Softening Characteristics And Numerical Modeling Of A6N01 Aluminum Alloy Welded Joints

Weight reduction is the main direction of high-speed trains'development.Owing to the increasing use of large wide thin-walled ribbed sheet and hollow profiles in the manufacture of automobiles,the use of Al-Mg-Si-based alloy A6N01 with good extrusion performance increased rapidly in the train industry.A6N01S-T5 aluminum alloy is aging strengthening alloy,welded joints would soften after welding,in order to simulate the stress-strain behavior of the welding process accurately,it is very necessary to study the distribution and mechanism of the softening of welded joints,establish a reliable mathematical model to reflect the softening of welded joints,and analysis the relationship between softening performance and temperature history during welding.The welding test of typical welded joints of A6N01 aluminum alloy roof was conducted.The softening behavior of the welded joints was studied by measurement of mechanical properties and microstructure observation with the means of OM,SEM.The results showed that:The hardness of the heat-affected zone was about 70%of the base material;the softening of HAZ is significant.The main reason for this phenomenon is that:First of all,the grain size in heat-affected zone increased during the welding thermal cycle,especially in the overaging zone,in which grain size was uneven,in that case the strengthening effect was reduced;On the other hand,precipitates in overaging zone dissolved incompletely at high-temperature,and the amount of precipitation of strengthening phase reduced in the cooling process.Furthermore the original undissolved metastable?'phase also gathered and grew into a steady strengthening?-phase under the influence of welding thermal cycle,reducing precipitation strengthening and solid-solution strengthening effect,forming the overaging softening zone.Softening behavior of A6N01 aluminum alloy is related to the fraction of dissolved strengthen phase X_d.The mechanical properties of A6N01aluminum alloy were predicted by the relationship between X_d and two extremes;namely,the behavior of material in its initial heated condition and in completely softened state.Satoh test was conducted for the validation of softening model.The results showed that:Residual stress values of simulation using softening model showed good agreement with the measured ones.Whereas values of simulation without using a softening model were much higher than the measured values.This softening model was applied to simulate welding process of A6N01 aluminum alloy typical welded joints and train roof,and values of simulation were in line with the measured ones.This model is of great importance to guide the longevity evaluation and safe operation of aluminum alloy train.