孔隙类型对激光选区熔化Inconel 625合金高温塑性的影响

Effect of Pore Type on High⁃Temperature Plasticity of Inconel 625 Alloy Fabricated by Selective Laser Melting

采用激光选区熔化成形了Inconel 625合金试样,研究了激光功率和扫描速度对合金孔隙的影响,制备了存在大尺寸圆形孔隙缺陷(key-hole模式下)、小尺寸圆形孔隙缺陷(conduction模式下)和不规则孔隙缺陷的拉伸试样,通过815℃高温拉伸试验探讨了孔隙类型对合金抗拉强度及塑性的影响。结果表明:孔隙类型对合金815℃高温抗拉强度及塑性有较大影响。其中,不规则孔隙缺陷试样的高温抗拉强度为374 MPa,塑性为5%;大尺寸圆形孔隙缺陷试样的高温抗拉强度为364 MPa,塑性为31%;小尺寸圆形孔隙缺陷试样的高温抗拉强度为363 MPa,塑性为37%。当不规则孔隙向小尺寸圆形孔隙转变时,高温抗拉强度约降低3%;当大尺寸圆形孔隙向小尺寸圆形孔隙转变时,高温塑性约提升19%。在各类815℃高温拉伸试样断口纵截面中均发现了大量的沿晶界扩展的高温失塑裂纹,这是合金失塑的主要原因。孔隙缺陷处的位错塞积造成较大的应力集中,阻挡了位错滑移和合金的塑性变形,使合金更易发生断裂。

Objective Inconel 625 superalloy is extensively used in combustion turbines because of its superior tensile,fatigue,and creep strengths at elevated temperatures.Selective laser melting(SLM),which is one of the most promising additive manufacturing(AM)methods for hightemperature superalloys,has attracted considerable attention in many fields,including aerospace,owing to its special technical advantages.However,plastic loss occurs in the service temperature range of nickelbased superalloys,which poses a serious threat to the stability of their mechanical properties during service.To ensure the stability of SLMfabricated Inconel 625 superalloy in hightemperature service environments,the different types of pore defects should be studied to understand the hightemperature plastic deformation behavior of this superalloy for engineering applications.Methods Different types of pore defect samples are prepared by adjusting the process parameters,and the samples are subjected to room-and hightemperature tensile tests.The influence of different types of pore defects on the hightemperature plastic loss of the superalloy is analyzed using fracture and fracture longitudinal sections,and the fracture mechanism is analyzed.Results and Discussions When the laser power is set to 300 W,with increasing scanning speed,the pore defects transit from large regular circular pore defects in“keyhole”mode to small regular circular pore defects in“conduction”mode.When the scanning speed is further increased,the laser energy input decreases,and irregular pore defects with typical tip shapes are obtained(Fig.3).In the tensile test,the influence of different pore types on the room-temperature plasticity and hightemperature plasticity of the alloy becomes more apparent.Among these pore types,irregular incomplete fusion defects exert the greatest impact on plasticity,with roomtemperature plasticity and hightemperature plasticity of 6.47%and 4.85%,respectively.The second most influential defects are the large circular pores in the“keyhole”mode,which exhibit roomtemperature and hightemperature plasticity of 20.45%and 11.5%,respectively.However,the regular small circular pore defects produced in the“conduction”mode have the least effect on plasticity,with roomtemperature plasticity of 39.19%and hightemperature plasticity of 36.84%(Fig.6).The stress–strain curve of the SLMformed Inconel 625 alloy at high temperatures shows a significantly different trend from that at room temperature.After the alloy stress reaches its peak value,the alloy softens with an increase in strain(Fig.5).In room-and hightemperature tensile fracture surfaces,keyholemode pores can be observed in a small number of larger pores,which are embedded with unfused powder particles;there are a small number of secondary cracks along the outer edges of the pores,and the fracture mode is the typical brittle fracture.In the conductionmode pores,along the outer edges,there are smaller,tough nests,which exhibit more evident ductile fracture characteristics.Irregular pores exhibit brittle fracture characteristics because of their larger number and sizes(Fig.8).The three different pore types exhibit more plastic cracks at high temperatures(Fig.9).With the movement of dislocations,the tips of the irregular pores easily cause dislocation accumulation,which affects the plastic deformation of the superalloy and leads to its rapid fracture.Compared with irregular pores,the circular pore boundary of the“keyhole”mode can reduce the degree of dislocation pileup;therefore,the hightemperature plasticity of these pores is higher.The circular pores in the“conduction”mode are very small,and their ability to weaken the dislocation pileup is the strongest;therefore,their hightemperature plasticity is the highest.Conclusions The results of static load tensile tests show that the stressstrain curves of tensile specimens with different pore types at high temperatures are obviously different from those at room temperature.After the superalloy stress reaches its peak value,the superalloy appears to soften with an increase in strain.The load fluctuates continuously and periodically in the softening stage and gradually decreases until fracture.The effects of different pore types on the hightemperature plasticity of the superalloy are significantly different;the hightemperature plasticity of the irregular pore sample is 4.85%,and those of large circular pores in the“keyhole”mode and small circular pores in the“conduction”mode are 30.69%and 36.84%,respectively.Although the hightemperature plasticity of the“conduction”mode pores is improved compared to the other types,it is still lower than that of the ascast Inconel 625 superalloy.Therefore,it is believed that the type of pore defect does not significantly improve the hightemperature plasticity of the superalloy.