Hardening response and precipitation behavior of Al-7%Si-0.3%Mg alloy in a pre-aging process

In order to improve the properties of an Al-7%Si-0.3%Mg alloy, an orthogonal test of four factors and two levels is designed to optimize pre-aging process. Hardness and tensile tests and TEM observations are used to evaluate the hardening response and precipitation behaviors. Range analysis of orthogonal test indicates the sequence of significance from high to low is as the second aging temperature, first aging temperature, second aging time and then first aging time. The pre-aging process of 120 °C/14 h+170 °C/6 h has the highest hardening response of 80.9HB in the orthogonal test. Compared with the underaging and peak aging processes, the pre-aging process leads to a considerable increase in the amount of β″ precipitates but an obvious reduction in their sizes. At the same time, both the amount and size of β′ phase are decreased, and the Si precipitate is completely suppressed. These alterations have an important influence on the hardening response. In the sample that has undergone the pre-aging course, a great number of β″ disperoids are precipitated, but their sizes are much smaller than that under the peak aging, thus the hardening response is weakened. It is mainly from the coherency strain strengthening, not the Orowan strengthening.