| With the rapid development of economy and manufacturing industry in China,the global warming and environmental pollution are becoming more and more serious,and the demands for energy saving,emission reduction and green manufacturing are also more urgent.Aluminum alloy has the advantages of low density,high specific strength,good resistance,etc.Moreover,aluminum has the highest content in the earth and has the great development prospects.Aluminum alloy is an ideal lightweight material and has been widely used in many fields such as highspeed trains,aerospace and automobiles.Aluminum alloy plays an important role in energy saving,emission reduction and sustainable development.Extrusion is an important production method of aluminum alloy.Among the aluminum alloy plastic processed products,the output of aluminum alloy profiles produced by extrusion stands in the second place,and they are only lower than the rolled aluminum products.According to the structure of extrusion dies,the extrusion can be divided into flat die extrusion and porthole die extrusion.The flat die extrusion is mainly used for the production of solid aluminum profiles,while the porthole die extrusion is mainly used to produce the hollow aluminum profiles.The profiles extruded by porthole die have several longitudinal weld seams along the extrusion direction,and the weld seams are usually the weakest parts of the profile.The heat treatments involved in the extrusion processes are homogenization of billets,solution and aging treatments of extruded profiles,all of which greatly affect the microstructure and properties of the extruded profiles.The homogenization of as-cast billets can reduce the segregation of components in the billets,which can make the structure of billets more uniform,and improve the extrusion performance of the billets.The solution treatment can dissolve the precipitates into matrix to obtain a supersaturated solid solution,and the precipitates reprecipitated from supersaturated solid solution during aging treatment.Currently,there are few studies on the effects of homogenization treatment of the as-cast billets on the microstructure and properties of the extruded profiles,and the effects of solution and aging treatments on the anisotropy of the profiles.To solve the above problems,6005 and 7075 aluminum alloys were chosen as the raw materials.The heat treatments involved in extrusion such as homogenization,solution and aging were carried out,and the influence of heat treatment parameters on microstructure,mechanical properties and corrosion resistance of profiles extruded by flat die and porthole die were investigated.The main contents and conclusions are as follows:(1)The as-cast billets of 6005-aluminum alloy were subjected to homogenization at different temperatures and holding time,then extruded by flat die.It was found that the coarse ?-Al(FeMnCr)Si in billets dissolved and transformed to ?-Al(FeMnCr)Si during homogenization,then ?-Al(FeMnCr)Si transformed to ?-Al(FeMnCr)Si.Moreover,the Mg2Si and Mn-containing phase in billets with low melting point completely dissolved.Increasing the homogenization temperature and extending the holding time could reduce the size and quantity of second phase in billets.Thequantity of second phase in profiles could be decreased by reducing the quantity of second phase in billets.Increasing the temperature and extending the holding time could both reduce the quantity and size of the second phase in profiles,and could also increase the strength and elongation of profiles at the same time,but the excessively long holding time of billet made profiles grain too coarse,resulting in a decreasing in strength.(2)Solution and aging treatment were conducted using 6005 extruded profiles.The grains grew up obviously after solution treatment,and GP zones,?" phase and ?'phase appeared in order with increasing aging time.Cube and Goss were the major textures in extruded profiles.The proportion of Cube decreased dramatically after solution and aging treatments,while the proportion of Goss increased.The peak aging time is 20 hours,when the hardness and strength could reach the peak values.Before reaching the peak aging time,the strength and hardness increased with extending time.Over-aging occurred after peak aging time,resulting in the decrease of strength and hardness.The elongation continued to decrease with the increase of aging time.The profiles exhibited obvious plastic anisotropy,and the anisotropy could be decreased with the increase of aging time.(3)The as-cast billets of 7075 aluminum alloy were subjected to homogenization with different temperatures and cooling rates.Then,the billets were extruded by porthole die to obtain the profiles with longitudinal weld seam.The lamellar(?+Al)in 7075 as-cast transformed into ? and S phases during homogenization,while the Fe-containing phase changed slightly.With increasing homogenization temperature,the quantity and size of the second phase in billets decreased,and the fine dispersoids precipitated when the cooling rate was low and distributed more uniform.The profiles extruded by porthole die were composed by matrix zone and welding zone.The welding zone was composed by fine equiaxed grains,and matrix zone was composed by fine equiaxed and coarse elongated grains.The quantity of second phase in profiles decreased with increasing homogenization temperature.And the strength of profiles was determined by both second phases and fine dispersoids,increasing homogenization temperature improved strength and decreased elongation of profiles.Homogenization and increasing temperature could improve the corrosion performance in welding zone,while the corrosion performance in matrix zone decreased. |
PDF Full Text Request