Study On The Effects Of Trace Elements On Microstructure And Properties Of Al-Si-Cu Alloys

This research comes from the practical problems encountered in production. Micro-alloying method was used to solve the problems that more impurity elements exist and lower mechanical properties in the alloy scraps after being remelting. Different levels of Zr, Sr, and RE elements were added to the Al-Si-Cu alloys, respectively, when the alloy scraps were remelting. The effects of each trace element on the microstructure and mechanical properties of the as-cast and heat-treated Al-Si-Cu alloys were investigated in this paper. The main results achieved were summarized as follows:The morphology of the eutectic Si and Al2Cu phase of the as-cast Al-Si-Cu alloys had no significant change when different contents of Zr were added into the alloys. However, a large quantity of plate-like or needle-like phases precipitated in the matrix when about 0.278wt.%Zr was added to the alloy. According to the semi-quantitative analysis of EDS, these interesting phases may be AlsSiZr2 and Al5Si(ZrTi)2. The plate-like eutectic Si particles transformed into small stick-like firstly, and finally appeared as fib dot-like with the increasing contents of Sr. The Cu-rich phases were segregated in the eutectic Si dendrite and the bone-like Fe phases were promoted to precipitated when a certain amount of Sr was added into the alloys. Adding trace RE element into the Al-Si-Cu alloys made the plate-like part of the eutectic Si particles bent and broken, and finally appeared as fine particles or rod clusters.The eutectic Si particles of the alloys dissolved, granulated and coarsened after T4 and T6 heat treatment. These plate-like primary AlSiZr and AlSiZrTi phases of the Zr-containing alloys still distributed unevenly in the matrix after T4 and T6 heat treatment because of their high temperature stability. The eutectic Si particles became smaller, more rounded and distributed more evenly in the Sr-modified alloys after heat treatment. However, the quantity of needle-like Fe phase increased when Sr were added to the alloys after heat treatment. Trace RE added to the alloys reduced the size of the eutectic Si particles and α(Al) after heat treatment.The ultimate tensile strength (UTS) and elongation (EL) values increased firstly and then decreased with an increasing of Zr addition to the alloys. The peak value of UTS is 235MPa when 0.1wt.%Zr was added to the as-cast Al-Si-Cu alloys. After T4 and T6 heat treatment, the peak values of UTS were in the alloys with 0.1wt.%Zr, which increased about 14.6% and 20.6%, respectively, compared with that of the alloy without Zr. The mechanical properties of the alloys were greatly enhanced when about 0.0134wt.%Sr was added. The UTS of the as-cast, T4 treatment and T6 treatment alloys reached 263.3MPa,345.7MPa and 373.3Mpa. However, the addition of Sr seems have no significant influence on the toughness of the Al-Si-Cu alloys. The effects of trace RE on the mechanical properties of the alloys were not obvious. The maximum tensile strength values of the as-cast, T4 treatment and T6 treatment were 250Mpa,333Mpa and 361.3Mpa, respectively, which increased only about 3.1%, 5.7% and 6.5%, respectively, compared with the alloys without RE addition.The mechanism of the change of the Fe-rich phases morphology in the alloys modified with Sr was discussed in this paper. Needle-like Fe phases were transformed into fishbone-like by changing the enrichment and distribution of Cu atoms in the Sr-modified alloys. However, this effect was significantly decreased for the decline of the enrichment of Cu atoms in the alloys after heat treatment.