Laser powder bed fusion (L-PBF) refers to an additive manufacturing (AM) process in which a high-intensity laser source melts powders in a layer-by-layer manner to fabricate parts based on a computer-aided design (CAD) model without almost any geometrical limitations. The development of the L-PBF process has provided an outstanding opportunity to manufacture unique parts which are practically impossible to be produced by conventional manufacturing methods. The L-PBF process also does not require intricate build tools and assembly processes. However, quality issues such as non-uniform microstructure or mechanical properties, porosities, and surface roughness deteriorate the quality of the parts fabricated by the L-PBF process. Therefore, the reliability and the repeatability of the process are required to be addressed.
This study deals with improving the quality of the part fabricated by the L-PBF process and making the process more reliable and repeatable. The control approach was employed to elevate the quality of the final part from three different aspects. First, making the microstructure and microhardness of the part uniform through a control approach was investigated. Three controllers, namely, proportional (P), adaptive P, and quasi sliding mode, were developed to control the melt pool temperature for the Inconel 625 superalloy. An analytical-experimental model was presented to evaluate the performance of controllers via simulation. A monitoring system consisting of a two-color pyrometer was utilized off-axially to monitor the melt pool temperature for use by the controllers as a feedback signal. The results indicated that the control approach led to microhardness and microstructure uniformity, resulting from the reduced variation in the primary dendrite arm spacing compared to the case with constant process parameters. Second, the control approach was utilized to produce optimum parts instead of using the energy density criterion. Temperature domains corresponding to the most common porosities, namely, lack of fusion (LOF), lack of penetration (LOP), and keyhole, were determined in a range of process parameters using a thermal imaging system. A safe zone was introduced by defining a lower and an upper limit based on the critical temperatures causing transitions from LOP to defect-free and from defect-free to keyhole zones, respectively. A proportional-integral-derivative (PID) controller was used to maintain the melt pool temperature within the safe zone during the L-PBF process for Inconel 625 and avoid the formation of porosities, regardless of the initial condition selected and the scanning speed employed. In all cases, a short settling time in the order of the printing time for a few layers was required to reach the steady-state condition at which defect-free parts could be obtained. Finally, minimizing the top surface roughness of the parts manufactured by the L-PBF process by deploying a Feedforward plus Feedback control system was targeted in this study. The most common factors affecting the surface quality, namely, balling, lack of inter-track overlap, overlapping curvature of laser scan tracks, and spatters, were investigated through a monitoring system consisting of a high-speed camera, a zooming lens, and a short pass filter. The desired melt pool width and the critical value for the level of spatters were determined using the imaging system and subsequent image processing. An experimental model was developed, and the control system was designed accordingly. Both simulations and experimental results showed excellent transient performance of the control system to reach the desired melt pool width only after printing a few layers. Also, the control system was evaluated at different scanning speeds and with different geometries. The results obtained from this study indicated that controlling the geometry of the melt pool can mitigate significant defects occurring during the process and minimize the top surface roughness. / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/27420 |
| Date | January 2022 |
| Creators | Hossein Rezaeifar |
| Contributors | Mohamed Elbestawi, Mechanical Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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