Weld metal composition change during conduction mode laser welding of aluminum alloy 5182
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I. INTRODUCTION
THE 2000, 5000, and 6000 series aluminum alloys chosen for automotive applications provide substantial specific strength, good crash-worthiness, and excellent corrosion resistance.[1] Laser beam welding is characterized by high welding speed, low heat input, and low weldment distortion; it is also amenable to automation. These attributes make laser beam welding potentially attractive for the joining of automotive components in drive train assemblies, suspension components, and various body panels.[2,3] Currently, however, hot cracking, porosity, and weld metal composition change are major concerns in the welding of aluminum alloys for automotive applications.[4] Weld metal composition change caused by selective vaporization of volatile alloying elements, especially magnesium, may affect the mechanical properties, corrosion resistance, and hot crack susceptibility of the weldment.[5,6,7] The yield strength of 5000 series aluminum alloys increases linearly with the concentration of magnesium, as shown in Figure 1.[5] Loss of magnesium results in loss of the yield strength of these alloys. Cieslak and Fuerschbach[6] investigated the loss of hardness resulting from the laser welding of aluminum alloys. They found that magnesium vaporization from the weld pool results in reduced weld metal hardness as compared with an equivalently treated base metal. For the alloys 5456 and 5086, they proposed that the loss of hardness resulted from a diminished solid solution strengthening effect caused by a lower magnesium concentration. The compositional and microstructural changes across the weldment may also deteriorate the corrosion resistance of the alloy.[5] Finally, it is known that the hot crack susceptibility of aluminum alloys, as shown in Figure 2,[7] is dependent on H. ZHAO, Graduate Assistant, and T. DEBROY, Professor, are with the Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. Manuscript submitted June 8, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
their composition. For Al-Mg alloys that contain more than 1.5 pct of magnesium, loss of magnesium from the weld metal substantially increases hot crack susceptibility. Experimental and theoretical investigations have been conducted to understand the vaporization of various species from the weld pool during fusion welding of several important engineering alloys and pure metals.[8–16] The weld metal composition change depends on the vaporization flux and the melting rate, the latter often being an important factor in determining the composition change.[17] In order to achieve a quantitative understanding of the vaporization and weld metal composition change, a comprehensive model is needed. In early studies, the Langmuir equation[10,18] was widely used for the estimation of vaporization rates during welding. This simple model is useful in predicting the relative vaporization rates of various alloying elements. However, since it was derived for vaporization in vacuum, where no significant cond
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