Abstract
In this study, 2198 Al-Li alloy, a low density and high-performance material for aerospace equipment, was welded using ultrahigh-frequency pulse alternating current with cold metal transfer (UHF-ACCMT). Influence of different ultrahigh-frequency on the formation, porosity, microstructure, microhardness and tensile strength of the welded joints were investigated. The results showed that the coupled ultrahigh-frequency current generated electromagnetic force to stir the liquid metal of molten pool. The weld formation became much better with metallic luster and uniform ripples at frequency of 60 kHz and 70 kHz. The porosity was the minimum at frequency of 60 kHz. Furthermore, the molten pool was scoured and stirred by the electromagnetic force which provided the thermal and dynamic conditions for nucleation and grain refinement, the width of fine equiaxed grain zone became larger, and the number of equiaxed non-dendrite grains increased. With the grain refining and crystallize transition, the average microhardness and tensile strength of the joints at frequency of 60 kHz reached up the highest value, 116 HV0.1 and 338 MPa, respectively. The fracture of the welded joints presented the characteristics of quasi-cleavage fracture.
Highlights
Al-Li alloys are good structural materials for aerospace due to low density, high strength, excellent performance at low temperatures, and high resistance for stress corrosion
Compared with AC CMT process, the base materials exhibit less less deformation and surface of the joints are smooth and homogeneous by coupled with deformation and surface of the joints are smooth and homogeneous by AC CMT coupled with ultrahigh-frequency pulse current
The coupled ultrahigh-frequency pulse current generated electromagnetic force that played an important role in stirring the liquid metal in molten pool
Summary
Al-Li alloys are good structural materials for aerospace due to low density, high strength, excellent performance at low temperatures, and high resistance for stress corrosion. Using the advantage of cleaning oxide-film, AC CMT with positive polarity pulse (EP-CMT-phase) and negative polarity pulse (EN-CMT-phase) has been proposed to effectively control the crystallization process of the weld pool, refine grains, and further improve properties of the joints [17,18]. In alternating polarity arc welding coupled with high-frequency pulse current, reasonable frequency of pulse current is likely to strongly affect solidification structure and crystallization process of the weld pool, which will improve mechanical properties of the welded joints [21,22,23,24]. Combining test measurements and welding metallurgy, the effects of pulse current frequency on the formation, porosity, microstructure, microhardness and tensile strength of the welded joints were studied. Under the action of ultrahigh-frequency pulse current, the interaction mechanism of microstructure and mechanical properties of the welded joints was studied
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