Microstructure and Mechanical Properties of Laser Welded and TIG Welded Joints Between Invar Steel and 06Cr19Ni10 Stainless Steel

Overview and Technical Background

The paper published in Transactions of the China Welding Institution (2025, Vol. 46, No. 11) by Zhang Shiwei and colleagues from the Aviation Welding and Joining Technology Key Laboratory at the China Aviation Manufacturing Technology Research Institute investigates the welding of a dissimilar material joint between Invar steel (4J36) and 06Cr19Ni10 austenitic stainless steel. This research was funded by the National Natural Science Foundation of China (Project No. U23A20627) and addresses a critical engineering challenge in aerospace manufacturing where dissimilar material joints must accommodate differential thermal expansion while maintaining structural integrity.

Invar steel, with its exceptionally low coefficient of thermal expansion (approximately 1.2 x 10^-6 /K between 20°C and 100°C), is widely used in aerospace structures requiring dimensional stability. However, joining Invar steel to austenitic stainless steels presents unique metallurgical challenges due to the significant differences in thermal conductivity, thermal expansion coefficient, and solidification behavior between the two materials. The selection of welding process is therefore critical, as it directly determines the thermal cycle, solidification microstructure, and residual stress state of the joint.

Comparative Welding Process Parameters and Geometry

The authors systematically compared laser welding and TIG welding for this dissimilar joint, selecting process parameters that represent practical industrial capabilities:

The laser welding parameters represent a high-power fiber laser system operating at a relatively high travel speed, which produces a narrow weld bead with deep penetration and minimal heat-affected zone. The TIG welding parameters represent a conventional manual or semi-automatic process with moderate current and voltage, producing a wider weld bead with more distributed heat input.

Microstructure Analysis and Comparison

The microstructural differences between the two welding processes are profound and directly attributable to the thermal cycle characteristics. Laser welding, with its high energy density and rapid travel speed, produces a very high cooling rate in the weld zone. This results in fine columnar crystals in the weld metal, which is a direct consequence of the rapid solidification conditions. The narrow heat-affected zone in laser welding means that the microstructural transformation is confined to a limited region adjacent to the fusion line.

In contrast, TIG welding produces a more moderate thermal cycle with lower cooling rates. The wider weld bead and larger heat-affected zone result in coarser microstructural features, including potentially larger grain sizes in both the weld metal and the heat-affected zone. The difference in cooling rate between the two processes is the primary driver of the observed microstructural differences.

The hardness measurements provide quantitative evidence of the microstructural differences:

The higher hardness in the laser weld zone (140-150 HV versus 121 HV) is consistent with the finer grain structure and potentially higher dislocation density resulting from rapid solidification. However, hardness alone does not determine joint performance, and the mechanical property comparison reveals a more nuanced picture.

Mechanical Property Comparison and Failure Analysis

The tensile strength comparison is particularly instructive. The laser weld joint achieved 431 MPa, which represents over 95% of the Invar steel base metal strength, while the TIG weld joint achieved only 395 MPa. More importantly, the fracture location differs significantly: in the laser weld joint, fracture occurred in the Invar base metal, indicating that the weld zone was stronger than the base metal, while in the TIG weld joint, fracture occurred in the weld zone, indicating that the weld was the weakest link.

This fracture location difference has profound implications for engineering design. A joint where the weld zone is stronger than the base metal is generally preferred because it ensures that the joint failure mode is ductile and predictable, occurring in the base metal where the material properties are well characterized. A joint where the weld is the weak link is less desirable because weld metal properties are more variable and less predictable.

The impact energy values (26.1 J for laser versus 25.5 J for TIG) are closely comparable, indicating that both processes produce joints with adequate toughness for the intended application. The bending tests at 180° without cracking confirm that both joints have adequate ductility and strain accommodation capability, which is essential for dissimilar material joints subject to thermal cycling.

The fracture surface analysis revealed fine grain features and ductile dimples in both joints, confirming that the failure mode was ductile in both cases. However, the finer features in the laser weld fracture surface are consistent with the finer microstructure and higher strength.

Engineering Practice Implications for Aerospace Applications

For aerospace applications involving Invar steel and stainless steel dissimilar joints, the laser welding results suggest that this process offers superior joint performance. The combination of higher strength, fracture in the base metal, and fine microstructure makes laser welding the preferred process for applications where joint integrity is critical.

However, several practical considerations must be weighed. Laser welding requires expensive equipment and is limited by the accessibility of the weld joint. In aerospace structures with complex geometries, laser welding may not be feasible for all joint configurations, and TIG welding may remain necessary for certain applications. The study provides valuable data for process selection decisions in aerospace manufacturing.

The 95% strength retention of the laser weld joint relative to the Invar base metal is an important design parameter. In structural analysis, this value can be used to determine whether the joint can be treated as a homogeneous material or whether a strength reduction factor is required. The fracture in the base metal provides confidence that the joint will not be the initiating site for catastrophic failure.

This study provides a valuable data set for aerospace dissimilar material welding, demonstrating that laser welding can produce joints with superior mechanical properties compared to TIG welding for Invar steel and austenitic stainless steel combinations. The findings support the adoption of laser welding for critical aerospace joints where strength and reliability are paramount, while acknowledging that TIG welding remains a viable alternative for applications with less demanding requirements.