Interlayer Stress Analysis and Process Optimization in Multi-Pass Metal Surfacing

Literature Overview

This paper by Xu Yan et al. (2017), published in Foundry Technology (Vol. 38, No. 7, pp. 1713-1717), presents a numerical simulation study of residual stress and deformation in single-pass multi-layer buildup welding, with process optimization validated by machining experiments. The research is conducted at Xinjiang University's School of Mechanical Engineering in collaboration with Xinjiang Weiao Technology Co., Ltd., funded by the National Natural Science Foundation of China (51665055), Xinjiang Uygur Autonomous Region High-Tech Research Project (201113129), and Xinjiang Youth Science and Technology Innovation Talent Training Project (gn2015yx008). The work addresses a critical practical challenge in additive manufacturing and multi-layer surfacing of large components where residual stress accumulation can lead to cracking, distortion, and dimensional inaccuracy.

Core Technical Findings

Using Visual-Weld software interfaced with the SysWeld finite element solver, the authors conducted thermomechanical simulations of multi-pass buildup welding. The primary findings include:

Stress concentration zones are identified at layer interfaces (interlayer boundaries) and at the arc start and arc stop locations—these constitute the critical danger zones for cracking.
Subsequent passes exert a stress-relieving effect on previously deposited layers, as the thermal input from later passes partially anneals the residual stresses in earlier layers.
Welding speed effects: Higher welding speeds increase longitudinal residual stress while decreasing transverse residual stress in the buildup zone.
Welding path effects: A reciprocating (back-and-forth) deposition path significantly reduces residual stress in both the weld stabilization zone and the base plate, while also minimizing welding deformation and improving arc crater quality at discontinuities.

Process Parameter	Effect on Longitudinal Stress	Effect on Transverse Stress	Effect on Deformation
Increased welding speed	Increase	Decrease	Moderate increase
Reciprocating path	Significant reduction	Significant reduction	Substantial reduction
Layer interface	Stress concentration peak	Stress concentration peak	Localized distortion
Arc start/stop locations	High stress concentration	High stress concentration	Crater cracking risk
Subsequent pass thermal input	Stress relief (annealing)	Stress relief (annealing)	Partial distortion correction

Finite Element Analysis Methodology and Results Interpretation

The numerical simulation approach employed a coupled thermomechanical model, where the temperature field solution from each pass is sequentially mapped onto the stress field calculation. This sequential coupling approach is standard practice for multi-pass welding simulations and provides reasonable accuracy for residual stress prediction.

The identification of interlayer boundaries as stress concentration zones is consistent with established welding metallurgy principles. At each layer interface, there exists a gradient in:

Chemical composition (due to dilution variation between passes)
Microstructure (due to different cooling rates at different depth positions)
Thermal expansion mismatch (between deposited metal and previously solidified layers)

The observation that subsequent passes provide stress relief is physically intuitive—the thermal cycle from a later pass reheats the earlier layer to temperatures approaching but below the Ac1 transformation temperature, allowing partial recovery and stress relaxation through viscoplastic flow.

Process Optimization and Experimental Validation

The optimized welding strategy recommended by the authors includes:

Reciprocating deposition path: Instead of unidirectional passes, the torch follows a back-and-forth pattern within each layer, which distributes heat input more uniformly and reduces thermal gradients.
Controlled welding speed: A moderate welding speed balances productivity with acceptable residual stress levels.
Interpass temperature management: Maintaining appropriate interpass temperatures to facilitate stress relief without compromising microstructural integrity.

The machining experiments confirmed that the optimized process parameters resulted in:

Reduced overall deformation of the buildup component
Improved dimensional accuracy after machining
Reduced cracking tendency at arc craters
Better surface quality of the finished component

Engineering Practice Integration

For engineers working on large-scale surfacing applications—such as turbine casing repair, pressure vessel overlay cladding, or large pipe fitting manufacture—the following practical recommendations emerge:

FMEA application: The identified stress concentration zones (interlayer boundaries, arc start/stop) should be included as critical failure modes in any FMEA for multi-pass surfacing operations. Countermeasures include:
Use of back-strap or backing material to support the root of the buildup
Controlled arc start and stop with proper crater filling techniques
Interpass temperature monitoring using infrared thermography or thermocouples
PDCA cycle implementation: The process optimization should follow a systematic PDCA approach:
Plan: Define target residual stress levels based on component criticality
Do: Implement reciprocating path and optimized parameters
Check: Measure residual stress using X-ray diffraction or hole-drilling methods
Act: Adjust parameters based on measurement results
5W2H framework for process documentation:
What: Multi-pass buildup welding with reciprocating path
Why: Reduce residual stress and deformation
Where: Large-diameter pipe fittings, pressure vessels
When: During manufacturing or repair operations
Who: Certified welders with FCAW or SAW qualification
How: Visual-Weld optimized parameters with interpass temperature control

Key Questions and Reflections

Several aspects of this study merit further consideration:

The simulation assumes idealized boundary conditions; in practice, clamping constraints, gravity effects, and pre-existing residual stresses in the base material significantly influence the actual stress state.
The study focuses on stress magnitude but does not extensively address stress direction and its implications for fatigue life. In cyclic loading applications, the principal stress direction relative to the service load is critical.
The stress relief effect of subsequent passes, while beneficial, may also affect microstructural properties. Repeated thermal cycling at interlayer boundaries could promote grain coarsening or intermetallic phase precipitation in certain alloy systems.

Study Insights and Implications

This paper provides a valuable methodological framework for residual stress management in multi-pass surfacing operations. The combination of numerical simulation with experimental validation offers a reliable approach to process optimization that reduces reliance on costly trial-and-error methods. For the piping and fitting industry, where large-scale surfacing is common in repair and overlay applications, the key practical insight is that welding path selection—specifically the reciprocating pattern—can dramatically reduce residual stress without requiring additional post-weld heat treatment. This represents a significant cost and time saving, particularly for large components where post-weld stress relief may be impractical or impossible. Engineers should consider incorporating the simulation-based optimization approach into their process development workflows, using it as a complementary tool to traditional empirical methods. The identified danger zones at interlayer boundaries and arc discontinuities should be incorporated into inspection protocols, with increased NDT coverage in these regions to ensure structural integrity of the finished component.