IV. MLab Weld Tracks
4.2.4 Weld Evaluation
After a single exposure at the selected power and speed settings, the pucks were imaged with a Zeiss Observer equipped with an Axiocam 103 to examine surface quality. Defects such as solidification cracking, balling, and uneven welds were used to exclude the parameter from further use. Pucks were then cut orthogonal to the weld tracks, to examine weld penetration and melt zone regime. The cut pucks were polished using diamond slurry in a succession of 9, 3, and 1 µm, then etched in a
(a) Uneven powder layer on the rear left puck prior to weld track exposure.
(b) Clearly visible streaks in weld tracks at- tempted on unevenly covered pucks.
Figure 4.6. Image of a poorly set up powder layer, and the resulting inconsistent weld tracks.
solution of 20 mL of nitric acid in 180 mL of methanol (Nital) for 12 seconds, or until the surface visibly dulled. The pucks were then neutralized in a sodium bicarbonate solution, rinsed in deionized water, rinsed in acetone, and dried in a jet of Nitrogen. Etched specimens were imaged under a Keyence light microscope at 500x as quickly as possible, as rust appeared on the cut surface in as little as one hour after etching. The depth and width of the weld tracks was measured using the native Keyence software. Due to the limited viewing window of the microscope at 500x, dots of black ink were applied with a permanent marker between parameter weld track sets in order to avoid mix-up. This resulted in a few of the individual weld tracks being obscured, but prevented data mislabeling. The weld penetration was then classified as shallow, conduction, penetration, or keyhole; shallow and keyhole welds were excluded from further use.
4.3 Results
4.3.1 Weld Appearance
The weld tracks were first evaluated from above using a Zeiss Observer optical microscope equipped with an Axiocam HRc camera. The welds were categorized by visual quality, with balling and the lack of a melt pool used to completely exclude settings, while elongation and inconsistencies in the appearance did not fully exclude settings from further contention. Examples of welds in these categories are shown in Figures 4.7 and 4.8. The weld track widths were measured in the Zeiss AxioVision software and later compared to the weld penetration width and used to calculate hatch spacing. Several weld tracks at 80 W and 100 W looked very similar to a classic ’stacked-coin’ Tungsten Inert Gas (TIG) weld, while welds at higher laser powers and speeds had highly elongated tails typical of automated laser welding procedures. Weld inconsistencies were far more prevalent in welds generated with powder, which may indicate that the inconsistency is due more to small differences in powder bed thickness than actual process flaws.
(a) Balling (b) Inconsistent melt
(c) Elongated (d) No melt pool
Figure 4.8. Representative images of different weld track surface quality flaws used to exclude settings.
4.3.2 Weld Penetration
After surface evaluation, the weld track pucks were sectioned, mounted, and etched with Nital before being examined on a Keyence VH-Z250R optical microscope. The welds were categorized by penetration depth as conduction/penetration, shallow, or keyhole. Some welds exhibited significant undercutting, which is generally attributed to an overly rapid processing speed in traditional welding. Representative images of these categories are displayed in Figure 4.9. Weld width and depth were measured in the native Keyence software, with width being used for hatch spacing calculation and depth used to exclude shallow welds or preferentially select for layer remelting. Top surface weld quality did not immediately appear to directly result in conduction
zone welds, as the ’stacked-coin’ welds were very shallow, and elongated welds more typically resulting in penetration depths adequate to prevent layer delamination.
(a) Conduction/Penetration (b) Shallow
(c) Undercut (d) Keyhole
Figure 4.9. Representative images of different weld track depth descriptors.
4.3.3 P -V Process Maps
Weld track quality evaluations were applied to the Power-Velocity (P − V ) pro- cess maps to define a cone of viable processing parameters. In the welds without powder, the best visual quality welds were produced at 80 W and 100 W settings, with inconsistencies and elongation prevalent at 140 W and 170 W settings, and
positive identification of the balling phenomenon at high fluence values. When the weld width, measured from the top, was compared to fluence, there appeared to be a positive linear correlation, with the narrowest weld for each value of fluence generally appearing as the highest quality. Plots of the surface evaluation of the no-powder welds are shown in Figure 4.10.
(a) P − V map of welds without powder, sorted by top surface quality.
(b) Weld track width vs fluence, sorted by top surface qual- ity.
Figure 4.10. P − V map and width vs fluence plots of weld tracks run without powder, top surface evaluation only.
The surface quality of the weld tracks with powder was similar to that of the weld tracks without powder, with the best looking welds occurring at lower powers, elon- gation appearing at higher powers and balling occurring at high-power, high-fluence parameter sets. The highest quality welds were again the narrowest for each value of fluence, though the addition of powder actually appeared to make this relationship more linear. Plots of the surface evaluation of the with-powder welds are shown in Figure 4.11. The weld track experiments indicate that a decision to carry parameters forward solely on top surface quality would be limited to laser powers of 100 W or less, and there would be no clear values of fluence to use as a bounding cone.
(a) P − V map of welds with powder, sorted by top surface quality.
(b) Weld track width vs fluence, sorted by top surface quality.
Figure 4.11. P − V map and width vs fluence plots of weld tracks run with powder, top surface evaluation only.
Analysis of the weld tracks sections indicated that the weld surface quality was not a reliable indicator of weld melt quality. While welds that were excellent in appearance
did in fact result in conduction mode penetration, the depth of penetration indicated that they would not remelt multiple previous layers, which is known to increase part density. Symmetric, penetrating weld profiles that are ideal for creating high density parts were associated with elongated welds. Tracks that did not have a clearly identifiable melt pool structure were those that resulted in undercutting around the sides of welds. High-power, high-fluence parameter sets that resulted in balling, as well as some of the elongated welds, were associated with keyhole mode welds. The P − V map produced for weld sections, displayed in Figure 4.12, positively identified a cone-shaped region for further development. Unlike the previous weld width vs fluence plots, the weld sections did not exhibit a linear quality relationship. The welds instead seemed to indicate that there was a fluence region of between 200 J/mm3 and 500 J/mm3 in which the parameters would produce the weld profiles
(a) P − V map of sectioned welds with powder, sorted by top surface quality
(b) Weld track width vs fluence, sorted by melt penetration mode.
Figure 4.12. P −V map and width vs fluence plots of sectioned weld tracks, with powder case.
4.3.4 Parameter Selection
After evaluating the weld tracks, a set of ten initial power and speed combinations were chosen for the generation of solid parts, shown in Figure 4.13. Parameter sets that did not achieve conduction mode melting were eliminated, and two points were chosen at each power level based on the top surface evaluation. Top surface evalua- tions of acceptable were chosen preferentially, with elongated welds chosen when no other options were available. The fluence values of these initial points ranged from 180 J/mm3 to 550 J/mm3.
Figure 4.13. The initial ten points chosen with weld track sectioning results for evalu- ation as solid cylinders, vs the set of 40 parameters tested as weld tracks.
4.4 Summary
The weld track experiments conducted on the MLab 200R resulted in potential settings for high density parts at each of the four power levels surveyed. The welds that had the appearance of classic TIG welds and those that appeared as smoothly
elongated versions of those welds were most likely to result in conduction or pene- tration mode processes. Welds that exhibited balling are likely to generate keyhole pores, and welds without a discernable melt pool are likely to be too shallow to ad- equately melt powder layers. Using a survey of power and speed settings used on similar materials successfully resulted in welds that are promising for further devel- opment as additive manufacturing settings, but no conclusions can yet be made on the use of fluence as a selection parameter.
V. Material Characterization
5.1 Overview
The weld track experiments covered in Chapter 4 resulted in the selection of ten power and speed settings that would be carried forward for solid part generation and porosity analysis. Following testing of these initial ten settings, additional test points were chosen based on fluence to test its viability as a scaling parameter. The solid parts generated were subjected to porosity analysis consisting of both sectioning and optical microscopy, and CT scanning. Preliminary tests were promising, with multiple parts exhibiting densities of greater than greater than 99.9%. This early success with high density parts led to extensive material characterization, including surface roughness, hardness, microstructure, chemistry, and tensile and impact strength.
5.2 Methodology