Effect Of Zn Addition On Precipitation Behavior During Aging In An Al-Mg-Si-Cu Alloy For Automotive Panels

Al-Mg-Si alloys are widely used as automobile body panels because of their strength,good formability and corrosion resistance.Al-Mg-Si alloys are considered to be the most promising candidates of light weight materials for automobiles.These alloy sheets should have low strength for good formability,while high bake hardening response for in-service dent resistance.Understanding the precipitation evolution of 6xxx series alloys and the relationship between precipitate microstructures and properties is essential for optimizing chemical composition and developing aging treatment.Designing the new alloys and aging treatment process is importance to improve the mechanical properties of these alloys for the successful application in industry.In this study,the Al-Mg-Si-Cu alloys with and without Zn addition are designed.The effect of Zn addition on the clustering and precipitation evolution under different aging conditions were systematically studied by means of transmission electron microscopy(TEM),high resolution transmission electron microscopy(HRTEM),three-dimensional atomic probe(3DAP),Vickers hardness measurement and tensile test.The intrinsic relationship between precipitation microstructure and mechanical properties is also established.The main conclusions include:The artificial aging response and precipitation behavior of the experimental alloys are investigated.During artificial aging at 170 ?,the addition of Zn enhances the aging hardening response.The hardness of the Zn-added alloy increases rapidly with the aging time increasing,and the peak hardness of the Zn-added alloy is obviously higher than that of the Zn-free alloy.The precipitation sequence of Zn-added alloy during aging at 170 ?is similar to that of Zn-free alloys:SSSS?solute clusters?GP zones??" phases.No Zn-rich clusters or precipitates have been observed in Zn-added alloy.During artificial aging process,Zn does not significantly partition into clusters or precipitates,and most of Zn remains in A1 matrix.The presence of Zn in the A1 matrix enhances the partitioning of Mg,Si and Cu atoms from A1 matrix to clusters and precipitates,and thus prompts the formation of solute clusters during early stage of aging,subsequently stimulates the transformation from solutes cluster to GP zones to ?" precipitates.The characterization of precipitation evolution caused by Zn addition results in the enhanced age hardening response.The precipitation behavior and mechanical properties of the experimental alloys during pre-aging and subsequent bake hardening response are investigated.During pre-aging at 100 ? for 3 h,the Zn can partition into clusters because of the strong Zn-Mg interaction,prompting the formation of clusters.During subsequent artificial aging at 180 ? for up to 240 min(peak hardness condition),the Zn does not significantly partition into clusters or precipitates,and the majority of Zn remains in the Al matrix.However,the presence of Zn in the matrix stimulates the transformation from clusters to GP zones to ?"phases.The enhanced formation of GP zones and ?" phases correlates well with the remarkable age-hardening response.Compared to pre-aging at 50 ? for 6 h,pre-aging at 150 ? for 20 min enhances bake hardening response of the Zn-added alloy.The Mg-Si clusters can form after pre-aging at 150 ? for 20 min and at 50 ? for 6 h.Cu and Zn atoms can be also incorporated into these Mg-Si clusters,the Cu/Mg ratio and Zn/Mg ratio in the clusters formed under the two different pre-aging conditions are similar.The difference between the two pre-aging treatments is that pre-aging at 150 ? for 20 min prompts the formation of larger clusters with Mg/Si ratio close to 1,which can easily transform to ?"phase.During subsequent paint baking treatment at 185? for 25 min,?"phases are formed and have a refine and dense distribution,resulting in the high bake-hardening response.The sample pre-aged at 150 ? for 20 min has the higher number density and volume fraction of ?"with a lower size than the sample pre-aged at 50 ? for 6 h.Therefore,the sample pre-aged at 150 ? for 20 min exhibits an enhanced bake-hardening response.The natural aging after pre-aging and subsequent baking hardening response of the experimental alloys are investigated.During natural aging(NA)after pre-aging at 80 °C for 15 min(PA),cluster growth is the dominant process in the Zn-free and Zn-added alloys,associating with the partitioning of solute atoms from A1 matrix into clusters.Si has a much stronger partitioning than Mg during NA after PA,which leads to the increase in number of Si-rich clusters.Cu can also partition into the clusters of the two alloys during the NA treatment after PA.Some Zn atoms are partitioned into the clusters under PA+NA condition.Partitioning of Zn may change the stability of clusters,increasing the growth rate of clusters.The strength of the two alloys increases with the increasing NA time.The volume faction of clusters is the important factor responsible to strength.The higher volume faction of clusters leads to the higher strength in the Zn-added alloy during NA after PA.The prolonged NA inhibits the transformation of clusters to GP zones and ?"phases during bake hardening(BH)treatment at 170 ? for 30 min in the Zn-free and Zn-added alloys,resulting in the lower BH response.The Zn does not significantly partition into clusters or precipitates,and the majority of Zn remains in the A1 matrix during BH treatment,prompting the transformation from solute clusters to GP zones and ?"phases.As a result,the strength of the Zn-added alloy after PA+NA+BH treatment is higher than that of the Zn-free alloy.The cooling pre-aging process and subsequent natural aging behavior and baking hardening response are investigated.Compared with isothermal pre-aging(IPA)at 90 ?for 3.5 h,cooling pre-aging(CPA)at 90?50 ? for 6 h prompts the formation of solute clusters,which provides more nucleating sites for the ?" phases.Thus the formation of ?"phase is enhanced during subsequent bake hardening(BH)treatment at 170 ? for 30 min,resulting in the higher BH response.Compared with IPA+NA process,the average spacing of clusters formed during natural aging(NA)after CPA is smaller and the cluster shear modulus is higher.This leads to the higher increase in strength during NA after CPA.The Si-rich clusters are formed under CPA+NA condition,due to the stronger partitioning of Si atoms than Mg atoms.These clusters can not be directly transformed into ?" phase during BH treatment,reducing the BH response of the experimental alloy during long-term NA after CPA.