Abstract Scope |
Introduction:
Laser Powder Bed Fusion (LPBF) additive manufacturing (AM) has the distinct advantage of producing complex parts with fine feature resolution due to the high energy density power source and powder feedstock. Through the manipulation of slicing and scanning pattern parameters, single laser pass exposures repeated in a vertical build direction yield ultra-thin fins, representative of difficult to print thin sidewalls and small scale components such as heat sinks. These thin fins also allow comprehensive metallographic analysis of LPBF part microstructural evolution when operating at the finest resolution possible. Having successfully isolated appropriate slicing and scanning techniques for producing ultra-thin fins, the effects of adjusting a binary argon-helium shielding gas mixture are evaluated. Building off of established advantages of utilizing helium in traditional laser welding, mixtures of argon and helium are understood to have higher thermal conductivity than either gas alone. This work translates an understanding of laser welding to the LPBF process by comparing samples produced under both a pure argon environment and that with equal parts argon and helium, and evaluating differences in fin thickness and morphology. Results show that those fins produced under the balanced argon-helium mixture yielded a lower thickness at a higher level of consistency. As the effect of shielding gas is better understood, its manipulation and effects on resultant parts will be established, further allowing the manipulation of this key LPBF parameter in industry.
Experimental Procedures:
A Concept Laser Mlab Cusing 100R (Lichtenfels, Germany) and 316L sold by Concept Laser are utilized for all experiments. The powder used is recycled powder, having been sieved across a 50-micron mesh using a Retsch AS-200 vibratory Sieve (Haan, Germany). Argon gas used was obtained from Praxair Inc. (Danbury, CT) at a purity of 4.8, with Delille Oxygen (Columbus, OH) providing 50% high purity helium balanced with argon, with the mixture being by volume and certified through gravimetric analysis. All specimens are designed using Autodesk Fusion 360 (Mill Valley, CA) as well as Materialise Magics (Leuven, Belgium). The sample design includes a rectangular sidewall surrounding fins oriented in reference from the x-y build plane to range from 45 degrees to 90 degrees in 5 degree increments.
Prior to removal from the build plate, each sample is filled with cold mount epoxy, specified as Pace Technologies (Tucson, AZ) Epoxy-Elite to support the delicate fins before cross sectioning the samples to observe a plane containing fin thickness and height. Collected images are stitched together and analyzed utilizing ImageJ, available in the public domain. The images are taken with an overlap of 20% and combined utilizing the grid/collection stitching tool in ImageJ moving upward and to the right. The location for points of measurement for x-projection and angle are determined by overlaying a series of horizontal lines spaced 500 microns apart moving from the base of the fin upwards. The x-projection and angle then inform corrected fin thickness through trigonometric relationships.
Results and Discussion:
Preliminary investigation of fins printed perpendicular to the build plate showed those ultra-thin features produced under an argon-helium atmosphere to be thinner and more consistent than those produced under the pure argon. However, corrected thickness values for 60-degree overhang fins are 100 and 105 microns for the pure argon and argon-helium mixture, respectively. The cross sections of the fins display a distinctive stepping effect, where a series of vertical columns are oriented diagonal to each other, a result of the LPBF process where the design angle is a function of deposited layers and hatch spacing. This effect yields a high range in measured values for x-projection and angle, each impacting the resulting corrected thickness. While observation of the x-projection alone would suggest thinner vertical columns for the argon-helium condition with an average of 140.6 microns compared to the 159.5 microns of the argon condition, it is therefore necessary to revisit the measurement methodology. To yield a more accurate depiction of fins displaying the stepping effect, continuing work shall include a revision of the ImageJ analysis stage, to increase the resolution at which the cross sections are analyzed. As the measurements better represent the entire profile of each fin, the stepping effect and realistic thickness across the fin will be more effectively quantified. Beyond the analysis of the metallographic cross section, it is further suggested to implement laser profilometry to study the surface quality of the fins.
Conclusion:
Fins printed perpendicular to the build plate show fins printed under an argon-helium atmosphere to be thinner and more consistent than those produced under the pure argon, the methodology is inconsistent for overhanging features. To evaluate overhanging features, a fin oriented at 60 degrees in relation to the building plane is analyzed in both thickness and build angle. This analysis yields an understanding of a stepping effect associated with the LPBF scanning parameters. The thinner Ar-He vertical columns promote increased variation in fin thickness and angle measurements when combined with the stepping effect. By placing these trends in reference of those conclusions made in preliminary work, increased cooling rate under the Ar-He atmosphere may prevent remelting of previous layers, yielding an overall thinner fin, which is desirable to produce ultra-thin features through the L-PBF process.
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