On-line monitoring of Laser Directed Energy Deposition with hot wire
Laser Directed Energy Deposition with metal wire (LDEDmw) is one of rapidly growing braches of Additive Manufacturing technologies. In this process a 3D shape is created by depositing an additive material in form of wire. The material is melted using a high-power density laser beam as the energy source. The LDEDmw process is commonly used for production of large scale products, e.g. structural elements of turbo-jet engines, where the deposition rate and the efficiency of material’s utilization is of high importance (Kumar and Krishnadas Nair, 2017). In the conventional LDEDmw, the wire entering the laser beam is at a room temperature. This is so called “cold wire” process.
In this work a preheating of the wire was introduced to improve the deposition process. This was achieved by using the resistive heating phenomenon. Thus, a voltage and a current was supplied to an electrical circuit containing the wire and the meltpool creating a so called “hot wire” effect. The needed electrical energy was created by an external electrical power source. This technology has already been applied to laser beam welding (Peng et al., 2016) and laser cladding (Liu et al., 2017) processes. An experimental investigation of depositing Alloy 718 with and without hot wire effect was conducted. The samples produced with hot wire effect showed increased penetration depth in comparison to the once produced by the cold process. While conducting experiments, the voltage and current signals were acquired and used for on-line monitoring of the process. Changes of the process like increased heat input and variations in the distance between the wire tip and the meltpool were indicated by the change of voltage and current levels as well as abrupt variations within the signals. Thus a monitoring system based on these signals gives an early indication of the drift from the nominal process window, allowing to conduct a counter actions to stabilize the process. In the future, the acquired voltage and current signals can be used to develop a closed loop, feedback control system.