Heat Treatment Development for a Rapidly Solidified Heat Resistant Cast Al-Si Alloy
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Existing heat treatment standards do not properly define tempers for thin-walled castings that solidified with high solidification rates. Recently emerged casting processes such as vacuum high pressure die casting should not require long solution treatment times due to the fine microstructures arising from rapid solidification rates. The heat treatment studies involving rapidly solidified samples with secondary dendrite arm spacing between 10 and 35 μm were conducted for solution times between 30 min and 9 h and temperatures of 510 and 525 °C and for various aging parameters. The metallurgical analysis revealed that an increase in microstructure refinement could enable a reduction of solution time up to 88%. Solution treatment resulted in the dissolution of Al2Cu and Al5Mg8Si6Cu2, while Fe- and TiZrV-based phases remained partially in the microstructure. The highest strength of approximately 351 ± 9.7 and 309 ± 3.4 MPa for the UTS and YS, respectively, was achieved for a 2-step solution treatment at 510 and 525 °C in the T6 peak aging conditions, i.e., 150 °C for 100 h. The T6 temper did not yield dimensionally stable microstructure since exceeding 250 °C during in-service operation could result in phase transformation corresponding to the over-aging reaction. The microstructure refinement had a statistically stronger effect on the alloy strength than the increase in solutionizing time. Additionally, thermal analysis and dilatometer results were presented to assess the dissolution of phases during solution treatment, aging kinetics as well as dimensional stability.
KeywordsAl cast alloy dilatometer analysis dimensional stability heat treatment mechanical properties solidification process thermal analysis
The authors would like to acknowledge the financial support of the Advanced Materials for Transportation (AMT) Program of Natural Resources Canada. The authors thank Dr. A. Javaid for wedge casting experiments and R. Zavadil B.Sc. and M. Aniolek M.Sc. Eng. from CanmetMATERIALS for their work on the microstructure analysis and thermal analysis data evaluation. Moreover, we thank K.W.S. Yeom and M. Mir from University of Waterloo, L. Nicodemou from McMaster University, L. Pushan and C. Ferhat from Ryerson University for their contributions to graphical data presentation done during their Co-op programs and summer research assistant program.
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