Elsevier

Materials & Design

Volume 31, Issue 6, June 2010, Pages 3121-3126
Materials & Design

Short Communication
Effect of laser offsets on joint performance of laser penetration brazing for magnesium alloy and steel

https://doi.org/10.1016/j.matdes.2009.12.035Get rights and content

Abstract

The experiments of laser penetration brazing for magnesium alloy and steel were carried out, and the effect of laser offsets on mechanical properties and microstructure of the welds was investigated. In the range of 0.4–0.8 mm, with the increase of laser offset, the tensile strength of the joints increased firstly and reduced afterward. In particular, at 0.6 mm laser offset, the average tensile strength could reach a maximum of 185 MPa, which was attributed to the defect-free joining of magnesium alloy to steel. If laser offset exceeded the optimized range (0.5–0.7 mm), the defects of welding crack or incomplete fusion would occur at the interface. As it could be noticed from hardness distribution, a rapid increase in hardness was found near the interface, which had some relations with the coarse microstructure and high element concentration of the interface.

Introduction

As the lightest metal material, magnesium and its alloys have been suggested in automotive, airplane and aerospace to improve the fuel efficiency, decrease air pollution by reducing its self-weight [1], [2], [3], [4]. It is well known that steels are the most common materials in modern industry. Hybrid structures of magnesium alloy to steel have a widely applied prospect in the field of automotive industry and aviation because of their unique characteristics, such as low weight, high performance, saving energy and so on. Therefore, it is receiving a remarkable attention to joining magnesium alloy and steel dissimilar metals.

It is well known that it is difficult to join magnesium alloy and steel by conventional welding technologies. The reason can be attributed to the great difference in their melting points and the nearly zero solubility of magnesium alloy and steel [5], [6]. As a solid-state welding technology, friction stir welding (FSW) process [7] has some advantages on the joining of dissimilar metals [8], [9], [10]. Chen and Nakata [5] studied the effect of tool geometry on microstructure and mechanical properties of friction stir lap welded AZ31 magnesium alloy and steel. It was found that the joints welded using a long probe fractured at the stir zone of magnesium alloy side, while those welded using a short probe fractured at the joining interface [5]. Watanable et al. [11] investigated the FSW of AZ31 magnesium alloy and SS400 steel, and reported the effect of rotation speed and pin position on the strength and microstructure of FSW butt joint. On the other hand, despite the difficulty of joining magnesium alloy to steel, some studies had been done by fusion welding process. The laser–TIG hybrid welding of AZ31B magnesium alloy and 304 steel had been investigated [6], [12]. It was found that the maximum strength of the joints was about 95 MPa, showing that the metallic oxides at the interface were the reason for the poor tensile strength of joints [6].

One of the advantages of laser welding versus other fusion welding, is the ability to provide high energy-density and low heat-input [13]. Therefore, it is a promising technique for joining magnesium alloy to steel by controlling reaction time and molten pool size [14], [15], and limiting the mixing of magnesium and steel. However, if laser beam radiates directly on the groove of butt joints, laser beam can make magnesium alloy and steel melt simultaneously because of its high energy intensity. As a result, it is very difficult to obtain the effective joining because of no intersolubility and reaction between magnesium alloy and steel.

Based on the experiments of laser welding, the technology of laser penetration brazing (LPB) for magnesium alloy and steel was developed. In the process, laser beam offset and radiated on the magnesium alloy side of butt joint, to melt magnesium alloy achieving full penetration, and to limit the melting of steel. The molten magnesium alloy, as filler metal, formed a laser welding–brazing joint of magnesium alloy to steel, which was called as laser penetration brazing.

The present work investigates the effect of laser offsets on laser penetration brazing for magnesium alloy and steel. The purpose of this investigation is to reveal the relationship between mechanical properties and microstructure of the welds, and to provide some foundations for selecting the proper laser offset and improving the welding quality.

Section snippets

Experimental procedure

AZ31B magnesium alloy (containing (wt.%): 2.5–3.5Al, 0.5–1.5Zn, 0.2–0.5Mn, 0.1Si, balance Mg) and Q235 steel (containing (wt.%): 0.14–0.22C, 0.3–0.65Mn, 0.3Si, balance Fe) were used as base metals in the investigation. The width and length of joining sheets were 40 mm and 100 mm, respectively. Considering the vaporizing loss of element Mg (Tvap = 1090 °C) and the collapsing of molten magnesium alloy, the thickness of selected magnesium alloy sheet (2.4 mm) was more than that of steel sheet (1.7 mm).

Effect of laser offsets on tensile properties

The tensile strength testing results of the joints at different laser offsets are presented in Table 2. The statistical data has been treated by analysis of variance (ANOVA). Fig. 2 shows the effect of laser offsets on average value and standard deviation of tensile strength. In the range of 0.4–0.8 mm, with the increase of laser offset, the tensile strength of the joints increases firstly and reduces afterward. In particular, at 0.6 mm laser offset, the average tensile strength can reach a

Conclusion

By investigating the effect of laser offsets on joint performance of laser penetration brazing for 2.4 mm-thick AZ31B magnesium alloy and 1.7 mm-thick Q235 steel, the major conclusion can be summarized as follows:

  • (1)

    Laser offset has a significant effect on the tensile strength of the joints. In the range of 0.4–0.8 mm, with the increase of laser offset, the tensile strength of the joints increases firstly and reduces afterward. In particular, at 0.6 mm laser offset, the average tensile strength can

Acknowledgments

The authors are very grateful to Prof. Chen Yanbin and Associate Prof. Wang Qing for their contributory assistance. This work is supported by Heilongjiang Postdoctorial Fund (LBH-Z08248), China.

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