Experimental Investigation on Corrosion Behavior of Ultrasonic Assisted stir Cast AA6061 Hybrid Metal Matrix Composite under DifferentAqueous Corrosive Environments

Aluminum Hybrid Metal Matrix Composites (Al-HMMCs) are contemporary materials that blend aluminum with additional reinforcing elements to enhance their mechanical, thermal, and other properties. These materials find applications across various sectors including the aerospace industry, automotive sector, military and defence, marine industry, as well as manufacturing industries. In this research, AA6061, zirconium dioxide (ZrO₂) and boron carbide (B₄C) have been selected as the matrix and reinforcement materials. AA6061/ZrO₂/B₄C (Al-HMMCs) is manufactured using Ultrasonic-assisted stir casting. The process parameters of ultrasonic assisted stir casting were optimized with the help of Taguchi optimization technique. The melting temperature (700, 750 and 800 ºC), stirring speed (10, 15 and 20 minutes) and AA6061/xwt.%B₄C/xwt.%ZrO₂ ( 6, 8 and 10) are selected as input parameters for optimization. The reinforcements ZrO₂ and B₄C are weighed by equally sharing by wt.% The micro hardness was selected as response parameter and it was determined by using Vickers micro hardness tester. The L9 Orthogonal Array (OA) was employed for optimization of input parameters of ultrasonic assisted stir casting. Taguchi results showed that medium level of melting temperature (750 ºC), higher level of stirring speed (300 rpm), lower level of stirring duration (10 minutes) and medium level of wt.% of ZrO₂ and B₄C (8wt.%) were the optimized combination of input process parameters of ultrasonic assisted stir casting for fabricating AA6061 HMMC. The AA6061/xwt.%B₄C/xwt.%ZrO₂ (x = 0,4,6,8 and 10) HMMCs were manufactured by utilizing optimized process parameters. The micro hardness of AA6061/xwt.%B₄C/xwt.%ZrO₂ (x = 0,4,6,8 and 10) was determined. . From the obtained hardness tests, it was observed that the hardness of AA6061 HMMCs was enhanced up to addition of 8wt. % of B₄C and ZrO₂ and decreased the hardness while adding 10wt.% of B4C and ZrO₂. The better AA6061/8wt.%ZrO₂/8wt.%B₄C HMMC undergone X-ray Diffraction Analysis (XRD) and Scanning Electron Microscope (SEM) testing to identify elements and examine the microstructure. The high peak of ZrO₂, B₄C and Al was obtained in 71.4, 35.68 and 37.81 2 theta values respectively. The SEM image of ultrasonic aided stir cast AA6061 based composite was confirmed the uniform distribution of zirconium di oxide and boron carbide reinforcements. There was no oxide formation on the surface of the stir casted AA6061 composites due to the execution of casting process in the inert environment. The casted AA6061/8wt.%ZrO₂/8wt.%B₄C surface of the composite exhibits no porosity occurrence due to the impact of usage of ultrasonic vibrator to prevent the formation of holes due to escape of unwanted gases. Subsequently, corrosion tests were conducted on the AA6061 HMMC in various corrosive environments such as NaOH, HCl, H₂SO₄, and NaCl to evaluate its electrochemical behavior. The surfaces of corroded specimens were analyzed using SEM and Energy Dispersive Spectroscopy (EDS). The research findings indicate that AA6061 composites exhibit the highest corrosion resistance when immersed in a NaCl medium. This enhanced resistance was attributed to the medium's lower corrosion potential, stronger charge transfer resistance (Rct), and lower double-layer capacitance (Cdl) compared to alternative environments. Additionally, SEM analysis illustrated that the incorporation of B₄C and ZrO₂ ceramics results in the formation of protective barrier layers, further enhanced their corrosion resistance. ZrO₂ particles were attracted and neutralized the corrosive ions and electrons, thereby reducing corrosive activity on the AA6061 aluminium matrix.

Keywords:
AA6061; ZrO2; B4C; Corrosion resistance; XRD; EDS; SEM