Layer-to-layer control of laser metal deposition processes

Laser Metal Deposition (LMD) is a promising technology for Solid Freeform Fabrication. It allows fabrication of functional metal parts directly from CAD solid models, which reduces the machining and material costs. Typically, constant process parameters are selected, resulting in a non-uniform track morphology, increased machining cost, powder waste, etc. To address these problems, this dissertation develops automatic control methodologies to implement closed-loop LMD process control. In PAPER I, a novel powder flow rate controller is designed to track time varying powder flow rate references. This methodology allows a constant distribution of powder per unit length. The experimental results demonstrate that the powder flow rate controller provides more consistent track morphologies. A layer-to-layer control methodology is developed and applied to layer height control in PAPER II. Layer-to-layer control employs online melt pool temperature measurement and part height measurement between layers to estimate process model parameters. Based on the updated model, the powder flow rate reference profile for the next layer is computed with respect to the height reference. The experimental results illustrate that the application of layer-to-layer height control produces a more predictable layer height increment and a more precise track height, as compared to the widely used constant process parameter strategy. In PAPER III, melt pool temperature is regulated using an online temperature controller based on an empirical model. The experimental results illustrate that online temperature control cannot produce a uniform track morphology in multi-layer deposition, although it tracks the temperature reference signal well. A layer-to-layer temperature controller is designed in PAPER IV. It utilizes process measurements, melt pool temperature profile and layer height profile, to estimate process parameters. The laser power reference profile is then computed with respect to the temperature reference. The experimental results show that layer-to-layer temperature control successfully regulates the melt pool temperature while maintaining a uniform track morphology.