| Selective laser melting(SLM)technology can be used for printing of superalloy in three dimensions and fabricating terminal or near-terminal metal parts directly.It is widely acknowledged as a 3D-printing method that has unique advantages in material forming.However,the presence of pores can always be found in the SLM forming parts,which greatly affects the mechanical properties of the parts.Among all forming defects,pores are the most typical one.Therefore,in this study,the formation and elimination of pores are systematically researched on GH4169 alloy by using the method combining computational fluid dynamics and experimental schemes.Orthogonal experiments are designed to study the effects various process parameters have on the shape of the molten channel in single-channel laser scanning,which reveals the characteristics of the melt pool shape in laser processing.Next,double-channel scanning and triple-channel scanning experiments are designed to explore the evolution law of molten channel shape.It has been found that in the low laser power area,the lower the energy density is,the stronger the remelting effect of the laser has on the matrix;the faster the laser scanning,the smaller the molten channel height is.Moreover,all the relatively deeper molten channel appears only at the scan speed that is not lower than 0.8m/s.Either under the condition of low power and low energy density or the condition of high power and high energy density,the penetration depth of the melt pool is large.The average height of the melt channel gradually decreases from 80 ?m to 41 ?m(very close to the thickness of the powder layer)while changing the scanning method from single-channel scanning to doublechannel scanning or triple-channel scanning.Observing the double molten channel and the triple molten channel,it can be found that the height of the firstly scanned molten channel is slightly higher than the later scanned ones.Evidence shows that the height of the melting channel gradually decreases with time.Observe and count the porosity defects on the SLM specimens.The results show that the keyhole defects are most serious in the sample with the laser power of 200 W and a scan speed of 0.4m/s.Keyholes distribute throughout the bottom of the molten channel and disappear gradually while the scan speed increases.Meanwhile,the pores between 2?m and 10?m in diameter are sparsely distributed in the sample,which is considered to be caused by internal pores in the powders made by atomization.It has also been found that the remolten zone of matrix helps to eliminate defects that have already existed in the matrix.Under the parameter of small heightdepth ratio,the remelting effect the laser has on the matrix is strong,leading to the stability of melt pool.Numerical simulation of SLM process is carried out with computational fluid dynamics to approximate the morphology of the keyholes and the morphological characteristics of the melt pool.Time and distance the molten pool needs to reach steady state or quasi-steady state under different process parameters are obtained.Taking the force of micro-bubbles of several micrometers equivalent to the force of the particles in the flow field and considering the flow field to simulate the motion of the bubble pre-positioned in the matrix when melt pool moves to the bubble position,it can be found that the bubbles are difficult to escape from melt pool,which means that laser remelting is required to eliminate such defects. |
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