Study On The Rheological Behavior And Gates Of The Thixoforming Of Semi-Solid Aluminum (A356) Alloy

The main purpose of this dissertation is to study the rheological behavior of semi-solid aluminum alloys (A356) in die-casting and the effects of gate upon the mechanical properties after the relative theory thoroughly. To study the rheologocal behavior and the ability of thin wall forming, many experiments are performed. Based on the experiment data, the viscosity model and designing of gate size are carefully studied.The main contributions of this paper are as follows:(a) Based on the researching status of semi-solid metals, we use tube viscosity apparatus to measure the shear rate and shear stress, which can calculate the apparent viscosity of fluid in the tube. This is the best way to close the conditions of the industrial operation of die-casting.(b) It is realized first that under the high shear rate and large solid fraction, the rheological behaviors of semi-solid aluminum alloys on thixoforming are different in the shear rates.(c) By calculating the Renold number and the apparent viscosity in the plate casting area, it is concluded that the flow, of metal mushy in the mould is laminar flow.(d) From the viewpoint of Poiseuille, during the process of the thixocasting of semi-solid Al alloy, the metal mushy obeys the non-Newtonian fluid's behavior. It is proved that the viscosity derived from this paper is a much better one to describe the metal mush'sflow behavior, rate distribution and temperature scatter.(e) We initially understand the flow behavior, the rheological behavior and the viscosity model in the thixocasting of semi-solid Al alloy. When the shear rate is 2000s-1-10000s-1, the rheological behavior is a kind of pseudoplasticity. But when the shear rate is over 100000s-1, it is dilatancy. Basing on the experiment data, a new viscosity model in die-casting condition is first proposed.(f) Using the new viscosity model, some simulation of the die-casting processing of semi-solid aluminum alloys is done. It shows the results of simulation are coincident with the results of experiments.(g) The effect of gate upon the mechanical properties of semi-solid Al alloy casting is investigated. Comparing the five different gate systems, it is concluded that the single light curve gate system can produce the best castings in both ultimate tensile and elongation.(h) After analysised effects of the gate upon the piece of casting sample, we first give the ideas of die gate design.(i) It is sure that semi-solid metal processing can form thin wall pieces thickness in 1 mm.