Online temperature monitoring and control of wire arc additive manufacturing

Abstract

Additive manufacturing (AM) is the process of joining materials to make parts or objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies. The Direct Energy Deposition (DED) is a series of several similar AM technologies that creates parts by melting and fusion process. The combination of an electric arc as heat source and wire as feedstock is referred to as wire arc additive manufacturing (WAAM). The WAAM uses gas metal arc welding (GMAW), gas tungsten arc welding (GTAW) and plasma arc welding (PAW) as a source for deposition. The GTAW or tungsten inert gas (TIG) welding is used in many applications such as surface hardening, melting, alloying, cladding, additive manufacturing, and many others despite its primary application of quality welding. The mechanical and geometrical properties of the product manufactured by the process are mainly dependent on the thermal characteristics. In this experimental study, in-situ monitoring and control of the generated temperature are proposed to obtain the desired properties. The proposed model consists of three parts, namely instrumental control, in-situ monitoring of temperature and its feedback control in LabVIEW-PID. The fast, error-free and emissivity independent temperature monitoring was complemented with the help of a ratio pyrometer. The primary constraint, i.e. the automation of the process was achieved by replacing the foot pedal of the TIG power source with an in-house developed control system consisting of a data acquisition (DAQ) system and relays. The TIG current was made variable automatically based on the desired or set point temperature. A wide range of experiments was performed to establish the process capability such as constant and dynamic temperature control, on the substrate with a stationary and moving heat source (TIG torch). The feedback temperature control was achieved successfully for the range of temperature between 400°C to 1600°C. A single bead was also deposited with a controlled temperature between 1200°C to 1400°C at different deposition speeds. It was observed that by controlling the bead deposition temperature, its geometrical properties could be altered which would eventually determines the mechanical property. The optimised PID constants were found to be 1.0, 0.04 - 0.08, and 0.0 for proportional, integral and derivative, respectively. The error in controlled temperature was found in between 0.3–1.67% (5-25°C) for different sets of the experiments, where the maximum error was realised for the dynamic and bead deposition temperature control 1–1.67% (15-25°C). It was observed that the stabilisation time for attaining the setpoint temperature was dependent on the nature of the heating source, i.e. stationary or moving. In the case of the stationary heat source, the time taken for controlling the temperature was lesser as compared to the moving heat source where the temperature changes rapidly.