Removal of Oxide Inclusions in Aluminium Scrap Casting Process with Sodium based Fluxes

The investigation of Oxide Inclusions removal in aluminium scrap casting process with sodium based fluxes has been carried out. The purpose of this research is to investigate the effect of Na2SO4 and NaCl based fluxes addition onto the fluidity and microstructure of aluminium product. The alloy which is used in this investigation is Al-Si which mixed with metal scrap using gravity casting method. The variation of melting temperature in this investigation are 700C, 740C, and 780C. In this research, material characterization was determined using DSC, EDAX, XRD, and fluidity test. The results show that the number of oxide inclusions decrease as the addition of 0,2% wt. flux, and completly removed after the addition of 0,4% wt. flux. The highest fluidity and tensile strength was obtained after the addition of 0,4% wt. flux. at 740C..


Introduction
Aluminium alloys is widely applied in many industrial product, such as transportation, packaging, construction, or houseware product due to its excellent properties, such as light weight, high corrosion resistance, high castability, and good conductor [1]- [4]. Recently, secondary aluminium is widely used in transportation, building, and packaging industries. Secondary aluminium is an aluminium which is produced by aluminium recycling process using aluminium scrap as its main source [5]- [6]. The utilization of aluminium scrap in the aluminium recycling process has a disadvantage as it contains many impurities element that is not desired in metal casting [7]- [9]. These impurities are considered as unwanted inclusions can reduce the quality of the final product. In metal casting process, inclusion can cause the presence of defect which can reduce the quality of the final product [10]- [12].
Oxides are the most general inclusions which can be formed in metal casting process. Without proper refining treatments of molten aluminum, oxide inclusions will be retained in the aluminum ingots after casting, and invariably causes many problems such as a diminishing of mechanical properties, poor machinability and surface quality [9].
In order to reduce the presence of defect on the final product, it is important to use molten aluminium which free from any impurities in the casting process [13]- [14]. Lately, the most widely accepted and frequently adopted method to remove inclusions such as oxide films from molten aluminum is solid flux refining (also known as refining agents, treatment agents or fused agents).
Chemical components that are used in a flux depends on the objective of casting process (alkali removal, cleanliness, dross separation) [15]. In this paper flux based Na 2 SO 4 and NaCl was used to reduce the percentage of oxide inclusions inside the molten aluminium by binding the inclusions into the melt surface. In this research, 20 kg of aluminium scraps were used for each experiment. These aluminium scraps were re-melted in eletrical crucible furnace to obtained molten aluminium. Next, the molten aluminium was treated with flux to determine the effects of flux on quality of the final product. Fluxing treatment were performed at various melting temperatures such as 700 0C, 740 0C, and 780 0C, under constant stirring condition for 20 minutes. In this research, fluxing treatment were performed using saltbase flux about 0.2% and 0.4% weight of of the total molten aluminium. The details of experimental condition for this research are given in Table 2. Table 2. Experimental Condition Fig 1 shows the mould design. In this research, the mould metal which we used is SKD 61 type with gravity casting system for pouring. The products are tensile test specimens which furthermore being through machining process as final step. Spesimen [16].

Experimental Methods
Based on Fig 2, it is shown that the flux based Na 2 SO 4 and NaCl has several peaks below 300 o C. Those peaks represent a reaction between organic compound and air. At higher temperature, another peak appeared at 696.81 o C, which is higher than the melting temperature of aluminum alloys, which is around 660 o C. This high melting point of flux was needed to ensure that the flux will not melted prematurely and also to prevent high temperature oxidation.    fig. 6, it is shown that the fluidity of molten aluminium increased as the temperature increased. Temperature 7800 C with the addition of 0,4% wt. flux has the highest flow rate with gradient 0.01. Temperature 7000C with the addition of flux 0,2% wt has gradient 0.009. Otherwise, the molten Al with melting temperature around 700oC but without using flux has the lowest fluidity, with gradient 0.005. The increasing of temperature affects the flux, as the viscosity of the flux reduced. Based on Green and Borne kinetic equation, molten salt viscosity (µ) is related to the root of inverse absolute temperature µ (1/T) 1/2 (1) From equation (1), we can see that viscosity and temperature has an inverse correlation. The viscosity of flux decreased as the melting temperature increased, thus the fluidity of flux inside the melt increased. Furthermore, the diameter of flux particles also decreased and the filtration efficiency of flux was improved. As the contact area between the flux and molten metal increase, the probability of contact between oxide inclusions with flux increased and wetting phenomenon happened [7]- [12]. Temperature also have an optimum point, after the optimum point is passed, the filtration efficiency of flux particles inside the molten metal reduced.   fig. 7, the amount of inclusions percentage was reduced as the number of flux weight percentage increased. The floatability of inclusion in molten metal is affected by the wettability of inclusion and oxides with the molten metal and molten flux. By increasing the amount of flux which used in casting, the contact surface for inclusion to be wetted by flux will be increased. As a result, the increasing weight percent of flux will increase the filtration efficiency and thus increases the rate of deletion of inclusion via flotation by gas bubbles [7].