No abstract

Cold spray (CS) is a new solid state coating technique that has attracted considerable attention as it is quite suitable for mass production of metallic materials or metal matrix composites. More recently, cold spray additive manufacturing (CSAM) has been proposed to fabricate/repair components or produce welding joint. However, inherently deficient in ductility/plasticity of CSAMed materials limits its application. Therefore, many hybrid processes have been proposed to further explore CS, which can be grouped together as 'cold spray +' hybrid processes (laser, shot peening and friction stir processing). This review will summarise the state-of-the-art of the 'cold spray +' processes to show their abilities and their effectiveness. Recommendations for further research and development have been incorporated to help widespread the process.

The interface microstructure and shear strength of Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints under thermal-cycle loading were investigated with scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and physical and chemical tests. The results show that an intermetallic compound (IMC) layer of Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints evolved gradually from the scalloped into larger wavy forms with increasing number of thermal cycles. The roughness and average thickness of IMC increased with thermal-cycle loading. However, at longer thermal-cycle loading, the shear strength of the joints was reduced by about 40%. The fracture pathway of solder joints was initiated in the solder seam with ductile fracture mechanism and propagated to the solder seam/IMC layer with ductile-brittle mixed-type fracture mechanism, when the number of thermal cycles increased from 100 to 500 cycles. By adding 0.05 wt.% Ni, the growth of the joint interface IMC could be controlled, and the roughness and average thickness of the interfacial IMC layer reduced. As a result, the shear strength of joints is higher than those without Ni. When compared to joint without Ni, the roughness and average thickness of 0.05 wt.% Ni solder joint interface IMC layer reached the minimum after 500 thermal cycles. The shear strength of that joint was reduced to a minimum of 36.4% of the initial state, to a value of 18.2 MPa.

During cold spray, the temperature of the particle carrier gas is at room temperature, which is lower than the preheated propulsive gas and affects the mixed gas temperature and particle acceleration. This study is investigating the associated effect of the powder carrier gas and powder characteristics on the optimal design of the nozzle using numerical simulations. The results show that the heat exchange of mixed gas stabilizes, and temperatures of mixed gas and particles are higher in the nozzle when a larger inlet diameter and a longer convergent section are used. In addition, particle impact velocity is inversely related to its size and density. Therefore, heavier particles require longer divergent lengths and larger expansion ratios for a longer acceleration distance and increased kinetic energy. The nozzle geometry can be designed depending on the selected particles and the chosen propulsive gas, for a maximum particle impact velocity.

Cold spray additive manufacturing (CSAM) shows great potential in titanium-alloy production as it is a solid-state process. However, data published so far have demonstrated the difficulty of producing dense and high-strength Ti alloy parts. Our previous studies have shown that nozzle design together with high-cost helium propulsive gas plays a crucial role in particle acceleration. In this work, special nozzles for Ti alloy were designed and validated experimentally with commercially available Ti6Al4V powder. Simulation results show that particle impact temperature increases remarkably for a long convergent length, while particle kinetic energy slightly increases, which is validated by experiments. The relationship between the particle impact temperature and practice diameter shows the first increase and then decrease. The experimental results show that as the nozzle convergent section becomes longer, the edges of the single-pass deposits become smoother, and the width, density, deposition efficiency, and microhardness of the single-pass deposits increase.