Analysis of the Process Conditions for the Coating of Grey Cast Iron Brake Discs through Laser Material Deposition

EHLA (Extreme High-speed Laser Material Deposition) is a process variant of LMD (Laser Material Deposition) that has been developed by Fraunhofer ILT and RWTH Aachen University. EHLA is characterized by the fact that the laser beam melts the powder particles above the melt pool, as opposed to conventional LMD where the particles melt only when they immerse into the melt pool. Due to this difference, EHLA processes can be set up with speeds of up to several hundred meters per minute. The changed process setup is achieved using specially designed coaxial powder nozzles and adapted process parameters. As previous work conducted at ILT has shown, EHLA allows to generate thin and uniform coatings on grey cast iron brake discs, which is desirable for different reasons, ranging from increased resistance to wear and corrosion to optical benefits and reduction of particulate emissions of vehicles. EHLA coatings as thin as 100 μm can be applied with high deposition rates (in this work up to 300 cm2/min). Compared to coatings produced by galvanic or thermal spray processes, EHLA layers exhibit a stronger metallurgical bonding to the grey cast iron substrate. In this work, a stainless steel powder material (316 L) is chosen to demonstrate the feasibility of coating grey cast iron brake discs. Process parameters based on previous experimental results are presented and compared for conventional LMD and EHLA. To underline the differences in process setup, a simulation of the two processes is performed and correlated with the experimental findings. The degree of transmission is calculated and compared as well as the temperature distribution in the substrate, focusing on the depth of the heat affected zone.