Extruded member of aluminum alloy excelling in flexural crushing performance and corrosion resistance and method for production thereof

Abstract

An extruded member of Al—M13 Si aluminum alloy specially composed of Mg, Si, Fe, Cu, Zn, Ti, etc. which has the equiaxed re-crystallized grain structure in which intergranular precipitates 1 μm or larger are separate from one another at large average intervals and there are many cube orientations over the entire thickness region thereof so that it excels in both flexural crushing performance and corrosion resistance. The extruded member is suitable for use as automotive body reinforcement members which need outstanding lateral crushing performance under severe collision conditions as well as good corrosion resistance.

1 - 6 . (canceled) 
     
     
         7 . A method for producing an extruded member of aluminum alloy, said method comprising:
 (1) soaking a cast billet of aluminum alloy at 500-590° C., said billet containing (in mass %) Mg:0.60-1.20%, Si:0.30-0.95%, Fe:0.01-0.40%, Mn:0.001-0.35%, Cu:0.001-0.65%, Zn:0.001-0.25%, and Ti:0.001-0.10%, with the remainder being aluminum and inevitable impurities,   (2) subjecting the soaked billet to forced cooling to 400° C. or below at an average cooling rate of 100° C./hr or above,   (3) reheating the cooled billet and subjecting the reheated billet to hot extrusion such that the extrudate reaches the solid solution temperature which is 500° C. or higher at the extruder exit,   (4) immediately subjecting the extrudate to forced cooling at an average cooling rate of 100° C./hr or above, and   (5) subjecting the cooled extrudate to aging,   so that the resulting extruded member has a 0.2% proof stress of 240 MPa or greater and also has the metallographic structure whose cross section perpendicular to the direction of extrusion shows the equiaxed recrystallized grain structure in which intergranular precipitates 1 μm or larger in terms of diameter of an equivalent circle are 3μm or more separate from one another in the observation under a TEM of 5000 magnifications and also the average areal ratio of the cube orientation is 15% or larger over the entire thickness region including the grain growth layer in the outermost surface in the cross section perpendicular to the direction of extrusion.   
     
     
         8 . The method for producing an extruded member of aluminum alloy according to  claim 7 , wherein the cast billet of the aluminum alloy further contains at least one of Cr:0.001-0.18% and Zr:0.001-0.18%, in a total amount of 0.30% or less. 
     
     
         9 . The method for producing an extruded member of aluminum alloy according to  claim 7 , wherein the cast billet of the aluminum alloy contains Mg and Si such that Mg(%)≧1.73×Si(%)−0.4; where Mg(%) and Si(%) denote the content of Mg and Si in mass %, respectively. 
     
     
         10 . The method for producing an extruded member of aluminum alloy according to  claim 7 , wherein the extruded member of aluminum alloy shows has an equiaxed recrystallized grain structure such that the average areal ratio of cube orientation is 20% or larger. 
     
     
         11 . The method for producing an extruded member of aluminum alloy according to  claim 7 , wherein the extruded member of aluminum alloy has flexural crushing performance such that the critical bending radius (R) is 3.0 mm or smaller which does not cause cracking in the 180° bending test according to JIS Z2248 in which the platy specimen is bent in the direction of extrusion, and has corrosion resistance such that the specimen does not suffer intergranular corrosion in the alternating immersion corrosion test according to ISO/DIS 11846B. 
     
     
         12 . The extruded member of aluminum alloy produced by the method of  claim 7 . 
     
     
         13 . The extruded member of aluminum alloy produced by the method of  claim 8 .