CLADDING TECHNOLOGY SHANXI CO., LTDMechanical Properties of Plasma Transferred Arc Cladding Alloys
Literature Overview
This study, published in 2007 by Liu Zhengjun, Liu Changjun, Wan Qian, and Yin Yijun, was funded by the Liaoning Provincial Natural Science Foundation (Grant No. 200412025). The research originates from Shenyang University of Technology and Shenyang Blower Fan Co., Ltd., and focuses on the mechanical properties of alloy overlay layers produced via plasma transferred arc (PTA) cladding. The work addresses a critical gap in understanding how PTA process parameters influence the final microstructure and mechanical performance of clad surfaces, which is essential for selecting appropriate parameters in industrial applications such as blower impellers, turbine components, and pressure vessel linings.
Core Technical Content
The study systematically investigates the relationship between PTA cladding parameters and the resulting mechanical properties of the overlay alloy. Key process variables examined include plasma arc current, arc voltage, powder feed rate, travel speed, and gas flow rate. The mechanical properties evaluated encompass hardness distribution across the overlay thickness, tensile strength of the bond interface, impact toughness, and microhardness gradients from the substrate to the top of the cladding layer.
| Parameter | Typical Range | Influence on Mechanical Properties |
|---|---|---|
| Plasma current | 100–300 A | Higher current increases dilution and reduces hardness uniformity |
| Arc voltage | 20–40 V | Affects arc stability and penetration profile |
| Powder feed rate | 100–500 g/min | Controls deposition rate and layer thickness |
| Travel speed | 50–200 mm/min | Higher speed reduces heat input and dilution |
| Shielding gas flow | 5–15 L/min | Insufficient flow leads to oxide inclusions and porosity |
The research demonstrates that dilution rate is the single most critical factor governing the mechanical properties of the PTA overlay. At dilution rates below 15 percent, the overlay retains its designed alloy composition and exhibits hardness values within the expected range. However, when dilution exceeds 25 percent, significant softening occurs due to the incorporation of substrate material, particularly carbon steel or low-alloy steel elements such as carbon, manganese, and silicon, which form ferrite-pearlite microstructures that reduce overall hardness and wear resistance.
Microstructural Analysis
Metallographic examination reveals that the PTA overlay microstructure is predominantly columnar dendritic near the fusion line, transitioning to equiaxed grains toward the top surface. The grain orientation is strongly influenced by the thermal gradient, which is in turn determined by the heat input per unit length. Lower heat inputs, achieved through higher travel speeds and lower currents, produce finer grain structures and superior mechanical properties. The presence of carbide phases, particularly in nickel-based and cobalt-based overlay alloys, provides the primary mechanism for wear resistance. However, excessive carbon content from substrate dilution can lead to the formation of brittle cementite networks at grain boundaries, which adversely affects toughness.
Engineering Practice Implications
From an engineering perspective, this research provides actionable guidance for setting PTA cladding parameters in industrial repair and manufacturing scenarios. The following recommendations emerge from the study:
- For maximum hardness retention, maintain dilution below 15 percent by adjusting powder feed rate relative to arc current.
- Multi-pass cladding is preferred for thick overlays; each pass should be individually parameterized to ensure consistent dilution throughout the build-up.
- Preheating the substrate to 150–200 degrees Celsius reduces residual stresses and minimizes cracking susceptibility in high-strength substrates.
- Post-weld heat treatment at 700–800 degrees Celsius for 2 hours relieves residual stresses and homogenizes the microstructure without significantly reducing hardness.
Defect Analysis and Countermeasures
Common defects observed in PTA cladding include porosity, cracks, lack of fusion, and excessive dilution. Porosity typically results from insufficient shielding gas coverage or moisture in the powder feedstock. Cracking, particularly hot cracking in the weld cap, is associated with high sulfur and phosphorus content in the base metal or excessive cooling rates. Lack of fusion at the substrate-overlay interface occurs when the initial heat input is insufficient to achieve wetting and bonding.
Study Insights and Reflections
This study, though published in 2007, remains highly relevant to contemporary PTA cladding practice. The fundamental relationship between dilution and mechanical properties has not changed, and the parameter windows identified continue to serve as reliable starting points for process optimization. What has evolved since then is the ability to monitor and control dilution in real time through optical and thermal monitoring systems, which can now provide feedback to adjust powder feed rates dynamically. For engineers working with bimetallic pressure vessels or clad equipment, the key takeaway is that PTA overlay is not merely a deposition process but a thermomechanical process where heat input management is paramount. The mechanical integrity of the overlay-substrate bond directly impacts the service life and safety of the component, making parameter optimization a non-negotiable step in any cladding qualification program.