Additive Manufacturing Melt Pool Physics Understood Through the Lens of Two-Color Thermal Imaging

<p dir="ltr">Metal additive manufacturing (AM) offers increased design freedom compared to conventional manufacturing methods, but the properties of the resulting parts can be unpredictable. Many of the defects in metal AM parts can be traced back to defects in the molten metal region created by the heat source, termed the melt pool. While machine parameters such as laser power and scanning velocity can be used to identify defect-free regimes, these comparisons do not provide significant insight into the physics of the melt pool. Multi-physics computational fluid dynamics models (CFD) are increasingly used in the field to understand the complex fluid flows and vaporization in the melt pool but lack rigorous experimental validation. This thesis outlines the use of a novel two-color thermal imaging system, which is used on numerous metal AM platforms including laser powder bed fusion (LPBF), powder-blown laser directed energy deposition (L-DED), and electron beam powder bed fusion (EB-PBF) to gain insight into the physics of the melt pool. </p><p dir="ltr">An experimental method was developed to image melt pool temperature with a single commercial color camera by leveraging the principle of two-color (i.e., ratiometric) thermal imaging. Two-color thermal imaging is advantageous because it reduces the need for a priori knowledge of melt pool emissivity, plume transmissivity, and the camera’s view factor, which are the major challenges of single-wavelength thermography. The color camera’s ability to accurately measure temperature was validated with both a National Institute of Standards and Technology (NIST) blackbody source and a calibrated tungsten filament lamp. In LPBF, a high-speed camera and high-magnification lens are used to measure melt pool temperature. Experimental melt pool temperature fields for 316L stainless steel at varying processing conditions show peaks above the vaporization temperature of the material. Vaporization is a key heat transfer mechanism in these melt pools, which can be modeled in a FLOW-3D® multi-physics CFD model. However, two key parameters relating to the absorption and vaporization in the melt pool, the Fresnel and accommodation iv coefficients, are not well characterized in the literature. Fitting the CFD model to ex-situ measurements of the melt pool cross-sectional geometry for 316L stainless steel identifies multiple combinations of these coefficients that lead to geometric agreement, but two of these combinations show more reasonable agreement with the thermal images than the others. This fitting motivates the need for thermal imaging as a means to advance the validation of multi-physics CFD models. </p><p dir="ltr">In L-DED, where the velocity is an order of magnitude lower and the laser spot size an order of magnitude higher than in LPBF, an inexpensive color camera was used to measure melt pool temperature. Due to the larger spot size, L-DED peak temperatures often do not reach the vaporization temperature of the material. In these melt pools, the temperature?dependent surface tension drives the fluid convection via the Marangoni effect. In-situ L-DED temperature measurements are combined with ex-situ cross-sectional geometry to determine the effective absorptivity and temperature-dependent surface tension coefficient of 316L SS in a multi-physics CFD model. Measured peak melt pool temperatures are important for understanding the convection within the melt pool, and measured cooling rates can be related to the resulting part microstructure. Peak temperatures from 1750 K to 3000 K show that below the boiling point, temperature increases with increasing laser power density and decreases less with increasing scanning velocity. Thermal images are used to estimate cooling rates in the melt pool tail, which increase linearly with the ratio of scanning velocity to power as previously shown in modeling literature. </p><p dir="ltr">The EB-PBF process operates in a vacuum with a spot size between LPBF and L-DED processes, a power on the order of the L-DED process, and a scanning velocity three orders of magnitude larger than LPBF. This set of parameters makes the process suitable for untraditional spot melting scanning strategies. Spot melting is of interest to the research community because it has the potential to increase build rates without the bead-up that occurs at high powers and scan velocities with traditional rastering. However, these unique strategies and the EB-PBF build environment present new challenges such as undesirable v melt-pool and layer-scale fluid effects as well as lack of fusion and keyhole porosity. With these new challenges also comes the improved preheating capability, allowing two-color thermal imaging of the entire layer. This layer-wise process monitoring tool in conjunction with ex-situ cross-sectional and surface analysis, and conduction-based thermal modeling is used to study the effectiveness of scan strategies. </p><p dir="ltr">In LPBF, two-color thermal images of nickel superalloy 718 and Ti-6Al-4V show compa?rable temperatures, with increased plume obstruction, especially in Ti-6Al-4V. At first, the plume was the plume was considered an experimental obstacle. However, the plume formed from the rapid vaporization of metal is also important to the process stability, especially for multi-laser machines, as it can attenuate the laser. This experimental study increases our understanding of the plume’s behavior at the melt-pool scale in LPBF and can be used to inform parameter selection for minimal melt-pool scale laser-plume interaction, validate multi-physics models of the plume, and provide optimum parameters for avoiding plume interference in melt-pool imaging.</p>