High-strength aluminum alloy extruded material with excellent corrosion resistance and method of producing the same

Abstract

The present invention provides a high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance and secondary workability and suitably used as a structural material for transportation equipment such as automobiles, railroad vehicles, and aircrafts, and a method of manufacturing the same. The aluminum alloy extruded product has a composition containing 0.6 to 1.2% of Si, 0.8 to 1.3% of Mg, and 1.3 to 2.1% of Cu while satisfying the following conditional expressions (1), (2), (3), and (4), 3%≦Si %+Mg %+Cu %≦4%   (1) Mg %≦1.7×Si %   (2) Mg %+Si %≦2.7%   (3) Cu %/2≦Mg %≦(Cu %/2)+0.6%   (4) and further containing 0.04 to 0.35% of Cr, and 0.05% or less of Mn as an impurity, with the balance being aluminum and unavoidable impurities. The cross section of the extruded product has a recrystallized structure with an average grain size of 500 μm or less. The manufacturing method includes, when extruding the aluminum alloy into a solid product by using a solid die, extruding the aluminum alloy by using a solid die in which a bearing length (L) is 0.5 mm or more and the bearing length (L) and the thickness (T) of the solid product have a relationship expressed as “L≦5 T”, and, when extruding the aluminum alloy into a hollow product by using a porthole die or a bridge die, extruding the aluminum alloy while setting the ratio of the flow speed of the aluminum alloy in a non-joining section to the flow speed of the aluminum alloy in a joining section in a chamber, where the billet reunites after entering a port section of the die in divided flows and subsequently encircling a mandrel, at 1.5 or less.

1 . A high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance, comprising an aluminum alloy which comprises, in mass%, 0.6 to 1.2% of Si, 0.8 to 1.3% of Mg, and 1.3 to 2.1% of Cu while satisfying the following conditional expressions (1), (2), (3), and (4),  
       3%≦Si %+Mg %+Cu %≦4%   (1)  Mg %≦1.7×Si %   (2)  Mg %+Si %≦2.7%   (3)  Cu %/2≦Mg %≦(Cu %/2)+0.6%   (4)  
     and further comprises 0.04 to 0.35% of Cr and 0.05 % or less of Mn as an impurity, with the balance being aluminum and unavoidable impurities, the aluminum alloy extruded product having a recrystallized structure with a grain size (average grain size; hereinafter the same) of 500 μm or less.  
   
   
       2 . The high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance, according to  claim 1 , wherein the aluminum alloy further comprises at least one of 0.03 to 0.2% of Zr, 0.03 to 0.2% of V, and 0.03 to 2.0% of Zn.  
   
   
       3 . A method of manufacturing a high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance, the method comprising: extruding a billet of the aluminum alloy according to  claim 1  into a solid product by using a solid die, in which a bearing length (L) is 0.5 mm or more and the bearing length (L) and a thickness (T) of the solid product to be extruded have a relationship expressed as “L≦5 T”, to obtain a solid extruded product of which a cross-sectional structure has a recrystallization texture with a grain size of 500 μm or less.  
   
   
       4 . The method of manufacturing a high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance according to  claim 3 , wherein a flow guide is provided at a front of the solid die, an inner circumferential surface of a guide hole in the flow guide being apart from an outer circumferential surface of an orifice which is continuous with the bearing of the solid die at a distance of 5 mm or more, and the flow guide having a thickness 5 to 25% of a diameter of the billet.  
   
   
       5 . A method of manufacturing a high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance, the method comprising: extruding a billet of the aluminum alloy according to  claim 1  into a hollow product by using a porthole die or a bridge die while setting a ratio of a flow speed of the aluminum alloy in a non-joining section to a flow speed of the aluminum alloy in a joining section in a chamber, where the billet reunites after entering a port section of the die in divided flows and subsequently encircling a mandrel, at 1.5 or less, to obtain a hollow extruded product of which a cross-sectional structure has a recrystallization texture with a grain size of 500 μm or less.  
   
   
       6 . The method of manufacturing a high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance according to  claim 3 , the method comprising: homogenizing the billet of the aluminum alloy at a temperature equal to or higher than 500° C. and lower than a melting point of the aluminum alloy; and heating the homogenized billet to a temperature equal to or higher than 470° C. and lower than the melting point of the aluminum alloy and extruding the billet.  
   
   
       7 . The method of manufacturing a high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance according to  claim 3 , the method comprising: a quenching step of maintaining a surface temperature of the extruded product immediately after extrusion at 450° C. or higher and then cooling the extruded product to 100° C. or lower at a cooling rate of 10° C./sec or more, or subjecting the extruded product to a solution heat treatment at a temperature of 480 to 580° C. at a temperature rise rate of 5° C./sec or more and then a quenching step of cooling the extruded product to 100° C. or lower at a cooling rate of 10° C./sec or more; and a tempering step of heating the extruded product at 170 to 200° C. for 2 to 24 hours.