Calorimetry study of the precipitation in an Al7075-graphite composite fabricated by mechanical alloying and hot extrusion

Differential scanning calorimetry (DSC) technique was used to study the effect of adding graphite particles as well as the effect of different amounts of zinc over the precipitation sequence in a 7075 aluminum alloy. Recent works have shown that the graphite can be used to enhance the mechanical properties of the 7xxx aluminum alloys. According to the DSC results, the peaks were identified as dissolution of Guinier–Preston zones, precipitation of η′, dissolution of η′, precipitation of η and dissolution of η. These transformations were studied by using heating rates of 5, 10 and 20 °C min−1 at different natural aging times (1, 5.5 h and 5 days). The Kissinger’s method was applied to calculate the activation energies from peak temperatures. Results showed that the greater heating rate is, the greater amount of energy is required; as well as the greater natural aging time, the smaller energy to generate the precipitation in this composite. An important change in the energy requirement for phase transformations, mainly for the η′ phase, was generated by the addition of graphite particles (C). Vickers microhardness was also used as a complementary technique to determine the effect of both graphite and zinc contents over the mechanical properties. The results showed that C promotes an important increase in microhardness in a short time compared with a conventional 7075 alloy. In addition, the mechanical properties of the composite increased in direct relation to the addition of graphite. These particles also decrease the amount of energy required to generate the phases.