| Additive manufacturing is an important part of the strategic plan of Smart Manufacturing 2025.With high processing efficiency,short response time and high automation integration,it is widely used in the fields of aerospace,deep-sea equipment,biomedical and military defense.Among them,selective laser melting technology is currently the most widely used additive manufacturing technology,which can directly obtain metal formed parts with high dimensional accuracy,high density and high mechanical properties.However,during the forming process,the great temperature gradient and cooling rate lead to extremely uneven temperature distribution inside the formed part,which causes expansion or contraction of the formed part,and then the inprocess thermal stress affects the microstructure evolution and the quality of the formed part.Therefore,it is important to reveal the changes of thermal history and microstructure evolution of formed parts during the forming process for their quality.In this study,Ti-6Al-4V alloy was selected as the object of study.Firstly,a prediction model of temperature field distribution under multi-channel single-layer linear reciprocal scanning in a semi-infinite area is established and considering the influence of material thermophysical properties with temperature variation.The effect of laser process parameters on the variation of the thermal history is analytically explored by temperature history extraction.Secondly,the temperature field is equated to the external load applied within the mechanical model,and the thermal stress model based on the Green's function method for point body loading is established to predict the thermal stress caused by the inhomogeneous heating during the forming process,and the thermal stress distribution law is analyzed and investigated according to the model predicted values.Finally,the microstructure evolution characteristics were analyzed by experimental method and the grain evolution model was established.According to Johnson-Mehl-Avrami-Kolmogorov(JMAK)theory,grain growth during heating is modeled by using the kinetics of phase transformation predicted by the negative exponential equation,and grain refinement during cooling is modeled by the extended classical reaction kinetic equation.The predicted grain size values were analyzed and compared with the experimental values and the grain evolution model was used to study the grain size changes under different laser process parameters.The main conclusions are as follows.During the selective laser melting and forming of titanium alloys,The effects of laser power and scanning speed on cooling rate both showed positive correlation.The effect of substrate preheating temperature on the thermal history and cooling rate is not significant,but it affects the peak temperature and thus induces phase transition.Increasing the scanning speed during the forming process increases the peak stress.During the stress relief phase,more stress is released at low scanning speeds.When the laser power is increased,the peak stress also increases,and more stress is released at high laser power.At laser power of 200 W and scanning speed of 1000mm/s,there is very little residual ?-phase in the microstructure which has not been transformed,the volume fraction is about 0.188%,and the average grain size is about 2.2?m.When the scanning pitch is adjusted from 110?m to 140?m,the volume fraction of residual ?-phase in the formed part is about 0.388%,and the length and width of martensite in the microstructure increase significantly.The aspect ratio was significantly larger than that when the scanning pitch was 110 ?m.In addition,the use of tessellation scanning will obtain more uniform fine martensite and the phase organization of the formed part is ?-phase.It is shown that the variation of scanning speed has a more significant effect on grain refinement than laser power,and the average grain size decreases with increasing scanning speed. |
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