Advanced lightweight materials, including high-strength steels, aluminum, magnesium, plastics, and reinforced polymer composites, are increasingly used in industry. Combinations of mixed materials are becoming commonplace in the design of structures. Adhesives can be used to join a wide range and combinations of materials. However, joining of materials depends on their specific characteristics. The choice of adherend material is one particular and important parameter that influences adhesively bonded joint performance, and its effect should be taken into consideration in the design of adhesive joints. This article overviews experimental and modeling investigations on the influence of adherend properties on the strength of adhesively bonded joints.

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.

In this paper we give an answer to Furstenberg’s problem on topological disjointness. Namely, we show that a transitive system $(X,T)$ is disjoint from all minimal systems if and only if $(X,T)$ is weakly mixing and there is some countable dense subset $D$ of $X$ such that for any minimal system $(Y,S)$, any point $y\in Y$ and any open neighbourhood $V$ of $y$, and for any non-empty open subset $U\subset X$, there is $x\in D\cap U$ such that $\{n\in \mathbb{Z}_{+}:T^{n}x\in U,S^{n}y\in V\}$ is syndetic. Some characterization for the general case is also given. By way of application we show that if a transitive system $(X,T)$ is disjoint from all minimal systems, then so are $(X^{n},T^{(n)})$ and $(X,T^{n})$ for any $n\in \mathbb{N}$. It turns out that a transitive system $(X,T)$ is disjoint from all minimal systems if and only if the hyperspace system $(K(X),T_{K})$ is disjoint from all minimal systems.

Thermal cycling of planar solid oxide cell (SOC) stacks can lead to failure due to thermal stresses arising from differences in thermal expansion of the stack’s materials. The interfaces between the cell, interconnect, and sealing are particularly critical. Hence, understanding possible failure mechanisms at the interfaces and developing robust sealing concepts are important for stack reliability. In this work, the mechanical performance of interfaces in the sealing region of SOC stacks is studied. Joints comprising Crofer22APU (preoxidized or coated with MnCo2O4 or Al2O3) are sealed using V11 glass. The fracture energy of the joints is measured, and the fractured interfaces are analyzed using microscopy. The results show that choosing the right coating solution would increase the fracture energy of the sealing area by more than 70%. We demonstrate that the test methodology could also be used to test the adhesion of thin coatings on metallic substrates.

Based on wettability and reaction interfaces previously reported, as well as on thermodynamic considerations, a likely mechanism has been proposed for the chemical interaction taking place at the metal/ceramic interface during wettability experiments carried out by the so-called “sessile drop” method. The experiments involved three Ag-Cu-based brazing alloys [Cusil (Ag-28wt.%Cu), Cusil-ABA (Ag-34.6wt.%Cu-1.58wt.%Ti) and Incusil-ABA (Ag-26.6wt.%Cu-12.4wt.%In-0.89wt.%Ti)] and as polished and pre-oxidized pressure-less sintered silicon carbide (PLS-SiC), with a total holding time of 90 minutes at 850 °C, under a Zr sponge-gettered vacuum of 10-4/10-5 Torr.

An old procedure used to carry out a graphical derivation of curves, which is based on the optical properties of plane mirrors, has been adapted for the measurement of the contact angle (θ) formed between a liquid drop and a flat solid substrate in wettability experiments carried out by the so-called “sessile drop” method. The method was tested for mercury on soda-lime glass at room temperature in air as well as for Cusil (Ag-28wt.%Cu) and Incusil-ABA (Ag-27wt.%Cu-12wt.%In-2wt.%Ti) brazing alloys on pressureless-sintered silicon carbide (PLS-SiC) at 850 °C, under a vacuum of 10-4/10-5 Torr. The proposed method is fast, simple and accurate enough from high (∼140°) to relatively low (∼10°) contact angles. Although the proposed method has been tested for metal-ceramic systems, it is of general application, so that it would be useful for any liquid-solid system. The method is applicable for any temperature, pressure and atmospheric experimental conditions employed, as well as for any chemical composition of liquid and solid. It is also useful for both low and high contact angles, as well as for reactive and non-reactive systems, as long as a photograph of a liquid drop resting on a flat solid surface is available for the studied system.

This article showcases details on enumerative information of dissimilar aluminum (Al) to steel welds manufactured using different friction-based welding processes with an emphasis on the description of the manufacturing process, influence of parameters, microstructural variations, formation of intermetallic compounds (IMCs), and variations in mechanical properties. Friction-based welding processes such as friction welding, friction stir welding, hybrid friction stir welding, friction stir spot welding, friction stir spot fusion welding, friction stir scribe welding, friction stir brazing, friction melt bonding, friction stir dovetailing, friction bit joining, friction stir extrusion, and friction stir assisted diffusion welding are analyzed for the formation of dissimilar Al–steel joints. It can be summarized that friction-based joining processes have great potential to obtain sound Al–steel joints. The amount of frictional heat applied decides the type and volume fraction of IMCs that subsequently affects mechanical joint properties. Process variations and novel process parameters can enhance joint properties.

Functionally graded materials (FGMs) in which the elemental composition intentionally varies with position can be fabricated using directed energy deposition additive manufacturing (AM). This work examines an FGM that is linearly graded from V to Invar 36 (64 wt% Fe, 36 wt% Ni). This FGM cracked during fabrication, indicating the formation of detrimental phases. The microstructure, composition, phases, and microhardness of the gradient zone were analyzed experimentally. The phase composition as a function of chemistry was predicted through thermodynamic calculations. It was determined that a significant amount of the intermetallic σ-FeV phase formed within the gradient zone. When the σ phase constituted the majority phase, catastrophic cracking occurred. The approach presented illustrates the suitability of using equilibrium thermodynamic calculations for the prediction of phase formation in FGMs made by AM despite the nonequilibrium conditions in AM, providing a route for the computationally informed design of FGMs.

The present study addressed the weldability of Hastelloy C-276 and Type 321 austenitic stainless steel (ASS) dissimilar combination used for manufacturing of high-temperature equipments in nuclear power plants. Investigation of the microstructural evolutions across the different welding passes and their subsequent effect on the mechanical properties and corrosion resistance would be helpful in better understanding and pave way for the frequent application of such dissimilar joints in the industrial applications. The problem of segregation associated with the multi-pass gas tungsten arc welding process was also investigated systematically. The fusion zone microstructures exhibited a transition from columnar to an equiaxed dendritic structure with varying passes. The topologically closed packed (TCP) phases (such as P and μ) were observed in the fusion zone as well as at the weld interface of Hastelloy C-276. Polarization test was performed to evaluate the corrosion resistance and results indicated that the Cr and Mo depleted zones formed around the TCP phases might be responsible for decreased Epit value for fusion zone. The novelty of this work is to explore the possibilities of substitution of an expensive Hastelloy C-276 with a cost-effective Ti-stabilized Type 321 ASS.

Persistent current joints are a critical component of commercial superconducting magnets.The standard jointing method widely used in the magnet industry for technological low temperature superconducting wires such as NbTi and Nb3Sn wires uses a superconducting solder (e.g. PbBi). In these joints the physical and superconducting properties of the solder materials inevitably play an important role in the overall performance of the joint. Key requirements for superconducting solders include low melting point to prevent degradation of the superconducting filaments during joining, good wettability of the superconducting filaments, suitable liquid phase viscosity, and finally adequate superconducting properties to enable sufficient supercurrent to pass through the joint under typical operating conditions (typically at 4.2K in a field of 1 T for an MRI magnet). PbBi solder satisfies all these criteria, but restrictions on the use of lead in the magnet industry are expected in the relatively near future, so new lead-free jointing techniques need to be developed.

One approach is the development of superconducting lead-free solder materials. In our work, we have focussed on the In-Sn system and ternary systems involving In and Sn as two of the elements. Thermodynamic modelling has been used to produce ternary phase diagrams of potential alloy systems, and various formulations have been fabricated in order to explore how microstructure and phase chemistry influence the superconducting properties of the solders. Alternative approaches to fabricating lead-free joints, including spot welding and cold-pressing, have also been investigated. These methods have the potential advantage of achieving direct NbTi-NbTi joints with no intermediate, lower performance superconducting material. The spot welding method produced joints with the best superconducting performance, signifiantly better than the currently used PbBi solder, but the lack of reproducibility in this technique may be a problem from an industrial point of view.

Additive manufacturing (also known as 3D printing) is considered a disruptive technology for producing components with topologically optimized complex geometries as well as functionalities that are not achievable by traditional methods. The realization of the full potential of 3D printing is stifled by a lack of computational design tools, generic material feedstocks, techniques for monitoring thermomechanical processes under in situ conditions, and especially methods for minimizing anisotropic static and dynamic properties brought about by microstructural heterogeneity. This article discusses the role of interdisciplinary research involving robotics and automation, process control, multiscale characterization of microstructure and properties, and high-performance computational tools to address each of these challenges. Emerging pathways to scale up additive manufacturing of structural materials to large sizes (>1 m) and higher productivities (5–20 kg/h) while maintaining mechanical performance and geometrical flexibility are also discussed.

The field of electronics packaging is undergoing a significant transition to accommodate the slowing down of lithographically driven semiconductor scaling. Three-dimensional (3D) integration is an important component of this transition and promises to revolutionize the way chips are assembled and interconnected in a subsystem. In this article, we develop the key attributes of 3D integration, the enablers and the challenges that need to be overcome before widespread acceptance by industry. While we are already seeing the proliferation of applications in the memory subsystem, the best is yet to come with the heterogeneous integration of a diverse set of technologies, the mixing of lithographic nodes and an economic argument for its implementation based on overall system function, and cost rather than a narrow component-based analysis. Finally, an extension to monolithic 3D integration promises even further benefits.

Gas tungsten arc welding-tungsten inert gas (GTAW-TIG) is focused in literature as an alternative choice for joining high strength low alloy steels; this study is performed to compare the differences between gas metal arc welding-metal inert gas (GMAW-MIG) and GTAW welding processes. The aim of this study is to characterize microstructure of dissimilar transformation induced plasticity steels (TRIP) and martensitic welded joints by GMAW and GTAW welding processes. It was found that GMAW process lead to relatively high hardness in the HAZ of TRIP steel, indicating that the resultant microstructure was martensite. In the fusion zone (FZ), a mixture of phases consisting of bainite, ferrite and small areas of martensite were present. Similar phase’s mixtures were found in FZ of GTAW process. The presence of these mixtures of phases did not result in mechanical degradation when the GTAW samples were tested in lap shear tensile testing as the fracture occurred in the heat affected zone. In order to achieve light weight these result are benefits which is applied an autogenous process, where it was shown that without additional weight the out coming welding resulted in a high quality bead with homogeneous mechanical properties and a ductile morphology on the fracture surface. Scanning electron microscopy (SEM) was employed to obtain information about the specimens that provided evidence of ductile morphology.

Al6061 and AZ31 plates were processed using accumulative roll bonding (ARB) method up to two passes to produce laminated composites. The sandwich stacks of Al6061/AZ31/Al6061 were held at 450 °C for 10 min in a cubical furnace and rolled together with reduction of 50% in one pass. The microstructural investigations were done using optical and scanning electron microscopes. The structures of the interface, mechanical and drop impact properties of the laminated composites after the first and second passes were investigated and compared with Al6061 and AZ31 alloy plates. It was found that Al6061 improved the elongation to failure property of AZ31 after the first pass of ARB process and the drop impact properties of AZ31 after the first and second passes. However, elongation to failure magnitude with the uniaxial tensile loading decreased with increase in the number of passes due to the formation of brittle intermetallic between the Al6061/AZ31 nonuniform interfaces.

The effect of the heat input on the mechanical and metallurgical properties of the welds has been investigated in the heat affected zone (HAZ) of welds joined with gas metal arc welding (GMAW), using normal production welding parameters. The thermal effect in the HAZ of the welds is important for the optimization of the welding parameters used when weld transformed induced plasticity (TRIP) steels, because this will have a great influence in the mechanical and metallurgical properties of the weld. In this work 3 samples was welded a high, average and low heat input, with the variation of welding parameters to obtain different thermal affectation to investigate the variations in different parts of weld joint: weld, HAZ and base metal, due the heat applied for the welding process used. Mechanical properties were evaluated by tension test, microhardness and fatigue testing and metallurgical evaluation with optical metallograpy, scanning electron microscopy (SEM), fractograpy and X-Ray diffraction (XRD).The results obtained shows that the mechanical properties of the tension test decrease when the heat input increase and the microhardness exhibit a softening zone in the HAZ with lower hardness and the fatigue life were similar for all heat inputs for the high stress levels, but only in low stress there is a difference. For metallurgical properties the metallographic evaluation shows ferrite, bainite - martensite and retained austenite, and the fractography analysis exhibit a ductile fracture in all cases and the content in volume fraction of retained austenite increases in the HAZ of welds when increasing heat input in to the base metal due the thermal effect.

One of the critical issues for development of the nuclear fusion demonstration reactor (DEMO) is the high heat flux on heat-resistant equipments, especially the blanket and divertor. Materials of such equipments require relatively high thermal conductivities. In this study, we developed iron-based composite materials with carbon nanotube (CNT) and copper, which have high thermal diffusivities, by means of Hot Pressing (HP) and Spark Plasma Sintering (SPS).

The thermal diffusivity in the iron/CNT composites was not high enough compared with that of pure iron, while iron/copper composite showed a relatively high thermal diffusivity in the joining conditions. One of the reasons not to be improved thermal diffusivity could be non-mono-dispersion of CNT by the formation of carbides in the matrix.

Al–7Si/gray iron bimetal composites with a sound metallurgical bonding were obtained by a gravity die casting process. The surface treatments of the gray iron specimen including fluxing and hot dipping were applied to forming a complete metallurgical bonding layer at the Al–7Si/gray iron interface. In addition, the effect of Mn in dipping bath on the microstructure of the Al–7Si/gray iron interfacial bond zone has been studied in an Al–7Si alloy containing five different levels of Mn ranging from 0 to 5 wt%. Microstructure analysis indicates that addition of Mn in dipping bath can eliminate the harmful needle-like phase (β-Al5FeSi) as the Mn content is no less than 1.5 wt% and also plays an important role in facilitating the growth of intermetallic phases [α-Al15(FexMn1−x)3Si2] and the metallurgical bonding layer. The sound metallurgical bonding formed at the Al–7Si/gray iron interface is attributed to combining the effect of surface treatments and selection of Mn content.

Capillary type underfill is still the mainstream underfill for mass production flip chip applications. Flip chip packages are migrating to ultra low-k, Pb-free, 3D and fine pitch packages. Underfill selection is becoming more critical. This paper discusses the performance and potential of underfills using a novel organic-inorganic hybrid polymer technology.

Compared to eutectic and high lead solder, tin-silver-copper solder has lower C.T.E., higher elasticity and greater brittleness. In light of these properties, it is generally better to select high Tg and lower CTE underfill in order to prevent bump fatigue during reliability testing. Given the brittleness of low-k dielectric layers of flip chips, the destruction of low-k layers by stress inside the flip chip packages has become a major issue. Underfills for low-k packages should have low stress, and the warpage should be small. It is expected that as the low-k trend expands, the underfill is required to provide less stress. Low Tg underfill shows lower warpage. New chemical technologies have been developed to address the needs of underfills for low-k/Pb-free flip chip packages, specifically organic-inorganic hybrid polymer compounds. The organic-inorganic hybrid polymer provides excellent cure properties which enable a balanced combination of low stress and good bump protection. The material properties of the underfill were characterized using Differential Scanning Calorimetry (DSC), Thermo-Mechanical Analysis (TMA), and Dynamic Mechanical Analysis (DMA). A daisy-chained test vehicle was used for reliability testing. A detailed study is presented on the underfill properties, reliability data, as well as finite element modeling results.

In order to investigate the formation of precipitates such as MC carbides and intermetallic compounds in the friction stir welded and post-heat-treated Inconel 718 alloy, this work was carried out. Furthermore, the microstructural and mechanical properties of welds and post-heat-treated material were evaluated to identify the effect on precipitates formed during post-heat-treatment. Friction stir welding (FSW) was performed at a rotation speed of 200 rpm and welding speed of 150 mm/min; heat treatment was performed after welding at 720 °C for 8 hours in vacuum. As a result, the grain size due to FSW was notably refined from 5–20 μm in the base material to 1–3 μm in the stir zone; this was accompanied by dynamic recrystallization, which resulted in enhancements in the mechanical properties as compared to the base material. In particular, applying heat treatment after FSW led to improvements in the mechanical properties of the welds—the microhardness and tensile strength increased by more than 50% and 40% in fraction, respectively, as compared to FSW alone.