Process induced multi-layered Titanium – Boron carbide composites via additive manufacturing

Highlights

• First report on non-equilibrium multilayered microstructures in additively manufactured TMCs.

• Ti-B₄C TMCs were successfully fabricated using LENS, a laser directed energy deposition process.

• A combined occurrence of the reactions, $3\mathit{Ti}+B_{4}C\to 2\mathit{Ti}{B}_2+\mathit{TiC}\mathit{and}5\mathit{Ti}+B_{4}C\to 4\mathit{TiB}+\mathit{TiC}$ is observed.

• The alternatively repeating layers exhibited TiB_2, TiB, TiC, retained-B₄C and α-Ti phases.

• Schematics based on Heipel-Roper theory were presented to discuss the evolution of the layered microstructures.

Abstract

Laser engineered net shaping (LENS) processing of an in-situ Ti-B₄C composite, results in the natural formation of a novel periodically layered structure, when a mixture of Ti powders and B₄C powders are used as a feedstock. One of the layers predominantly consisted of TiB₂ and inter-dendritic TiC phases, while the other alternating layer exhibited a rather complex microstructure, comprising of TiB, TiC, partially melted-B₄C and α-Ti phases. Increasing the laser power (300–700 W) results in an increase in the height/thickness of these layers, as well as the number density of in-situ formed ceramic precipitates (TiB, TiC) in the TiB + TiC + α-Ti layer. Additionally, the Heipel-Roper theory of weld pool dynamics was employed to rationalize the unconventional microstructural evolution in these multi-layered LENS processed Ti-B₄C composites. Microhardness and wear properties revealed that among the three powers, the 700 W condition exhibited the best combined wear and hardness which can be attributed to reduced porosity, and an increase in hardness of both layered regions due to an increase in number density of precipitates in the TiB + TiC + α-Ti layer. Such AM process induced naturally layered composites open up a new avenue for design and development of hybrid materials for future engineering applications.

Introduction

Over the past three decades, several Titanium Matrix Composites (TMCs) have been evaluated for various aerospace applications, specifically the engine and airframe components. Their high specific strength/stiffness along with excellent wear and heat resistance make them ideal candidates for such applications [1], [2]. For instance, titanium aluminide based TMCs have been shown to possess remarkable high-temperature abilities, up to 760 °C, while offering nearly 50% weight savings compared to the nickel-based superalloys [2], [3]. A wide variety of ceramic particles like TiB, TiC, TiN, SiC, TiB₂, Graphene, CNTs, La₂O₃, etc. have been used for reinforcing the titanium matrix [2], [4], [5], [6], [7]. Among these, fine TiB whiskers and ultra-fine TiC particles have been identified as most suitable candidates due to their excellent chemical compatibility with titanium [8], [9]. Many conventional manufacturing techniques such as powder metallurgy, ingot castings, and melt spinning have been employed in the past for the fabrication of such TMCs [10], [11], [12], [13]. However, complexities like non-uniform distribution/agglomeration of the reinforcement particles, long processing times, substantial energy consumptions and the need for post processing make their implementation less industrially viable [14]. In light of this, additive manufacturing (AM) techniques, due to their high cost/energy efficiencies, extremely high process control, and rapid solidification rates, can be sought as an attractive alternative [15], [16].

In-situ TMCs are a class of TMCs wherein the reinforcements are synthesized during the fabrication process via chemical reaction between the titanium and the feedstock elements like boron, carbon, and nitrogen [2]. In-situ composites are known to perform better than ex-situ composites due to the excellent interfacial coherent and chemical bonding between matrix and the ceramic phase, good thermodynamic stability of the reaction product, and finer scale distribution of the reinforcements within the metal matrix [17]. These advantages can be well exploited using laser additive manufacturing [18], [19], [20], [21]. Laser additive manufacturing also opens up the avenue to process functionally graded near-net shape composites, involving a gradation in the volume fraction of the in-situ reinforcement phase(s) with site specific properties; something very difficult to achieve with conventional manufacturing [22], [23].

A large number of studies have focused on the laser additive manufacturing of Ti-TiC, Ti-TiB, and other Ti-matrix composites using powder blends of Ti with B [24], [25], [26], [27], TiB₂ [28], [29], [30], [31], TiC [22], [32], [33], N₂ gas [34] and have been largely successful in obtaining near-net-shapes with excellent mechanical properties. While these studies mainly focused on in-situ synthesis of single type of reinforcement particles (TiC, TiB, TiB₂, or TiN), Zhang et. al for the first time, have demonstrated the in-situ formation of multiple reinforcement particles by employing B₄C as a starting material [18]. The injected B₄C particles serve as a stock source for B and C, which upon dissolving, react with titanium in the laser melt pool forming ceramic phases like Ti₂C, TiB, TiB₂, TiC, etc. [35], [36], [37], [38]. However, the prediction of the type of ceramic phase evolved based on the initial concentration/loading fraction is not straightforward due to dynamic/non-equilibrium conditions prevailing in the laser melt-pool. For instance, Kühnle et al. in their study on Ti-28 (wt%) B₄C, had reported to observe Ti₂C and TiB phases [35], while in our previous study on similar composition, TiC and TiB phases with negligible amounts of TiB₂ were observed [38]. On the other hand, Pouzet et al. have shown the formation of same precipitates, TiB and TiC, for a lower loading fraction (3 wt%) of B₄C particles [36]. The melt-pool characteristics associated with the type of AM process employed and associated thermokinetics, the kind of feedstock material used, and the initial loading fraction can be considered critical in determining the final microstructure. However, AM of in-situ TMCs is still surrounded by uncertainty and besides, the literature lacks in detailed microstructural characterization and holistic understanding of the sequence of formation of ceramic phases during the fabrication process. The primary aim of this study is to investigate the microstructural evolution in a LENS (a direct laser deposition technique) processed TMC loaded with 35 wt% B₄C. Additionally, wear behavior and mechanisms were also investigated for the alloy.