简介概要

Microstructure and tribological behavior of tungsten inert gas welding arc brazing tin-based babbit

来源期刊：Rare Metals2020年第2期

论文作者：Fang-Ming Zhou Qing-Ya Zhang Ming-Xiao Shi Hui Li Jun-Wu Guo

文章页码：193 - 199

摘要：The microstructure of the tin-based babbit obtained by the method of tungsten inert gas welding（TIG）arc brazing was studied by optical microscopy（OM） and X-ray diffraction（XRD）.Tribological behavior was investigated by high-temperature friction and wear testing machine（HTFWT）,laser scanning confocal microscopy（LSCM）,scanning electron microscopy（SEM） and energydispersive spectrometer（EDS）.It can be found that the higher welding current of the melting tin-based babbit makes it possible to form isomer structure with fine crystals of the cubic SnSb compounds and large star Cu₆Sn₅ compounds so that a higher hardness can be achieved,and a lower wear rate can be obtained over the entire distance of sliding friction.When the applied load is 2 N,the contact surface is oxidized due to the accumulation of friction heat and oxide,which plays a role of lubricated film.Also,the softer Sn-based solid solution forms obvious furrow under abrasive wear,and the harder SbSn and Cu₆Sn₅ intermetallic compounds shatter and leave a hole under friction.

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稀有金属(英文版) 2020,39(02),193-199

Microstructure and tribological behavior of tungsten inert gas welding arc brazing tin-based babbit

Fang-Ming Zhou Qing-Ya Zhang Ming-Xiao Shi Hui Li Jun-Wu Guo

Jiangsu Province Key Laboratory of Advanced Welding Technology,Jiangsu University of Science and Technology

作者简介：*Fang-Ming Zhou,e-mail:fangmingzhou@just.edu.cn;

收稿日期：24 September 2016

基金：financially supported by the University Natural Science Research Project of Jiangsu Province (No. 15KJA460006);

Microstructure and tribological behavior of tungsten inert gas welding arc brazing tin-based babbit

Fang-Ming Zhou Qing-Ya Zhang Ming-Xiao Shi Hui Li Jun-Wu Guo

Jiangsu Province Key Laboratory of Advanced Welding Technology,Jiangsu University of Science and Technology

Abstract：

The microstructure of the tin-based babbit obtained by the method of tungsten inert gas welding(TIG)arc brazing was studied by optical microscopy(OM) and X-ray diffraction(XRD).Tribological behavior was investigated by high-temperature friction and wear testing machine(HTFWT),laser scanning confocal microscopy(LSCM),scanning electron microscopy(SEM) and energydispersive spectrometer(EDS).It can be found that the higher welding current of the melting tin-based babbit makes it possible to form isomer structure with fine crystals of the cubic SnSb compounds and large star Cu₆Sn₅ compounds so that a higher hardness can be achieved,and a lower wear rate can be obtained over the entire distance of sliding friction.When the applied load is 2 N,the contact surface is oxidized due to the accumulation of friction heat and oxide,which plays a role of lubricated film.Also,the softer Sn-based solid solution forms obvious furrow under abrasive wear,and the harder SbSn and Cu₆Sn₅ intermetallic compounds shatter and leave a hole under friction.

Keyword：

Tin-based babbit; TIG arc brazing; Microstructure; Tribological behavior;

Received： 24 September 2016

1 Introduction

With the rapid development of manufacturing industry,ships,vehicles,aerospace and other heavy machinery industries have witnessed higher requirements for the antifriction layers of sliding bearing.Babbit alloys (also called white alloys) are one of the various anti-friction materials [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] ,which have been widely used for the sliding bearings operated in oil lubrication because of its excellent anti-friction,anti-occlusion and embeddability.A bearing works in a stable manner when a proper film thickness is formed and maintained between the shaft and bearing under normal operation.However,when poor supply of oil occurs,the shaft and babbit layers come into partial contact in the process of sliding friction,and this condition is called boundary lubrication [ 11, 12, 13, 14, 15] .

Iron substrate and babbit alloys are not compatible metals because of their different melting points (1538℃for Fe and 230℃for tin-based babbit).Casting is a traditional technology applied for bonding babbit alloys to iron substrate,but there are many drawbacks such as pore,peeling and cracking,and a certain thickness layer must be removed by machining,which raises the manufacturing cost and wastes resources [ 11] .Many technology processes [ 11, 16, 17] have been used to obtain the babbit layers;however,high machining allowance of the babbit layers and tinning raises the cost of bearing fabricated.Therefore,it is receiving a remarkable attention to obtain babbit layers by new procedure and meet the requirements of green manufacturing [ 18] .Nowadays,TIG arc brazing has been successfully used to join the dissimilar metal [ 19, 20, 21, 22] .The iron substrate and filler wire metals were heated or melted by arc in this technique.

In this study,a kind of tin-based babbit wire was applied to prepare the babbit layers through TIG arc brazing at different welding heating inputs.The aim of this study is to research the effect of welding heating input on the micro structure and tribological properties of tin-based babbit.And it will have certain theoretical and practical significance.

2 Experimental

The tin-based babbit wire with the diameter of 4 mm and Q235B steel with the thickness of 8 mm were used for the study,and their chemical composition is listed in Tables 1and 2.And the argon with the concentration of 99%was used as the shielding gas.Besides,the steel substrate was cut into the size of 50 mm×50 mm,and the surface of which was cleaned before TIG arc brazing.The substrate was fixed,and welding schematic diagram is shown in Fig.1.The welding parameters are listed in Table 3.

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Table 1 Chemical constitution of ZChSnSbl1-6 (wt%)

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Table 2 Chemical constitution of Q235B (wt%)

Fig.1 Schematic of babbitt metal arc surfacing

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Table 3 Welding parameters of TIG arc brazing

The thickness of the tin-based babbit layer is 4 mm,and2-mm-thickness layer would be removed by line cutting.The weld seams were cut into the size of15 mm×15 mm×10 mm,which was used as samples.The cross section of samples was pre-grinded and then polished.And then,they were corroded by 3%-5%nitric acid alcohol corrosion.The microhardness was carried out by MH-5 microhardness tester with the applied load of0.25 N and dwelling time of 10 s.

The friction and wear tests were carried out under the dry friction.Wear rate is based on the formula:I=ΔV/ΔtF,where I is wear rate,ΔV is volume wear loss,At is time and F is load.The specimens with a friction radius of4 mm run in high-temperature friction and wear testing machine (HTFWT).The friction ball was made from GCr15 with the diameter of 3 mm,and its chemical composition is listed in Table 4.Different loads and speeds were used for the friction and wear experiment.The experimental time was 15 min,and experimental temperature was 25℃.The samples were fixed by four screws,and a friction ball was fixed in a bar,and friction and wear diagram are shown in Fig.2.

Micros truc ture of the tin-based babbit was studied by optical microscope (OM,Axioskop 2 MAT) and X-ray diffractometer (XRD,XRD-6000).The wear mechanism and morphology were analyzed through laser scanning confocal microscope (LSCM,LEXT OLS4000) and scanning electron microscope (SEM,JSM-6480) equipped with energy-dispersive spectrometer (EDS).

Fig.2 Friction and wear schematic diagram

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Table 4 Chemical composition of GCr15 (wt%)

3 Results and discussion

3.1 Microstructure of tin-based babbit layers

It is well known that the tribological property of tin-based babbit is determined by microstructure characteristics.XRD was used to identify the phase types of tin-based babbit,and the results are shown in Fig.3.It can be seen that the tin-based babbit obtained by the parameters above is composed of three kinds of phases:αphase (copper and antimony solution in Sn matrix),βphase (SnSb compound)andεphase (Cu₆Sn₅ compound) [ 10, 11] .Increasing welding current changes the phase content,and the highest contents of Sn-based solid solution and SbSn and Cu₆Sn₅compounds are obtained when the welding current is 70 A.

Fig.3 XRD analysis of tin-based babbit with different welding currents

The effect of welding current on the microstructure of tin-based babbit is shown in Figs.4 and 5.It can be seen from Fig.4a-c that the straight interface can be obtained when the welding current is<90 A,as shown in Fig.4a,b.While the welding current is 90 A,steel substrate is melted and obvious penetration forms,as shown in Fig.4c.

The microstructural characteristics of tin-based babbit are controlled by the arc brazing current,and the higher welding current can make the melted babbit crystallized form isomer structure of SbSn and Cu₆Sn₅.As shown in Fig.4d-f,the SbSn cubics are clearly identified in Sn matrix,and the Cu₆Sn₅ intermetallic compounds that are distributed in Sn matrix have characteristic star shapes that can be easily identified.However,the size of SbSn is fined and the size of Cu₆Sn₅ is larger as the welding current increases from 50 to 90 A.As shown in Fig.5,with the welding current increasing,the grain size of SbSn decreases from 47.8 to 37.7μm.It is analyzed that since the solidification temperatures of Cu₆Sn₅-,SbSn-and Sn-based solid solution are 375,273 [ 23] and 230℃,Cu₆Sn₅ compounds are firstly formed by over eutectic reaction [ 24] and precipitate from the melting tin-based babbit and distribute like star shape,then SbSn compounds are formed through mono tec tic reaction [ 25] and precipitate from the melting tin-based babbit and distribute like cubic and finally Snbased solid solution precipitates from the melting metal during solidification process.The grain growth of SbSn and Cu₆Sn₅ is determined by the temperature of liquid metal under the process of slow cooling.However,cooling rate is larger under the condition of TIG arc brazing,which can make the grain be fined.The melted babbit will be overheated when the welding current is 90 A.

Fig.4 OM images of babbit layers at different welding currents:a,d 50 A,b,e 70 A and c,f 90 A

Fig.5 Grain size of SbSn compounds

Fig.6 Microhardness of samples and phases at different currents

The tribological property of the tin-based babbit is also associated with the microhardness.Figure 6 shows the microhardness of each sample,αphase,βphase andεphase.It can be seen that microhardness of each sample andαphase is basically constant,while the microhardness ofβphase andεphase increases with the welding current increasing.It is analyzed that the higher welding current makes the isomer structure of SbSn be fined and uniformly distributed Cu₆Sn₅,which improve the microhardness of SbSn and Cu₆Sn₅,and the tin-based babbit is overheated when the welding current is 90 A,which also increases the microhardness of SbSn and Cu₆Sn₅.

3.2 Friction and wear behavior of tin-based babbit surfaces

Tribological property tests were carried out for the tinbased babbit layers,and the experimental results are shown in Fig.7.It can be seen that the friction coefficient is approximately constant for sliding time over 1 min,and this constant value increases with the current increasing.Friction coefficient of the tin-based babbit is directly depended on the roughness of the sample's surface.At the initial stage of friction and wear,the relative roughness of the sample's surface is so large that the friction coefficient changes apparently.

Figure 7b shows the friction coefficient of sample at welding current of 70 A under loads of 2 N and rotation of300 r·min^-1.It can be seen that friction coefficient is bigger and changes more intensely than that in Fig.7a.It is analyzed that the sample's surface is oxidized because the contact surface is heated by the accumulation of friction heat,which makes Sn atoms in tin-based babbit react with O atoms in air.Moreover,according to EDS analysis(Table 5),the oxide is SnO₂.As lubricated film,these oxides play the role in reducing friction coefficient,but the friction coefficient would increase when the oxides were peeled off under the action of friction [ 1, 26] .Changes in the friction coefficient are accompanied by the cycle of oxides formation-lubrication-peeling off.

Fig.7 Friction behavior of tin-based babbit layer at different conditions:a 0.5 N,300 r·min^-1;b 2.0 N,300 r·min

In addition,the friction coefficient is closely related to the micro structural characteristics of tin-based babbit.The soft Sn-based solid solution would become softer,even plastical deformation takes place,due to the friction heat accumulated on the sample.The friction operation is supported by the harder SbSn and Cu₆Sn₅ intermetallic compound particles,which decreases the contact area and increases the toughness of contact surface [ 27] .However,these hard particles would be broken under the action of friction,which increases the toughness of contact surface.Therefore,the friction coefficient would also change with the operation of friction ball on hard intermetallic compounds.

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Table 5 EDS analysis of characteristic points in Fig.10 (at%)

Fig.8 Wear rate of tin-based babbit layer

The wear morphology was investigated by SEM LSCM to provide a quantitative measurement for the friction and wear of tin-based babbit.It can be directly seen from Fig.8that the wear rate decreases obviously with the welding current increasing from 50 to 90 A.As shown in Fig.9a-c,wear width,wear cross-sectional area and wear volume decrease and the wear surface becomes rougher and rougher with the welding current increasing.It is analyzed that SbSn is refined and Cu₆Sn₅ compounds uniformly distribute in Sn-based solution,so that the hard particles would be broken more seriously.Therefore,the wear behavior is attributed to the microstructural characteristics of the tin-based babbit and the microstructure of the tin-based babbit,which can be controlled by optimizing welding parameters.

SEM and EDS analyses were carried out to further explore the wear mechanism of the tin-based babbit.Figure 10a,b shows wear morphology at load of 2 N and speed of 300 r·min^-1,and the results of characteristic points are listed in Table 5.Not only the oxide would affect the friction coefficient,but also the abrasive wear would change it.The soft Sn-based solid solution is melted because of the large amount of friction heat,and the furrow would be formed under the action of abrasive wear [ 28, 29, 30] .The hard-intermetallic compounds still can maintain the shape of solid particles because of their high melting point,but these hard SbSn and Cu₆Sn₅ compound particles shatter and leave a hole under the action of friction.

3.3 Shear strength of TIG arc brazing tin-based babbit

Shear strength tests were carried out to measure the interfacial bonding property,and the results are shown in Fig.11.It can be seen that the interfacial shear strength obviously increases from 73 to 155 MPa as the welding current increases from 50 to 90 A.The interfacial bonding property is apparently strengthened with the current increasing.

Fig.9 A SEM images and inset LSCM mappings of wear morphologies of tin-based babbit at different welding currents:a 50 A,b 70 A and

Fig.10 a SEM images of wear morphology at friction and wear parameters of 2.0 N and 300 r·min^-1 and b enlarged image on the hole under

Fig.11 Shear strength of TIG arc brazing tin-based babbit

4 Conclusion

In this study,TIG arc brazing was used to fabricate the tinbased babbit anti-friction layers under different welding currents.Experimental results show that the higher welding current makes it possible to form isomer structure of the SbSn and Cu₆Sn₅,and the higher hardness and better tribological property are due to these hard intermetallics.Oxidative wear and abrasive wear are found when applied load is 2 N.EDS analysis results show that the oxidative production is SnO₂.Furrow forms on the contact surface at the softer Sn-based solid solution under abrasive wear,while harder SbSn and Cu₆Sn₅ will be shattered and a hole under friction is left.