MECHANICAL AND MICROSTRUCTURAL CHARACTERIZATION OF COLD METAL TRANSFER WELDEDALUMINIUM METAL MATRIX COMPOSITE JOINTS

Abstract

Technologists are being encouraged to develop useful and efficient methods and procedures for the production and joining of Al-alloys and their metal matrix composite by the growing application of aluminum in numerous industries. It has been noticed that AA7475 plates and sheets are currently used in high-performance aviation applications. However, due to the challenging-to-weld nature of the Al-alloy, the primary obstacle to these applications is the appropriate joining technique. This investigation primarily aims to evaluate and assess the ripeness of the Cold Metal Transfer welding process for hybrid aluminum metal matrix composites (AMMC). The CMT or cold metal transfer, refers to a modified MIG welding process. Researchers are getting more interested in CMT as an advanced welding technique. The latest developments in this field, include the improved mechanical properties of Al alloys and their metal matrix composite, and the development of efficient welding procedures. To evaluate the weld efficiency, measurements were made of its mechanical characteristics and microstructure, including its tensile strength, yield strength, elongation, microhardness, interface structure, microstructure, and fracture process. To produce welded connections with the best mechanical qualities, an attempt is made to determine the welding technique that is free from all welding imperfections, such as gas porosity, spattering, and intermetallic production. The objective of the present investigation is to characterize the mechanical and microstructural behaviours of aluminium metal matrix composite (AMMC) of AA7475 alloy in T7351 temper conditions by using a bottom pouring stir casting machine. The hybrid composite was formed using SiC, B4C, MoS2, and Gr as reinforcements in different sets of combinations. The mechanical properties were studied of all three sets of fabricated composites. The composite sample showing the best mechanical properties is re-casted using a stir-casting machine with a squeeze-casting arrangement operated by the human-machine interface (HMI). The mechanical properties were vii tested and reported for the chosen composite of Al7475 alloy. To check the weldability of the composite a sample was prepared and welding was performed using a robotic CMT welding setup. To join the samples, the Taguchi orthogonal array of L9 was used to optimize the welding parameters. The mechanical characteristics, fracture behaviour, and morphology of the welded joints were examined. As compared to the base metal (BM) and heat-affected zone (HAZ) the weld metal (WM), has a lower value of tensile strength and hardness. All the welded joints fail at the weld metal. The weld metal is the weakest link of the welded sample. The weld metal (WM) has the lowest hardness, which is 64.69 % of the base metal (BM). The microstructure of the weld area was analysed for the welded joint. The final welding parameters were used to perform the confirmatory test. The mechanical and microstructural properties of the confirmatory test sample validate the final welding process parameters. The final welding parameters were also used to weld the samples of AA7475 alloy in T7351 temper conditions as a confirmatory test. The samples were prepared and welding was performed. The mechanical properties were tested and the microstructure was analysed and reported. After analysing the tensile strength (TS), yield strength (YS), and percentage elongation, it was found that the weld metal (WM) had an elongation of 6.2%, which is considerably less than 13.2% of the base metal (BM) and a bit lower than 8.4% of the heat affected zone (HAZ). Hardness tests were conducted, and the results showed that the weld metal had reduced hardness values. The WM has the lowest hardness, just 58.24 % of the BM. Typical metallographic images of CMT welded samples were analysed for the AA7475-T7351 joint. At base metal, HAZ, and weld metal, the micrograph and associated Energy Dispersive X-ray (EDX) examination were performed. The dispersion of precipitates throughout the weld was assessed using EDX. Analysis and discussion were carried out regarding the EDX data obtained from the top, middle, and root portions of the weld.The fractography of WM from the fractured tensile sample was also studied and reported. Weld metal (WM), base metal (BM), as well as the generation of the secondary phase in the heat-affected zone (HAZ), were all evaluated using EDX and SEM micrographs.