Dynamic Recrystallization Behabior Of Wrought Al-Si Alloys

Al-Si alloys are widely used in aerospace and automobile industry as cast alloys for their excellent wear and corrosion resistance, low density, low coefficient of thermal expansion and good strength and castability. However, application of Al-Si alloys as wrought alloys has not been reported. In order to achieve the industrialization of wrought Al-Si alloys as structural materials, direct chill (DC) casting was employed to produce Al-15Si alloy and Al-2Si alloy billets in the present research. The Al-2Si alloy was prepared as a reference material. Hot rolling and hot compression were performed on the alloys following pre-heat-treatment. The microstructure evolution and dynamic recrystallization behavior in the Al matrix during hot deformation were studied systematically. It is expected to provide fundamental knowledge for the microstructure control during hot plastic deformation.The Al-15Si alloy and Al-2Si alloy billets with fine microstructure were produced by DC casting. Following heat-treatment with temperature of 450-520? and holding time of 1?3h, the coral-like structure of eutectic Si phase changed into spheroidal Si particles and then coarsened, and grain boundaries formed between the spheroidal Si particles on the Al matrix. In Al-15Si alloys, the grain boundaries with Si particles composed to integrated grains in the eutectic regions. And the size of Si particles and the Al matrix grains in the eutectic regions increases with increasing of heat-treatment temperatures and holding times.The microstructure evolution of Al-15Si alloy hot rolled at 450-500? showed that the Si particles are more homogeneously distributed in the Al matrix with increasing thickness reduction. Meanwhile, the dynamic recrystallized area fraction in the Al matrix increases progressively. The microstructure of the Al matrix was refined. The fine dynamic recrystallized grains are uniformly distributed in the Al matrix with thickness reduction of 70%. Dynamic recrystallization can be accelerated and the size of recrystallized grains became smaller with deformation temperature decreasing.The flow behaviors of Al-15Si alloyand Al-2Si alloy hot compressed at deformation temperature of 300?500? and strain rate of 0.00 1?5s-1 showed that the hot compression deformation behavior of alloys exhibit steady-state flow characteristics. The flow stress decreases with increasing deformation temperature and increases with increasing strain rate. Under the same condition, the flow stress of Al-15Si alloy is higher than those of Al-2Si alloy, but the differential value between the alloys decreases with increasing deformation temperature and decreasing strain rate. In the range of experimental conditions investigated, the hot compression deformation of the alloys is a process of thermal activation. The average thermal activation energy of the Al-15Si alloy and Al-2Si alloy is 180.96kJ/mol and 152.62kJ/mol, respectively.The investigation on the microstructure of Al-15Si alloy hot compressed at deformation temperature of 300?500? and strain rate of 0.001-5s-1 with the same strain indicated that the microstructure of the alloy was affected by the deformation temperature and strain rate (Zener-Hollomon parameter, Z). With deformation temperature decreasing or strain rate increasing, the fracture frequency of the Si particles increased. And it is found that only dynamic recovery occurres in the Al matrix when 21.18?lnZ?23.48, dynamic recrystallization grains formed near the Si particles at grain boundaries when 25.35?lnZ?28.09 and dynamic recrystallization grains formed near the Si particles (include Si particles at the grain interior and grain boundaries) when 29.7?lnZ?39.5. With the deformation conditions for dynamic recrystallized occurrence, the dynamic recrystallized area fraction in the Al matrix increasing and the size of dynamic recrystallized grains became smaller with decreasing deformation temperature or increasing strain rate. With deformation temperature decreasing and strain rate increasing, dynamic recrystallization can be accelerated.In the Al-15Si alloy and Al-2Si alloy without other alloying elements, Si particles stimulated dynamic recrystallization is possible in these alloys during hot rolling and hot compression. And higher silicon particles content in the alloys leads to higher area fraction of dynamic recrystallization and smaller dynamic recrystallized grains with the same deformation condition.In the Al-15Si alloy and Al-2Si alloy without other alloying elements, the dynamic recrystallized grains are mainly formed at the vicinity of Si particles during hot rolling and hot compression. Results manifest that the dynamic recrystallized grains in the Al matrix evolve from subgrains formed during deformation by dynamic recovery. The operation of continuous dynamic recrystallization as predominant mechanism is anticipated. Subgrain rotation play an important role for the occurrence of continuous dynamic recrystallization.