Lightweighting of vehicles and portable structures is an important undertaking. Multimaterial design is required to achieve conflicting design targets such as cost, stiffness, and weight. Friction stir welding (FSW) variants, such as friction stir dovetailing and friction stir scribe, are enabling technologies for joining of dissimilar metals. This article discusses how FSW variants are capable of joining aluminum to steel in particular. The characteristics of metallurgical bonding at the dissimilar materials interface are strongly affected by weld temperature. Control of FSW process temperature enables metallurgical bonding with suppressed formation of intermetallics at the dissimilar materials interface, resulting in improved mechanical properties relative to competing techniques. Temperature control is thus a powerful tool for process development and ensuring weld quality of dissimilar materials welds.

The transportation sector is the largest contributor to greenhouse gas emissions in the United States. One method being used to reduce greenhouse emissions related to the transportation sector is improving vehicle fuel efficiency through mass reduction. Reducing the mass of on-highway passenger vehicles by 10% can result in vehicle fuel economy improvements of as much as 6–8% if the powertrain is downsized to maintain equivalent performance. Some of the materials being investigated and implemented to reduce passenger vehicle mass include advanced high-strength steel, aluminum, magnesium, and polymer composites. Additionally, multimaterial structures that allow for optimal combinations of lightweight materials to achieve maximum weight reduction with lowest cost and best structural performance have recently become of particular interest. However, assembling multimaterial structures can be challenging due to differences in melting temperature and coefficient of thermal expansion of different materials, as well as formation of intermetallic compounds and galvanic corrosion potential. Joining technologies for lightweight multimaterial structures must address these challenges to be successful. This article highlights advances made in five different joining techniques: nondestructive evaluation of resistance spot-welded aluminum to steel, modeling of structural adhesives, temperature control of friction stir welds, ultrasonic welding of magnesium, and vapor foil actuation welding.

Conventional silicon-based electronics have faced challenges in the realization of soft bioelectronics, such as wearable and implantable integrated devices, which necessitate electrically and mechanically interactive biotic–abiotic interfacing without disturbing the daily life of the user or posing biocompatibility issues. Recently, much effort has been directed at overcoming the mechanical limitations of conventional rigid electronics by replacement of bulky, thick, and rigid electronic materials with biocompatible, soft, and nanoscale electronic materials, which exhibit intrinsic mechanical deformability as well as superior electrical properties. Recent advances in the synthesis of unconventional nanomaterials, surface functionalization methods, and integrated device fabrication techniques have resulted in further improvements in the performance of nanomaterials-based soft bioelectronics. Numerous studies have focused on the biological, electrical, and mechanical analyses of heterogeneous nanomaterial–biosystem interfaces as well as the development of efficient integration processes of soft nanomaterials into devices. In this article, we summarize the latest advances and future prospects in nanomaterials synthesis, processing, and integration strategies for flexible and stretchable bioelectronics, and their application to wearable and implantable devices.

Enthalpies of formation and heat capacities are key parameters for thermodynamic assessments using the CALPHAD method and were determined in this work in the ZrO2–Y0.5Ta0.5O2 quasibinary system. ZrxY0.5−x/2Ta0.5−x/2O2 samples with compositions in the monoclinic and tetragonal ZrO2-based solid solutions (0.65 ≤ x ≤ 1) as well as the two monoclinic (M′ and M) polymorphs of the Y0.5Ta0.5O2-based solid solutions (0 ≤ x ≤ 0.2) were investigated. The enthalpies of the two monoclinic polymorphs M′ and M are essentially the same within experimental uncertainty and have significant energetic stability (enthalpies of formation near −45 kJ/mol) relative to their component oxides. The monoclinic and tetragonal zirconia-rich solid solutions have little energetic stability with respect to ZrO2, Ta2O5, and Y2O3 and presumably owe their existence to the configurational entropy. The heat capacities of M′ and M phase are similar and can be estimated from their constituent oxides using the Neumann–Kopp rule.

Ultra-rapid microwave sintering of ceramics has been recently demonstrated by the authors. In the experiments with oxide ceramic samples carried out in a 24 GHz gyrotron system for microwave processing of materials, full density was achieved in the sintering processes with a duration of the high-temperature stage of one to several minutes and zero hold at the maximum temperature. The implementation of the ultra-rapid microwave sintering processes was made possible due to fast and efficient control over the temperature of the materials and the supplied microwave power. The absorbed microwave power density was typically in the range of 10–100 W/cm3, which is within the same order of magnitude as the power of Joule heat in the DC electric field–assisted flash sintering processes. At this power level, a thermal instability is triggered by the volumetric heating, which results in a drastic enhancement of mass transport. In addition, possibility of ultra-rapid microwave sintering of powder metals has been demonstrated within a model accounting for the effective electromagnetic properties and resonant absorption effects.