Laser based Directed Energy Deposition
Laser based Directed Energy Deposition (DED) is an additive manufacturing technique that deposits metal powder particles in a melt pool created by a laser beam. The metal particles are carried by a shielding gas towards the melt pool in order to minimize oxidation and the resulting defects in the deposited track. A constant temperature distribution during the metal deposition process is required in order to guarantee quality.
Additive manufacturing offers the possibility to CREATE components with almost any geometrical freedom. Parts no longer need to be redesigned for manufacturing due to the endless geometrical freedom obtained through additive manufacturing. This design freedom is further extended by using hybrid additive subtractive production processes. However, in order to MAKE parts with structural load bearing capabilities, one should MONITOR the metal deposition process. This is the only way to guarantee consistency in the laser metal deposition process, which is inherently plagued by many uncertainties and variations in the metal deposition process. Our group focusses on the development of monitoring systems in order to ensure parts consistency by metal deposition process CONTROL. Using such a closedloop metal deposition process we IMPROVE the overall geometrical and structural quality of the metal deposited parts.
Numerical and experimental modelling of the laser metal deposition provides a better insight into the many physical phenomena involved when a gas powder flow is combined with the heat of a travelling laser source. Numerical modelling of both the heat input as well as the gas powder flow allows to better understand the influence of uncertainty in the material and process variables, which can be cross validated with the obtained experimental models.
The modelling of the temperature distribution as a result of the laser spot travelling over the surface of an object being built provides many of the required parameters to define the optimum process parameters for laser based metal deposition. The unique strength of the hybrid manufacturing group at the Vrije Universiteit Brussel results from the powerful combination of numerical and experimental techniques. Initial setpoints can easily be defined from the numerical models. However, due to the potential large variability in the laser metal deposition process, building models from experimental data allows to fine tune the deposition process for each specific case.
Obtaining accurate additive manufacturing parts with structural load bearing capabilities requires good quality control, which already starts during the manufacturing process. Due to local changes in the substrate, variations in thermal and mechanical properties of the deposited powder and geometry, as well as variation of the build geometry, the monitoring of the actual melt pool is essential.
An in-house developed camera system allows to monitor the melt pool temperature distribution in real-time. The data is acquired at rates above 1000 frames per second, in order to generate the required temperature information for the feedback closed-loop laser metal deposition process control. High-speed image recording of rates up to 180 000 frames per second even allows to study the melt pool dynamics upon impact of 20 μm metal powder particles in the molten substrate. Robustness of the system is guaranteed by the use of commercial available and industry accepted components.
Our unique monitoring system can also be used to actually control the machine parameters such as laser output power, resulting in a consistent laser metal deposition process. The closed-loop system is capable of generating deposed metal beads with a specified temperature time history as illustrated for the square wave profile. The large influence of the process deposition temperature on the resulting tracks is clearly visible from the change in width and surface roughness of the beads.
Closed-loop control of the laser metal deposition process allows the accurate control of the metal deposition rate, track width and height, as well as the surface roughness. In addition to these enhanced geometrical characteristics, the precise control of the heat reduces the residual stresses and results in improved mechanical characteristics when compared with traditional openloop metal deposition processes.
The research question is to develop strategies for the detection of manufacturing defects during the printing process itself. Not only is this more convenient than having to inspect the complex structures post-build, but detecting flaws during manufacturing enables the introduction of a dynamic intervention mechanism and minimal loss of material and time. The aim of this research is to validate at least two in-process inspection methods: IR thermography and laser ultrasonic testing. These technologies will be studied for their capability to detect internal defects as well as residual stresses and the results will be used to optimize quality inspection procedures and closed-loop control of the printing process.
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