简介概要

Improved mechanical properties of Ni-rich Ni₃Al coatings produced by EB-PVD for repairing single crystal blades

来源期刊：Rare Metals2017年第7期

论文作者：Jing-Yong Sun Yan-Ling Pei Shu-Suo Li Hu Zhang Sheng-Kai Gong

文章页码：556 - 561

摘要：Active control of turbine blade tip clearance for aircraft engine continues to be a concern in engine operation,because turbine blades are subjected to wear and therefore cause an increasing tip clearance between the rotating blades and the shroud and also reduce the engine efficiency.In this work,a Ni-rich Ni₃Al coating with γ’/γtwo-phase microstructure was deposited by electron beam physical vapor deposition（EB-PVD）,which worked as repairing the worn blade tips of single crystal blades.Nb molten pool was used to increase the molten pool temperature and thus to enhance the deposition rate.The microstructures and mechanical properties can be modified by the deposition temperatures and the following heat treatments.All coatings consist of γ’ and γ phases.At deposition temperature of 600 ℃,a dense microstructure can be achieved to produce a coating with grain size of ¹ urn and microhardness of ^HV 477.After being heated for 4 h at a temperature of 1,100 ℃,the coatings have a more uniform microstructure,and microhardness maintains at a high level of HV 292.Effect of Hf and Zr on EB-PVD Ni₃Al repair coating will be further investigated.

稀有金属(英文版) 2017,36(07),556-561

Improved mechanical properties of Ni-rich Ni₃Al coatings produced by EB-PVD for repairing single crystal blades

Jing-Yong Sun Yan-Ling Pei Shu-Suo Li Hu Zhang Sheng-Kai Gong

School of Materials Science and Engineering,Beihang University

收稿日期：25 April 2014

基金：supported by the Postdoctoral Science Foundation of China(No.2013M540037);

Improved mechanical properties of Ni-rich Ni₃Al coatings produced by EB-PVD for repairing single crystal blades

Jing-Yong Sun Yan-Ling Pei Shu-Suo Li Hu Zhang Sheng-Kai Gong

School of Materials Science and Engineering,Beihang University

Abstract：

Active control of turbine blade tip clearance for aircraft engine continues to be a concern in engine operation,because turbine blades are subjected to wear and therefore cause an increasing tip clearance between the rotating blades and the shroud and also reduce the engine efficiency.In this work,a Ni-rich Ni₃Al coating with γ'/γtwo-phase microstructure was deposited by electron beam physical vapor deposition(EB-PVD),which worked as repairing the worn blade tips of single crystal blades.Nb molten pool was used to increase the molten pool temperature and thus to enhance the deposition rate.The microstructures and mechanical properties can be modified by the deposition temperatures and the following heat treatments.All coatings consist of γ' and γ phases.At deposition temperature of 600 ℃,a dense microstructure can be achieved to produce a coating with grain size of ¹ urn and microhardness of ^HV 477.After being heated for 4 h at a temperature of 1,100 ℃,the coatings have a more uniform microstructure,and microhardness maintains at a high level of ~ HV 292.Effect of Hf and Zr on EB-PVD Ni₃Al repair coating will be further investigated.

Keyword：

Repair coating; Ni₃Al; Electron beam physical vapor deposition; Blades tips; Single crystal; Heat treatment;

Author： Jing-Yong Sun e-mail:sunjingyong@mse.buaa.edu.cn; Sheng-Kai Gong e-mail:gongsk@buaa.edu.cn;

Received： 25 April 2014

1 Introduction

Rotating turbine blades are subjected to wear damage by turbine shroud during operation,resulting in an increase of tip clearance and a low efficiency [ 1, 2, 3, 4, 5] .Current studies indicate that a tighter tip clearance would reduce specific fuel consumption linearly,and at the same time lower the exhaust gas temperature [ 6, 7, 8, 9] .In benefits of additional service life and higher operating efficiency,nickel-based single crystals,which contain a large amount of refractory and precious elements,are widely used in the high-temperature turbine blades [ 10, 11] .Therefore,an effective method for repairing worn blade tips of single crystal blades,rather than replacing the whole costly blades,is desired.This could promote engine efficiency,reduce the maintenance costs and extend the service time.

Gas tungsten arc welding (GTAW),plasma transferred arc welding (PTAW) and laser beam welding (LBW) are the three main methods for the repair of blade tips [ 12, 13, 14, 15, 16, 17, 18, 19] .Compared with the high heat input of GTAW and PTAW,LBW minimizes heat affected zones,which is more suitable for the repair of single crystal blades.However,LBW depends critically on a complicated process control for the point-of-case blade tips repair [ 17, 18, 19] .Excellent thermal shock and interfacial bond strength made the electron beam physical vapor deposition (EB-PVD) be successfully used in the metal coatings and ceramic thermal barrier coatings on blades and vanes.During the deposition of EB-PVD coatings,the nickel-base single crystal which was heated below its melting point can effectively avoid thermal cracking.In addition,the EB-PVD process offers many other desirable characteristics such as high deposition rates(up to 150μm·min^-1),controlled composition,tailored micros true ture,and flexible deposition parameters,all of which make it a potential and general method to repair single crystal blade tip [ 20, 21, 22] .

Fig.1 Versatile schematics of modified EB-PVD system for repairing tips of single crystal blades.1 worn blade,2 repair fixture,3electron gun for evaporating,4 electron gun for heating the substrate,5 partition,6 evaporated target,7 water-cooled copper crucible,8 gun chamber,9 vertical axis,10 working chamber

In the EB-PVD repair coating process,which is different from conventional metal and ceramic thermal barrier coatings,the preheating energy is localized at the tip of the worn blade to reduce the heat affected zone,with repair coating thickness reaching a millimeter-level.

In this paper,we focus on the feasibility of repair coating prepared by EB-PVD and the influence of deposition temperature and heat treatment temperature on the micro structure and mechanical properties of the repair coating.Ni₃Al alloy with ordered Ll₂ structure was selected as the repair material because of its excellent oxidation resistance,wear resistance and superior strength at elevated temperatures,and its well-matched thermal expansion coefficient with the single crystal superalloy substrate [ 23, 24, 25] .Ni-rich Ni₃Al repair coatings were produced by EB-PVD with different microstructures obtained by modifying the deposition temperature and heat treatment temperatures.The microstructures and microhardness of the coatings were also investigated.

2 Experimental

The repair coating was prepared in a modified EB-PVD system (Fig.1).In the repair process,the worn blades held in the fixture could prevent the deposition of repair material on the sites rather than the blade tips.The electron beam used for heating the blades which focused on the blade tips can reduce the heat affect zone of blades.During the repair process,deposition on the surface of the blade tips was directly heated to the desired temperature by heat guns.

In this paper,rectangle-shaped steel substrate cut from a mirror-like stainless steel sheet was proposed for the repair coating rather than using an expensive blade.The external dimension of the substrate (length×width) was 125 mm×60 mm.Prior to coating deposition,the substrates were ultrasonic ally cleaned in the acetone and thoroughly dried.The target material of Ni-12.73A1 (wt%) was produced by vacuum induction melting (diameter of 68 mm and thickness of 150 mm).Calcium fluoride (CaF₂) was pressed into a cylindrical shape,which was 20 mm in diameter and10 mm in thickness.CaF₂ was first deposited on the substrates as an isolation layer,followed by depositing the Ni₃A1 coating onto the CaF₂ layer.Nb molten pool was used to increase the molten pool temperature and thus to enhance the deposition rate of Ni₃Al coating [ 26] .The evaporation process was carried out in EB-PVD working chamber under vacuum ranging from～2×10^-3 to～2×10^-2 Pa.Under different substrate temperatures of 400,600 and800℃,different coating micros truc tures were obtained.After deposition,the coating layer was separated from the substrates to study its properties.For further investigation,the coatings prepared at 600℃were heat treated at 900,1,000 and 1,100℃for 4 h in order to achieve different microstructures with a vacuum pressure of～3×10^-3 Pa.

The surface morphologies were surveyed with atomic force microscopy (AFM,Icon),and the concentration of elements on the surface was analyzed by JXA-8100 electron probe microanalyzer (EPMA).Microstructural analysis was performed in scanning electron microscope (SEM,QUANTA200F),X-ray diffraction (XRD,D/max2200pc,Cu Kα) and transmission electron microscopy (TEM,JEM-2100).The coatings were etched with 4 g CuSO₄,20 ml HCl and 20 ml H₂O.

3 Results and discussion

3.1 Effect of deposition temperatures on composition and microstructure

The Ni-rich Ni₃Al coatings (～300μm) were produced at400,600,and 800℃,labeled as Coatings A,B and C,respectively.All the coatings have fairly smooth outer surfaces and display a shiny metallic luster.

Figure 2 shows the XRD patterns of Coatings A,B and C.All as-deposited coatings consist ofγ'-Ni₃Al phase and y-Ni phase,which can be determined from the high angle diffraction of the XRD patterns.Compared with the standard diffraction peaks ofγ'-Ni₃Al,the diffraction peaks shift slightly to higher angles,as shown in the inset of Fig.2.The primary reason may be that the lattice spacing is reduced because of theγphase with the shorter lattice spacing.The broadening of diffraction peaks could be attributed to the fine crystallite size [ 27] .

Fig.2 XRD patterns of Coatings A,B,and C

Figure 3 shows the AFM surface morphologies of the coatings.The particles on the surface are small in dimension.As deposition temperature rises,the particle size of the surface increases.The average size of the particles of Coatings A,B and C are～0.8,～1.5,and～3.0μm,respectively.The surface roughness of Coatings A and B is similar (～200 nm),but drops sharply to 85 nm when the deposition temperature increases to 800℃.EPMA backscattered electron (BSE) images of the surface of Coatings A,B and C are shown in Fig.4.The grain size also becomes larger with the increase of deposition temperature,which is consistent with the AFM results.

The aluminum contents of Coatings A,B and C are very close to～10 wt%(Table 1).No Nb is found because of its very low vapor pressure during the deposition.Composition deviation is found between target material and the coatings,and it is observed in the coatings produced by EB-PVD [ 28] .But there are few changes in chemical composition detected among the repair Coatings A,B and C at different deposition temperatures.

From the results of the coatings prepared at different temperatures (400,600,and 800℃),it shows that the deposition temperature has little effect on phase composition and chemical composition of the coatings.However,the surface morphology and grain size are significantly affected by the deposition temperature.The increase of surface temperature of substrates increases the level of thermal activation of adsorbed atoms in transition and interaction with other adsorbed atoms [ 29] .

As temperature increases,they become so activated that they can jump easily to adjacent regions.As a result,it can reduce the defects and increase the density of the deposited alloys.However,when the deposition temperature is above600℃,the deposition surface becomes uneven during the deposition process,which would cause a pattern of banding.Therefore,the deposition temperature of 600℃is chosen to obtain the repair coatings.

3.2 Microstructural evolution during heat treatment

The coatings,deposited at 600℃,were continued to be heat treated at 900,1,000,and 1,100℃for 4 h.After the heat treatment,the results of the surface morphologies and cross-sectional morphologies of the coatings are shown in Figs.5 and 6,respectively.

Fig.3 AFM surface images of coatings prepared at different deposition temperatures:a Coating A,b Coating B,c Coating C;d R_a value and depth value

Fig.4 EPMA surface images of Coatings a A,b B,and c C

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Table 1 Aluminum contents of coatings and target

There are few changes in the surface morphologies of coating as-deposited and heat treated at 900℃(Fig.5a,b),and the cros s-sectional structures of the two coatings are both characterized by the columnar structure (growth direction shown along the arrows in Fig.6).As heat treatment temperature increases to more than 1,000℃,the grain size of the coatings grows (Fig.5c,d) and the columnar structure changes to an equiaxed structure (Fig.6c,d).The average grain size of the coatings is～1.5.～2.0,and～3.0μm at900,1,000,and 1,100℃,respectively.It is worth noting that the grain size of the coatings in Fig.6d is fairly uniform,which is different from that of the other coatings.

Fig.5 SEM surface images of coatings at different heat treatment temperatures:a as-deposited,b 900℃,c 1,000℃,and d 1,100℃

After heat treatment,the results of the XRD patterns of the coatings can be found in Fig.7.All the heat-treated coatings consist ofγ'andγphases.The morphology and distribution of theγ'andγare identified by TEM as shown in Fig.8.Theγ'phase presents uneven size and irregular shape.The three dark field images,which are performed using primaryγ'phase superlattice reflection (Fig.8d,f,h),are used to determine the heat-treated microstructures of the coatings.

Fig.6 SEM cross-sectional images of coatings at different heat treatment temperatures:a as-deposited,b 900℃,c 1,000℃,and d 1,100℃

Fig.7 XRD patterns of coatings at different heat treatment temperatures

Among the bright field images (Fig.8c,e,g),γphase occurs on the one side of same grains during subsequent annealing at 900,1,000,and 1,100℃for 4 h.Numerous fine secondaryγ'particles precipitating fromγphase show the same orientation relationship with primaryγ'phase in the same grain.Further observation discovers the presence of twinned Ni₃Al in the as-deposited coatings,as shown in Fig.8a.Dislocations are not found in any of the coatings.

3.3 Influence of heat treatment on mechanical properties

After different heat treatments,the data of the microhardness of the coatings are shown in Table 2.The microhardness of the as-deposited coating is～HV 477,which is higher than that of Ni₃Al alloys produced by combustion synthesized (CS) or self-propagation high-temperature synthesis and hot extrusion (SHS/HE) [ 30, 31] .The microhardness decreases with the increase of heat treatment temperature.However,they still maintain a high level of HV 292.The fine-grain strengthening contributes to the high microhardness of coatings as the increasing grain boundaries handicap the movement of dislocations and suppress the deformation,which could enhance the microhardness of the coatings [ 32, 33, 34] .

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Table 2 Mechanical properties of Ni₃Al coatings or alloys at RT

For non-work-hardening materials,the relationship between yield stress and hardness is given by Meyers [ 35] .The calculated results show that the coatings have superior yield strength.

whereσ_y is the yield strength and H is the Vickers microhardness.The calculated results show that the coatings could have superior yield strength in Table 2.

4 Conclusion

Ni-rich Ni₃Al repair coatings were prepared at 600℃by EB-PVD,and their thicknesses are～300μm.There is no significant effect on the phases and chemical compositions of the coatings under different deposition temperatures,but an obvious effect on grain size.The coatings based onγ'andγphases have dense microstructure.No Nb is found in the Ni₃Al coating.The grain size increases from～1.5 to～3.0μm as the heat treatment temperature increases.All the heat-treated coatings areγ'/γtwo-phase microstructure.The columnar structure changes to an equiaxed structure after heat treatment over 1,000℃for 4 h.The Ni₃Al coatings exhibit very high microhardness due to the finegrain strengthening.The microhardness of the as-deposited coating (at 600℃) is about HV 477.The microhardness decreases with the increase of heat treatment temperature but maintains a high value of～HV 292 at 1,100℃for4 h.

Fig.8 TEM images of coatings:a bright field image of as-deposited,b SAED pattern[100]of as-deposited;bright field images treated at c 900℃,e 1,000℃,and g 1,100℃;dark field image using superlattice reflection at d 900℃,f 1,000℃,and h 1,100℃

The studies show that the high strength Ni-rich Ni₃Al coating by EB-PVD is proved to be feasible.This novel process can provide a promising repair approach for the worn blade tips of the nickel-base single crystal blades.

Acknowledgments This work was financially supported by the Postdoctoral Science Foundation of China (No.2013M540037).