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Rare Metals2018年第7期

Interface microstructure and bond strength of 1420/7B04 composite sheets prepared by diffusion bonding

Fan Wu Wen-Long Zhou Bing Zhao Hong-Liang Hou

School of Materials Science and Engineering, Dalian University of Technology

Beijing Aeronautical Manufacturing Technology Research Institute

Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province),School of Materials Science and Engineering,Dalian University of Technology

作者简介：*Fan Wu e-mail: wufandut@163.com;

收稿日期：4 January 2018

基金：financially supported by the Major State Basic Research Development Program of China(No.2011CB012803);the National Natural Science Foundation of China (No. 51334006);

Interface microstructure and bond strength of 1420/7B04 composite sheets prepared by diffusion bonding

Fan Wu Wen-Long Zhou Bing Zhao Hong-Liang Hou

School of Materials Science and Engineering, Dalian University of Technology

Beijing Aeronautical Manufacturing Technology Research Institute

Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province),School of Materials Science and Engineering,Dalian University of Technology

Abstract：

The effect of temperature on interface microstructure and shear strength of 1420 Al-Li alloy and7 B04 A1 alloy composite plates prepared by diffusion bonding were investigated. The results indicate the optimum temperature for bonding the composite plates is520 ℃, a sound bonding interface without continuous intermetallic compound layers and interfacial voids is obtained, and the shear strength value of bond joints can be as high as 190 MPa. An interfacial transition zone is formed due to the alloying elements mutual diffusion during the bonding process. Meanwhile, the effect of temperature on diffusion of alloying elements and interface reaction were discussed in detail, the results show that the higher temperature can increase the diffusion of alloying elements fluxes across the bonding interface, which can accelerate the closure of interfacial voids; meanwhile,when Mg atoms diffuse across the bonding interface, it can react with and break up the surface oxide films into discrete particles, and the removal of interface oxides increases the metal to metal bond areas and improves the bond quality.

Keyword：

Diffusion bonding; Alloying elements diffusion; Interface microstructure; Shear strength;

Received： 4 January 2018

1 Introduction

Diffusion bonding (DB) is a solid-bonding process in which two prepared materials are joined at elevated temperature ranging from 0.5 T_m to 0.8 T_m (where T_m is the absolute melting temperature) under pressure [ 1, 2, 3] .Recently,there is considerable interest in extending the diffusion bonding with superplastic forming (DB/SPF)technology to aluminum alloys [ 4, 5, 6] .Although several high-strength aluminum alloys have been designed and specifically processed to have excellent superplastic property [ 7, 8, 9, 10, 11] ,diffusion bonding of aluminum alloys is difficult,because the surface oxide film inhibits metal to metal contact and hinders elements diffusion during bonding [ 12, 13] .According to statements in previous study [ 14, 15] ,there is a general agreement that all metals can bond together if two cleaned surfaces are brought within the range of interatomic force.Hence,to obtain a sound bond interface for aluminum alloys,it is necessary to remove the surface oxide or at least partially to disrupt its continuity [ 16] .

In order to break the oxide film and achieve metallic bonds in aluminum alloy substrate,many methods have been attempted to obtain bond joints with significant strengths,including large deformation,high temperature and protective atmosphere.Thin interlayer is generally produced by ion plating [ 17] ,vapor deposition [ 18] ,or plasma spraying,and good bonds were produced with the interlayer.From the practical point of view,the methods mentioned above are not applicable due to their complicated processes or high costs.When diffusion bonding of Al/Mg2Si [ 19] ,the effect of cooper as interlayer was investigated,and a sound bond joint with various diffusion layer along the bonding interface was obtained,while the brittle diffusion layer causes brittle fracture.

The liquid gallium can be used as a flux to break the continuous oxide film,and this has been successful in achieving aluminum to aluminum bond [ 20] .However,the quantity of gallium must be controlled precisely because the liquid gallium penetrates along the grain boundary and affects the mechanical properties of the base materials [ 21] .The liquid metal brittleness (LMB) of Al alloys was caused by the liquid gallium penetration along the grain boundary.Active alloying elements Mg and Li were used to break the oxide film when diffusion bonding aluminum-based alloys [ 22] .They can chemically react with and break up continuous oxide films to form an array of discrete particles which may increase metal to metal interface and improve the bonding quality [ 14, 23] .

To the best of our knowledge,diffusion bonding between 1420 Al-Li alloy and 7B04 A1 alloy has rarely been studied yet [ 24, 25] .In this study,a composite plate was prepared by diffusion bonding between 1420 Al-Li alloy and 7B04 Al alloy under different bonding temperatures;the concentration gradient of alloying elements was established across the bonding interface.The effect of bonding temperature on the bonding quality was investigated in detail by the interface microstructural observation and bonding strength evaluation.The effects of diffusion of alloying elements and interface reaction during bonding on the interface bond formation were investigated in detail.

2 Experimental

2.1 Materials

The materials used in the present work were 1420 Al-Li alloy and 7B04 A1 alloy and supplied in the form of plates by Central South University.The chemical composition and mechanical properties of the alloys used in this study are shown in Tables 1 and 2,respectively.The chemical composition was determined by the inductively coupled plasma (ICP-Thermo iCAP 7400) after the surface treatment before bonding.The mechanical properties of the alloys were conducted on WDW-50E universal testing machine.For each material,5 tensile samples were tested and the average values were taken.The deviation was kept in the range of±10 MPa.The samples prepared for bonding were processed by wire-electrode cutting to a dimension of 15.0 mm×10.0 mm×2.5 mm.Prior to diffusion bonding,the contacting surfaces of samples were polished by 1200 mesh SiC paper and degreased with acetone;then the samples were ultrasonically cleaned in deionized water.

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Table 1 Chemical compositions of aluminum alloys used in present study (wt%)

2.2 Bonding procedure

The bonding experiment was conducted on Gleeble-3500thermal simulation testing machine.The temperature was measured by a K-thermocouple welded on the 7B04 Al alloy specimen.Then the diffusion bonding couple was heated to bonding temperature with a heating rate of60℃·min^-1 and held at the objective temperature for1 min to minimize temperature gradient and then the bonding pressure was loaded.After bonding,pressure was released and the bonded sample was cooled to room temperature in the work chamber.The parameters of diffusion bonding are listed in Table 3.Parameters such as bonding temperature,pressure and time were automatically controlled during bonding.In order to protect the samples from oxidizing,the work chamber was pumped for vacuum to1×10^-3 Pa;then argon gas was introduced to maintain protective atmosphere.

2.3 Evaluation tests and characterization methods

After bonding,samples were processed into the shear specimens,for the diffusion bonding samples were not large enough for standard shear test,according to the former study [ 13] ,and a non-standard shear specimen was devised to measure the shear strength of bonding joints.The size of specimens for interface bonding test is 15 mm×10 mm,and the schematic illustration is shown in Fig.1.The shear test was carried out on computer controlled electronic universal testing machine (WDW-50E) with a head speed of0.3 mm·min^-1.Shear strength was calculated after the shear test,for each bonding condition,three bonded joints were tested,and the average value was taken.The calculation formula of shear strength was shown as follow:

whereτ_DB is the shear strength of the bonded joints,F_max is the maximum tensile force,ωl₀ is the area of shear zone,and in this study,ωis 1 mm and 1₀ is 10 mm,as shown in Fig.1.The sample was prepared using standard metallographic techniques,and the Keller's reagent was used for etching.The microstructure analysis was conducted by scanning electron microscope (SEM,ZeissSupra55) equipped with energy-dispersive spectroscope (EDS).The chemical composition distributions were examined across the interface by electron probe micro-analysis (EPMA,ShimadzuEPMA1720).Thin foils with a thickness of 0.5 mm for transmission electron microscope (TEM,JEM2100) measurement were machined from the bonding interface.These foils were thinned to 40μm before they were cut into wafers of 3 mm in diameter.They were then twin jet electro-polished in 10 ml HClO₄+90 ml C₂H₅OH solution and then examined by TEM operated at 300 kV.

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Table 2 Mechanical properties of aluminum alloys used in present study

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Table 3 Diffusion bonding parameters for preparing 1420/7B04composite sheets in present study

Fig.1 Schematic illustration of shear strength testing pieces (unit mm)

3 Results and discussion

3.1 Interface microstructure of bonded joints

The interface micros truc ture between 1420 Al-Li alloy and7B04 Al alloy after bonding was studied to evaluate the bonding quality.Special attention was focused on the micro structural evolution of the bond areas and the interfacial voids under different temperatures.SEM images of interface microstructure bonded at different temperatures are shown in Fig.2;the interface is positioned horizontally in the center of each image.As shown in Fig.2a,only few metallic bond areas are obtained when bonded at 430℃,the bond areas are separated by long and continuous interfacial voids,indicating poor bond quality under this bonding temperature.To further increase the bonding temperature to 460℃,the bond areas increase and the edge of interfacial voids tends to be smooth;there are still several large voids existing along the interface (the arrows pointed in Fig.2b).The bond areas increase obviously when the bonding temperature increases to 490℃,the long interfacial voids disappear,and there are occasional isolated ellipse voids along the interface,as shown in Fig.2c.Figure 2d shows the interface bonded at 520℃,the interfacial voids can be seldom found and the bond interface is difficult to distinguish with the adjacent grain boundary (Fig.2d),indicating a sound bond joints obtained.The results show that the bond temperature has immediately influence on the increase in bond areas.Meanwhile,there is obvious interfacial transition zone(ITZ) formed when bonding at 490 and 520℃,as shown in Fig.2c,d,which is ascribed to the diffusion of alloying elements during bonding in the ITZ on 7B04 side,as shown in Fig.2c,d with rectangle.And there are some smaller grains formed,as shown in Fig.2e with ellipse,compared with the grains of the substrate.

According to the statement of previous studies [ 26, 27] ,the void shrinkage during diffusion bonding process is an important process for achieving high-quality joints which is generally ascribed to several physical mechanisms:(1) the plastic flow of materials around voids including plastic deformation and creep deformation;(2) the atomic diffusion including surface diffusion,interface diffusion and volume diffusion.The increase in bonding temperature will reduce the flow stress of materials,enhance the plastic flow of materials,and thereby force more mass from adjacent regions into voids.Simultaneously,void shrinkage is further accelerated by increasing atomic diffusion,while the atomic diffusion is strongly depended on the bonding temperature according to Arrhenius type equation:

Fig.2 SEM images for interface microstructure bonded at different temperatures:a 430℃,b 460℃,c 490℃,and d,e 520℃

where D is the diffusion coefficient;D_o is the proportionality constant (m²·s^-1),independent of temperature for Eq.(2) is valid;Q is the activation energy (J·mol^-1);R is the molar gas constant (8.314 J·mol^-1·K^-1);T is the temperature (K).According to Eq.(2),the diffusion coefficient of alloying atoms can be accelerated by increasing bonding temperature.

There is clear evidence of local migration of interface boundaries leading to the formation of triple junctions between grains across the interface,as shown in Fig.2e.It is obvious that the formation of metallic bond between interface grains is highly dependent on the bonding temperature,and the results show that the optimum condition for diffusion bonding in the present work is 520℃with applied pressure of 6 MPa for 60 min.Meanwhile,according to the micros truc tural observation of the bonded interface,no evidence of detrimental intermetallic compounds is found throughout the interface,indicating excellent metallurgical bonding between 1420 Al-Li alloy and 7B04 Al alloy.

3.2 Diffusion of alloying elements

In the present study,the diffusion of alloying elements is actuated by the concentration gradient of alloying elements across the bonding interface.An obvious ITZ (Fig.2c,d) is formed due to the diffusion of alloying elements.To further investigate the influence of bonding temperature on diffusion of alloying elements,the content distribution of alloying elements after bonding was investigated in detail.As the lithium atom is too light to be detected by EDS and EPMA,in this study,the main alloying elements Mg and Zn were chosen to investigate the diffusion of alloying elements.The content distribution of Mg and Zn after bonding at different temperatures is shown in Fig.3.The thickness of ITZ is defined as the distance across the interface over which Mg content decreases from the stable value of approximately 5.2 wt%in 1420 substrate to the stable value of 1.8 wt%in 7B04 substrate.The results show that there are two platforms on both sides of the interface corresponding to the 1420 substrate and 7B04substrate;meanwhile,there is an interfacial transition zone(ITZ) where the concentration changes continuously.The difference of ITZ thickness can be ascribed to the different diffusion flux of alloying elements under different bonding temperatures.When the bonding temperature increases from 430 to 520℃,the thicknesses of ITZ are 110,130,170 and 230μm,respectively,as shown in Fig.3.The results show the diffusion of alloying elements flux across the interface promoted by the bonding temperature.The diffusion flux was determined by the concentration gradient of alloying elements across the bonding interface and temperature according to Eq.(2) and the following Eq.(3),and the relationship between diffusion coefficient and diffusion distance is shown in Eqs.(3) and (4):

where J is diffusion flux;D is diffusion coefficient(m²·s^-1);dC/dx is concentration gradient;δis the thickness of ITZ;A is a constant;t is the time of diffusion bonding (s).The diffusion flux and diffusion distance of alloying atoms are directly influenced by bonding temperature.Hence,the thickness of ITZ is determined by diffusion distance of alloying elements.According to Eq.(4) and the ITZ thickness under different temperature,the relationship between diffusion coefficient of Mg and bonding temperature is shown as follows:

Fig.3 Distribution of alloying elements after diffusion bonding at different temperatures:a 430℃,b 460℃,c 490℃,and d 520℃

The results show that the diffusion coefficient of Mg increases obviously with the increase in temperature.

3.3 Interface reaction

When diffusion bonding of aluminum alloys,the principal difficulty is the existence of continuous oxide films along the bonding interface.The oxide films impede the metal to metal contact and hinder the diffusion of alloying elements during bonding.In order to obtain a sound bond interface,it is necessary to remove the surface oxide during bonding.According to previous study [ 14] ,the active alloying elements (such as Mg and Li) can react with and break up the oxide films to form an array of discrete particles which may increase metal to metal contact and improve bonding quality.Mg is more effective than Li in increasing bonding quality.The reaction between active alloying elements and alumina can happen spontaneously according to the following reactions [ 14, 28] :

As mentioned earlier,Li atom is too light to be detected;special attention is focused on the diffusion of Mg and its interface reaction.Since the concentration gradient of Mg exists,when Mg diffuses across the bonding interfaces,the interface reaction happens.To investigate the process of alloying atoms diffusion and interface reaction during bonding,the distribution of alloying elements around bonding interface after bonded at different temperatures was analyzed by EDS,as shown in Fig.4.In this image,the X-X is the bonding interface obtained at 430℃,and the Y-Y shows the interface obtained at 520℃.Figure 4a is the interface microstructure bonded at 430℃,Fig.4b,c is the distribution of alloying elements O and Mg corresponding to Fig.4a,respectively.When bonded at430℃,there are obvious interfacial voids along the bond interface,the O distributes uniformly in the substrate while some O gathers around the interfacial voids,meaning that the residual oxide films are along the bonding interface.Meanwhile,Mg diffuses from 1420 side to 7B04 side,and some Mg gathers specially on the same zone with the residual oxides.The results show that when Mg diffuses across the bonding interface (Fig.4c),they gather and react spontaneously with the interface oxide films.When bonding temperature increases to 520℃,the micros truc ture of bonding interface is shown in Fig.4d,and the corresponding distribution of alloying elements O and Mg is shown in Fig.4e,f.The results show that a sound bonding interface without interfacial voids is obtained.There is no concentrated zone of O atoms along the bonding interface except some small concentrated zones exist on the 7B04 substrate.Meanwhile,Fig.4f shows obvious concentration gradient of Mg distribution after bonding,as the arrow pointed.The results show that under the low bonding temperature,as shown in Fig.4a-c bonded at 430℃,due to the low diffusion coefficient and diffusion fluxes of alloying elements,the reaction between magnesium and oxide film is insufficient to remove the oxide film and eliminate the interfacial voids totally.When bonded at 520℃,the increased diffusion flux of alloying elements removes the oxide film and eliminates the interfacial voids effectively.A previous study [ 29] expressed similar conclusion by investigating the relation between diffusion of alloying elements and bonding quality.

Fig.4 SEM images of and corroding alloying elemental mappings of interface after bonded at a-c 430℃and d-f 520℃

The diffusion and reaction processes mentioned above can be verified by the TEM observation of the bonding interface.Figure 5 shows TEM images of bonding interface of the composite sheets obtained at 520℃with6 MPa for 60 min.The results show obvious amorphous nanoscale particles existing in the interior of 7B04 grains,and the EDS analysis shows that the particles mainly contain Al,Mg and O,meaning that the surface oxide film was modified by Mg and moved toward to the 7B04 side with Mg diffusion.There are still some residual amorphous nanoscale particles existing along the bonding interface and triple-junction grain boundary,as shown in Fig.5b.The results show that it is difficult to eliminate the oxide on the bonding interface totally.

In the present study,the removal of surface oxide films can be summarized as follows.With the diffusion of alloying elements flux across the bonding interface,the alloying element Mg can react spontaneously with the surface oxide film,hence the Mg gathers at the bonding surface.Through the reaction,the surface continuous oxide film can transform into discrete oxide particles containing Al,Mg and O.Then the complex oxides move into 7B04substrate along with Mg diffusion from 1420 side to 7B04substrate.As the surface oxide film is removed,more diffusion paths form due to the increase in metal to metal contact areas between 1420 substrate and 7B04 substrate.Hence,the bonding quality improves effectively as the diffusion flux of alloying elements increases under higher temperature.

3.4 Shear strength of bonded joints

To evaluate the bond strength of joints,shear strength test was carried out.The shear strength of the joints bonded under different temperatures is shown in Table 4.The results show the shear strength of joints bonded at 430,460,490 and 520℃are 37.5,75.2,136.5 and 190 MPa,respectively.The results show that the tensile shear strength increases with bonding temperature increasing.The shear strength value obtained at 520℃(190 MPa) is higher than that of 7B04 base materials (165 MPa).In a similar study [ 30] ,when diffusion bonding between aluminum alloy and magnesium alloy,the results show that shear strength of joints increases with bonding temperature.When solid-state diffusion bonding of 7075 aluminum alloy was conducted at various bonding parameters and the bond strength was determined by shear test,the increasing temperatures and pressures promote the bond strength [ 31] .

Fig.5 TEM images of bonding interface bonded at 520℃:a oxide particles along bonding interface and residual oxide on triple-junction grain boundary

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Table 4 Shear strength and specific shear strength of joints bonded at different temperatures

In general,the absolute strength value of bonded joints could not be used to evaluate the extent of bonding because the specimens with different thermal histories would have different shear strengths.Hence,to exclude these effects,the specific shear strength (μ) was introduced to assess the shear strength of joint [ 32] ,and is defined as follows

whereτ_DB is the shear strength of bonded joint,whileτ_B is the shear strength of the base material under the same bonding condition.In this study,shear strength of 1420 AlLi alloy-based material was tested and used asτ_B,and the shear strength value of 1420 based material is 206 MPa.The specific shear strength under various temperatures is shown in Table 4.The results of shear strength test show that the fracture for all the samples took place at the bond interface because shear force is concentrated along the bond plane.In the present work,the shear strength of bonded joint obtained at 520℃can be as high as190 MPa,andμcan reach as high as 92%.It is difficult to compare the shear strength values with those reported for other Al alloys because of differences in composition,bonding conditions and post-bonding heat treatment.The best shear strength measured in the present work is higher than the average value of 150 MPa as mentioned in previous study [ 33] when diffusion bonding of 7475 Al alloys.

4 Conclusion

In this study,diffusion bonding of dissimilar aluminum alloys under different temperatures was investigated.The influence of bonding temperature on diffusion of alloying elements and bonding quality was discussed.The following conclusions can be drawn from the diffusion bonding experiment above.

A sound bonding interface without interfacial voids between 1420 Al-Li alloy and 7B04 A1 alloy composite sheets is obtained when bonding at 520℃under applied pressure of 6 MPa for 60 min,the shear strength of the bond joints is 190 MPa,and the specific shear strength can be 92%compared with the shear strength of 1420 substrate.The diffusion of alloying elements results in the formation of interfacial transition zone ITZ,and the thickness of ITZ can be 230μm when bonding at 520℃.No evidence of continuous intermetallic compounds layers which are detrimental to the bond quality is found;the results indicate excellent metallurgical bonded joints obtained.When the magnesium atoms diffused across the bonding interface,they can react with the oxide film.Then the interfacial reaction breaks the continuous aluminum oxide film into nanoscale discrete particles.The concentration gradient of alloying elements and increasing temperature elevate the diffusion flux of alloying elements;hence the bonding quality improves significantly.