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JMEPEG https://doi.org/10.1007/s11665-018-3835-3

Effects of Thermal Exposure on the Microstructure and Mechanical Properties of Al-Si-Cu-Ni-Mg-Gd Alloy Yudong Sui, Lina Han, and Qudong Wang (Submitted January 16, 2018; in revised form September 6, 2018) The present study focused on the effect of thermal exposure on the phase evolution and properties of the cast-T6 Al-12Si-4Cu-2Ni-0.8Mg-xGd (x = 0, 0.1, and 0.2) alloy used for cast piston.The thermal stability of the alloys was studied in the temperature range of 200-300 °C for exposure duration up to 500 h, thus obtaining the relationship between the microstructural evolution and its impact on the mechanical properties. The fine Al2Cu phase grew into the coarse h-Al2Cu phases during the thermal exposure, resulting in a decrease in the mechanical strength of the alloy. The thermal stability and stiffness of the Al-Si-Cu-Ni-Mg alloys were improved with the increase in Gd content from 0 to 0.2%. Keywords

Al-Si-Cu-Ni-Mg-Gd alloy, microstructure, physical properties, tensile properties, thermal exposure

1. Introduction Al-Si alloys are important aluminum alloys widely used in manufacturing engine components, especially pistons. They have excellent corrosion resistance, good abrasion resistance, high strength-to-weight ratio, low coefficient of thermal expansion, and excellent high-temperature performance (Ref 1-6). In particular, the use of aluminum alloys can effectively reduce the vehicle weight and lower the environmental impact. When the engine is running, the temperature of the combustion chamber will reach up to  200-400 C under a peak pressure of approximately 20 MPa. Therefore, it is necessary to develop Al-Si-Cu-Ni-Mg alloys with 3-5 wt.% Cu, 1-3 wt.% Ni, and 0.8-1.3 wt.% Mg contents to achieve excellent high-temperature strength and fatigue performance (Ref 7-9). Many researchers stated that the mechanical properties at ambient and elevated temperatures can be improved by the addition of other trace alloying elements to Al-Si-Cu-Ni-Mg alloys (Ref 10-13). However, a piston working at an in-service temperature above 200 C for a long time will lead to a severe decrease in the mechanical properties of the Al-Si alloy, thus limiting its

use (Ref 14-16). Generally, h-Al2Cu and M-Mg2Si precipitates in the Al-Si-Cu-Mg alloy show excellent coarsening resistance at temperatures below 200 C but grow up rapidly above it. The existence of large precipitates results in a loss of their effectiveness in hindering the dislocation movement of alloys. A study by Tzeng et al. (Ref 17) stated that M-Mg2Si phases coarsen very quickly and exhibit low stability in an Al-7Si0.6Mg alloy maintained above 250 C for 100 h, causing deterioration in the mechanical properties. Mohamed et al. (Ref 18) reported that the h-Al2Cu precipitates lose coherency with the matrix and coarsen rapidly in the Al-Si-Cu alloy under thermal exposure at temperatures exceeding 190 C, resulting in a decrease in the tensile properties. In our previous work, Al-12Si-4Cu-2Ni-0.8Mg-0.2Gd cast alloy showed b