CLADDING TECHNOLOGY SHANXI CO., LTDTemperature Field and Microstructure-Properties Relationship in CNC Cladding Additive Manufacturing
Literature Overview and Research Background
Additive manufacturing (AM) has fundamentally transformed the paradigm of overlay welding and cladding technology by enabling precise, layer-by-layer deposition of metallic materials with controlled thermal cycling. The study under review focuses on numerically controlled (CNC) cladding additive manufacturing, specifically investigating the relationship between the temperature field evolution during multi-track, multi-layer deposition and the resulting microstructure and mechanical properties of the cladding layer. This research bridges the gap between process simulation and metallurgical outcomes, providing engineers with predictive tools for process parameter optimisation in complex cladding geometries.
Core Technical Findings
Temperature Field Modelling
The numerical model employs a three-dimensional finite element approach with a moving heat source to simulate the thermal history during CNC cladding. The heat source is typically modelled as a conical or double-ellipsoidal distribution, calibrated against measured thermocouple readings. The simulation accounts for material properties as functions of temperature, including thermal conductivity, specific heat, and density for both the base material and the deposited overlay material.
Key thermal parameters extracted from the simulation include peak temperature, cooling rate, and thermal cycling frequency. For a typical CNC cladding process using gas metal arc welding (GMAW) with a wire feed rate of 6–8 m/min and a welding current of 180–220 A, the peak temperature in the molten pool reaches approximately 1800–2100°C. The cooling rate from 1300°C to 800°C (the critical range for austenite-to-ferrite transformation in austenitic stainless steel overlays) typically ranges from 5 to 50 °C/s, depending on the thermal mass of the underlying layers and the base material.
| Process Parameter | Typical Range | Effect on Temperature Field |
|---|---|---|
| Wire Feed Rate | 6–8 m/min | Higher rate increases deposition rate but may reduce penetration |
| Welding Current | 180–220 A | Higher current increases peak temperature and pool volume |
| Travelling Speed | 300–600 mm/min | Higher speed reduces heat input and increases cooling rate |
| Track Spacing | 0.6–0.9 × wire diameter | Affects overlap and inter-track thermal interaction |
| Layer Thickness | 1.5–3.0 mm | Thinner layers increase thermal cycling frequency |
Microstructure Evolution
The cooling rate directly governs the solidification microstructure of the cladding layer. At cooling rates below 10 °C/s, coarse columnar dendrites form, often with significant grain boundary segregation of alloying elements. As the cooling rate increases beyond 30 °C/s, the microstructure transitions to finer equiaxed grains, and at rates exceeding 80 °C/s, cellular and micro-dendritic structures with significantly reduced grain size (below 10 μm) develop. This refinement is particularly beneficial for austenitic stainless steel overlays (such as 308L or 316L), where fine grain structures improve both toughness and corrosion resistance.
The thermal cycling effect from subsequent layers creates an additional refinement mechanism in previously deposited layers. Each new layer acts as a heat sink during solidification but then provides a reheating effect on the underlying layer during its own solidification. This repeated thermal cycling promotes precipitation of secondary phases (such as Cr23C6 in stainless steel or Mo-rich carbides in nickel-based alloys) and can either improve or degrade mechanical properties depending on the specific alloy system and the number of cycles experienced.
Mechanical Properties Correlation
The hardness of the cladding layer exhibits a strong correlation with the cooling rate and thermal cycling history. For austenitic stainless steel overlays, hardness typically increases from approximately 200 HV at the bottom layers to 250–280 HV at the top layers, reflecting the progressive grain refinement and increased precipitation hardening. Tensile strength of the overlay material generally exceeds 600 MPa for 308L compositions, with elongation values of 30–40%, demonstrating that the AM process does not compromise ductility despite the rapid solidification.
The interlayer bond strength is a critical quality parameter in CNC cladding. Cross-sectional tensile testing and shear testing reveal that bond strengths typically exceed 300 MPa when the track overlap is maintained at 70–80% and the interpass temperature is controlled below 150°C. Insufficient overlap or excessive interpass temperature leads to incomplete fusion between tracks, creating weak interfaces susceptible to cracking under service loading.
Process Optimisation and Engineering Practice
Parameter Selection Strategy
The study recommends a systematic approach to parameter selection based on the desired microstructure and properties. For applications requiring high hardness and wear resistance (such as pump impeller repair or valve seat cladding), higher cooling rates are achieved by reducing wire feed rate and increasing travelling speed, while maintaining adequate track overlap. For applications requiring high toughness and corrosion resistance (such as nuclear-grade cladding or marine applications), lower cooling rates are preferred, achieved by increasing wire feed rate and reducing travelling speed, with careful control of interpass temperature to prevent excessive grain growth.
Defect Prevention
The temperature field analysis also identifies regions susceptible to defect formation. Hot cracking is most likely to occur at the top surface of each track where the cooling rate is highest and the solidification shrinkage is most constrained. The study identifies a critical cooling rate threshold of approximately 40 °C/s above which hot cracking susceptibility increases significantly for 308L-type alloys. To mitigate this, the wire composition may be adjusted by increasing the niobium and titanium content to promote grain refinement and reduce the temperature range of the brittle phase.
Porosity formation is another concern, particularly in multi-layer builds where gas entrapment between layers can occur. The thermal model predicts regions of low fluidity in the molten pool where porosity is most likely to nucleate. These regions correlate with the trailing edge of the weld pool where solidification is incomplete and the metal is still in a mushy state. Increasing the thermal input slightly (by reducing travelling speed) or adding a small amount of aluminium to the wire composition can improve fluidity and reduce porosity.
Key Questions and Reflections
The research demonstrates that CNC cladding AM offers unprecedented control over the thermal history of the overlay material, enabling targeted microstructure engineering that is not achievable with conventional single-pass or multi-pass cladding methods. However, the complexity of the temperature field in multi-track, multi-layer builds introduces challenges for real-time process monitoring and quality assurance. Engineers must develop robust process windows that account for variations in base material thermal properties, ambient conditions, and geometric complexity.
A significant practical challenge is the scalability of CNC cladding from laboratory-scale components to full-size industrial parts. The thermal interaction between adjacent tracks increases with part size, and the cumulative thermal load can lead to distortion and residual stress accumulation that must be managed through process planning, fixture design, and post-weld stress relief.
Study Insights and Conclusions
The relationship between the temperature field and the resulting microstructure and properties in CNC cladding additive manufacturing is complex but predictable through proper numerical modelling and experimental validation. The key insight is that cooling rate and thermal cycling frequency are the primary levers for controlling the overlay microstructure, and these can be precisely managed through CNC-controlled welding parameters. Engineers adopting this technology should invest in robust process simulation capabilities and develop thorough qualification procedures that address the unique metallurgical characteristics of AM-produced cladding layers. The ability to tailor the microstructure through process control represents a paradigm shift in cladding technology, enabling performance optimisation for specific service conditions that was previously limited to material selection alone.