Investigation On The Fabrication Of Semi-solid Billets Of Mg-RE Alloy Capable Of Forming Long Period Stacking Ordered Structure

The semi-solid forming has many advantages,such as smooth filling,near-net shaping,and low molding temperature,which can be used for the manufacturing of Mg-based components by solving the problems of oxidization and burning.The most work on the semi-solid forming of magnesium alloys have devoted to the Mg-Al or Mg-Zn alloy systems.However,the mechanical properties,especially high-temperature creep performance,of Mg-Al and Mg-Zn alloys are relatively low,which cannot meet the requirements of high performance magnesium alloys.Accordingly,high-performance Mg-RE alloys,especially with the addition of transition elements to form a new long period stacking ordered(LPSO)structure,have attracted great attentions.Therefore,it is necessary to develop new semi-solid high-strength Mg-RE alloys and corresponding semi-solid billet preparation technology.In this work,the fabrication of semi-solid billets of three LPSO-forming alloys(Mg-Gd-Zn-Zr、Mg-Gd-Nd-Ni and Mg-Gd-Ni alloys)were investigated using optical microscope(OM),scanning electron microscopy(SEM),Energy Dispersive X-ray Detector,X-ray diffraction(XRD)and differential thermal analysis(DTA).The main results are the following.1.The semi-solid billet Mg-15Gd-2Zn-0.6Zr magnesium alloy with spherical or nearly-spherical a-Mg solid phase was successfully fabricated using isothermal heat treatment method at a temperature between the second phase melting point and initial melting temperature of a-Mg phase.2.A high-quality Mg-15Gd-2Zn-0.6Zr semi-solid billet,which has α-Mg grain size of 75μm,shape factor of 0.84 and liquid fraction of about 20%,was produced by an isothermal heat treatment at 560℃ for 10~15min.The evolution of solid α-Mg phase during the heat treatment was as follows: fine dendritic crystal→dendritic crystal coarsen → irregular polygonization → spheroidization → final coarsening.The composition of the alloy did not change significantly.3.A new method,which was a combination of pulsed magnetic field and isothermal heat treatment,was developed to fabricate the Mg97Gd1Nd1Ni1 semi-solid billet with large as-cast grain size.The pulsed magnetic field was performed to reduce the grain size.4.The refinement effect is the most significant when the pulse frequency of pulsed magnetic field is 10Hz;the grain size was reduced to about 80μm.After the isothermal heat treatment at 560℃ for 20 min,the size and the shape factor of solid α-Mg phase is 85μm and 0.82,indicating a high-quality semi-solid billet of Mg97Gd1Nd1Ni1 alloy.5.Four Mg-Gd-Ni alloys with different solid phase and liquid phase were obtained by composition design;with the increase of the mass fraction of solute elements in the alloy components,the average size of solid particles in the semi-solid microstructure decreases and the liquid fraction increases gradually.6.The Mg-15Gd-1.5Ni(Alloy 1),Mg-20Gd-1Ni(Alloy 2),Mg-20Gd-1.5Ni(Alloy 3),and Mg-20Gd-2Ni(Alloy 4)semi-solid billets were obtained by isothermal heat treatment.The optimal isothermal temperature of the four alloys are 570℃,550℃,550℃ and 550℃,respectively,and the corresponding holding time is 15 min.Meanwhile,the sizes of the primary α-Mg in the semi-solid microstructure of the four alloys were 68μm,43μm,35μm and 27μm,respectively;the liquid fractions were 32%,35%,37% and 43% respectively;the shape factors are similar with each other.7.In the semi-solid isothermal heat treatment,the forming of the liquid has great effects on the formation of the semi-solid microstructure;the volume fraction of the second phase and the mass fraction of the solute elements of the as-cast alloy can affect the formation and content of the liquid phase;meanwhile,the high volume fraction of the second phases in the alloy is beneficial to promote the formation of liquid phase.