Study On The Plane Strain Fracture Toughness Of AZ31B Magnesium Alloy And Its Welded Joints

The research and application of magnesium alloy has attracted increasingly great attention from the world academic and industrial circles As a series of advantages such as easy to recycle the high-strength lightweight, and it is known as green construction materials which have potential of development in the 21st century . Magnesium alloy has become the third largest construction materials after after steel and aluminum .However, connectivity problems must face up to when was widely used as structural members. Welding would be a preferred connection method. Plane strain fracture toughness KIC is the parameters which characterize the ability to resist the crack extension , and one of the essential basic parameters to measure material properties during modern engineering design. In this article, KIC of magnesium alloy and its welded joints were studied for the fracture toughness is important factor that affects the security of welded structural components.Three-point bending (SEB) tests have been carried out on AZ31B magnesium alloy and its welded joints and the plane strain fracture toughness were studied in this paper. The Force vs. crack opening displacement curves (P-V) curve of AZ31B magnesium alloy base metal specimens, weld center and HAZ of AZ31B magnesium alloy TIG welded joints specimens , and vertical joints (cracks across the weld) specimens were tested in this paper. And their value in plane strain fracture toughness KIC were calculated. Their micro and macro fracture morphology were analyzed and compared, and the impact of the extrusion direction on value of plane strain fracture toughness also specifically considered. The three-point bending (SEB) tests have been respectively carried out on the specimens which extrusion direction of the notch orientation parallel and vertical to, and angle of 45°to the .and P-V curves were obtained and the value of their KIC were calculated. The value of the KIC of these specimens which notch orientation parallel to extrusion direction range from 8.94 MPa m ~ 10.13 MPa m . That of these specimens which notch orientation angle of 45°to extrusion direction range from 9.06 MPa m ~ 10.15 MPa m . And that of these specimens which notch orientation vertical to extrusion direction range from 8.91 MPa m ~ 11.71 MPa m .The three-point bending (SEB) tests have been respectively carried out on the welded center and HAZ of magnesium alloy AZ31B specimens, their P-V curves were obtained and the value of their KIC were calculated. Their value of KIC respectively range from15.75 MPa m ~ 18.47 MPa m and 12.89 MPa m ~ 14.67 MPa m .The three-point bending (SEB) tests have been respectively carried out on the vertical welded specimens, their P-V curves were obtained and the value of their KIC were calculated. Their value of KIC respectively range from 15.74 MPa m ~ 16.45 MPa m .The value of KIC of each specimens were compared and analyzed, and the performance of the resistance to crack growth are lowered accordingly: AZ31B magnesium alloy weld center specimens, the vertical welded specimens, HAZ specimens, the notch orientation vertical to extrusion direction specimens, the notch orientation tilted 45°to extrusion direction specimens, and the notch orientation parallel to extrusion direction specimens .These fracture of each specimen from the beginning to the end of the crack were observed by Scanning electron microscopy (SEM). And found that most of the fracture overall showed a gradual increase cleavage fracture morphology. In a few fractures, there are fatigue striations, which are due to the complex role of the internal stress and external force during pre-fatigue crack making. The value of KIC of AZ31B magnesium alloy and its welded joints were smaller, and uniquely associated with the observed cleavage fracture morphology.By taking a look into the macro pictures of the fracture of each specimens, we can see that Plastic deformation were found in fracture of Nearly all specimens except for the which notch orientation parallel to the extrusion direction. And the fracture of specimens which notch orientation parallel to the extrusion direction are flush, almost no deformation, while in the fracture of other specimens, there are hill-like torn edges on both sides of the thickness position . These torn edges were about 3mm high. And there is a positive correlation between the size of the plastic deformation and the value of KIC.