Fusion zone grain refinement in aluminum alloy welds through magnetic arc oscillation and its effect on tensile behavior
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Fusion Zone Grain Refinement in Aluminum Alloy Welds through Magnetic Arc Oscillation and Its Effect on Tensile Behavior G.D. Janaki Ram, R. Murugesan, and S. Sundaresan (Submitted 11 September 1998; in revised form 6 April 1999) Grain size reduction in weld fusion zones confers the advantages of an increased resistance to solidification cracking and an improvement in mechanical properties. Oscillation of the welding arc through an imposed alternating magnetic field is one of several approaches to modify weld solidification structures. In this study, gas tungsten arc welds were produced in two high strength, age hardenable aluminum alloys with and without an external magnetic field. Metallographic characterization revealed the degree of structural refinement produced by magnetic arc oscillation. The decrease in grain size was found to increase tensile elongation, while the effect on strength and age hardening response was only meager. The improvement in ductility was partially maintained in the peak aged condition also.
Keywords
aluminum alloys, grain refinement, welding
1. Introduction The formation of fine equiaxed grains in weld fusion zones helps in reducing solidification cracking and in improving the mechanical properties of the weld metal including ductility, fracture toughness, and fatigue life (Ref 1, 2). While in relation to castings, grain refinement techniques are seldom practiced in the welding industry, many methods for controlling the grain structure in weld metals have been reported in literature. These include inoculation by the use of grain refining agents, vibration of the welding torch, current pulsation, and magnetic arc oscillation (Ref 2). If the natural convection existing in the weld pool could be enhanced by artificial agitation, advantages in terms of grain refinement could be realized. One method of inducing such disturbance is to cause the welding arc to oscillate by using an alternating external magnetic field. It is well known that the interaction of the arc current and its own magnetic field leads to Lorentz forces that cause fluid flow and a self-induced stirring effect, particularly when the current density is large. Reinforcing the natural flow with an external magnetic field enhances this effect. The forces on the arc due to the external field depend on their relative orientation. If the field is coaxial with the arc, the induced forces will be perpendicular to both the magnetic field and the radial component of the diverging current through the arc. This will result in the rotation of the arc and in an annular flow of the molten metal in the weld pool. The flow will be reversed periodically if an alternating magnetic field is used, thus leading to electromagnetic stirring (EMS). In the situation where the external magnetic field is oriented parallel to the welding direction, the interaction of this alternating field with G.D. Janaki Ram, R. Murugesan, and S. Sundaresan, Department of Metallurgical Engineering, Indian Institute of Technology, Madras 600
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