Multi-laser powder bed fusion of Ti-6.5Al-2Zr-Mo-V alloy powder: Defect formation mechanism and microstructural evolution
Graphical abstract
Introduction
Titanium alloys possess many desirable properties such as high strength, corrosion resistance and low density, which are widely utilized in aerospace and automobile applications [[1], [2], [3]] As an important structural material in the aerospace industry, near-α Ti-6.5Al-2Zr-Mo-V alloy (refers to TA15 in Chinese industry) is used to manufacture aircraft bulkheads, sidings, and casings with high operating temperature and complex structure. Early research on Ti-6.5Al-2Zr-Mo-V mainly focused on deformation behavior [4,5], heat treatment [6], and welding [7]. However, with the aerospace equipment moving towards high-performance, lightweight and heavy-duty, higher requirements are placed on traditional complex component manufacturing.
Under this background, researchers introduce additive manufacturing (AM) technology to fabricate Ti-6.5Al-2Zr-Mo-V complex-structure components. Since bulkhead, siding, and casing structures tend to large sizes, current studies about additive manufacturing Ti-6.5Al-2Zr-Mo-V mainly concentrated on laser melting deposition (LMD) [[8], [9], [10]]. Nevertheless, shortcomings of LMD, such as the limited precision and design freedom, limit to fabricate complex structures. Laser powder bed fusion (L-PBF) is generally regarded as another promising laser-based AM technique [2,11,12]. It has attracted much attention because of its advantages of directly fabricating near-full-density, high-precision, and complex-shaped metal parts. However, the conventional L-PBF technique is not capable to build large-size parts due to the limited building dimension and efficiency. Some works demonstrated that near-full-dense Ti-6.5Al-2Zr-Mo-V parts were produced by LMD and L-PBF. This shows that there is no obvious difference in density for the Ti-6.5Al-2Zr-Mo-V samples obtained by L-PBF and LMD [10,[13], [14], [15], [16]]. The differences between the two technologies are mainly reflected in manufacturing efficiency, dimensional accuracy, and microstructure.
Multi-laser powder bed fusion (ML-PBF) combines the advantages of high-precision and high-efficiency, which is more suitable in building large-size and complex-structured Ti-6.5Al-2Zr-Mo-V components than the conventional LMD and L-PBF techniques [17]. Thus far, only a few studies related to ML-PBF are publicly reported. Tsai et al. [18] used a diffractive optical element to build a multi-spot system through a single-laser L-PBF machine. The multi-spot scanning time was decreased by 38% of the conventional single-laser L-PBF. Roehling et al. [19] developed a multi-laser system that combines a fiber laser and four laser diodes to in situ control the residual stress during L-PBF. Heeling and Wegener [20] utilized a dual L-PBF system to fabricate full-density 316 L stainless steel samples. One laser beam was used to melt powders and the other one was used to heat the vicinity of the molten pool, the pre- and post-heating can be therefore applied.
Although ML-PBF naturally owns advantages over the single L-PBF, many issues remain to be addressed. The multi-laser overlap zone usually widths up to several millimeters to ensure the joining quality between each single-laser processing region. Owing to the multiple laser scanning of the overlap zone, it is challenging to ensure the densification, microstructure, and performance uniformity between the single-laser processing region and the overlap zone. Andani [21] demonstrated more spatter formation and greater recoil pressure in the dual L-PBF. They found the unmelted regions resulted from two-lasers working closely were detrimental to the mechanical properties of specimens. Li et al. [17] found a stair-step effect that affected the surface topography of the overlap zone in ML-PBF processing Ti6Al4V. However, the density in the overlap zone was not mentioned. Using the same equipment, Zhang et al. [22] prepared near full-density AlSi10Mg samples. They found the density and the grain size of the overlap zone are slightly lower than the ones in the single-laser processed area. As demonstrated by Wei et al. [23], many keyhole defects may form in the overlap zone if the multi-laser encounter together. Besides, when multi lasers scan closely, the defect will be promoted due to the laser-plume interaction [24].
Moreover, the scan strategy applied to the overlap zone requires attention to reduce or avoid multi-laser induced effects. In the early stage of ML-PBF development, multi-laser scan strategies are mostly parallel, meaning multi-laser beams work independently in the overlap zone [18,25]. In a dual-laser parallel scanning experiment with a small offset between two lasers, Zhang et al. found periodic coalescence between the two scanning molten pools [26]. Later, Heeling and Wegener [27] proposed a spatially synchronized multi-laser scan strategy. It let the multi-laser beams group together to melt the powders, thereby resulting in a higher energy density. The higher temperature gradient induced by the multi-laser scan strategy [28] implies the potential change of defect, microstructure, and mechanical property of the overlap zone. Yet few studies related to the above concerns.
In the present study, a self-developed four-laser PBF system was applied to fabricate Ti-6.5Al-2Zr-Mo-V parts. The densification deterioration of the multi-laser overlap zone under two different scan strategies was investigated to deepen the understanding of the multi-laser induced defects. The defect formation mechanism was clarified. In addition, the differences in defect characteristics, microstructure, and microhardness of the single-, dual- and quadruple- laser processed samples were investigated.
Section snippets
Raw material
The starting material is gas-atomized Ti-6.5Al-2Zr-Mo-V powders (6.47 Al, 2.09 Zr, 1.62 Mo, 2.12 V, wt%) supplied by Sino-Euro Materials Technologies of Xi'an Co., Ltd. The particle size was measured using the Malvern Mastersizer 3000 laser-based particle-size analyzer. The morphology and particle distribution of the spherical powders are shown in Fig. 1a and b, respectively.
ML-PBF process
The ML-PBF equipment (NRD-SLM-500) and CAM software are both self-developed. To keep consistent during multi-laser
Multi-laser induced defect characteristics
Following the optimal L-PBF parameters, cubic samples were fabricated with the laser overlap zone which was built using the sequential and synchronized scan strategies. It should be noted that under both the two scanning strategies, four laser beams synchronously process their scanning tasks according to the divided areas. The two strategies are different when scanning the overlap zone. For the sequential scan strategy demonstrated in Fig. 4a, laser #1 scans the powder layer firstly, after
Conclusions
In the present work, a four-laser PBF system was used to prepare Ti-6.5Al-2Zr-Mo-V parts under two different multi-laser scan strategies. By investigating the defect, microstructure, and microhardness characteristics of the samples, the defect formation mechanism and the microstructural evolution induced by ML-PBF are clarified. Conclusions can be drawn as follows:
- (1)
With more laser beams involving ML-PBF, the sample density gradually decreases when sequential and synchronized scan strategies are
CRediT authorship contribution statement
Shuhan Li: Writing - original draft, Data curation, Investigation. Jingjing Yang: Methodology, Investigation. Zemin Wang: Conceptualization, Writing - review & editing, Supervision.
Declaration of Competing Interest
None.
Acknowledgments
This work was supported by the Civil Aerospace Pre-research Project: research on additive manufacturing of core components in the liquid rocket engine, and the Fundamental Research Funds for the Central Universities through Program no.2019kfyXMPY005 and no. 2019kfyXKJC042. The authors thank the Analytical and Testing Center of HUST for the EBSD analysis.
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