| Although6xxx series aluminum alloy sheets have been the perfect light materials for automotive body sheet, there are still two severe problems which baffled their extensive application to automotive body. One is that their formability can not satisfy the request during the practical pressing of automotive body, and the other is that their aging strengthening response after paint baking process of automobile body fails to meet in-service strength requirements. This work is part of National High-Tech Research and Development Program (863program)"Manufacture Technology of High Formability Aluminum Alloy Sheet for Automotive Body Sheet". The purposes of the present study are to exploit a suitable heat treatment technique by which both the pressing formability and the bake-hardening effect during paint baking can be improved. The effects of solution temperature and solution time on the dissolution of alloy phases, recrystallization, formability and the bake-hardening ability of cold rolled Al-1.3Mg-1.3Si-0.6Cu-0.3Mn-0.3Fe aluminum alloy, also the effects of pre-aging temperature within the range of60℃～400℃, pre-aging time within the range of10s～15min on the formability and the bake-hardening ability during paint baking of sever kinds of Al-Mg-Si-Cu alloy sheets, which were holding at room temperature(RT) before pre-aging for lOmin,8h,48h and2week respectively after solution at550℃for5mins then water quenched, were examined integrally by means of metallographic microscopy, XRD, orientation distribution function (ODF) analysis, tensile and cupping test, forming limit diagram(FLD) measurement and differential scanning calorimetry (DSC). The results are as follows:1. The recrystallization of cold rolled Al-1.3Mg-1.3Si-0.6Cu-0.3Mn-0.3Fe alloy sheets is very fast, which can be completed within10s during solution treatment at540℃～560℃, and the recrystallization texture are mainly Cube+ND20orientation. The orientation and size of recrystallization grains have little changes when the solution holding time prolonged up to120min. 2. There are insoluble phases Al3FeSi and Al6Mn and soluble phases Al1.9CuMg4.1Si3.3, Mg2Si and Al2Cu in as cold rolled Al-Mg-Si-Cu alloy sheets. When the sheets were solution treated in the range of540℃to560℃, elevating the solution temperature the number of remained second phases drops off, while the size and morphology of the recrystallization grains in the matrix change slightly, the strength of sheets was enhanced, but the elongation and the n, r value didn’t change much with the solution temperature increasing.3. The appropriate solution treatment of cold rolled Al-1.3Mg-1.3Si-0.6Cu-0.3Mn-0.3Fe aluminum alloy sheets is540℃～560℃for10min～1min, and the solution time can be shortened with the solution temperature elevating.4. It can be found that the shorter the stand-to time after solution water quenching at RT before pre-aging, the more effectual of pre-aging treatment on improving the formability and bake-hardening of Al-Mg-Si-Cu alloy sheets.5. For6xxx series aluminum alloy sheets, when the stand-to time at RT after quenching is10min, the optimal pre-aging condition is pre-aging at140℃for12min, and σ0.2,σb,δU, δb, n, r15of aluminum alloy sheets after pre-aging treatment are142MPa,266MPa,18.4%,24%,0.26and0.66, respectively, and σ0.2of alloy sheets increases to225MPa after paint baking at170℃for30min.6. When the stand-to time at RT after quenching is8h, the optimal pre-aging condition is pre-aging at190℃for9min, and σ0.2,σb,δu,δb, n, r15of aluminum alloy sheets after pre-aging treatment are150MPa,269MPa,18%,22.7%,0.24and0.44, respectively, and σ0.2of alloy sheets increases to215MPa after paint baking at170℃for30min.7. When the stand-to time at RT after quenching is48h, the optimal pre-aging condition is pre-aging at300℃for3min, and σ0.2, σb,δu,δb, n, r15of aluminum alloy sheets after pre-aging treatment are157MPa,264MPa,16%,22%,0.21and0.68, respectively, and σ0.2of alloy sheets increases to220MPa after paint baking at170℃for30min.8. When the stand-to time at RT after quenching is2weeks, the optimal pre-aging condition is pre-aging at240℃for4min, and σ0.2, σb,δu,δ8b, n, r15of aluminum alloy sheets after pre-aging treatment are166MPa,305MPa,16%,22.4%,0.23and0.7, respectively, and σ0.2of alloy sheets increases to240MPa after paint baking at170℃for30min. |
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