Investigation On The Typical Defects Of Low Pressure-Lost Foam Casting (LP-LFC)Process With Mg Alloy

In the last 10 years, there is a rapid growth of application of magnesium (Mg) alloys in the auto and electronic industries because of its low density and high strength-to-weight ratio, and the lost foam casting (LFC) process is a precision forming technology capable of casting complicated parts with low manufacturing costs. In order to overcome the shortcomings of melt Mg alloy quick oxidation and mould filling difficulty, the Huazhong University of Science and Technology developed a new Low Pressure-Lost Foam Casting (LP-LFC) process for producing Mg alloy castings with thin-wall and complex geometries by combining benefits of LFC with the superiority of low pressure casting (LPC). This paper analyzes the principle of the LP-LFC process,presents the physical and mathematical models of mould filling of the new process. The patterns including window-plate, ladders cluster, electromotor frame and exhausting manifolds are poured using LP-LFC process with AZ91D alloy, and the mould filling course is tracked with electrode touch point method. Based on above tests and experiments,the casting defects including short runs, cold laps, trapped gas,metal penetration, and so on, were investigated. The appearance characteristics,birth causes and influence factors of above defects are discussed in the article,diagrammatize modes of the defects forming is also given, and the measures of reducing or avoiding defects are put forward. The investigation and practice results demonstrate that the mould filling ability of Mg alloy with LP-LFC is much better than that of LFC,and the new process can largely reduce or overcome the defects of short runs and cold laps encountered in LFC. The complex thin-walled Mg alloy (or Al alloy) castings can be produced perfectly by LP-LFC, so the new process is very suitable for casting Mg alloy (or Al alloy) parts with high precision and complex thin-walled. In LP-LFC process, the mould filling and metal freezing are carried out under the controllable pressure and gas environment. The filling course and the defects of short runs and trapped gas are mainly depended on the pressure and flux of filling gas, foam pattern property, pouring temperature, coating performance, gating system,vacuum level, etc. The various defects can be eliminated ultimately by optimizing technology data and rationally arranging those data.