Effect of Welding Parameters on Microstructure and Microhardness of Ni60 Alloy Plasma Cladding Layer

Literature Overview

The research by Ge Yanliu, Deng Dewei, Tian Xin, and Guan Meng, published in China Surface Engineering in 2011, investigates the systematic influence of plasma transferred arc (PTA) welding parameters on the microstructural evolution and mechanical properties of Ni60 alloy cladding layers. Funded by the National Natural Science Foundation of China (Grant No. 11072045), this work bridges fundamental materials science with industrial application at Shenyang Blower Works Corporation, a leading manufacturer of high-performance pumps and compressors.

Core Technical Content

Ni60 alloy, a nickel-cobalt-cromium austenitic weld overlay material, is widely used in tribological applications due to its exceptional wear resistance, which develops through martensitic transformation during cooling. The study examined the effects of key PTA parameters on the resulting microstructure:

Parameter	Range Studied	Primary Effect
Arc current	80–160 A	Dilution ratio, grain size
Travel speed	200–500 mm/min	Cooling rate, martensite fraction
Powder feed rate	300–800 g/min	Layer thickness, dilution
Shielding gas flow	8–16 L/min	Oxidation, gas porosity
Powder particle size	30–75 μm	Melt pool stability, dilution
Standoff distance	3–8 mm	Arc stability, spatter

Microstructural Analysis

The microstructure of PTA-cladded Ni60 layers exhibits a characteristic austenite-martensite dual-phase morphology:

Austenite (γ-phase): Primary solidification phase, stable at high temperatures, providing ductility and corrosion resistance.
Martensite (α′-phase): Forms during cooling below the Mₛ temperature (approximately 300–400°C for Ni60), providing hardness and wear resistance.
Carbide phases: M₇C₃ and M₂₃C₆ chromium carbides precipitate at grain boundaries, contributing to secondary hardening.

The critical finding is that the martensite fraction is highly sensitive to cooling rate, which is primarily governed by the ratio of heat input to travel speed (H/V ratio). Higher H/V ratios produce slower cooling rates, resulting in increased austenite retention and reduced martensite fraction.

Mechanical Property Correlation

The microhardness distribution across the cladding layer follows a predictable pattern:

Depth from Surface	Typical Microhardness (HV)	Dominant Phase
0–0.5 mm	450–550	High martensite fraction
0.5–1.0 mm	380–480	Mixed austenite-martensite
1.0–1.5 mm (near fusion boundary)	250–350	Higher austenite, dilution effects

The dilution ratio, defined as the weight percentage of base metal in the overlay layer, is the single most influential parameter on final properties. Acceptable dilution for Ni60 cladding is typically maintained below 25–30%, as excessive dilution introduces carbon and alloying elements from the substrate that alter the Mₛ temperature and phase composition.

Process Optimization

Based on the experimental matrix, the following parameter combinations yield optimal results for wear-resistant Ni60 cladding:

High hardness requirement: Current 100–120 A, travel speed 300–400 mm/min, powder feed 500–600 g/min, producing cooling rates of 20–40 °C/s and martensite fractions exceeding 60%.
Balanced toughness-hardness: Current 130–150 A, travel speed 250–350 mm/min, powder feed 600–700 g/min, yielding mixed microstructures with hardness of 400–500 HV.
Low dilution strategy: Multi-pass deposition with thin layers (0.8–1.2 mm per pass) and high powder-to-current ratios maintains dilution below 20%.

Common Defects and Countermeasures

Defect Type	Root Cause	Countermeasure
Cracking at fusion boundary	High dilution, carbon enrichment	Reduce current, increase feed rate, preheat
Gas porosity	Insufficient shielding, wet powder	Increase gas flow, dry powder storage
Excessive spatter	High current, long standoff	Reduce current, optimize standoff distance
Uneven layer thickness	Unstable powder feeding	Calibrate feeder, use constant feed rate control
Insufficient bond strength	High dilution, poor wetting	Optimize heat input, clean substrate

Study Insights and Implications

This research provides engineers with a systematic understanding of how PTA parameters translate into microstructural and mechanical outcomes for Ni60 cladding. The practical implication for pump and compressor manufacturing is significant: by controlling the cooling rate through parameter optimization, operators can tailor the martensite fraction to achieve the desired balance between wear resistance and fatigue life. The study also highlights the importance of maintaining dilution ratios below critical thresholds, which requires careful process design rather than simple parameter adjustment. For industrial implementation, in-situ monitoring of cooling rates through thermocouple measurement combined with post-build metallographic verification creates a robust quality assurance framework.