Research paperMechanisms on inter-track void formation and phase transformation during laser Powder Bed Fusion of Ti-6Al-4V
Introduction
Laser Powder Bed Fusion (PBF) is a major Additive Manufacturing (AM) process involving rapid scanning of the laser heat source on prepacked powder feedstocks. Solid deposit is generated from the powder particles which experience a melting and solidification process. The quality of the deposited part largely depends on various accompanied issues such as the formation of voids and undesired phases in the final microstructure [[1], [2], [3]]. Massive pores in laser PBF components were reported under a large variety of process conditions of different alloys [[4], [5], [6]]. Moreover, the existence of undesired martensite phase was widely observed in as-deposited Ti-6Al-4V part produced by laser PBF [[7], [8], [9]]. A good understanding of the formation mechanisms of defects and microstructure of laser PBF parts would be thus helpful to the prediction and control of the final product quality.
Porosity is a major type of laser PBF defects and can be classified into several categories considering the formation sources. For example, lack of fusion of adjacent tracks and layers [10], keyhole induced pores [11], originally trapped voids in powder feedstocks [12,13], and vaporization of specific alloying elements during the printing process [14] are all causative factors. The origins of pores in PBF could be broader than that of other AM approaches such as wire arc Directed Energy Deposition (DED) considering the supplied forms of the feedstock materials. During PBF, large number of gaps exist among the pre-packed solid powders. These gaps need to be removed from the molten pool during the melting and solidification process, which otherwise would remain as trapped voids in the selectively melted zone.
Formation of voids in irregular shapes were widely reported for low heat input laser PBF conditions [[15], [16], [17]]. Stef et al. observed pores with various morphologies including near-spherical shapes located in the overlap region of adjacent tracks [6]. Note that laser PBF is a highly transient process and the local metallurgy environment around the molten pool largely depends on several critical process variables such as laser power, scanning speed, and hatch spacing between adjacent tracks [1]. The characterization of porosity in the as-deposited samples by 3D CT is an important approach [6,15], but not sufficient when examining the temporal variations of the pores as it is a post process technique. The real time observations of porosity variations through experimental approaches such as synchrotron XRD [4,11,18] and synchrotron micro CT [15] are effective ways to study the evolution of pores. However, the comprehension of the formation mechanisms of the voids necessitate more critical information such as the accompanied 3D transient temperature and liquid metal flow distributions.
To this end, comprehensive phenomenological models need to be developed to examine the melting and solidification behaviors of the powder feedstocks and the resultant evolution of the voids considering the accompanied complex physical conditions. Several related researches have been reported. For example, Mukherjee et al. examined the formation of lack of fusion defects during PBF for various alloys through a multi-track multi-layer numerical model [19,20]. Khairallah et al. [21] explored the formation of pores during single-track laser PBF using ALE3D. Panwisawas et al. [22] and Gu et al. [23,24] reported open pores observed from the deposit surface when the scanned tracks were insufficiently melted. Tang and Tan et al. explored the formation of keyhole induced pores during laser PBF of SS316 [25,26]. Bayat et al. studied the porosity evolution during laser PBF of Inconel 718 using a numerical model developed on Flow-3D [27]. However, a systematic study on the formation mechanisms of the inter-track voids and the comprehensive examination of their 3D and 2D dimensions and morphologies through a comprehensive powder-based numerical model is still absent.
Ti-6Al-4V is the mostly used titanium alloy in laser PBF. One major issue during PBF of Ti-6Al-4V is the formation of martensite in the as-built parts under various process conditions [17,[28], [29], [30]]. The as-deposited laser PBF components composing mainly martensite were characterized by high strength but relatively low ductility [31]. For example, the yield strength can achieve over 1300 MPa [32], but with low tensile elongation of 4.54% which was well below the minimum threshold of 10% suggested for critical structural applications [32]. Thus, laser PBF part with mainly martensitic structure is particularly undesirable for engineering applications requiring toughness in high-cycle fatigue operations [33]. Transformation from β phase to fine martensite α' occurs during the fast cooling process of laser PBF. The formation and decomposition of martensite in the as-built Ti-6Al-4V parts were reported in various process conditions though [7,34], the mechanisms remain unclear due to the complexity of the thermal cycles. Quantitative examinations of the multiple thermal cycles and the cooling rates for various laser PBF conditions are thus of great significance. Measurement of the temperature fields is difficult due to the fast scanning speed of the heat source, the minuscule dimensions of the molten pool, and the steep temperature gradient in the solid deposit [35,36]. Alternatively, transport phenomena based numerical models could be used to compute the complex physical process of laser PBF and further examine the temporal variations of the 3D temperature fields.
In this work, a comprehensive numerical model was developed to compute the transport phenomena during laser PBF of Ti-6Al-4V. The model is capable of examining the heat transfer, liquid metal flow, formation of inter-track voids, and full temperature-time histories. The comprehensive results obtain from the powder-based numerical model demonstrate that variously shaped voids including spherical ones could be caused by insufficient heating and melting at excessively large hatch spacings. The cooling rates were obtained from the computed thermal cycles, which were further used to assess the metallurgical conditions for the solid-state phase transformations from the β phase to various α phase variants. The novel scientific discoveries from this research is helpful to the understanding of formation of voids with various morphologies and dimensions, and the estimation of the potential phase transformations and the resultant microstructure during laser PBF of typical dual phase titanium alloys.
Section snippets
Methodologies
In this work, exemplary cases of single-track and multi-track Ti-6Al-4V builds were deposited through laser PBF. A powder-based phenomenological model was developed to examine the temperature and velocity fields, the molten pool and deposit profiles, and the formation of inter-track voids. The model was validated by comparing the computed results against the experimental data. The experimental conditions and the development of the numerical model for laser PBF of Ti-6Al-4V are presented in this
Results and discussions
In this section, the heat transfer and fluid flow during laser PBF for both single- and multi-track cases will be presented first, which serves as the basis for subsequent studies. The formation, examination, and elimination of inter-track voids due to insufficient heating and melting of the powder feedstocks will be discussed in the following part. Finally, the monitored thermal cycles will be discussed, with specific focus on the cooling rates in the temperature range for solid state phase
Conclusions
A comprehensive numerical model was developed in this work to compute the complex transport phenomena during laser Powder Bed Fusion (PBF) of Ti-6Al-4V. The transient temperature and velocity fields during single- and multi-track laser PBF were computed from the numerical model considering powder packing, melting, and solidification. Critical metallurgical variables including the molten pool characteristics and thermal cycles were obtained. The cooling rates in various PBF process conditions
CRediT authorship contribution statement
H.L. Wei: Conceptualization, Methodology, Writing - original draft, Supervision, Funding acquisition. Y. Cao: Data curation, Visualization. W.H. Liao: Funding acquisition. T.T. Liu: Project administration, Funding acquisition, Formal analysis.
Declaration of competing interest
One of the authors of this article is part of the Editorial Board of the journal. To avoid potential conflict of interest, the responsibility for the editorial and peer-review process of this article lies with a different Editor. Furthermore, the authors of this article were removed from the peer review process and had no, and will not have any access to confidential information related to the editorial process of this article.
Acknowledgement
We acknowledge the support from The National Natural Science Foundation of China (No. 51805267), The Natural Science Foundation of Jiangsu Province (No. BK20180483), and The Fundamental Research Funds for the Central Universities (No. 309181A8803).
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