Fabrication Of N-SiCp Reinforced AZ91 Composites By Two-Step Cyclic Closed-die Forging

Magnesium alloys have widely potential applications because of their light weight.However,the development of magnesium alloys are hindered because of their relatively low strength and ductility at room temperature.Generally,particulate reinforcing and grin refinement are two ways to improve the mechanical properties of magnesium alloys.In this study,we combined these two methods and fabricated n-SiCp?0%,0.5%,1.5%?AZ91composites by two-step cyclic closed-die forging?CCDF?.In this research,finite element simulation,microstructure observation and property characterization were carried out.The influence of processing temperature,processing pass,SiC content and pre-solution treatment to the microstructures and mechanical properties of the composites were studied.Besides,the effect that severe plastic deformation has on the nanoparticle distribution was also investigated.The finite element simulation shows that CCDF is a stable processing method.Multi-pass CCDF promotes the uniform distribution of strain within the whole sample and leads to a uniform bulk material.Processing temperature is the primary factor affecting the microstructures and properties of the composites during CCDF,and it also affects the shape of?-Mg₁₇Al₁₂.For the as-cast composites,after 3 passes of CCDF at 400 ?,the grain size is reduced to 22²6?m and the?phase dissolves into Mg matrix.After 2 subsequent passes of CCDF at 300 ?,the grain size is further decreased to 2.5?m,and the?precipitates again as particles along the DRXed grain boundaries.During CCDF process,the n-SiCp brings the composites a higher yield strength due to Orowan strengthening,and it hinders grain growth by pinning effect.What's more,the n-SiCp promotes the precipitation of?phase.The SiC clusters existing in the as-cast composites are eliminated during CCDF,and the n-SiCp are dispersed uniformly.After the two-step CCDF,the hardness distribution of the bulk sample is uniform,and the AZ91-0.5%composite gains a yield strength of 242 MPa,an ultimate strength of 366 MPa and an elongation of 8%.The microstructures of the composites are totally different if a prior solution treatment is conducted.On the one hand,after 3 passes of CCDF at400 ? and 6 passes of CCDF at 300 ?,the grain size is 6.8?m,coarser than that without solution treatment.On the other hand,the?phase precipitates discontinuously as lamellae after 1 pass of CCDF at 300 ? and turns to particles within and between grains after 6 passes of CCDF at 300?.The main reason for these differences is the diffusion of Al during solution treatment,which directly affects the morphology and distribution of?precipitation.In this way,solution treatment also influences the recrystallization and grain refinement.Compared with the composites without solution treatment,a pre-treatment before CCDF provides the composites a higher elongation but a lower strength.Considering the material properties and processing costs,CCDF without solution treatment is a better choice.