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

Abstract

An extruded member of Al—Mg—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 lager 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 . An extruded member of aluminum alloy which contains (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, and 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 lager in terms of the diameter of an equivalent circle are 3 μm or more separate from one another in the observation under a TEM of 5000 magnifications and also the average areal ratio of 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. 
   
   
       2 . The extruded member of aluminum alloy as defined in  claim 1 , which 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. 
   
   
       3 . The extruded member of aluminum alloy as defined in  claim 1 , which has the equiaxed recrystallized grain structure such that the average areal ratio of cube orientation is 20% or larger. 
   
   
       4 . The extruded member of aluminum alloy as defined in  claim 1 , which contains at least either of Cr: 0.001-0.18% or Zr: 0.001-0.18% in a total amount of 0.30% or less. 
   
   
       5 . The extruded member of aluminum alloy as defined in  claim 1 , which 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 which has corrosion resistance such that the specimen does not suffer intergranular corrosion in the alternating immersion corrosion test according to ISO/DIS 11846B. 
   
   
       6 . An energy absorbing member formed from the extruded member of aluminum alloy defined in  claim 1 , which crushes under load in the direction perpendicular to the direction of extrusion. 
   
   
       7 . A method for producing an extruded member of aluminum alloy, said method comprising a step of 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, a step of subjecting the soaked billet to forced cooling to 400° C. or below at an average cooling rate of 100° C./hr or above, a step of 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, a step of immediately subjecting the extrudate to forced cooling at an average cooling rate of 100° C./hr or above, and a step of 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 lager in terms of diameter of an equivalent circle are 3 μm 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 as defined in  claim 7 , wherein the cast billet of said Al—Mg—Si aluminum alloy further contains at least either of Cr: 0.001-0.18% or Zr: 0.001-0.18% in a total amount of 0.30% or less.