are many unbounded particles and voids between the layers and tracks,as well as coarse columnar grains perpendicular and penetrate all layers.The microstructure inside a layer and between two layers is both acicular martensite,but that inside a layer is much finer.The microstructure of SLM(Selective Laser Melting)sample is much homogeneous,mainly consists of acicular martensite.Fewer defects except very small voids are dispersed in its matrix.Tension test performed on distinct directions of SLM sample revealed that there is no difference on Young's modules and elasticity limit in each direction and all will failed through brittle fracture.While the direction normal to layers has the highest elongation of 2.1%and ultimate strength of 1100 MPa,which shows the anisotropy in above mechanical properties.The parameters in phase transformation model of TC4 alloy were determined by dilation test and CALPHAD.Thermal physic parameters were determined by laser flash system.The temperature and microstructure coupled numerical modal,plugged in finite element software and user subroutines,were employed to predict the temperature and fraction of each phase during the laser additive manufacturing processes.Simulation results are able to explain the microstructure character at distinct position in the as built sample reasonably:the largest temperature gradient appear in the direction normal to layers,which results to columnar grains grow along the direction;the as-built sample only experience primary thermal cycle and the cooling rate is large enough,which results to full martensite microstructure;temperatures at the top of a layer is always higher than that inside a layer,which results to finer microstructure inside a layer than that at layer band.