| As a promising and environmentally-friendly structural material in 21st century, the development and application of magnesium alloy is of far reaching importance for human sustainable development. But the existing magnesium alloys are far from the demand due to the lesser alloy systems and lower mechanical properties. In this dissertation, two new styles magnesium alloy with high tensile and high tough were developed by adding rare-earth element into AZ31 alloy system. Moreover, the welding metallurgy performance of AZ31B alloy and mechanical properties of the welded joint were studied.The effect of Y addition on microstructure, room temperature mechanical property and fracture mechanism of AZ31-Y alloy were studied. Massive Al2Y was formed and the microstrure was composed ofα-Mg, Mg17Al12 and Al2Y. The toughness of AZ31-Y alloy was improved, and the tensile strength and elongation of as cast AZ31-Y was 212Mpa and 7%, respectively. On the other hand, the tearing ridge and dimple appearanced on the tensile fracture, which revealed that the fracture mechanism transformed from cleavage fracture to quasi-cleavage fracture.The modification effect and modification mechanism of La in AZ91 alloy was studied. With additon of La element, the grain was effectively refined, and the continous, massive Mg17Al12 of being reticular transformed to be uncontinous, fine and dense. At the refining temperature of 993K, the perfect properties and the least alloy waste were realized When La addtion was 0.2%. Comparing with standard AZ91, The rigidity and impact energy, the tensile strength and the elongation of AZ91-La alloys were improved 100%, 73% and 700% respectively.The welding metallurgy performance of TIG self fluxing welding, TIG filling wire welding and MIG welding of AZ31 alloy were investigated. And the formation and characteristic of cracks in the above three welding were analyzed. Welding hot cracks of AZ31 alloy formed near the centre of the welded seam, and then growed rapidly into fusion area, in the course which branch cracks happened. The cracks and branch cracks both growed along the grain boundaries.The mechanical properties of AZ31 magnesium alloy and its welded joint were studied. The tensile strength of AZ31 plates as rolling used in testing is 285.2 Mpa. After the TIG welding, the tensile strength of welded joint reduces about 10% Compared with base metal. The tensile strengths of transverse and longitudinal cruciform welding joints are 255.9 Mpa and 258.0 Mpa respectively. Moreover, the microhardness testing result of welded joint reveals that the microhardness in the weld joint zone is higher, while that in heat-affected zone is at the least.Effect of temperatures with a rang of -80 ~ 340℃on impact toughness evolving were studied. The result indicates that in AZ31 alloy tough-brittle transition phenomena happens at about 140℃which is measured with energy standard and ductility standard. The impact property of AZ31 alloy is very sensitive to indentation, and no matter how small the indentation is, the impact toughness of AZ31 alloy decreases markedly. For example, the impact toughness of the sample with a indentation of 0.1mm depth is 15.95 J/cm2, while the impact toughness of the sample without any indentations is higher than 50 J/cm2. the result of instrument impact test reveals that the fracture processes of AZ31 magnesium alloy parent metal and welded joint are quite different.The fatigue properties of base metal, TIG butt joint, transverse cross joint and longitudinal cross joint were tested. The results indicate that when the cycle index N is 2×106 times, the fatigue resistances of base metal, butt joint, transverse cross joint and longitudinal cross joint were 57.81 MPa, 24.60 MPa, 17.25 Mpa and 20.14 Mpa, respectively. It is obvous that the fatigue resistances of the welding joints is far lower than that of base metal. Moreover, SEM analysis of the fatigue fracture finds that there is no continuous and legible fatigue stripe on the fractures of the welding joints which is mainly composed of dense cleavage step, sector pattern fracture and river pattern fracture where there are secondary cracks with different sizes.The was put forward to improv fatigue strength of welding joint of AZ31 alloy. After treated by the ultrasonic peening method, Fatigue life is improved three times in certain stress extent. The fatigue resistance of longitudinal cross joint (2×106) is improved 54.5%, which is 84.2% of parent metal and that of transverse cross joint (2×106) is improved 43.6%, which is 68.6% of parent metal.The corrosion behaviors of AZ31 magnesium alloy and its welded joints in 3.5wt% NaCl solution were studied. The corrosion potential (Ecorr), corrosion current (Icorr), corrosion rate (Ψ) of AZ31 alloy were -1.425 V, 1.093×10-2 A and 139.8 mm/year respectively, which reveals that AZ31 magnesium alloy is very easily corrupted in cl- solution. On the other hand, the corrosion resistance of the welded joint is lower than that of AZ31 parent alloy because due to some defects in the welded joint.Metal coated layer was obtained on the surface of magnesium alloy by electroless plating method, and the chemical composition and structure of coated layer were studied using XRD, SEM and TEM. The coated layer is mainly composed of amorphous structures and a few microcrystalline structure. The corrosion current and corrosion potential of the original coated layer are 4.52μA/cm2 and -250 mV respectively. Moreover, there are no corrosion spots when the coated layer is exposedin in salt mist for 8h. |
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