Elsevier

Powder Technology

Volume 384, May 2021, Pages 100-111
Powder Technology

Multi-laser powder bed fusion of Ti-6.5Al-2Zr-Mo-V alloy powder: Defect formation mechanism and microstructural evolution

https://doi.org/10.1016/j.powtec.2021.02.010Get rights and content

Highlights

  • Directional pores distributed along the overlap line due to laser switching.

  • The synchronized scan strategy led to a significant densification deterioration.

  • Multi-laser powder bed fusion resulted in near-full martensite α' microstructure.

  • Martensite α' coarsened and hardness decreased with increasing the laser number.

Abstract

Multi-laser powder bed fusion (ML-PBF) has attracted much attention due to its advantage of directly building complex-structured and large-size components. In this work, this technique was conducted to fabricate Ti-6.5Al-2Zr-Mo-V (TA15) parts using sequential and synchronized scan strategies. The defect formation mechanism and the effect of the laser beam number (up to four) on density, microstructure, and microhardness were clarified. We find that the densification gradually deteriorates as increasing the number of involved laser beams. The laser-switching induced pores are dominated in the sequential scan strategy, which cause a directional pore distribution along the overlap lines. The synchronized scan strategy has a similar phenomenon. Moreover, the laser intersection induced by the synchronized scan strategy leads to a potential transition of molten pool mode and an increase of recoil pressure, which significantly deteriorates the densification. ML-PBF processed Ti-6.5Al-2Zr-Mo-V parts are composed of near-full acicular martensite α'. ML-PBF induces the low angle grain boundary to transfer to the high angle grain boundary due to the heat accumulation effect. Consequently, slow grain coarsening of acicular martensite α' occurs with more laser beams involving in. This leads to the microhardness of single-, dual-, and quadruple- L-PBF processed samples value from ~423 Hv to ~388 Hv. The microstructure-property correlation in ML-PBF is in good agreement with the Hall-Petch relationship.

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.

References (61)

  • J. Jiang et al.

    Mechanical properties and microstructural evolution of TA15 Ti alloy processed by selective laser melting before and after annealing

    Mater. Sci. Eng. A

    (2020)
  • Y. Zhu et al.

    Microstructure evolution and layer bands of laser melting deposition Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy

    J. Alloys Compd.

    (2014)
  • X. Wang et al.

    Internal pores in DED Ti-6.5Al-2Zr-Mo-V alloy and their influence on crack initiation and fatigue life in the mid-life regime

    Addit. Manuf.

    (2019)
  • F. Li et al.

    Microstructures and mechanical properties of Ti6Al4V alloy fabricated by multi-laser beam selective laser melting

    Mater. Lett.

    (2017)
  • C.-Y. Tsai et al.

    Synchronized multi-spot scanning strategies for the laser powder bed fusion process

    Addit. Manuf.

    (2019)
  • J.D. Roehling et al.

    Reducing residual stress by selective large-area diode surface heating during laser powder bed fusion additive manufacturing

    Addit. Manuf.

    (2019)
  • T. Heeling et al.

    The effect of multi-beam strategies on selective laser melting of stainless steel 316L

    Addit. Manuf.

    (2018)
  • M. Taheri Andani et al.

    Spatter formation in selective laser melting process using multi-laser technology

    Mater. Des.

    (2017)
  • C. Zhang et al.

    A comparative study on single-laser and multi-laser selective laser melting AlSi10Mg: defects, microstructure and mechanical properties

    Mater. Sci. Eng. A

    (2019)
  • K. Wei et al.

    Multi-laser powder bed fusion of Ti–6Al–4V alloy: defect, microstructure, and mechanical property of overlap region

    Mater. Sci. Eng. A

    (2021)
  • C. Tenbrock et al.

    Effect of laser-plume interaction on part quality in multi-scanner laser powder bed fusion

    Addit. Manuf.

    (2021)
  • W. Zhang et al.

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

    Acta Mater.

    (2020)
  • T. Heeling et al.

    Computational investigation of synchronized multibeam strategies for the selective laser melting process

    Phys. Procedia

    (2016)
  • J. Yin et al.

    High-power laser-matter interaction during laser powder bed fusion

    Addit. Manuf.

    (2019)
  • J. Yang et al.

    Role of molten pool mode on formability, microstructure and mechanical properties of selective laser melted Ti-6Al-4V alloy

    Mater. Des.

    (2016)
  • S. Luo et al.

    Selective laser melting of an equiatomic AlCrCuFeNi high-entropy alloy: Processability, non-equilibrium microstructure and mechanical behavior

    J. Alloys Compd.

    (2019)
  • K. Wei et al.

    Effect of laser remelting on deposition quality, residual stress, microstructure, and mechanical property of selective laser melting processed Ti-5Al-2.5Sn alloy

    Mater. Charact.

    (2019)
  • W. Yu et al.

    Influence of re-melting on surface roughness and porosity of AlSi10Mg parts fabricated by selective laser melting

    J. Alloys Compd.

    (2019)
  • C. Weingarten et al.

    Formation and reduction of hydrogen porosity during selective laser melting of AlSi10Mg

    J. Mater. Process. Technol.

    (2015)
  • A.M. Mancisidor et al.

    Reduction of the residual porosity in parts manufactured by selective laser melting using skywriting and high focus offset strategies

    Phys. Procedia

    (2016)
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