| Ti6Al4V alloy has been widely used in aerospace,medical and automotive fields due to its excellent mechanical properties and high specific strength.Selective laser melting processing of titanium alloys has advantages such as near-net-shape forming,low-volume production cost,and support for complex geometric structure processing,which accelerates the design and application of integrated functional structure of parts,and reduces processing costs to a certain extent.The action of laser and titanium,a short-time and physical-model-complex process,could be investigated through numerical simulation method as it could show the temperature and fluid field in great detail during processing.In addtion,it is a time-saving and low cost method.Lattice Boltzmann method,as a mesoscopic fluid mechanics method that has developed rapidly in recent years,has become a powerful tool for fluid calculation due to its advantages of clear physical meaning,simple boundary processing and efficient processing of multiphase flow boundaries,however,it is rarely used in the field of selective laser melting simulation.In this paper,Lattice Boltzmann Method(LBM)was employed to build a D3Q15 double distribution function numerical model for laser-powder bed fusion(LPBF)investigation.The code of this model was programmed by Python based on its expansion packages such as Numpy,math,Matplotlib and Mayavi,in which iteration process is computed by array so as to run efficiently(90 times faster than before).All three processes–model building,computing and data visualization–are completed in Python.The model is composed of 150 × 150 × 50 grids.Without parallel computing,the CPU computing time on the Intel Core i3-4160 quad-core computer is 22 h,and the computational efficiency is significantly improved compared with traditional methods.In order to deal with solid-liquid phase change,enthalpy is encouraged to calculate energy equation and temperature and volume distribution function are the functions of enthalpy respectively.In this case,physical quantities such as solid-liquid surface and interface force would change smoothly.Physical models,including interface force,Marangoni effect and surface tension are considered in this model.To be as similar as the real processing,border scanning and hatch scanning are contained for a meodel with four borders and three hatches.The temperature of borders and hatches are approximately1900 K and 3300 K respectively due to the different laser power and scanning velocity.Some special phenomenons,for example,melting pool widen,melting pool temperature rises,tail of last melting pool deepen and more violent flow,are observed at the corner of melting pool.However,the temperature would decrease smoothly to about 3200 K with slight fluctuation and fluid would not as violent as before when the laser beam moved to middle of melting pool.The main way of heat transfer in border is thermal diffusion so that the depth and width of melting pool is as similar as about 428).The shape is like a sphere.However,the countpart in hatch melting pool is heat convection,in which fluid could flow to the lower temperature area.It's shape is like "comet" and the depth and width of melting pool are 528)and 798)respectively.Heat effected zone of borders are widely,yet only 100 K increased in hatches compared with the hatches with no border scanning.Different from the dramatically increase of temperature at the beginning of the first hatch with no border,the counterpart with border keeps steady because of close to the tail of fourth border where the basic temperature is about 1600 K.The effect of heat accumulation are gradually decrease when the laser beam is more close to the middle of melting track.Specially,the temperature of mid-point of second melting track increases only 200 K when laser beam approach the same point of first melting track.The reason is inefficient heat conduction(1/10 compared with ordinary alloys)of Ti6Al4 V and enough time for cooling in longer scanning distance. |
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