激光选区熔化GH3536镍基高温合金组织与性能研究

Microstructure and Properties of GH3536 Nickel-Based Superalloys by Selective Laser Melting

研究了激光选区熔化(SLM)GH3536合金扫描面与建造面的组织与性能。采用光学显微镜(OM)、 X射线衍射仪(XRD)、扫描电镜(SEM)和电子背散射衍射(EBSD)对激光选区熔化GH3536合金金相、物相、微观组织和晶粒特征进行研究。结果表明,通过优化成形参数可以减少合金中孔隙与微裂纹,但是无法消除。半椭圆形熔池广泛分布于建造面,其宽深比约为1.5。激光选区熔化GH3536合金由单一的面心立方γ奥氏体组成。扫描面与建造面都分布着大量胞状与柱状亚晶,建造面熔池交界处存在沿建造方向的微裂纹。建造面的平均晶粒尺寸(145.1μm)约为扫描面晶粒尺寸的4.5倍,织构强度约为扫描面的2倍。横向与纵向试样的拉伸性能存在明显差异,横向试样的屈服强度和极限抗拉强度分别为645 MPa和781 MPa,分别比纵向试样高4.1%和7.0%。激光选区熔化GH3536合金断口呈明显韧性断裂,存在大量韧窝。本研究有望为激光选区熔化GH3536合金扫描面与建造面组织与性能差异提供有效的参考。

GH3536 superalloy was prepared by selective laser melting(SLM)and the effect of forming parameters on the superalloy microstructure was studied in this paper.The difference in microstructures and properties between the scanning plane and the building plane was studied.The metallographic,phase,microstructure and grain characteristics of SLMed GH3536 alloy were studied by optical microscope(OM),X-ray diffractometer(XRD),scanning electron microscope(SEM)and electron backscattered diffraction(EBSD).The pores and microcracks could be reduced by optimizing the forming parameters,but could not be eliminated.The forming parameters of SLM played a decisive role in the internal defects of GH3536 superalloy according to the types and quantities of defects.The forming parameter interval could be divided into three regions.When the energy density reached a low level,many irregularly shaped unfused pores and a large number of tiny pores could be observed.The keyholes were formed when the energy density reached a high enough level.By optimizing the process parameters of SLMed GH3536 superalloy,the optimal forming parameters were finally obtained:laser power(P)=200 W,scanning speed(v)=800 mm·s^(-1),hatch spacing(h)=0.08 mm,layer thickness(t)=0.03 mm.GH3536 alloy consisted of a single face-centered cubicγaustenite.Due to its high solidification rate(1×10^(4)~1×10^(6) K·s^(-1))and the phenomenon of"solute capture"caused by rapid solidification,the precipitation phase cooled to form a supersaturated solid solution before it precipitated.Most of the alloying elements were solid-dissolved into the nickel matrix to form a singleγaustenite.However,there were significant differences in the organizational preference orientation between the scanning planes and building planes.The scanning plane had the strongest diffraction peak at 74.5°,indicating that its texture grew along(220)crystal plane.While the building plane had the strongest diffraction peak at(200),indicating that GH3536 alloy had a 002 fibrous texture on the building planes.A large number of fish-scale molten pools were arranged layer by layer.The molten pools formed by Gaussian heat source were closely arranged in a semicircle.The width of the molten pool was about 200μm and the depth was about 120μm,and the ratio was about 1.5.The depth of the molten pool run through the thickness of 3~4 layers of powder,indicating that the metallurgical bond was good during the forming process.This indicated that the formation of the molten pool was a conduction mode during the selective laser welding of GH3536 alloy.A large number of cellular and columnar sub-crystals were distributed on both the scanning plane and the building plane.Some solidification microcracks were randomly distributed at the junction of the molten pool along the building direction due to the fast-cooling rate of SLM.The alloying elements could not be completely uniformly distributed in the molten pool during the solidification process.The low melting point eutectic microstructure was forming at the end of solidification.Distinct grain tearing features could be observed near the cracks.The texture index and texture strength of the scanning plane and the building plane were both greater than 1,indicating that the alloy microstructure exhibited obvious anisotropy.Whether it was the scanning plane or the building plane,a large number of small-angle grain boundaries were distributed inside the columnar crystals.The volume fraction of the small-angle grain boundaries on the scanning plane was 62.9%,while the building plane were as high as 82.9%.From kernel average misorientation(KAM)images of the scanning plane and the build plane,it could be seen that a large number of dislocations were distributed at the high-angle grain boundaries,indicating that the dislocations in GH3536 alloy mainly existed near the high-angle grain boundaries.The average KAM of the scan plane and the build plane were 1.41 and 1.04,respectively.The average grain size(145.1μm)of the building plane was about 4.5 times that of the scanning plane,and the texture strength was about 2 times that of the scanning plane.The tensile properties of transverse and longitudinal specimens were significantly different.The yield strength,ultimate tensile strength and elongation of the transverse specimens were 645 MPa,781 MPa and 24.3%,respectively.Their strengths were comparable to the as-forged GH3536 alloy.The yield strength and ultimate tensile strength of the transverse specimen were 4.1%and 7.0%higher than those of the longitudinal specimens,respectively,while the elongation was 11.5%lower.This was mainly because the distribution of solidification cracks along the building direction reduced its plasticity.The fracture of GH3536 alloy by SLM showed obvious ductile fracture,and there were a lot of dimples and their average size was about 0.5μm.This study was expected to provide an effective reference for the differences in microstructure and properties between the scanning plane and the building plane of GH3536 alloy by SLM.