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Because of its low density and high strength, magnesium (Mg) and its alloys are being sought after in the automotive industry for structural applications. Although many road-going cars today contain cast Mg parts, in the fabrication of chassis structural members the wrought alloys are required. One of the challenges of fabrication with wrought Mg is welding and joining the formed sheets. Because of the commonly observed difficulties in fusion welding of Mg such as hot cracking and severe Heat Affected Zone (HAZ), this work aimed to establish the feasibility of the solid-state process Magnetic Pulse Welding in producing lap welds of Mg sheet. Mg AZ31 alloy sheets have been lap-welded with Magnetic Pulse Welding (MPW), an Impact Welding technique, using H-shaped symmetric coils connected to a Pulsar MPW-25 capacitor bank. MPW uses the interaction between two opposing magnetic fields to create a high speed oblique collision between the metal surfaces. The oblique impact sweeps away the contaminated surface layers and forces intimate contact between clean materials to produce a solid-state weld. Various combinations of similar and dissimilar metals can be welded using MPW. Other advantages of MPW are high speed, high strength, and the possibility of being mounted on a robotic arm. The present research focuses on the feasibility and mechanical performance of an MPW weld of 0.6 mm AZ31 Mg alloy sheets made in a lap joint configuration. Tensile shear tests were carried out on the joints produced. Load bearing capacity showed linear increase with capacitor bank discharge energy up to a certain value above which a parabolic increase was seen. Strength was estimated to be at least as high as base metal strength by measuring the fracture surface area of selected samples. The fracture surface of samples welded at higher discharge energy showed two regions. In the beginning of the bond a platelet-featured fracture brittle in appearance and a ductile, micro-voiding fracture in the latter part. The joint cross section morphology consisted of a flattened area that had two symmetric bond zones 1 mm wide each separated by an unbonded centre zone ~3mm wide. Reasons for the morphology were presented as a sequence of events based on the transient nature of the oblique collision angle. The interface microstructure was studied by optical and electron microscopy. Examination of the bonds has revealed sound and defect free interfaces. No microcracking, porosity, resolidification, or secondary phase formation was observed. Metallographic examination of the unbonded centre zone revealed anisotropic deformation and a lack of cleaning from the interface. This zone is formed as a result of normal impact in the initial stage of collision. The bond zone interface of the joint was characterized by a smooth interface consisting of refined grains. In samples welded at higher energy interfacial waves developed in the latter half of the bond zone. Transmission electron microscopy (TEM) of the bond zone revealed a continuous interface having an 8-25 μm thick interlayer that coincided with the waves and had a dislocation cell structure and distinct boundaries with adjacent material. Equiaxed 300 nm dynamic recrystallized (DRX) grains were found adjacent to the interlayer as well as other slightly larger elongated grains. The interlayer is thought to be formed in plasticized state at elevated temperature through severe shear strain heating. The interlayer corresponds to a ductile fracture surface and, along with the interfacial waves, is responsible for the joint’s high strength.

Because of its low density and high strength, magnesium (Mg) and its alloys are being sought after in the automotive industry for structural applications. Although many road-going cars today contain cast Mg parts, in the fabrication of chassis structural members the wrought alloys are required. One of the challenges of fabrication with wrought Mg is welding and joining the formed sheets. Because of the commonly observed difficulties in fusion welding of Mg such as hot cracking and severe Heat Affected Zone (HAZ), this work aimed to establish the feasibility of the solid-state process Magnetic Pulse Welding in producing lap welds of Mg sheet. Mg AZ31 alloy sheets have been lap-welded with Magnetic Pulse Welding (MPW), an Impact Welding technique, using H-shaped symmetric coils connected to a Pulsar MPW-25 capacitor bank. MPW uses the interaction between two opposing magnetic fields to create a high speed oblique collision between the metal surfaces. The oblique impact sweeps away the contaminated surface layers and forces intimate contact between clean materials to produce a solid-state weld. Various combinations of similar and dissimilar metals can be welded using MPW. Other advantages of MPW are high speed, high strength, and the possibility of being mounted on a robotic arm. The present research focuses on the feasibility and mechanical performance of an MPW weld of 0.6 mm AZ31 Mg alloy sheets made in a lap joint configuration. Tensile shear tests were carried out on the joints produced. Load bearing capacity showed linear increase with capacitor bank discharge energy up to a certain value above which a parabolic increase was seen. Strength was estimated to be at least as high as base metal strength by measuring the fracture surface area of selected samples. The fracture surface of samples welded at higher discharge energy showed two regions. In the beginning of the bond a platelet-featured fracture brittle in appearance and a ductile, micro-voiding fracture in the latter part. The joint cross section morphology consisted of a flattened area that had two symmetric bond zones 1 mm wide each separated by an unbonded centre zone ~3mm wide. Reasons for the morphology were presented as a sequence of events based on the transient nature of the oblique collision angle. The interface microstructure was studied by optical and electron microscopy. Examination of the bonds has revealed sound and defect free interfaces. No microcracking, porosity, resolidification, or secondary phase formation was observed. Metallographic examination of the unbonded centre zone revealed anisotropic deformation and a lack of cleaning from the interface. This zone is formed as a result of normal impact in the initial stage of collision. The bond zone interface of the joint was characterized by a smooth interface consisting of refined grains. In samples welded at higher energy interfacial waves developed in the latter half of the bond zone. Transmission electron microscopy (TEM) of the bond zone revealed a continuous interface having an 8-25 μm thick interlayer that coincided with the waves and had a dislocation cell structure and distinct boundaries with adjacent material. Equiaxed 300 nm dynamic recrystallized (DRX) grains were found adjacent to the interlayer as well as other slightly larger elongated grains. The interlayer is thought to be formed in plasticized state at elevated temperature through severe shear strain heating. The interlayer corresponds to a ductile fracture surface and, along with the interfacial waves, is responsible for the joint’s high strength.

Das Magnetpulsschweißen ermöglicht das stoffschlüssige Fügen verschiedenartiger Metalle, wobei die intermetallische Phasenbildung im Gegensatz zu herkömmlichen Schmelzschweißverfahren deutlich reduziert werden kann. Im Rahmen dieser Arbeit erfolgt die Entwicklung eines neuartigen optischen Messsystems, welches sich zum Bestimmen der axialen und radialen Kollisionsgeschwindigkeit eignet und die Berechnung des Kollisionswinkels ermöglicht. Es wertet das charakteristische Prozessleuchten aus, das bei der Fügepartnerkollision während dieses Pressschweißverfahrens entsteht. Experimente in Vakuumatmosphäre belegen, dass die Temperatur im Fügespalt bei kleinen Kollisionswinkeln deutlich über den Siedetemperaturen der beteiligten Werkstoffe liegen kann. Die aus dem Fügespalt strömende heiße Partikelwolke schmilzt die Fügepartneroberflächen an, bevor diese aufeinander treffen, sich stoffschlüssig verbinden und schließlich rasch abkühlen. Metallografische Analysen belegen die angeschmolzenen Bereiche in der Fügeverbindung und bilden den Ausgangspunkt für ein numerisches Modell, welches das Aufheiz- und Abkühlverhalten der Oberflächen abschätzt. Das patentierte Messsystem hilft außerdem bei der Prozesseinstellung und -überwachung mit möglichst geringer Impaktgeschwindigkeit, wobei der Einfluss verschiedener anlagenbedingter und geometrischer Faktoren untersucht wird. Der Wärmeeintrag in die Verbindungszone kann außerdem durch exotherm reagierende Zwischenschichten erhöht und dadurch die benötigte Impaktgeschwindigkeit reduziert werden. Die genannten Maßnahmen tragen dazu bei, die thermischen und mechanischen Belastungen auf die Werkzeugspulen zu reduzieren und damit ihre Lebensdauer zu erhöhen.:1 Einleitung 2 Stand der Kenntnisse beim Magnetpulsschweißen 2.1 Verfahrenseigenschaften und Anwendungsgebiete 2.2 Wirkprinzip und Einflussgrößen beim elektromagnetischen Umformen 2.3 Theorien zum Fügemechanismus beim Kollisionsschweißen 2.4 Erscheinungsbild und Eigenschaften der Verbindungszone 2.5 Messtechnisches Erfassen von Prozessparametern 2.6 Strategien für eine höhere Prozesseffizienz 2.7 Zwischenfazit zu Kapitel 2 3 Zielsetzung 4 Versuchsaufbau und Bewerten des Schweißvorgangs 4.1 Versuchsaufbau 4.2 Bewerten des Energieeinsatzes 4.3 Bewerten des Schweißergebnisses 4.4 Zwischenfazit zu Kapitel 4 5 Erfassen der kinetischen Kollisionsparameter 5.1 Entwickeln eines Messsystems zum Erfassen des Impaktblitzes 5.2 Numerisches Modell zum Bestimmen der Kollisionsparameter 5.3 Experimentelles Bestimmen der Impaktgeschwindigkeit 5.4 Experimentelles Bestimmen der Kollisionspunktgeschwindigkeit 5.5 Weitere Anwendungsmöglichkeiten der Blitzauswertung 5.6 Zwischenfazit zu Kapitel 5 6 Experimentelle Analyse der Partikelwolkeneigenschaften 6.1 Einfluss der Kollisionsbedingungen auf die Temperatur der Partikelwolke 6.2 Charakterisieren der Partikelwolke 6.3 Zwischenfazit zu Kapitel 6 7 Schweißmodell 7.1 Unterscheidung von Schweißmechanismen 7.2 Zwischenfazit zu den experimentellen Ergebnissen 7.3 Metallurgische Effekte 7.4 Aufbau des temperaturbasierten Schweißmodells 7.5 Einfluss der thermischen und kinetischen Prozessbedingungen 7.6 Zwischenfazit zu den numerischen Ergebnissen 7.7 Wellenbildung 7.8 Zwischenfazit zu Kapitel 7 8 Einstellen der kinetischen Kollisionsparameter 8.1 Frequenzeinfluss 8.2 Wandstärkeeinfluss 8.3 Fügespalt- und Wirklängeneinfluss 8.4 Prozessrobustheit bei geometrischen Abweichungen 8.5 Experimentelle Hinweise zum Ermitteln des Schweißfensters 8.6 Zwischenfazit zu Kapitel 8 9 Exotherm reagierende Zwischenschichten 10 Zusammenfassung / Magnetic Pulse Welding is a pressure welding process that enables material joints between dissimilar metals. Compared to conventional fusion welding processes, the intermetallic phase formation can be minimized to an uncritical minimum due to the reduced and localized heat input. Although the process is already applied in industrial production for hybrid parts, the underlying principle of the bond formation is not yet completely explored. One of the main reasons for this is the difficulty in process monitoring, which also hinders process adjustment or the targeted support of the joining mechanism. Both aspects are of great importance for an efficient welding process and increased life-times of the tool coils. In the present thesis, a new optical measurement system has been developed to get insights into the kinetic conditions during collision welding processes. It evaluates the characteristic flash that occurs during the high-speed collision of the joining partners above a certain impact velocity. Furthermore, the second velocity component of the collision front in axial direction can be measured, which enables the calculation of the collision angle. Experiments in vacuum atmosphere reveal for small collision angles, that the temperatures in the joining gap can exceed the vaporization temperatures of the involved materials. Since the ejected cloud of particles is very hot, the surfaces of the parts are melted before they are pressed together. Afterwards, the bond is formed and the joining zone is cooled down rapidly. Metallographic analysis evidenced melted regions in the joining zone, which serve as an input variable for the numerical model. This model predicts the heating and cooling behavior of the surfaces and shows for large collision angles, that the surfaces are already solidified before they come into contact. This fact inhibits the identified welding mechanism based on fusion. The patented measurement device helps studying the influence of certain machine-related and geometrical parameters during the process adjustment with low impact velocities and serves as a quality assurance system. Furthermore, exothermic reactive interlayers can increase the heat input in the joining zone and thus, decrease the minimum impact velocities. These strategies may contribute to a significant reduction of thermal and mechanical shock loading of the tool coils to increase their life-time.:1 Einleitung 2 Stand der Kenntnisse beim Magnetpulsschweißen 2.1 Verfahrenseigenschaften und Anwendungsgebiete 2.2 Wirkprinzip und Einflussgrößen beim elektromagnetischen Umformen 2.3 Theorien zum Fügemechanismus beim Kollisionsschweißen 2.4 Erscheinungsbild und Eigenschaften der Verbindungszone 2.5 Messtechnisches Erfassen von Prozessparametern 2.6 Strategien für eine höhere Prozesseffizienz 2.7 Zwischenfazit zu Kapitel 2 3 Zielsetzung 4 Versuchsaufbau und Bewerten des Schweißvorgangs 4.1 Versuchsaufbau 4.2 Bewerten des Energieeinsatzes 4.3 Bewerten des Schweißergebnisses 4.4 Zwischenfazit zu Kapitel 4 5 Erfassen der kinetischen Kollisionsparameter 5.1 Entwickeln eines Messsystems zum Erfassen des Impaktblitzes 5.2 Numerisches Modell zum Bestimmen der Kollisionsparameter 5.3 Experimentelles Bestimmen der Impaktgeschwindigkeit 5.4 Experimentelles Bestimmen der Kollisionspunktgeschwindigkeit 5.5 Weitere Anwendungsmöglichkeiten der Blitzauswertung 5.6 Zwischenfazit zu Kapitel 5 6 Experimentelle Analyse der Partikelwolkeneigenschaften 6.1 Einfluss der Kollisionsbedingungen auf die Temperatur der Partikelwolke 6.2 Charakterisieren der Partikelwolke 6.3 Zwischenfazit zu Kapitel 6 7 Schweißmodell 7.1 Unterscheidung von Schweißmechanismen 7.2 Zwischenfazit zu den experimentellen Ergebnissen 7.3 Metallurgische Effekte 7.4 Aufbau des temperaturbasierten Schweißmodells 7.5 Einfluss der thermischen und kinetischen Prozessbedingungen 7.6 Zwischenfazit zu den numerischen Ergebnissen 7.7 Wellenbildung 7.8 Zwischenfazit zu Kapitel 7 8 Einstellen der kinetischen Kollisionsparameter 8.1 Frequenzeinfluss 8.2 Wandstärkeeinfluss 8.3 Fügespalt- und Wirklängeneinfluss 8.4 Prozessrobustheit bei geometrischen Abweichungen 8.5 Experimentelle Hinweise zum Ermitteln des Schweißfensters 8.6 Zwischenfazit zu Kapitel 8 9 Exotherm reagierende Zwischenschichten 10 Zusammenfassung