Using a dual-laser system to create periodic coalescence in laser powder bed fusion

doi:10.1016/j.actamat.2020.09.071

Acta Materialia

Volume 201, December 2020, Pages 14-22

https://doi.org/10.1016/j.actamat.2020.09.071 Get rights and content

Abstract

Conventional laser-based powder bed fusion of metals (PBF-LB/M) currently faces technological challenges in scalability due to its low building rate and manufacturing throughput. One approach to address this issue is to parallelize multiple laser beams to increase processing flexibility. Recent research has studied, for instance, the improvements to mechanical properties of final products when using two or more laser beams in PBF-LB/M. However, some obstacles still need to be addressed involving the proximity of molten pools and their interaction mechanism. In particular, interactions between two close, parallel molten pools have not been fully understood yet. In this study, two lasers create two parallel-running molten pools with a small spatial offset in between. With different spatial offsets, experimental results reveal that besides the completely merged and completely separated regimes, there exists a new regime which yields periodic coalescence between the two molten pools. High-speed imaging shows two different mechanisms for the formation of such coalescence, what we denote as head-to-head and head-to-tail coalescence. By changing processing parameters including laser power and spatial offset, periodic structures with various wavelengths can be engineered using this dual-laser approach.

Graphical abstract

Introduction

Additive manufacturing (AM), also referred to as 3D printing and rapid prototyping, is a process for producing 3D objects in a layer-by-layer fashion. Currently, metal AM is widely used in aerospace, automotive and many other industries due to its ability to manufacture products easily and economically [1]. Laser-based powder bed fusion of metals (PBF-LB/M) is one of the most popular methods for metal AM. In conventional PBF-LB/M, a Gaussian laser beam is used as the energy source to fuse regions of a powder bed to create the final product [2], [3]. However, slow building rate and other technological challenges prevent PBF-LB/M from gaining greater market share [1].

There have been studies on how to address the current challenges in PBF-LB/M. For example, extensive research has been done to explore the potential benefits using multiple beams or a tailored laser beam [4], [5]. Numerical simulations have also been developed to explain how novel energy distribution geometries, like an elliptical Gaussian beam, affect the resulting structures and mechanical properties [5], [6]. In the field of laser welding, novel beam profiles generated through a diffractive optical element (DOE) have offered greater control of weld pool dimensions and improved surface roughness [7], [8]. Also, to specifically address the slow building rate, recent research has focused on using multiple beams to improve the processing efficiency. Renishaw has introduced an AM machine with four independently controlled laser sources [9]. Hong et al. utilized this particular Renishaw machine to compare the product properties constructed from multiple-laser PBF-LB/M with those from single-laser PBF-LB/M [10]. Zhang et al. conducted similar experiments with a multiple-laser machine developed in-house [11]. Slodczyk et al. showed that square arrays of beams created using a DOE can increase melt rates while maintaining melt pool stability [12]. Sundqvist et al. also solved the analytical solution for the temperature field for a spatially and temporally modified beam, which can help to quickly predict the temperature profiles for multi-spot welding [13]. Furthermore, Tsai et al. built a three-spot PBF-LB/M system by integrating a DOE into the set-up. Shorter processing time as well as lower surface roughness of the final product were achieved [14].

It is clear that the key to facilitating wider adoption of PBF-LB/M is increasing manufacturing flexibility, and parallelizing multiple laser beam is one approach. In previous research on utilizing two laser beams, the molten pools created by the two beams are either completely separate [10], [11] for higher building rate or completely merged together [12], [15]. In the latter case, one beam serves as a preheating or reheating source to reduce temperature gradient and improve mechanical properties [16]. We believe there is a gap between the two cases, namely the little-understood transition between the fully-merged and fully-separated regimes. For example, there was no clear establishment of the resolution limit in the parallel beams. Also, although research has been conducted on the macrostructure and morphology produced by a single molten track [17], [18], critical questions regarding the macrostructure resulting from parallel-running beams remain unsolved.

To understand the questions raised above, we use two identical, parallel-running laser beams as energy sources for PBF-LB/M. By placing two molten tracks close to each other, we investigate the molten pools’ interactions on the cusp of merging. By doing so, the resolution between the two molten tracks is established. At the same time, we come to understand how the molten tracks transition from being fully merged to fully separated as the distance between the two tracks increases.

In addition to the lateral spatial offset between the two molten pools, we include a temporal offset between the two lasers, which effectively produces an in-line spatial offset. The introduction of the temporal offset allows us to further investigate how two close pools interact with each other within this broader parameter space. We find a new regime where periodic structures are produced at certain spatial offsets at each of the different laser powers tested. Additionally, adjusting spatial offsets within this regime changes the wavelengths of the periodic structures.

Section snippets

Experimental system

Fig. 1 shows the set-up for the dual-laser experiments. In this set-up, two 1070 nm wavelength lasers with Gaussian-shaped beams operating in continuous-wave (CW) mode are used as the laser sources. The laser beams, after being narrowed by focusing telescopes, pass through 3D scanning systems consisting of beam expanders, 2D scanners and F-θ lenses. Each of the scanning systems has a scan field of 178 × 178 mm². By putting two scanners side-by-side, an overlapping scan area of 20 × 178 mm²

Effects of hatch spacing

Fig. 3 is a confocal image showing how different hatch spacings affect the resulting molten tracks when the perpendicular offset is kept constant. The image is captured by an Olympus confocal microcope. Different colors represent various heights for the sample, ranging from purple at the height of the undisturbed substrate surface to yellow at a height of around 100 µm above the substrate surface. In this case, both lasers operate with a power of 80 W and move at a scan speed of 150 mm/s. The

Two coalescence scenarios

High-speed imaging reveals the detailed mechanism behind the periodic coalescence observed in the previous sections. Schematic diagrams to elucidate selected high-speed images are shown in Figs. 7 and 8, and the corresponding high-speed images are in Figs. (C.13) and (C.14). Video clips corresponding to Figs. 7 and 8 are included in the Supplementary materials. In each figure, images (a)–(c) show a sequence of schematic diagrams for molten pool dynamics at 0, 80 and 160 µs, respectively, where

Conclusion

In conclusion, we introduce two parameters, hatch spacing and perpendicular offset, to arrange two identical laser beams in PBF-LB/M experiments. We find that a distinct phase exists between the completely merged and completely separated phases. This transition phase yields periodic structures that would otherwise be difficult to manufacture with conventional PBF-LB/M using only a single laser beam. For a given laser power, periodic structure arises only within a specific envelope defined by

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We would like to acknowledge the Princeton University Eric and Wendy Schmidt Fund for the financial support of this project.

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Full length articleUsing a dual-laser system to create periodic coalescence in laser powder bed fusion

Abstract

Graphical abstract

Introduction

Section snippets

Experimental system

Effects of hatch spacing

Two coalescence scenarios

Conclusion

Declaration of Competing Interest

Acknowledgments

Acta Mater.

Acta Mater.

J. Mater. Process. Tech.

Phys. Procedia

Opt. Lasers Eng.

Mater. Sci. Eng. A

J. Mater. Process. Technol.

Addit. Manuf.

Addit. Manuf.

Acta Mater.

Acta Mater.

Acta Mater.

J. Mater. Process. Technol.

J. Mater. Process. Technol.

Full length article
Using a dual-laser system to create periodic coalescence in laser powder bed fusion