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.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19708号 / 工博第4163号 / 新制||工||1642(附属図書館) / 32744 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 白井 泰治, 教授 乾 晴行 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The improvement in mechanical strength offered by ultra fine- (UF) and nanocrystalline (NC) sized grains is very attractive for potential applications of structural metals. Accumulative Roll-Bonding (ARB) is one of the promising new techniques for producing bulk UF grained metals. There are numerous reports on the monotonic mechanical behavior of various ARBed metals, however there are few, if any, on the cyclic deformation behavior of such metals. The primary objective of this study is to investigate the cyclic deformation behaviour and the related micro-mechanisms of ARBed metals from a fundamental perspective. To achieve this, the microstructure and the deformation behavior of commercial purity aluminum, OFHC copper, and DLP copper after ARB processing have been systematically characterized. The as-ARBed microstructure is found to be composite natured, with constituents of different grain sizes. The three constituents are: (i)UF grained matrix, (ii)NC primary discontinuities, and (iii)conventional sized pre-existing coarse grains. Due to this composite nature, three different cyclic strain accommodation mechanisms were found in the ARBed OFHC copper: (i)conventional dislocation patterns in the large grains, (ii)reactivation of pre-existing shear bands, and (iii)stress/strain driven grain coarsening at sites of strain localization. The order of activation of the mechanisms can be described with a composite approach based on activation energy. The occurrence of grain coarsening is the major contributor to the cyclic softening response observed in OFHC copper. Conversely, the lesser extent of cyclic softening in the other two metals is likely due to the higher microstructure stability of the initial as-ARBed materials. The microstructure stability is believed to be the primary influencing factor for the extent of grain coarsening and cyclic softening. The applied cyclic plastic strain is a secondary influencing factor, although this is generally overshadowed by the limitation of grain coarsening due to the short cyclic lifespan of these metals. The occurrences of shear banding and grain coarsening reported in the present ARBed metals are similarly reported for UF grained metals from other processes, e.g. ECAPed metals. Thus, its relationship to the cyclic deformation response and governing factors are believed to be applicable for UF grained metals in general.

The improvement in mechanical strength offered by ultra fine- (UF) and nanocrystalline (NC) sized grains is very attractive for potential applications of structural metals. Accumulative Roll-Bonding (ARB) is one of the promising new techniques for producing bulk UF grained metals. There are numerous reports on the monotonic mechanical behavior of various ARBed metals, however there are few, if any, on the cyclic deformation behavior of such metals. The primary objective of this study is to investigate the cyclic deformation behaviour and the related micro-mechanisms of ARBed metals from a fundamental perspective. To achieve this, the microstructure and the deformation behavior of commercial purity aluminum, OFHC copper, and DLP copper after ARB processing have been systematically characterized. The as-ARBed microstructure is found to be composite natured, with constituents of different grain sizes. The three constituents are: (i)UF grained matrix, (ii)NC primary discontinuities, and (iii)conventional sized pre-existing coarse grains. Due to this composite nature, three different cyclic strain accommodation mechanisms were found in the ARBed OFHC copper: (i)conventional dislocation patterns in the large grains, (ii)reactivation of pre-existing shear bands, and (iii)stress/strain driven grain coarsening at sites of strain localization. The order of activation of the mechanisms can be described with a composite approach based on activation energy. The occurrence of grain coarsening is the major contributor to the cyclic softening response observed in OFHC copper. Conversely, the lesser extent of cyclic softening in the other two metals is likely due to the higher microstructure stability of the initial as-ARBed materials. The microstructure stability is believed to be the primary influencing factor for the extent of grain coarsening and cyclic softening. The applied cyclic plastic strain is a secondary influencing factor, although this is generally overshadowed by the limitation of grain coarsening due to the short cyclic lifespan of these metals. The occurrences of shear banding and grain coarsening reported in the present ARBed metals are similarly reported for UF grained metals from other processes, e.g. ECAPed metals. Thus, its relationship to the cyclic deformation response and governing factors are believed to be applicable for UF grained metals in general.

This diploma thesis describes properties of ultra-fine grain Cu prepared via ECAP procedure. The influence of fatigue loading with positive mean stress on S-N curve (i.e. fatigue life), cyclic plastic behaviour and grain size was investigated. It was found that tensile mean stress leads to shorter lifetime in comparison with fatigue loading with zero mean stress. During main part of the lifetime, significant hardening of UFG Cu was observed. There is no distinct effect on microstructural orientation and stability.