| Magnesium alloy has a series of advantages such as low density, highstrength, being prone to recycle, etc.. Although it has been expected to be themost promising structure materials in modern high and new technology, thewelding problems have become one of bottlenecks of its wide application. Atthe point of view of fracture mechanics of engineering materials, somecharacteristics of magnesium alloy will directly influence its mechanicalproperty as welded structures. So it is important to make further studies on thefracture behavior and mechanism of magnesium alloy and its welded joint underimpact load and it is also one of the urgent problems that should be solved.These studies will be of extraordinary importance from both fundamental andtechnological considerations for its wide application.The tensile strength of AZ31 magnesium alloy plate of 8 mm thickness ofrolling condition and its welded joint was studied. The results showed that thetensile strength of AZ31 magnesium alloy plate was 234.48 MPa and that of thebutt joint was 185.68 MPa, which was 79 percent of that of the base metal. Andthe tensile specimen of butt joint fractured at HAZ because of embrittlement ofcoarse grain in this area, which would be one of the key factors of poormechanical properties of the welded joint of magnesium alloy. These studies hadimportant directing action for the design of magnesium alloy welded struture.In the range of -80℃to 340℃, the notch impact toughness of AZ31magnesium alloy plate was studied. The results showed that, in the range of -80℃to 80℃, the impact energy of AZ31 magnesium alloy was very low and thevalues of impact toughness were only from 5.48 to 8.75 J·cm-2 and fracturesurfaces were quasi-cleavage patterns of brittle rupture; in the range of 80℃to200℃, the values of impact toughness were from 8.75 to 17.44 J·cm-2, fracture surfaces were quasi-cleavage and dimple patterns of admixture rupture; in therange of 200℃to 260℃, the values of impact toughness were about 18 J·cm-2and the fracture surfaces were many dimples patterns of ductile rupture; whentemperature exceeded 260℃, the impact toughness began to decrease becauseof high temperature softening of AZ31 magnesium alloy and the fracture patternwas still dimple. In the range of -80℃to 260℃, AZ31 magnesium alloyhaving ductile-brittle transition phenomenon was found and the ductile-brittletransition temperature was about 140℃. This was one of finds of this paper.The notch sensitivity of AZ31 magnesium alloy of different surfacesituation at room temperature was studied. The impact toughness of the impactspecimen without notch was greater than 50 J·cm-2 and that with a notch of 0.1mm depth was only 15.95 J·cm-2. The impact toughness of AZ31 magnesiumalloy decreased greatly with increase of notch depth, and the minimum was 7.44J·cm-2. In consideration of the effect of notch stress concentration on impacttoughness, the stress concentration coefficient was stimulated with finiteelement modeling (FEM) and then the impact toughness was amended. As aresult, impact toughness increased with the notch depth, which indicated thatAZ31 magnesium alloy was very sensitive to notch. This was also one of findsof this paper.The notch impact toughness of the TIG-welded butt joint of AZ31magnesium alloy was studied. The weld metal had different impact toughness inwelding direction and in thickness. The maximum of 8.66 J·cm-2 of impacttoughness of the former is in the seam center, which was bigger than that of thebase metal of 8.01 J·cm-2, and HAZ was the weak link of the whole joint. Impacttoughness of the latter had a minimum of 6.21 J·cm-2 in the seam center. Theimpact toughness increased dramatically with the distance from the centerlinetill it reached its peak value and then became even gradually. The fracturesurface of AZ31 magnesium alloy welded joint was quasi-cleavage pattern andmany little dimple pattern.Oscillography impact test was adopted to study the impact fracture processof magnesium alloy base metal and welded joint. The results displayed that fracture process of AZ31 magnesium alloy as base metal and welded joint wasvery different. The force-displacement curve of the impact specimen of basemetal without notch was E type and all force eigenvalues such as Fgy,Fm,Fiu andFa were in existence while that with a notch along the welding direction in thebase metal, seam center and the part 3 mm away from the centerline were all Dtype, only Fgy,Fm were in existence. Force-displacement curves of the last twowith a notch in thickness were all B type, and only Fm was in existence. Thesedifferent type curves were of great value for studying impact fracturemechanism.Electro-chemical corrosion property of AZ31 magnesium alloy and itswelded joints were studied. The results showed that Ecorr and icorr of the basemetal were -1.425 V and 0.01093 A respectively and Ecorr of the weld metal,welded by TIG with 3 different currents, were -1.497 V, -1.489 V, -1.468 Vrespectively, and icorr were 0.02323 A, 0.01645 A, 0.01598 A respectively. It wasapparent that the corrosion resistance of the weld metal was poorer than that ofthe base metal. |
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