Study On The Microstructures And Mechanical Properties Of EW75Alloy During Multidirectional Forging

The main reason that restricts the application of magnesium alloys in primary-load-carrying structure is their low strength at room and elevated temperatures. The Mg-Gd-Y series alloys have a great potential on high strength both at room and high temperatures and have become a research hotspot at home and abroad. In this paper, large-size round ingot (Φ500mm×1000mm) of Mg-7Gd-5Y-1Nd-0.5Zr (EW75) alloy is researched. The microstructures and mechanical properties of EW75alloy during uniaxial compression and Multi-directional forging (MDF) have been investigated systematically by OM, SEM, TEM, EBSD, XRD and mechanical test to provide instructive information for the preparation of heavy magnesium alloy forging stock.The as-cast microstructure of EW75alloy consists of the α-Mg matrix, skeleton-like eutectic structures, small cuboid-shaped phases and Zr-rich particles. The optimum homogenization treatment condition for the cast alloys is480℃×6h+535℃×16h. The eutectic structure in the as-cast ingot had dissolved and the fracture characteristic is change from intergranular fracture to transgranular fracture. The as-cast and homogenized samples have grains with random crystallographic orientation.From the true stress-strain curve, the typical characteristics of dynamic recrystallization could be obtained. The microstructure of alloy is affected by deformation temperature and strain rate. At temperature of450℃, strain rate0.05s-1and strain20%, cavities appear in the knot of grains, which grow up first and turn down later with deformation degree increased, as deformation degree reach to80%, cavities healed, according to microstructure evolution, we get the evolution model of cavity. The nucleation of dynamic recrystallization occurs at deformation band and around grain boundaries.The microstructures during MDF are influenced by deformation temperature, strain path and pass number. The volume fraction of recrystallized grains increase with increasing the initial forging temperature and dynamic precipitation is inhibited at high temperature. The grains are refined during MDF, the average grain size decreased to5.1μm after6pass,Basal texture is formed after1pass, and the intensity of texture is decreased with increment of pass number; it may be ascribed to DRX, dynamic precipitation and the rotation of force axis. Deformation uniformity is influenced by frictional force and temperature. The mechanical properties are increase with increment of pass number, after6pass, the ultimate tensile strength, yield strength and elongation to failure of0°are318MPa,232MPa and7%. the ultimate tensile strength, yield strength and elongation to failure of90°are320MPa,253MPa and7.5%. Static recrystallization take place during the re-heating process, after improvement of processing, the ultimate tensile strength, yield strength and elongation to failure are345MPa、247MPa、12%.