Robot-Based Hardfacing Additive Manufacturing Process and Methods

Literature Overview

This paper, published in Manufacturing Automation (2013, Vol. 35, No. 11, pp. 145-147) by Zhang Yu et al. from Nanjing University of Aeronautics and Astronautics, Capital Aerospace Machinery Company, and Tianjin University, explores the application of robotic hardfacing for additive manufacturing of metal components. The work addresses two critical challenges in hardfacing-based rapid prototyping: arc crater collapse and surface roughness, and proposes process solutions to overcome these limitations.

Core Technical Content

Arc Crater Collapse Problem and Solutions

Arc crater collapse is one of the most significant quality issues in hardfacing additive manufacturing. When the arc is extinguished at the end of a weld pass, the molten pool cools rapidly, and the lack of material filling the crater leads to a depression or collapse. This defect not only affects the surface quality but can also lead to porosity and reduced mechanical properties in the deposited material.

The authors propose three complementary process measures to address arc crater collapse:

1. Reducing the heat input to minimize the volume of the molten pool and thereby reduce the extent of crater formation.
2. Incorporating an arc crater filling action into the robot program instructions, where the robot pauses at the end of each pass and applies additional material to fill the crater before extinguishing the arc.
3. Slightly overlapping the welding path to ensure that each subsequent pass covers the crater from the previous pass, effectively self-correcting the defect.

These measures, when applied in combination, effectively eliminate arc crater collapse and produce deposits with consistent geometry and surface quality.

Surface Roughness Improvement

Surface roughness is another critical issue in hardfacing additive manufacturing, as it directly affects the dimensional accuracy and functional performance of the manufactured part. The authors propose electrolytic machining (electrochemical machining) as a post-processing technique to achieve smooth surfaces on hardfacing-built components. This approach is particularly advantageous because it is a non-contact process that does not introduce additional stresses or material removal issues associated with traditional machining methods.

Process Demonstration Results

The authors successfully demonstrated the process on both low-carbon steel and aluminum alloy substrates, producing hardfacing-built parts with acceptable surface quality and dimensional accuracy. The robot-based approach provides the repeatability and precision required for consistent part production, which is essential for industrial applications.

Engineering Practice Integration

The robotic hardfacing additive manufacturing approach has significant implications for the repair and manufacturing of complex-shaped components in the aerospace and industrial sectors. For example, turbine blades, impellers, and other complex geometries can be built up layer by layer using robotic hardfacing, followed by electrolytic machining to achieve the required surface finish. This approach offers cost advantages over traditional manufacturing methods, particularly for low-volume production or repair applications.

The heat input reduction strategy is consistent with the general principle in welding that lower heat input leads to finer microstructures and reduced distortion. In the context of additive manufacturing, this means that each deposited layer should have minimal dilution with the previous layer, which helps maintain the intended composition and properties of the deposited material.

The robot programming approach for arc crater filling represents an elegant solution that leverages the flexibility of robotic systems. The robot can be programmed to execute precise movements at the end of each pass, applying additional material to fill the crater while the arc is still active. This level of process control is difficult to achieve with manual welding and highlights the advantage of robotic systems in additive manufacturing applications.

Key Questions and Reflections

The study does not provide detailed information on the mechanical properties of the hardfacing-built parts, such as tensile strength, fatigue resistance, and microstructural characterization. For industrial applications, particularly in aerospace, these properties are critical for design qualification. Additionally, the study does not address the build rate and productivity of the process, which is an important economic consideration.

The applicability of the electrolytic machining post-processing technique is limited to conductive materials, which may restrict its use for certain alloy systems. Alternative surface finishing methods, such as chemical polishing or abrasive flow machining, should be considered for non-conductive or difficult-to-machine materials.

Study Insights and Implications

This paper demonstrates that robotic hardfacing additive manufacturing is a viable approach for producing metal components with acceptable quality. The key insight is that process optimization must address both in-process issues (such as arc crater collapse) and post-process issues (such as surface roughness) to achieve production-quality parts. The combination of reduced heat input, robotic arc crater filling, and path overlap represents a practical and effective solution set that can be implemented with existing robotic welding systems. Engineers considering robotic hardfacing additive manufacturing should focus on process parameter optimization and robot programming to achieve consistent, high-quality deposits.