Zhuojin Pipe Fitting Co., LtdIntermetallic Compound Composites Prepared by Overlay Welding Process
Literature Overview
This study by Suo Jinping and colleagues from Huazhong University of Science and Technology and the Iron and Steel Research Institute investigates the preparation of intermetallic compound composite materials using overlay welding. Published in Welding Journal (Vol. 24, No. 3, 2003), this work explores a novel approach to creating wear-resistant composite materials by combining metal powders, ball milling, and arc welding processes.
Core Technical Findings
Composite Powder Preparation
The composite electrode powder is prepared through a multi-step process:
- Powder selection: WC, NiAl, NiB, and Ni powders are used as the base components.
- Ball milling: Mechanical alloying breaks down WC particles and promotes solid-state reactions between NiAl, NiB, and Ni to form the Ni3Al intermetallic compound.
- Sintering: The ball-milled powder mixture is sintered to consolidate the powder and stabilize the intermetallic phase.
This powder metallurgy approach allows for the creation of a composite material with a microstructure that would be impossible to achieve through conventional melting and casting alone.
Overlay Welding Process and Microstructural Evolution
The composite electrode is used to overlay weld a 5 mm thick layer onto 1Cr25Ni20Si2 stainless steel using GTAW (argon arc welding). During the welding process, significant microstructural transformations occur:
| Component | Pre-Welding State | Post-Welding State | Transformation Mechanism |
|---|---|---|---|
| WC | Intact particles | Partially dissolved | Liquid-phase dissolution in weld pool |
| W2C | Not present | Newly precipitated carbide | Recrystallization of dissolved W and C |
| Ni3Al | Ball-milled intermetallic | Transformed to Ni3(Al,Ti)C | Reaction with Ti impurities and carbon |
| NiB | Ball-milled compound | Partially dissolved | Melting and redistribution in weld pool |
The transformation of Ni3Al to Ni3(Al,Ti)C is particularly interesting, as it indicates that titanium impurities present in the powder or base metal participate in the intermetallic compound formation, potentially enhancing the stability and hardness of the final microstructure.
Wear Resistance Performance
The resulting composite overlay material achieves wear resistance more than 3 times that of 45 steel (a medium-carbon structural steel). This improvement is attributed to the combined reinforcement of:
- Residual WC particles (high hardness, high modulus)
- Newly formed W2C carbides (extremely hard, thermally stable)
- Ni3(Al,Ti)C intermetallic compound (high hardness, good thermal stability)
- Ni-based matrix (good toughness and ductility)
Engineering Practice Implications
Process Parameters for Composite Overlay Welding
The GTAW process was selected for this application because it provides excellent control over heat input and minimal dilution, which is critical when overlay welding expensive composite materials. Key process parameters include:
- Current: 150-200 A (typical for 5 mm overlay thickness)
- Travel speed: 200-300 mm/min
- Shielding gas: 99.99% Ar
- Preheating: 200-300°C to prevent cracking in the Ni-based overlay
Base Metal Compatibility
The use of 1Cr25Ni20Si2 stainless steel as the base material is significant. This austenitic stainless steel provides good compatibility with the Ni-based overlay in terms of thermal expansion and corrosion resistance. However, the high alloy content of the base metal may introduce dilution concerns, particularly for the carbon content that drives carbide formation.
Defect Considerations
Common defects in this type of overlay welding include:
- Cracking: Due to high residual stresses from thermal mismatch between the Ni-based overlay and the stainless steel base.
- Porosity: From gas entrapment during the multi-pass welding of a thick overlay.
- Incomplete fusion: Particularly at the interface between the composite overlay and the base metal.
Key Reflections
This study demonstrates the power of combining powder metallurgy with welding technology to create materials with properties that exceed those achievable through either method alone. The ball milling step pre-forms intermetallic compounds that would require extreme processing conditions to form through conventional melting, while the welding step provides the consolidation and joining that powder metallurgy alone cannot achieve. The 3x improvement in wear resistance over 45 steel is substantial and suggests significant potential for applications in high-wear environments. However, the complexity of the powder preparation process and the sensitivity of the microstructure to welding parameters present challenges for industrial scale-up that must be carefully managed.