Sammendrag
One large societal challenge today is the large emissions of CO2 related to the automotive industry. Introducing more lightweight materials into the vehicle structure will not only reduce the weight of the vehicle, but also contribute to reducing the energy consumption and environmental gas emissions. Lightweight materials could be integrated in vehicles by replacing parts of the traditional steel structure with aluminum and joining the parts using welding as the manufacturing technique. However, one of the main challenges with joining of dissimilar metals such as steel and aluminum, is the formation and growth of Fe-Al intermetallic phases. These phases are inevitable during joining at elevated temperatures due to the limited solubility between the metals. The intermetallic phases have been reported to be very brittle and their presence is detrimental for the strength of the produced joint.
Through investigations of roll bonded steel-aluminum composites, this thesis aims to increase the understanding of all the main aspects of steel-aluminum joining, i.e. the joining process, the intermetallic phase formation and growth, and the mechanical properties of the final joint. Advanced microscopy techniques, including SEM and TEM, have been used to analyze the produced roll bonded composites, and the mechanical properties such as the bond strength have been evaluated based on peel tests and glued tensile tests.
The first part of this thesis highlights the complexity of the rolling process. The influence of several process parameters on the interface characteristics and properties of the final composite has been investigated and discussed, resulting in the establishment of the optimal roll bonding procedure. The results presented in this study clearly show the influence of the base metals stress-strain relationship, fastening method used during rolling and the selected roll bonding process route on the produced composites with regards to deformation behavior, interface characteristics and most importantly the bond strength.
To further increase our understanding of the Fe-Al intermetallic phases, the second part of the thesis focuses on some important alloying elements found in steel and aluminum and how they influence the intermetallic phase formation and growth. Commercially pure aluminum AA1080, AA5083 and AA6082 were roll bonded with both a low-alloyed 355 steel and a 316L stainless steel. Hence, the influence of chromium and nickel found in stainless steel, and silicon and magnesium found in the aluminum alloys, were investigated. Through extensive experimental work, the formation and growth sequence of the intermetallic phase layers for the different material combinations has been identified, some which have not been previously reported in the literature. The combination of silicon with nickel and chromium seems to be optimal not only with regards to minimizing the intermetallic layer growth, but these composites also showed superior tensile bond strength compared to the other material combinations during the glued tensile tests.
Lastly, the concept of adding a metal interlayer between the steel and aluminum base metals to eliminate the formation and growth of the brittle and unwanted Fe-Al intermetallic phases, was investigated. The possibility of using both nickel and silver as interlayers was investigated as part of this thesis, and the results showed that nickel was the best-suited metal of the two. The results clearly show the large potential in using interlayers such as nickel in joining of steel and aluminum as composites with a high bond strength could be produced by rolling followed by post-rolling heat treatments.
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