Research On The Effect Of Natural Aging On The Mechanical Properties And Bake Hardening Properties Of An Al-Mg-Si Alloy

With the current increasingly severe global environmental problems and increasingly stringent automotive exhaust emission standards,reducing vehicle weight has become one of the urgent tasks for all automobile manufacturers.Increasing the use of lightweight alloys materials in automotive parts and replacing steel with lightweight alloys are the effective and immediate methods to reduce the vehicle weight.AA6014 alloy is currently the mainstream aluminum alloy material used in the exterior panels of middle and high-end passenger cars due to its excellent hemming property and good corrosion resistance,so it is necessary to conduct in-depth research on it.When the Al-Mg-Si alloys are placed at room temperature,as time goes by,mechanical properties change significantly and always results in a delayed and compromised artificial aging hardening behaviour.This phenomenon is often referred to as the “negative effect” of natural aging characteristic of Al-Mg-Si alloys.At present,many studies have characterized the typical nano-scale precipitates of Al-Mg-Si alloys and basically determined the precipitation sequence.However,it is rare to be seen the research that quantitatively describes the changes of mechanical properties and bake hardening properties of Al-Mg-Si alloys during a long period of time.Therefore,this subject is developed around the following two core purposes: First,quantitatively and systematically determine the changes in mechanical properties of AA6014 aluminum alloy sheets during the 6-month validity period.The second is to study the effects of different natural aging durations on the bake hardening properties of AA6014 aluminum alloy sheets within 6 months.The main research conclusions of this paper are as follows:(1)Within 6 months,the 0.2% offset yield strength and ultimate tensile strength of AA6014 aluminum alloy sheet change significantly along 0°,45° and 90° directions.The average increment is 45 MPa and 34 MPa,and the average percent of increase is 46% and16%.The ratios of 0.2% offset yield strength and ultimate tensile strength show an upward trend with the extension of natural aging time,ranging from 0.46 to 0.6.Along the three directions,the strength and the strength increment along the 0° direction are higher than the other two directions,and the 45° direction and the 90° direction are basically the same.(2)The elongation does not decrease significantly with the extension of natural aging time,but fluctuate within a certain range.Along the three directions,the average values of plastic extension at maximum force and elongation after fracture are basically the same,and the overall average values are 21.58% and 23.34%,respectively.Throughout the whole experiment time period,plastic extension at maximum force maintains above 18%,and elongation after fracture maintains above 20%.The value of plastic extension at maximum force after bake hardening are above 12% and the elongation after fracture are above 13%.Overall,elongation decreases by ～6.5% due to bake hardening.(3)Before bake hardening,along the three directions,n value shows a downward trend with the extension of the natural aging time,and all the n values are in the range of0.21 to 0.28.After bake hardening,with the extension of the natural aging time samples undergone,n values along the three directions remain stable in the first four months,and shows an upward trend in the 4-6 months of natural aging.The overall range is between0.14 and 0.17.Among them,no matter before and after bake hardening,n values along 90°direction are the largest.(4)After bake hardening,the strength obtained after bake hardening along the three directions all show waving changes.However,with the different length of natural aging time experiencing before bake hardening,the increment of 0.2% offset yield strength and ultimate tensile strength show an obvious downward trend.Along the three directions,during the whole experiment period,the increments of yield strength decrease from130 MPa to 87 MPa,144MPa to 85 MPa,and 131 MPa to 78MPa;the increments of ultimate tensile strength decrease from 80 MPa to 44 MPa,92MPa to 45 MPa,and 75 MPa to 43 MPa.We obtain six mechanical properties(including hardness,0.2% Offset Yield Strength,Ultimate Tensile Strength,Plastic extension at maximum force,Elongation after fracture,and Strain Hardening Exponent)by a large amount of micro-hardness measurements and tensile tests.The data reveals the changes of each mechanical property during natural aging and after bake hardening heat treatment within 6 months in detail,which can not only be used to provide data reference for industrial production of automobile manufacturers,but also can be used to reveal the microscopic mechanism of the natural aging process.