Friction Stir Welding Of Magnesium Alloy And Research On Microstructure And Properties

Nowadays, the joint of magnesium alloy is completed by conventional fusion weldingand riveting, therefore hot crack, hole, a large deformation and a poor joint performanceoccur. As a new solid joint method, friction stir welding has a series of incomparableadvantages, and provides a new solution for the welding of magnesium alloy. In the thesis,the10mm thick Mg-Gd-Y magnesium alloy plates are friction stir single-welded anddouble-welded, and the microstructure and mechanical properties are analyzed. The AZ31magnesium alloy plates are welded by friction stir T-type welding, after observing themicrostructure, the effect of the process parameters on mechanical properties is discussed.For the10mm thick Mg-Gd-Y magnesium alloy plates, when the rotation speed is400r/min and the welding speed is120mm/min, the friction stir single-welding butt jointcan be achieved. According to the Z factor, weld nugget zone is judge the dynamicrecrystallization occurs, generating tiny uniform grain size, and grain size reducing in turnfrom top to bottom. The dynamic recrystallization partly occurs in the thermal-mechanicalaffected zone. Affected by thermal cycle alone, the grains in heat affected zone grow up.Base material does not change. The straight three bifurcation boundary is observed in TEMmorphology of parent metal grain, and no grain of high density of dislocations is found.Because of nano-scale precipitated phase β’, there is an obvious precipitation strengthening.A high density of dislocation and the crystal boundary exist in welding nugget zone, sodoes the completed dynamic recrystallization grain, and different grains are at differentstages of recrystallization. Heat affected zone has low density dislocation areas and largeangle grain boundary after multilateral. Due to large amounts of β phase and incoherentwith matrix, the strength and hardness decrease.Micro-hardness has a w-shaped distribution in horizontal direction, the highest parentmetal, welding nuclear area second, the advancing TMAZ the lowest. In the thicknessdirection, micro-hardness increases from the weld surface to bottom gradually. Themaximum tensile strength of joint is264.0MPa, equal to88%of the parent metal. Tensilefracture is located in the advancing TMAZ. The highest impact toughness is in the WNZ,HAZ secondly, lowest parent metal.When strain rate is over103s-1, strain rate effect in HAZ and BM is not obvious, whilein WNZ there is a certain degree. At the similar strain rate, the compression strength of BM is the highest, and HAZ is the lowest. The BM fracture strain is close to HAZ. Impactabsorption energy in WNZ is the highest, and in HAZ is the lowest.WNZ grain gets agrown up. There are lots of twins, and quantity increases after decreases along with theincrease of strain rate. Impact plastic deformation in WNZ is slip, while in HAZ and BMslip and twinning coexist.When the rotation speed is600r/min and the welding speed is80mm/min, the frictionstir double-welding butt joint can be achieved."S" line is found in both directions.Dynamic recrystallization occurs in WNZ, generating tiny uniform grain size. Part of thedynamic recrystallization and plastic deformation occur in TMAZ. Under the effect ofthermal cycle, HAZ grains grow up.Micro-hardness has a w-shaped distribution in the horizontal direction, themicro-hardness is probably same between parent material and WNZ, TMAZ the least.TMAZ are on both sides of welding center in the same direction welding, causing the leastto be on both sides, while the different direction welding is in the same side.Micro-hardness distribution is V-shape, top and bottom surface highest, overlap zone in themiddle lowest. Compared to different direction welding and single-welding, the samedirection has a higher tensile strength, equal to95.2%of the parent metal.Tensile fracture of same direction welding is in WNZ, while the different direction isin TMAZ. Shock absorption function, and impact toughness are highest in WNZ, HAZsecondly, parent metal lowest. Impact performance of the double-sided welding is betterthan the single-side welding and the parent metal.For the12mm thick AZ31magnesium alloy plates, when the rotation speed is400r/min and the welding speed is100mm/min, the friction stir T-type welding can beachieved. Dynamic recrystallization occurs in WNZ, generating tiny uniform grain size.Dynamic recrystallization partly occurs in TMAZ. Under the effect of thermal cycle, HAZgrains grow up. Parent material does not change.The maximum tensile strength and bend strength separately are196.7MPa and310.3MPa, equal to73.4%and72.7%of the parent metal. With the increase of rotation speed,welding speed and P value(rotation speed/welding speed),tensile strength and bend strengthincrease firstly, then drop. When the ratio P is equal to4, they are the maximum.Micro-hardness distribution shows "VW" shape, and WNZ is the highest, plate centerlowest, the bottom higher than the top.