TIG Arc Additive Manufacturing of TC4 Frame Structures

Literature Overview

This paper by Ma Qian, Ma Shuyuan, Liu Changmeng, and Chen Guangsen, published in Heat Processing Technology in 2018 (Vol. 47, No. 1, pp. 189-194), investigates the application of TIG arc additive manufacturing (AM) technology to the fabrication of typical TC4 (Ti-6Al-4V) frame structures. The research is supported by the National Natural Science Foundation of China (Grant 51505033), Beijing Natural Science Foundation (Grant 3162027), and Beijing Institute of Technology Outstanding Young Teacher Funding Program (Grant 2015YG 0302). The work was conducted at Beijing Institute of Technology and the Academy of Armament Technology.

Process Development and Parameter Optimization

The study employs a systematic approach to develop the TIG arc AM process for TC4 frame structures. The first phase involves investigating the effect of process parameters on single-track macroscopic morphology using the controlled variable method. This approach isolates the influence of each parameter (current, voltage, travel speed, wire feed speed, shielding gas flow rate) on the resulting track geometry, allowing the establishment of stable single-track formation parameter ranges.

The controlled variable method is a fundamental experimental technique in welding research. By varying one parameter at a time while keeping others constant, the researchers can clearly identify the influence of each parameter on the outcome. This approach is particularly important in additive manufacturing where process stability is critical for the successful fabrication of complex geometries.

Key Technical Innovation: Rotational Approach for Isotropic Forming

The most significant technical contribution of this paper is the discovery that rotating the welding torch and wire feed rod assembly by 10 degrees during the fabrication of square frame structures achieves isotropic and stable forming. This simple yet effective solution addresses a well-known challenge in arc AM: the anisotropy of deposited material properties and geometry due to the directional nature of the welding process.

In conventional arc AM, the heat input direction and solidification pattern create inherent anisotropy in the deposited layers. This anisotropy manifests as differences in mechanical properties, residual stress distribution, and dimensional accuracy depending on the orientation relative to the build direction. The 10-degree rotation strategy effectively distributes the thermal and mechanical effects more uniformly, resulting in more consistent properties throughout the structure.

Path Planning for Complex Geometries

For the typical "田" character (a grid-like frame structure with cross-intersections), the authors developed a specialized path planning strategy. At the cross-intersection positions, arc-shaped overlap paths were employed to ensure good surface quality at these critical locations. Cross-intersections in frame structures are stress concentration areas and require particular attention to weld quality and geometric accuracy.

The path planning approach demonstrates an understanding of the thermal accumulation effects in multi-pass AM. By using arc-shaped overlaps at intersections, the researchers minimize the risk of excessive heat input and thermal distortion at these locations. This is particularly important for titanium alloys, which have low thermal conductivity and are prone to distortion when subjected to concentrated heat inputs.

Process Parameters and Quality Considerations

Engineering Practice and Quality Control

For engineers implementing TIG arc AM of TC4 frame structures, several quality control measures are essential. First, the shielding gas coverage must be maintained at all times to prevent nitrogen pickup, which is particularly detrimental to titanium alloys. Even small amounts of nitrogen can significantly embrittle the deposited material and compromise its mechanical properties.

Second, the inter-pass temperature should be carefully controlled. While some preheating can help reduce residual stresses and prevent cracking, excessive temperatures can lead to grain coarsening and reduced mechanical properties. The optimal inter-pass temperature window for TC4 AM should be established through systematic experimentation and verified through microstructural analysis.

Third, the residual stress state of AM parts should be characterized and, if necessary, relieved through appropriate post-processing such as stress relief annealing or solution treatment followed by aging. The residual stresses in AM parts can be significant and may affect dimensional stability and fatigue performance.

Study Insights and Implications

This paper demonstrates that TIG arc AM is a viable technology for fabricating complex titanium alloy structures with good surface quality and controlled geometry. The rotational approach for achieving isotropic forming is a practical and elegant solution to a fundamental challenge in arc AM. The path planning strategy for complex geometries provides a methodology that can be adapted to other structural configurations.

The work also highlights the importance of process development methodology in AM. The systematic approach of first establishing single-track parameters and then extending to multi-track and complex geometries is a sound engineering methodology that can be applied to other AM processes and materials. The combination of experimental investigation and path planning optimization represents a comprehensive approach to process development.

Overall, this research contributes valuable knowledge to the growing field of arc AM for titanium alloys. The findings provide a foundation for further development of AM process windows, quality models, and design guidelines for AM-produced titanium alloy components. The practical focus on frame structures, which are common in aerospace and defense applications, enhances the relevance and applicability of the research findings.