Modification of AlSi7Mg alloy with Sr and Sb

Hypoeutectic aluminium-silicon alloys called hypoeutectic silumins have been used for foundry structural elements. Their properties depend directly on the microstructure. They are one of the few metal alloys that are characterized by a simultaneous increase in strength and elongation. This is possible by changing the shape of the eutectic silicon precipitates from the lamellar form to the dendritic or, less frequently, spheroidal form. Hard and at the same time brittle silicon plates are natural notches occurring in silumins. They are also places of nucleation and development of microcracks leading to the destruction of the material. Modification of the shape of this unfavourable phase to an oval shape leads to an increase in the properties of the alloy. A number of silumin modification processes are known in the literature. These processes can be divided into technological ones, in which the geometric form of eutectic silicon is changed, e.g. by controlling the crystallization of the alloy by means of a temperature gradient. The second group of processes are chemical modification processes. During these processes, chemical additives introduced into the liquid alloy by affecting the crystallization process cause changes in the microstructure. The influence of a very large number of chemical elements on the alloy crystallization process is known. Unfortunately, among so many reports, there is contradictory information about the impact of individual additives. In this work, it was decided to check the repeatability of the test results found in the literature in other technological conditions. Typical hypoeutectic silumin containing 7% silicon with the addition of magnesium was tested. The alloy was modified only with antimony and a double modification consisting in modification with strontium and then with antimony was carried out. The contents of antimony and strontium were taken from the literature. As a result of the tests, it was noticed that the addition of antimony leads to the modification of the microstructure, and thus to the increase of the mechanical properties of the alloy. After double modification with strontium and then with antimony, changes in mechanical properties oscillating around the measurement error were obtained. It can therefore be assumed that modification of the alloy with antimony after prior permanent modification with strontium is not very effective.