Magnesium Loss in Nd:YAG Pulsed Laser Welding of Aluminum Alloys
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RODUCTION
PULSE laser welding is now considered a favorable process to be applied to Al alloys due to advantages, including high heat intensity and low heat input. The presence of Mg in Al alloys increases strength through solid solution strengthening; the presence of both Mg and silicon produces a compound of Mg-silicide (Mg2Si). In addition to strength, the resistance of the weld metal to solidification cracking is affected by Mg content.[1] Michaud et al. measured the effect of base metal composition on solidification cracking in pulsed laser welding of Al-Cu alloys.[2] The loss of alloying elements can also result in significant changes in the microstructure and degradation of mechanical properties of the weld.[3–5] Vapor pressures of Al and Mg at 1473 K were 2 Pa and 2.5 9 105 Pa, respectively.[6] It is reported that the temperature of the weld pool in pulsed laser welding can reach 2273 K and Mg is evaporated
Z. MALEKSHAHI BEIRANVAND, F. MALEK GHAINI, and H. NAFFAKH-MOOSAVY are with the Department of Materials Engineering, Tarbiat Modares University, Tehran 14115-143, Iran. Contact e-mail: [email protected] M. SHEIKHI is with the Department of Materials Engineering, Bu-Ali Sina University, Hamedan 65178-38695, Iran. M.J. TORKAMANY is with the Iranian National Center for Laser Science and Technology (INLC), Tehran 14665-576, Iran. Manuscript submitted December 4, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
both in conduction and keyhole modes.[6,7] However, a higher level of Mg evaporation during keyhole mode is also reported.[8] Tenner et al. developed an experimental and numerical model for measuring the density of metal vapor and pressure inside the keyhole during laser welding.[9] The evaporation rate of volatile elements was mainly controlled by equilibrium between vapor pressure and temperature.[6,10–12] Hence, in this respect, it is essential to estimate the surface temperature of the melt pool.[6] However, there are other factors such as temperature distribution profile, chemical composition, and weld surface area that can affect the evaporation rate.[13] By means of computed temperatures, mass loss, due to the vaporization of alloying elements, has been calculated.[4] In another research, the physical processes influencing the changes in chemical composition due to the evaporation in the weld seam during laser beam welding were analyzed.[14] Langmuir presented a simple equation to calculate vaporization of a pure metal in a vacuum; this approach has been employed to estimate the vaporization fluxes of alloying elements.[6,15,16] Block-Bolten and Eagar calculated the evaporation rate of alloying elements in GTA welding of Al alloys using Langmuir equation. Likewise, Collur et al. studied the evaporation mechanisms of alloying elements during laser welding in the conduction mode.[17] It was shown that Mg evaporation could occur in both continuous and pulsed Nd:YAG laser welding of 5086 and 5456 Al alloys, and that the amount of Mg loss in weld metal was affected by welding parameters.[7] In other
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