New Type Of Aluminum Alloy For Automotive Body Sheet And Its Bake Hardening Properties

Due to the advantages of medium strength,excellent formability,low density and ability to be strengthened through heat treatment,Al-Mg-Si-Cu alloy has great application prospects in lightweight vehicles for energy-saving and emission-reduction.Based on the application of 6xxx aluminum alloy in automobile panel,this thesis studies the effect of different pre-treatment on paint bake hardening in Al-Mg-Si-(Cu)alloy with different alloy elements through micro-hardness test,tensile test.At the same time,the effect of pre-treatment on precipitation behavior during paint bake process and relationship between micro structure and mechanical property are investigated using differential scanning calorimetry(DSC)and transmission electron microscopy(TEM).This thesis is organized as follows:1.effect of pre-ageing treatments on paint bake response for Al-Mg-Si alloy;2.effect of pre-ageing treatments on paint bake response for Al-Mg-Si-Cu alloy;3)effect of pre-straining and pre-deformation on paint bake response for Al-Mg-Si-Cu alloy.Main conclusions of the study are summarized as below:(1)The hardness and elongation to fracture of the Al-Mg-Si(AA6016)alloy treated by pre-ageing could remain below 65HV and above 25%,respectively,after the duration of one month at room temperature.Then after paint bake cycle for a short time,the hardness and yield strength increase to above 100HV and 220MPa,separately.Compared to samples treated by one-step pre-ageing,stability of the mechanical properties as well as the hardness and strength after paint bake of samples treated by two-step pre-ageing is higher.The quick bake hardening response in AA6016 alloy is associated with the marked growth of the fine precipitates formed in the pre-aged samples during paint bake cycle.The fine,dense and coherent particles,whose lengths are about 2-3 nm,precipitate in the pre-aged alloys.These particles grow remarkably in the paint bake cycle and become needle-like precipitates with a length above 10 nm.High-resolution TEM observations reveal that the precipitates are monoclinic GP zones and pre-β".(2)Pre-treatments including both pre-ageing and pre-straining are effective to enhance paint bake hardening response by eliminating negative influence of natural ageing.The alloys pre-aged at 120℃ or 150℃ could keep in low strength and high ductility even after two-month natural ageing,and the increase of yield strength could reach 80MPa～100MPa during paint bake treatment.(3)Pre-ageing imparts the same degree of strength enhancement in the three Al-Mg-Si-Cu alloys(total alloying element content is similar,Mg/Si=2,1,0.5)during paint bake treatment.This means that the Al-Mg-Si-Cu alloy with a high ratio of Mg possesses significant quick bake hardening response and could be used as automotive body panel alloy.In addition,this alloy is more stable during room temperature storage and quick paint bake response remains after longer room-temperature storage.What’s more,compared to alloy with high content of Si,hardness of this alloy could keep even after long thermal exposure.Thus longer service life and higher safety performance are expected.This finding provides new insights into the chemistry design for automobile panel.(4)Though the introduced dislocations could slightly enhance the paint bake hardening response,pre-straining and pre-deformation have little effect on eliminating negative influence of natural ageing.Pre-straining and pre-deformation are more beneficial in alloys with Mg/Si ratio not exceeding 1.Though treated with the same 4%pre-straining,the yield strength increase after bake treatment of alloy with Mg/Si ratio of 0.5 could be 50MPa higher than that of alloy with Mg/Si ratio of 2.(5)Continuous precipitations along dislocation are found in alloys with 4%pre-straining processed before natural ageing,but not in the same alloy with 2%pre-deformation after pre-ageing and natural ageing.The steady nuclei have formed during pre-ageing,dislocations act as atomic transport corridor to promote the transformation from clusters to needle-like β",and the formation of coarsening precipitated phases.