Micro-Deformation Behavior of SUS316L/TA18 Bimetallic Composite Tube Bending Based on Real Microstructure

Literature Overview and Research Background

This study investigates the micro-deformation behavior during the bending forming process of SUS316L stainless steel / TA18 titanium alloy bimetallic composite tubes, with particular emphasis on how the real microstructure of each layer influences the deformation response. In the context of nuclear power, petrochemical, and aerospace industries, bimetallic composite tubes are increasingly demanded where a corrosion-resistant inner lining must be combined with a structurally strong outer shell. The SUS316L/TA18 combination is particularly attractive in applications involving aggressive chemical environments coupled with cryogenic or high-temperature service conditions. The research addresses a critical gap in the forming technology of such composite tubes: conventional forming analyses often treat the two layers as homogeneous materials, neglecting the actual microstructural features that govern deformation heterogeneity.

Core Technical Points and Microstructural Analysis

The study employs a multi-scale approach to characterize the real microstructure of both the SUS316L stainless steel and TA18 titanium alloy layers. For SUS316L, the microstructure typically consists of austenitic grains with delta ferrite phases distributed along grain boundaries, which directly affects the plastic deformation capacity and work hardening behavior. For TA18, the microstructure is characterized by alpha-beta phases, where the volume fraction and morphology of the alpha phase significantly influence the forming limit and anisotropy.

Key Microstructural Parameters

Parameter	SUS316L Layer	TA18 Layer
Grain size	30-60 μm	15-40 μm
Phase composition	Austenite + δ-ferrite	α + β phases
δ-ferrite content	3-8%	N/A
α-phase fraction	N/A	40-70%
Tensile strength	550-700 MPa	550-650 MPa
Elongation	40-55%	15-25%
Young's modulus	193-200 GPa	90-110 GPa

The significant difference in Young's modulus between the two materials (approximately a factor of 2) is the primary driver of deformation mismatch during bending. This modulus disparity means that during bending, the TA18 layer experiences significantly lower stress levels compared to the SUS316L layer for the same imposed strain, leading to asymmetric stress distributions through the wall thickness.

Deformation Mechanisms and Bending Behavior

During the bending process, the outer surface of the bend undergoes tensile deformation while the inner surface experiences compressive deformation. In a homogeneous tube, the neutral axis coincides with the geometric center. However, in the SUS316L/TA18 composite tube, the neutral axis shifts toward the stiffer SUS316L layer due to the modulus mismatch. The study reveals that this shift is not constant throughout the bending process but evolves dynamically as plastic deformation initiates and propagates through the wall thickness.

Deformation Characteristics at Different Strain Levels

Strain Level	SUS316L Behavior	TA18 Behavior	Interface State
Elastic regime	Linear stress-strain	Linear stress-strain	No separation
Yield onset	SUS316L yields first	TA18 remains elastic	Stress redistribution
Plastic deformation	Work hardening active	Elastic-plastic transition	Interfacial shear develops
Large deformation	Necking risk increases	Potential cracking	Delamination risk

The interfacial bonding quality becomes critically important during large deformation. The study demonstrates that the bond interface, whether produced by explosion welding, roll bonding, or flash butt welding, must possess sufficient shear strength to withstand the differential deformation between the two layers. A weak interface can lead to interfacial delamination, which is a catastrophic failure mode in pressure-containing components.

Engineering Practice Implications

From a fabrication standpoint, the findings have direct implications for the design of bending operations on bimetallic composite tubes. The minimum bend radius must be increased compared to single-material tubes to accommodate the higher deformation mismatch. The study suggests that for SUS316L/TA18 tubes with a wall thickness ratio of approximately 60:40 (SUS316L:TA18), the minimum bend radius should be at least 3.5D to 4.0D, compared to 1.5D to 2.0D for homogeneous tubes.

Process Recommendations

The TA18 layer should be positioned on the inner (compressive) side of the bend to minimize the risk of cracking, since titanium alloys have lower ductility compared to austenitic stainless steels.
Multi-pass bending with intermediate annealing may be beneficial for tight bend radii, allowing partial stress relief between passes.
The interface bond strength must be verified to exceed 150 MPa in shear to ensure reliable performance during forming.
Post-bending non-destructive examination (particularly ultrasonic testing with contact or immersion methods) should focus on the interface region for potential delamination.

Study Insights and Reflections

The most valuable insight from this study is the recognition that "real microstructure" matters significantly more than idealized material models in predicting forming behavior. In engineering practice, we often rely on average material properties from standard test specimens, but the actual microstructural state in a production tube—affected by manufacturing history, heat treatment, and rolling direction—can dramatically alter the forming response. The study also highlights the importance of interface engineering: the bond quality is not merely a static property but must be evaluated under the specific deformation conditions of the intended forming operation. This perspective should be incorporated into the qualification procedures for bimetallic tube forming operations, particularly for critical applications in nuclear and chemical processing industries. The work represents a meaningful step toward physics-based forming simulations for composite tubes, which could eventually replace empirical trial-and-error approaches in production settings.