Alluminum alloy pipe and aluminum alloy structural member for automobile using the same

Abstract

An Al—Mg-based aluminum alloy pipe for hot working, having an alloy composition having from 2.5% by mass to 2.8% by mass of Mg, 0.25% by mass or less of Si, 0.35% by mass or less of Fe, and from 0.25% by mass to 0.35% by mass of Cr, with the balance being inevitable impurities and Al, wherein an area ratio of cavities after hot working is 2.3% or less; and an aluminum alloy structural member for automobile using the same.

1 . An Al—Mg-based aluminum alloy pipe for hot working, having an alloy composition comprising from 2.5% by mass to 2.8% by mass of Mg, 0.25% by mass or less of Si, 0.35% by mass or less of Fe, and from 0.25% by mass to 0.35% by mass of Cr, with the balance being inevitable impurities and Al, wherein an area ratio of cavities after hot working is 2.3% or less. 
     
     
         2 . The Al—Mg-based aluminum alloy pipe for hot working according to  claim 1 , wherein the area ratio of cavities after hot working is 1.0% or less. 
     
     
         3 . The Al—Mg-based aluminum alloy pipe for hot working according to  claim 1 , wherein a minimum pipe wall thickness of the pipe after pipe expanding by hot working is 83% or more of an average thickness of the pipe wall thickness. 
     
     
         4 . The Al—Mg-based aluminum alloy pipe for hot working according to  claim 2 , wherein a minimum pipe wall thickness of the pipe after pipe expanding by hot working is 90% or more of the average thickness of the pipe wall thickness. 
     
     
         5 . The Al—Mg-based aluminum alloy pipe for hot working according to  claim 3 , wherein a crystal grain diameter after hot working of the aluminum alloy pipe is 300 μm or less. 
     
     
         6 . The Al—Mg-based aluminum alloy pipe for hot working according to  claim 4 , wherein a crystal grain diameter after hot working of the aluminum alloy pipe is 300 μm or less. 
     
     
         7 . An Al—Mg-based aluminum alloy pipe obtained by hot working the Al—Mg-based aluminum alloy pipe for hot working according to  claim 1 , which has a tensile strength from 175 to 235 MPa and a proof stress from 70 to 95 MPa. 
     
     
         8 . An automobile structural member made of an aluminum alloy, obtained by hot working an Al—Mg-based aluminum alloy pipe, having an alloy composition comprising from 2.5% by mass to 2.8% by mass of Mg, 0.25% by mass or less of Si, 0.35% by mass or less of Fe, and from 0.25% by mass to 0.35% by mass of Cr, with the balance being inevitable impurities and Al, wherein an area ratio of cavities after hot working is 2.3% or less, a crystal grain diameter after hot working of the aluminum alloy pipe is 300 μm or less, a minimum pipe wall thickness of the pipe after pipe expanding by hot working is 83% or more of an average thickness of the pipe wall thickness, and a tensile strength is from 175 to 235 MPa and a proof stress is from 70 to 95 MPa after hot working of the aluminum alloy tube, respectively. 
     
     
         9 . An automobile structural member made of an aluminum alloy, obtained by hot working an Al—Mg-based aluminum alloy pipe, having an alloy composition comprising from 2.5% by mass to 2.8% by mass of Mg, 0.25% by mass or less of Si, 0.35% by mass or less of Fe, and from 0.25% by mass to 0.35% by mass of Cr, with the balance being inevitable impurities and Al, wherein an area ratio of cavities after hot working is 1.0% or less, a crystal grain diameter after hot working of the aluminum alloy pipe is 300 μm or less, a minimum pipe wall thickness of the pipe after pipe expanding by hot working is 90% or more of an average thickness of the pipe wall thickness, and a tensile strength is from 175 to 235 MP and a proof stress is from 70 to 95 MPa after hot working of the aluminum alloy tube, respectively. 
     
     
         10 . An aluminum alloy automobile structural member using the Al—Mg-based aluminum alloy pipe for hot working according to  claim 1  after hot working, wherein a tensile strength is from 175 to 235 MPa and a proof stress is from 70 to 95 MPa after the hot working, respectively, and wherein fluctuations of the tensile strength and proof stress are 10 MPa or less, respectively. 
     
     
         11 . An aluminum alloy automobile structural member using the Al—Mg-based aluminum alloy pipe for hot working according to  claim 1  after extrusion followed by hot working, wherein a fatigue strength upon 1×10 7  times after hot working is 70 MPa or more, and a fluctuation of the fatigue strength upon 1×10 7  times after hot working is 20 MPa or less. 
     
     
         12 . An aluminum alloy automobile structural member using the Al—Mg-based aluminum alloy pipe for hot working according to  claim 1  after hot working, wherein a tensile strength is from 175 to 235 MPa and a proof stress is from 70 to 95 MPa after the hot working, respectively, fluctuations of the tensile strength and proof stress are 10 MPa or less, respectively, a fatigue strength upon 1×10 7  times after hot working is 70 MPa or more, and a fluctuation of the fatigue strength upon 1×10 7  times after hot working is 20 MPa or less. 
     
     
         13 . A structural member for motor bicycles or four-wheel automobiles made of the aluminum alloy according to  claim 10 .