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Rare Metals2017年第11期

Joint interface during arc milling brazing of aluminum alloy to low carbon steel with cutter milling at various rotation speeds

Hui-Bin Xu Song Tang Hai-Feng Xi Peng-Yu Gao Mo-Yang Li

School of Materials Science and Engineering, Chongqing University of Technology

State Key Laboratory of Advance Welding and Joining, Harbin Institute of Technology

收稿日期：9 June 2017

基金：financially supported by the State Key Laboratory of Advanced Welding and Joining of China (No.AWJ-Z16-02);

Joint interface during arc milling brazing of aluminum alloy to low carbon steel with cutter milling at various rotation speeds

Hui-Bin Xu Song Tang Hai-Feng Xi Peng-Yu Gao Mo-Yang Li

School of Materials Science and Engineering, Chongqing University of Technology

State Key Laboratory of Advance Welding and Joining, Harbin Institute of Technology

Abstract：

In this study, 5052 aluminum alloy and Q235 steel were joined by a new way of arc milling brazing, with the Zn15%Al filler metal and no use of flux. Effect of rotation speed on mechanical properties and microstructure of joint interface was investigated. The results show that increasing rotation speed is in favor of formation of weld and spread of filler metal on substrate. The fine grain with homogeneous composition in brazed seam can be realized by the stronger stirring in pool induced by milling at more higher rotation speed. And, a composite joint reinforced with needle-like or block-like Fe-Al-Zn intermetallic compounds(IMCs) particle can be obtained by arc milling brazing, which comes from the reaction between steel scrape and Zn-A1 filler alloy. The test on strength of joint interface shows that with the increase in rotation speed,there is a peak value in variation ranges of strength of joint interface. This is to say, when rotation speed is 2720 r·min^-1, the strength of joint has maximum value(182.01 MPa). Moreover, a thin and discontinuous IMCs layer at joint interface can be obtained by increasing rotation speed. But at a low or high rotation speed, there also is a crack at joint interface, which weakens the strength of joint interface. Only at 2720 r·min^-1, a good joint interface without an obvious crack can be realized.

Keyword：

Author： Hui-Bin Xu e-mail:hbxu@cqut.edu.cn;

Received： 9 June 2017

1 Introduction

Owing to high strength of steel and light weight of aluminum alloy,compound structure of steel and aluminum alloy was considered as an economic and energy-saving choice in these areas,such as automobile,military arming,and aerospace.However,poor weldability of aluminum to steel was caused by a large difference on physical and chemical properties for aluminum and steel,which will severely hinder applications of their compound structure [ 1] .

Nowadays,joining of aluminum alloy to steel mainly focuses on these methods,such as fusion welding [ 2, 3, 4, 5, 6, 7] ,stirring friction welding [ 8, 9, 10, 11, 12] ,diffusion bonding [ 13, 14] ,ultrasonic joining [ 15, 16, 17] ,laser brazing [ 18, 19, 20, 21] ,arc brazing [ 22, 23, 24, 25, 26, 27, 28, 29, 30] .Many intermetallic compounds and residual stress were caused by high thermal cycle in fusion welding of aluminum alloy to steel,which will lead to the formation of crack in joint.The stirring friction welding is one of reliable joining methods of aluminum alloy to steel,for a higher strength of joint obtained by this way.But an inevitable formation of intermetallic compounds (IMCs)and segregation of IMCs on zone near axial shoulder were related to direct contact of aluminum alloy to steel and inhomogeneous heat production in friction stir welding(FSW) process,respectively.They will lead to the decrease in joint strength.

Being considered as an economic,flexible,and a lowtemperature welding method,arc brazing is more and more applied in joining process of aluminum alloy to steel.However,a thick,continuous,and straight IMCs layer caused by arc brazing limited the increase in weld strength.Furthermore,flux was inevitable to be used in the above process.

Fig.1 Principle of arc milling brazing

To overcome the above problems,it was put forward that a new arc milling brazing technology on joining of aluminum alloy to steel.In this process,stirring induced by milling promotes the spreading and wetting of filler metal on surface of steel,instead of using of flux.And,disruption of IMCs layer is accomplished by milling on surface of steel.In addition,scrap iron can be extensively distributed in liquid filler metal by stirring induced by milling.Its joining principles are illustrated in Fig.1.In this paper,influence of rotation speed on the appearances,microstructure,and mechanical properties of welds were investigated in detail.

2 Experimental

In this paper,5052 aluminum alloy and Q235 steel was joined with a dimension of 60 mm×50 mm×3 mm.The Zn15%Al welding wires was used as a filler metal with diameter in 1.6 mm.The chemical compositions of base metal and filler metal are listed in Tables 1 and 2,respectively.

The self-made equipment for arc milling brazing is shown in Fig.2.The equipment is the combinations of milling machine,tungsten inert gas welding (TIG) welder,and welding fixture.Welding parameters are as follows:The angle between milling cutter and surface of base metal is 60°,welding current is 70 A,welding speed is44 mm·min^-1,feeding speed of wires is 720 mm·min^-1,and flow of argon is 10 L·min^-1.Rotation speed changes from 660 to 5440 r·min^-1.Before brazing,welds were fixed on welding platform,by a butted way.And the butted gap is kept at about 2 mm.The joining process is as follows:After molten filler metal had fill up the butted gap and changed into a welding pool,surface of steel was immediately treated by milling cutter.At the same time,the welding pool was strongly stirred by milling.

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Table 1 Chemical compositions of 5052 alloy and Q235 steel (wt%)

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Table 2 Chemical compositions of Zn15%Al filler metal (wt%)

Fig.2 Equipment of arc milling brazing

Fig.3 Schematic diagram for shear jig of joint

The specimens were cut into ones with dimension of20 mm×8 mm×3 mm,by wire-electrode cutting.Then,they were fixed in a special shear jig (Fig.3),in order to measure strength of joint by an electronically controlled tension machine (WDW-E200).In addition,the other one was sectioned along line perpendicular to welding direction by wire-electrode cutting.Then,they were polished to metallographic ones.The microstructure and fracture of joint were examined by scanning electron microscope (SEM,JSM-6460LV) equipped with an energy-dispersive spectrometer (EDS).

Fig.4 Appearances of joints at different rotation speeds:a 660 r·min^-1-front,b 660 r·min^-1-back,c 2720 r·min^-1-front,d 2720 r·min^-1-back,e 5440 r·min^-1-front,and f 5440 r·min^-1-back

3 Results and discussion

3.1 Appearance of joint

As shown in Fig.4,with the increase in rotation speed,appearance of joint becomes regular and uniform,moving trace of milling cutter is obviously found on the surface of weld seam.In addition,spread area of filler metal on surface of substrate is obviously enlarged by increasing rotation speed,especially on surface of aluminum alloy.In the process of arc milling brazing,the liquid filler metal with steel scraps is considered as a newly formed semisolid fluid.According to theory of“shear thinning”in the semisolid formation process [ 31] ,viscosity of filler metal decreases by increasing rotation speed,which is in favor of filling up the butted gap and spread on substrate for filler metal.

3.2 Microstructure of joint

As shown in Fig.5 and according to Zn-Al binary phase diagram [ 32] ,microstructure of joint is composed ofη-Zn phase andα-Al phase.And,the amount ofη-Zn phase in the center zone of brazed seam decreases (Fig.5b,d,f),with the increase in rotation speed.It is indicated that there is a tendency on the composition uniformity of joint during the process of increasing rotation speed,which was deduce by a sufficient mixture of compositions in different micro zones in joint.At the same time,with the increase in rotation speed,the fine and non-dendritic grain can be realized in the center of brazed seam,as shown in Fig.5b,d,f.This should be related to stronger stirring in pool,which is related to milling at a higher rotation speed.

In addition,after a sufficient stirring at a higher rotation speed,there is an extensive distribution of steel scrap in whole brazed seam (Fig.5e,f).Moreover,as shown in Table 3 and according to Al-Fe binary phase diagram [ 32] ,steel scraps are transmitted into needle-like or block-like Fe-Zn-Al IMCs by metallurgical interface reaction between steel scraps and liquid Zn-Al alloy.Similar metallurgical reaction was confirmed in a previous work [ 22] .Besides,no obvious crack is found at interface between steel scraps and liquid Zn-Al alloy at a higher rotation speed,as shown in Fig.5e,f.It is indicated that interface reaction between steel scraps and liquid Zn-Al alloy is promoted with the increase in rotation speed.So,by rising rotation speed,a composite joint can be obtained,where Fe-Zn-Al IMCs is extensively distributed in whole joint as a reinforced particle.Certainly,it will be in favor of improving the strength of brazed seam.

Fig.5 SEM images of fusion line and brazed seam in joint at different rotation speeds:a fusion line at 660 r·rnin^-1,b brazed seam at660 r min^-1,c fusion line at 2720 r·rnin^-1,d brazed seam at 2720 r·min^-1,e fusion line at 5440 r·min^-1,f brazed seam at 5440 r·min^-1

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Table 3 Chemical compositions of microzones in Figs.5,6 and 8(wt%)

Based on the microstructure,elemental distribution at join interface (Fig.6) and composition analysis in Table 3,it is deduced that interface zone is composed of Fe-Zn-Al IMC layer.Furthermore,it is clear that the maximum thickness of IMC layer gradually decreases from 25μm at660 r·min^-1 to 13μm at 5440 r·min^-1.Simultaneously,there is an obvious change on distribution of interface zone,where a flat and continuous IMC layer at 660 r·min^-1change into bending and discontinuous ones at5440 r·min^-1.It is illustrated that a thin and bending IMC layer on the interface can be obtained by rising rotation speed.

Fig.6 SEM images and EDS results of joint interface at different rotation speeds:a,b 660 r·min^-1,c,d 2720 r·min^-1,e,f 5440 r·min

Fig.7 Shear strength of joint interface at different rotation speeds

But,it is worth noticing that there are cracks at joint interface at a low or high rotation speed,as shown in Fig.6a,e.At a low rotation speed (660 r·min^-1),a thick and flat IMC layer exists on the interface,where the thermal stress during cooling stage can result in a crack between IMCs layer and Zn-Al Filler metal.The similar results are also found in other research works [ 2] .However,at a high rotation speed (5440 r·min^-1),a minimum thickness of IMC layer is very thin and evenly close to zero,which should weak metallurgical bonding between Zn-Al filler metal and steel.This is because the formation of IMC layer with a thin thickness at joint interface is a key to reliable bonding of aluminum to steel [ 1] .So,only at a moderate rotation speed (2720 r·min^-1),an ideal joint interface without obvious microcrack can be realized,where a thin and discontinuous IMC layer exist.

3.3 Mechanical properties of joint

As illustrated in Fig.7,the strength of joint interface increases rapidly in the rotation speed ranging660-2720 r·min^-1.Then,the strength of joint interface declines gradually in the rotation speed range of2720-5440 r·min^-1.When rotation speed is 2720 r·min^-1,the strength of joint interface is up to maximum,182.01 MPa.This should be related to an ideal joint interface without microcrack at 2720 r.min^-1,as mentioned above.

3.4 Fracture of joint

On base of fracture morphology of joint in Fig.8 and composition of fracture surface (Table 3),joint mainly fractured along IMCs layer and brazed seam close to steel.This is related to bending and discontinuous IMCs layer on interface of joint.

4 Conclusion

5052 aluminum alloy and Q235 steel can be joined by an imnovative arc milling brazing method,instead of using of flux.In this paper,experiment at various rotation speeds was carried out.And the effect of rotation speed on micro structure and mechanical properties of joint was discussed.The appearance of joint is improved by rising rotation speed.At same time,spread of filler metal on substrate is enhanced.The increase in rotation speed is in favor of composition uniformity and grain refinement in joint.Moreover,by increasing rotation speed,there is an extensive distribution of steel scrap in whole joint zone.At this time,metallurgical reaction occurs between steel scrap and liquid Zn-Al alloy,resulting in the formation of FeZn-Al IMCs on base of steel surface.So,a composite joint reinforced with IMCs particle can be obtained by arc milling brazing.There is a thin and bending IMC layer on the interface,by boosting rotation speed.But,only at a moderate rotation speed such as 2720 r·min^-1,an ideal joint interface without obvious microcrack can be obtained.The shear strength of joint first increases and then declines with the increase in rotation speed.When rotation speed is2720 r·min^-1,it reaches the maximum value(182.01 MPa).Interface of joint partially fractures along IMCs layer,and partially fractures along brazed seam close to steel.