Stirring brazing of dissimilar Al/Mg alloy without flux in air
摘 要：
AZ31B magnesium alloy and 2024 aluminum alloy were successfully jointed at aid of mechanical stirring with Sn–Zn–Al filler metal.The microstructure, fracture morphologies, and mechanical properties of joint were investigated.The results show that Mg–Al intermetallic compounds can be avoided by the process.But, a small quantity of porosity is found in the joint.The shearing strength of joint interface adjacent to magnesium alloy is35.4 MPa for formation of Mg–Sn intermetallic compounds, which is about 46%of that of filler metal.While, the shearing strength of joint interfaces adjacent to aluminum alloy is 70.4 MPa for formation of Zn–Sn–Al solid solution, which is about 92%of that of filler metal.
收稿日期：23 January 2013
基金：supported by the Natural Science Foundation Project of Chongqing (No. cstc2011jjA50001);the State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology (No. AWPTM12-07);
Stirring brazing of dissimilar Al/Mg alloy without flux in air
Hui-Bin Xu Hui-Bin Sun Hong-You Chen
School of Materials Science and Engineering, Chongqing University of Technology
Chen State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology
Abstract：
AZ31B magnesium alloy and 2024 aluminum alloy were successfully jointed at aid of mechanical stirring with Sn–Zn–Al filler metal. The microstructure, fracture morphologies, and mechanical properties of joint were investigated. The results show that Mg–Al intermetallic compounds can be avoided by the process. But, a small quantity of porosity is found in the joint. The shearing strength of joint interface adjacent to magnesium alloy is 35.4 MPa for formation of Mg–Sn intermetallic compounds, which is about 46 % of that of filler metal. While, the shearing strength of joint interfaces adjacent to aluminum alloy is 70.4 MPa for formation of Zn–Sn–Al solid solution, which is about 92 % of that of filler metal.
Keyword：
2024 Al alloy; AZ31B Mg alloy; Stirring brazing;
Author： Hui-Bin Xu e-mail: hbxu@cqut.edu.cn ;
Received： 23 January 2013
1 Introduction
As their attractive mechanical, metallurgical properties, and high commercial value, aluminum alloys and magnesium alloys are becoming increasingly important lightweight structural materials for applications such as aerospace, automotive, and so on[1–3].So, it is unavoidable that joint Mg alloys and Al alloys to form a compound structure, owing to their lower weight and cost[4–6].However, for the difficulty in joint Al alloys and Mg alloys, associating with different physicochemical properties, a novel reliable joining method is urgent needed to develop[7–9].
It was reported that brittle intermetallic compounds were found in the fusion zone when fusion welding Al alloy to Mg alloy[10, 11].Recently, much attention was brought in friction stir welding Al alloy to Mg alloy.Sato et al.[5]and Kostka et al.[12]reported that Al12Mg17and Al3Mg2intermetallic compounds were formed in weld center, which caused higher hardness.Another feasible method was diffusion welding.Dietrich et al.[13], Shang et al.[14], and Liu et al.[15]investigated the diffusion welding Mg alloy to Al alloy.They reported that a new layer, which consists of Mg–Al intermetallic compounds, was formed between Al alloy and Mg alloy.The layer also results in a higher hardness and less ductility.
In the article, AZ31B Mg alloy and 2024 Al alloy were brazing at aid of mechanical stirring using a Sn–Zn–Al filler metal.In the process, the filler metal was used to avoid the formation of Mg–Al intermetallic compounds.And, the mechanical stirring was used to disrupt oxide film on the substrates.Microstructure and mechanical properties of joint were investigated.
2 Experimental
AZ31B Mg alloy and 2024 Al alloy, with dimensions of60 mm 9 40 mm 9 3 mm, were selected as the experimental materials for stirring brazing.The chemical compositions of the two materials were shown in Table 1.The microstructure of the filler metal was illustrated in Fig.1.The chemical composition of filler metal was 61 wt%Sn, 29 wt%Zn, and 10 wt%Al, examined by energy dispersive spectroscopy (EDS) .The solidus-liquids temperature of filler metal was 197–470°C[16].
Table 1 Chemical compositions of AZ31B and 2024 alloys (wt%) 下载原图
Table 1 Chemical compositions of AZ31B and 2024 alloys (wt%)
Fig.1 Microstructure of filler metal
Fig.2 Schematic of stirring brazing equipment
Before brazing, the 2024 Al alloy and AZ31B Mg alloy plates were placed on moving table.The filler metal, of which the width was 1.9 mm, was mounted between the faying surfaces of the Al/Mg substrates.The thermocouple was connected near the bond line to measure the temperature during process.The samples were heated up to350°C by a resistance heating plate, which were to remain temperature constant during subsequently process.Then a stirrer, with a 1.8 mm diameter, was introduced into the mushy filler metal immediately at 1, 570 r?min-1rotation speed approximately.At same time, the moving table with speed maintained at 15 mm?min-1, was start to move.While, another specimen was joined by the above process, except without stirring.When specimen was heated up to350°C, the stirring pin without stirring was inserted into mushy filler metal and pulled out.During that, moving table was kept stationary.After brazing, specimens cooled down in air similarly.The process and experiments setup of the stirring brazing were illustrated in Fig.2.
A series samples were sectioned by a line cutting machine, and the cross section were prepared for metallographic analysis by standard polishing techniques.In addition, the microstructure was characterized by scanning electron microscopy (SEM, JSM-6460LV) equipped with an energy dispersive X-ray spectrometer (EDS) and X-ray diffraction (XRD) .And shear strength was tested by an electronically controlled tension machine (WDW-E200) at a cross-head speed of 0.5 mm?min-1at room temperature.Vickers microhardness was detected along the line vertical welding line using a 0.490 N load, 15 s dwelling time.
3 Results and discussion
The macrostructure, microstructure, and element distribution of the joint without stirring at 350°C are shown in Fig.3.A joint interface with crack can be clearly seen between the filler metal and base metal in Fig.3a.As seen from Fig.3b, a bond with liner interface between Al base metal and filler metal is found.As seen from Fig.3c, the distribution of Al exist a break in interface, where the concentration of Al decreases rapidly from Al base metal to filler metal.Also, there is almost no Sn and Zn in interface and base metal.It is indicated that there is no diffusion at joint interface adjacent to Al alloy.As seen from Fig.3d, a joint interface with oversize porosity between filler metal and Mg base metal is found.As seen in Fig.3e, the distribution of Mg exists a break in interface, and there is no Sn and Zn in interface and base metal also.It is indicated that there is no obvious diffusion at joint interface adjacent to Mg alloy.It is shown that there is no metallurgy bonding at joint interface without stirring.Based on the above results, it can be inferred that the oxide film on the substrate cannot be disrupted without stirring of stirrer, which can hinder element diffusion at joint interface.
Figure 4 shows the macrostructure, microstructure, and element distribution of the joint brazed with stirring at350°C.Figure 4a shows that nonliner joint interface without crack is found.It is indicated that the oxide film on the substrates can be disrupted during stirring brazing.At same time, a quantity of void is found in joint.This porosity is associated with a higher temperature of brazing processes.As seen from Fig.4b, a diffusion layer with about 12 lm width is observed at joint interface adjacent to Al alloy.Figure 4c shows the contents of Al elemen decrease gradually from Al base metal to filler metal.A the same time, Sn and Zn in filler metal diffuse into bas metal.It is indicated that there is obvious diffusion at join interface adjacent to Al alloy.The chemical compositions in the diffusion zone marked by square frame in Fig.4b are consisted of 64.99 wt%Al, 16.60 wt%Zn, and 18.41 wt%Sn, which is examined by EDS.So, according to Sn–Zn–Al ternary phase diagrams[17]and EDS results, the diffusion layer may be composed of Zn–Al–Sn solid solution, which will enhance the strength of joint interface.As seen in Fig.4d, an interaction layer about 27 lm width can be found at joint interface adjacent to Mg alloy.In the layer, the size of deep dark Zn–Al solid solution and light gray columnar Zn grain is smaller than original microstructure size of filler metal.This is mainly because the original grains are break into smaller ones under the force of stirring during process.Figure 4e shows that there is obvious elemental diffusion at joint interface.Here, in Fig.4e, the distribution of Mg element is much similar to that of Sn element.According to Mg–Sn binary phase diagrams[18]and EDS results in Table 2 (Fig.4f) , it can be illustrated that the Mg–Sn intermetallic compounds may be generated in the layer.Above all, it can be inferred that stirring can promote element diffusion at joint interface.In comparison with joining without stirring, mechanical stirring can promote the formation of metallurgy bonding at joint interface by intensive interaction between filler metal and substrates.
Fig.3 Macrostructure of joint brazed without stirring a, microstructure of interface near Al base metal b, elements distribution in Fig.3b c, microstructure of interface near Mg base metal d, and elements distribution in Fig.3d e
Fig.4 Macrostructure of joint brazed with stirring a, microstructure of interface near Al base metal b, elements distribution in Fig.4b c, microstructure of interface near Mg base metal d, elements distribution in Fig.4d e, and microstructure of diffusion layer near Mg base metal under higher multiples f
Table 2 EDS results taken from different positions as denoted in Fig.4f (wt%) 下载原图
Table 2 EDS results taken from different positions as denoted in Fig.4f (wt%)
The microhardness distribution of joint brazed without and with stirring is shown in Fig.5a, b.In Fig.5, the average hardness values of Al base metal and Mg base metal are HV93 and HV 52, respectively.In the filler metal zone, the hardness value is nonuniform, owing to its complex microstructure and porosities.In Fig.5a, the hardness value of base metal decreases sharply to that of filler metal on the joint interface, for existing of the crack.In Fig.5b, the hardness value of Al alloy decreases gradually to that of filler metal at joint interface.The formation of Zn–Al–Sn solid solution should be responsible for the change of hardness values at joint interface.However, a peak in hardness values is observed at joint interface adjacent to Mg alloy.Similarly, the formation of Mg2Sn intermetallic compounds should be responsible for the change of hardness values at joint interface.It was reported that the microhardness of Mg–Al intermetallic compounds is HV 200 in average[3].In the process, the microhardness values of Mg2Sn intermetallic compounds are obviously less than that of Mg–Al intermetallic compounds.So, the effect of Mg2Sn intermetallic compounds on weak shear strength of joint interfaces is nonstronger than that of Mg–Al intermetallic compounds.
The shear strength of joint interfaces and test equipment is illustrated in Fig.6.The shear strength of joint interface adjacent to Al alloy is up to 70.4 MPa, which is about 92%of that of the filler metal.This is because the formations of Zn–Al–Sn solid solution will enhance strength of joint interface.But, the shear strength of joint interface adjacent to Mg alloy is35.4 MPa.The value is about 46%of the shear strength of filler metal.This is because that the formation of intermetallic compounds will weak the strength of joint interface.
The fracture surface morphologies of joint are presented in Fig.7.In Fig.7a, a ductile fracture surface is shown at joint interface adjacent to Al alloy.It is confirmed that the ductile fracture is related to the formation of Al–Sn–Zn solid solution on interface.In Fig.7b, a brittle fracture surfaces are shown at joint interface adjacent to Mg alloy.Specially, a large number of Mg2Sn and a small quantity of Al5Mg11Zn4intermetallic compounds are observed on joint interface near Mg alloy in Fig.7c, examined by XRD.It is confirmed that the brittle fracture is related to formation of intermetallic compounds on joint interface, which will weaken the strength of joint interface.
Fig.5 Microhardness distribution of the joints brazed a without and b with stirring
Fig.6 Shear strength of filler metal and interface near both Al and Mg substrates a, shear strength text equipment b, and dimension of shear strength test specimen c
Fig.7 SEM images and XRD results of fracture surfaces of interface:fracture surface of a Al side and b Mg side, and c XRD pattern from Mg side
4 Conclusion
In the article, a novel brazing method of Mg alloy and Al alloy was carried out in air.The 2024 Al alloy and AZ31B Mg alloy can be successfully brazed at aid mechanical stirring by adding of Sn–Zn–Al filler metal, without using flux in air.
The results show that a small quantity of porosity is also found in the joint, which is associated with a higher temperature of brazing processes.At joint interface, metallurgy bonding can be formed by diffusion of elements during stirring brazing.Zn–Al–Sn solid solution forms at joint interface near Al alloy during stirring brazing.While, Mg2Sn intermetallic compounds are found at joint interface near Mg alloy, instead of Mg–Al intermetallic compounds.The shear strength of interface near Al alloy can be up to70.4 MPa.So, a ductile fracture surfaces are observed at joint interface.And shear strength of interface near Mg alloy is 35.4 MPa.A brittle fracture surfaces are also found at joint interface.
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