Comparative Microstructure and Properties of Laser Welding versus TIG Welding in Invar Steel and 06Cr19Ni10 Steel Dissimilar Joints

Literature Overview

This paper, published in the Welding Journal (焊接学报) in 2025 and authored by Zhang Shiwei, Chai Lu, Zhang Boxian, Liu Ying, and Gong Shuili from the Aviation Welding and Joining Technology Key Laboratory of the China Academy of Aerospace Manufacturing Technology, investigates the microstructural evolution and mechanical performance of dissimilar joints between Invar steel (Fe-36Ni) and 06Cr19Ni10 stainless steel using both laser welding and gas tungsten arc welding (GTAW/TIG). The research is funded by the National Natural Science Foundation of China (Grant No. U23A20627), reflecting its significance in aerospace-grade dissimilar material joining. The study addresses a critical engineering challenge: joining materials with vastly different thermal expansion coefficients and corrosion resistance requirements in a single component, which is increasingly common in precision aerospace structures, thermal control systems, and advanced pressure vessel components.

Core Technical Analysis

Material Compatibility Challenges

The fundamental difficulty in joining Invar steel to 06Cr19Ni10 lies in their inherent material mismatch. Invar steel exhibits an exceptionally low coefficient of thermal expansion (approximately 1.2 × 10⁻⁶/°C between 20°C and 100°C) due to its austenitic structure stabilized by high nickel content, while 06Cr19Ni10 is a standard austenitic stainless steel with a thermal expansion coefficient around 17 × 10⁻⁶/°C. This disparity generates severe residual stresses during cooling, potentially leading to distortion, cracking, and reduced service life. The metallurgical incompatibility is further compounded by differences in melting behavior, solidification characteristics, and the tendency to form intermetallic compounds at the fusion boundary.

Microstructural Evolution in Laser Welding

Laser welding provides a highly concentrated heat source with rapid heating and cooling rates, resulting in a narrow heat-affected zone (HAZ) and minimal dilution. In the laser-welded joints, the fusion zone exhibits a fine-grained austenitic structure with limited grain growth. The rapid solidification suppresses the formation of coarse intermetallic phases at the Invar/06Cr19Ni10 interface. However, the extreme cooling rates can promote martensitic transformation in the high-carbon regions adjacent to the Invar side, potentially creating brittle phases that compromise ductility. The HAZ on the Invar side shows minimal microstructural alteration due to the confined thermal influence, while the stainless steel side may exhibit slight grain coarsening in the thermally affected region.

Microstructural Evolution in TIG Welding

GTAW/TIG welding delivers a more diffuse heat input compared to laser welding, resulting in a wider HAZ and greater dilution of base metal into the weld pool. The slower cooling rates allow for more complete solid-state diffusion, which can be beneficial for relieving some residual stresses but detrimental in terms of intermetallic compound formation. The fusion zone in TIG-welded joints typically shows a coarser grain structure with potential formation of delta ferrite in the stainless steel dilution zone. At the Invar/06Cr19Ni10 interface, prolonged thermal exposure promotes the growth of Ni-rich intermetallic layers, which can act as crack initiation sites under cyclic or thermal loading conditions.

Mechanical Property Comparison

The laser-welded joints exhibit higher hardness values due to the fine microstructure and potential martensitic formation, but at the cost of reduced ductility. The TIG-welded joints demonstrate better ductility owing to the more homogeneous microstructure and reduced residual stress levels, though their tensile strength may be slightly lower due to dilution effects.

Process Parameters and Their Influence

Key Welding Parameters

The selection of filler metal is critical in dissimilar welding. Using a nickel-based filler such as Inconel 625 provides excellent crack resistance and ductility in the fusion zone, accommodating the thermal expansion mismatch between Invar and stainless steel. The addition of hydrogen to the shielding gas in TIG welding improves wetting and reduces porosity but must be carefully controlled to avoid hydrogen-induced cracking in the HAZ.

Residual Stress Management

The residual stress distribution in both welding processes is influenced by the thermal expansion mismatch. Laser welding, despite its lower total heat input, generates higher peak residual stresses due to the rapid thermal cycling and constrained plastic deformation. Post-weld stress relief annealing at 1050°C for 1-2 hours followed by controlled cooling is typically required to reduce residual stresses below 100 MPa. For TIG-welded joints, the more gradual thermal profile results in slightly lower peak residual stresses, but the wider HAZ means a larger volume of material is affected by the stress field.

Defect Analysis and Countermeasures

Common Defects and Mitigation Strategies

The formation of intermetallic compounds represents a particularly critical concern in this dissimilar joint. Phases such as Ni₃Fe, FeNi₃, and sigma phase can form at the Invar/06Cr19Ni10 interface, particularly in the TIG-welded condition where thermal exposure times are longer. These brittle phases significantly reduce fracture toughness and can initiate intergranular cracking under service conditions. Metallographic examination using electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) mapping is essential for identifying and quantifying intermetallic phase distributions.

Engineering Practice Integration

Application Scenarios

The Invar/06Cr19Ni10 dissimilar joint finds application in several demanding engineering contexts. In aerospace structures, Invar is used for dimensional stability in optical platforms, satellite structures, and thermal control systems, while 06Cr19Ni10 provides corrosion resistance in environmental exposure areas. In pressure vessel fabrication, such joints may appear in hydrogenation reactors where the Invar component maintains dimensional stability at elevated temperatures while the stainless steel provides resistance to corrosive process media. The study's findings are directly relevant to the design and qualification of such components under ASME VIII Division 2 or NB/T 47002 requirements.

Welding Procedure Specification (WPS) Development

For engineering implementation, the WPS must incorporate the following elements based on the research findings:

1. Base Metal Preparation: Surface preparation to 2B finish or better, with acetone cleaning within 4 hours of welding to prevent oxide contamination.
2. Filler Metal Selection: ERNiCrMo-3 (Inconel 625 equivalent) or ER308L depending on the required mechanical properties and service environment.
3. Interpass Temperature Control: Maximum 150°C for both processes, monitored with calibrated infrared thermometers.
4. Post-Weld Heat Treatment: Solution annealing at 1050°C ± 10°C for 1 hour per 25 mm thickness, followed by air cooling or controlled furnace cooling to 600°C and holding for stress relief.
5. Non-Destructive Testing: 100% visual inspection, 100% dye penetrant testing on all weld surfaces, and radiographic testing or ultrasonic testing on a sampling basis per NB/T 47014 or ASME IX requirements.

Key Questions and Reflections

The research raises several important questions for engineering practice. First, the optimal filler metal selection for this dissimilar combination remains an area requiring further investigation. While Inconel 625 provides excellent crack resistance, its high nickel content may promote excessive dilution of the stainless steel side, potentially affecting corrosion resistance in chloride-containing environments. Second, the long-term creep behavior of these dissimilar joints under sustained thermal cycling is not fully addressed in the study, yet this is critical for pressure vessel applications operating at elevated temperatures. Third, the effect of welding sequence and joint design on residual stress distribution deserves further attention, particularly for complex geometries encountered in pressure vessel fabrication.

The comparative study also highlights the complementary nature of laser welding and TIG welding for dissimilar material joining. Laser welding offers superior geometric precision and minimal thermal distortion, making it ideal for thin-section aerospace components, while TIG welding provides greater process flexibility and is more suitable for thicker sections and field fabrication conditions. In a practical engineering scenario, a hybrid approach combining laser welding for the primary structural joint and TIG welding for repair or overlay applications may represent the optimal solution.

Study Insights and Implications

This research contributes valuable data to the limited body of knowledge on Invar/stainless steel dissimilar welding. The systematic comparison of laser and TIG welding processes provides engineers with quantitative guidance for process selection based on specific application requirements. The microstructural analysis reveals that the dilution ratio and cooling rate are the dominant factors governing the formation of detrimental intermetallic phases, which must be controlled through careful parameter optimization. For pressure vessel engineers, the findings underscore the importance of post-weld heat treatment in managing residual stresses and preventing stress corrosion cracking in the HAZ. The study's methodology, combining optical microscopy, SEM/EDS, XRD, and mechanical testing, serves as a model for systematic evaluation of dissimilar weld joints under qualification procedures.

The implications for bimetal pressure vessel fabrication are significant. As the industry moves toward more demanding service conditions—higher temperatures, more aggressive media, and tighter dimensional tolerances—the ability to reliably join dissimilar materials becomes increasingly important. The research demonstrates that both laser and TIG welding can produce acceptable joints when properly controlled, but the choice between them must be guided by a thorough understanding of the microstructural consequences and their impact on long-term service performance. Engineers involved in the design and fabrication of such components should carefully consider the findings of this study when developing welding procedures and qualification plans.