Microstructure and Properties of TC4 Titanium Alloy TIG Arc Additive Manufacturing

Literature Overview

This paper by Wei Zhixiang, Li Guoxuan, Wang Yueyong, Wang Chaoning, Wang Qipeng, and Kong Jian, published in "Nonferrous Metals Engineering" (2021, Vol. 11, No. 10, pp. 14-19), investigates the microstructure and mechanical properties of TC4 (Ti-6Al-4V) titanium alloy produced by TIG arc additive manufacturing (AM). Funded by the National Defense Basic Research Program (JCKY2017206B002), this research demonstrates that TIG arc AM of titanium alloy produces a microstructure with significant heat-treated characteristics—distinct from the typical cast-like microstructure observed in arc AM of steel—achieving a room-temperature tensile strength of 998 MPa with an elongation of 8.7%.

Core Technical Concept

TIG arc additive manufacturing of titanium alloy is fundamentally different from conventional arc welding and from other AM processes such as laser powder bed fusion (LPBF) or directed energy deposition (DED) using laser sources. The key distinction lies in the thermal characteristics: titanium has relatively low thermal conductivity (approximately 6-7 W/m·K for TC4), which means that the deposited material retains significant heat during subsequent deposition layers. This retained heat creates a unique thermal history that drives microstructural evolution distinct from both wrought and cast conditions.

Microstructural Evolution During Deposition

Deposition Stage	Temperature Condition	Microstructure	Crystallographic Feature
Initial layer	High cooling rate from arc	Dendritic solidification	Strong texture, directional growth
Intermediate layers	Repeated reheat cycles	Dendrite dissolution, β grain elongation	Anisotropic, columnar β grains
Final layers	Lower peak temperature, slower cooling	α + β basket-weave transformation	Near-isotropic, fine α lamellae in β matrix

The paper's central finding is that the final microstructure of TIG arc AM TC4 alloy exhibits a basket-weave α + β structure that closely resembles the microstructure achieved through conventional solution treatment and aging of wrought TC4 alloy. This is remarkable because it means the AM process inherently produces a microstructure with properties comparable to heat-treated wrought material, without requiring post-processing.

Mechanical Properties and Comparison

Property	TIG Arc AM TC4	Wrought TC4 (as-received)	Wrought TC4 (solution treated + aged)	Cast TC4
Tensile strength (MPa)	998	950-1000	900-1050	800-900
Elongation (%)	8.7	10-14	10-13	5-8
Microstructure	Basket-weave α + β	Widmanstätten α + β	Widmanstätten α + β	Equiaxed α + β (dendritic)
Grain size (μm)	Fine (5-15)	Medium (20-50)	Medium (20-50)	Coarse (100-300)

The tensile strength of 998 MPa achieved by the TIG arc AM process is at the upper end of the wrought TC4 specification range, while the elongation of 8.7% is slightly below typical wrought values but significantly above cast values. This combination of high strength and reasonable ductility makes the AM-produced material suitable for structural applications.

Metallurgical Mechanism Analysis

The formation of the basket-weave microstructure in TIG arc AM TC4 can be explained through the following sequence:

Initial solidification: The first deposited layer solidifies from the arc-heated melt with a high cooling rate (estimated 10-50 K/s), producing a dendritic solidification structure with strong crystallographic texture aligned with the thermal gradient.
Thermal homogenization: During subsequent layer deposition, the retained heat from the TIG arc (which has lower energy density than laser sources) maintains the previously deposited material at elevated temperatures (above the β-transus of approximately 995°C for TC4). This sustained high-temperature exposure dissolves the dendritic solute segregation and allows the β grains to coarsen and elongate in the direction of heat flow.
Phase transformation: As the final layers are deposited and the part cools, the temperature drops below the β-transus. The elongated β grains transform to a basket-weave α + β microstructure. The α phase forms as fine lamellae within the β matrix, and the β phase exists in a supersaturated state with nano-scale precipitates.
Final state: The resulting microstructure exhibits near-isotropic properties because the multiple thermal cycles have randomized the crystallographic texture. The fine α lamellae provide high strength through Hall-Petch strengthening and precipitation hardening, while the β matrix provides ductility.

Comparison with Steel Arc AM

The paper highlights a critical distinction: arc AM of steel typically produces a cast-like microstructure with dendritic features, coarse grains, and segregation. In contrast, arc AM of titanium produces a heat-treated-like microstructure due to the fundamentally different thermal behavior of the two materials. This distinction has profound implications for:

Post-processing requirements: Steel AM parts typically require full heat treatment to achieve acceptable properties, while titanium AM parts may require minimal or no post-treatment.
Property prediction: The microstructure-property relationships for titanium AM can be modeled using conventional heat treatment kinetics rather than solidification-based models.
Process optimization: The key variable for titanium AM is the interpass temperature (controlled by deposition rate and layer thickness), which determines the degree of thermal homogenization.

Engineering Practice Implications

For titanium alloy component manufacturing, this research suggests several practical applications:

Repair and remanufacturing: TIG arc AM can be used to rebuild worn or damaged titanium components with properties matching or exceeding the original wrought material.
Custom geometry fabrication: Complex titanium geometries that are difficult or impossible to produce by conventional forging can be manufactured by AM with acceptable mechanical properties.
Pipeline component fabrication: For titanium pipeline components operating in aggressive environments (such as sour gas service), AM could produce custom fittings and connections with optimized microstructures.

The key process parameters for production implementation include: wire feed rate (1.5-3.0 m/min), welding current (150-250 A), travel speed (5-15 mm/min), interpass temperature control (maintained above 800°C for homogenization), and shielding gas (high-purity argon at 20-30 L/min to prevent nitrogen and oxygen pickup).

Study Insights and Reflections

This research represents a paradigm shift in how we think about arc additive manufacturing of titanium alloys. The finding that the AM process inherently produces a heat-treated microstructure—without any explicit heat treatment step—suggests that the process parameters of TIG arc AM can be tuned to achieve any desired microstructure within the TC4 phase diagram range. By controlling the interpass temperature through deposition rate, layer thickness, and arc power, engineers can target specific microstructural features and thus specific mechanical properties.

The near-isotropic properties achieved through multiple thermal cycles are particularly significant for structural applications, as they eliminate the need for orientation-dependent design considerations. The combination of high strength (998 MPa) and reasonable ductility (8.7%) places this material in a favorable position for aerospace and marine structural applications where titanium is already widely used. The research validates TIG arc AM as a viable manufacturing route for titanium components, with the added advantage of lower equipment costs compared to laser-based AM systems. This technology has the potential to significantly reduce the cost and lead time of titanium component fabrication while maintaining or improving material properties.