Structural Automotive Part Made From an Al-Zn-Mg-Cu Alloy Product and Method of its Manufacture

Abstract

A method of manufacturing a formed aluminium alloy body-in-white (“BIW”) part of a motor vehicle, the BIW part having a yield strength of more than 500 MPa after being subjected to a paint-bake cycle. The method includes providing a rolled aluminium sheet product of an AlZnMgCu alloy and having a gauge in a range of 0.5 to 4 mm and subjected to a solution heat treatment (SHT) and quenched following SHT, and wherein the SHT and quenched aluminium sheet product has a substantially recrystallized microstructure, forming the aluminium alloy sheet to obtain a formed BIW part, assembling the formed BIW part with one or more other metal parts to form an assembly forming a motor vehicle component, subjecting the motor vehicle component to a paint bake cycle, wherein the aluminium alloy sheet in the formed BIW part has a yield strength of more than 500 MPa.

1 . Method of manufacturing a formed aluminium alloy body-in-white (BIW) part of a motor vehicle, the BIW part having a yield strength of more than 500 MPa after being subjected to a paint-bake cycle, the method comprising:
 a. providing a rolled aluminium sheet product having a gauge in a range of 0.5 to 4 mm and comprising an aluminium alloy being subjected to a solution heat treatment (SHT) and having been quenched following said SHT, and wherein the SHT and quenched aluminium alloy of the sheet product has a substantially recrystallised microstructure, and a chemical composition of, in weight percent,
 Zn 6.9 to 8.0, 
 Mg 1.2 to 2.4, 
 Cu 1.3 to 2.4, 
 Mn <0.3, 
 either 0.05 to 0.25 of Cr or Zr, 
 Si <0.3, 
 Fe <0.35, 
 Ti <0.1, 
 impurities and others each <0.05, total <0.2, balance aluminium; 
   b. forming the aluminium alloy sheet to obtain a formed BIW part,   c. assembling the formed BIW part with one or more other metal parts to form an assembly forming a motor vehicle component;   d. subjecting said motor vehicle component to a paint bake cycle, wherein the paint bake cycle comprises at least one heat treatment of holding the assembly forming the motor vehicle component at a temperature in a range of 140 ° C. to 190 ° C. for a period of 10 to less than 40 minutes, and wherein the aluminium alloy sheet in the formed BIW part has a yield strength of more than 500 MPa.   
     
     
         2 . Method according to  claim 1 , wherein the aluminium alloy has Zr in a range 0.04% to 0.25%. 
     
     
         3 . Method according to  claim 1 , wherein the aluminium alloy has a Cu content in a range of 1.4% to 1.8%. 
     
     
         4 . (canceled) 
     
     
         5 . Method according to  claim 1 , wherein the whole aluminium alloy sheet within 10 hours prior to forming in step b. has been heated to a temperature in a range of 400° C. to 490° C. and soaked at this temperature for a period of 3 sec. to 15 min. and then rapidly cooled. 
     
     
         6 . Method according to  claim 1 , wherein the whole aluminium alloy sheet within 10 hours prior to forming in step b. has been heated to a temperature in a range of 450° C. to 480° C. and soaked at this temperature for a period of 3 sec. to 15 min. and then rapidly cooled. 
     
     
         7 . Method according to  claim 1 , wherein the aluminium alloy sheet has been artificially aged to a yield strength of 500 MPa or more prior to forming in step b. 
     
     
         8 . Method according to  claim 1 , wherein the rolled aluminium sheet product has a core layer of the aluminium alloy and a clad layer on at least one side of the core layer. 
     
     
         9 . (canceled) 
     
     
         10 . (canceled) 
     
     
         11 . Method according to  claim 1 , wherein the aluminium alloy has a Zn content in a range of 6.9% to 7.8%. 
     
     
         12 . Method according to  claim 10  or  11   1 , wherein the aluminium alloy has a Mg content in a range of 1.4% to 2.1%. 
     
     
         13 . (canceled) 
     
     
         14 . (canceled) 
     
     
         15 . (canceled) 
     
     
         16 . Method according to  claim 1 , wherein the aluminium alloy has Zr in a range 
     
     
         0 . 07% to 0.18%. 
     
     
         17 . Method according to  claim 1 , wherein the aluminium alloy has a Si content in a range of 0.1% to 0.25%. 
     
     
         18 . Method according to  claim 1 , wherein the aluminium alloy has a Fe content in a range of 0.1% to 0.25%. 
     
     
         19 . Method according to  claim 1 , wherein the aluminium alloy has a Cr content in a range of 0.05% to 0.25% and an absence of Zr. 
     
     
         20 . Method according to  claim 1 , wherein the rolled aluminium alloy sheet product has a gauge in a range of 0.7 to 3.5 mm. 
     
     
         21 . Method according to  claim 1 , wherein the aluminium alloy sheet has been artificially aged to a T6 or T7 temper prior to forming in step b. 
     
     
         22 . Method according to  claim 1 , wherein the aluminium sheet product has been artificially aged to a yield strength of at least 540 MPa, prior to forming in step b. 
     
     
         23 . Method according to  claim 1 , wherein the whole aluminium alloy sheet within 1 hour prior to forming in step b. has been heated to a temperature in a range of 400° C. to 490° C. and soaked at this temperature for a period of 3 sec. to 15 min. and then rapidly cooled. 
     
     
         24 . Method according to  claim 1 , wherein the whole aluminium alloy sheet within 10 hours prior to forming in step b. has been heated to a temperature in a range of 400° C. to 490° C. and soaked at this temperature for a period of 3 sec. to 10 min. and then rapidly cooled. 
     
     
         25 . Method according to  claim 1 , wherein the whole aluminium alloy sheet within 10 hours prior to forming in step b. has been heated to a temperature in a range of 400° C. to 490° C. and soaked at this temperature for a period of 3 min. to 10 min. and then rapidly cooled. 
     
     
         26 . Method according to  claim 1 , wherein the SHT and quenched aluminium alloy sheet is subjected to forming in step b. within less than 2 weeks after the quench. 
     
     
         27 . Method according to  claim 1 , wherein the SHT and quenched aluminium alloy sheet is subjected to forming in step b. within less than 4 days after the quench. 
     
     
         28 . Method according to  claim 1 , wherein the SHT and quenched aluminium sheet is subjected to forming in step b. before it reaches a yield strength of about 400 MPa. 
     
     
         29 . Method according to  claim 1 , wherein during step b. forming is by means of stamping. 
     
     
         30 . Method according to  claim 1 , wherein during step b. forming is by means of deep drawing. 
     
     
         31 . Method according to  claim 8 , wherein the aluminium sheet product comprises a layer of the aluminium alloy having a gauge in the range of 0.5 to 4 mm. 
     
     
         32 . Method according to  claim 8 , wherein each said clad layer consists of an AA5xxx-series alloy having more than 3.8 wt. % Mg, wherein each said clad layer has a thickness in a range of 2% to 30% of the thickness of the core layer. 
     
     
         33 . Method according to  claim 8 , wherein each said clad layer is an AA5xxx-series alloy having, in wt.%:
 Mg 3.8% to 7.0%,   Zn 0.6% to 2.8%,   Mn 0 to 1.0%,   Cu 0 to 2.0%,   optionally at least one element selected from the group consisting of: Zr 0.04 to 0.3%, Cr 0.04 to 0.3%, Hf 0.04 to 0.3%, and Ti. 0.01 to 0.2%,   Fe max. 0.3%,   Si max. 0.3%,   inevitable impurities,   balance aluminium, and   
       wherein the range for the Zn-content is a function of the Mg-content according to:
 lower-limit of the Zn-range: [Zn]=0.34[Mg]−0.4, and 
 upper-limit of the Zn-range: [Zn]=0.34[Mg]+0.4, 
 
       wherein each said clad layer has a thickness in a range of 2% to 30% of the thickness of the core layer.