Studies On The Forging Process Of High Strength Mg-Gd-Y-Zn-Mn Alloy

Because of its low density, high specific strength, good damping, easy recycling, excellent electromagnetic shielding performance and so on, Magnesium alloys have become a hot research area of materials.However,the industry development of this alloy is limited by its low absolute strength.Nowadays, lots of researchers are paying attention to high strength Mg-RE-Zn series.To the author's knowledge, the extrusion and rolling of high strength Magnesium alloys have been studied systematically,but little is known about the forging of high strength Magnesium alloys,which is also significant processing in industries. It is hard to obtain high strength Mg-RE-Zn alloys with uniform microstructures and isotropic mechanical properties,which is the bottleneck.Therefore,high or low strain rate forging were carried out on as-cast,as-heat treatment and as-extruded Mg-Gd-Y-Zn-Mn to obtain massive or columnar high strength forging specimens,with the particular emphasis on the effect of forging on microstructure and mechanical properties of Mg-Gd-Y-Zn-Mn.Firstly, to save the cost of forging experiments and get the opitimum forging processing parameters,the hot deformation behavior and processing maps of Mg-Gd-Y-Zn-Mn were investigated by the hot compression tests,which were performed at a temperature range from 623K to 773K and a strain rate range from 0.001 S-1 to 1 S-1.The activation energy of the as-homogenized Mg-Gd-Y-Zn-Mn was obtained to be 260.94kJ/mol.Based on the hyperbolic since law, the constitutive equation is ?= 6.55X1017[sinh(0.008376?)]5.40314 exp(-260.94/8.314T) The processing maps of as-homogenized Mg-Gd-Y-Zn-Mn were established. According to the maps, The processing map developed at the strain of 1.2 shows two safety domains:one occurring in the temperature range of 733K to 773K and in the strain rate range of 0.001 S-1 to 1 S-1;the other occurring in the temperature range of 623K to 773K, and in the strain rate range of 0.001 S-1 to 0.005S-1.,providing the guidance for the following forging experiments. Meanwhile,the corresponding microstructure evolutions as well as the deformation mechanism of two kinds of LPSO phases(MgZnY LPSO phases and MgGdZn LPSO phases) in this alloy, have been also discussed:the kink band was observed in the gray bulk LPSO phase, which was caused by the motion of dislocation.Moreover, the lamellar LPSO phase became the stream- shaped phase after the hot deformation.Secondly,low strain rate multidirectional forging by the hydraulic machine were carried out on as-homogenized and as-extruded Mg-Gd-Y-Zn-Mn based on the results of hot deformation simulation above.The as-homogenized alloys ware forged successfully for 4 passes and the cracks occured during forging in the fifth pass.The grains were refined to a certain extent and the second phases did not change apparently. The alloy multi-directionally forged (MDFed) for 6 passes exhibited the best mechanical properties:UTS(Ultra Tensile Strength)=256MPa, YS(Yield Strength)=231Mpa,EL(Elongation)=3.1%.The as-extruded alloy were forged successfully for 14 passes and were not continued because of the titled specimens.The alloy multi-directionally forged (MDFed) for 12 passes exhibits the best mechanical UTS=410Mpa, YS=300Mpa, EL=15.1%.The high strength forged alloy has good ductility,which is increased by 300%,in comparison with that of the specimen before forging.The refinement strengthening, second-phase strengthening solution strengthening and the weakened texture induced by the forging processing are the factors to the high strength and ductility of the forged Mg-Gd-Y-Zn-MnFinally,to improve the efficiency of forging and obtain massive or columnar high strength forging specimens with higher mechanical property,high strain rate multidirectional forging were carried out on as-cast and as-extruded Mg-Gd-Y-Zn-Mn based on the results of hot deformation simulation and high strain rate multidirectional forging above. As a result,the second formability of the as-cast Mg-Gd-Y-Zn-Mn was enhanced by the forging processing.The alloy was homogenized at 540 for 5 h and 450 for 10 h,then forged to the total compression of 20%.The forged specimens were extruded following by the aging treatment at 200 for 50.After those processing,the alloy has the outstanding mechanical properties:UTS=503Mpa, YS=395Mpa, EL=5.2%?High strain rate multidirectional forging by the air hammer were carried out on refined by to acquire the cuboid forging specimens.UTS of these alloys did not change apparently,which was between 314Mpa and 341Mpa.EL of these alloys was increased by 3 times after the forging,which was between 7% and 14%.The excellent ductility was attributed to the uniform micro structure refined by and a lot of isometric crystals refined by the forging.Moreover, as-extruded Mg-Gd-Y-Zn-Mn was processed by'the first extrusion +forging+the second forging'After the aging treatment, the excellent mechanical properties was that UTS=460 Mpa, YS=377 Mpa,EL=4.7%.