Research On Effect Of AZ91D-Y Semi-Solid Billets Preparing By Different Methods On Thixoforging

Based on the policy guidance of energy-saving and emission reduction and the deteriorating situation of metal mineral resources supply, we attach great importance to the development of magnesium alloys because of their low density and great abundance. Accelerated research on the forming technology of magnesium alloys is the one of important measures to realize sustainable development. Currently, the forming technology of magnesium alloys is dominated by diecasting, in combination with liquid forging and forging. However, these technologies have their own drawbacks. For example, although forging can meet the requirement of forming parts with good mechanical properties, it is difficult for this technology to form parts with complex shape. Semi-solid thixoforging is the one of near-net shape technologies to form parts with complex shape. The semi-solid microstructure is characterized by spherical grains. The complexity of parts formed by semi-solid thixoforging exceeds that formed by forging. The mechanical properties of parts formed by semi-solid thixoforging exceed those formed by diecasting and liquid forging. The aim of this paper is to research the preparation of semi-solid billets by casting, near-liquidus forging and equal channel angular extrusion (ECAE), respectively. The microstructure evolution during reheating, the effect of the formation and characteristics of spherical grains on thixoforging and the mechanical properties of parts formed by thixoforging are also studied. Moreover, the application scope and possibility of semi-solid billets prepared by different methods are discussed.The semi-solid billets are prepared by casting, near-liquidus forging and ECAE, respectively. By means of metallographic microscope, scanning electronic microscope, X-ray diffraction and universal material testing machine, microstructure and mechanical properties of semi-solid billets prepared by three different methods are analyzed. The results show that the addition of Y to AZ91D alloy refines primaryα(Mg) matrix andβ(Mg17Al12) phase. The addition of Y forms rod-shaped Al2Y phase. In the as-cast billet, coarse equiaxed grains are obtained and columnar crystals are not obvious. In the near-liquidus forged billet, fine dendritic microstructures are produced and with increasing applied pressure the mechanical properties of billets are improved. In the ECAE-formed billet, increasing temperature is helpful for obtaining equiaxed grains but unfavorable for improving yield strength and ultimate tensile strength. Moreover, increasing the number of pass promotes the formation of fine equiaxed grains.Semi-solid billets prepared by casting, near-liquidus forging and ECAE are reheated in the semi-solid temperature range. Microstructure evolution and characteristics of semi-solid billets prepared by casting, near-liquidus forging and ECAE during reheating are also studied. The results show that in the as-cast billet, coarse dendritic structures evolve into large blocked structures, then into cloddy pulverescent structures, finally irregular grains are spheroidized and coarsening mainly by coalescence. In the near-liquidus forged billet, fine dendritic structures evolve into blocked structure, then into cloddy pulverescent structure, finally irregular grains are spheroidized and coarsening mainly by Ostwald ripening mechanism.In the ECAE formed billet, deformed structures transform into recrystallised structures in the early stage. Then recrystallised structures are penetrated by liquid and disintegrated into seperated grains. These seperated grains are spheroidized and coarsening mainly by Ostwald ripening mechanism. Compared with that of as-cast billet, the mean grain sizes of near-liquidus forged and ECAE formed billets are much smaller. Amoung these three methods, the microstructure evolution for ECAE formed billet is the fastest and the degree of spherpidization is also best.Thixotropic behaviors of billets prepared by casting, near-liquidus forging and ECAE are studied in the semi-solid state by thermal simulation experiments. The results show that under low stress the steady state filling is achieved for the ECAE formed billet and the thixotropic fluidity of the ECAE formed billet is best, that of near-liquidus formed billet the second and that of as-cast billet the third. The effect of processing parameters and methods of preparing billets on the mechanical properties of thixoforged billets are studies by universal material testing machine. The results show that when the preheating temperature of billet is 560℃and applied pressure is 200MPa, the best mechanical properties of thixoforged parts prepared by casting can be obtain after 30min isothermal holding; the yield strength, ultimate tensile strength and elongation to fracture are 131MPa,280MPa and 9.2%, respectively. The best mechanical properties of thixoforged parts prepared by near-liquidus forging can be obtain after 20min isothermal holding; the yield strength, ultimate tensile strength and elongation to fracture are 161MPa,302MPa and 9.7%, respectively. The best mechanical properties of thixoforged parts prepared by ECAE can be obtain after 15min isothermal holding; the yield strength, ultimate tensile strength and elongation to fracture are 164MPa,307MPa and 11.8%, respectively. As for the four-pass ECAE formed billet, the yield strength, ultimate tensile strength and elongation to fracture are 220MPa,333MPa and 16.1%, respectively under the same conditions with the one-pass ECAE formed billet. The mechanical properties of thixoforged parts prepared by ECAE are first, those prepared by near-liquidus forging are second and those prepared by casting are third.The comparative studies mentioned above demonstrate that casting, near-liquidus forging and ECAE have their own advantages and these methods can satisfy the different requirements for shape, size and mechanical properties of thixoforged magnesium alloy parts. Considering the cost of preparing billet, the complexity of parts and mechanical properties, near-liquidus forging has competitive advantage and is likely to become one of main methods to producing billets with high performance.