Effect of a silicon micro addition on the thermal stability of hardening and structure of θ′-phase particles during annealing of 2219 alloy with Sn addition

A comparative analysis of the effect of aging temperature in the interval of 175–250 °C as well as holding time on the stability of the achieved hardening and structural evolution of the aging products (θ′-phase particles) is carried out for experimental wrought Al–6Cu–0.5Mn, Al–6Cu–0.5Mn–0.1Sn, and Al–6Cu–0.5Mn–0.1Sn–0.2Si alloys having tin and silicon micro additions in their compositions. Experimental studies into the structure are conducted using the methods of electronic scanning and transmission microscopy, while studies of physical and mechanical properties are conducted by analyzing changes in hardness (HV) and specific electrical conductivity during isothermal aging of alloys obtained in the form of flat products. A micro addition of tin significantly increases the peak hardness of alloys in the entire studied interval of aging temperatures. The increase in temperature of up to 250 °C, compared to aging at a lower temperature of 175 °C, leads to a natural decrease in the peak hardness of the studied alloys, although to varying degrees. The peak hardness of tin-containing alloys decreased only by 10% (up to 140 HV) compared to 18% (up to 110 HV) in the base Al–6Cu–0.5Mn alloy. A gradual decrease in hardness of all alloys at different rates after achieving peak hardening during prolonged holding and aging at 250 °C is shown. At the same time, the rate of decrease in hardness of the Al–6Cu–0.5Mn–0.1Sn–0.2Si alloy with a silicon micro addition during ongoing annealing is significantly lower than that of other two alloys. An analysis of the microstructure showed that the proportion of more dispersed particles is significantly higher in the Al–6Cu–0.5Mn–0.1Sn–0.2Si alloy with a silicon micro addition compared to that in the Al–6Cu–0.5Mn–0.1Sn alloy. This circumstance may explain the observed higher hardness of the alloy with a small silicon addition after long-term high-temperature annealing.