Deformation Behavior of Overlay Bimetal During Cold Upsetting

Literature Overview

This paper by Hui Youke, Wang Huajun, Xie Bing, and Chen Wen, published in "Forging and Stamping Technology" in 2017 (Volume 42, Issue 10, pages 1-4), investigates the deformation behavior of overlay bimetal during cold upsetting. The research was funded by the National Natural Science Foundation of China (Grant No. 51475346) and conducted at the School of Materials Science and Engineering, Wuhan University of Technology.

Technical Background

Overlay bimetal is a composite material created by depositing a hardfacing or wear-resistant alloy onto a ductile base material through welding processes such as plasma arc surfacing. This approach combines the wear resistance of the overlay layer with the toughness and ductility of the base material. However, the feasibility of subsequent forming operations, such as cold upsetting, is limited by the deformation behavior of the bimetal interface.

Plasma Arc Surfacing Process

Plasma arc surfacing (PAS) is a widely used technique for creating overlay bimetal. The process involves:

A high-velocity plasma arc melts the base metal surface
A filler wire or powder is introduced into the arc
The molten pool solidifies to form a dilution-controlled overlay layer

Parameter	Typical Range	Effect
Plasma Current	80-200 A	Controls dilution and penetration
Arc Voltage	20-40 V	Affects arc stability and energy density
Travel Speed	200-500 mm/min	Controls cooling rate and layer thickness
Shielding Gas Flow	10-20 L/min	Protects the molten pool from oxidation
Wire Feed Speed	200-400 mm/min	Controls deposit thickness

Experimental Methodology

The researchers prepared bimetal specimens using plasma arc surfacing technology and then subjected them to cold upsetting tests. The experimental approach involved:

Microhardness measurement: To characterize the hardness distribution across the overlay layer and base metal
Cold upsetting tests: To observe the deformation and failure modes of the bimetal
Microstructural analysis: To understand the metallurgical changes during deformation

Microhardness Distribution

The key finding from the microhardness measurements is that the weakest region in the overlay bimetal is near the fusion line. This is a critical observation because:

The overlay layer has significantly higher hardness than the base metal
The fusion line represents the interface between the two materials
The hardness gradient at the fusion line creates a stress concentration zone

Region	Relative Hardness	Characteristics
Overlay Layer	High (2-3× base)	Hard, wear-resistant, but brittle
Fusion Line	Lowest	Stress concentration, potential crack initiation site
Base Metal	Moderate	Ductile, provides toughness
Heat-Affected Zone	Variable	May have altered microstructure

Deformation Behavior Analysis

Cold Upsetting Process

Cold upsetting is a bulk deformation process where a specimen is compressed at room temperature to increase its cross-sectional area. The process involves:

Uniaxial compressive stress
Multi-axial strain state
Plastic deformation of both the overlay and base metal

Observed Deformation Sequence

The researchers observed a distinct deformation sequence during cold upsetting:

Initial stage: The base metal region deforms first due to its lower hardness and higher ductility
Intermediate stage: The overlay layer resists deformation due to its high hardness
Advanced stage: The bottom region of the overlay layer begins to deform plastically
Final stage: The overlay layer exhibits plastic deformation potential

Stress Distribution

The deformation behavior can be understood through stress analysis:

Stage	Base Metal	Overlay Layer	Fusion Line
Initial	High strain	Low strain	Stress concentration
Intermediate	Continued strain	Elastic deformation	High stress
Advanced	Strain hardening	Plastic deformation begins	Stress redistribution
Final	Uniform strain	Plastic deformation	Stress equilibrium

Engineering Implications

Feasibility of Cold Forming

The research demonstrates that overlay bimetal can undergo plastic deformation during cold upsetting, but with important limitations:

Deformation capacity: The overlay layer limits the total deformation that can be achieved
Interface integrity: The fusion line is the critical weak point
Stress distribution: The hardness gradient creates complex stress states
Ductility requirement: The base metal must have sufficient ductility to accommodate deformation

Applications and Limitations

Application	Feasibility	Key Considerations
Small deformation (< 10%)	High	Interface integrity maintained
Moderate deformation (10-30%)	Moderate	Requires careful parameter control
Large deformation (> 30%)	Low	Risk of interface failure
Complex geometries	Variable	Depends on strain distribution

Key Reflections

This research provides valuable insights into the forming behavior of overlay bimetal, which is directly relevant to manufacturing processes that require both wear resistance and complex geometries. The finding that the fusion line is the weakest region has important implications for:

Process design: Forming operations must be designed to minimize stress at the fusion line
Material selection: The base metal must have adequate ductility to accommodate deformation
Quality control: The fusion line must be carefully inspected for defects
Process optimization: Multi-step forming may be necessary to achieve large deformations

The research also highlights the importance of understanding the metallurgical interface in composite materials. For engineers working with overlay-welded components, this knowledge is essential for predicting component performance during service and for designing appropriate forming processes.