ADVANCED JOINING TECHNIQUES FOR MULTI-MATERIAL STRUCTURES: A REVIEW ON ULTRASONIC CONSOLIDATION, COLD SPRAY, AND ELECTRON BEAM MELTING

Recently, high-performance lightweight materials with outstanding mechanical properties have opened up their way to some sophisticated industrial applications. As one of these systems, aluminum matrix composites/nanocomposites (AMCs) offer an outstanding combination of relative density, hardness, wear resistance, and mechanical strength. Until now, several additive manufacturing methods have been developed for fabrication of 3D metallic components among them, selective laser melting (SLM), electron beam melting (EBM), laser metal deposition (LMD), Wire+Arc additive manufacturing (WAAM), and ultrasonic additive manufacturing (UAM) are of prime significance. Unlike other methods, in ultrasonic additive manufacturing, the ultrasonic waves are used instead of applying the sintering process. This technique is well-known for its ability to produce 3D components by repeating the alternative welding and machining procedures at low temperatures. This is why it can overcome the technological issues arisen from the high-temperature sintering. The present review strives to provide an inclusive introduction to the principles of ultrasonic additive manufacturing method and recent advances in ultrasonic additive manufacturing of aluminum matrix composites/nanocomposites. Also, the challenges of this new emerging technique, i.e. its dependence to the applied weld power, is addressed in the paper. The authors attempt to give some perspectives to the researchers for further investigations in this new-emerging field.

Residual stress induced by cold spray coating of magnesium AZ31B extrusion

In cold spray, high gas temperatures and high velocity impact of particles introduce complex thermal and stress histories in the coating layers and the substrate. It is crucial to understand the interplay between temperature evolution and residual stress development as the residual stresses could potentially cause distortion and/or delamination. In this study, a system-level cold-spray deposition model is developed. The model includes a moving heat source, material addition and thermo-mechanical stress analyses. The heat source model is first validated by qualitatively comparing the thermal history with experimental measurements from literature. The system-level model is then used to calculate the residual stresses and distortion in the cold-sprayed Cu/steel system. The influence of the spray path on the residual stress development is systematically analyzed. The corresponding interface residual stress states are also examined for the different spray paths. Simulation results indicate that the thermo-mechanical analysis using the developed system-level model can provide an effective way to assist in optimizing the cold-spray coating deposition strategy and improve the coating performance.

The in-plane residual stress in thin films greatly affects their properties and functionality as well as the substrate bending, and hence is an important factor to be controlled. In order to obtain general knowledge on the development of residual stress in sol-gel-derived oxide thin films, the in-plane residual stress was measured for yttria stabilized zirconia gel films on Si(100) wafers as a function of firing temperature by measuring the substrate curvature. The films showed a rather complex variation in residual stress, and the mechanism of the residual stress evolution was discussed, referencing the intrinsic stress and the x-ray diffraction data. At low annealing temperatures of 100–200 °C, the residual tensile stress decreased and became compressive partially due to the structural relaxation occurring during cooling. When the firing temperature was increased over 200 °C, the residual stress turned tensile, and increased with increasing annealing temperature, which was attributed to the increase in intrinsic stress due to film densification as well as to the reduced structural relaxation due to the progress of densification. The residual tensile stress slightly decreased at firing temperatures of 500–600 °C, which was attributed to the reduction in intrinsic stress due to thermally activated atomic diffusion as well as to emergence of thermal stress. At firing temperature over 600 °C, the residual tensile stress increased again, which was attributed to the increase in thermal stress generated during cooling due to the increased Young’s modulus of the film. Although appearing to be complicated, the whole variation of residual stress with firing temperature could be understood in terms of film densification, structural relaxation, atomic diffusion, progress of crystallization and thermal strain. The illustration presented in the work may provide a clear insight on how the residual stress could be developed in a variety of functional sol-gel-derived, crystalline oxide thin films.

Cold Spray Additive Manufacturing (CSAM) is a solid-state metal deposition process that enables the generation of dense, high-performance components via the supersonic impact of fine metal particles onto a substrate. Unlike conventional additive manufacturing processes that rely on melting, CSAM does not entail high-temperature phase changes, thereby preserving the microstructure of the feedstock and minimizing oxidation, grain growth, and thermal distortion. Bonding in CSAM is triggered by severe plastic deformation at the particle–substrate interface via adiabatic shear instability and localized heating without melting. Successful bonding is governed by a critical velocity threshold below which particles rebound and above which solid-state adhesion is attained through mechanical interlocking and metallurgical bonding. Material response to high strain rate impact is significant to optimize the process. Constitutive models such as Johnson–Cook, Zerilli–Armstrong, and the Mechanical Threshold Stress (MTS) model are frequently used to simulate the complex thermo-mechanical behavior during deposition. Residual stresses, which are mainly compressive in nature owing to kinetic impact, are intrinsic to coating performance, as they enhance fatigue resistance and crack propagation is impeded. Excessive or inadequately controlled residual stress, nevertheless, can lead to structural instability. Post-deposition heat treatment is also essential to increase bonding, reduce porosity, and stabilize microstructures by transforming metastable phases. Strength and hardness are also increased by dynamic recrystallization and grain refinement at particle interfaces. The article presents a general review of the interaction among bonding mechanisms, material modeling, residual stress development, and post-spray microstructural evolution with insights for CSAM process design and optimization for emerging structural applications.

In this study, the formation of fast, hard, well-adhered and scratch-resistant in-situ composite coatings on electron beam melted (EBM) Ti-6Al-4V alloy were proposed by successive applications of cold spray (CS) and induction heat treatment techniques. The total treatment time to achieve such properties was around only 5 min. In this context, pure aluminum (Al) was first coated by CS technique on EBM Ti-6Al-4V alloy, and the coated samples were heat treated by induction heating method for different times (10 and 20 s). The coating layer that bonded well to the substrate material was obtained using the CS technique. After the induction heating, TiAl3 intermetallics were formed in a sawtooth-like morphology as an interfacial layer at the substrate-coating interface. Similar to the interface, fiber and plate-like TiAl3 compounds were found within the CS’ed coating layer. As the induction heating time increased to 20 s, the thickness of the interfacial layer and the fraction of intermetallics in the coating layer correspondingly increased. The highest hardness and scratch resistance were obtained in the 20 s induction-heated sample. Finally, this study demonstrates that the cold spray followed by induction heating is a rapid technique to obtain hard, well-adhered and scratch-resistant composite coatings in-situ on EBM Ti-6Al-4V alloy.

Eight-millimeter-thick high-strength AA2219 aluminum alloy plates were successfully welded by variable polarity tungsten inert gas welding, and the effect of cold spraying (CS) process on the residual stress, microstructure and mechanical properties of the welded joints were investigated. The CS experiment was performed under different driving gas temperatures and pressures and with different powder types. The microstructure of the as-welded joints and as-coated joints was analyzed by optical microscopy and scanning electron microscopy, while the residual stress was measured by the blind-hole-drilling method. The results show that the CS process could significantly modify and re-distribute the residual stress of the welded joints. The maximum residual tensile stress of the welded joint has been significantly reduced by 111.9% and distributed more uniformly compared to the as-welded state. With the increase in gas temperature and/or pressure in CS, the residual tensile stress of joints was reduced, and even some compressive stress appeared. Among the powders of Cu, Al, and Ni, Cu particles have the best-improving effect. In addition, double-sided spraying treatment can reduce the residual tensile stress of both sides more effectively. Besides, the grain size of the joint surface after CS was refined.

In this paper, we report the effect of multi-layer cold spray deposition on the residual stress formation in the coating and substrate. A method is proposed to separately measure the thermal and mechanical residual stresses induced in cold spray coating. Fiber Bragg Grating (FBG) sensors were employed for in situ monitoring of the strain evolution during the cold spray of multi-layer coating Al7075-Zn on AZ31B Magnesium substrates. Utilizing the capability of the FBG sensors in recording both thermal and mechanical strain gradients, first the effect of temperature on the substrate was investigated when the sample was only treated under carrier gas temperature. Then, the sensors were employed to evaluate the mechanical strain behavior of substrate during the coating process and cooling. Therefore, the effect of thermal mismatch on inducing mechanical strains was observable during the process. Finally, the interaction between the peening process of cold spray and thermal mismatch after cooling was studied. It is shown that the thermal expansion coefficient (CTE) plays a critical role in residual stress development in the substrate and consequently affects the mechanical properties of the coated sample. Hence, careful selection of layers in multilayer deposition can provide desired residual stress in the coating and substrate.

Residual stress development in cold sprayed Al, Cu and Ti coatings

A contact model that accounts for interfacial cohesion and thermal conduction is developed to investigate the influence of bonding on the final residual stresses build-up in cold spray. The residual stress evolution in the cold-sprayed Al-6061 coating on an Al-6061 substrate is investigated via three-dimensional single-particle and multi-particle impact simulations. It is shown that the interface bonding mainly affects the local residual stress distribution near the interfaces. The residual stresses are largely due to the kinetic peening and bonding effects. The thermal cooling has negligible influence. In general, this work finds that peening introduces compressive stress, while bonding causes relaxation. The balance between the peening and the bonding effects, which depends strongly on the local bonding environment, determines the final residual stress in the system. This work suggests that the interface bonding should be considered as one of the essential factors in numerical modeling of the residual stresses evolution in cold spray.

Cold gas dynamic spraying (CGDS) is a relatively new branch of surface engineering that involves modification of the surface of substrates to provide specific engineering advantages, which the substrate alone cannot provide. Cold spraying, as a metal deposition technique, involves spraying of typically 10-40 μm particles which are accelerated by a propellant gas to 300-1200 m/s at a temperature well below the melting point of material, and upon impact deform and adhere to the substrate. The deposition process in cold spraying occurs in a solid state which results in reduced oxidation and absence of phase changes; whereas, in thermal spraying deposition occurs of molten or semi molten particles. Over the last decade the interest in cold spraying has increased substantially. Considerable effort has been invested in process developments and optimization of coatings like copper. However, bonding in cold spraying is still a matter of some debate. The most prevalent theory is that when a particle travels at a minimum required velocity the particle deforms at a very high strain rate upon impact and during this deformation thermal softening dominates over work hardening in impact zone and a material jet is produced. This material jet removes oxides from the surface of the materials and the metal-to-metal contact is established between the freshly exposed surfaces. However, precisely how this high strain rate deformation behaviour of material promotes bonding is still unclear and requires further investigations. This article provides a comprehensive review of the current theories of bonding in cold spraying based on numerical modelling of impact and experimental work. The numerical modelling of the impact section reviews adiabatic shear instability phenomena, critical velocity, critical particle diameter, window of deposition of particles, particle impact on various substrates and the role of adhesion and rebound energy. The review of the experimental section describes the shear lip formation, crater formation on the substrates, role of surface oxides, characterization of bond formation, role of substrate preparations, coating build up mechanisms and contributions of mechanical and metallurgical components in bonding. Cold spraying of copper and aluminium has been widely explored in the last decade, now it is of growing interest to the scientific and engineering communities to explore the potential of titanium and its alloys. Titanium and its alloys are widely utilized in many demanding environments such as aerospace, petrochemical, biomedical etc. Titanium components are very expensive to manufacture because of the costly extraction process of titanium and their difficult to machine properties. Therefore, additive manufacturing from powder and repair of titanium components are of great interest to the aerospace industry using technologies such as cold gas spraying. Titanium coating as a barrier layer has a great potential for corrosion resistant applications. Cold spraying has a great potential to produce oxygen-sensitive materials, such as titanium, without significant chemical degradation of the powder. In-flight oxidation of materials can be avoided to a great extent in cold spraying unlike thermal spraying. This review article provides a critical overview of deposition efficiency of titanium powder particles, critical velocity, bond strength, porosity, microhardness, microstructural features including microstrain and residual stress, mechanical properties reported by various research groups. A summary of the competitor warm sprayed titanium coating is also presented in this article.

On the evolution of substrate's residual stress during cold spray process: A parametric study

A contact model that accounts for interfacial cohesion and thermal conduction is developed to investigate the influence of bonding on the final residual stresses build-up in cold spray. The residual stress evolution in the cold sprayed Al-6061 coating on an Al-6061 substrate is investigated via three-dimensional (3D) single-particle and multi-particle impact simulations. It is shown that the interface bonding mainly affects the local residual stress distribution near the interfaces. The residual stresses are largely due to the kinetic peening and bonding effects. The thermal cooling has negligible influence. In general, the peening effect introduces a compressive stress while the bonding effect results in a relaxation to this compressive stress. This work suggests that the interface bonding should be considered as one of the essential factors in numerical modeling of the residual stresses evolution in cold spray.

Influence of spray angle in cold spray deposition of Ti-6Al-4V coatings on Al6061-T6 substrates

The evolution of microstructure and residual stress during the tempering of 700 L low-carbon micro-alloyed steel was studied using a crack compliance method for measuring residual stress. Additionally, a non-isothermal tempering dilatation test, Vickers micro-hardness test, and transmission electron microscopy were used. The evolution of residual stress during tempering consists of two stages. The first stage coincided with cementite precipitation. Under the initial residual stress, the transformation plasticity due to cementite precipitation leads to partial relaxation of the micro-stress evoked by the austenite-to-ferrite transformation during quenching. It also caused the material surface and the core to exhibit different residual stress evolution trends. After tempering at 300 ∘ C for 30 min, the residual stress was reduced from 487 MPa to 200 MPa; however, the elastic strain energy remained unchanged. The second stage coincided with alloy carbide precipitation and Mn partitioning, but the precipitation of the alloy carbide only reduced the elastic strain energy by 8.7%. Thus, the change in activation energy was the main reason for the relaxation of residual stress at this stage. After tempering at 600 ∘ C for 30 min, the residual stress was reduced to 174 MPa, the elastic strain energy was reduced by 72.72%, and the residual stress was controlled.