Common Tube End Forming Defects and Prevention
author: Huashun Machinery
2025-06-27
Flaring Process
| Issue Symptoms – Flaring Process | Root Causes | Solutions |
| Tube end cracking | Insufficient material ductility, excessive flaring ratio (>1.5), flaring speed too high | Perform annealing (heat stainless steel to 800°C), apply staged flaring (expand 10% per step), reduce flaring speed to 1–3 mm/s |
| Uneven wall thickness | Die misalignment > 0.1 mm, large variation in initial tube wall thickness | Use laser alignment to calibrate dies, add tube rotation deformation step |
| Surface orange peel texture | Die surface roughness exceeds limit (Ra > 1.6 μm) | Polish die surface to Ra 0.8 μm, switch to PTFE-coated dies |
| Flaring angle deviation | Die taper wear, hydraulic pressure fluctuation ±5% | Inspect die angle every 500 cycles, implement closed-loop pressure control system |
Preparation:
- Material Inspection:
- Measure tube outer diameter and wall thickness (tolerance ±0.1 mm). Inspect tube ends for burrs and cracks (visual + tactile check). Confirm material ductility (e.g., stainless steel must be preheated to 800°C).Die Preparation:
- Select appropriate flaring angle (30° / 45° / 90°). Check die surface finish (Ra ≤ 0.8 μm). Install guide sleeve to ensure concentricity (≤ 0.05 mm).Equipment Setup:
- Set hydraulic pressure (aluminum tube: 5–10 MPa; steel tube: 15–30 MPa). Adjust feed rate (soft metals: 3–5 mm/s; hard metals: 1–2 mm/s). Perform dry-run stroke test to ensure die clearance during retraction.Auxiliary Measures:
Apply high-temperature lubricant (graphite-based, thickness 0.01–0.03 mm). For thin-walled tubes (t/D < 0.05), insert internal mandrel for support.
Reducing Process
| Issue Symptoms – Reducing Process | Root Causes | Solutions |
| End wrinkling | Wall thickness < 1 mm with no internal mandrel support; reduction ratio > 1.8 | Install carbide mandrel, switch to spinning reduction process |
| Ovality in the reduced area | Uneven die clearance; material anisotropy | Use floating-type reducing dies, add reshaping step with 0.5s dwell time |
| Inner wall scratching | Damaged mandrel surface; poor lubrication | Replace mandrel every 2,000 cycles, switch to molybdenum disulfide lubricant |
| Dimensional springback | High-carbon steel with yield strength > 500 MPa | Apply 0.3–0.5 mm over-reduction to compensate for springback, perform low-temperature stress relief at 200°C |
Preparation:
- Material Pretreatment:
- Check tube end ovality (≤ 0.3% of tube diameter). Mark reducing position (laser marking accuracy ±0.2 mm). Perform annealing treatment (e.g., heat high-carbon steel to 650°C and hold for 10 minutes).Die Setup:
- Select appropriate die type (tapered/stepped/radius). Calculate reduction ratio (Original ID / Final ID ≤ 1.5). Install anti-wrinkle support mechanism.Process Parameter Configuration:
- Program staged reduction (e.g., 3 stages with 10% reduction per stage). Set dwell time (0.5–2 seconds) to reduce springback. Configure cooling system (water-cooled die temperature ≤ 60°C).Quality Control Points:
Prepare go/no-go gauges for post-reduction inspection. Prepare metallographic samples to examine grain deformation.
Flanging Process
| Issue Symptoms – Flanging Process | Root Causes | Solutions |
| Insufficient flange height | Excessive blank holder force, restricting material flow | Adjust blank holder force to 25% of main forming pressure, preheat material (aluminum alloy to 200°C) |
| Edge tearing | Die fillet radius R < 2t (t = material thickness); flanging speed > 15 mm/s | Increase die fillet radius to 3–5× material thickness (t), reduce flanging speed to 5–8 mm/s, and enhance lubrication |
| Uneven flange face | Die parallelism deviation > 0.05 mm; unsynchronized ejector system | Calibrate die using dial indicator, upgrade ejector system to servo-driven type |
| Hole position misalignment | Locating pin wear (clearance > 0.1 mm) | Replace locating pins with carbide pins (HRC62), add vision-based positioning compensation system |
Preparation:
- Tube End Preparation:
- Machine a lead-in chamfer (45° bevel, depth = 1–2x wall thickness). Clean oil residues using acetone wipe. Pre-punch locating hole if multi-sided flanging is required.Die System Check:
- Ensure die-to-holder clearance is 1.05–1.1× material thickness. Confirm ejector stroke (≥ flange height + 2 mm). Preheat die (for aluminum dies, heat to 150–200°C).Process Validation:
- Produce a first-article sample (verify flange height tolerance ±0.1 mm). Adjust blank holder force (typically 20–30% of forming force). Set up infrared thermometry to monitor forming temperature (600–800°C for hot flanging).Safety Measures:
Install scrap auto-collection system. Use dual-hand start buttons (minimum safety distance ≥ 300 mm).
Key Comparison Table:
| Preparation Items | Flaring Process | Reducing Process Setup | Flanging Process Setup |
| Core die | Tapered flaring head | Ring-type reducing die | Stepped flanging die |
| Lubrication requirements | High lubrication (both inner and outer surfaces of tube required) | Medium lubrication (outer surface only) | Low lubrication (only on the blank holder surface) |
| Temperature control | Optional heating | Mandatory temperature control (±20°C) | Die preheating required |
| Inspection tools | Angle gauge | Plug gauge inspection | Height gauge for measurement |
Ultimate Solutions to Common Issues Across All Three Processes
- Material Defect Failures
- Implement a 3-level incoming material inspection system:Level 1: Spectral analysis (chemical composition)
- Level 2: Ultrasonic testing (internal defects)
- Level 3: Pre-forming trials (flaring/bending simulation)
- Insufficient Die Life
| Repair / Treatment Processes | Service Life Improvement | Applicable Scenarios |
| TD (Thermo-Diffusion) treatment | 3–5×increase | Flaring dies for carbon steel / stainless steel |
| Laser cladding | 8–10×increase | High-precision flanging dies |
- Apply appropriate die surface treatments (compare options like nitriding, TiN coating, hard chrome plating).Intelligent Prevention Measures
- Install IoT-based sensors for proactive maintenance:Vibration sensors (detect die misalignment)
- Infrared thermometers (real-time temperature monitoring)
- Pressure curve analysis (predict cracking risk)
Notes and Precautions
- General Requirements:
- All dies must undergo magnetic particle inspection (monthly). Machines must be level-calibrated to within 0.02 mm/m.Material Specifics:
- Titanium Tubes: Must be processed under argon shielding.
- Copper Tubes: Do not use sulfur-containing lubricants.
- Emergency Preparedness:
Equip machines with quick-change die systems (swap time ≤ 5 minutes). Include emergency die-release protocol for stuck tubes (hydraulic relief valve system).
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