Prediction and Experimental Verification of Buckling Deformation in Thin Plate TIG Overlay Welding

Literature Overview

This paper, published in Welding Journal (2019, Vol. 40, No. 10, pp. 116-120), addresses the complex buckling deformation problem caused by TIG (GTAW) overlay welding on thin plates. The research was conducted by Henan University of Science and Technology's Key Laboratory of Mechanical Design and Transmission Systems, with collaboration from Xi'an Jiaotong University. Funded by Henan Province's Major Science and Technology Program (161100210900) and Key Research Project for Higher Education Institutions (17A460012), the study combines finite element simulation with digital image correlation (DIC) experimental verification to predict and validate buckling deformation patterns.

Theoretical Framework and Modeling

The researchers established a finite element prediction model based on thermal-elastoplastic theory, which is essential for capturing the nonlinear material behavior and geometric instability that characterize welding buckling. The model incorporates:

Gaussian heat source model — Accurately represents the concentrated heat input of the TIG arc, with heat flux distribution following a Gaussian profile.
Nonlinear transient heat conduction boundary conditions — Accounts for temperature-dependent thermal properties, convective and radiative heat loss, and the moving heat source.
Temperature-dependent material properties — Includes elastic modulus, yield strength, thermal expansion coefficient, and heat capacity as functions of temperature.

Key Modeling Parameters

Parameter	Description	Typical Value / Range
Heat source type	Double-ellipsoidal or Gaussian	Gaussian
Arc efficiency	Fraction of arc power entering the workpiece	0.7-0.9 for TIG
Welding speed	Travel speed of the arc	50-150 mm/min (thin plate)
Plate thickness	Base material thickness	1-3 mm (thin plate range)
Material	Carbon steel or stainless steel	Q235, 304, etc.
Thermal boundary	Convection + radiation	h = 5-25 W/m²·K

Experimental Verification Methodology

The study introduces digital image correlation (DIC) as a non-contact, full-field deformation measurement technique to validate the finite element predictions. DIC was employed to:

Dynamically capture the buckling deformation — Unlike traditional contact-based strain gauges or displacement transducers, DIC provides full-field surface displacement and strain data throughout the welding process.
Verify the buckling mode — The saddle-shaped (saddle-like) deformation pattern observed experimentally was compared with the finite element prediction.
Quantify deformation magnitude — The DIC measurements provided quantitative displacement values at multiple points across the plate surface.

The researchers designed a specialized thin plate welding deformation testing apparatus that accommodates the DIC optical system while providing stable welding conditions. This apparatus design is itself a contribution to the field, as many laboratories lack the infrastructure to perform high-temperature DIC measurements on welding specimens.

Buckling Mechanism and Results

The cooling of the TIG overlay weld on a thin plate produces a characteristic saddle-shaped deformation, where the plate edges near the weld start and end deflect upward while the center deflects downward, or vice versa depending on the constraint conditions and welding direction. This buckling mode arises from the interaction of:

Thermal contraction — The weld zone contracts upon cooling, pulling the surrounding material inward.
Geometric instability — Thin plates have low bending stiffness, making them susceptible to buckling under compressive thermal stresses.
Sequential heating and cooling — Each weld pass creates a new thermal cycle that interacts with the residual stress state from previous passes.

Comparison of Predicted and Measured Deformation

Measurement Location	Predicted Deflection (mm)	Measured Deflection (mm)	Deviation
Weld centerline, mid-length	Saddle-shaped	Saddle-shaped	Within acceptable range
Plate edge near weld start	Upward	Upward	Qualitative agreement
Plate edge near weld end	Downward	Downward	Qualitative agreement
Maximum out-of-plane displacement	Consistent	Consistent	Good agreement

The study confirms that the thermal-elastoplastic finite element model with Gaussian heat source and nonlinear boundary conditions provides high-accuracy predictions of buckling deformation. The dynamic temperature field and stress field data obtained from the simulation offer mechanistic insight into the buckling initiation and propagation.

Engineering Practice Integration

Thin plate overlay welding is commonly used in applications such as:

Wear-resistant plate fabrication — Adding hardfacing layers to thin sheet metal for mining and material handling applications.
Corrosion-resistant cladding — Applying corrosion-resistant alloys to carbon steel plates for chemical processing equipment.
Repair and restoration — Restoring worn or corroded thin plates in situ.

The buckling deformation predicted in this study has direct implications for:

Flatness requirements — Overlay welded plates for structural applications must meet flatness tolerances, and the predicted saddle deformation can be compensated by pre-bending or by adjusting the welding sequence.
Stress state assessment — Buckling deformation indicates significant compressive stresses in the plate, which may affect fatigue life and stress corrosion resistance.
Process optimization — The welding parameters (current, voltage, speed) can be adjusted to minimize buckling, and the simulation model can be used to identify optimal parameter combinations.

Defect Analysis and Countermeasures

Defect	Cause	Countermeasure
Saddle buckling	Thermal contraction in thin plate	Reduce heat input; use backing plate; adjust welding sequence
Edge curling	Asymmetric heating	Use symmetric welding; apply edge clamps
Weld cracking	Excessive residual stress	Reduce welding speed; preheat; post-weld stress relief
Porosity	Gas entrapment in thin weld pool	Use proper shielding gas flow; clean base material

Study Insights and Reflections

The integration of finite element simulation with DIC experimental verification represents a rigorous approach to welding deformation analysis. The DIC technique, while requiring specialized equipment and expertise, provides data quality that far exceeds traditional contact-based measurement methods. For engineering organizations that invest in DIC capability, the payoff is significant: validated simulation models can be used to predict and control welding deformation across a range of geometries and parameters.

The saddle-shaped buckling mode identified in this study is characteristic of thin plate welding in general, not limited to overlay welding. The same deformation pattern is observed in fillet welding, seam welding, and multi-pass welding of thin plates. The mechanistic understanding gained from this study can be extended to other welding configurations through appropriate model modification.

The temperature-dependent material properties are critical for accurate buckling prediction. Many engineering analyses use room-temperature material properties throughout the simulation, which leads to significant errors in predicted deformation. The study's use of temperature-dependent properties, particularly for elastic modulus and yield strength, is essential for capturing the thermal-elastoplastic behavior that drives buckling.

This research provides a validated methodology that engineers can adopt for their own thin plate welding applications. The key recommendation is to always validate simulation models against experimental measurements before using them for process optimization or quality control decisions.