Enhancement of the integrity of hypoeutectic aluminium castings by high frequency induction

Abstract

Aluminium is among the most widely used metals in the world. Unlike copper that most of its alloys are good for both casting and wrought purposes, aluminium alloys differ substantially between the forging series(hypoeutectic) and casting series (eutectic or hypoeutectic). Casting is relatively inexpensive but the oxygen affinity nature of aluminium results in porous castings while forging entails expensive mold charges but comes up with internally sound shapes with good mechanical properties and machinability. These benefits are unique to the series itself (either forging or casting) and mutually exclusive to the other series. This is a major limitation in aluminium processing industry as current castings require high internal integrity and ease of surface finishing. The industry has to resort to the tedious and expensive process of machining a shape from a block billet of aluminium. Huge amount of money in the order of billion dollars is consumed annually through billet machining and the great wastage in the form of machining chips and turnings is also environmentally undesirable. This research explores the possibility of merging the benefits of these two series of alloys together by attempting to cast wrought aluminium alloy in an industrial scale with high internal integrity which resembles forging results. The ultimate objective is to save up substantial expenditure for the entire aluminium processing industry. Previous researchers in their small, laboratory scale crucibles had proven the effectiveness of ultrasonic treatment (UST) in degassing and grain refinement of the melt, which translates into better mechanical properties and internal soundness. Their numerous research results laid down the ground work for this research. Yet these discrete, small-scale experiments did not ripen into industrial application mainly because molten aluminium improvement in the crucible or furnace itself did not collate with the subsequent casting quality improvement. Besides, repeatability of experimental results was not emphasized in most experiments. This research aims to ripen the UST process into an industrial manufacturing scale. Therefore it was designed in an industrial scale with casting samples of around 4.5 Kg, the usually medium-sized aluminium casting in the industry. Application of UST was not to the stagnant crucible aluminium melt approach done by previous researchers, it was applied in a dynamic way, to a mold-filling situation where transitory freezing phases progressed through solidification. Only the mushy zone of the freezing process was to be vibration treated, therefore successfully locating the mushy zones during solidification was of cardinal importance. A series of simulations were done to isolate the different zones during solidification. The simulation results were then verified by processing them in the experiments to see if they bore resemblance to the empirical results. Only validated parameters were applied to the experiments. An accurate zoning of solidification process was simulated so that the minimum application time for UST to the mushy zone could be focused. This avoided excessive energy ingress into the melt to become hot spots which resulted in hot cracks and grain enlargement. Repeatability is the most important aspect for this research because it's an advanced manufacturing process which is intended to mature into a production technology. And repeatability comprises the experimental results, the mechanical property data and even the experiment-engendered defects. The experiment required a purpose-built casting facility with consistent temperature control, UST delivery and filling speed control. In the experiments, ultrasonic waves of different frequencies were activated towards the mushy zone of the casting and there were several trials for the same frequency. The samples were examined by tensile testing machine, and optical microscope for the grain size, X ray scanning for investigating the internal void, the porosity volume and distribution orientation. The results showed that ultrasonic frequencies focused at mushy zone only could reduce the volume of internal void of the castings. Several castings so treated were found with no porosity detected at all, meaning a complete absence of porosity is possible. It also revealed that UST could enhance the mechanical properties of the castings in both ultimate tensile strength and ductility. It was also found that UST application in a controlled way (along the mushy zone) yielded better improvement than fully switched on UST during the entire solidification. While further work still needs to be done, the research confirmed that UST applied to mushy zone only could yield the ultimate porosity-free casting. It therefore strikes out the demarcation that separates the casting and forging aluminium process,enables forging internal integrity to be achievable through casting. The industry will be the ultimate beneficiary of this low-cost casting method to produce shapes with integrity that matches the quality through forging process.