Effects Of Impurity Elements On The Microstructure And Mechanical Properties Of2A12Aluminum Alloy

The ratio of alloy elements and heat treatment are important means to improve the micro structure and mechanical properties of aluminum alloys. This paper studies two different purity2A12aluminum alloy, according to the different proportion contents of impurity alloy elements Fe and Si, the alloys are divided into two models as L1and L2. The experiments use optical microscope and TEM to observe the micro structure of the alloy, X-ray diffraction analysis of precipitates in the alloy and scanning electron microscopy analysis tensile fracture morphology. Through experiments analysis the impact of micro structure changes on strength material strength, toughness and dimensional stability properties, we can get the conclusions as follow:(1) After aging treatment, the contents and size of the coarse second phase of L1-type alloy is highly more than the L2-type’s.Through observe the alloys’microstructure, it can be found that grains are stretched along the L direction, the coarse phase of L1-type alloy distributes within the crystal grains and grain boundaries as chain, and as discontinuous distribution as L2-type’s compound.(2) Though the TEM analysis conclude that the mainly aging strength phase consists of fine needle phase S"(Al2CuMg),it presents dispersed. The size of L1-type’s S’phase is larger, however, the density is lower; L2Alloy S’phase dispersed fine, evenly distributed, small pitch and high density.(3) By the X-ray diffraction analysis, the amounts of the impurity coarse phase Al7Cu2Fe and Al6MnFe of L1-type alloy are obviously more than L2-type’s.The coarse second phase can produce the stress concentration and crack source easily, it would reduce the alloy mechanical properties and the fracture toughness.(4) The yield strength and tensile strength of L1-type alloy are lower than L2-type about110MPa and80MPa, respectively. The tensile mechanical properties of L2-type alloy is better than L1-type, the amounts and distribution of strengthening precipitates S’(Al2CuMg) are the main factors which affect the tensile properties.(5) The fracture toughness of L2-type alloy is higher than L1-type about20%. The fracture toughness of the alloy is greatly affected by the coarse second phase. Due to the high impurity content in L1-type alloy, the number and size of coarse second phase become larger, the fracture toughness is poor.(6) The tensile properties and fracture toughness of2A12alloy showed obvious anisotropy, L direction performance is higher than T direction; The main reason of the anisotropic is own to grain along the L direction, the crack extended transgranular encounters with more grain boundaries, resulting in secondary crack resume more energy and the coarse second phase content is low. while the T direction, because the grain boundary area is smaller than perpendicular L direction, grain boundaries are not in the same plane, grain boundary space is larger, it could result of the distribution of grain boundary coarse second phase in two directions are different, then the properties of the alloy appear anisotropy.(7) After thermocycling heat treatment (T7) process, tensile strength, yield strength and micro dimensional stability of2A12aluminum alloy have been increased dramatically than the traditional process, the best heat treatment process is T7process.