SOME ADVANTAGES OF BUTT JOINTS OF THIN WROUGHT ALUMINIUM ALLOYS AMg5M AND AMg6M PRODUCED BY FSW, COMPARED TO TIG-WELDED JOINTS

摘要

Comparative studies were performed of microstructure, degree of softening, ultimate strength, crack initiation and propagation susceptibility and fatigue resistance of welded joints of wrought aluminium alloys AMg5M and AMg6M 1.8 mm thick, produced by nonconsumable-electrode argon-arc welding and friction stir welding, as well as of the levels of residual longitudinal stresses developing in them. It is shown that application of FSW enables formation of permanent joint with minimum level of stress concentration in weld-to-base metal transition areas, and allows avoiding defects in the form of pores, oxide film macroinclusions and hot cracks in welds due to metal melting and solidification in fusion welding. Intensive plastic deformation of metal results in formation of a uniform disoriented structure under the tool shoulder and in the weld nugget with grain size of 3-4μm and disperse (not more than 1μm) phase precipitates, and grain elongation and distortion in the direction of plasticized metal displacement occur in the adjacent areas. This leads to increase of metal hardness in the joint zone, sample ultimate strength at uniaxial tension, their fatigue strength and lowering of crack initiation and propagation susceptibility. Lowering of temperature of welded edges heating leads to lowering of maximum level of residual longitudinal stresses in welded joints by 25%, compared to TIG welding.