Researches On The Magnetoplasticity Effect In 7055 Aluminum Alloy

Being a kind of aluminum alloy with high strength, the 7055 aluminum alloy had a wide utilization in the fields of aerospace, aviation and vehicle transportation. The restrictive step was focused on its low plasticity. In the paper, with the assistance of advanced measurement techniques including X-ray diffraction(XRD), optical microscopy(OM), scanning electronic microscopy(SEM) and transmission electronic microscopy(TEM) together with the mechanical properties and residual stress, on the basis of magnetoplasticity effect, the effect of magnetic field on the microstructure and properties of 7055 aluminum alloy, especially plastic deformation capability, was investigated in the static test under static magnetic field and tensile test under pulsed high magnetic field. The main research results have been summarized as the following four aspects.(1) As for the result of static treatment in the presence of static magnetic field, the tensile strength of the treated alloy is reduced, and the elongation exhibits the tendency of first increase followed by the decrease. Furthermore, the residual stress decreases at first then increases afterwards. Compared with the untreated sample(σb=565MPa, δ=7.5%, σv=122MPa), the σb,δ, σv of 3T sample are 555 MPa,10.5% and 38 MPa, whose amplifications are-1.8%, 40% and-68.9% separately. That is the optimal comprehensive performance. It is analyzed that entangled dislocations become regular, that is dislocation network and dislocation wall appear. A chain of η phases(Mg Zn2) are soluble in the crystal and the semicoherent η’phases form in the grains, which promotes the plasticity of the material.(2) When compared the research results of static experiments in static magnetic field and pulsed high magnetic field, it can be seen that the main difference lies in the dislocation characteristic. In stable static magnetic field, dislocations are mainly annihilated and show a characteristic of "order and low density"; Dislocations show proliferation during transient single pulse and appear the characteristic of "high entanglement and high density". There are also something in common: Dislocation depinning is an intrinsic mechanism of magnetic treatment, which obtains the best plasticity and residual stress of the material at 3T. B=3T is a best parameter of the magnetic field, which is independent of the type of the magnetic field.(3) As for the result of pulsed high magnetic field tensile tests, the tensile strength and elongation changes synchronously. It exhibits the tendency of initial increase and subsequent decrease. At B=3T the optimal properties is arrived at. Compared with the control experiments, at B=3T, the tensile strength(σb=587MPa) and the elongation(δ=8.3%) increased by 3.9% and 10.7% respectively. The microstructure analysis shows that the dislocation density increases with the increase of the magnetic induction intensity when B<3T. Dislocation strengthening promotes the increase of tensile strength. Dislocation structure is arranged, which improves the plasticity. While at B>3T, the microstructure including dislocation becomes more chaotic together with the decrease of the magnetic permeability. The magnetoplasticity effect becomes weak due to the appearance of dislocation pile-up, which is detrimental to the performance of the material.(4) The variation of material strength and toughness is due to the magnetoplasticity effect on the microstructure of materials, and internal organizational changes mainly lie in dislocation and alloy phase. The causes of these changes should be explained in the atomic scale and electronic scale. Magnetic field mainly affects the electronic movement, contributing to electron pairs change from the reverse spin to the same direction spin and the chemical bond between the atoms is weakened or even broken. Atomic activity enhanced, which is conducive to migration and recombination. In the motion process of atoms, the internal defects(vacancies and dislocations) of material and alloy phase changed. Dislocations appear depinning and orderly movement, besides, solution and re-precipitation of alloy phase change the original shape. As a result, material properties change. From the view of quantitative analysis, mechanism of dislocation dynamics on magnetic field includes: expansion of dislocation core and regularization of entangled dislocations decrease resistance of dislocation movement, which improves mobility of dislocation.