Effect of solution heat treatment and additives on the hardness, tensile properties and fracture behaviour of Al-Si (A413.1) automotive alloys

A study was carried out to determine the role of Mg, Cu, Be, Ag, Ni, and Zn additives during the solution heat treatment of grain refined, Sr-modified eutectic A413.1 (Al-11.7% Si) alloy, and their consequent effect on mechanical properties. For comparison purposes, some of the alloys were also studied in the non-modified condition. The alloys were cast in the form of test bars using a steel permanent mold preheated at 425degreesC that provided a microstructure with an average dendrite arm spacing (DAS) of similar to22 mum. The test bars were solution heat treated at 500 +/- 2degreesC for times up to 24 h, followed by artificial aging at 155degreesC for 5 h (T6 treatment). Tensile and hardness tests were carried out on the heat-treated test bars. Details of the microstructural evaluation are reported in a previous article [1]. With respect to the mechanical properties, it is found that the hardness and strength (YS, UTS) of Mg-containing alloys decrease with the addition of Sr due to the sluggish dissolution of the Al5Cu2Mg8Si6 phase during solution treatment, and a delay in the precipitation of Mg2Si or Al2MgCu phases during artificial aging thereafter. The properties of the Cu-containing alloys, however, remain unaffected by the addition of Sr. With the exception of Ni, all alloying elements used improve hardness and strength, particularly after heat treatment. In the case of Ni, addition of up to 1.41% Ni is observed to decrease the mechanical properties in the T6 condition. Fracture of non-modified alloys takes place through crack initiation within the brittle acicular Si particles without the crack passing through the ductile Al matrix. In the Sr-modified alloys, the fracture is of ductile type, as evidenced by the pinpoint nature of the alpha-Al dendrites on the fracture surface. The number of cracked Si particles and intermetallics beneath the fracture surface increases in proportion to the increase in alloy strength. (C) 2003 Kluwer Academic Publishers.