Defect Analysis and Bending Forming Process Optimization of Small Bend Tubes

Literature Overview

This paper by Luo Xin, Liu Jinping, Zhu Hui, Chen Jinfang, Ye Lifeng, and Fu Jun, published in Copper Industry Engineering in 2015, Issue 5, addresses a practical manufacturing challenge in the production of small-diameter copper bend tubes used in air conditioning systems. The authors from Jiangxi Copper Group and Jiangxi University of Science and Technology investigate the formation of defects during the bending process of small-diameter, thin-walled copper tubes and propose an optimized process based on finite element analysis. The work is particularly relevant to the growing trend of miniaturization in air conditioning tubing, where tube diameters continue to decrease while wall thicknesses become thinner.

Core Technical Content and Methodology

The study focuses on copper tubes with a nominal specification of 7 mm outer diameter and 0.41 mm wall thickness, which are representative of the small-diameter, thin-walled tubes used in modern air conditioning systems. The bending process involves forming a 180-degree bend using a mandrel (core rod) to support the tube wall and prevent collapse or wrinkling.

The authors employed the DEFORM finite element software to simulate the bending process and analyze the stress and strain distributions that lead to defect formation. The key process parameters investigated include the mandrel diameter, the mandrel lead-in length (advance amount), and the bending angle.

The finite element analysis provides insight into the stress states at the inner and outer walls of the bend during the forming process. The outer wall experiences tensile stresses that can lead to cracking if the strain exceeds the material's ductility limit, while the inner wall experiences compressive stresses that can cause wrinkling if the compressive strain is too high.

Defect Analysis and Mechanism

The study identifies three primary defect types that can occur during the bending of small-diameter copper tubes:

1. Outer wall cracking: This occurs when the tensile strain at the outer wall exceeds the material's fracture strain. The paper finds that outer wall cracking is most likely to occur at bending angles approaching 180 degrees, where the accumulated strain is highest. The crack initiates at the outer wall and propagates inward, which is characteristic of tensile failure in bending.
2. Inner wall wrinkling: This occurs when the compressive strain at the inner wall exceeds the critical buckling strain. The paper finds that wrinkling is most likely to occur in the bending angle range of 0 to 90 degrees, where the initial compression is established. The mandrel is essential for preventing wrinkling by providing internal support against compressive deformation.
3. Tube mouth flattening: This is a geometric defect where the circular cross-section of the tube becomes elliptical due to asymmetric deformation at the bend entry. This defect can affect the subsequent crimping or brazing operations and is a common quality issue in small-diameter tube bending.

The finite element analysis reveals that the stress distribution is highly non-uniform, with the maximum tensile stress concentrated at the outer wall of the bend and the maximum compressive stress at the inner wall. The mandrel plays a critical role in redistributing these stresses and maintaining the circular cross-section of the tube.

Process Optimization Results

Based on the finite element analysis, the authors determined the optimal mandrel parameters for the 7 mm x 0.41 mm copper tube:

• Optimal mandrel diameter: 5.80 mm, which corresponds to a mandrel-to-tube ratio of approximately 0.83. This ratio provides sufficient internal support to prevent wrinkling without creating excessive friction that could induce cracking.
• Optimal mandrel lead-in length: 2 mm, which allows the mandrel to enter the tube smoothly without causing premature deformation at the tube mouth. This lead-in length is critical for preventing the flattening defect at the bend entry.

The bending tests conducted with these optimized parameters confirmed that no outer wall cracking, inner wall wrinkling, or tube mouth flattening defects were observed. The resulting bend tubes exhibited uniform wall thickness, smooth surface finish, and correct geometric dimensions.

Engineering Practice Integration

The findings of this study have direct practical value for manufacturers of air conditioning copper tubing. The trend toward smaller diameters and thinner walls in air conditioning systems, driven by the need for compact heat exchangers and reduced refrigerant charge, makes the bending process increasingly challenging. The optimization of mandrel parameters is a straightforward and cost-effective solution to the defect problems that arise from miniaturization.

The finite element analysis approach demonstrated in this study can be adapted to other tube specifications by adjusting the material properties and geometric parameters. Manufacturers can use this methodology to develop process parameter databases for different tube sizes, reducing the need for extensive trial-and-error testing during new product development.

The study also highlights the importance of process control in small-diameter tube bending. The margin between defect-free forming and defect occurrence is narrow for thin-walled tubes, and small variations in mandrel positioning, bending speed, or material properties can lead to quality issues. Implementing in-process monitoring and statistical process control is recommended to maintain consistent quality.

Key Questions and Reflections

One question that arises is how the optimized parameters would change for different copper alloys. The study focuses on a specific copper alloy used in air conditioning tubing, but other alloys such as copper-beryllium or copper-nickel may have different mechanical properties that would require different mandrel parameters. The optimization methodology would need to be repeated for each alloy specification.

Another reflection concerns the effect of bending speed on defect formation. The study does not explicitly address the influence of bending speed, but in practice, faster bending speeds can increase the strain rate and potentially alter the material's ductility behavior. For thin-walled tubes, the interaction between bending speed and mandrel parameters should be investigated to ensure robust process design.

The study also raises the question of how the optimized parameters would perform under different ambient conditions. Temperature and humidity can affect the lubrication conditions between the mandrel and the tube, which in turn affects the friction coefficient and the stress distribution during bending. Process robustness under varying environmental conditions should be evaluated for production environments.

Study Insights and Implications

This study provides a practical and technically sound approach to solving the defect problems encountered in small-diameter copper tube bending. The use of finite element analysis to identify the root causes of defects and optimize process parameters is a methodology that can be widely adopted in the tubing industry. The specific optimized parameters of 5.80 mm mandrel diameter and 2 mm lead-in length for the 7 mm x 0.41 mm tube are directly applicable in production settings.

For engineers involved in the design and manufacturing of air conditioning components, this study underscores the importance of process optimization in the context of miniaturization trends. As tube diameters continue to decrease, the margin for error in the bending process narrows, and systematic analysis becomes essential for maintaining quality. The finite element analysis approach provides a powerful tool for predicting defect formation and optimizing process parameters without extensive physical testing, reducing development time and cost. Future work should extend this methodology to include the effects of bending speed, material variability, and environmental conditions to develop more comprehensive process windows for production use.