Characterization of the bistable melting regime for processing of copper with an NIR laser under LPBF conditions

LPBF of pure copper using an infrared laser is considered challenging because only limited parameter ranges are usable to achieve highest densities (>99.5%). The absorption coefficient of copper for the infrared wavelength is often used as a central influencing parameter but does not reflect the characteristic transition behavior. An abrupt transition to keyhole welding when the energy input is increased limits the usable parameter range. In the present work, the transition behavior is characterized on basis of single weld tracks generated under LPBF conditions. In addition to the process parameters laser power, beam diameter and scanning speed, different surface qualities are included in the consideration of the transition behavior. In this context, a process parameter range defined by a melt pool alternating between heat conduction and keyhole welding is observed. This bistable range can be characterized, among other things, with the quotient of laser power and beam diameter.