| The industry use of cast aluminium alloys has greatly increased during the past decade. ZL107 alloy is one of the widely used industry alloys especially in automotive engine blocks and cylinder heads because it has not only good castability but also good air seal property. The increasing demand for accuracy and reliability of automotive parts by modern industry requires more and more dimensional stability of cast aluminium alloy. The dimensional stability of the cast aluminum component in elevated-temperature applications is an important physical property. Certain cast aluminum alloy parts (such as ZL107 alloy) can undergo dimensional changes when they exposed to the engine operating temperatures over long periods of time; when these changes occur, the shape of the casting is distorted and the properties of the part may be diminished. Therefore, it has been an important proposition of material researchers how to improve the mechanical properties and dimensional stability of ZL107 alloy.Universal material testing machine, optical microscope, scanning electronic microscope (SEM), energy disperse spectroscopy (EDS), and many kinds of other testing technology were used to research the effects of rare-earth Y on the microstructures and properties of ZL107 alloy in this paper. On the basis, the effects of different heat treatments on the microstructures and properties of ZL107 alloy were studied. The effects of the exposure temperature and exposure time on the dimensional stability of ZL107 alloy were observed in the thermal growth test. Furthermore, the effects of rare-earth Y and different heat treatments on dimensional stability of ZL107 alloy were studied. The main research results are as follows:The following results are attained through adding element yttrium in ZL107 alloy. Rare earth element yttrium is capable of refining the microstructure of ZL107 alloy and improving the distribution and the shape of the eutectic Si phase. The formation of the needle-like eutectic Si phase is inhibited, while the globule-like eutectic Si phase can be obtained. Furthermore, The shape ofα-Al phase in ZL107 alloy was greatly improved and the secondary dendrite arm spacing (SDAS) was shortened by the addition of element yttrium. When the addition of Y was 0.1%(mass percent), there appeared the best microstructure in ZL107 alloy which led to the most ideal mechanical properties. Under gravity casting with metal die, this new kind of ZL107 alloy added with Y can achieve a better as-cast mechanical properties:tensile strengthσb=203.4MPa, elongation rateδ=2.6%, and brinell hardness (HBS) =59.4HB.Different heat-treatments result in different effect on the microstructure and mechanical properties of ZL107 alloy modified by either rare-earth Y or not. The strengthening of Al-Si-Cu alloys can be realized by solid solution strengthening, grain refinement strengthening and the second-phase strengthening. But the most important strengthening method for ZL107 alloy was the second-phase precipitation strengthening. The more the second-phase particles are and the smaller the spacing of second-phase particles is, the more evident the strengthening effect is. The results indicated that the best heat-treatment for ZL107 alloy modified by 0.1% rare-earth Y was T6 heat treatment (495℃×8h+190℃×5h).The dimensional stability of the ZL107 alloy in the T4 ("naturally-aged), T6 ("peak-aged") and T7 ("over-aged") heat-treated conditions was studied by exposing the samples at the temperatures of 100℃,180℃and 250℃for up to 1000 hours. The results show that the T7 heat-treatment provides the most ideal dimensional stability for all three of the exposure temperatures. The samples by T4 and T6 heat-treatments have only dimensional stability at 100℃. The significant thermal growth (up to 0.09%) can occur at time as short as 100 hours at 180℃and 250℃.
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