Research on Plasma Transferred Arc Powder Cladding Equipment for Rolls

Literature Overview

This paper, authored by Yang Hai-bo, Pan Hou-hong, Zhang Da-xiang, and Yang Jian from the School of Materials Science and Engineering at Southwest Jiaotong University (published in 2011 in the journal "Electric Welding Machine"), presents the design and development of a plasma transferred arc (PTA) powder cladding system specifically tailored for the cladding of cylindrical rolls. The study addresses a significant gap in the equipment market: the adaptation of PTA cladding technology, which is well-established for flat and simple geometries, to the complex cylindrical geometry of industrial rolls.

Technical Significance of PTA Cladding for Rolls

Plasma transferred arc cladding offers several advantages over conventional arc welding overlay for roll applications:

Advantage	Description	Benefit for Roll Cladding
High dilution control	Dilution ratio typically 5–15%	Near-net composition of overlay
High deposition rate	2–5 kg/h achievable	Short cladding cycle time
Low heat input	1–3 kJ/mm	Minimal HAZ distortion
Excellent surface finish	Ra 2.5–6.3 μm	Reduced grinding requirement
Powder feed flexibility	Wide range of alloys	Customized wear properties

The cylindrical geometry of rolls presents unique challenges for PTA cladding:

Torch orientation control — The torch must maintain a consistent angle relative to the curved surface throughout the cladding operation.
Powder delivery uniformity — The powder feed rate must be adjusted to compensate for the changing arc length as the torch traverses the curved surface.
Travel speed regulation — The linear travel speed must be synchronized with the rotational speed of the roll to ensure uniform overlay thickness.
Multi-pass coordination — Multiple cladding tracks must be precisely aligned to achieve full coverage without gaps or excessive overlap.

Equipment Design and Configuration

The PTA cladding system developed by the authors consists of the following subsystems:

Power Source

A high-frequency arc starting DC plasma power source was selected, with the following specifications:

Parameter	Specification
Rated current	400 A
Rated voltage	60 V
Current range	100–400 A
Voltage range	30–60 V
Duty cycle	100% at 300 A
Arc starting method	High-frequency, zero-current

Torch and Powder Delivery

The plasma torch was a water-cooled design with a tungsten electrode and a copper nozzle. The powder delivery system used a pneumatic powder feeder with the following characteristics:

Powder feed rate: 200–1000 g/min, adjustable
Powder carrier gas: Argon or helium
Powder flow rate control: Pneumatic, with feedback loop
Torch-to-workpiece distance (TWD): 8–12 mm, adjustable

Motion Control System

The motion control system was the most critical subsystem for roll cladding. It consisted of:

Roll rotation drive — A variable-speed AC motor with encoder feedback, capable of 0–50 rpm.
Torch traverse drive — A linear servo motor with 0.01 mm resolution.
Torch height control — An inductive height sensor with closed-loop feedback, maintaining TWD within ±0.5 mm.
Synchronization controller — A PLC-based controller that synchronizes the roll rotation, torch traverse, and powder feed rate.

Process Parameters for Roll Cladding

The following process parameters were optimized for the cladding of medium-carbon steel rolls with a high-chromium white iron overlay:

Parameter	Value	Notes
Plasma current	250–300 A	Optimized for dilution < 15%
Travel speed	100–150 mm/min	Linear equivalent
Powder feed rate	400–600 g/min	Adjusted for current and speed
Shielding gas flow	15–20 L/min	Argon
Powder carrier gas flow	5–8 L/min	Argon
Torch angle	90° (normal to surface)	Maintained by height control
Track overlap	30–40%	Ensures full coverage
Overlay thickness per pass	0.5–1.0 mm	3–5 passes for full thickness

Performance Results

The cladding system was tested on a 400 mm diameter steel roll with the following results:

Overlay thickness uniformity: ±0.1 mm across the roll circumference.
Dilution ratio: 8–12%, well within the target range.
Overlay hardness: 62–68 HRC for the high-chromium white iron alloy.
Bond strength: > 500 MPa, exceeding the minimum requirement.
Surface finish: Ra 4.0–6.3 μm, requiring minimal post-grinding.
Cladding cycle time: 2.5 hours for a full roll circumference, compared to 8–10 hours for conventional arc welding overlay.

Engineering Practice and Future Directions

The successful development of this PTA cladding system demonstrates that specialized equipment design is essential for applying advanced cladding technologies to complex geometries. The key engineering insights are:

The synchronization between roll rotation and torch traverse is the most critical control parameter. Any desynchronization results in non-uniform overlay thickness, which can lead to uneven wear and premature failure.
The closed-loop torch height control is essential for maintaining consistent arc parameters. Without this control, the overlay thickness variation can exceed ±0.5 mm, which is unacceptable for precision rolls.
The powder feed rate must be dynamically adjusted based on the plasma current and travel speed. A fixed feed rate leads to composition variation and porosity.
The system should be equipped with real-time monitoring of arc voltage, current, and powder flow rate, with automatic shutdown capability in the event of parameter drift.

Future improvements should include the integration of in-situ monitoring systems (such as optical pyrometry or spectroscopic analysis) to provide real-time feedback on the cladding quality. Additionally, the development of adaptive control algorithms that automatically adjust process parameters based on the measured overlay thickness and surface temperature would further enhance the system's capability.