Effect Of Deformation And Ageing Process On The Microstructure And Properties Of A 7N01 Alloy

Compared with ordinary steels, aluminum alloy may have a similar strength with less than a half of weight, and therefore have been widely applied as important lightweight structural materials in high-speed train and aircraft indusntry and defence equipments. The medium strength, non-copper-containing Al-Zn-Mg series aluminum alloys which with the advantages of natural ageing strengthening, excellent corrosion-cracking resistance, good hot workability, low sensitivity to hot cracking, low quench sensitivity and good weldability are widely used in high-speed vehicle chassis and other mechanical parts as a lightweight structural materials. Al-4.5Zn-1.32Mg (wt.%) aluminum alloy (7N01 alloy) is the most widely used 7xxx series aluminum alloy, which occupies a very important position in the development and construction of the national rail transit. Due to the practical application in continuous improvement of train speed and safety requirements, so the performance of the material also put forward higher requirements, how to keep plastic to improve its strength is currently needs to be solved urgently in practical industrial production and application problems.Adopting combined deformation and ageing process, the objective of present work aims to explore the effect of the optimized parameters for the mechanical properties of 7N01 alloys by pre-ageing such as no ageing, natural ageing or artificial ageing and rolling deformation such as 20%,40%,60%,80% as well as post-annealing tempers such as 80?,100?,120?,140?,160?. The effect of deformation and ageing process on the mechanical properties and microstructure evolution were investigated by hardness test, tensile test, differential scanning calorimetry(DSC), scanning electron microscopy(SEM), electron backscatter diffraction(EBSD), and transmission electron microscopy(TEM). The main conclusions are as follows:(1) By suitable pre-ageing treatment,7N01 alloy can not only maintain good ductility, the strength can be more than the corresponding T6 peak ageing treatment alloy. The one reason is deformation process before subsequent ageing introduces a large number of dislocation which have dislocation strengthening effect on the alloys. The other reasion is high density dislocation increase the subsequent precipitation nucleation sites which make the matrix precipitate phase size is as fine as T6 alloy.(2) For 7N01 alloy, the deformation of the alloy is not conducive to the mechanical properties of the material, and the strength and elongation of the alloy is decreased with the increase of the deformation. The reason is in the material by pre-deformation dislocations introduces and the number of subsequent ageing strengthening of nano size precipitated phase particle type, the size and distribution have a key role, thus affecting the final material mechanical properties. It is unfavourable for the mechanical properties of the Al-Zn-Mg alloy by using large deformation in the combined process. Because the large deformation introduced a large of dislocation defects which can promote the nucleation and growth of precipitate in subsequent artificial ageing and result the size and the distribution of precipitates is different with small deformation.(3) Choose appropriate subsequent re-ageing temperature and time can optimize alloy comprehensive performance. At above 120?ageing, ageing temperature is too high, precipitated phase coarsening rate and dislocation degradation rate is too fast, makes a rapid decline in mechanical properties of the alloy. Under the age of 100?, the ageing temperature is too low, the ageing precipitation response is too slow, the strengthening of the speed is slow. With the subsequent artificial ageing temperature control precipitation rate and degradation rate of dislocation, so that the two can make the appropriate combination of the properties of the alloy increased significantly.(4) The different pretreatment result in element segregation in the matrix during the deformation ageing process, which affect the dislocation patterns and content, and then influence the recrystallization grain structure of the alloy during the subsequent high temperature annealing.