Effect of coherent and incoherent precipitates upon the stress and strain fields of 6xxx aluminium alloys: a numerical analysis

This paper investigates the stress and strain fields induced by the coherent/incoherent precipitates of type Mg_xSi_y in the 6xxx series aluminium alloys using numerical means. During welding of 6xxx series alloys, the heat-affected zone experiences high temperature heating and cooling cycles and hence results in the non-uniform stress and strain states both at macro- and micro-scales. In the present work, the macro-scale finite element (FE) simulations were carried out to calculate and compare the bulk material properties with those of experimental findings. Since the microstructure of 6xxx alloys comprises mostly of soft α-matrix and coherent/incoherent Mg_xSi_y precipitates, the properties of the individual constituents were used to perform micro-scale FE simulations. Temperature dependent theoretical yield strengths for the precipitates were also determined. The simulated micro-stress and strain fields showed strong dependency over the nature (coherent/incoherent), shape morphology (needle-like, random etc.) and distribution of precipitates. Additionally, unlike coherent precipitates, the plastic deformation of the matrix due to incoherent impurities was found to be highly non-uniform and localized. FE analysis was also performed to characterize the effect of grain size upon the stress state of material.