Deformation Behavior of Bimetallic Overlay During Cold Upsetting

Literature Overview

This paper by Hui Youke, Wang Huajun, Xie Bing, and Chen Wen, published in Forging and Stamping Technology (锻压技术) in 2017, Volume 42, Issue 10, investigates the deformation behavior of bimetallic materials produced by plasma arc surfacing during cold upsetting. The research, funded by the National Natural Science Foundation of China (Grant No. 51475346), explores the feasibility of forging bimetallic workpieces, which could enable the production of components with functionally graded properties.

Research Background and Significance

Bimetallic materials produced by overlay welding combine the properties of two distinct materials in a single component. For example, a hard, wear-resistant overlay on a tough, ductile substrate could provide both surface durability and structural integrity. However, the deformation behavior of such materials during plastic forming processes is poorly understood, particularly at the interface region where mechanical property discontinuities exist.

The authors prepared bimetallic specimens using plasma arc surfacing (PASM), a process that offers high energy density, low dilution, and good bonding quality. The overlay material was deposited onto a base substrate, creating a distinct interface with a narrow heat-affected zone.

Microstructure and Hardness Characterization

The authors conducted microhardness measurements across the cross-section of the bimetallic specimens. The key findings are summarized below:

The critical observation is that the weakest region in terms of hardness is not the overlay or the substrate, but the fusion line or dilution zone near the interface. This is because the dilution zone contains a mixture of overlay and base metal elements, potentially forming softer phases or having a coarser microstructure due to the thermal cycle experienced during surfacing.

Cold Upsetting Deformation Analysis

Cold upsetting is a bulk forming process where a cylindrical specimen is compressed axially, causing lateral expansion. The authors analyzed the deformation and failure modes of bimetallic specimens during this process:

1. Initial stage: The base substrate region deforms plastically first due to its lower yield strength compared to the overlay. The overlay layer remains largely elastic.
2. Intermediate stage: As deformation progresses, the stress concentration at the interface increases. The overlay layer begins to deform plastically, particularly at its base region adjacent to the dilution zone.
3. Advanced stage: The overlay layer exhibits significant plastic deformation, demonstrating that it has the capacity to undergo substantial plastic strain without delamination or cracking.

The deformation behavior can be understood through the concept of strain compatibility. Since the overlay and substrate are bonded, they must deform compatibly. The softer substrate deforms first, imposing strain on the overlay through the interface. The overlay, being harder, initially resists deformation but eventually yields under the imposed strain.

Interface Integrity and Failure Modes

The interface between the overlay and substrate is the critical region for structural integrity. The authors observed:

• No delamination: The fusion bond between overlay and substrate maintained integrity throughout the cold upsetting process, indicating good metallurgical bonding.
• Crack initiation: Any cracks, if present, would initiate at the dilution zone due to its lower hardness and potential microstructural weakness.
• Strain localization: The strain distribution is non-uniform, with higher strains in the substrate and lower strains in the overlay, particularly near the surface.

Feasibility Assessment for Forging Applications

Based on the experimental results, the authors conclude that bimetallic workpieces produced by plasma arc surfacing have the potential for cold forging. Key considerations include:

Study Insights and Reflections

This paper makes a significant contribution to the understanding of how bimetallic materials behave under plastic deformation. The finding that the overlay layer can undergo plastic deformation, even though it is harder than the substrate, is counterintuitive but physically consistent with the mechanics of bonded interfaces.

From a practical standpoint, this research opens the door to several engineering applications:

• Functionally graded components: Shafts with hard, wear-resistant surfaces and tough cores could be produced by first forging the substrate and then applying a wear-resistant overlay, or vice versa.
• Repair and enhancement: Existing components could be enhanced with overlay materials and then subjected to additional forming operations if needed.
• Tool and die manufacturing: Bimetallic tools with hard working surfaces and tough backing structures could be produced through combined welding and forging processes.

However, several challenges remain to be addressed:

1. The dilution zone weakness must be mitigated, possibly through multi-pass surfacing with intermediate annealing or by using overlay materials with closer thermal and mechanical properties to the substrate.
2. The forming parameters must be carefully controlled to avoid excessive strain in the dilution zone, which could lead to cracking.
3. The interaction between residual stresses from the surfacing process and the stresses introduced during forging needs further investigation.

This research represents a promising intersection of welding and forming technologies, and I believe it has significant potential for industrial development in the coming years.