Investigations on ultra-high-speed laser material deposition as alternative for hard chrome plating and thermal spraying
The development of new coating technologies for the wear and corrosion protection of large, high-quality components in the manufacturing industry is becoming more significant, not only from an economic but also from an ecological perspective. In various branches such as aerospace, oil and gas, automotive, papermaking, and others, hard chrome plating (HCP) is a widespread, standard process. However, the implementations of EU directives such as 1999/13/EC (VOC), 2011/65/EU (RoHS), and 2012/19/EU (WEEE) as well as the EU regulation EC 1907/2006 (REACH) to environmental protection, to environmental protection, CO2-reduction, and energy efficiency are leading to considerable market upheavals in the future. In HCP, toxic and carcinogenic hexavalent chromium (Cr6+) is used, which can only be used with authorization after the so-called sunset date in September 2017. Similar applies to the use of nickel in electroplating have been classified as dangerous for the environment and toxic by the World Health Organization. Coating technologies which are considered capable of replacing HCP are thermal spray technologies, especially high-velocity oxygen-fuel (HVOF) thermal spraying and laser material deposition (LMD). With HVOF thermal spraying, coatings out of a large range of materials can be applied, which feature high wear resistance at relatively low investment costs. Nevertheless, thermal spray coatings are technologically constrained in regard to limited adhesion strength due to the poor mechanical bonding between coating and substrate. Moreover, thermal spray coatings are typically difficult to repair and often exhibit porosity levels in the range of 1%-2%. In the production of wear and corrosion protection layers, LMD is only established in individual applications. With LMD, high-quality pore-and crack-free coatings out of a large range of materials can be produced with metallurgical bonding and low dilution. However, typical coating thicknesses (> 500 mu m) are commonly too large for the wear and corrosion protection and attainable surface rates in the range of 10-50 cm(2)/min far too small for the coating of large components. In this paper, a novel approach for LMD of Inconel 625 with surface rates up to 500 cm(2)/min and deposition speeds up to 200 m/min is presented. The influence of the main processing parameters on layer thickness and appearance of bonding zone is investigated. With this new process variant much thinner, pore-and crack-free wear and corrosion protection layers with a thickness in the range of approximately 10-250 mu m can be produced.