Using cold spraying (CS), a surface layer with a modified microstructure and enhanced mechanical properties was formed on a 3.2 mm thick friction stir welded (FSW) AA2024-T3 joint. The combined effect of “shot peening effect (SPE)” and “heat flow effect (HFE)” during CS were used to enhance joint mechanical properties. The microstructure evolution of the FSW AA2024-T3 joints in the surface layer following CS coatings and their effect on mechanical properties were systematically characterized with electron back-scattered diffraction, transmission electron microscopy, differential scanning calorimetry and mechanical tests. Based on these experiments, a grain refinement, finer and more S phases, and improved amount of Guinier-Preston-Bagaryatsky (GPB) zones produced by CS treatments are proposed. The deposition of aluminium coating on the joint, lead to hardness recovery in the stir zone and the development of two low hardness zones as the density of GPB increased. The tensile properties of FSW AA2024-T3 joints improved with the application of the aluminium coatings. Experiments and analysis of the enhanced mechanical properties mechanism indicate that SPE with a high plastic deformation and HFE with an intensive heat flow are necessary for the production of refined grains and increased numbers of GPB zones.

An innovative hybrid process which combines the two very effective solid-state techniques of cold spraying (CS) and friction stir processing (FSP), was proposed to fabricate a high-strength ultrafine-grained Cu-Zn coating. Results show that the CS coating had an elongated microstructure with 78.42% of low-angle grain boundaries. Following FSP, there appear ultrafine grains with 90.47% of high-angle grain boundaries and a composition of α, β' and γ phases while the CS coatings was mainly α. Significant mechanical properties enhancement is achieved, i.e. with the ultimate tensile strength increasing from 87.2 MPa to 257.5 MPa and fracture elongation increasing from 0.17% to 0.81%. The precipitates have a significant effect on the fracture behavior of FSP coatings.

A low-cost, low-pressure (less than 1 MPa) cold spray unit was used to deposit tungsten carbide (WC)-based metal matrix composite (MMC) coatings on low carbon steel substrates. The coatings were then friction-stir processed (FSP) by using a flat cylindrical tool. Scanning electron microscopy (SEM), image analysis, micro-hardness testing, and ASTM Standard G65 dry abrasion wear testing were conducted to study the influence of FSP on the coating properties and its wear rate. It was found that porosity increased following FSP on the coating due to insufficient flow of the metal matrix material (nickel). The hardness of the WC-based MMC coating decreased after FSP as a result of increase in porosity and possible decarburization of the WC caused by the heat of the FSP. The SEM images taken from the cross sections of the FSPed coatings confirmed the effectiveness of FSP in distributing the WC particles within the matrix to produce a coating with uniform distribution of WC particles in the matrix. As a result, the abrasion wear resistance of the coatings after FSP increased compared to that of the as-sprayed coatings. This suggested that FSP can be considered as a method to improve the wear properties of MMC coatings.

This paper presents a concept to develop a practical method to join dissimilar materials by the cold spray process currently not possible with conventional technologies. The novel aspect of the process includes the development of a joint design using cold spray as the enabling technology to join dissimilar materials. The emphasis in this study will be on the method used in the joining of cast ZE41A-T5 magnesium and wrought 6061 aluminum by the cold spray process (CSP) in combination with friction-stir welding (FSW) and the subsequent materials characterization. Microstructural analysis by electron and optical microscopy, along with mechanical test results will be presented. Comparisons will be made to conventional joining techniques and the importance, as well as the applications of this technique, will be discussed.