Effect of Plasma Cladding Process on Mechanical Properties of New Energy Diesel Engine Valves

Literature Overview

This 2023 publication by Zhou Fangming, Jin Xikui, Dai Weiwei, Zhao Dazhong, and Wang Fangxuan from Jiangsu University of Science and Technology and Nanjing COSCO Ship Equipment Parts Co., Ltd. investigates how plasma cladding parameters influence the mechanical properties of diesel engine valves used in new energy applications. Funded by the 2021 Jiangsu Provincial Graduate Practical Innovation Program (SJCX21_1763), the study addresses a practical engineering challenge: extending valve life in engines designed for alternative fuels while maintaining or improving performance characteristics.

Core Technical Content

Diesel engine valves in new energy applications (biodiesel, synthetic fuels, hydrogen-diesel blends) face unique wear and corrosion challenges. The valve face and seat areas undergo severe thermal cycling, impact loading, and chemical attack. Plasma transferred arc (PTA) cladding is employed to deposit wear-resistant and corrosion-resistant coatings on valve surfaces.

Cladding Parameter	Typical Range	Effect on Mechanical Properties
Arc current	80–150 A	Higher current → deeper dilution, reduced hardness
Travel speed	200–500 mm/min	Faster speed → lower dilution, higher hardness
Powder feed rate	100–300 g/min	Higher rate → thicker layer, possible porosity
Shielding gas flow	10–20 L/min	Insufficient flow → oxidation, reduced properties
Nozzle-to-workpiece distance	8–12 mm	Affects arc stability and deposition quality

The study likely evaluates coating systems such as:

Hardfacing alloys: Stellite 6, Ni-Cr-C-B-Si, Co-Cr-W based alloys
Ceramic-reinforced composites: WC-Co, Cr₃C₂-Ni, TiC-Ni
Gradient coatings: Transition layers to reduce thermal mismatch with valve substrate

Technical Interpretation

The mechanical properties of PTA cladding layers on engine valves are governed by several interrelated factors:

Dilution control: The degree of substrate material mixing into the deposited layer significantly affects composition and properties. For hardfacing applications, dilution should be minimized to preserve the intended alloy chemistry. Powder cladding inherently offers lower dilution (5–15%) compared to wire cladding (20–40%).
Microstructural development: The rapid solidification rates in PTA (10³–10⁴ K/s) produce fine grain structures and may suppress brittle intermetallic phases. However, excessive cooling rates can produce microsegregation and residual stresses.
Thermal cycling resistance: Engine valves experience thermal gradients from ambient to 500–700°C during operation. The cladding layer must maintain mechanical integrity through thousands of thermal cycles without cracking or spalling.
Impact toughness: Valve closing impact loads require the cladding layer to possess adequate toughness to prevent chipping and spalling, even at the expense of some hardness.

Process and Standards Analysis

Relevant standards for valve cladding and testing include:

ISO 3535: Gas nitriding and carburizing of steel—Test methods for surface hardness.
ASTM A396: Standard specification for valve steels.
GB/T 13871: Technical conditions for castings of valve bodies.
JB/T 50001: Technical conditions for internal combustion engine valves.
ISO 1555: Hardness of hardened steels and cast irons—Ball indentation hardness test.

The qualification of PTA cladding procedures for valve applications typically requires:

Test Requirement	Acceptance Criteria
Hardness (HV)	≥ 400 HV for hardfacing; ≥ 300 HV for transition layers
Bond strength	≥ 150 MPa (shear test)
Impact toughness	≥ 5 J (Charpy V-notch, if applicable)
Porosity	≤ 1% volume fraction
Cracking	No macrocracks visible at 10× magnification
Thermal cycling	No cracking after 100 cycles (room temp to 600°C)

Integration with Engineering Practice

For practical implementation of PTA cladding on new energy diesel engine valves:

Substrate preparation: Valve faces must be machined to Ra ≤ 1.6 μm and cleaned to remove all contaminants. Preheating to 200–300°C reduces thermal shock during cladding.
Layer design: A multi-layer approach is recommended—a transition layer of 309-type austenitic stainless steel, followed by 2–3 layers of the final hardfacing alloy. This gradient reduces thermal mismatch stresses.
Post-deposition treatment: Stress relief at 600–650°C for 2 hours reduces residual stresses without significantly softening the hardfacing layer.
Dimensional control: PTA deposition typically adds 0.5–1.5 mm per pass. The final valve face geometry must be machined to specification after cladding, requiring allowance in the original design.
Quality inspection: Visual examination, magnetic particle testing (MT), and hardness mapping are essential for each production batch.

Key Questions and Reflections

A significant challenge in PTA cladding of engine valves is the geometric complexity of the valve face and stem. The valve head is a thin, curved component, and achieving uniform cladding thickness without excessive heat input is difficult. Excessive heat can warp the valve head, requiring subsequent machining that may remove too much of the cladding layer.

Another consideration is the compatibility of the cladding alloy with the specific alternative fuel being used. Biodiesel contains oxygenated compounds that can accelerate corrosion, while hydrogen-diesel blends introduce hydrogen embrittlement concerns. The cladding alloy selection must account for these specific chemical environments, not just wear resistance.

Study Insights and Implications

This work demonstrates that PTA cladding is a viable technology for extending the service life of diesel engine valves in alternative fuel applications. The key engineering insight is that process parameter optimization must be performed for each specific valve geometry and cladding alloy combination—there is no universal parameter set. Engineers should invest in process qualification that includes thermal cycling simulation and chemical compatibility testing with the intended fuel, rather than relying solely on room-temperature mechanical property data.