A Study About The Mechanism Of Static Recrystallization Of Al-12.7Si-0.7Mg Alloy

Al-Si alloys have been widely used in automobile and aircraft industries for their excellent wear resistance, low density, low coefficient of thermal expansion and good strength and castability. Recrystallization is a very important process after the deformation of metal.In this work, Optical microscopy and hardness testing were used to study the microstructure and property of the hot/cold rolled and annealed alloys. Meanwhile, the mechanism of the recrystallization was discussed. The main results are listed as follows:1. The pretreatment before hot rolling spherifys the eutectic Si in casting condition, resulting in equiaxial Mg2Si phase.2. The dendritic crystal and the particle of Si were broken after hot rolling. Dynamic recrystallization started and refined the crystal.The size of crystal in step sample increased with the growth of the percentage of reduction.3. The grain of Al-12.7Si-0.7Mg alloy was elongated after cold rolling; the particles of Si were twisted and some large ones were broken; the hardness increased with the growth of the percentage of reduction.4. With the increase of the annealing temperature, the Al-12.7Si-0.7Mg alloy recrystalized. The hardness of Al-12.7Si-0.7Mg alloy increased firstly and then dropped.5. After cold rolling (the percentage of reduction is70.56%) and then annealing (the holding temperature is500℃),the grain size increased with increasing annealing holding time. When the holding time is5s,the diameter is5.89μm10s,the mean diameter is7.23μm,when30s the mean diameter is7.33μm,when60s,the mean diameter is7.54μm,and when180s,the mean diameter is7.78μm It is difficult to see the changing of Si particle along with the growth of the holding time. The hardness increased firstly and then dropped.6. When the annealing temperature is500℃and the holding time is10seconds, the mean diameter of the crystal increase along with the increasing of the percentage of reduction. When the percentage reduction is30.85%, the mean diameter of the crystal is6.63μm; when the percentage reduction is51.06%, the mean diameter of the crystal is6.4μm; when the percentage reduction is61.7%, the mean diameter of the crystal is6.41μm; when the percentage reduction is70.56%, the mean diameter of the crystal is7.92μm.7. The speed of the recrystallization of Al-12.7Si-0.7Mg alloy is so high that its difficult to see the nucleation.The nucleation mechanism is not been found. Coalescence of subgrains, strain-induced boundary migration mechanism, and the twins may be the spot of the nucleation.8. The nucleation and growth of the crystal after annealing is influenced by the Si and Mg2Si phase. If the particle size is large, t will promote the process of the recrystallization, and if the particle size is small, it will restrict the process of the recrystallization.