稀有金属(英文版) 2020,39(10),1181-1189

Microstructure and mechanical behavior of Inconel 625 alloy processed by selective laser melting at high temperature up to 1000℃

Xiao-An Hu Gao-Le Zhao Fen-Cheng Liu Wei-Xiong Liu

School of Aircraft Engineering,Nanchang Hangkong University

Airbreathing Hypersonic Technology Research Center,China Aerodynamic Research and Development Center

作者简介：*Xiao-An Hu,e-mail:hu_xiao_an@163.com;

收稿日期：4 September 2018

基金：financially supported by the Youth Science and Technology Project of Jiangxi Education Department (No.GJJ160726);the National Natural Science Foundation of China (No.51865036);Gulf Research Program (US)(No.2015CB057400);Major Research Plan (No.51865036);

Microstructure and mechanical behavior of Inconel 625 alloy processed by selective laser melting at high temperature up to 1000℃

Xiao-An Hu Gao-Le Zhao Fen-Cheng Liu Wei-Xiong Liu

School of Aircraft Engineering,Nanchang Hangkong University

Airbreathing Hypersonic Technology Research Center,China Aerodynamic Research and Development Center

Abstract：

Mechanical behavior of Inconel 625 alloy manufactured by selective laser melting(SLM) was experimentally studied.Tensile tests were performed at various temperatures(20,540,760,815,870,950 and 1000℃).The microstructures and fracture mechanism were investigated using optical microscopy(OM),scanning electron microscopy(SEM) and energy dispersive spectroscopy(EDS).The SLM Inconel 625 exhibits excellent plastic deformation at low temperature.However,significant embrittlement for the SLM Inconel 625 was found at elevated temperatures compared to low temperature and traditional forging process.Through microstructure studies,the non-homogenous microstructure is a key factor causing the intergranular cracking mode at elevated temperature.Moreover,carbides were found to form at the grain boundary at elevated temperatures.These carbides could weaken the strength of the grain boundary at elevated temperatures,which decreased the tensile strength and ductility of the alloy processed by SLM.

Keyword：

Nickel-based superalloy; Selective laser melting; Embrittlement; High temperature; Microstructure;

Received： 4 September 2018

1 Introduction

Additive manufacturing (AM) of metals provides great flexibility in producing components with low complexity of design process,short time for design iteration,complex structure design,low wastage of materials and reduced part's number [ 1, 2] .Selective laser melting (SLM),one of the most promising AM methods for high-temperature superalloy,has attracted a considerable amount of work in many fields including aerospace,automobile and biomedical industries due to their special technical advantages [ 3, 4, 5] .Extensive studies have been carried out to focus on the SLM process conditions [ 2, 6, 7, 8] and post-process conditions [ 9, 10, 11, 12, 13] such as heat treatment and hot isostatic pressing (HIP).However,as a result of high reliability requirement for the application of SLM superalloys,their micro structure and failure mechanism should be carefully investigated,especially at high temperatures [ 14] .

During SLM process,the line-by-line scanning of a high-powered laser is essentially a nonequilibrium process [ 15] .Highly localized heats with considerable power act on the powder in a very short time.The powder in a limited space experiences a complex thermal cycle with rapid heating up to several thousand K·s^-1.A steep temperature gradient occurs from the local melted material to the part buckets [ 16] .After the laser moves away,the material solidifies rapidly.Many parameters including those of powder (quality and performance) and process (i.e.,laser power,scan speed and layer thickness) will influence the quality of the SLM components.Compared to traditional forged and cast super alloy,a high level of residual stress,inhomogeneous microstructures and phase distributions exist in as-built SLM superalloy [ 3] .Thus,heat treatment(HT) and HIP are considered as the most effective methods to improve the mechanical behavior of the as-built SLM superalloys.Although the mechanical performance of the material can be improved through the optimization of the SLM process and post-process methods,harmful microstructures and phases cannot be totally avoided.Furthermore,material performance requirements for components subjected to different loads and service environments cannot be the same.Though the mechanical behavior of SLM superalloy at room temperature has received many attentions,the high-temperature mechanical behavior is not clearly understood.

The microstructure and mechanical behavior of SLM superalloy have been intensively studied with respect to various parameters.Kanagarajah et al. [ 17] studied the effect of microstructure and temperature on the mechanical properties of SLM Inconel 939 superalloy.They found that the samples tested at 750℃showed a clearly decreased ductility compared to those tested at room temperature.They attributed the reason to the embrittlement of the specimens which was caused by the precipitates formed at elevated temperatures.The mechanical behavior of SLM Inconel 625 has been extensively studied by many researchers [ 18, 19, 20, 21] .The room temperature mechanical properties such as fracture strength,yield strength and elongation were compared to those obtained from traditional processes.Most of the researches confirm sufficient static strength and ductility for AM superalloy except for Yadroitsev et al.’s finding [ 21] where embrittlement (only9%elongation) was found.Combination of brittle and

ductile fracture from scanning electron microscopy (SEM)observation was primarily explained.

Kreitcberg et al. [ 22] studied the microstructure and mechanical properties of the Inconel 625 alloy processed by laser powder-bed fusion after the following post-treat-ments including stress relief (SR) annealing,solution treatment and HIP.Vertical specimens are found to be significantly more ductile than their horizontal counter-parts.After SR process,the overall elongation is signifi-cantly improved compared to that of the as-built specimens.The ductility of the material at 760℃decreased compared to room temperature ductility.The results were not coincided with traditional forged or castInconel 625.Trosch et al. [ 23] studied mechanical properties of SLM Inconel 718 at room temperature and elevated temperatures (450 and 650℃).The parts processedby SLM have better mechanical properties than forged and cast material at room temperature.Meanwhile,they have similar properties with forged material at elevated tem-peratures.The optimization of microstructure with a high proportion of intragranularδ-phase in SLM Inconel 718was an effective way to improve high-temperature mechanical property.Mostafaei et al. [ 24] investigated the effect of solution and aging on the microstructure and mechanical properties of powder-bed binder jet-printed nickel-based superalloy 625.They found that the aging treatment resulted in the formation of intermetallic phases such as Ni₃Nb and Ni₂ (Cr,Mo) as well as carbides such as NbC and Cr₂₃C₆ in the microstructure.While the microhardness and tensile strength of the aged specimen increased due to these phases,the ductility value decreased.Li et al. [ 25] studied microstructural evolution characteristics of Inconel 625 alloy from SLM to HT.The result revealed that after annealing at high temperatures,random grain growth accompanied by dislocation annihilation and twinning occurs.Ozgiir et al. [ 26] discovered that aging HT led to the formation of some carbides and intermetallic phases in the microstructure.Therefore,the microstructure has an important effect on the mechanical properties of additional manufactured superalloy,especially the carbides and intermetallic phases formed during the manufacture process and HT process.

On the whole,most of the published works on SLM superalloy (i.e.,Inconel 625) were performed at room temperature [ 18, 19, 20, 21] .Only few works were carried out at elevated temperature [ 17, 22] with maximum temperature of 760℃.Because Inconel 625 is typically a promising candidate material in chambers and low-pressure turbine blades for gas turbines,aero engines and rocket engines [ 24, 27, 28] ,their service temperature typically designed for 850℃with some hot spots that can be up to 1000℃in combustion chambers.Since the SLM Inconel 625 is manufactured by line-by-line process,the microstructure and high-temperature mechanical property should be carefully addressed for its application.Thus,service temperatures up to 1000℃were considered in the present work.The microstructure,mechanical properties and failure mechanism were discussed.

2 Experimental

2.1 Powder

In the present work,gas-atomized spherical powder material of Inconel 625 with an average particle size of about 12μm (2-24μm) was used for the SLM process.The detailed size distribution of the Inconel 625 powder is shown in Fig.1.Using the scanning electron microscopy(SEM)(Nova Nano SEM450),the microstructure of the powder is obtained as shown in Fig.2.Some dendrites are shown in Fig.2b.The chemical composition of the Inconel625 powder is shown in Table 1.

2.2 Specimen preparation

The building direction is parallel to the axial of the specimen as shown in Fig.3a.The specimen is a plate and the thickness of the gage length is the same as that of the plate.The semi-finished specimens were fabricated by a laser powder-bed fusion system (BLT-S300) with a margin of0.5 mm in each direction based on the size shown in Fig.3b.The beam spot size is 0.1 mm.The layer thickness is 0.04 mm.After SLM process,the as-built specimens were subjected to solution HT (1100℃/1 h/argon) to relieve the residual stress and homogenize the material’s micro structure.No hot isostatic pressing was performed on the specimens.Then,the specimens were machined to final size of test specimens (Fig.3b) with a surface roughness of0.4μm and the thickness of the specimen is 3.0 mm.

Fig.1 Powder size distribution of Inconel 625 alloy

2.3 Mechanical testing procedure

The mechanical test on the specimens was performed on Instron 8801 test system.Seven temperatures including 20,540,760,815,870,950 and 1000℃were selected for the testing environment.The samples were heated up to target temperature at heating rate of 1℃·s^-1 and maintained at the test temperature for 15 min using infrared radiant heating chamber shown in Fig.4.Three K-type thermocouples were put in the furnace to control the temperature and its gradient.High-temperature extensometer was used to measure the strain of the tested specimen.

Fig.2 SEM images of powder of Inconel 625:a shape of powder and b detail of granules shape

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Table 1 Chemical compositions of Inconel 625 powder

Fig.3 a Specimen building direction (vertical direction) and b tensile samples (mm)

Fig.4 Furnace,specimen,extensometer,grips and thermocouples on Instron 8801 test system

2.4 Microstructure

In order to relieve the residual stress and homogenize the material microstructure,solution heat treatment processing was applied after the specimens were produced by SLM.Subsequently,so as to clarify the failure mechanism of SLM Inconel 625,the fracture surfaces of the tested specimens were systematically studied by the optical microscope (OM,Axio Scope Al),scanning electron microscopy (SEM,Nova Nano SEM450) and energy dispersive spectroscopy (EDS,INCA 250 X-Max 50).As shown in Fig.5,the primary grains are mainly equiaxed and twins are uniformly distributed.However,the grains were not homogenous,which may result in inconsistent deformation among grains.

3 Results and discussion

3.1 Mechanical properties

The engineering stress and strain curves of the SLMInconel 625 in vertical direction tested at 20,540,760,815,870,950 and 1000℃are shown in Fig.6.The mechanical properties of the SLM Inconel 625 show strong temperature dependency.

Fig.5 OM image of specimen before tensile test

As shown in Fig.6,the material exhibits significant hardening behavior at room temperature,but les s hardening at higher temperatures.The plastic deformation behavior is the same as that for traditional material.The temperature dependence of ultimate strength and yield strength of the tested specimen are shown in Fig.7.Both the ultimate and yield strengths decrease as temperature increases.The ultimate strengths of SLM Inconel 625 compared to traditional forged one reveals a slight decrease within 10%,as well as the yield strengths,decreased within 15%.Thus,SLM Inconel 625 has excellent strength compared to that produced by forged process.

An interesting phenomenon is found.The ductility of the SLM Inconel 625 decreases with temperature increasing in Fig.8.When the temperature is below540℃,the elongation of the SLM Inconel 625 is slightly higher than that of forged Inconel 625.It also shows that the elongations of conventional and SLM Inconel 625remain steady between room temperature and 540℃,which suggests that they have poor sensitivity to temperature.However,there is a slight drop in elongation of forged Inconel 625 (decrease to about 30%) from 540 to600℃.Subsequently,the elongation increases as the temperature further increases.The trend of elongation of SLM Inconel 625 is different from that of forged Inconel625 when the temperature is above 540℃.The elongation of SLM Inconel 625 is constantly decreasing when the temperature continually increases from 540 to 1000℃,indicating a poor ductility.As a whole,Fig.8 shows the comparison of temperature-dependent elongation for forged Inconel 625 and SLM Inconel 625.The results reveal a similar characteristic from room temperature to540℃but a significant difference in behavior above540℃.For most of the metals produced by traditional process,the ductility increases at high temperature.The lowest ductility of SLM Inconel 625 is only 4.35%which is observed at 1000℃.Therefore,some new mechanismsmay control the high-temperature deformation for SLM Inconel 625 in the present research.

Fig.6 Engineering stress and strain curves at various temperatures for SLM Inconel 625:a global view and b local view

Fig.7 a Ultimate stress and b yield stress comparison of forging and SLM Inconel 625

Fig.8 Elongation comparison of forging [29] and SLM Inconel 625at fracture

3.2 Deformation and failure mechanism

In order to study the deformation and failure mechanism ofthe SLM Inconel 625 at various temperatures,OM,SEM and EDS were used to analyze the micros truc ture in longitudinal section of the specimens after tensile tests.

Fig.9 SEM images for fracture surface at a room temperature (20℃) and b high temperature (870℃)

Figure 9 shows the fracture surface examination of the specimens tested at room temperature and 870℃.A large number of dimples are found on the fracture surface at room temperature.The distribution of the dimples is nonhomogenous,indicating non-homogeneity in the microstructure of the alloy processed by SLM.Though it is detrimental to the deformation coordination,the material still exhibits excellent plastic deformation at low temperature,and it is consistent with the elongation measurement from the tensile test.However,dimple does not exist on the fracture surface for specimens deformed at high temperatures.The cracks grow along the grain boundary which could be treated as intergranular cracking.Cleavage features of the fracture surface indicate that typical brittle fracture occurs at high temperature.Additionally,Fig.10shows microstructure of the surface near the fracture section for the specimens tested at 20,540,760,815,870 and1000℃.Obviously,the grains were deformed in loading direction at 20 and 540℃.However,the grains in Fig.l0c-f is almost the same with those in Fig.5 before tensile test,indicating that small plasticity occurs among the grains.Plenty of intergranular cracks are observed for specimens tested at 760,815,870 and 1000℃.

Owing to the non-homogenous grain distributions,inconsistent deformations occur among the grains,resulting in grain boundary cracking at high temperatures.Therefore,non-homogenous grain causes intergranular cracking,resulting in a poor ductility of the superalloy.In brief,based on microscopic observations,failure of the SLM Inconel 625 at low temperature is ductile,whereas brittle failure occurs at high temperatures.A valuable information that low ductility for SLM Inconel 625 is mainly dependent on the intergranular cracking at high temperatures is established.It may be a key factor for low plastic tolerance for SLM Inconel 625 superalloy at high temperatures.

SEM and EDS analysis were conducted on fractured specimens after tensile testing,as shown in Figs.11 and12.Figure 11 shows the microcracks along the grain boundary at high temperatures,especially among the triple intersections.Because the orientations among the adjacent grain are different,the elastic and plastic properties are not the same.In order to satisfy the compatibility equation of deformation before crack initiation,large stresses were developed in these locations,resulting in microcracks with1-2μm in length indicated by the arrows in Fig.11.After the microcracks initiate along the grain boundary,the adjacent cracks link together to form relative larger cracks,and the bearing area of the specimens decreases until failure.

No obvious slip bands are observed at high temperature as shown in Fig.11c-e.However,slip bands are clearly shown in Fig.11b at room temperature,and it indicates that the SLM Inconel 625 has a large plastic deformation at room temperature.This phenomenon is caused by the movement of the dislocation inside the crystal under the tensile stress.In addition,the grains are stretched in the loading direction.A few scattered micro holes are found in the grain boundary and within the grain.However,they do not participate or develop during the fracture procedure,and have tiny influence on the failure.

From Fig.12b,c,it is observed that there are precipitates at the grain boundary.In addition,there is evidence that most of the precipitates along grain boundary are often accompanied by the formation of microcracks (Fig.12b).This offers a valuable information that the precipitates along grain boundary weaken the strength of the grain boundary by promoting the formation of microcracks at elevated temperature,leading to intergranular fracture of SLM Inconel 625 during tensile testing.

Fig.10 OM images of surface near fracture section at various temperatures:a 20℃,b 540℃,c 760℃,d 815℃,e 870℃,and f 1000℃

Fig.11 a Schematic diagram of tensile specimen and SEM images of tensile specimens at b 20℃,c 760℃,d 815℃and e 870℃

Fig.12 SEM images of carbide precipitation at a 540℃,b 950℃and c 1000℃;d EDS spectra of spots in b;elemental mappings of carbides in c:e Mo,f Ni,g C,and h Nb

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Table 2 EDS results of secondary phases and matrices in Fig.12b(wt%)

To clarify the identity of the precipitates,the matrix and the precipitate along grain boundary were selected for the element analysis using EDS (Fig.12b,c).From EDS analysis,it is evident that the precipitates are richer in Mo and Nb compared to matrix.Moreover,C,Mo and Nb are accumulated across the precipitates at grain boundary,as shown in Fig.12d-h and Table 2.This indicates that the precipitates are carbides.However,carbides are not formed at the temperature of 540℃(Fig.12a).Kreitcberg et al. [ 22] think that the carbide particles which form and grow at the grain boundaries during tensile testing could weaken grain boundary,resulting in severe embrittlement at elevated temperature.Also,they found the segregation of Mo and Nb in the area of carbides (M₆C) aggregation.Since Mo and Nb are uniformly distributed in Fig.12b,c,M₆C carbides [ 24] are formed.Therefore,the carbides accumulated along the grain boundary could weaken the strength of the grain boundary.Moreover,these carbides would intensely reduce the ductility of the alloy at elevated temperatures.Since the carbides are commonly found within the grain boundary at high temperature,they are absent at room temperature tests.It could be asserted that these carbides accelerate the formation of microholes or cracks at high temperatures.Additionally,formation of carbide precipitates at the grain boundary prevents the grain boundary from sliding,leading to the poor ductility displayed at high temperature.

4 Conclusion

Temperature effect on mechanical behavior of Inconel 625alloy manufactured by SLM was experimentally studied from room temperature up to 1000℃.Abnormal embrittlement was firstly found at high temperature and explained by microstructure examinations.

The ultimate stress and yield stress of SLM Inconel 625showed a slight decrease compared to those of forged Inconel 625.However,the elongation of the SLM Inconel625 decreased with temperature increasing,which was much different from that of forged Inconel 625 as well as most of the metal materials.Intergranular cracking decreased the ductility of SLM Inconel 625 at high temperatures.The non-homogenous distribution of grains and the carbides formed at the grain boundary is the critical reason for the intergranular cracking mode at elevated temperature.The heat treatment method may be an effective way for the future research to improve the ductility of the SLM materials like Inconel 625 investigated in the present work.

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