Study On Semi-solid Structure Of Al-Cu Alloy And Its Composites Prepared By Low Voltage Pulsed Magnetic Field

Semi-solid billets of2024, Al-4.5Cu aluminum alloy and TiB2/Al-4.5Cu compositewere prepared by low voltage pulsed magnetic field (LVPMF) processing in thispaper,and then the semi-solid billets were reheated,finally thixotropic die casting.Theeffect and mechanism of LVPMF on the organization of semi-solid billets,reheatingorganization evolution and the reasons of grain growth rate differences are discussed.The results showed that: when LVPMF was introduced,the total free energy changeand the nucleation radius were decreased so that the nucleation rate was increased andprimary α-Al particles were refined. At the same time,around the temperature gradientof primary α-Al particles was reduced,so growth of dendritic grains wererestrained.With the increase of discharging voltage,average grain size of primary α-Alparticles of semi-solid billets of three kinds of material decreased;when dischargingvoltage reached a maximum250V,average grain size of every semi-solid billet wasminimum.Average grain size of primary α-Al particles decreased with the treatmenttime varying from0to60s,and then changed a little when the treatment time exceeded60s.When the mould preheating temperature was from460℃to530℃,average grainsize of primary α-Al particles of semi-solid billet of three kinds of material decreasedfirst,and then average grain size of primary α-Al particles increased as the mouldpreheating temperature exceeded a certain temperature.The more casting temperatureneared liquidus temperature,the more average grain size was small.During reheating,primary α-Al particles gradually grew and liquid phase increasedwith the increas of temperature and holding time.The particles roundness of2024aluminum alloy and TiB2/Al-4.5Cu composite were continuously improved;but thenumber of spheroidization particles of Al-4.5Cu was small,and even a part of particlescoarsened seriously.Through the calculation found that semi-solid billet of2024aluminum alloy and TiB2/Al-4.5Cu composite were reheated, particles growth rateconstant K value was significantly lower than the Al-4.5Cu particles’s K value.when thealloy elements(the second phase particles formed) and TiB2particles wereintroduced,the grain boundarys were pinned and grain growth were restrained;So thethermal stability was enhanced.Thixotropic die casting parts hardness of2024aluminum alloy and TiB2/Al-4.5Cu composite were higher than hardness of the ordinary liquid forming part and liquid diecasting part.