The influence of the processing parameters on selective laser melting of copper using a green laser source
Selective laser melting (SLM) of pure copper using conventional Nd:YAG lasers presents a formidable challenge due to the high optical reflectivity in the infrared range (98%) and a very high thermal conductivity (400 W/mK). These two properties combined leads to power requirements on the order of 1 kW in order to avoid balling defects. These type of defects occur when the melt does not penetrate deep enough to allow the underlying substrate to be wetted. One way to decrease the threshold for this type of defects to form is to increase the absorptivity of the powder bed. This can be done by using a laser with a wavelength which is more strongly absorbed by copper. Green light has a reflectivity from copper of around 60%, and laser systems with a beam quality and power suitable for SLM have recently become commercially available at industrially relevant prices.
In this work the transition from balling to the conduction welding regime of selective laser melting of single layers of pure copper with a green laser, infrared laser, and combinations thereof is investigated. The mechanisms for the defect formation is explained using thermal and fluid dynamics simulations and verified using high speed imaging of the melting process. The influence of the commonly used process parameters (power, scanning speed, hatch distance and layer thickness) on the transition is investigated and explained, with a focus on how copper differs from other materials commonly used for selective laser melting.