Page 10 - 3DMP Summer 2022

3D AM INSIGHTS By Alber Sadek
Effect of Laser-Stirring Scan Strategy on Laser Powder-Bed Fusion As-Built Microstructure
In recent years, laser powder-bed fusion (LPBF) additive manufacturing (AM) has stepped out of the prototyping realm and matured to become a production-ready manu- facturing technology. The big-picture advantages of LPBF:
• The most well-developed supply chain of all metal-AM processes, including service providers, and feedstock and equipment manufacturers
• The best geometric fidelity and complexity, making it a natural choice for AM applications that seek to maximize per- formance and cost savings via advanced design
• The widest range of qualified materials and materials property data.
LPBF does have some disadvantages, however, stemming from the physics of the process and current process limitations: • Results in complex residual stresses and thermal gradients
in the component, which must be managed
• Exhibits microstructural anisotropy—in particular the
growth of columnar grains—mandates complex post-print thermal heat treatments to rectify the microstructure
• Undergoes high cooling rates, resulting in microcracking when printing alloy systems with relatively poor weldability, making alloys such as Al7075, 6061, tungsten, CM-247 and sim- ilar Ni-alloy systems very difficult to process
• Experiences slow in-layer material consolidation.
These issues all can be addressed, in part by deliberately modifying the chemical composition of the alloy systems with the addition of trace elements, nucleating agents or even non- metallic reinforcement particles. While the chemistry-based solutions are gaining in popularity, they still must overcome the hurdles of material property allowable generation and qualification.
In response, EWI has devised a patented “laser-stirring” approach driven by improvements in the LPBF process. The approach alters the laser-scan strategy used in the entire LPBF process and allows for the use of feedstock materials that already have been certified and relied upon in other manufac- turing technologies.
Alber Sadek is a senior technology leader for EWI’s materials group. His current work involves the selection of metals and alloys as they are applied in different industrial sectors, covering their physical properties, material characterization, weldability (similar/dissimilar alloys), corrosion, wear, fatigue and creep properties. He currently leads several projects aimed toward developing heat treatment procedures of additively manufactured alloys.
Laser Stirring Overview
Laser stirring has been adapted from electromagnetic arc stirring, a technique that combines the electromagnetic stir- ring of arc welds with electron-beam oscillation. This process has been shown to break up the dendritic microstructure of aluminum alloys, encourage an equiaxed grain structure, reduce thermal gradients at the edge of the melt pool and decrease mean grain size. Each of these attributes proves bene- ficial in reducing solidification cracking in high-strength alu- minum alloys and austenitic stainless steels.
During LPBF, laser stirring replaces linear hatches with repeated circular or elliptical scan paths (Fig. 1). EWI’s laser- stirring process aims to:
• Produce a more equiaxed and isotropic as-printed LPBF microstructure
• Mitigate solidification and liquation cracking in crack- prone alloys.
Experimental Work
We conducted an investigation to compare and contrast the results of using stirred and linear-hatch techniques, as well as the effects of process-parameter changes (Fig. 2).We used test coupons from three conventional LPBF alloy systems: Type 316L stainless steel, Ti-6Al-4V and Inconel 718.
The powders were produced via inert-gas atomization, with the initial melt per-
formed under a vacu-
um or inert-gas cover.
Powders with parti- cle-size range con- forming to -325 Mesh/+10 m (45/+10
m) were purchased. All builds were car-
ried out on EWI’s open-architecture LPBF system (OAS), developed in 2016 as part of the Measure- ment Science Innova- tion Program for Additive Manufactur- ing, funded by NCDMM and NIST. A
D2
D1
Travel Direction
Fig. 1
8 | 3D METAL PRINTING • SUMMER 2022
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