Oheil, Mazin2025-08-282025-08-282025-08-282025-08-26https://hdl.handle.net/10012/22302The increasing demand for lightweight vehicles to improve fuel efficiency and reduce greenhouse gas emissions has encouraged the use of materials such as aluminum (Al) and magnesium (Mg) in automotive design. These metals offer excellent strength-to-weight ratios and recyclability but present challenges when joined together due to the formation of brittle intermetallic compounds (IMCs) during traditional welding processes. Resistance spot welding (RSW), a common method in automotive manufacturing, is particularly limited when directly joining Al and Mg because of these IMC-related issues. This study investigates the use of cold spray (CS) technology to enable reliable RSW of dissimilar Al/Mg joints. Cold spray is a solid-state deposition method that can create interfacial coatings, acting as a physical barrier between the base metals and reducing IMC formation. In this research, nickel (Ni) and titanium (Ti) powders were selected as interlayer materials due to their high melting points and favorable metallurgical properties. These coatings help isolate the Al and Mg during welding, improving bond quality and weld durability. Ni and Ti coatings were applied to Al 6022-T4 aluminum and AZ31B magnesium alloy sheets using a low-pressure cold spray system. Ni was deposited at 485–500 °C and 1.5 MPa, while Ti was applied at 500 °C and 1.4 MPa using nitrogen as the carrier gas through a DeLaval nozzle. Coating thicknesses ranged from 80–120 µm for Ni and 260–350 µm for Ti. RSW was then conducted using optimized welding parameters: 27 kA current for Ni-coated and 22 kA for Ti-coated samples, both under 4 kN force with two pulses of 15 cycles each. Lap shear testing showed maximum strengths of 4.3 kN for Ni and 4.2 kN for Ti, significantly higher than the 0.833 kN strength observed in direct Al/Mg welds. Post-weld characterization using optical microscopy (OM) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX) revealed minimal IMC formation at the Al/Mg interface for both coating types. Mg was detected at the Ni-to-Ni and Ti-to-Ti interfaces within the weld nugget, indicating some elemental diffusion but no significant degradation of joint quality. Since the welding was carried out using a novel approach, both the weld mechanism and the behavior of magnesium during the process represent unobserved phenomena. This reveals a unique joining mechanism, offering new insights into how Mg reacts under such conditions and emphasizing the innovative contribution of this study to dissimilar metal welding research. Also, Ti coatings provided slightly better IMC suppression, attributed to Ti’s strong affinity for Mg and its role in forming a stable transition layer. Fatigue and monotonic testing were conducted to assess long-term joint performance under cyclic and static loading. The results demonstrated substantial improvements in both durability and mechanical performance when using cold spray coatings. On average, joint strength increased by a factor of 4.6 with a Ti coating and 4.9 with a Ni coating compared to direct RSW joints that achieve 0.833kN load. For the cyclic load, the fatigue strength at 2M cycles reached 1.28 kN max load for Ni-coated Al/Mg joint and reached 1 kN max load for Ti coated Al/Mg joint. It is worth to mention that the Ti-coated joint reached its level of strength and fatigue performance after approximately 3% of Al powder was mixed with the Ti powder and then deposited only on the Al sheet using the cold spray technique, which underscored the critical role of powder mixing in improving joint integrity and mechanical performance. This study highlights the excellent role of cold spray technology in enabling reliable resistance spot welding of dissimilar aluminum and magnesium alloys. By applying Ni and Ti coatings through cold spray, intermetallic compound formation is significantly reduced, enhancing joint strength and durability. The coatings act as diffusion barriers and improve metallurgical compatibility, making cold spray a key innovation for joining lightweight metals in automotive applications.enCold sprayMg/AlDissimilar Resistance Spot WeldsNi interlayerTi interlayercrack detection in Resistance Spot WeldsDoctoral Thesis