The influence of two-step homogenization on microstructures and properties of AZ40M magnesium alloy were investigated by the scanning electronic microscopy (SEM), X-ray diffraction analysis (XRD) and tensile experiment. Through comparing the microstructure and mechanical properties of the alloy before and after homogenization treatment, a two-step homogenization process of the semi-continuous casting AZ40M magnesium alloy ingot was determined. The result shows that the as-cast AZ40M alloy mainly contained two kinds of second phase, β-Mg17Al12 and Al8Mn5. Treating at temperature of 380 °C for 8 h as the first step homogenization treatment and then treating at temperature of 420 °C for 20 h as the second step homogenization treatment made the eutectic phase (α-Mg/β-Mg17Al12) completely eliminated and the alloying elements such as Al, Zn distributed uniformly. Al8Mn5 is the residual second phase in the homogenized AZ40M alloy. During the homogenization process, the β-phase dissolved which made the content of Al increased in the matrix. The decomposition of β-phase benefits the tensile strength and elongation of the alloy. The tensile fractures of as-cast and homogenization alloys are characteristic of ductile fracture.

Different cooling parameters, including the initial cooling temperature, finishing cooling temperature and cooling rate, have a significant effect on the final microstructures and properties for pipeline steels. In present work, Gleeble-1500 thermal-mechanical simulator was used to investigate the microstructural evolution of X70 pipeline steels under different cooling conditions, and the microstructures obtained were analyzed using optical microscope and transmission electron microscope. The experimental results showed that when the initial and finishing cooling temperatures are controlled in the range of 740~760°C and 500~520°C respectively, the microstructure of X70 pipeline steels reveals a proper content (about 12%) of pre-eutectoid ferrite besides acicular ferrite and M/A (Martensite/Austenite) island, which can guarantee an optimum combination of strength and ductility. In contrast to the conventional way using CCT curve, this methodology shows a higher accuracy and operability, and used in industrial production to achieve good effect.

The effect of online solution and subsequent age heat treatment on microstructure and mechanical properties of die-cast AZ91D +1wt. %RE alloy were researched. The results indicated that online solution heat treatment led to inhibit some β-Mg17Al12 phase precipitation, and remained the Al element in matrix plays a role of solid solution strengthening. The online solution samples were aged at 160°C, 180°Cand 200°C. Microstructure results suggested that age treatment caused some lamellar β-Mg17Al12 phases precipitated near grain boundaries. The tensile properties of samples under air cool, online solution and subsequent age heat treatment were tested, which UTS were 194 MPa, 243 MPa and 244 MPa, and the elongation were 3.67%, 3.97% and 1.6%, respectively. The results indicated that the online solution could enhance the mechanical properties significantly, which the subsequent age heat treatment could not improve.

Cooling curves and residual deflection during spray quench process are measured by using thermometric instrument and spiral micrometer, separately, in extruded EW75 magnesium alloy. By using Numerical analysis, temperature and stress with the change of time during quenching were also investigated. The results showed that: when the spray speed was 6L/min, the steam film stage was obvious, as the speed reached to 9L/min, steam stage disappeared. The residual deflection increased with the spray speed increased. When the spray speed was 6L/min, the residual deflection was 3.38mm and increased to 3.88mm as the spray speed increased to 9L/min. But with the increase of spray speed, the growth rate of deflection induced. The results of the numerical simulation indicated that during the quench process, the surface cools faster than body and shrink more severely at first, leading stress distribution is compress at surface while tensile in body. Plastic deformation happens at this time. And then along with the surface cooling speed reduced, the body cool speed is greater than surface; the shrink in body is also greater, thus the compress stress turn to be tensile and contrary change take place at surface. With the increasing distance from the cross section, the tensile stress is reduced gradually, in 5.3mm, there exists a zero stress layer, then turns to compressive stress.

Using the wire of the same material as base metal, EW75 magnesium alloy (Mg-7Gd-5Y-1Nd-0.5Zr wt. %) plate with the thickness of 6 mm was welded by TIG to investigate the welding property. Optical microscopy (OM), scanning electron microscopy (SEM) and universal tensile testing machine were used to analyze the appearance of the welding joint, organization performance and mechanical properties.The results revealed that the fusion zone (FZ) was composed of fine equiaxed grains. Precipitated phase distributed on the grain boundary as network structure. The grain size in heat affected zone (HAZ) was coarse because of overheating. The grain size of the FZ increased sharply when the welding current was more than 125A. With the increase of welding current, the grain size of the HAZ remained relatively stable. The hardness curve presented typical “W” shape, and the microhardness of the FZ was the highest, then was the base metal (BM), while the HAZ was the lowest. The welding joint showed uniform scale shape when the welding current was125A.The tensile strength of welding joint was 228 MPa, about 89.4% of the BM, the fracture location was in the FZ and the fracture surface was characterized by mixture fracture.