Effect of Post-weld Heat Treatment on Mechanical Characteristics of AZ31 Magnesium Alloy Welded Joints

Tungsten inert gas（TIG） welding was performed on 2.7 mm thick commercial extruded AZ31 B magnesium alloy plates. We investigated the effect of post-weld heat treatment（PWHT） on the microstructure, mechanical properties and precipitated phase of the weld joints. The results showed that during the annealing treatment（200 ℃-1 h, 250 ℃-1 h, 300 ℃-1 h, 350 ℃-1 h, 400 ℃-1 h, and 450 ℃-1 h）, the average grain size in the weld seam was the minimum after annealing at 400 ℃ for 1 hour, and then abnormally grew up after annealing at 450 ℃ for 1 hour. The mechanical properties enhanced when the joints were processed from 200 ℃-1 h to 400 ℃-1 h but sharply decreased with increasing annealing temperature. In contrast to the annealing treatment, solution treatment（250 ℃-10 h, 300 ℃-10 h, 350 ℃-10 h, 400 ℃-10 h, and 450 ℃-10 h） exhibited a better ductility but a slight deterioration in tensile strength. Especially speaking, no eutectic compounds（such as Mg17 Al12） were observed in the weld seam. The supersaturated Al atoms were precipitated in a coarse spherical shape dispersed in the weld seam. The precipitated Al atoms dissolved in the matrix substances at the condition（400 ℃-1 h） or（250 ℃-10 h）. The solution treatment caused grain coarsening and precipitated Al atoms dissolved in the weld seam substantially, which resulted in a drop in micro-hardness at the weld seam compared to the area of the annealed joints.

Novel Coil Winding Method to Realize Pot Heated Evenly

To solve the problem about the inhomogeneous thermal effect of pot heated by coils along the circumference, a novel coil winding method is proposed and compared with the general winding method in the paper. First, based on the Biot-Savart law and Ampere＇s rule, the magnetic induction generated by a straight current carrying conductor and a current loop is discussed, respectively. Then the novel coil winding method is developed by adjusting the location of inhomogeneous joints. The joints are periodically scattered along the circumferential direction and symmetrically designed around the central axis.Numerical results show that the quite non-uniform temperature in the base of pot at the circular direction is effectively improved by using the proposed winding method. The potential danger produced by high temperature at some region of coils plate is minimized.It is energy-efficient and safe for residential appliances.

Numerical Studies of Estimation of Fracture Parameters for Mismatched Welded Joints with HAZ Cracks

An idealized tri-material assumption is established to describe the constitutive relationship of mismatched welded joints by considering the influence of heat affected zone（HAZ）.The fracture parameters Jand C＊are estimated for mismatched welded joints with HAZ cracks by finite element analysis with ABAQUS.A middle crack tension（M（T））specimen is utilized in the analysis for different material properties and geometries of the weldment.The influence of mechanical property and geometry on the fracture parameters Jand C＊of the specimen is discussed for the welded joints with HAZ crack.The results suggest that the HAZ property is a significant factor in the estimation of Jand C＊for the mismatched welded joint with HAZ crack.