Towards high-strength cold spray additive manufactured metals: Methods, mechanisms, and properties

Cold spray, as a solid-state additive manufacturing process, has been attracting increasing attention from both scientific and industrial communities. However, cold-sprayed deposits generally have unfavorable mechanical properties in their as-fabricated state compared to conventionally manufactured and fusion-based additive-manufactured counterparts due to the inherent microstructural defects in the deposits (e.g., porosity and incomplete interparticle bonding). This downside reduces its competitiveness and limits its wide applications as an additive manufacturing process. In the past years, many strengthening technologies have been developed or introduced to improve the microstructure and mechanical properties of cold-sprayed deposits. The term "strengthening" in this work specifically refers to improving the mechanical strength, particularly the tensile strength of the cold-sprayed bulk deposits. According to the stage that the strengthening technologies are used in the cold spray process, they can be classified into three categories: pre-process (i.e., powder heat treatment), in-process (i.e., powder preheating, in-situ micro-forging, laser-assisted cold spray), and post-process (i.e., post heat treatment, hot isostatic pressing, hot rolling, friction stir processing). Therefore, a comprehensive review of these strengthening technologies is conducted to illuminate the possible correlations between the strengthening mechanisms and the resultant deposit microstructures and mechanical properties. This review paper aims to help researchers and engineers well understand the different strengthening methods and provide guidance for the cold spray community to develop new strengthening strategies for future high-quality mass production.