Study On Dynamic Recrystallization In Al-Mg-Si Alloy

AA6013 aluminum alloy is widely used in automobile manufacturing,mobile phones and other electronic and electrical industries.However,there are some problems,such as poor molding at room temperature and inhomogeneous distribution of microstructure during hot deformation.Therefore,the machinability of 6013 aluminum alloy at high temperature is limited.In this paper,the hot deformation behavior of Al-MgSi alloys was investigated by isothermal compression tests.The constitutive model and hot processing map were established to obtain the optimal hot working parameters(temperature,strain and strain rate)during hot deformation.The effects of different strains on microstructure,dislocation density and dynamic recrystallization(DRX)volume fraction were studied by using the inhomogeneous distribution of strain.Using the JMAK dynamic recrystallization model and DEFORM-3D finite element software to simulate the industrial hot extrusion process and study the effects of different dies and extrusion speeds on the strain,damage value and DRX volume fraction.The dynamic softening mechanism was discussed through micro structure analysis,and the relationship between Zener Hollomon parameter(Z parameter)and DRX mechanism was established.The main conclusions are as follows:1.The flow stress increased with increasing temperature and decreasing strain rate.Based on compensation for strain,there was an excellent correlation between the experimental and predicted stress.Besides,the optimum processing conditions were in the temperature range of 470-550℃ and strain rate range of 0.001-0.06 s-1 according to the hot process maps.2.Under a certain strain,the higher temperature and lower strain rate would lead to higher DRX volume fraction.After hot compression,the distribution of strain of the samples was inhomogeneous,and the micro structure in the nominal strain zone was coarse near equiaxed grains,while the grains in the maximum strain zone were flat.The critical dislocation density(ρc)and the average residual dislocation density(p)decreased with increasing temperature and decreasing strain rate,and the average dislocation density increased with the increase of strain.At the same strain rate and temperature,ρc was higher than ρ.3.The DRX fractions increased with the decrease of In Z.Three different DRX mechanisms:continuous dynamic recrystallization(CDRX),discontinuous dynamic recrystallization(DDRX)and geometric dynamic recrystallization(GDRX)were observed in the experimental materials.DDRX happened through grain boundary bulging,subgrain rotation,and grain boundary migration.CDRX can happen through purely subgrain rotation or subgrain rotation+grain boundary migration.GDRX happened through thinning of grains,serration of grain boundary,pinch-off of original grains by one or two original grain boundaries(≥35°).Both dynamic recovery(DRV)and DRX happened during deformation at elevated temperatures.DRV dominated at high In Z and DRX dominated at low In Z.Three different DRX mechanisms,CDRX,DDRX and GDRX were observed.The main DRX mechanism depended on In Z,i.e.,GDRX dominated at low In Z,CDRX dominated at middle ln Z,while DDRX dominated at high In Z.4.Using the acquired JMAK model and DEFORM-3D,thermal compression and industrial thermal extrusion were simulated.The results showed that the DRX volume fraction were in good agreement with the experimental results.The damage value,strain and DRX fraction increased with the increase of extrusion speed.When the extrusion speed exceeded 2.5mm/s,the extrusion force changed significantly.The deformation degree of flat die sheet was small and uniform and the extrusion surface quality was relatively better,while the deformation degree of concave die sheet was relatively larger which was easy to produce greater strain and damage,so the DRX was greater,but the strain distribution along the thickness and horizontal direction was inhomogeneous.