Effect of thermo mechanical treatment on Al-Mg2Si alloy containing magnesium in excess

The effects of the pre aging and the deformation on the Al-1.0mass%Mg2Si-0.4mass% Mg (excess Mg) alloy were investigated by hardness measurement, tensile test and transmission and scanning electron microscopy to improve its mechanical properties, especially elongation. The time achieving to the maximum hardness (Uax) in the specimen with deformation by 1% prior to final aging at 473K was shortened and the value of maximum hardness (HVmax) was not changed to be compared with that in the specimen without deformation. In the tensile test of this specimen, the value of the elongation was higher, and values of the ultimate tensile strength (U.T.S.) and the 0.2% proof stress (0.2% P.S.) were slightly lower than those of the specimen without deformation. The value of U.T.S., 0.2% P.S. and elongation of the specimen final aged at 473K to HVmax after pre aging at 423K for 6ks were higher than those of the specimen without pre aging. The thermo mechanical treatment (TMT), which means combination of the deformation by 1%, pre aging at 423K for 6ks and final aging at 473K to HVmax, improved U.T.S., 0.2%P.S. and elongation of the specimen and a total time of aging treatment was shortened. The TMT specimen was almost revealed the fracture surface of the transgranular fracture, while fracture surfaces of specimens improved their elongation by pre aging or deformation prior to final aging at 473K to HVmax was mixed feature with the transgranular fracture in the intergranular fracture. The width of the precipitate free zone in the TMT specimen was more narrow than those of specimens either with deformation or with pre aging. When manganese of 0.5mass% was added to the excess Mg alloy, the grain size of this alloy was refined from 550/jm to 50/mi and its U.T.S., 0.2%P.S. and elongation were further improved. Finally, 290MPa of U.T.S. and 18% of the total elongation were obtained in the excess Mg alloy containing 0.5mass%MnbyTMT.