Study on Low Temperature Brazing of Magnesium Alloy to Aluminum Alloy Using Sn–Xzn Solders ⇑ Zhi Wang, Hongyang Wang, Liming Liu

Materials and Design 39 (2012) 14–19

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Materials and Design

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Study on low temperature brazing of magnesium alloy to aluminum alloy using Sn–xZn solders ⇑ Zhi Wang, Hongyang Wang, Liming Liu

Key Laboratory of Liaoning Advanced Welding and Joining Technology, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China article info abstract

Article history: In this article, a series of Sn–xZn solders are designed for joining Mg/Al dissimilar metals by low temper- Received 2 December 2011 ature brazing. The effect of Zn content in Sn–Zn solders on microstructure evolution and mechanical Accepted 13 February 2012 properties of the different brazed joints are investigated. The experimental results indicate that Sn– Available online 20 February 2012 30Zn alloy is identiﬁed as the optimized solder. Al–Sn–Zn solid solutions form and disperse in the brazing zone of the Mg/Sn–30Zn/Al brazed joint, decreasing the risk of embrittlement of the brazed joint. The Keywords: average shear strength of Mg/Sn–30Zn/Al brazed joint can reach 70.73 MPa. The joint fractures in the A. Non-ferros metals and alloys coarse blocky Mg Sn intermetallic phases in the center of the brazing zone. D. Brazing and soldering 2 Ó 2012 Elsevier Ltd. All rights reserved. F. Microstructure

1. Introduction The Sn–Zn system lead-free solder shows valuable in advanced research and development due to its no-poisonous, low cost, and Magnesium alloys are extensively applied in automotive, elec- fine mechanical properties [16,17]. Zn and Sn elements in the sol- tronics and aerospace industries, due to their outstanding physical der can form solid solution with Al element; Zn has the same crys- and mechanical properties such as high strength-to-weight ratio, tal lattice with Mg, and Sn has a certain degree of solubility in Mg. good castability, ease of machinability, high damping capacity In addition, low melting point of Sn–Zn solder benefits the brazing and a large recycling potential [1–3]. It is urgently demanded to of Mg/Al dissimilar metals. join Mg alloys to other metals in order to further exploit the poten- In the present study, the Sn–xZn solders (x = 3 wt.%, 9 wt.%, tial application of Mg alloys. At present, welding Mg/Al dissimilar 15 wt.%, 30 wt.%, 40 wt.%, 50 wt.%) are selected to braze Mg/Al dis- metals becomes a hard task and a hotspot. A variety of methods similar metals at low temperature. The aim of current work is to are used to join Mg alloys to Al alloys, such as tungsten inert gas investigate the effect of Zn content in Sn–Zn solders on microstruc- welding [4], resistance spot welding [5], laser welding [6,7], fric- ture evolution and mechanical properties of the Mg/Al brazed joints. tion stir welding [8–10], reactive brazing [11] and vacuum diffusion welding [12,13]. To some extent, brittle Mg–Al intermetallic 2. Experimental procedure compounds (IMCs) would be formed when using the approaches above and deteriorate the performance of Mg/Al joint seriously The base materials used for brazing were AZ31B Mg alloy sheet [14,15]. When the reaction temperature reaches or exceeds the and 6061 Al alloy sheet with dimensions of 50 mm 6mm Mg–Al eutectic temperature (437 °C), diffusion between 1.7 mm. The chemical compositions of these alloys are shown in Ta- magnesium and aluminum occurs and results in the formation of ble 1. The Sn–xZn (x = 3, 9, 15, 30, 40, 50) solders used in present Mg–Al IMCs. Therefore, it is necessary to decrease the reaction work were produced by melting pure tin(99.99%) and pure temperature and add another alloy layer to avoid direct contact be- zinc(99.9%) in a crucible electrical resistance furnace at 500 °C for tween Mg alloys and Al alloys, in order to eliminate the negative 2 h, and then the molten Sn–Zn alloys were cooled and curdled to effect of Mg–Al IMCs on the strength of the joints. Brazing is con- ingots in the furnace. A pure argon flow was furnished for gas shield- sidered as a proper method of bonding dissimilar materials, be- ing throughout the whole process. cause the elemental composition of filler alloy could be adjusted The Sn–xZn filler alloys were first combined with Al alloy sheet flexibly to meet the requirements of phase constituent and by hot-dipping technique at suitable temperature as listed in Table mechanical properties of the joint together. 2. Prior to brazing, all the surfaces of hot-dipping specimens and Mg alloy sheet were ground by SiC paper and cleaned in ethanol, in order to remove the oxide films, oil and other contaminants. A ⇑ Corresponding author. Tel./fax: +86 411 8470 7817. lap joint configuration was used with overlap distance approxi- E-mail address: [email protected] (L. Liu). mately 5 mm and the pressure of 2 MPa as shown in Fig. 1a. Ther-

Table 1 microstructure of Sn–9Zn alloy presents eutectic feature, while Chemical compositions of AZ31B Mg alloy and 6061 Al alloy (wt.%). other Sn–Zn alloys exhibit hypoeutectic and hypereutectic fea- Al Zn Mn Si Mg ture. Fig. 2a shows the metallographic structure of the hypoeu- AZ31B 2.5–3.5 0.5–1.5 0.2–0.5 0.1 Bal. tectic solder of Sn–3Zn, Sn and Sn–Zn eutectic structure are 6061 98.0331 0.0005 0.0038 0.5527 0.9678 distributed uniformly in the solder. As shown in Fig. 2b, the microstructure of Sn–9Zn solder reveals typical eutectic feature. Microstructures of hypereutectic Sn–Zn solders of Sn–15Zn, Sn– Table 2 30Zn, Sn–40Zn and Sn–50Zn are exhibited in Fig. 2c–f, respec- Brazing parameters on the joining of Mg alloy with Al alloy using Sn–xZn solders. tively, and they are composed of Zn and Sn–Zn eutectic struc- Sn–xZn Hot-dipping Brazing Holding time tures. The increase of the Zn content in these hypereutectic system temperature (°C) temperature (°C) (s) Sn–xZn solders causes more Zn phases to precipitate in the sol- Sn–3Zn 215 225 5 ders, and Sn–Zn hypereutectic solder alloys with high Zn content Sn–9Zn 200 210 5 have higher strength than Sn–9Zn eutectic solder. Based on this Sn–15Zn 230 240 5 theory, Nagaoka et al. [19] used Sn–Zn hypereutectic solder al- Sn–30Zn 315 325 5 loys to obtain high-strength aluminum joints. Sn–40Zn 340 350 5 Sn–50Zn 355 360 5 Fig. 3 shows the microstructures of different layers of Sn–xZn solders on Al base metal prepared by hot dipping technique. When the Sn–3Zn, Sn–9Zn and Sn–15Zn solders are used, their layers on Al base metal almost keep the original microstructure character- izations of the solders as shown in Fig. 3a–c, indicating that tiny Al element diffuses into the solders due to the low temperature in the hot-dipping process. However, the diffusion between Al base metal and solder accelerates as the hot-dipping temperature rises. The microstructures of Sn–30Zn, Sn–40Zn and Sn–50Zn prepared by hot-dipping technique are shown in Fig. 3d–f. It can be observed that many black phases marked with circle form in the solders, and the EPMA results indicate that the composition ranges of these phases are 13.952–20.724 at.% Al, 76.554–69.455 at.% Sn and 10.494–8.821 at.% Zn. According to the Al–Sn–Zn ternary phase diagram [20,21], it is concluded that these phases are Al–Sn–Zn solid solutions. Note that the Al element diffuses into the Sn–30Zn solder, leading to variation of Zn from rod-shaped phase (Fig. 2d) to blocky-shaped phase (Fig. 3d). This could be accounted for the fact that a small quantity of Zn dissolve into Al and Sn to form Al–Sn–Zn solid solution.

3.2. Mechanical properties and fractures of the joints

The relationship between Zn content in Sn–xZn solders and shear strength of the brazed joints is shown in Fig. 4. As can be seen from the diagram, with the increase of Zn content in the Sn–xZn solders, Fig. 1. Sketch of experimental setup (a) and dimensions of lap shear tensile the shear strength increases firstly and then decreases. When the specimen (b) with supporting plates in the shear test (mm). three solders of lowest Zn content are used, especially the solder of Sn–9Zn, the joints show insufficient strength. The shear strength mocouple was attached to the joint, to measure the actual temper- increases to a maximum average value of 70.73 MPa with the Sn– ature of the joint during the brazing process. The experiments were 30Zn solder used. Fernandus et al. [22,23] reported that the maxi- performed in a tubular resistance furnace. The argon gas was uti- mum shear strength obtained by diffusion bonding between Mg al- lized for the protection during the brazing process. The optimized loy and Al alloys without filler metal was less than 60 MPa. The brazing parameters are shown in Table 2. shear strength of the Mg/Sn–40Zn/Al joint is lower than that of The cross-sections of the brazed specimens were cut and pre- Mg/Sn–30Zn/Al joint, and the Mg/Sn–50Zn/Al joint presents unbe- pared for metallographic examination. The microstructure was re- lievably poor shear strength. vealed with an etchant (5 vol.% HCl + 3 vol.% HNO + 92 vol.% 3 Fig. 5 shows the fracture positions of varying brazed joints. It is C H OH). The microstructures of the solders were observed by 2 5 obvious that the fracture locations of the joints are different by optical microscope (OM). The microstructures and element compo- using different Sn–Zn solders. Fig. 5a–c shows the fracture posi- sitions of the brazed joints were investigated by scanning electron tions of the brazed joints used solder of Sn–3Zn, Sn–9Zn and Sn– microscopy (SEM) and electronic probe micro-analyzer (EPMA), 15Zn, respectively. All of these joints fracture at the AZ31/solder respectively. The shear strength of the specimen as shown in interfaces. In the case of Sn–40Zn and Sn–50Zn as shown in Fig. 1b was measured by universal material machine of type Fig. 5e and f, the joints fail at the 6061/solder interfaces. It means DNS100 at a cross-head speed of 2 mm/min. that all of the brazed joints mentioned above fracture at the base metal/solder interfaces. However, when the Sn–30Zn solder is used 3. Results and discussion to braze Mg/Al dissimilar metals, the joint fractures in the center of the brazing zone as can be seen in Fig. 5d. The base metal/solder 3.1. Microstructures of the solders interfaces are not the weak area in this joint. In brief, the fracture positions of the brazed joints can be classi- Metallographic structures of the Sn–xZn solders are shown fied into three types: type I joint fails at AZ31/solder interface; type in Fig. 2. According to Sn–Zn binary phase diagram [18], II joint fails in the center of brazing zone; type III joint fails at 6061/ Download English Version: https://daneshyari.com/en/article/830644

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