Effect of laser power on microstructure and properties of laser assisted cold sprayed copper coatings on steel

Abstract

Laser assisted cold spraying (LACS) is a promising technology for preparing high performance copper-steel composite materials for grounding electrode applications. Laser power is a critical LACS parameter, however, in literature there is a lack of studies on the influence of laser power on coatings. In this study, the effect of laser power on microstructure and properties of LACS copper coatings on steel is investigated. The results show that with the increase of laser power, the copper coating generally shows reduced porosity and microhardness, as well as increased bonding strength. However, the grains are first refined and then coarsened, while the electrical conductivity and corrosion resistance both increased and then decreased. This is because when the laser power exceeds 4 kW, the grain is coarsened and the amount of large angular grain boundaries (HAGBs) increases because excessive temperature increases the recrystallization driving force, ultimately leading to decrease in coating electrical conductivity and corrosion resistance. 4.0 kW, 5 MPa, 800 °C is considered as optimal LACS parameters for preparation of pure copper coatings on Q235 steel. The coating has a porosity of <0.04 %, grain size of 0.634 μm, microhardness of 99.4HV0.1, bonding strength exceeds 62 MPa, electrical conductivity of 99 % IACS, and corrosion current density of 6.092 × 10−8 A/cm2. In addition, mechanisms of LACS particle deposition and corrosion of copper coatings in simulated soil solution were discussed. With the increase of laser power, more heat absorbed by the powder and the substrate promotes thermal softening and plastic deformation, resulting in stronger mechanical bonding. The corrosion mechanism gradually changes from pitting corrosion to uniform corrosion due to the reduced porosity and tighter grain boundaries, and finally to intergranular corrosion due to grain coarsening and high HAGBs content. Overall, this study provides guidance and reference for the practical application of LACS.