Numerical Simulation of Temperature Field and Microstructure Evolution in CNC Surfacing Additive Manufacturing

Literature Overview

The study by Zhang Yu, Luo Zhen, Li Yang, and Bi Jing (2016), published in Welding (No. 4, pp. 42–45), presents a three-dimensional finite element thermal analysis of the CNC-controlled surfacing additive manufacturing process for thin-walled components. Funded by the Ministry of Education Doctoral Fund (20130032110004), this research addresses a critical gap in understanding how thermal cycling during multi-pass automated surfacing affects the final microstructure and mechanical properties of the built part. The work is highly relevant to engineers working on additive manufacturing of pipeline components, repair of worn pipe surfaces, and production of complex-shaped corrosion-resistant overlays.

Core Technical Findings

Thermal Field Simulation

The researchers developed a 3D transient heat transfer finite element model using a moving heat source to simulate the CNC surfacing process on thin-walled workpieces. The key thermal findings include:

The repeated reheating of previously deposited material is a fundamental characteristic of additive manufacturing processes. Each new pass acts as a secondary thermal input on the underlying layers, creating a complex thermal history that differs markedly from conventional single-pass welding. This cumulative thermal cycling has profound implications for grain growth, phase transformation, and residual stress development.

Microstructural Differentiation

One of the most significant findings is the microstructural differentiation between the internal and surface regions of the built part:

This microstructural gradient arises because the internal regions experience repeated thermal cycles that fully austenitize the material and allow sufficient time for complete recrystallization and grain refinement. In contrast, the surface and end regions experience more rapid cooling due to their proximity to the free surface and the trailing edge of the heat source, resulting in a harder, more complex microstructure with martensitic constituents.

Thermal Cycle Characteristics

The thermal cycling in CNC surfacing is fundamentally different from conventional welding. In a multi-pass surfacing operation, a given point in the deposited material may experience 3–5 thermal cycles, with peak temperatures ranging from the solidification temperature (>1400°C) in the first pass to progressively lower temperatures in subsequent passes. This thermal history is analogous to a controlled multi-step annealing process, which can actually improve the microstructure of the deposited material compared to a single-pass deposit.

Process-Structure-Property Relationships

Understanding the process-structure-property chain is essential for optimizing CNC surfacing parameters:

1. Heat input per pass – Controls the initial solidification microstructure and dilution level. Higher heat input increases dilution and coarsens dendrite spacing.
2. Interpass temperature – In CNC surfacing, this is naturally controlled by the deposition sequence and travel speed. A lower interpass temperature increases the effective cooling rate for previously deposited material.
3. Deposition sequence – The path planning algorithm directly determines the thermal history of each region. A serpentine or zig-zag pattern produces different thermal cycles than a circular or spiral pattern.
4. Travel speed – Higher travel speed reduces heat input and increases cooling rate, promoting finer grain structures but potentially increasing residual stress.

Comparison with Conventional Surfacing

Engineering Practice Implications

For engineers applying CNC surfacing to pipeline components or large structural weld repairs, the following considerations are critical:

1. Thermal simulation should precede production – A 3D FEA thermal model should be developed for each component geometry to predict the thermal history and identify regions prone to excessive hardness or cracking.
2. Post-build heat treatment – Despite the self-annealing effect, a final stress relief treatment at 550–650°C (for low-alloy steels) or 700–800°C (for austenitic overlays) is recommended to homogenize the microstructure and reduce residual stresses.
3. Surface hardness monitoring – The surface region, with its martensitic constituents, may exhibit hardness values exceeding 400 HV, which could be problematic for sour service applications. Engineers should verify hardness compliance with API 5L or NACE MR0175 limits.
4. Process parameter validation – The CNC program parameters (travel speed, wire feed rate, heat source power) should be validated through trial builds with full metallographic and mechanical testing before production deployment.

Study Insights and Reflection

This paper makes a valuable contribution to the understanding of additive manufacturing thermal effects, particularly the self-annealing phenomenon that naturally occurs during multi-pass deposition. The finding that internal regions achieve fine, uniform microstructures through repeated thermal cycling is encouraging, as it suggests that CNC surfacing can produce materials with properties comparable to or better than conventional welding. However, the surface microstructural differentiation remains a concern, as the harder martensitic regions at the surface may be more susceptible to corrosion and fatigue cracking. Engineers should view CNC surfacing not merely as an automated deposition process but as a thermally complex manufacturing operation that requires careful process design, simulation, and validation. The integration of thermal simulation with microstructural prediction models represents a promising direction for future process optimization, and this study provides a solid foundation for such developments.