On the weldability, composition, and hardness of pulsed and continuous Nd:YAG laser welds in aluminum alloys 6061,5456,
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I. INTRODUCTION LASERwelding of aluminum alloys is inherently difficult due to the low absorptivity of aluminum in the wavelength range of commercial laser welding systems. ~,2 Among the higher power (>0.1 kW) laser welding systems, the solid state Nd:YAG laser offers better absorptivity than the CO2 laser. ~ Thus, the Nd:YAG laser offers greater latitude in those processing situations involving small hardware and weld joints, where controlled penetrations of less than 0.5 mm are often required. Heat transfer restraints due to the solid state operation of the Nd:YAG laser presently limit its output power to generally less than 1 kW. To achieve higher short-term powers, pulsed Nd:YAG lasers have been developed. 25 Joules per pulse at a pulse frequency of 10 Hz is not uncommon. A representative laser pulse of 5 msec would then have an effective power of 6 kW. For a beam focused to a spot diameter of 0.05 cm, the power density would be > 106 watts/cm 2. This high energy fluence may affect the weld pool chemistry by evaporating alloying elements which have a high vapor pressure. This phenomenon was observed recently by Khan and DebRoy 3 for continuous wave CO2 laser welds on 202 stainless steel. In the case of 202 stainless steel, the volatile component was Mn. Other fusion welding processes have been shown to result in alloying element vaporization. Schauer et al. 4 have shown that Mg and Zn evaporation occurs during electron beam welding of aluminum alloys and that this vaporization limits the weld pool surface temperature. Using infrared pyrometry, single pass weld pool surface temperatures of ~ 1525 K to 2075 K were measured. More recently, Block-Bolten and Eagar 5 have observed vaporization of Mg and Zn during stationary gas-tungsten-arc (GTA) welds. Blake and Mazumder6 have also observed Mg evapoM.J. CIESLAK and P.W. FUERSCHBACH are with the Process Metallurgy Division of Sandia National Laboratories, Albuquerque, NM, 87185. Manuscript submitted July 10, 1987.
METALLURGICALTRANSACTIONS B
ration during high-power, continuous wave (7 to 8 kW) CO2 laser welding of aluminum Alloy 5083. They have suggested a method for control of Mg evaporation by imposing a high pressure gas jet on the surface of the pool during welding. It is clear from a review of the literature that while high-power, continuous wave CO2 laser welding has been studied in some detail analogous work on pulsed and continuous Nd:YAG laser welding does not exist. There are several possible metallurgical ramifications of alloying element(s) vaporization. From a welding perspective, hot-cracking susceptibility may be affected. Early work by Jennings et al. 7 established that the hot-tearing tendency of aluminum alloy castings is a strong function of composition. In the specific cases of A1-Mg, A1-Si, and A1-Mg-Si alloys, the resistance to hot tearing is increased as alloying element content increases. This knowledge led to the development of both wrought alloys with improved weldability (5086, 5083, 5456) and superior welding consumable alloys (4043,404
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