On-line melt pool temperature control in L-PBF additive manufacturing

Laser powder bed fusion is a promising additive manufacturing technology which has enabled the fabrication of complex-shape, custom-designed, and cost-effective parts with no need for expensive tools and dies. Despite the numerous advantages of this technology, inconsistency in the microstructure and, consequently, the mechanical properties of the fabricated components in the building direction makes it quite challenging to obtain uniform parts in the as-built state. This issue originates from the layer-by-layer melt pool temperature variation caused by the layer-wise nature of this process when a fixed set of process parameters is applied. Accordingly, a layer-wise melt pool temperature control system is beneficial in manipulating the process parameters and therefore adjusting the melt pool temperature. In this study, three different controllers, namely, simple proportional (P), adaptive P, and sliding mode, were designed to control the melt pool temperature in the building direction for Inconel 625 superalloy. An analytical-experimental model was introduced to evaluate the performance of controllers through simulation. A monitoring system having a two-color pyrometer was used to online monitor the temperature for use by the controllers as a feedback signal. The microstructure and microhardness of the final products were evaluated prior to and after employing the melt pool temperature controllers. Compared to the scenario with constant process parameters, the implementation of these controllers led to improved microhardness and microstructure uniformity, resulting from the reduced variation in the primary dendrite arm spacing. The lessons learned from this study can assist in the fabrication of functionally graded materials with engineered microstructures.