CLADDING TECHNOLOGY SHANXI CO., LTDCNC Weld Overlay Additive Manufacturing Process Temperature Field and Microstructure-Property Relationship Research
Literature Overview
This study, conducted by Zhang Yu, Luo Zhen, Li Yang, and Bi Jing at the School of Materials Science and Engineering, Tianjin University (2016), investigates the relationship between the thermal field generated during computer numerical control (CNC) weld overlay additive manufacturing and the resulting microstructure and mechanical properties of the deposited layers. Funded by the Ministry of Education Doctoral Point Fund (20130032110004), the research addresses a critical gap in understanding how rapid thermal cycling in automated overlay processes influences the metallurgical quality of the build-up material. The work is particularly significant as it bridges the gap between traditional weld overlay practices and emerging additive manufacturing concepts applied to surface engineering.
Core Technical Content
The research focuses on the transient temperature field evolution during CNC-controlled weld overlay deposition, where the welding parameters are precisely programmed and executed by a numerical control system. Unlike conventional manual or semi-automated overlay welding, the CNC approach allows for repeatable thermal input, controlled interpass temperatures, and optimized travel speeds that can be systematically varied to study their effects on the final microstructure.
The key findings relate to how the thermal gradient, cooling rate, and heat affected zone (HAZ) characteristics are directly governed by process parameters such as welding current, voltage, travel speed, wire feed rate, and the sequence of deposition layers. The authors demonstrate that the peak temperature in the melt pool, the thermal gradient at the solidification front, and the cooling rate at 800°C are the dominant factors controlling grain morphology, phase transformation, and ultimately the hardness and toughness of the overlay layer.
| Process Parameter | Typical Range | Effect on Microstructure |
|---|---|---|
| Welding current | 200-400 A | Higher current increases melt pool volume and grain size |
| Travel speed | 200-600 mm/min | Higher speed increases cooling rate, refines grains |
| Interpass temperature | 50-250°C | Controlled to prevent excessive grain growth |
| Wire feed rate | 4-8 m/min | Affects dilution rate and dilution ratio |
| Number of layers | 2-10 | More layers increase thermal input accumulation |
Microstructure and Property Analysis
The study reveals that the microstructure of the CNC weld overlay deposits exhibits a columnar grain structure oriented normal to the substrate interface, with equiaxed grains appearing near the top surface of the final layer. The thermal cycling effect from successive passes produces a "recrystallization-reprecipitation" cycle in the HAZ, which can either refine or coarsen the microstructure depending on the interpass temperature control.
Key mechanical properties observed include:
- Hardness: typically 250-450 HV depending on the alloy system and cooling rate
- Tensile strength: 450-700 MPa for martensitic overlay systems
- Dilution rate: 5-25% depending on the number of passes and heat input
- Bond strength: meets or exceeds 95% of the base metal tensile strength
The research highlights that the CNC approach enables systematic optimization of the dilution rate by controlling the number of passes and the heat input per pass. This is particularly important for overlay applications where the desired surface properties must be achieved while maintaining adequate metallurgical bonding with the substrate.
Engineering Practice Implications
The findings have direct relevance to several industrial applications:
- Hydrogenation reactor cladding: Where Inconel 625 or 316L overlay layers must maintain low carbon dilution, the CNC process allows precise control of heat input to minimize dilution below 5%.
- Wear-resistant overlay on rotating equipment: The thermal cycling effect can be leveraged to produce a surface layer with fine carbide distribution, improving wear resistance.
- Repair and restoration of worn components: The repeatability of CNC parameters ensures consistent overlay quality across multiple repair operations.
The study also identifies common defects such as porosity (primarily gas porosity from hydrogen absorption), lack of fusion at the interpass boundaries, and cracking in high-carbon martensitic systems. Countermeasures include preheating to 150-250°C for high-carbon systems, using low-hydrogen flux, and maintaining interpass temperatures below 200°C.
Study Insights and Reflections
After reviewing this research, I find the systematic approach to thermal field analysis particularly valuable for engineers who need to justify process parameter selections to quality assurance personnel. The correlation between measured thermal profiles and resulting microstructure provides the quantitative basis needed for welding procedure qualification under standards such as ASME IX or NB/T 47014. The work demonstrates that CNC control is not merely an automation convenience but a fundamental tool for achieving metallurgical objectives that would be impossible to maintain consistently with manual operation. The thermal modeling component also opens the door to finite element simulation for predicting overlay quality before physical trials, which could significantly reduce development costs for new overlay applications.