Additive Manufacturing of Ti-6Al-4V Using Micro-Plasma Arc Surfacing Technology

Literature Overview

This paper by Luo Zhen, Zhang Yu, and Jia Peng, published in the journal Welding (2016, Issue 4, pp. 13–16), presents the development and experimental validation of an additive manufacturing system based on micro-plasma arc surfacing (MPAS) for Ti-6Al-4V titanium alloy components. The research is supported by the Ministry of Education Doctoral Point Fund (20130032110004) and represents a significant advancement in the application of plasma arc technology to titanium alloy fabrication. The authors from Tianjin University and Civil Aviation University of China have constructed a comprehensive system integrating robotic deposition, multi-path atmospheric protection, and visual monitoring.

System Architecture and Process Design

The additive manufacturing system comprises three principal subsystems: a dedicated 6-degree-of-freedom (6-DOF) surfacing robot, a triple-path protective atmosphere system, and a supporting visual monitoring system. The 6-DOF robot provides the spatial flexibility necessary for complex three-dimensional deposition geometries, enabling the construction of near-net-shape components without the need for extensive post-machining.

The triple-path atmospheric protection is a critical innovation for titanium alloy processing. Titanium is notoriously reactive with oxygen, nitrogen, and hydrogen above approximately 400°C, and even trace contamination can severely degrade mechanical properties. The three protection paths are: (1) gas shielding at the welding gun nozzle, (2) gas trailing shield (gas drag cover) for post-arc protection of the cooling weld pool, and (3) fixture jig atmosphere protection for the surrounding workpiece area. This multi-layered approach addresses the well-documented challenge of maintaining inert conditions during plasma arc deposition of reactive alloys.

Process Configuration Summary

Microstructural Characterization

The as-deposited overlay layers exhibit coarse columnar grains with growth direction perpendicular to the welding direction. This columnar morphology is a direct consequence of the directional heat flow during plasma arc deposition, where the thermal gradient (G) is oriented opposite to the solidification front velocity (R), resulting in a high G/R ratio that favors columnar growth. The basket-weave microstructure observed within the grains is characteristic of the Widmanstätten α' phase in Ti-6Al-4V, formed by the α → β → α + β phase transformation sequence during the rapid heating and cooling cycles of plasma arc processing.

The distinct layer-by-layer morphology visible between successive deposition passes is another expected feature of additive manufacturing. Each layer represents a discrete thermal cycle, and the interfaces between layers may exhibit differences in grain orientation, residual stress state, and potentially microsegregation patterns. These interlayer features are critical for understanding the anisotropy of mechanical properties in additively manufactured titanium components.

The authors specifically note that oxidation causes significant reductions in both strength and ductility of titanium alloy workpieces. This observation underscores the paramount importance of atmosphere protection in plasma arc additive manufacturing of titanium. Even minor oxygen pickup—on the order of 0.1 wt% above the nominal 0.2 wt% oxygen content of Ti-6Al-4V—can reduce elongation by 50% or more and increase yield strength through solid solution strengthening, resulting in a brittle microstructure unsuitable for structural applications.

Mechanical Properties and Process Optimization

The alternating constant-current and pulse deposition mode is a sophisticated approach to heat input management. Constant-current mode provides continuous, stable energy input suitable for building up material volume, while pulse mode allows for brief high-energy bursts separated by low-energy or zero-current intervals. This alternation enables several process advantages: (1) controlled peak temperatures that prevent excessive grain growth, (2) inter-pulse cooling that reduces overall heat accumulation, (3) improved arc stability through periodic arc restart, and (4) potential for tailored microstructure through controlled thermal cycling.

For titanium alloy applications, the heat input must be carefully balanced. Excessive heat input promotes the formation of coarse acicular α' and Widmanstätten α, which are associated with reduced ductility and fatigue resistance. Insufficient heat input may result in incomplete melting, lack of fusion between layers, and poor metallurgical bonding. The optimal heat input window for plasma arc deposition of Ti-6Al-4V is typically in the range of 0.5–2.0 kJ/mm, depending on the specific application requirements.

Engineering Practice and Application Outlook

The development of this MPAS-based additive manufacturing system has significant implications for aerospace and aviation industries, where titanium alloy components are widely used for their excellent strength-to-weight ratio. Repair and remanufacturing of turbine blades, compressor disks, and structural brackets represent near-term applications where the system's capabilities could provide substantial economic and logistical benefits. The ability to deposit titanium material in-situ on existing components eliminates the need for complete part replacement, reducing both cost and lead time.

From a quality assurance perspective, the integration of visual monitoring into the deposition system enables real-time defect detection and process parameter adjustment. This closed-loop control capability is essential for ensuring the repeatability and reliability of additively manufactured components, particularly for safety-critical aerospace applications governed by standards such as AMS 7000 series and ASTM F2924.

The columnar grain structure observed in the as-deposited state may require post-processing through hot isostatic pressing (HIP) or solution treatment and aging (STA) to achieve equiaxed α + β microstructure for optimal fatigue and fracture toughness properties. The STA treatment typically involves heating to the β transus temperature (approximately 995°C for Ti-6Al-4V), holding for 1–2 hours, followed by air cooling and aging at 540°C for 4 hours.

This research demonstrates that micro-plasma arc surfacing is a viable and promising technology for the additive manufacturing of titanium alloys. The systematic approach to atmosphere protection and the intelligent alternation of deposition modes represent meaningful engineering innovations that address the fundamental challenges of reactive metal processing. Future work should focus on scaling up deposition rates, optimizing multi-layer thermal history for improved fatigue performance, and developing comprehensive qualification procedures for aerospace certification.