Effect Of Processing Parameters On The Microstructure And Properties Of D357 Aluminum Investment Castings

The mechanical properties depend on the chemical composition of alloys, melt treatment, casting and the heat treatment. This paper study the shell preheat temperatures, modification, casting section thickness and heat treatment parameters on the D357 aluminum investment castings. This paper mainly includes the following research.When the shell preheat temperature at 100℃, the alloy has homogeneous microstructure and better mechanical properties, but the poor mold-filling capacity results in that the thin parts can not be fulfilled. When the shell preheat temperature at 200℃, the casting cavity can be basically filled. When the shell preheat temperature at 300℃, the casting cavity can be filled well, but the mechanical properties are poor.Comparing to the alloy without modification, through the D357 alloy with the modification of Na or Sr, the silicon turns into fiber from lath at the same time. With the modification of Sr and Na, the alloy have smaller refined organizations and smaller eutectic silicon phase.The difference of casting wall thickness causes different mechanical properties. Alloy with thin wall thickness can get fine grain size, uniform and good mechanical properties. With the increasing of wall thickness, the mechanical performance declines.The relations between the solution treatment technology and the mechanical properties are investigated by variation of solution treatment parameter. With the solution temperature of 540℃and holding time of 8h, D357 aluminum alloy have the best mechanical properties. The tensile strength is 335MPa, the hardness is 94.5HV, and the elongation is 13.2%. When the solution temperature is 555℃, the mechanical properties meet a huge decline.The relations between the aging heat treatment technology and the mechanical properties are discussed by changing aging technology parameters.When the aging temperature is fixed, with the holding time prolonged, the the tensile strength and hardness increase, but the elongation decline. When the aging heat treatment technology are 160℃×8h and 160℃×8h, the mechanical properties are well matched. With the solution time increasing, the silicon phase reduce and the eutectic Si grows up more and more isolated and coarsening. When solution treatment technology is 540℃×8h, the silicon phase become round and fine. It is good for the mechanical properties.With the solution temperature increasing and the solution time prolonging, the α-Al become round and biger.By analysising microcosmic specimen fracture surface, we know that all of them is ductile fracture. With a suitable solution and the aging process, its uniform is dimple and deep, the plastic is better.