A Study On Precipitation Strengthening And Corrosion Behaviors Of Medium,High Strength Al Alloys And Their Sintered Rapidly Solidified Ultra-fine Grain Materials

Aluminum alloys is a preferred metalzllic material for light weight vehicles.7075 aluminum alloy is a widely used lightweight high-strength structural materials.It has a large number of applications in aerospace,transportation and other fields,but sensitive to stress corrosion.7075 contains not only a large number of different alloying elements,but also a relatively high content of alloying elements.It possesses a large amount of strengthening intermetallic compounds and reduces the plasticity and corrosion resistance of the material.Some of these intermetallic compounds are difficult to be re-dissolved into the matrix during the solutionization process,and also in the subsequent heat treatment.Using rapid solidification can increase the solid solubility of the alloying elements in the matrix,and the microstructure can be refined to improve the strength and toughness of the material.6xxx series aluminum alloy has excellent formability,it is the only aluminum alloy that is insensitive to stress corrosion.In recent years,large quantities have been used in the automotive industry to replace steel as body panels.To make the 6xxx series aluminum alloy reach the mechanical requirements of the automotive body panel,"bake hardening" is a key process.However,the natural aging will have a "negative effect" on the hardening of the aluminum alloy used for automotive body panel.It is necessary to find a way to overcome this problem.Pre-aging and pre-strain are effective ways to solve this problem,but there are few reports that combine these two processes to conduct in-depth research.In this project,a newly developed 6014 aluminum alloy,which is suitable to be used as body panel is taken as the research object,and the processes of solution treatment,natural aging,artificial aging,pre-aging,and pre-strain are used.The 6014 aluminum alloy was treated to study the effect of various processes on the precipitation behavior and mechanical properties of the material.Aluminum alloy used in cars has become an upsurge.In the near future,there is a large amount of aluminum panel scraps that need to be recycled.This subject has has to be taking care of.The preparation of Al alloy ribbons and their sintered materials were studied in order to achieve the purpose of recycling the scraped aluminum alloys.In this paper,ultra-fine grain ribbons of 6014 and 7075 aluminum alloys were prepared by single-roller melt spin-quenching method.Then fine-grained aluminum alloys 6014 and 7075 were prepared by spark plasma sintering(SPS)and hot extrusion methods,respectively to fabricate powder metallurgy blocks and rods.Use different fly wheel speed,air pressure,superheat temperature and other parameters to explore the best processing parameters.The optimum ribbon sintering process was explored with different temperatures and extrusion ratios.The 6014 and 7075 aluminum alloy ribbons and their powder metallurgical materials were characterized and their mechanical properties were measured.The sintered materials were treated with solutionization,natural aging,artificial aging and pre-ageing,and the effect of various processes on the precipitation and mechanical properties of the strenthening phases of the sintered materials was studied.Corrosion performance were also studied.7075 high-strength aluminum alloy is widely used in military vehicles,aircraft and large machinery.However,its corrosion resistance is relatively poor,and it is vulnerable to corrosion in the harsh environment of open field or marine climate.In the ever-changing battlefield,the rapid repair of equipment on the site can improve combat effectiveness.In this study,a 7075 aluminum alloy repair coating was prepared on the surface of 7075 aluminum alloy plate using cold spray technology.The influence of Ar gas and nitrogen gas as powder carrier on the density,metal structure and mechanical properties of the prepared coating was studied.The experiments including electrochemical corrosion test and immersion experiments were performed to compare the corrosion behavior of the 7075 plate and coatings in sodium chloride solutions with different concentrations.The corrosion mechanisms were also studied.This study found that: 1)Pre-ageing effectively eliminates the "negative effects" of natural aging.The effects of combining both pre-strain and pre-ageing is similar to the effect of pre-ageing alone;2)SPS sintered materials and hot extruded rods have high mechanical properties than those of the plates.It can greatly shorten the solutionization time.3)The corrosion resistance of the sintered 6014 and 7075 aluminum alloy is slightly lower than that of their corresponding plates.4)He gas can be used as a carrier for cold spray to obtain a dense nearly void-free 7075 coating,with a corrosion resistance slightly lower than that of the corresponding plate.The corrosion characteristics and the degree of corrosion of the coating are directly related to the porosity of the material and the severity of the plastic deformation of the particles.Sub-micron grain and material discontinuities,such as interconnected voids and weak metallurgical bonding between grains,play an important role in the corrosion behavior of the coating.