Zhuojin Pipe Fitting Co., LtdTesting and Analysis of Copper Strip Hardfacing on Projectile Bodies
Literature Overview
This paper by Liu Shuyan, Hou Xiujun, Wang Shuhua, and Deng Hua from the Physical Chemistry and Metrology Center of Factory 123, published in Chemical Analysis and Metering in 2011 (Vol. 20, Suppl. S1, pp. 66-69), presents an innovative approach to joining copper alloy strips to steel projectile bodies using TIG arc hardfacing. The traditional method of mechanically embedding copper or copper alloy rings into grooves machined on the projectile body was found to be inadequate for certain applications, leading to deformation and loosening during the forming process.
Problem Statement and Traditional Approach Limitations
The projectile body in question is a thin-walled component that undergoes significant plastic deformation during the forming process. The traditional mechanical connection method involves:
- Machining a circumferential groove on the projectile body surface.
- Fabricating a ring-shaped copper or copper alloy strip.
- Press-fitting the copper ring into the groove.
This mechanical connection suffers from several critical deficiencies:
| Deficiency | Description | Consequence |
|---|---|---|
| Thin-wall deformation | The projectile body deforms during forming | Dimensional inaccuracy |
| Copper strip loosening | Mechanical fit is not permanent | Functional failure |
| Stress concentration | Sharp groove edges create stress risers | Crack initiation |
| Limited design flexibility | Groove geometry constrains design | Performance compromise |
| Assembly complexity | Press-fitting requires precision | Production inefficiency |
Hardfacing Process Development
The authors proposed replacing the mechanical connection with a TIG (Tungsten Inert Gas) arc hardfacing process, where copper alloy is directly deposited onto the steel projectile body surface. This approach eliminates the need for groove machining and mechanical fitting, creating a metallurgical bond between the copper alloy layer and the steel substrate.
Welding Process Parameters
The TIG hardfacing process involves several critical parameters that must be carefully controlled:
- Shielding Gas: Argon (Ar) or helium-argon mixture to prevent oxidation of both the molten copper alloy and the steel substrate.
- Electrode Material: Filler wire or rod of the appropriate copper alloy composition.
- Current Density: Must be carefully balanced to achieve adequate fusion with the steel substrate without excessive melting of the thin-walled projectile body.
- Travel Speed: Controls the deposit thickness and heat input per unit length.
- Preheating: May be required to reduce thermal stress in the thin-walled component.
Key Technical Challenges
The primary challenge in this application is the significant difference in thermal expansion coefficients between copper alloy (approximately 17 × 10⁻⁶ /°C) and steel (approximately 12 × 10⁻⁶ /°C). This mismatch creates thermal stresses during cooling that can lead to cracking or delamination at the interface. Additionally, the thin wall thickness of the projectile body limits the allowable heat input, as excessive heating can cause distortion or even melting through.
Microstructural and Compositional Analysis
The authors conducted comprehensive characterization of the hardfacing deposits using multiple analytical techniques:
Energy Dispersive X-ray Spectroscopy (EDS)
EDS analysis was used to determine the iron content in the copper alloy layer, which directly reflects the dilution rate. The iron content in the copper alloy layer is a critical indicator of process quality, as excessive dilution can degrade the electrical and mechanical properties of the copper layer.
Spectrographic Analysis
Spectrographic analysis provided detailed composition profiles of the copper alloy layer, the interface region, and the steel substrate. The compositional gradient across the interface reveals the extent of interdiffusion between copper and iron during the welding process.
Scanning Electron Microscopy (SEM)
SEM examination revealed the microstructural morphology of the copper alloy layer, the interface zone, and the steel substrate. Key observations include:
- The copper alloy layer microstructure and grain morphology.
- The nature of the iron-copper intermetallic compounds formed at the interface.
- The heat-affected zone (HAZ) in the steel substrate.
- The quality of the metallurgical bond at the interface.
Optical Metallography
Optical microscopy provided additional information on the overall microstructural features, including:
- The thickness and uniformity of the copper alloy deposit.
- The presence or absence of defects such as porosity, cracks, or lack of fusion.
- The morphology of intermetallic phases at the interface.
Mechanical Property Analysis
The mechanical properties of the projectile body with the copper alloy hardfacing were evaluated to assess the functional performance of the joint. Key properties examined include:
| Property | Significance | Acceptance Consideration |
|---|---|---|
| Tensile Strength | Load-bearing capacity | Must meet projectile design requirements |
| Hardness | Interface integrity | Gradient from copper to steel |
| Elongation | Ductility retention | Must withstand forming deformation |
| Impact Energy | Toughness | Resistance to fracture during service |
The hardfacing process must not compromise the mechanical properties of the projectile body, particularly its ability to undergo the forming deformation without cracking. The thin-walled geometry makes this requirement especially challenging.
Engineering Practice Implications
This study demonstrates the viability of hardfacing as an alternative to mechanical connection for copper-steel joints in precision components. The key engineering considerations include:
- Process Window: The TIG welding parameters must be tightly controlled to balance fusion quality with heat input limitations imposed by the thin wall thickness.
- Interface Quality: The metallurgical bond at the copper-steel interface is critical for functional performance. Intermetallic compound formation must be managed to ensure adequate toughness.
- Dimensional Control: The hardfacing deposit must maintain the required surface finish and dimensional accuracy of the projectile body.
- Consistency: The process must be repeatable across production quantities, requiring stable power sources and well-defined procedures.
Study Insights and Reflections
This paper addresses a practical manufacturing challenge with an elegant solution. The transition from mechanical connection to hardfacing represents a paradigm shift in joining philosophy, moving from a mechanical interlock to a metallurgical bond. The comprehensive characterization approach, combining EDS, spectrographic analysis, SEM, optical metallography, and mechanical testing, provides a thorough understanding of the joint quality.
The study is particularly relevant to engineers working on precision components where traditional joining methods are inadequate. The use of TIG welding for hardfacing copper alloys onto steel substrates is a well-established technique in the hardfacing industry, but its application to thin-walled projectile components presents unique challenges related to heat input control and thermal stress management. The successful implementation of this process demonstrates that careful process development and rigorous quality assessment can overcome these challenges. The findings have broader applicability to any application requiring a reliable copper-steel metallurgical bond on thin-walled components, including electrical contacts, heat exchanger tubes, and precision instruments.