Laser cladding has many advantages, including the generation of parts that are not manufacturable by conventional processes, due to their complex shapes, gradual material composition, or undercuts. The powder nature of the filler material and steep thermal gradients can lead to unwanted changes in microscopic and macroscopic appearances. Especially powder processes like laser cladding for additive
manufacturing and laser powder bed fusion (LPBF) are prone to defects inside the 3D-printed metal (Zhang et al. 2017). The application of novel but hard-to-weld materials results in increasingly complex processes with narrow process and parameter windows. Inevitably, defects can occur during the print process. Irregularities of a laser cladding process can range from small pores to larger heat-induced cracks as well as bonding defects and cavities, as shown in Fig. 1.3. Bonding defects are typically a result of insufficient energy input that results in elongated pores. Pores are gas-filled, whereas cavities and cracks are a result of shrinkage.
Inclusions inside the printed part can range from segregation to impurities and oxidation. Oxidation is a challenge due to the large build volume that only allows for local inert atmospheres. During the process, oxygen can react with the heated material and form unwanted oxides. This can lead to microscopic defects and irregularities. Another common macroscopic error is a lack of dimensional accuracy. The thermal capacity changes with every layer; especially at corners, where the heat is concentrated. An additional limitation of 3D printing is the compromise between complexity and deposition rate. Fine structures with a resolution of 100 μm are possible with powder-based laser cladding. At this resolution, the deposition rate decreases to 0.5–1 g per minute (Kaierle et al. 2012). Larger focus diameters reduce the fidelity significantly but allow deposition rates up to 10 kg/h.
To counteract these defects, efforts are being made to mitigate these effects and make the process more robust. Controlled process environments monitored with different sensors are key to a reliable and repeatable process. Developments in computing power allow fast image processing of the melt pool. From the captured meltpool image, the contour is extracted and a convex hull is superimposed (see Fig. 1.4a, b). In-process evaluation of temperature, shape, area, stability, and uniformity with feedback control to the powder mass flow, feed rate, and laser power greatly improves the printed results. These data in conjunction with acoustic and deposition measurements allow the creation of a visual representation of the data (see Fig. 1.4c).
