| In efforts to reduce vehicle weight and reduce emissions of automobiles,aluminum alloys have been widely utilized in Europe and North America.Al-Mg-Si(6xxx)alloy sheets are easily formed and strengthened during the paint baking process in the automotive factory.Resistance to fracture of these 6xxx aluminum sheet parts in service can also be an important consideration.Energy absorption and predictable deformation behavior during an impact are essential to the safety of automotive components.Therefore different 6xxx alloys are employed across a broad field of application depending on their combination of strength and ductility.However,due to the research of aluminum alloy is immaturity,the use of aluminum auto body sheet is still at a low level in domestic Chinese automobile industry.Hence,a detailed study of the relationship between heat treatment,microstructure and properties of 6xxx alloys can provide significant theoretical support and technical guidance in automotive industry.In the present work,Quenching and artificial aging were used to create a range of microstructures and properties for alloy 6016 sheet.Commercially produced AA6016-T4 temper sheet was used as a reference material.Re-solution heat treatment was carried out at 560 ~oC for 5 min,and these samples were quenched to room temperature by two different media:25 ~oC water and 25 ~oC air,then artificially aged was performed at 180 ~oC.Properties of samples in each tempers were characterized via Vickers hardness and electric conductivity,testing of bend specimens and an~100micron deep surface notched specimen in both underage and T6 tempers was used to obtain the bendability,notched tensile strength and elongation.Scanning Electron Microscope(SEM)and Transmission Electron Microscope(TEM)were carried out to examine the microstructure and fracture surface of the specimens.The main conclusions were drawn as follow:(1)Fast quenching rate results in high hardness and strength.Since the matrix is rich in supersaturated solid solution,during artificial ageing,large numbers of cluster(2)were formed and then transformed into strengthening?''phase,hence,the hardness of water quenched specimens increases fastest in the beginning of artificial ageing and the peak aged hardness is the highest.(2)Water quenched specimens show better bendability,while air quenched specimens have worst bendability in T6 temper.Slow quenching(AQ)specimens showed wide precipitate free zones(PFZ)and grain boundary precipitate particles,which promoted intergranular brittle fracture and poor bendability.(3)Non-resolution heat treated(AR)specimens have high strength and elongation,and the fracture mode dominated the tensile elongation values after artificial ageing.AR specimens showed mixture fracture mode and broken in high elongation,whereas,brittle fracture was the major fracture mechanism in AQ specimens and resulted in low elongation.(4)The fracture toughness correlated with the yield strength and the n-value of the notched tensile test(NTT).AR specimens have higher notch tensile elongation,static toughness,n-value and fracture toughness at similar strength compared to air quenched material.This confirms the critical role of quenching and grain boundary microstructure on fracture toughness of AA6016 artificially aged automobile body sheets(ABS).(5)The use of a surface-notched specimen has distinct advantages over the bending test as a toughness indicator for thin aluminum ABS.The surface notch initiates a through-thickness crack directly,NTT does not require prior deformation to form shear bands to initiate failure,and does not suffer from spring back.Unlike the bending test,the notch tensile test fracture depends more clearly on microstructure and generates a toughness parameter that can be more readily used in fracture models. |
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