Activity 002: MIG Welding — T-Joint Fillet Welds

Activity ID: U6M2-ACT-002 Duration: 45 minutes Objective: Students will MIG weld T-joint fillet welds in the flat/horizontal position, applying proper torch angle, travel speed, and parameter selection for complete fusion. Group Size: 2 students

Overview

Building on the flat-plate bead running skills, students will weld their first structural joint — the T-joint fillet. This is the most common weld in fabrication and teaches the critical skill of directing heat to both joint members simultaneously.

Materials & Equipment Needed

• MIG welder (same setup as Activity 001: ER70S-6, C25, 0.030" wire)
• 8 mild steel coupons: ⅛" × 3" × 6" (4 T-joint assemblies, tack-welded from Activity U6M1-ACT-001 or newly prepared)
• Welding magnets (90° squares)
• Wire brush and chipping hammer
• Fillet weld gauge
• Angle grinder with flap disc (for cleanup)
• Full welding PPE
• Fire extinguisher within 15 feet

Instructions & Procedure

Phase 1: Joint Preparation (5 minutes) 1. If using previously tack-welded T-joints from U6M1, verify tacks are secure and angle is 90° 2. If preparing new joints: grind joint surfaces to bright metal, tack weld at both ends, verify 90° with square 3. Set MIG parameters for ⅛" material: - Wire feed speed: 300 IPM (~120A) - Voltage: 19V - Gas flow: 22 CFH

Phase 2: First T-Joint Fillet (15 minutes)

Weld 1 (one side): 1. Position the T-joint with the vertical plate pointing up, weld running left-to-right 2. Set torch work angle at 45° — bisecting the angle between the horizontal and vertical plates 3. Set travel angle at 10° push 4. Direct the arc into the ROOT of the joint (where the two plates meet) 5. Start the arc at one end, watch the weld pool form — it should wet out equally on both plates 6. Travel at a steady pace — the pool should be approximately ⅜" wide 7. If the pool favors one plate, adjust the torch angle slightly toward the unfused plate 8. Complete the 6" fillet weld, release trigger

Weld 2 (opposite side): 9. Flip or rotate the assembly to weld the opposite side 10. Apply the same technique 11. After completing, let cool 2 minutes

Inspection: 12. Wire brush both welds 13. Measure fillet leg size with the fillet gauge — target: ⅛" legs (matching base metal thickness) 14. Check for: equal leg lengths, consistent bead width, toe fusion (no cold lap), no undercut 15. Record measurements and observations on worksheet

Phase 3: Second T-Joint (Practice for improvement) (15 minutes) 1. Use a fresh tacked T-joint assembly 2. Based on observations from the first joint, adjust ONE parameter if needed 3. Weld both sides with focused attention on the area that was weakest in the first attempt 4. Inspect and measure — compare to the first joint

Phase 4: Comparison & Assessment (10 minutes) 1. Place both completed T-joints side-by-side 2. Compare: - Bead consistency (width, height, ripple pattern) - Leg size uniformity - Toe fusion quality - Overall appearance 3. Did the second joint improve? What specific change made the difference? 4. If time permits, instructor performs a destructive test (bend or break test) on one sample to show penetration

Discussion Points

• Why is the 45° torch angle critical for T-joint fillet welds?
• What happens if you weld too cold (not enough amperage) on a T-joint?
• How would you adjust your technique for a ¼" T-joint vs. the ⅛" you welded?
• Why might one leg be longer than the other?

Expected Outcomes

• Students produce T-joint fillet welds with ⅛" ± 1/16" leg size
• Students demonstrate fusion to both plates (no cold lap on either member)
• Students show improvement between their first and second attempt

Assessment Rubric

Criteria	Excellent (4)	Proficient (3)	Developing (2)	Beginning (1)
Fillet Size	⅛" legs ± 1/16", equal both sides	Within ±⅛", reasonably equal	Inconsistent sizing	Cannot produce measurable fillet
Fusion Quality	Complete fusion both plates, no cold lap	Fusion to both plates with minor issues	Fusion to one plate only in places	Lack of fusion on one or both plates
Bead Appearance	Consistent width, even ripple, smooth profile	Generally consistent with minor variations	Significant width variation	Poor appearance, inconsistent
Technique	Correct angles, stable arc, steady travel	Mostly correct with minor corrections	Multiple technique issues	Cannot maintain proper technique

Safety Considerations

• Full welding PPE mandatory — T-joint welding produces more spatter upward toward the welder
• The inside corner of the T-joint concentrates heat — the assembly gets hot quickly
• Use pliers to handle completed T-joints — they will be 300°+ F
• Welding vertical plate can cause drips of molten metal — ensure pant legs cover boot tops
• Fume extraction should be positioned close to the joint — T-joints create a confined area that traps fumes

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Last Updated: 2026-03-19