选区激光熔化成形Fe-Mn-Cr-Ni中熵合金的微裂纹形成机理及成形质量调控

Mechanism of microcrack formation and forming quality control of Fe-Mn-Cr-Ni medium-entropy alloy prepared by selective laser melting

研究了Fe-Mn-Cr-Ni中熵合金在选区激光熔化(SLM)制备中的微裂纹形成机理,并通过优化单道、块体成形试验中激光功率(P)和扫描速度(v),以激光能量密度为评估参数,调控微裂纹形成从而提升成形质量。结果表明:微裂纹可分为微孔聚集性裂纹和热裂纹两类,其中微孔聚集性裂纹与孔隙引发的应力集中有关,热裂纹是由于Mn元素偏析及热应力所引起的。通过减少孔隙形成和控制能量输入可抑制微裂纹形成。随着线能量密度(E_(L))的增加,单道表面形貌先变优后变差,熔池尺寸逐渐增大,熔池模式由传导向过渡再向匙孔模式转变。当E_(L)为0.29~0.40 J/mm时,单道整体形态和熔池尺寸可满足致密块体成形要求。较高体能量密度(E_(V))下微孔聚集性裂纹和热裂纹均会存在,较低E_(V)下以微孔聚集性裂纹为主,提高E_(V)有助于优化块体表面质量,而块体试样的相对密度随E_(V)的增加先上升后下降。在优化的E_(V)=65.10 J/mm^(3)(P=175 W,v=640 mm/s)下,块体试样的内部微裂纹得到有效抑制,表面质量优异,相对密度可达99.71%。

The mechanism of microcrack formation in Fe-Mn-Cr-Ni medium-entropy alloy during selective laser melting(SLM)preparation was studied.By optimizing the laser power(P)and scanning speed(v)in single track and bulk-forming experiments,the laser energy density was used as the evaluation parameter to regulate microcrack formation and improve forming quality.The results indicate that the microcracks can be divided into two types:microporous aggregation cracks and thermal cracks.Among them,microporous aggregation cracks are related to stress concentration caused by pores,while thermal cracks are caused by Mn element segregation and thermal stress.The formation of microcracks can be suppressed by reducing pore formation and controlling energy input.With the increase of line energy density(E_(L)),the surface morphology of the single track first becomes better and then worse,the size of the melt pool gradually increases,and the melt pool mode transitions from conduction to transition and then to keyhole mode.When the E_(L)is 0.29-0.40 J/mm,the overall morphology of the single track and the size of the melt pool can meet the requirements for dense bulkforming.At higher volumetric energy density(E_(V))both microporous aggregation cracks and thermal cracks will exist,while at lower E_(V),microporous aggregation cracks are mainly present.Increasing E_(V)helps to optimize the surface quality of the bulk sample,while the relative density of the bulk sample first increases and then decreases with the increase of E_(V).Under the optimized E_(V)=65.10 J/mm^(3)(P=175 W,v=640 mm/s),the internal microcracks of the bulk sample are effectively suppressed,with excellent surface quality and relative density of 99.71%.