Preparation And Tissue Evolution Of A356 Semi-solid Slurry

Semi-solid forming technology had attracted extensive attention from many scholars at home and abroad due to its high production efficiency,near-net shape and high performance of formed parts as one of the most promising metal forming processes in the 21 st century.It was the primary prerequisite to obtain a small and uniform primary globular structure in the semi-solid forming process.For this purpose,this paper developed two processes for preparing slurry from the actual situation of the equipment: pure electromagnetic cooling and stirring process and self-cultivation rod + electromagnetic stirring compound process,trying to prepare a high quality semi-solid slurry through reasonable control of experimental parameters by selecting widely used A356 alloy in industry,investigating the formation and evolution mechanism of globular crystals and providing some ideas for related research and industrial production by observing the evolution process of a-Al under different process conditions combined with the related theories of thermodynamics,kinetics and electromagnetism.The main conclusions of this paper are as follows: 1)A large number of spherical primary phases with uniform size can be prepared by controlling the melt cooling rate,the stirring temperature range and the stirring frequency in the pure electromagnetic cooling and stirring process.The size and morphology of the primary phase become better and better as the melt cooling rate slows down and the adhesion between the grains can be significantly improved.With the expansion of the stirring temperature range,the change rule of the primary phase size first decreases and then increases and the degree of the shape regularity increases.With the increase of the stirring frequency,the change of the primary phase size is firstly reduced and then increased and the shape does not change significantly.The slurry quality is the best when the furnace preheating temperature is 600°C,the stirring temperature range is 640-600°C and the stirring frequency is 30 Hz.2)The mechanism of the primary phase nucleation and growth in the pure electromagnetic cooling stirring process is that the electromagnetic stirring exacerbates the segregation of the relatively stable Al-Al and Si-Si atomic clusters on the microscopic scale,small and mutually agglomerated cotton-like primary phases are precipitated firstly,and the small primary phases gradually merge into large-sized spherical crystals as the stirring continues and the temperature is further reduced.3)The self-cultivation + electromagnetic stirring compound process has higher pulping efficiency than the pure electromagnetic stirring process and the obtained primary phase size is significantly reduced.The shape of the primary phase becomes more rounded and the size of the primary phase first decreases and then increases with the increase of the insertion temperature of the self-cultivation rod.The size of the primary phase gradually increases and the roundness gradually decreases as the stirring frequency increases.The slurry quality is the best when the temperature range of the self-cultivation rod is 640°C-610°C and the stirring frequency is 20 Hz.4)There are two main sources of spherical and nearly spherical primary phases in the selfcultivation + electromagnetic stirring compound process:(i)a large number of nonspontaneous nucleation are detached from the surface of the cultivation rod and grow into the spheroidal primary phase when the cultivation rod is inserted into the melt(ii)Coarse dendrites formed by intense temperature changes are gradually melted in the "secondary heating" in the furnace and gradually evolved into a nearly spherical primary phase.