Elsevier

Additive Manufacturing

Volume 58, October 2022, 103023
Additive Manufacturing

Schmid factor crack propagation and tracking crystallographic texture markers of microstructural condition in direct energy deposition additive manufacturing of Ti-6Al-4V

https://doi.org/10.1016/j.addma.2022.103023Get rights and content

Abstract

Metallic additive manufacturing (AM) provides a customizable and tailorable manufacturing process for new engineering designs and technologies. The greatest challenge currently facing metallic AM is maintaining control of microstructural evolution during solidification and any solid state phase transformations during the build process. Ti-6Al-4V has been extensively surveyed in this regard, with the potential solid state and solidification microstructures explored at length. This work evaluates the applicability of previously determined crystallographic markers of microstructural condition observed in electron beam melting powder bed fusion (PBF-EB) builds of Ti-6Al-4V in a directed energy deposition (DED) build process. The aim of this effort is to elucidate whether or not these specific crystallographic textures are useful tools for indicating microstructural conditions in AM variants beyond PBF-EB. Parent β-Ti grain size was determined to be directly related to α-Ti textures in the DED build process, and the solid state microstructural condition could be inferred from the intensity of specific α-Ti orientations. Qualitative trends on the as-solidified β-Ti grain size were also determined to be related to the presence of a fiber texture, and proposed as a marker for as-solidified grain size in any cubic metal melted by AM. Analysis of the DED Ti-6Al-4V build also demonstrated a near complete fracture of the build volume, suspected to originate from accumulated thermal stresses in the solid state. Crack propagation was found to only appreciably occur in regions of slow cooling with large α+β colonies. Schmid factors for the basal and prismatic α-Ti systems explained the observed crack pathway, including slower bifurcation in colonies with lower Schmid factors of both slip systems. Colony morphologies and localized equiaxed β-Ti solidification were also found to originate from build pauses during production and uneven heating of the build edges during deposition. Tailoring of DED Ti-6Al-4V microstructures with the insight gained here is proposed, along with cautionary insight on preventing unplanned build pauses to maintain an informed and controlled thermal environment for microstructural control.

Introduction

Metallic additive manufacturing (AM) enables the production of custom geometry parts and rapid design iteration during the overall design process [1], [2], [3]. This enables manufacturing without the requirement for extensive tooling, long production lead times, or removing excess material in substantial quantities, drastically reducing unit time and production cost. Three primary types of metallic AM have come to prominence as defined by ASTM: laser powder bed fusion (PBF-LB), electron beam melting powder bed fusion (PBF-EB), or directed energy deposition (DED) [4]. Other AM build processes exist (e.g., binder jetting or cold spraying), but these do not typically employ a sequential melting process such as those previously described. Regardless of build process, metallic AM struggles to guarantee repeatable microstructural control during solidification and in solid state transformations throughout a build process [5], [6]. This results in many AM parts performing outside of specification, relegating the manufacturing technique to low-risk applications [2], [7]. Therefore, there is an urgent need to develop methods of microstructural control in metallic AM to realize the time and cost savings this manufacturing technique can bring to higher-risk applications.

Additively manufactured Ti-6Al-4V, an α+β titanium alloy, has been the focus of many microstructural studies. This has been due to the potential savings and design advantages AM can realize for the biomedical and aerospace industries where Ti-6Al-4V is commonly implemented [4]. AM Ti-6Al-4V solidifies as the high temperature body centered cubic (BCC) β-Ti phase with a {001}β texture parallel to the solidification direction [8], [9]. The {001}β texture is primarily associated with columnar solidification, described more below. The solidification direction is often aligned with the build direction, but not always [10], [11], [12]. The solidification morphology of β-Ti is dictated by the thermal gradient (G) and solidification velocity (V) of the solid-liquid interface [13]. Both factors are sensitive to build parameters such as scan strategy, feedstock, and heat source [14]. The majority of AM Ti-6Al-4V solidifies with β-Ti in a columnar dendritic morphology [6], [8], [9], [12], [14], [15], [16], [17], [18], though the scale of this microstructure is a function of the cooling rate (G x V) [13]. Generally, DED builds have the coarsest solidification microstructures, followed by PBF-LB builds and finally PBF-EB. Columnar dendritic microstructures are highly directional both at the microstructural and crystalline scales irrespective of alloy system, potentially leading to anisotropic material properties [2], [6]. Thus, previous work has pursued achieving equiaxed dendritic solidification across alloys, especially Ti-6Al-4.

Traditional inoculation techniques to achieve equiaxed dendritic solidification for aluminum and other alloy types are typically ineffective in Ti-6Al-4V. Some success has been found with oxide additions and targeted solute additions [19], [20], [21], [22], [23], [24], [25]. However, bimodal equiaxed-columnar dendritic and close to pure equiaxed dendritic solidification has been achieved through careful manipulation of build thermal conditions. Previous work discovered changing scan strategy to reduce thermal gradients and increasing solidification velocity promoted the formation of finer columnar and mixed columnar-equiaxed β-Ti in Ti-6Al-4V. This was confirmed by β-Ti reconstruction, solidification modelling, and microstructural characterization [10], [26], [27] for Ti-6Al-4V, and has been explored more broadly for general AM build processes [28], [29].

Upon cooling below ~ 980 °C, Ti-6Al-4V β-Ti transforms into the lower temperature α-Ti phase. This hexagonal close packed (HCP) phase takes on one of 12 different orientations (each known as a variant) according to the Burgers Orientation Relationship (OR) given by {0001}α || {110}β and <112̅0>α || <111>β [30]. It is worth noting at ~ 25 °C and assuming equilibrium conditions, Ti-6Al-4V is expected to consist of 90% α-Ti and 10% β-Ti by phase fraction. AM Ti-6Al-4V often exhibits lower β-Ti phase fractions. The preferential selection of one or a few α-Ti variants is known as variant selection. This can lead to anisotropic properties if sufficient variant selection is achieved or different variants in other parent grains align coincidentally [15], [16]. From this transformation process, α-Ti takes on different microstructural morphologies depending on the cooling rate and parent β-Ti grain sizes [31]. Faster cooling rates through the β-Ti transus will produce martensitic α and suppress the presence of any equilibrium high temperature β-Ti [18], [32], [33], [34], [35]. This is often seen in PBF-LB build processes. Intermediate cooling rates (~ 50 °C/sec) [31], [32] will typically produce Widmanstätten α-Ti in basketweave or elongated plate morphologies and a reduced β-Ti phase fraction. Slower solid state cooling rates will result in characteristic α+β colony microstructures Ti-6Al-4V is known for. These cooling rates are sensitive to the size of parent β-Ti grains, with smaller grains exhibiting smaller mean-free paths for diffusion, and thereby requiring faster cooling rates for non-colony microstructures [31]. Widmanstätten α-Ti is often observed in PBF-EB and DED build processes [11], [36], while colonies are most commonly found in PBF-EB builds [10], [18]. It is important to note these generalizations are not all inclusive, as PBF-EB builds can still produce martensitic microstructures and DED colony structures when thermal conditions are sufficiently altered. Finer parent β-Ti grain sizes have also been linked to the formation of α+β colony microstructures. This was attributed to the reduced mean-free path for diffusion in smaller grains enabling closer to equilibrium partitioning [10].

Preferential orientation (also known as crystallographic texture) of α-Ti crystals in AM adds another layer of complexity to the microstructural evolution of AM Ti-6Al-4V. Faster cooling rates typically reduce α-Ti textures, while slower cooling rates can produce up to 4x the preferential orientations in the solid state microstructure [10]. Texture has been shown to change minimally as a function of build height, but local thermal changes can still result in deviations in preferred orientations of either parent or product phases. Considering the Burgers OR is almost always followed during the βα transformation, the orientation and size of the as-solidified β-Ti likely also influences the product α-Ti texture.

The combination of variable solidification and solid state microstructures with changes in crystallographic texture demonstrate how challenging microstructural control is in AM builds of Ti-6Al-4V. In a step towards this goal, previous work on PBF-EB Ti-6Al-4V found specific preferential α-Ti orientations was associated with both changes in parent β-Ti grain size and the morphology of the α-Ti microstructure [10]. The presence of a {011̅2}α fiber texture parallel to the build direction was found consistently in the presence of finer parent β-Ti grains and with α+β colonies formed by diffusion from slower cooling rates. Moreover, the intensity of this fiber texture was found to correspond directly to the quantity of colonies in the solid state microstructure and inversely to the parent β-Ti grain size. Conversely, a {112̅0}α fiber texture parallel to the build direction was observed for faster cooling rate solid state microstructures. It was suggested the {011̅2}α fiber texture could be used as a marker of microstructural condition (e.g, estimate the size of as-solidified β-Ti and the type of solid state microstructure present) via techniques such as X-ray diffraction (XRD), bypassing the need for extensive characterization work in validating build quality. However, it was not clear whether the {011̅2}α fiber corresponded to both parent β-Ti grain size and solid state microstructure, or if it was a specific marker of microstructural condition to PBF-EB build processes [10].

This work expands the previously mentioned PBF-EB efforts by exploring the relationship between crystallographic texture and microstructure in DED Ti-6Al-4V. DED build processes are typically employed for larger build volumes and higher throughputs, making this AM variant more applicable to on-demand structural applications than PBF-EB or PBF-LB. DED also enables the elucidation of if the {011̅2}α fiber texture parallel to the build direction is a hallmark of finer β-Ti grains and/or diffusional microstructures, and if this marker of microstructural condition can be used for other AM processes besides PBF-EB.

Section snippets

Material production

DED Ti-6Al-4V samples were extracted from the larger bowl build volume observed in Fig. 1a via electrical discharge machining (EDM). The full build geometry was produced on a modified1

Microstructural characterization

Microstructural characterization of the four primary DED Ti-6Al-4V specimens identified four dominant microstructural morphologies throughout the build volume. These included fine and coarse Widmanstätten basketweave morphologies (Figs. 2a and 2b respectively). A distinct categorization between these two microstructures was established considering the different cooling rates required to form the distinct scales of the same morphology. Two other morphologies, elongated “bow-tie” colonies (Fig. 2

Layered microstructures

The layered microstructure observed for the morphologies in Fig. 2a-d is thought to originate from differences in solid state cooling rates in layers below the current deposition [43]. Immediately after deposition, material just below the deposited layer will be heated above the β-Ti transus, completely removing any established α+β microstructures. Following the application of the most recent layer, this material will rapidly cool into a fine basketweave morphology (dark green region in Fig. 15

Conclusions

By employing centimeter scale EBSD, neutron diffraction, SEM characterization, and orientation simulations, the following findings were reported in this work.

  • 1)

    The presence of as-solidified fiber textures along the solidification direction are indicators of parent grain size, while the specific orientation in each solid state phase fiber identifies the type of solid state microstructure present. A mechanistic explanation is proposed explaining how the 011̅2α and {112̅0}α fibers can only exist

CRediT authorship contribution statement

Alec I. Saville: Writing – review & editing, Writing – original draft, Visualization, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Jake T. Benzing: Writing – review & editing, Data curation. Sven C. Vogel: Software, Methodology, Data curation. Jessica Buckner: Writing – review & editing, Supervision, Investigation, Conceptualization. Collin Donohoue: Writing – review & editing, Resources, Conceptualization. Andrew B. Kustas: Writing –

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.

Acknowledgements

AIS gratefully acknowledges the National Science Foundation Graduate Research Fellowship, USA, under Grant No. 2019260337 for supporting this work. All authors thank the Center for Advanced Non-Ferrous Structural Alloys (CANFSA), a National Science Foundation Industry/University Cooperative Research Center (I/UCRC), [Award No. 1624836] at the Colorado School of Mines (Mines), USA, for support completing this work. Scanning electron microscopy work completed here was supported by the National

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