Discharge Mechanism of Rotating Electrode Spark Deposition Cladding

Literature Overview

The 2019 study by Han Hongbiao, Guo Jingdi, and Jiao Wenqing, published in the Journal of Welding (焊接学报), investigates the discharge mechanism of rotating electrode spark deposition (RES) cladding. Conducted at Henan University of Science and Technology under the National Natural Science Foundation of China (Project 51375146) and the Henan Provincial Key Research Project (17A460012), this research explores a novel cladding technique that combines rotating electrode technology with spark discharge to achieve high-efficiency, low-dilution deposition of alloy coatings.

Rotating Electrode Spark Deposition Process Description

Rotating electrode spark deposition (RES) is an advanced thermal spray-like process that uses a rotating electrode to generate a high-velocity plasma arc and simultaneously feed molten metal droplets onto the substrate. The process combines the advantages of rotating electrode plasma spraying (REPS) with spark discharge ignition to achieve precise control over the deposition process. The key features of the RES process include:

1. Rotating electrode: A consumable electrode (typically copper or nickel alloy) rotates at high speed (20,000-40,000 rpm) to generate a centrifugal force that feeds molten metal droplets toward the substrate.
2. Plasma arc: A high-velocity plasma arc is generated between the rotating electrode and the substrate, providing the energy source for melting and accelerating the droplets.
3. Spark discharge: Periodic spark discharges occur between the rotating electrode and the substrate, creating localized high-temperature zones that enhance droplet melting and bonding.
4. Deposition: Molten droplets are accelerated by the plasma arc and deposited onto the substrate, where they rapidly solidify to form a dense, well-bonded coating.

Discharge Mechanism Analysis

The discharge mechanism in RES cladding involves complex interactions between the rotating electrode, plasma arc, and substrate. The study identifies several key discharge phenomena that influence the deposition process:

Arc Initiation and Stabilization

The plasma arc is initiated between the rotating electrode and the substrate through a spark discharge. The rotation of the electrode creates a dynamic arc root that moves continuously along the electrode surface, preventing localized overheating and ensuring uniform electrode consumption. The arc voltage and current are controlled by the power supply, with typical values of 30-50 V and 200-400 A, respectively.

Spark Discharge Characteristics

The spark discharge in RES cladding occurs periodically as the rotating electrode approaches the substrate. The spark discharge creates a localized high-temperature zone (exceeding 10,000 K) that melts the droplets and enhances their bonding to the substrate. The spark discharge parameters include:

Droplet Formation and Acceleration

The rotating electrode generates molten metal droplets through centrifugal force and plasma arc heating. The droplets are accelerated by the plasma arc and directed toward the substrate. The droplet characteristics include:

Microstructural Characteristics of RES Coatings

The RES-deposited coatings exhibit distinctive microstructural features compared to conventional thermal spray coatings:

1. Dense structure: The high droplet velocity and localized heating from spark discharge result in dense coatings with porosity below 1%.
2. Refined grain structure: The rapid solidification of droplets upon impact produces fine grains (1-10 μm), which improve mechanical properties through grain boundary strengthening.
3. Low dilution: The RES process achieves low dilution (<10%) with the substrate, preserving the intended chemistry of the coating alloy.
4. Good bond strength: The localized heating from spark discharge enhances metallurgical bonding between the coating and substrate, achieving bond strengths exceeding 50 MPa.

Wear and Corrosion Performance

The RES-deposited coatings exhibit excellent wear and corrosion resistance, attributed to the dense structure, refined grain structure, and low dilution. The wear resistance is particularly enhanced by the presence of hard carbides or intermetallic phases in the coating alloy, which act as load-bearing elements that resist abrasive attack.