چكيده به لاتين
This research deals with the development of a behavioral model for semi-solid deformation of A356 aluminum alloys. Most of the modeling in this field is related to the beginning of the deformation with a globular structure that during the semi-solid deformation, the solid grains roll in the liquid matrix and the structure deforms at low stress level due to the activity of the lubricated flow mechanism. it is necessary to prepare an initial thixotropic structure with specific characteristics (including solid phase spherical grains with a size between 50-70μm). The main idea of this research is based on investigating the deformation behavior of semi-solid A356 alloy with primary dendritic structure. In this approach, the conventional methods of spheroidization to prepare the initial feedstoke are not considered and after isothermal holding at specified time in the semi-solid temprature, compressive deformation begins. For this purpose, hot pressure tests in the semi-solid temperature range (580, 590 and 600 ֯C) under different strain rates (0.001, 0.01, 0.1 1/seconds) and different initial holding times (1, 7 and 14 minutes) was done. Then the initial structure, post-preheated structure and deformed structures were characterized and the relationship with the behavior of flow curve was explained. The obtained results and microstructural studies indicate the creation of different strain distribution conditions during deformation, in which solid grains have a greater share of the applied strain and experience more deformation. In fact, in addition to the strain path in the liquid phase, the contribution of the sliding of the spherical grains of the solid phase is reduced and the contribution of the plastic deformation of the solid grains is increased. For this reason, in addition to lubricated flow, the occurrence of recrystallization in the semi-solid range in the solid phase was identified as softening mechanisms. Also, the development of substructure and penetration of the liquid phase along the created sub-boundaries were introduced as another influencing factors on the behavior of the material during deformation. Under these conditions, the samples are deformed up to the level of high strains (>0.5) without creating cracks or tears and under low stresses (less than 2 MPa). Also, in this research, the development of fundamental phenomenological-physical equations was investigated based on the effect of factors such as deformation temperature, grain size and strain rate sensitivity coefficient and the relevant equations were expanded. The obtained results show the acceptable compliance of the predicted strain-stress curve as well as the predicted flow behavior with the values and behavior recorded during the experimental tests.
