Process Development Of Low Pressure Casting For An Extra-large Aluminum Casting And The Role Of Grain Refining In It

Process development of low pressure casting for an extra-large aluminum casting and the role of grain refining in it were studied in this paper, of which the aim is to produce qualified extra-large casting through designing an appropriate process and grain refining.For process developing, according to the size and structure characteristics of the extra-large SAYS203 aluminum casting, by specially designing the runner-gating system and adjusting the process parameter of pressure and pressure rise speed, and evaluating the filling and solidifying processes by MAGMASoft simulation, combined with defect detection by Ultrasonic Testing (UT), X-ray Testing (RT) and naked eyes check, an appropriate process of low pressure casting has been developed for producing qualified extra-large SAYS203 casting as following:adopting single runner-gating system, setting chills only over all the inner side of the casting while no chills over the outside, designing the size of in-gates as 80x80mm2, controlling the filling process of runner-gating system joined into the lifting stage in order to decrease the flow velocity of melt at the in-gates, increasing the dwell pressure and dwell time to 130kPa and 5000s respectively to enhance the feeding effect. Thus no defect is found on the surface and the defect level in the interior of the trial-manufactured production meets the requirement of industrial production, indicating the process development has succeeded.For grain refining, effects of Al-10Sr, Al-3B and Al-10La, added independently or together, on the microstructure and mechanical properties of A356 were investigated by microstructure observation and mechanical property testing. Results show that:for the same melt treatment such as sample S, solidification speed of the melt slows down with the thickness of the casting and thus the mechanical property decreases; second, with increasing the content of B, the refinement effect of a-Al dendrites becomes better while the modification effect of Si is weakened slightly, because of the poisoning effect between Sr and B; third, both the number and size of pore defects in the sample increase with the content of B, resulting in the decrease of the mechanical properties; fourth, adding 0.2%La in melt with Sr+B combined melt treatment can effectively solve the mutual poisoning effect between Sr and B and the mechanical properties are increased, however, excessive addition of La reduces the grain refining effect and modification effect, as a result, low mechanical properties is obtained; last, the addition of appropriate B and La in the melt modified by Sr can improve the fluidity of alloy melt.