Interface Diffusion Analysis of WC-Co Cemented Carbide TIG Weld Joints

Literature Overview

This 2005 publication by Xu Peiquan, Yang Dexin, Zhao Xiujuan, Lu Fenggui, and Yao Shun, published in the Journal of Shanghai Jiao Tong University, investigates the interface diffusion behavior in WC-Co cemented carbide joints welded by TIG (gas tungsten arc welding). The research originates from Shanghai Jiao Tong University and Dalian Railway Institute, combining metallurgical expertise with railway engineering applications. Cemented carbide components are widely used in railway cutting tools, wear parts, and mining equipment where extreme hardness and wear resistance are required.

Core Technical Content

WC-Co cemented carbides present unique welding challenges due to their heterogeneous microstructure consisting of hard tungsten carbide (WC) particles embedded in a ductile cobalt (Co) binder matrix. The thermal conductivity mismatch between WC (85 W/m·K) and Co (100 W/m·K), combined with the extreme hardness of WC (2300 HV), creates significant challenges for arc welding processes.

The interface diffusion analysis focuses on several critical phenomena:

• Cobalt redistribution at the weld interface during solidification
• Dissolution of WC particles in the molten pool
• Formation of brittle intermetallic phases at the carbide-metal boundary
• Thermal stress-induced cracking due to coefficient of thermal expansion mismatch

Interface Diffusion Mechanisms

During TIG welding of WC-Co cemented carbide, the molten pool experiences complex diffusion processes. The cobalt binder melts at approximately 1495°C, while WC decomposes at temperatures above 1200°C in the presence of liquid cobalt. The dissolution reaction proceeds as:

WC + 3Co → Co₃W + C (at elevated temperatures)

This reaction leads to carbide dissolution in the weld pool, with subsequent precipitation of new carbide phases during solidification. The study identifies three distinct microstructural zones:

Welding Process Parameters

The study examines TIG welding parameters specifically optimized for cemented carbide joining:

• Welding current: 40-80 A (DCEN polarity)
• Shielding gas: Argon (99.99%) or Argon-Helium mixture
• Travel speed: 3-8 mm/min (significantly lower than steel welding)
• Preheating temperature: 200-300°C (to reduce thermal gradients)
• Post-weld cooling rate: Controlled below 100°C/min

The low travel speed is critical to ensure complete melting of the cobalt binder without excessive carbide dissolution. Higher travel speeds lead to incomplete melting and poor metallurgical bonding at the interface.

Diffusion Behavior Analysis

The interdiffusion coefficient at the WC-Co interface during welding follows Arrhenius-type behavior:

D = D₀ × exp(-Q/RT)

Where D₀ represents the pre-exponential factor, Q is the activation energy for cobalt diffusion (approximately 180-220 kJ/mol), R is the gas constant, and T is the absolute temperature. The study demonstrates that:

1. Cobalt diffusion into WC particles is negligible at welding temperatures due to the extremely low solubility of Co in the WC lattice.
2. WC dissolution in liquid Co is the dominant interfacial reaction mechanism, governed by the Gibbs-Thomson effect at particle surfaces.
3. Carbon redistribution from dissolving WC creates localized carbon enrichment zones that promote new carbide precipitation during cooling.

Microstructural Evolution and Property Assessment

The significant property degradation in the HAZ represents the primary weakness of TIG-welded cemented carbide joints. The dissolution of WC particles reduces local hardness by 50-70%, creating a soft band susceptible to wear and mechanical failure.

Engineering Practice Considerations

For railway applications where cemented carbide components are welded (such as rail grinding tools or switch point inserts), several practical considerations emerge:

• Post-weld heat treatment can partially restore HAZ properties through controlled carbide precipitation.
• The weld design should minimize the HAZ width by using lower heat input parameters.
• Multi-pass welding with controlled interpass temperature (150-200°C) improves joint integrity.
• Surface finishing after welding can remove the degraded HAZ surface layer, exposing the harder weld metal.

Study Insights and Reflections

This research provides fundamental understanding of the metallurgical processes governing cemented carbide TIG weldability. The interface diffusion analysis reveals that carbide dissolution, rather than traditional diffusion bonding, dominates the interfacial behavior during arc welding. For engineers involved in bimetal product manufacturing where cemented carbide is used as a wear-resistant overlay or facing material, these findings underscore the importance of controlling heat input and cooling rates. The property gradient across the weld interface suggests that joint design should incorporate geometric features that reduce stress concentration in the weakened HAZ region. Future work should explore alternative joining methods such as brazing or friction stir welding that may better preserve the original carbide microstructure.