Intermetallic Compound Composites Prepared by Overlay Welding on Stainless Steel

Literature Overview

Published in Welding Journal (2003, Vol. 24, Issue 3, pp. 44–47), this paper by Suo Jinping and colleagues from the State Key Laboratory of Plastic Forming Simulation and Die Technology, Huazhong University of Science and Technology, and the Institute of High Temperature Alloys, Institute of Metal Research, Chinese Academy of Sciences, presents a novel approach to fabricating metal-intermetallic compound composite materials by overlay welding. The research was supported by the National 863 Program (Project No. 715-005-0010). The composite electrode was prepared by mixing, ball-milling, and sintering WC, NiAl, NiB, and Ni powders. The composite electrode was then used to deposit a 5 mm thick intermetallic compound wear-resistant composite layer on the surface of 1Cr25Ni20Si2 stainless steel using tungsten inert gas (TIG) welding.

Core Technical Findings

The study reveals several important metallurgical phenomena during the overlay welding process:

Ball-milling effects: During the ball-milling process, WC particles were fractured into smaller fragments, and NiAl, NiB, and Ni reacted to form the intermetallic compound Ni3Al. This solid-state reaction was facilitated by the mechanical energy input from ball milling, which increased the reactivity of the powder surfaces and promoted interdiffusion.
Welding-induced transformations: During the TIG welding process, partial dissolution of WC occurred, leading to the precipitation of a new carbide phase, W2C. Simultaneously, Ni3Al transformed into a new intermetallic compound, Ni3(AlTi)C, incorporating titanium from the substrate. These transformations indicate significant metallurgical activity at the high temperatures reached during welding.
Wear resistance: The resulting intermetallic compound composite material exhibited wear resistance more than three times that of 45 steel, demonstrating the effectiveness of the composite approach for wear protection.

Component	Ball-Milling Effect	Welding Transformation
WC	Particle fragmentation	Partial dissolution; W2C precipitation
NiAl	Solid-state reaction with NiB, Ni	Transformation to Ni3(AlTi)C
NiB	Reacts with NiAl, Ni	Contributes to intermetallic formation
Ni	Reacts with NiAl, NiB	Matrix phase; solid solution strengthening

Interpretation of Technical Points

The fabrication of metal-intermetallic compound composites by overlay welding is a creative approach that combines the advantages of powder metallurgy and welding. The ball-milling step serves a dual purpose: it reduces the particle size of WC to improve dispersion and reactivity, and it promotes solid-state reactions between the metallic powders to form intermetallic phases. The sintering step then consolidates the powder mixture into a coherent electrode that can be used for welding.

The welding process introduces additional metallurgical complexity. The high temperatures achieved during TIG welding are sufficient to partially dissolve the WC particles and promote the formation of new phases. The precipitation of W2C is significant because W2C is a hard, wear-resistant carbide with a high melting point (approximately 2800 °C) that can provide excellent resistance to abrasive wear. The transformation of Ni3Al to Ni3(AlTi)C is also noteworthy, as it indicates interdiffusion between the deposit and the substrate. The incorporation of titanium from the 1Cr25Ni20Si2 stainless steel substrate into the intermetallic phase suggests that the welding process creates a chemically bonded interface between the composite layer and the base metal, which is beneficial for adhesion and load transfer.

The threefold improvement in wear resistance compared to 45 steel is substantial and demonstrates the potential of intermetallic compound composites for wear protection applications. The combination of hard carbide and intermetallic phases in a ductile nickel-based matrix provides a synergistic effect, where the hard phases resist abrasive wear and the ductile matrix accommodates plastic deformation and prevents catastrophic failure.

Engineering Practice Implications

The approach described in this study has several practical implications for engineers designing wear-resistant coatings:

Composite electrode fabrication: The ball-milling and sintering process for composite electrode fabrication is relatively straightforward and can be adapted for various powder combinations. However, the ball-milling parameters, including milling time, ball-to-powder ratio, and milling atmosphere, must be carefully controlled to achieve the desired powder characteristics and avoid contamination.
Welding process selection: TIG welding was used in this study, which provides a stable, low-splatter arc suitable for precise deposition. However, other welding processes, such as plasma arc welding (PAW) or laser beam welding, may be more suitable for certain applications, particularly where high deposition rates or deep penetration are required.
Layer thickness and build-up: The 5 mm thick composite layer deposited in this study is relatively thick and may require multiple passes. The interpass temperature and heat input must be controlled to avoid excessive grain growth or phase coarsening, which could reduce the wear resistance of the final deposit.
Interface characterization: The metallurgical activity at the deposit-substrate interface, as evidenced by the formation of Ni3(AlTi)C, should be carefully characterized to ensure adequate adhesion and compatibility. Interface cracking or delamination is a potential failure mode that must be prevented through appropriate welding procedure design.

A limitation of this study is that it does not address the long-term stability of the intermetallic compound composite under elevated temperatures or in corrosive environments. Intermetallic compounds can be susceptible to oxidation and corrosion, and the long-term durability of the composite layer in harsh service conditions should be evaluated before widespread industrial application.

Summary and Reflections

This paper presents an innovative approach to fabricating wear-resistant composite materials by combining powder metallurgy and overlay welding. The formation of new intermetallic and carbide phases during the welding process, along with the threefold improvement in wear resistance compared to 45 steel, demonstrates the significant potential of this approach. The metallurgical transformations that occur during welding, including WC dissolution, W2C precipitation, and Ni3Al transformation to Ni3(AlTi)C, highlight the complexity of the process and the need for careful control of welding parameters. The study provides a valuable foundation for further research into the design and application of intermetallic compound composites for wear protection in demanding industrial environments.