Study On The Evolution Of Microstructure And Mechanical Properties In The Twin-roll Cast 3003 Alloy Foil Manufacturing Process

Non-heat treatable 3xxx alloys combine moderate strength,excellent formability and good corrosion resistance,and are widely used for raw materials in the manufacturing semi-rigid container foils.Twin-roll casting(TRC)is a green metallurgy method with low cost and high efficiency,and is gradually taken as an alternative technique to traditional direct chill casting(DC)in the fabrication of 3xxx Al-foil stocks.Generally,due to its specific casting features,the stocks produced by TRC have a heterogeneous microstructure and a high degree of supersaturated solid solution.Subsequent cold rolling followed by intermediate annealing could bring about non-homogeneous microstructure with a very coarse grained structure,which is detrimental to the mechanical properties and surface quality of the final foils.Up to date,TRC 3xxx Al-foils still have many problems in the industrial production,such as low ductility and poor surface quality.For the sake of solving the above problems,the microstructural and mechanical properties evolution during pre-treatment,intermediate annealing and final annealing of TRC 3003 alloys,were systematically studied to give technical guidance for the TRC 3xxx Al-foil production.In this thesis,the main results can be drawn as follows:Three kinds of pre-treatment process were carried out in a TRC 3003 alloy sheet,i.e.I: 460?/12 h,II: 610?/12 h and III: 610?/12 h+460?/12 h.The evolution of grain structure and recrystallization texture were investigated in the pre-treated sheets after cold rolling and subsequent annealing at different temperatures.The results show that the optimized pre-treatment is determined to be pre-treatment III.Particle coarsening occurs at high temperature stage and the Mn concentration in solid solution decreases dramatically at low temperature stage,both of them could promote the recrystallization nucleation rate.With the increase of deformation and annealing temperature,the sheets recrystallize much faster,leading to a decreased effect of precipitation on recrystallization and hence a finer grain structure.For the TRC 3003 sheets after Pre-treatment III,90% cold rolling and annealing at 500 ?,it can be found that particle stimulated nucleation occurs in the vicinity of large particles,which promotes the nucleation rate and results in a reduced texture intensity;in addition,precipitation hardly happens during annealing and thus the inhibition of recrystallization nucleation is negligible.Compared to the sheets with other techniques,a finer grain size is obtained,but the grain structure is non-uniform through the thickness of the sheet,with a grain size variation from 94.6 ?m at the surface to 32.8 ?m at the center.A two-step annealing(TSA)treatment consisting of a pre-recovery process(450 ?/5 h)and a following recrystallization process(530 ?/15 h)was developed for a cold-rolled TRC 3003 alloy.By compared experiments(Single-step annealing,SSA),the effect of pre-recovery on recrystallization behavior and mechanical properties were systematically studied.Our results show that pre-recovery stimulates the formation of precipitates-free zones around constituent particles.Within such zones,substantial sub-grains can readily grow to critical size for nucleation,which contribute to enhanced nucleation rate.Simultaneously,pre-recovery also decreases the net pressure for grain growth and thus decelerate the growth rate.As a result,the average grain size of the pre-recovered samples is much finer than that of the samples without pre-recovery(35.2 ?m vs.152.4 ?m).In addition,the pre-recovery process greatly reduces the texture intensity,and completely removed the planar anisotropy of tensile properties.A ?r value of 0.03 exists in the TSA sheet,much lower than that of sample(0.31)subjected to SSA.The sheet after SSA and TSA were multi-pass rolled by a total reduction of 98.6% to 75 ?m in thickness,followed by annealing at 150~330 ?.The recrystallization behavior and resulting mechanical properties of TRC 3003 alloy foils with two different intermediate annealing treatments were systematically investigated.Our results show that the two foils exhibits different recrystallization mechanism during annealing,which results in distinct grain structure and texture.Continuous recrystallization occurs during annealing of the cold-rolled TSA-foils at 150~310 ?,and abnormal grain growth happens at 330 ?;while discontinuous recrystallization in the SSA-foils during annealing at 150~330 ?.After recrystallization,the average grain sizes were measured to be 8.9~16.5 ?m in the SSA-foils after annealing at 250~310 ?,and 0.95~1.35 ?m in the TSA-foils at 250~310 ? and a rapid increase in grain size to ~8.0 ?m at 330 ?.A strong Cube{001}<100> texture formed during discontinuous recrystallization in SSA-foils,which the texture components in the cold-rolled TSA-foils hardly change(retained rolling texture)after continuous recrystallization.Due to the size effect,the ductility of foils increases with the increasing ratio of thickness and grain size of the foil.Consequently,the elongation to fracture of the recrystallized TSA-foils is up to 20.5%~23.9% after recrystallization annealing,which is nearly triple that of the SSA-foils with a maximum of ~8.3%.Based on the above results,a novel technical route for the production of TRC 3003 Al-foils is developed,i.e.two-step intermediate annealing process + cold rolling to very large strain + final annealing at low temperature.Significantly improved ductility was achieved in the final TRC 3003 Al-foils.At the same time,the evolution of microstructure and mechanical properties for the TRC 3xxx sheets/foils after annealing were elucidated to provide a technical guidance for congeneric products.