Study On Microstructure Of7075Aluminum Alloy Sheets Produced By Electromagnetic Twin-roll Casting

As a kind of ultra-high strength aluminium alloy, alloy7075is widely used in aerospace and other fields because it has many advantages, such as low density, high strength and good corrosion resistance. The traditional method for producing7075sheet is hot rolling, which involves many processes and thus has high cost. Compared with hot rolling, twin-roll casting has simple working procedures, low energy consumption, small investment and low cost. There are many technical problems in producing7075alloy sheet with twin-roll casting and7075sheet produced with this method has many microstructural defects such as well-developed dendritic crystal and severe segregation. These problems and defects are mainly caused by two reasons:high elements content and wide solidification range in7075alloy and technical features of twin-roll cast. In this paper, several kinds of electromagnetic field are applied in the cast-rolling process to study their effects on the microstructure of7075sheet so as to provide some theoretical and experiential references for the practical proposition of7075sheet produced by electromagnetic twin-roll casting.Effects caused by electromagnetic field on the microstructure of7075sheet are discussed in this paper by applying a DC electromagnetic field, a DC electromagnetic field mixed with half-wave electric field and a DC electromagnetic field mixed with alternating electric field in the casting zone, respectively. The effects of temperature on the impact of electromagnetic field are studied by setting different casting temperatures. The effects caused by electromagnetic field on solid solubility and second phases in the grain boundary are also studied. A series of results are obtained as follows:(1) The dendritic crystal is refined but not smashed under DC electromagnetic field. The dendritic crystal is both refined and smashed and become isometric a certain extent under half-wave oscillating field and alternating oscillation field. The impact of alternating oscillation field is more notable than that of half-wave oscillating field and is strengthened after the magnetic field intensity is enhanced. (2) Compared with common condition, the hardness in cross section and outside surface of7075sheet both increased under DC electromagnetic field, half-wave oscillating field and alternating oscillation field and the impacts of the three fields increase successively.(3) Compared with common condition, the macrosegregation in the side zone and central zone of7075sheet both are alleviated under DC electromagnetic field, half-wave oscillating field and alternating oscillation field and the impacts of the three fields increase successively. There is no macrosegregation can be seen under the alternating oscillation field.(4) Impacts of the oscillation fields are enhanced when the cast temperature is increased.(5) Microsegregation in7075sheet can be alleviated under DC electromagnetic field, half-wave oscillating field and alternating oscillation field and the impacts of the three fields increase successively.(6) The size of second phases in the grain boundary decreases under DC electromagnetic field, half-wave oscillating field and alternating oscillation field and so does the distribution continuity and the impacts of the three fields increase successively.(7) The second phase in the grain boundary of7075sheet is mainly T(AlZnMgCu) phase. The content of the second phase decreases under the three fields while its composition remains unchangeable.