CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of TC4 Titanium Alloy TIG Arc Additive Manufacturing
Literature Overview
This 2021 study published in Nonferrous Metals Engineering, conducted by researchers from Nanjing University of Science and Technology and the China Shipbuilding Industry Corporation, investigates the microstructure and mechanical properties of TC4 titanium alloy (Ti-6Al-4V) produced by TIG arc additive manufacturing. Funded by the National Defense Science and Technology Plan (JCKY2017206B002), this work addresses a rapidly growing area of manufacturing technology with significant implications for aerospace, marine, and biomedical applications. The study provides critical data on the relationship between TIG-based additive manufacturing process parameters and the resulting material properties.
Process Description and Parameter Optimization
TIG arc additive manufacturing involves the sequential deposition of metal layers using a TIG welding arc as the heat source, with wire feed providing the filler material. Unlike conventional TIG welding, where the goal is to join two existing components, additive manufacturing builds up material layer by layer to create a net-shape or near-net-shape component. The process parameters differ significantly from conventional TIG welding and require careful optimization.
Key Process Parameters
| Parameter | Typical Range | Effect on Microstructure |
|---|---|---|
| Welding current | 100-250 A | Controls heat input and dilution |
| Wire feed rate | 200-600 mm/min | Controls deposition rate and layer thickness |
| Travel speed | 5-30 mm/min | Controls cooling rate and grain morphology |
| Shielding gas flow | 10-20 L/min | Prevents oxidation of titanium |
| Layer height | 0.5-2.0 mm | Affects residual stress and distortion |
| Scan strategy | Single pass, multi-pass | Controls thermal history and defects |
The cooling rate in TIG arc additive manufacturing is significantly lower than in laser or electron beam-based processes, typically in the range of 10 to 100 K/s. This slower cooling rate promotes the formation of coarse acicular alpha-beta microstructure, which is characteristic of the as-deposited condition of Ti-6Al-4V. The alpha phase forms as needle-like structures within the beta matrix during solidification and subsequent cooling, with the alpha phase fraction increasing with each successive layer due to the cumulative thermal history.
Microstructural Characteristics
As-Deposited Microstructure
The as-deposited microstructure of TIG arc additive manufactured TC4 exhibits several distinctive features:
- Coarse acicular alpha phase with widths of 2 to 8 micrometers, significantly coarser than the lamellar alpha in conventionally wrought material
- Widmanstatten pattern of alpha plates within the beta grain matrix
- Prior beta grain boundaries that are clearly visible due to alpha precipitation along the boundaries
- Columnar grain structure in the build direction, with grains growing epitaxially from layer to layer
- Possible presence of retained beta phase in regions with slower cooling rates
The coarse acicular microstructure results from the relatively slow cooling rates inherent to the TIG process. The high thermal mass of the deposited material and the large heat-affected zone contribute to lower cooling rates compared to laser-based additive manufacturing, where cooling rates can exceed 1000 K/s.
Microstructural Evolution with Thermal History
Each successive layer in the additive manufacturing build undergoes a thermal cycle that modifies the microstructure of previously deposited layers. The thermal history can be characterized as follows:
| Layer Position | Peak Temperature | Cooling Rate | Microstructure |
|---|---|---|---|
| Newly deposited | 1600-2000°C | 10-100 K/s | Coarse acicular alpha |
| One layer below | 600-800°C | 1-10 K/s | Partially transformed alpha-beta |
| Two or more layers below | 400-600°C | <1 K/s | Recrystallized alpha-beta |
This microstructural heterogeneity through the build height is a significant challenge for TIG arc additive manufacturing, as it leads to property variation and potential anisotropy in the final component.
Mechanical Properties and Performance
The mechanical properties of TIG arc additive manufactured TC4 are characterized as follows:
| Property | As-Deposited | After Annealing (700°C/2h) | Conventional Forged |
|---|---|---|---|
| Yield strength (MPa) | 950-1100 | 850-950 | 880-950 |
| Tensile strength (MPa) | 1050-1200 | 950-1050 | 950-1100 |
| Elongation (%) | 8-12 | 12-15 | 10-14 |
| Hardness (HV) | 380-420 | 340-380 | 350-380 |
The as-deposited material exhibits higher strength but lower ductility compared to conventionally wrought TC4, a result of the coarse acicular microstructure and residual stress. Annealing at 700°C for 2 hours significantly improves ductility while maintaining adequate strength, as the alpha phase undergoes coarsening and spheroidization.
Anisotropy and Directional Properties
TIG arc additive manufactured components exhibit significant mechanical anisotropy. The tensile strength and elongation in the build direction (Z-axis) are typically lower than in the horizontal direction (X-Y plane), with differences of 10 to 20 percent. This anisotropy is attributed to the columnar grain structure and the preferential orientation of alpha plates along the build direction.
Defects and Countermeasures
The study identifies several common defects in TIG arc additive manufactured TC4:
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Porosity | Gas entrapment, keyhole instability | Optimize gas flow, reduce wire feed rate |
| Lack of fusion | Insufficient heat input | Increase current, reduce travel speed |
| Cracking | Residual stress, hydrogen pickup | Pre-heat, post-weld annealing |
| Distortion | Thermal stress accumulation | Fixture design, scan strategy optimization |
| Surface roughness | Arc instability, wire oscillation | Stabilize arc, use oscillating torch |
Study Insights and Engineering Implications
This study provides essential data for engineers evaluating TIG arc additive manufacturing as a viable production technology for titanium alloy components. The key findings are that while the as-deposited microstructure and properties are acceptable for many applications, post-processing heat treatment is essential for achieving optimal mechanical performance. The coarse acicular microstructure, while providing high strength, compromises ductility and fatigue resistance, necessitating annealing to achieve a balanced property set.
For pressure vessel and marine applications, where titanium components are used in corrosive environments, the additive manufacturing route offers the advantage of producing near-net-shape components with reduced material waste. However, the microstructural heterogeneity and potential anisotropy must be carefully considered in the design and qualification of additively manufactured components.
The study also highlights the importance of process parameter optimization in achieving consistent quality. Unlike conventional welding, where the process parameters are optimized for a specific joint configuration, additive manufacturing requires parameters that produce acceptable quality across multiple layers with varying thermal histories. This complexity demands rigorous process development and qualification protocols.
In conclusion, this literature provides a comprehensive technical foundation for understanding the microstructure-property relationships in TIG arc additive manufactured TC4, offering practical guidance for engineers seeking to apply this technology to demanding aerospace and marine applications.