Pressure working properties of magnesium alloys

Magnesium metal's properties

The pressure machinability of metallic materials depends on two relatively independent factors of the material: the machinability related to stress state (SOS) and the machinability inherent in the material itself.  The machinability of SOS depends on the geometrical relationship of the deformation zone, and the workpiece is in three-dimensional stress state in the deformation zone.  Therefore, SOS machinability is only related to the machining process, and has nothing to do with the properties of the material itself.  For example, SOS machinability during rolling can be optimized by appropriate rail pass design for strip, SOS machinability during forging can be optimized by appropriate preform design, SOS machinability during forging can be optimized by appropriate cavity geometry.  In general, in order to obtain good SOS machinability, it is necessary to ensure that the components of static pressure in the deformation zone are essentially compressible.  

Factors that influence the inherent machinability of metal materials, such as microstructure, chemical composition, premachining condition, deformation rate, temperature, strain state sensitivity during machining.  This sensitive state can be described by the changes of flow stress and temperature, deformation velocity and strain state, and can be expressed by a basic mathematical formula. This behavior can be expressed by the deformation and fracture mechanism diagram.  

Magnesium and magnesium alloy are dense hexagonal lattice, the pressure processing performance is not good at room temperature, must be carried out at high temperature, therefore, in addition to the processing tools and processes required by high temperature forming, magnesium alloy forming process and equipment, and steel, aluminum, copper and other forming process and equipment is roughly the same.  Magnesium alloy forming process and typical temperature are shown in the table.  

Magnesium alloy has high specific stiffness and strong damping capacity, so it is used to manufacture automobile, aircraft and rocket parts. However, magnesium and magnesium alloy are flammable and corrosive, so they must be protected by SO2 gas during smelting, casting and heat treatment, and should be covered with protective layer to prevent oxidation.  

Adding a small amount of Zn, Mn and Zr to mg-Al, Mg-Zn, Mg-Re and Mg-Th industrial alloys, and adding Zn to Mg-Zn-Mn alloys (ZM series) can improve the solid solubility of elements, and adding Mn can improve the corrosion resistance and hot working performance of the alloys.  When 10% ~ 13%Li is added to aerospace Mg-Li alloy, its density can be reduced to 1.35g/cm3, and it has a body-centered cubic lattice and superplasticity.  In order to improve the creep resistance of Mg-Li alloy and keep its property stable, a small amount of Al and a small amount of Zr can be added to the alloy to refine the grain size.  

Mg and Mg-Zn-Mn alloys can be forged by hydraulic forging machine at 450℃ ~ 500℃ with the deformation speed of 0.1mm/s.  When the deformation rate is higher, local rheological phenomenon will appear in the as-cast material, so the ingot should be pressed before hot rolling to make the material dynamic recrystallization.  The mg-Li-Al alloy is superplastic over a wide temperature range and has excellent machinability. Moreover, it has dynamic recrystallization property with the addition of Zr.