Activity 002: TIG Welding — Butt Joint on Mild Steel

Activity ID: U6M3-ACT-002 Duration: 45 minutes Objective: Students will TIG weld a butt joint on ⅛" mild steel, demonstrating proper technique for joint welding including tack welding, fit-up, and full-length welding with filler. Group Size: 2 students

Overview

The butt joint is the most fundamental weld joint in TIG. Students will prepare the joint, tack it, and weld the full length, focusing on consistent penetration, bead width, and appearance. This builds directly on the bead-running skills from Activity 001.

Materials & Equipment Needed

• TIG welder (DCEN, set for mild steel)
• 3/32" lanthanated tungsten (freshly ground)
• 3/32" ER70S-2 filler rods
• 4 mild steel coupons: ⅛" × 3" × 6" (for 2 butt joints per student pair)
• Welding magnets and heat sink clamps
• Wire brush, file, angle grinder with flap disc
• Combination square, gap gauge
• Full TIG welding PPE
• Fire extinguisher within 15 feet

Instructions & Procedure

Phase 1: Joint Preparation (8 minutes) 1. Grind both mating edges of two coupons to bright, clean metal 2. File edges flat and square — use combination square to verify 90° 3. Clean surfaces with acetone within 1" of the joint 4. Position the two pieces edge-to-edge with a 1/16" gap (use a 1/16" rod or feeler gauge as a spacer) 5. Ensure pieces are flat and level on the welding table 6. Tack weld at both ends and center (3 tacks, each ~¼" long) 7. Remove the gap spacer 8. Check alignment — pieces should be flat with no offset between surfaces

Phase 2: Student A — Full Butt Weld (15 minutes) 1. Set machine: DCEN, 90-110A max, 20 CFH argon, 3/32" tungsten, post-flow 10 seconds 2. Position comfortably — view the joint at an angle for best visibility of the pool and both edges 3. Start at one end of the joint: - Position tungsten 3/32" above the surface at the joint - Ramp up the foot pedal gradually - Watch for the base metal to melt — a pool should form spanning the gap and wetting both edges 4. Begin the dip-and-withdraw rhythm: - Add filler to fill the gap and build slight reinforcement - Watch the edges of the pool — they must wet out onto both coupons - If one side melts faster, slightly redirect the arc to the cooler side 5. Travel at 3-5 IPM, maintaining: - Consistent pool size (approximately ¼" wide) - Even filler addition - 15°-20° trailing torch angle 6. At the end: crater fill — reduce foot pedal gradually over 2-3 seconds while adding a final dip of filler 7. Hold for post-flow (10 seconds) 8. Allow cooling for 2 minutes

Phase 3: Inspection (5 minutes) 1. Wire brush the weld thoroughly 2. Visually inspect: - Bead width consistency - Even ripple pattern - Proper tie-in to both edges (no cold lap) - Appropriate reinforcement (⅟₁₆"–⅛" above the surface) - No porosity, undercut, or cracks 3. Flip the coupon and check the back side: - Slight discoloration along the joint line indicates good penetration - A visible line of fused metal on the back = full penetration (ideal) - No change on the back = incomplete penetration (needs more heat or slower travel) 4. Record observations on the evaluation worksheet

Phase 4: Student B — Full Butt Weld (15 minutes) Swap roles. Student B welds a fresh butt joint assembly.

Phase 5: Comparison (2 minutes) Compare both students' welds side-by-side. Discuss what each person did differently and what produced a better result.

Discussion Points

• Was it harder to maintain penetration at the start or end of the weld? Why? (Heat buildup)
• How did you know the pool was wetting into both pieces?
• What happened at the tack welds — did you have to adjust technique there?
• How would you improve your next butt joint weld?

Expected Outcomes

• Students produce a butt joint weld with visible penetration and proper fusion
• Bead width varies less than ⅛" over the length of the weld
• Students demonstrate crater fill technique

Assessment Rubric

Criteria	Excellent (4)	Proficient (3)	Developing (2)	Beginning (1)
Joint Preparation	Clean, square edges, consistent gap	Good preparation with minor issues	Adequate but inconsistent	Poor preparation
Bead Appearance	Even width, consistent ripple, proper height	Generally consistent with minor variation	Significant variation	Poor appearance
Penetration	Full or near-full penetration (visible on back)	Partial penetration most of length	Inconsistent penetration	No visible penetration
Technique	Proper angles, rhythm, and crater fill	Mostly correct with minor issues	Multiple technique issues	Cannot complete the weld

Safety Considerations

• All standard TIG welding PPE required
• The gap in a butt joint allows UV light through to the underside — ensure no skin is exposed below
• Filler rods extend significantly — keep the non-working end away from eyes and others
• Hot coupons: use pliers, mark "HOT"
• Tungsten grinding between welds (if contaminated): safety glasses, dedicated wheel, dust mask

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Last Updated: 2026-03-19