Creep and microstructures of slurry die cast AI-Si-Mg alloy

Abstract

In this research work, Gas Induced Simi-Solid (GISS) technique was used for preparing a slurry of semi-solid metal alloy which contains a low solid fraction and liquid phases. Al-Si-Mg-Fe aluminum alloy (AC4C) was used in the study. The slurry of AC4C alloy was prepared by GISS technique then cast into a simple plate product using a conventional die casting machine. The purposes of this research work were to investigate microstructures and mechanical properties of GISS die casting (GISS-DC) ACC alloy and compare with those of conventional die casting (CL-DC) one. The studies were divided into five parts. In the first part, the microstructures and tensile properties of as-cast specimens from different locations of die cast plates were studied. The results show that the specimens from the bottom location, near the gate section and from the middle location of die cast plates are stronger and more ductility than those from the top one. The top location of die cast plates contains more defects, such as shrinkage pores than those from other locations. The shrinkage porosity defects are resulted in a lower strength and ductility of the as-cast product. The second part, the tensile properties and microstructures of GISS die- cast ACC alloy after T6 heat treatment (GISS-DC-AC4C-T6) were studied and compared with those of conventional die cast AC4C alloy (CL-DC-AC4C-T6). In the as-cast GISS-DC and CL-DC specimens, -Chinese script and B phases were generally observed. After T6 heat treatment, the eutectic-Si particles were spheroidized and B phase was remained. Tensile tests were performed at 25, 100, 175 and 250 °C. The UTS, YS and elongation of GISS-DC-AC4C-T6 and CL-DC-AC4C-T6 decreased with increasing temperatures. At 25 °C, the GISS-DC-AC4C-T6 alloy gained UTS and YS of 300.9 MPa and 244.5 MPa, respectively. With increasing temperature, the fracture changed from brittle to ductile mode. Work hardening exponent (n) decreases with increasing strain. Work softening and dimples on fracture surfaces were observed at 250 °C. The transition from brittle to ductile fracture was observed in both GISS-DC-AC4C-T6 and CL-DC-AC4C-T6 alloys. The intermetallic particles and micro voids are responsible for tensile properties of the alloys. The third part, the microstructures and creep behavior of GISS-DC- AC4C-T6 alloy were investigated at temperature range of 300-360 °C and stress range of 20-40 MPa and compared with those of CL-DC-AC4C-T6 alloy. After 16 heat treatment, spheroidization and coarsening of eutictic-Si particles occured. Shapes of primary a-Al phases in GISS-DC-AC4C-T6 alloy are rosette while those in CL-DC-AC4C-T6 alloy are dendritic. Creep stress exponents (n) and apparent activity energy (Qc) for creep of GISS-DC-AC4CT6 and CL-DC-AC4C-T6 alloys were evaluated. The stress exponents n of CL-DC-AC4C-T6 alloy at 300 °C, 330 °C and 360 °C are 3.95, 3.66 and 4.94, while n of GISS-DC-AC4C-T6 are 3.52, 4.82 and 5.74, respectively. Based on the stress exponents n, creep mechanism of both alloys may be governed by the dislocation glide-climb process. The average activation energy for creep (Qc) are about 257.8 kJ/mol which is higher than the activation energy for self-diffusion of pure aluminum (143 kJ/mol). The fourth part, the analysis of creep rupture data using the Momkman- Grant model, the damage tolerance parameter and the theta projection method for predicting creep life was studied. The creep rate and rupture time are linear relationship and well fitted to the Monkman-Grant model. The creep damage tolerance parameter was analyzed for predicting creep life and creep fracture mechanisms. Based on creep curve data from the present work, constitutive models for fitting creep curves were created by using the theta projection procedure. In the final fifth part, the stress-change creep tests were peformed at temperatures of 25, 250, 280 and 300 °C on GISS-DC-AC4C-T6 and CL-DC-AC4C-T6 alloys. The result of stress-dip (or stress decrease) is responsible for an action of the internal backstress. If the stress reduction is very small, the internal backstress will quickly be in equilibrium with the applied stress. This situation gives a normal creep curve. In the present study, the responds of stress-dip are too fast to be detected. For the large stress-dip test, the stress reduction is too large to allow the microstructures to change quickly into a balance situation. So, the back flow and internal backstress are associated with the evolution of microstructures such as subgrains fromation.