CLADDING TECHNOLOGY SHANXI CO., LTDTIG Weld Overlay Process for Copper Alloy Molds of Xiali Automobiles
Literature Overview
This paper, authored by Bai Jin-sheng, Lin Jia-ming, and Han Chun-lan from the Tianjin Welding Research Institute (published in 1995 in the journal "Welding Technology"), describes the development and application of gas tungsten arc welding (GTAW/TIG) overlay technology for copper alloy molds used in the production of Xiali (a Chinese automobile model) body panels. The study addresses the challenge of manufacturing high-quality copper alloy molds for automotive stamping, where the mold surface must exhibit excellent wear resistance, thermal conductivity, and dimensional accuracy.
Technical Context and Requirements
Automotive body stamping molds are subjected to:
- High contact pressure — During stamping, the blank exerts pressures of 200–500 MPa on the mold surface.
- Frictional wear — Each stamping cycle causes micro-slip between the blank and the mold, leading to progressive surface degradation.
- Thermal cycling — The mold surface experiences rapid temperature changes during stamping, causing thermal fatigue.
- Adhesion wear — At elevated temperatures, the steel blank can adhere to the copper mold surface, causing material transfer and surface damage.
The base material of the molds is typically a copper alloy (such as CuCrZr or CuBe), which provides good thermal conductivity and formability but limited surface hardness and wear resistance. The TIG overlay layer must therefore provide a hard, wear-resistant surface while maintaining good thermal conductivity and a strong bond to the base metal.
Overlay Material Selection
The authors evaluated several overlay materials for the copper alloy molds:
| Overlay Material | Composition | Hardness (HV) | Wear Resistance | Thermal Conductivity | Compatibility with Cu |
|---|---|---|---|---|---|
| Pure copper | Cu 99.9% | 40–50 | Poor | 390 W/(m·K) | Excellent |
| Cu-Cr alloy | Cu-1.5Cr | 80–100 | Good | 200 W/(m·K) | Good |
| Cu-Ti alloy | Cu-2Ti | 120–150 | Excellent | 150 W/(m·K) | Moderate |
| Cu-Cr-Ti alloy | Cu-1Cr-0.5Ti | 100–130 | Excellent | 180 W/(m·K) | Good |
| Ag-Cu alloy | Ag-30Cu | 150–180 | Excellent | 120 W/(m·K) | Moderate |
The Cu-Cr-Ti alloy was selected as the optimal overlay material, providing an excellent balance of hardness, wear resistance, thermal conductivity, and metallurgical compatibility with the copper base.
TIG Overlay Process Development
The TIG overlay process was developed with the following parameters:
| Parameter | Value | Rationale |
|---|---|---|
| Welding current | 80–120 A | Sufficient for Cu-Cr-Ti wire melting |
| Welding voltage | 12–15 V | Stable arc, good wetting |
| Travel speed | 80–120 mm/min | Controlled cooling rate |
| Tungsten electrode | Φ2.4 mm, thoriated | Sharp arc, good penetration |
| Filler wire | Φ1.6 mm Cu-Cr-Ti | Matching composition |
| Shielding gas | Argon, 15–20 L/min | Complete protection |
| Preheat temperature | 100–150°C | Reduce thermal cracking risk |
| Interpass temperature | ≤ 200°C | Prevent grain growth |
| Pulse frequency | 5–10 Hz | Controlled heat input |
| Pulse ratio | 3:1 (on:off) | Reduced total heat input |
The pulse TIG process was selected over conventional DC TIG to reduce the total heat input, minimize distortion, and achieve a finer microstructure in the overlay layer. The pulse parameters were optimized to ensure complete melting of the filler wire while maintaining a low average current.
Process Optimization and Quality Control
Dilution Control
The dilution ratio between the copper base and the Cu-Cr-Ti overlay was a critical parameter. The following measures were employed to control dilution:
- The first pass was applied with a lower current (60–80 A) to minimize penetration into the base metal.
- A "back-step" welding pattern was used to reduce the effective heat input.
- The filler wire was fed manually with a slight forward angle to promote surface deposition over deep penetration.
The resulting dilution ratio was 20–30%, which was acceptable for maintaining the overlay composition and properties.
Defect Prevention
The following defects were identified and controlled during process development:
| Defect Type | Cause | Prevention Measure |
|---|---|---|
| Cracking | Thermal stress, hydrogen | Preheat, low-hydrogen consumables |
| Porosity | Gas inclusion, insufficient shielding | Increased gas flow, clean surface |
| Lack of fusion | Insufficient heat input | Increased current, proper technique |
| Excessive dilution | High heat input, deep penetration | Reduced current, pulse TIG |
| Surface irregularity | Unstable arc, manual feed | Consistent technique, automation |
Quality Inspection
The overlay layer was inspected using:
- Visual inspection — Surface finish, uniformity, and absence of visible defects.
- Hardness testing — Vickers hardness measured at 5 locations across the overlay surface, with an average of 110–130 HV.
- Bond strength testing — Shear bond strength measured at 350–400 MPa, exceeding the minimum requirement of 250 MPa.
- Metallographic examination — Cross-sectional examination confirmed a sound interface with no cracks or unmelted base metal.
- Wear testing — Pin-on-disk abrasion tests showed 2–3 times the wear resistance of the uncoated copper base.
Engineering Practice and Service Experience
The TIG overlay process was successfully applied to the production of Xiali automobile body panel molds. The following results were achieved:
- Mold life extension — The overlay extended the mold service life from approximately 50,000 stampings to over 150,000 stampings, a threefold improvement.
- Surface quality — The overlay surface achieved a finish of Ra 1.6–3.2 μm after light grinding, meeting the requirements for high-quality automotive panels.
- Dimensional accuracy — The overlay maintained the mold dimensional accuracy within ±0.05 mm, with minimal distortion.
- Cost effectiveness — The overlay process cost was approximately 15% of the cost of mold replacement, providing significant economic savings.
The study demonstrates that TIG overlay is a versatile and effective technology for improving the surface properties of copper alloy molds. The pulse TIG process, in particular, offers superior control over heat input and dilution, making it suitable for precision applications where dimensional accuracy and surface quality are critical.
Study Insights and Implications
This paper, although published in 1995, remains relevant to modern engineering practice. The fundamental principles of TIG overlay process development — dilution control, defect prevention, and quality verification — are timeless and continue to guide the application of weld overlay technology across diverse industries. The paper's emphasis on the use of pulse TIG for reduced heat input and improved microstructure is particularly noteworthy, as it anticipates the current trend toward advanced welding processes that offer superior control and quality.
The successful application of TIG overlay to automotive stamping molds highlights the importance of understanding the specific service conditions and material requirements of each application. The selection of the Cu-Cr-Ti overlay material, the optimization of pulse parameters, and the implementation of rigorous quality control measures all contributed to the successful outcome. This case study serves as a valuable reference for engineers considering the application of weld overlay technology to other precision components where surface properties and dimensional accuracy are critical.