简介概要

Soldering of Zr-based bulk metallic glass and copper by Au-12Ge eutectic alloy

来源期刊：Rare Metals2019年第1期

论文作者：Jun Wang Jing Cui Hong-Chao Kou Heng Guan Jin-Shan Li

文章页码：52 - 58

摘要：Zr-based bulk metallic glass and copper with different surface roughness were soldered using low temperature eutectic Au-12 Ge（wt%） solder on a thermomechanical simulator. The cross-sectional microstructures of the brazed joints were analyzed by scanning electron microscopy（SEM） and transmission electron microscope（TEM） in detail, and the compositional distribution along the interface was analyzed by energy-dispersive spectrometer（EDS）. Results show that the surface roughness of base metals plays an important role in the quality of the brazed joint because the surface roughness can enlarge the effective contact area, which can improve the brazing surface quality between two materials. A moderate roughness of treated Zr-based metallic glass of 18 μm is shown to be the best for the soldering, while the surface roughness has a weak effect on the soldering behavior of Au-12 Ge solder on copper. After soldering, long-range diffusion of atoms occurs between the base metal and solder, and five distinct regions are formed at the joint region.

稀有金属(英文版) 2019,38(01),52-58

Soldering of Zr-based bulk metallic glass and copper by Au-12Ge eutectic alloy

Jun Wang Jing Cui Hong-Chao Kou Heng Guan Jin-Shan Li

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University

作者简介：*Jun Wang e-mail:nwpuwj@nwpu.edu.cn;

收稿日期：8 June 2015

基金：financially supported by the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2014JM6234);the Specialized Research Fund for Doctoral Program of Higher Education (No. 20136102120007);the Program of Introducing Talents of Discipline to Universities (No. B08040);

Soldering of Zr-based bulk metallic glass and copper by Au-12Ge eutectic alloy

Jun Wang Jing Cui Hong-Chao Kou Heng Guan Jin-Shan Li

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University

Abstract：

Zr-based bulk metallic glass and copper with different surface roughness were soldered using low temperature eutectic Au-12 Ge(wt%) solder on a thermomechanical simulator. The cross-sectional microstructures of the brazed joints were analyzed by scanning electron microscopy(SEM) and transmission electron microscope(TEM) in detail, and the compositional distribution along the interface was analyzed by energy-dispersive spectrometer(EDS). Results show that the surface roughness of base metals plays an important role in the quality of the brazed joint because the surface roughness can enlarge the effective contact area, which can improve the brazing surface quality between two materials. A moderate roughness of treated Zr-based metallic glass of 18 μm is shown to be the best for the soldering, while the surface roughness has a weak effect on the soldering behavior of Au-12 Ge solder on copper. After soldering, long-range diffusion of atoms occurs between the base metal and solder, and five distinct regions are formed at the joint region.

Keyword：

Bulk metallic glass; Soldering; Au-Ge eutectic alloy; Interface;

Received： 8 June 2015

1 Introduction

Bulk metallic glasses (BMGs) have attracted much attention because of their unique micros true ture,which contributes to many excellent properties compared with crystalline alloys,such as high strength and hardness,excellent corrosion resistance and superplastic in supercooled liquid region [ 1, 2, 3, 4, 5] .Although large-sized BMG with a diameter of 73 mm has been fabricated recently [ 6] ,BMGs still cannot satisfy the requirement of engineering application due to its limited ductility at room temperature.Therefore,joining and bonding with other metallic glasses or crystal materials are of great important.Many bonding methods were adopted to join BMGs,such as explosive welding [ 7, 8] ,electron beam welding [ 9, 10] ,pulse-current welding [ 11] ,friction welding [ 12, 13] ,friction stir welding [ 14, 15] and diffusion bonding method [ 16, 17, 18, 19] .Considering the above joining methods,explosive welding is dangerous,there is a large heat affected zone in electron beam welding and friction welding methods,and the strength is low when joined by diffusion bonding method.Meanwhile,metallic glass which is in a metastable state will transform to crystal one when heated above the crystallization temperature.Therefore,new bonding method should be used at low temperature to joint bulk metallic glass.Brazing method is a more promising approach because the brazing temperature is lower than the crystallization temperature of metallic glass and the solder time is short,which has a minor effect on the stability of metallic glass.

Much effort was made to investigate the wetness of traditional solder on bulk metallic glass [ 20, 21, 22] ,but there are still very limited researches on the solderability of metallic glass using precious metal solder.The Au-12Ge eutectic alloy shows an interesting combination of low melting temperature,good mechanical properties and good corrosion resistance [ 23] .Therefore,Au-Ge alloys can be proposed as possible low temperature lead free solder materials for highly loaded components such as high power microelectromechanical systems (MEMS) devices and corrosion resistance solders for components in space technology [ 24] .In this study,research on Zr-based bulk metallic glass and pure copper brazed by eutectic Au-12Ge(wt%) alloy was studied,and factors affecting the soldering process were discussed and the interface was characterized.

2 Experimental

The BMG with nominal composition of Zr_41.2Ti_13.8-Cu_12.5Ni₁₀Be_22.5 (at%) was used in this study.The ingots of the BMG were prepared by arc melting.The pure elements(purity all above 99.99%) were melted under a high purity argon atmosphere in a water-cooled copper crucible.The ingots were re-melted several times to ensure the compositional homogeneity.Cylindrical specimens with 8 mm in diameter and 25 mm in length were prepared by injection casting method (injecting the melted ingot into a watercooled copper mold under a high purity argon atmosphere).The amorphous structure of the specimen was confirmed by X-ray diffraction (XRD).Then the amorphous specimen was turned into short cylinder samples with a dimension of8 mm in diameter and 5 mm in length.The thermal stability of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 was measured by differential scanning calorimeter (DSC,Netzsch STA449C) with a heating rate of 10 K·min^-1.The glass transition temperature (T_g),the onset crystallization temperature (T_x) and the melting temperature (T_m) were determined to be 625,686 and 937 K,respectively.

The solder with a nominal composition of Au-12Ge(wt%) was in ribbon form with the thickness of 80μm,which was provided by Kunming Institute of Precious Metals.The melting point of the Au-12Ge alloy was measured to be 629 K.The diameter of copper rod was the same as that of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5.

Before experiment,Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5/copper samples were treated by SiC sandpapers with 400,800,1000 and 1500 mesh to achieve surface roughness of 38,18,13 and 9μm,respectively.After that,all the samples were ultrasonic ally cleaned in acetone for 5 min and dried by air before soldering experiment.Soldering tests were carried out on a Gleeble 3500 thermomechanical simulator in a high vacuum about 1×10^-5 Pa at 653 K for 30 min.The heating rate was 10 K·min^-1.In order to make sure the close contact on the interface between the base metals,a small press about 5 MPa was applied during the soldering process.Au-12Ge foil pieces of 9 mm×9 mm were placed between the two base alloys.The thermocouple was welded on Zr_41.2Ti_i3.8Cu_12.5Ni₁₀Be_22.5 near the joining interface.The state of metallic glass before and after soldering was confirmed by XRD (DX-270,China,Cu Kαradiation).The cross-sectional interface microstructures of the joints were observed using scanning electron microscopy (SEM,TESCAN VEGAIILMH).The compositional distribution along the interface was analyzed by energydispersive spectrometer (EDS,TESCAN VEGAII LMH)and electron probe micro-analyzer (EPMA,EPMA-1720).The detailed interface was characterized by transmission electron microscope (TEM,Tecnai G2F30).

Therefore,the pre-processed samples were placed in Gleeble 3500 thermomechanical simulator and the Au-12Ge solder was placed between Zr_41.2Ti_13.8Cu_12.5-Ni₁₀Be_22.5 and Cu rods with 5 MPa preload to stabilize the samples.Then the temperature was raised to 653 K with the heating rate of 10 K·min^-1 and held for 30 min in high vacuum.After that,the soldered samples were taken out to do the further analysis.

3 Results and discussion

3.1 Effect of surface treatment on soldering

Before experiment,the characteristic temperatures of Au-12Ge eutectic alloy and the two base alloys were measured by DSC at a heating rate of 10 K·min^-1,and the results are shown in Table 1.The melting temperature of Au-12Ge alloy is higher than the glass transition temperature and lower than the crystallization temperature of Zr_41.2Ti_13.8-Cu_12.5Ni₁₀Be_22.5.The brazing temperature is usually25-60 K higher than the melting temperature of solder [ 25] .Thus,the soldering temperature is selected as 653 K which is in the supercooled liquid region of Zr_41.2Ti_13.8-Cu_12.5Ni₁₀Be_22.5.Table 2 shows the experimental parameters and results of the soldering tests.All the samples have formed brazed joints regardless of the surface roughness of base alloys shown in Fig.1.Part of solders were squeezed out from the joining interface because of the pressure.It also can be found that the macroscopic plastic deformation only occurs at the side of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5.The main reason is that the equilibrium viscosity of metallic glass decreases sharply and exhibits superplasticity when the temperature is in the supercooled liquid region.Moreover,the bonding pressure of 5 MPa exceeds the flow stress of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 at this temperature;thus,the metallic glass can deform easily.While for copper,the bonding pressure is lower than the flow stress so the macroscopic plastic deformation cannot happen.The plastic deformation can promote the spreading of solder on the alloy surface and enhance the wettability of Au-12Ge solder on base alloys.

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Table 1 Characteristic temperature of base metals and solder

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Table 2 Parameters and results of soldering tests between Zr-based metallic glass and Cu

Fig.1 Photograph of brazed joints of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5and Cu

To avoid crystallization,the state of Zr_41.2Ti_13.8Cu_12.5-Ni₁₀Be_22.5 BMG before and after the soldering experiment was confirmed by XRD,as shown in Fig.2.The broad diffraction peak indicates that the metallic glass is still in the amorphous state after soldering for 30 min at 653 K.However,the decrease of the broad peak width in Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 after soldering indicates that structural relaxation might have occurred during the soldering test [ 20, 26] ,and the lower intensity after the soldering test is due to the decrease of the diffraction area of the sample.

Fig.2 XRD patterns of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 before and after soldering experiment

To further analyze the contact condition of the surface,the cross-sectional microstructures of Zr_41.2Ti_23.8Cu_12.5-Ni₁₀Be_22.5/Cu joints were observed by SEM,and the back scattered electron (BSE) images are shown in Fig.3.It can be seen that Cu/Au-12Ge joint always has great brazing surface quality regardless of the surface roughness,but the Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5/Au-12Ge joint does not,indicating that the wettability of Cu/Au-12Ge joint is better than that of Zr_41.2Ti_13.8Cui_12.5Ni₁₀Be_22.5/Au-12Ge joint.For the brazing surface quality of Zr_41.2Ti_13.8Cu_12.5-Ni ₁₀Be_22.5/Au-12Ge joint,surface roughness plays an important role.As shown in Fig.3,the brazing surface quality is getting better with the surface roughness increasing.As known,two materials have the same wettability in a same condition.Thus,the surface roughness plays an important role in the brazing surface quality.When two materials do not have a proper wettability,increasing the surface roughness can enlarge the effective contact area,which can improve the brazing surface quality between the two materials.Therefore,it is clear that the surface roughness of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5/Au-12Ge joint of 9μm has the worst brazing surface quality,but the surface roughness of 38μm has the best brazing surface.Because Cu/Au-12Ge joint has better wettability,the brazing surface quality does not depend on the surface roughness as much as that of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5/Au-12Ge joint does.

Table 3 shows the detailed quantitative results of EDS analysis of the spots in Fig.3d.The Au-12Ge solder still remains eutectic state at the interface near Zr_41.2Ti_13.8-Cu_12.5Ni₁₀Be_22.5 and middle of solder,while at the interface close to Cu,a reaction layer with a composition of Au-42Cu-6Ge is formed,indicating that Cu atoms have diffused from based metal to the solder.This phenomenon is corresponding to the result in Ref. [ 24] .

3.2 Influencing factors of soldering

During soldering process,the temperature and time will significantly affect the welding process.The metallic glass is very sensitive to temperature and time due to the crystallization.Thus,the soldering and brazing parameters were selected by the thermal stability of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5.Figure 4 shows the surface images of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5/Cu joints at 653 K for different brazing time.It can be seen that when the time is short (15 min in Fig.4a),the brazing time is not long enough to form thick diffusion layer,while a better layer can be formed when the brazing time increases to 30 min,as shown in Fig.3b.And a clear,well-developed reaction layer along Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 side can be formed when the brazing time is elongated to 45 min,as shown in Fig.4b.The forming of well reaction layer is due to the enough brazing time,but the long annealing time can also cause the crystallization of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5,which will make the BMG become brittle during deformation at room temperature.Based on this consideration,the best brazing time should be around 30 min,because the BMG can still maintain amorphous state after brazing,which is confirmed in Fig.2.Moreover,the brazing temperature plays the same role as brazing time.A good brazing joint can be obtained at 663 K,but it is not easy to have a good brazing surface when temperature is lower than 653 K,and Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 is crystallized easily when the temperature is above 663 K.

Fig.3 SEM images of cross-sectional microstructure of Zr_41.2Ti_13.8Cu_12-5Ni₁₀Be_22.5 and Cu brazed joints with different surface roughness of soldering surfaces:a 38μm,b 18μm,c13μm,and d 9μm

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Table 3 EDS analysis results of phases formed at interface for spots shown in Fig.3d

3.3 Interface characteristics after soldering

As shown in Fig.3b,there are five distinct regions observed at the brazed joint.The detailed quantitative results of the EDS analysis for the regions in Fig.3b are listed in Table 4.It can be seen that the composite of brazed joint is much different from the original one.According to the brazing principle and former research,a complete brazed joint basically consists of three regions which are the diffusion region,the interface region and the central region,respectively [ 25] .RegionsⅠand V near the base alloys are the diffusion regions formed by atomic diffusion from solder to base alloys.RegionsⅡand IV are the interface regions which are formed by dissolution reaction from base alloys to solder.In the middle of the brazed joint,a wide RegionⅢis the central region while the composition is not the same as that of original solder.Furthermore,the elemental contents of these regions were analyzed by EDS and the results are listed in Table 4,which is consistent with the region pision.In the diffusion regions,the content of element Ge is higher than that of Au,indicating that the diffusion rate of Ge atoms is much faster than that of Au atoms.In the interface regions,the dissolution rate of Cu atoms into solder is faster than those of other atoms.It also can be seen that Cu atoms have diffused throughout the entire brazed joint,indicating that the reaction between Cu and liquid solder is very strong.

Fig.4 SEM images of cross-sectional microstructures of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 and Cu brazed joints processed at a 653 K for 15 min and b 653 K for 45 min

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Table 4 EDS analysis results of regions shown in Fig.3b (at%)

Figure 5 shows the elemental mappings along the interface of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5/Cu brazed joint investigated by electron probe micro analysis method.The surface roughness of the base alloys is 18μm.It can be seen from Fig.5 that the element Au diffuses from the middle solder into Zr_41.2Ti_13,8Cu_12.5Ni₁₀Be_22.5,and the element Ge segregates at the interface between solder and base alloys,which is consistent with the results of EDS analysis shown in Table 4.The map of Cu element shows a rapid change from a high content in the Cu part to a low content in the Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 part,and Cu atoms have diffused throughout the entire Au-based alloy solder.While for Zr,Ti and Ni elements in Zr_41.2Ti_13.8-Cu_12.5Ni₁₀Be_22.5,the element contents change sharply at the interface between the metallic glass and the solder,indicating that the distance of atoms diffused into the solder is very narrow.It also can be seen in Fig.5b,d that Ge atoms have a strong reaction with Zr atoms in Region V shown in Fig.3b.The result indicates that Au element has the excellent diffusivity in the superplastic state BMG.

Figure 6 shows TEM image and selected area electron diffraction (SAED) patterns of the interface of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5/Au-12Ge solder with surface roughness of 18μm.It can be seen that Zr_41.2Ti_13.8-Cu_12.5Ni₁₀Be_22.5 is still in amorphous state (SectionⅠin Fig.6) after soldering,while multi-grained feature is characterized in the joint interface judging from diffraction pattern shown in SectionⅡin Fig.6,indicating the existence of reaction between the amorphous alloy and Au-Cu solder which can form various intermetallic phases.The interface is continuous without any void or crack and soldered in atomistic scale of metallurgic bonding,showing that Au-12Ge eutectic alloy is a promising candidate for the jointing of BMGs and crystalline metals.

4 Conclusion

In this study,brazed joints with composition of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass and copper were fabricated using low temperature Au-12Ge eutectic alloy solder.The surface roughness of Zr_41.2Ti_13.8Cu_12.5-Ni₁₀Be_22.5 has great effect on the brazing quality of Au-12Ge solder,and the brazing quality increases with the surface roughness increasing due to the increasing contact area,while for copper,the influence is very weak due to the great wettability between the solder and copper.The best brazing time and temperature should be around 653-663 K and 30 min,respectively.Five distinct regions are observed at the brazed joint,and the eutectic microstructure of Au-12Ge solder does not exist because of the longrange diffusion between the solder and base alloys.

Fig.5 Elemental mappings along interface of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5/Cu brazed joint with surface roughness of base alloy of 18μm:a Au,b Ge,c Cu,d Zr,e Ti,and f Ni

Fig.6 TEM image and corresponding SAED patterns of Zr_41.2Ti_13.8-Cu_12.5Ni₁₀Be_22.5/Cu brazed joint at metallic glass side

Acknowledgments This work was financially supported by the Natural Science Basic Research Plan in Shaanxi Province of China(No.2014JM6234),the Specialized Research Fund for Doctoral Program of Higher Education (No.20136102120007) and the Program of Introducing Talents of Discipline to Universities (No.B08040).