Cold welding of steel and aluminum alloys - Joining processes, intermetallic phases and bond strength
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Steel and aluminum are two of the most commonly used metals in today’s society and can be found everywhere, in vehicles, buildings, ships, and in most new electronic devices to name a few. Both metals have desirable mechanical properties, making them unique and well suited for different applications. Traditionally, steel has been the main constituting metal in vehicles, due to its high strength and low costs. However, one large societal challenge today is the large emissions of CO2 related to the automotive industry. One part of the solution is to reduce the weight of vehicles by altering the vehicle design and by introducing more lightweight materials into the structure. By replacing parts of the steel structure with aluminum and combining the parts using welding as the manufacturing technique, more lightweight materials can easily be introduced into the vehicle structure. This will create a multi-material product which jointly utilize the mechanical properties of both metals. The ability to produce dissimilar metal joints is attractive, as it will offer design flexibility in many situations without achieving a loss in performance or quality. In order to produce dissimilar metal joints with the optimal mechanical properties, it is necessary to understand the individual base metals to be joined and how they interact with each other during and after joining. One main challenge to overcome is the formation and growth of intermetallic phases. The formation of these intermetallic phases is inevitable during joining at elevated temperatures for steel and aluminum 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. Nevertheless, the formation and growth of these phases are influenced by the chemical composition of the base metals and the used joining technique. Therefore, increasing our understanding of these phases and how to minimize their growth is vital in order to produce steel-aluminum joints of high quality with optimal mechanical properties. 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 and the bond strength of the composites have been evaluated based on peel tests and glued tensile tests. In general, roll bonding is an efficient, low-cost technique used to join dissimilar metal sheets at solid-state, but the first part of this thesis highlights the complexity of the rolling process. The influence of several process parameters such as surface preparation and the fastening method used to prevent movement between the layer during rolling, the chosen base metals, their mechanical properties, and bond strength of the final composite has been investigated and discussed. Finite element simulations were conducted in order to further investigate the experimental observations regarding the relationship between the thickness reduction achieved in the individual metal layers, the stress-strain curves of the base metals and the fastening method used during the rolling process. The results showed the large influence of the used fastening method on the deformation behavior of the metal layers during rolling, especially when performing roll bonding of metals with large differences in their stressstrain response. The influence of the mechanical properties of the base metals during joining was also investigated by cold pressure extrusion, in collaboration with PtU Darmstadt. Compared to roll bonding, this technique was found to be more sensitive to differences in the mechanical properties of the steel and aluminum base metals. With this joining process, when the difference in the initial hardness was large and the aluminum had a low work-hardening ability, the base metals would move relative to each other during the joining process. This terminated any bond that possibly formed early in the joining process, hence reducing the bond strength. This was a clear limitation for the cold pressure extrusion process, limiting the possible material combinations which can be joined. 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 alloys and how they influence the intermetallic phase formation and growth. Commercially pure aluminum, 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 the influence of silicon and magnesium found in the aluminum alloys, were investigated. The influence of the alloying elements was studied and discussed, both individually and combined. Through extensive experimental work, insight into the formation and growth sequence of the intermetallic phase layers for the different material combinations were established. The results showed that the alloying elements have a strong influence on the formation and growth of the intermetallic phases. Magnesium was found to accelerate, while silicon, nickel and chromium decreased the growth rate. Which phases that formed were found to be strongly dependent on the combination of alloying elements in the steel and aluminum. However, Fe2Al5 and Fe4Al13 formed adjacent to the steel for all material combinations. The combination of silicon, nickel and chromium present in the composites consisting of AA6082 and 316L stainless steel, were concluded to be a good combination, restricting the formation and reducing the growth rate of the intermetallic phases at elevated temperature, and at the same time achieving a high tensile bond strength in the conducted 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. The results showed that nickel was the best-suited metal to be used as an interlayer during roll bonding of steel and aluminum. Post-rolling heat treatments were found to be necessary to produce a high bond strength between the layers, as they resulted in metallurgical bonding between the nickel and steel layers, while simultaneously producing an intermetallic phase layer along the aluminum-nickel interface. The intermetallic phase layer was found not to weaken the strength of the joint when kept below a thickness of 5 μm.