Study On Process, Microstructures And Properties Of Magnesium-Alloy Investment Castings

Investment casting has been restricted to apply in the field of magnesium alloys, because the key problems have not been settled, such as the oxidation and ignition phenomena and metal-mold interface reaction during magnesium alloys being melting and casting. In this paper, the oxidation- and ignition-proofed effects of different agents on magnesium-alloy investment castings were investigated. And the influences of various conditions on the microstructures and properties of investment castings were also studied. Finally, an effective forming technology of the complex thin-walled components were explored.gooIt was found that the surface of AZ91D investment castings was smooth and silvery-white after the additions of carbon or pyrite or boracic acid into the investment ceramic shells. The effects of carbon and pyrite flame retardants were attributable to the compact protective coatings of MgO+C and 2MgO+MgS, which formed during the reactions between magnesium melts and CO₂ or SO₂ gas. The boracic acid transformed into boride, which could dissolved magnesia and form 3MgO?B2O3 enamel film, and protected the casting surface at the elevated temperature.When the shell mold temperature and the cast thickness were changing, the microstructures and mechanical properties of AZ91D step-like investment castings were also changing as a result of the different cooling conditions. Magnesium melt solidified quickly at the thin wall of the castings, and fine as-cast microstructures and high mechanical properties were achieved. As the cast thickness increased, the second phases coarsed and connected with each other in as-cast microstructure, and the castings obtained poor mechanical properties consequently.The investment casting process of the complex thin-walled components was studied as well as the microstructures and properties of different magnesium-alloy castings. The defects, such as misrun, shrinkage depression, cold shut, gas inclusion, heat cracking, etc., existed in the investment castings which were poured under gravity. But with low pressure casting, these defects were eliminated efficiently, and the castings had smooth surface, accurate dimensions and precise profiles. Moreover, in all of magnesium alloys, the second phases were mostly solutionized after solid solution, and then precipitated dispersedly in the castings during ageing heat treatment. This enhanced the strength of magnesium alloys and improved the mechanical properties of the castings. In addition, the microstructures and properties of investment casting samples, which could be the potent references, were similar with the components under different states. Ultimately, the complex thin-walled magnesium alloy components could obtain intact configuration, fine microstructures and good properties using investment casting process. But those measures should be taken during the process, namely, shell mold being affused with boracic acid, magnesium melts being poured under low pressure, castings being solid solution and ageing heat treated.