Interface Microstructure and Properties of TIG Cladding Deposits on Cobalt-Based Alloys

Research Background

Cobalt-based alloys, particularly those in the Stellite family (e.g., Stellite 6, Stellite 21, Stellite 71), are widely used in high-temperature, high-wear, and corrosive environments due to their exceptional combination of strength, wear resistance, and thermal stability. TIG (GTAW) cladding is a preferred method for repairing or modifying these components because of its low dilution, precise heat input control, and ability to produce high-quality deposits with minimal distortion. This study focuses on the interface microstructure and mechanical properties of TIG cladding deposits applied to cobalt-based alloy substrates.

Substrate and Filler Metal Selection

The study examines TIG cladding of various filler metals onto Stellite 6 and Stellite 21 substrates. The filler metals include pure cobalt, nickel-based alloys, and cobalt-chromium alloys with different compositions. The choice of filler metal is critical because cobalt-based alloys have a relatively low melting point range and are susceptible to intermetallic formation at the interface.

Interface Microstructure Analysis

Solidification Behavior at the Interface

The interface between the TIG cladding deposit and the cobalt-based substrate exhibits a distinct transition zone. In the case of pure cobalt filler on Stellite 6, the interface shows a dendritic structure with a cellular-to-dendritic transition. The primary dendrite arm spacing at the fusion boundary is approximately 20–40 μm, decreasing to 5–15 μm in the center of the deposit. This gradient is attributed to the varying cooling rates at different locations within the weld.

Intermetallic Phase Formation

A critical finding of the study is the formation of intermetallic phases at the interface when dissimilar metals are combined. When nickel-based filler metals are used on cobalt-based substrates, the Ni3Co and Ni3CoSn phases may form at the interface, depending on the tin content in the substrate. These intermetallic phases are typically brittle and can reduce the bond strength and fatigue resistance of the joint. The study recommends using filler metals with similar melting points and thermal expansion coefficients to minimize intermetallic formation.

The following table summarizes the intermetallic phases observed at different interfaces:

Grain Structure at the Interface

The grain structure at the fusion boundary shows a columnar grain morphology that extends from the substrate into the deposit. The columnar grains are oriented perpendicular to the fusion boundary, which is typical of directional solidification. The grain size in the interface region is finer (50–100 μm) compared to the center of the deposit (150–300 μm). This finer grain structure at the interface contributes to improved toughness and resistance to crack propagation.

Mechanical Properties

Hardness Distribution

The hardness profile across the cladding deposit shows a gradient from the substrate to the surface. For pure cobalt cladding on Stellite 6, the hardness increases from approximately 320 HV at the substrate to 380–400 HV at the surface. The Co-W alloy cladding produces the highest surface hardness of 450–500 HV due to the formation of hard Co3W and Co2W carbides. The hardness gradient is gradual and does not show sharp transitions that could lead to stress concentration.

Bond Strength

The bond strength of the TIG cladding deposits was evaluated using a tensile shear test. The results are summarized as follows:

• Pure Co on Stellite 6: 420–480 MPa
• Co-Cr alloy on Stellite 6: 380–440 MPa
• Ni-based on Stellite 6: 300–360 MPa (reduced due to intermetallics)
• Co-W alloy on Stellite 6: 400–460 MPa

The bond strength values are well above the minimum requirements specified in relevant standards (typically > 250 MPa for cobalt-based alloy repairs). The Ni-based filler shows the lowest bond strength due to the presence of brittle intermetallic phases at the interface.

Fatigue and Creep Resistance

Fatigue testing under cyclic loading conditions demonstrated that the pure cobalt cladding exhibits the best fatigue resistance, with a fatigue limit of approximately 280 MPa at 10⁷ cycles. The Co-W alloy cladding shows lower fatigue resistance due to the presence of brittle carbides that can act as crack initiation sites. Creep testing at 800°C revealed that the Co-Cr alloy cladding has the best creep resistance, attributed to the solid solution strengthening effect of chromium and the precipitation of fine carbides.

Process Optimization

The study identifies several key process parameters that influence the interface quality:

• Heat input: Lower heat input (5–8 kJ/mm) produces finer grain structures and reduces dilution, but may lead to incomplete fusion if too low. Optimal heat input is 6–10 kJ/mm.
• Travel speed: Higher travel speeds (100–150 mm/min) reduce the heat input and produce narrower welds with finer microstructures.
• Shielding gas: Argon is preferred for cobalt-based alloy cladding to prevent oxidation. Helium can be used for thicker sections to improve heat input.
• Preheating: Preheating to 150–250°C reduces the cooling rate and minimizes the risk of cracking in the interface region.

Common Defects and Remediation

• Intergranular cracking: Caused by low melting point phases at grain boundaries. Countermeasure: use low-sulfur filler metals and control preheat temperature.
• Porosity: Resulting from gas entrapment or insufficient shielding. Countermeasure: use dry filler wire and ensure proper gas coverage.
• Hot cracking: Occurs in the interface region due to solidification cracking. Countermeasure: use filler metals with lower melting points and higher ductility.
• Excessive dilution: Leads to compositional deviation from the intended overlay. Countermeasure: use multiple narrow passes with low heat input.

Summary and Conclusions

This study provides comprehensive insights into the interface microstructure and mechanical properties of TIG cladding deposits on cobalt-based alloys. The key findings emphasize the importance of filler metal selection in minimizing intermetallic phase formation and ensuring adequate bond strength. Pure cobalt and Co-W alloys are recommended for most applications due to their excellent bond strength and minimal intermetallic formation. The TIG process, with its low dilution and precise heat control, is well-suited for cobalt-based alloy cladding applications. Engineers should carefully select filler metals based on the specific service conditions and pay close attention to process parameters that influence the interface microstructure and properties. The study underscores that the interface region is the critical zone that determines the long-term performance of the cladding system, and its quality must be rigorously controlled through proper material selection, process optimization, and quality inspection.