Analysis of heat treatment strengthening mechanism of 7075 alloy by rheo squeeze casting

In the process of solid solution, the alloy elements are dissolved into the base metal, the hardness and strength of the alloy will be improved, forming solid solution. According to dislocation theory, the strengthening of alloy is mainly controlled and influenced by the multiplication, interaction and movement of dislocations. The dislocation degree of the matrix increases and the lattice distortion occurs when alloying elements are incorporated into the matrix metal. The elastic stress field around the dislocation interacts with the stress field produced by the distortion, which makes the atoms of alloy elements gather near the dislocation. When the dislocation movement meets the solute phase, the particle phase will hinder the forward movement of the dislocation and play a pinning role on the dislocation. At the same time, the elastic constant, cohesive force, diffusion coefficient and atomic arrangement defects of the solid solution will be changed after the atoms of alloy elements dissolve into the alloy matrix, which will make the dislocation line bend and increase the resistance of dislocation movement, thus strengthening the material. Therefore, the strengthening effect of alloy elements with high solid solubility increases with the increase of the content of alloy elements. In aluminum alloy, copper and magnesium are the main alloy elements, mainly because of their high solid solubility. At the same time, in order to improve the strengthening effect of the alloy, supersaturated solid solution is usually obtained by solution treatment before aging treatment. On the one hand, the solute atoms are supersaturated, on the other hand, the crystal defect vacancies are also supersaturated. The interaction between the vacancy concentration and the solute atoms in the solid solution has a great influence on the atom diffusion in the subsequent aging process. The higher the vacancy concentration, if there are excess vacancies in the quenched structure, the atomic diffusion speed will be doubled, which makes the aging power increase. The diffusion rate of atoms can be expressed as follows:

Where QF is the activation energy for vacancy formation, Q0 is the activation energy for vacancy movement; Q (QF + Q0) and Q are the activation energy for one atom and 1mol atom diffusion, respectively; K is the gas constant. According to relevant data, for Al Cu alloy, QF = 0.7eV, Q0 = 0.5EV, total q = 1.2ev. If there are excess vacancies in the alloy, the diffusion energy Q = Q0 = 0.5EV can increase the diffusion rate of copper atoms by 1010 times. It can be seen that supersaturated vacancies have a considerable effect on aging dynamics.

Aging process is a kind of solid phase transformation, which refers to the precipitation process of the second phase from supersaturated solid solution, and also a diffusion process. The reason of strengthening is generally explained by dislocation theory. Because the structure and properties of the new second phase particles are different from that of the matrix, there will be an interactive stress field around the particles. When there are particles on the slip surface through which the dislocation passes, it is necessary to overcome the resistance of the stress field and the particle phase itself, and the dislocation movement is difficult, which makes the dislocation movement produce internal strain strengthening. With the increase of aging, the solute atoms continue to gather and the distance between the stress fields is widened, which makes the dislocation wires bend around the stress field and hardens the alloy. The relationship between bending radius and stress is as follows

Where R is the bending radius, G is the shear modulus, B is the Bernoulli vector of the dislocation, and τ is the corresponding shear stress. The more the solute atoms gather, the smaller the bending radius of dislocation, the greater the stress and the stronger the hardening. The dislocation cuts through the particles and hardens the alloy. When the dislocation movement meets the precipitate, it can cut through it and force through, which will produce a new interface, increase the surface energy, cause the mismatch between the particles and the matrix, generate the stress field, change the proximity of solute solvent atoms, and cause chemical strengthening. When the precipitate phase is hard or the interphase distance is too large, the dislocation can continue to move around the precipitate, which is easier than cutting through the particles

Where l is the distance between particles R. The smaller the particle spacing, the greater the stress. When the dislocation continues to move around the particle phase, the dislocation ring around the particle phase will be left, the dislocation density will increase, the particle spacing will decrease, and the resistance will gradually increase due to the proliferation of dislocation, so as to achieve the effect of precipitation strengthening or dispersion strengthening.

To sum up, 7075 alloy (belonging to Al Zn mg Cu alloy) rheo squeeze casting alloy starts from age hardening, from precipitation phase G.P. zone to η′→ η→ t phase. Due to the increasing lattice distortion caused by coherent formation, the range of interaction stress field is wider and wider, the dispersion is higher and higher, and the age hardening gradually reaches the maximum. In 7075 aluminum alloy, Mg, Cu and other alloy elements have high solubility at a certain temperature. Therefore, the best solution temperature is obtained through experiments, which helps these alloying elements dissolve into the matrix more completely. During aging treatment, they precipitate in the form of strengthening phase in large amount and distribute uniformly in the alloy matrix, which has a good strengthening effect.

The alloy 7075 for rheo squeeze casting experiment has more spherulites after squeeze casting deformation, and the grains are fine and uniform. In the process of solution treatment, there are a lot of dislocations, vacancies and other defects in the matrix, which will effectively accelerate the diffusion and dissolution speed of alloy elements, especially copper, and shorten the solution time significantly, thus improving the production efficiency. At the beginning of aging treatment, due to vacancy and dislocation, Mg, Si, Cu and other solute atoms will aggregate rapidly and form G.P. region. With the further aging, the solute atoms cluster and tend to be ordered, and grow rapidly to form the transitional precipitate phase η′→ η. The coherent relationship between the transition phase and the matrix results in a large number of distortion zones in the matrix, the movement of dislocations is hindered, the stress increases, and the strength and hardness of the alloy are significantly improved. At the later stage of aging, the amount of equilibrium precipitated phase t (al2zn3mg3) particles gradually grows up, the coherent relationship with the matrix is destroyed, the elastic strain of the surrounding matrix gradually decreases, and the resistance of the strain field to dislocation motion begins to decrease. The dispersed and fine precipitates still play a pinning role in dislocation movement, resulting in precipitation strengthening, and the alloy still has good mechanical properties. Therefore, the main characteristic parameters of heat treatment properties of 7075 alloy are particle size, morphology, precipitation phase structure and distribution during aging. In the process of solution and aging treatment, the size, distribution and morphology of solute phase in the alloy matrix were improved by optimizing the process parameters, so as to obtain the dispersed precipitates with appropriate size, which made the strength and hardness of the alloy get the best strengthening.