The Research On Friction Stir Welding Process Of Magnesium Alloy And Corrosion Protection Of The Weld

Magnesium alloy as a newly high-performance structural material is inevitable to use welding structure in practical application. The traditional welding method may cause residual stress after welding, and stress corrosion cracking sensitive, but also easy to produce gas pores, slag inclusion, spatter and other defects. These problems greatly limit the applications of magnesium and magnesium alloy in the aviation, aerospace, automotive and ship, which have become urgent problems to be solved. In addition, as a result of the chemical active, the weld zone corrosion of magnesium alloy is also one of the key factors which restrict the applications in various fields. Friction Stir Welding (FSW) technology as a new type solid-state connection method has incomparable advantages. It does not cause melting process in the joint position, so various kinds of welding defects caused by metal melting do not exist. Therefore, it is of great theoretical significance and practical value to research the friction stir welding process of magnesium alloy and corrosion protection of the weld.In this paper, the welding process of AZ31B magnesium alloy and AZ31B magnesium alloy added with 0.5%Ce were systematically researched using friction stir welding (FSW), and the friction stir welding process of magnesium alloys was optimized with orthogonal test. Studied the microstructure, mechanical properties and corrosion resistance of the welded joints, in order to obtain well corrosion resistance weld by adding rare earth elements and using anodic oxidation treatment methods based on the poor corrosion resistance of magnesium alloy weld, and made comprehensive evaluation on the two ways improving the corrosion resistance of the weld.First, through the research on the friction stir welding process of AZ31B magnesium alloy, it was found the rotation speed of the stir head (R), welding speed (v) and the press amount of the shoulder are the most important factors impacting forming of the welded joints.1) It could make the weld temperature close to the melting temperature and lead to the weld local overheating and even melting when the rotation speed of the stir head is too fast or the welding speed is too slow. Otherwise, the weld will have tunnel-type defects in the internal weld or trench on the surface when the rotation speed of the stir head is too low or the welding speed is too fast; Defects such as loose structure, gas holes, tunnel inside the weld or trench on the weld surface are easily formed when the press amount of the shoulder is not enough, even lead to the metal in the weld zone spillover due to the shoulder can not play a closed function role; when the press amount of the shoulder is too big, which is bound to cause the stir head subject increasing frictional resistance, and easily lead to weld depression, flash and burr on weld surface.2) It was found that weld with good formation on the surface and without gas holes and tunnel-type defects inside was obtained when the rotational speed between 1200r/min and 1500r/min, the welding speed between 30mm/min and 60 mm/min, the press amount of the shoulder between 0.15mm and 0.2mm through the welding process test.3) The optimum technological parameters on friction stir welding of AZ31B magnesium alloys were obtained by orthogonal experiment analysis as following: the press amount of the shoulder 0.19mm, the rotational speed 1400r/min, the welding speed 40mm/min; For AZ31B magnesium alloys added 0.5%Ce, the optimum technological parameters as follows: the press amount of the shoulder 0.17mm, the welding speed 40mm/min, the rotational speed 1300r/min.4) The results based on verification test of the optimum technological parameters on friction stir welding of AZ31B magnesium alloys added 0.5%Ce were as follows: the tensile strength of welded joint was 250.76MPa, 92.7% of base metal tensile strength, elongation at break was 8.56%, the fracture location of the welded joint was heat-affected zone.Second, the main conclusions were obtained by microstructure analysis, mechanical properties analysis, fracture morphology analysis and XRD analysis on the welded joints of AZ31B magnesium alloys and AZ31B magnesium alloys added 0.5%Ce as follows:1) Base metal of AZ31B magnesium alloy was mainly consist of dynamic recrystallization grains, but it was clear there were deformation twins and coarse-grains existing in the microstructure as well as elongated grains in the rolling process. Added with 0.5%Ce, the grains of AZ31B magnesium alloy can be seen refined obviously, more uniform distribution, only containing a small number of deformation twins and coarse-grains.2) FSW welded joints of AZ31B magnesium alloy and AZ31B magnesium alloy added with 0.5%Ce mainly consist of weld nugget zone (WZ), thermo-mechanical affected zone (TMAZ) and heat affected zone (HAZ). The structure in the weld nugget was well distributed and equiaxed grains due to dynamic recovery and recrystallization after friction stir welding. The grains in the TMAZ have also become equiaxed and recrystallized, the grains in the TMAZ were not as fine as the weld nugget zone which ware characterized by deformed and elongated grains. The HAZ did not recrystallized, the grains in the HAZ were coarse relatively simply due to the role of heat conduction.3) The majority of the FSW welded joints were observed to fail in the heat affected zone, the failure of the tensile samples suggested a 45°shear fracture. For AZ31B magnesium alloy, the tensile strength of friction stir welded joints(No.6)under the conditions of rotational speed 1300rpm, welding speed 30mm/min and press amount of the shoulder 0.19mm was best, its tensile strength was 257.35MPa, 87.9% of the base metal, its elongation at break was 6.16%. The tensile strength of No.6 ample weld was 261.57MPa,89.3% of base metal. For AZ31B magnesium alloy added with 0.5%Ce, the tensile strength of friction stir welded joints(No.2)under the conditions of rotational speed 1400rpm, welding speed 40mm/min and press amount of the shoulder 0.19mmwas best, its tensile strength was 237.97MPa, 88.0% of the base metal, its elongation at break was 6.63%. The tensile strength of No.2 sample weld was 270.02MPa,99.9% of base metal, almost equated with the tensile strength of base metal.4) The tensile fracture of FSW welded joints was ductile-brittle mixed fracture. Dimple number increased and cleavage crack decreased obviously with the welding heat input increased, the fracture mode changed from ductile-brittle mixed fracture to ductile fracture.5) The micro-hardness of AZ31B magnesium alloy welded joints (No.6) distributed between 55.8Hv and 67.9Hv, and the micro-hardness of AZ31B magnesium alloy added with 0.5%Ce welded joints (No.2) distributed between 58.3Hv and 69.2Hv. The welded joints added with 0.5%Ce had bigger micro-hardness than which without Ce. The weld nugget zone had bigger micro-hardness than TMAZ and HAZ, but its micro-hardness was lower than base metal. The grain size of TMAZ and HAZ was coarse relatively, so the micro-hardness was lower than weld nugget zone. And micro-hardness increased from the welded joints upper surface of to lower surface.6) Weld zone of AZ31B magnesium alloy was mainly composed ofα-Mg phase and a very small amount ofβ- Mg17Al12 phase. Added with 0.5%Ce, the weld zone was mainly composed ofα-Mg phase and a small amount ofβ- Mg17Al12 phase too, in addition, the weld contained a small amount of Rare Earth phase Al4Ce.Finally, compared and analyzed the corrosion resistance of base metal, welded joints and welded joints added with Ce, as well as welded joints after micro-arc oxidation treatment by chemical and electrochemical methods. The conclusions were as follow.1) The corrosion of base metal and FSW welded jionts by salt water was localized corrosion and uneven, some black precipitate was produced on the surface of the specimens after corrosion test. Energy spectrum analysis shows that chemical elements of the corrosion products were mainly consist of Mg, O, Cl, and the composition was almost the same, Mg oxide as the main ingredient.2) The corrosion potential of base metal and welded joints AZ31B magnesium alloy added with 0.5%Ce increased by about 250mV, 250mV than AZ31B magnesium alloy. The rare earth Ce can improve corrosion resistance of the base metal and FSW welded joints.3) The coatings morphology of AZ31B magnesium alloy and AZ31B magnesium alloy added with 0.5%Ce FSW welded joints after micro-arc oxidation treatment were almost the same, porous ceramic-like layer formed on the samples surface. Energy spectrum analysis shows that the MAO coatings composition of AZ31B magnesium alloy and AZ31B magnesium alloy added with 0.5% Ce FSW welded joints were basically the same, the chemical elements of the MAO coatings were mainly consist of Mg, Al, O. XRD analysis, The micro-arc oxidation coatings were mainly composed of MgAl2O4 phase by XRD analysis.4) The corrosion resistance of AZ31B magnesium alloy and AZ31B magnesium alloy added with 0.5%Ce FSW welded joints had increased significantly, and the corrosion potential of FSW welded joints increased by about 500mV, 600mV after micro-arc oxidation treatment. In addition, the MAO coatings of FSW welded joints of AZ31B magnesium alloy added with 0.5%Ce had better corrosion resistance than the coatings of welded joints AZ31B magnesium alloy, and corrosion potential of MAO coatings increased by about 150mV. Added with 0.5%Ce then micro-arc oxidation treatment, the welded joints could have excellent corrosion resistance.