On the microstructure of superalloy pattern ed by selective laser melting-刘锦辉.doc

On the microstructure of superalloy pattern formed by selective laser meltingLiu Jinhui1, Wei Qingsong2, Zhang Jia1,Xiao Shengbing1, Xie Wenjuan1, Shi Yusheng21 Modern Manufacture Engineering Center, University of Heilongjiang Sci. & Tech. Harbin, 150027, China2 State Key Lab. Of Material Processing and Moulding, Huazhong University of Sci. & Tech. Wuhan, 430074,ChinaAbstract: Supperalloy components have been built by selective laser melting (SLM), but the application environment of superalloy components is usually different from other one. Owing to be applied in aerospace, the remarkable properties of these components should be presented in the end. According to the determination relationship of microstructures to performance, their microstructure must meet a corresponding criterion. The investigation on the state of superalloy components formed by SLM is carried on in this work. Under the condition of SLM process, the microstructures from different areas of one pattern are analyzed depending on the theoretical results of solidification and phase evolvement. Optical microscopy is used for the main testing measure. The forming and variation phenomenon can be discovered to provide knowledge for the future SLM forming of superalloy components. Keywords: Selective laser melting; superalloy; Microstructure1 Introduction Polycrystalline nickel base superalloys, are widely used as high-temperature, high-strength components in gas turbines such as stators, compressors and turbine discs, due to their outstanding mechanical property 1-3. Among them, Inconel 718 (IN718), a precipitation-hardenable, niobium-modified, nickel-base superalloy that is used extensively in the aerospace, petrochemical, and nuclear industries since it exhibits good strength, corrosion resistance and welding characteristics4. But manufacturing an aerospace component with IN718 through traditional processing method is difficult owing to its remarkable mechanical properties at the same time. Selective laser melting (SLM) is one of successful rapid manufacturing technologies, which is used for forming complicated metal components5,6. It is in particular applied for manufacturing components that can hardly be deal with under regular fabrication routes. Nowadays, SLM begins to step into the aerospace manufacturing domain. But it is a long way to go for SLM to replace other conventional methods before it has been tested in practical applications. In the views of microstructures, whether IN718 components formed by SLM is appropriate to be served as aerospace ones is also one of the topics to be further discussed. This work mainly discusses the state of microstructures of SLM formed IN718 patterns and presents a reference for the ongoing investigations. 2 Experiment2.1 Materials Gas atomized IN718 powders with the diameter range of 10-38m (Fig. 1) were used in this work. Seen in fig. 1, particles were all almost spheres and have good fluidity. Its chemical compositions (mass fraction,%) are : C 0.015, Co 12, Cr 18, Ti 1.0, Al 0.5, Mo 3, Ta 3,B 0.018, Nb 4 and Ni balance. The content of oxygen in powders is under 300ppm.Fig.1 SEM picture of IN718 powders2.2 Experiment procedures SLM system in this experiment was HRPM-B type machine made in China. The machine was equipped with a 500W continuous wave fiber laser and a vacuum system. The air in formation capsule was pumped out and a vacuum state was made. Then Ar gas filled into the capsule and made the content of oxygen was lower than 10 ppm before SLM process. Under the processing parameters of scanning interval 0.16mm, scanning speed 1350mm/s, laser power 400W, layer thickness 0.03mm, a flake of 10mm long and 5mm high consisted of five continuous scanning lines in each layer (fig.2a)and a block of 10mm(L)10mm(W)15mm(H) were formed. Then the left and right surfaces of the flake and its bottom were processed to make samples for optical microscopy test. Different parts of the block showed in fig.2b were also processed as samples for optical microscopy test. The etchant was 100ml CH3CH2OH +100mlHCl+5gCuCl2. 10mm123Fivelines (a)74685915mm10mm10mm(b)5mm Fig.2 SLM patterns for optical microscopy testing: (a) flake; (b) a block3 Results and discussionThe optical microstructures photographs of different parts of samples are shown as followings. Fig. 3 is the optical photographs of microstructures of sample (a) in fig.2 and fig.4 is the ones of sample (b). (1) (2)300m (3)Fig.3 the optical photographs of microstructures of sample (a) in fig.2There are 3 photographs in fig.3(1). They are all taken from the left side of the flake, but different areas. In general, the morphologies of the microstructures are different. The first one contains fine spot phase but the microstructure in the second one is almost strip ones. It is interesting that both the spot phases and strip ones are in the third photograph. In this experiment, laser power is set as 400W which means a very high energy loading on a micro-scale laser circle spot. Furthermore, laser scanning speed (1350mm/s) is relative high either. Then, heat transferring in laser exposure areas can almost not be affected by other ones. One exposure should induce the same temperature variation with another one definitely. Therefore, every parts of the sample should undergo the same thermal process during SLM period, and the microstructures of these parts should be similar in the end. But the microstructure morphologies in fig.3 appeal two individual ones. The reason of this is the superalloy melts will solidify based on its solidified parts. The cooling direction of the metal melts can not hold at a fixed one. So the growth directions of each crystalline on the interface will point any directions, which causes the random growing of oncoming crystalline. Now it is not difficult to explain that there must be different growth directions which induce the individual morphologies in the aforementioned microstructures. They coexist within a micro scope in the third photograph in fig.3(1) after all. Furthermore, it is determined that the strip phases are growing parallel to the inspected plane, but they appear to be the spot shaped ones when they grow perpendicularly to the inspected plane. Similarly, the microstructures in the other two places of the flake are similar. (4) (5) (6) (7)300m (8) (9)Fig.4 the optical photographs of microstructures of sample (b) in fig.2Fig.4 consists of six photographs (5-10) which show the microstructures of six parts of the block correspondingly. These parts cover the typical places which stand for different thermal routes during SLM process. Seen from the microstructures, they all spot or strip shaped ones and are similar with those in the flakes. It is determined that the microstructures from all the part of IN718 pattern formed by SLM method are all similar. The formation reason of these similar microstructures depends on the great difference between the instant melting temperature of IN718 by laser and the one in the forming capsule. The difference runs up to at least 10007 when one layer formed. Owing to the high solidification rate, the aforementioned phases cant grow fully. Furthermore, the former solidification phases even can not transformation due to the very short time heating by laser when the upper layer is scanned by laser. In another word, laser heating treatment during this short period has done nothing with the solidified part of the IN718 pattern. Therefore, the microstructures in SLM IN718 pattern can not vary with the samples taken from different part of the pattern. In general, the microstructures in IN718 are non-equilibrium and very fine. The improvement of the deformability and mechanical properties of superalloy relies on grain refinement method, such as fine grain casting including the thermal control method, the chemical approach and the dynamic method8.Fig.5 is the micrograph of the as-cast microstructures of IN718, and comparable to these, the ones in IN718 pattern formed by SLM is finer. In a sense, SLM also provide a way to refine the grain as well as it fabricates the complex metal structure. 300mFig.58 The as-cast microstructures of IN7184 Conclusions The microstructures of IN718 pattern formed by SLM show the characteristic of out-of-equilibrium. They appeal complete similarity within the whole pattern and are not affected by the different areas of the pattern. They are also finer than the as-casting ones. Therefore, better deformability and mechanical properties of IN718 components may be obtained through SLM method. It is hoped that SLM provides a new way to manufacture IN718 components with both complex structure and outstanding properties. Acknowledgement This work is financially supported by Key Project of Nature Science Foundation of Heilongjiang province(ZD201104) and Program for New Century Excellent Talents in University of Heilongjiang province(125-NCET-008).References1 Zhong Zhou, Amrinder S. Gill, Dong Qian, S.R. Mannava, Kristina Langer, Youhai Wen,Vijay K. Vasudevan, A finite element study of thermal relaxation of residual stress in laser shock peened IN718 superalloy. International Journal of Impact Engineering 38 (2011) 590-5962ZHOU Yuangui , PENG Zhifang , CHEN Fangyu , REN Yaoyao. Microstructure and typical elements distribution of IN718 alloy after aging treatment Heat treatment of metals , V 34 n 6, 2009, 19-24)。3Ulrich Krupp, William Kane, Jeffrey A. Pfaendtner, Xinyu Liu, Campbell Lairdb, Charles J. McMahon Jr. Oxygen-Induced Intergranular Fracture of the Nickel-Base Alloy IN718 during Mechanical Loading at High Temperatures.V 7, N 1, 35-41, 2004.4 DW J T