Biju Patnaik Central Library

Thesis Ph.D/M.Tech (R) National Institute of Technology, Rourkela

To decrease fuel consumption and environmental impact, the use of more aluminum-based alloys in automobiles is the current trend which necessitates reliable aluminum - steel dissimilar joint. For this, traditional fusion joining techniques due to their high heat input which promotes the excessive formation of thick intermetallics at the interface of the joint that leads to low strength and premature failure. Brazing is a probable solution which utilizes a filler material and eliminates the melting of the steel and can potentially avoid/decrease intermetallics formation but requires careful selection of the filler metal and process parameters. In the present study laser brazing technique was employed to weld-braze 2 mm thick galvanized interstitial free (IF) steel (mentioned as GI) with 2 mm thick aluminum alloys (i.e., AA6082 (AlMg-Si) and AA5082 (Al-Mg)) in flange joint configuration, using 2 mm diameter solid AlSi12 (4047) filler wire. Experiments were performed with a constant spot size (1.7 mm) and varying other laser parameters, i.e., laser power (2.0-4.0 kW), wire feed rates (2.0 – 3.6 m/min) and at a scan speed of (2-2.5 m/min). Subsequently, the joints were subjected to phase evaluation study (microstructure, XRD), mechanical characterization (hardness, tensile and low cycle fatigue) and corrosion studies (immersion, salt-spray, potentiodynamic polarization, EIS). From macrographs, it was noticed that the size and shape of the weld-braze joint appearance vary with respect to the laser parameters. Microstructural observation of the etched specimens under SEM demonstrated columnar dendritic structure at the brazed zone, while eutectic structure at the inter-dendritic region. At the aluminum side, epitaxial growth was noticed, and a two-layered (planar and needle structured) interface towards the galvanized steel in both the compositions (i.e., AA6082/GI and AA5083/GI). EDS point analysis and X-ray diffraction technique were used to study the interface, and the results confirmed that the planar and needle structures are comprised of ternary Alx-Fey-Siz and binary Alx-Fey intermetallic phases respectively in both the combinations (i.e., AA6082/GI and AA5083/GI). The asymmetrical hardness profile throughout the braze joint with the lowest value in the brazed region was noted. In the case of nanoindentation testing, intermetallics have shown higher hardness values compared to base materials. From the tensile testing, it was pointed out that joints produced with AA6082/GI has exhibited higher strength and failed at the base material (aluminum-side). AA503/GI joints demonstrated marginally lower strength and recorded interfacial failure. The fatigue life of the laser brazed joints (AA6082/GI and AA5083/GI) at lower stain amplitude recorded higher fatigue life, and it was close to aluminum base material value. Fatigue tested specimens recorded brazed zone failure and revealed flow lines, intergranular cracking, and typical fatigue striations. From the salt spray test, it was revealed that galvanized steel was highly susceptible to corrosion compared to the Al-rich phase in both the joint combinations (i.e., AA6082/GI and AA5083/GI). Polarization results showed a substantial change in corrosion resistance from the steel interface to the brazed region due to variation in the microstructure. AA5083/GI brazed joints have shown marginally higher corrosion resistance as compared to the joints made with AA6082/GI. Post corrosion microstructure revealed pit formation at the brazed region