Why welding selection matters
HVAC duct fabrication uses a surprising variety of welding methods. The right choice depends on sheet thickness (0.5-3 mm typical), material (galvanised, stainless, aluminium), production volume, finished appearance requirement, and code compliance (AWS, EN, ASME). The wrong choice produces distortion, burn-through, contamination, or excessive labour cost. Workshop choices made at equipment purchase lock in the welding capability for the next decade.
Method comparison overview
- Resistance spot welding: best for discrete points on rectangular duct corners, fast (0.5-1 s per spot), low capex, no consumables. Limited to overlapping joints; not for butt or T-joints.
- Resistance seam welding: continuous overlapping resistance welds via rotating wheel electrodes. Best for cylindrical duct longitudinal seams and tank-style construction. Faster than TIG/MIG on long seams.
- Stitch welding: pneumatic-driven discrete welds at programmed intervals along a seam. Used for tack-welding before final continuous weld, or as the final weld where intermittent attachment is sufficient.
- TIG (GTAW): gas-tungsten arc welding with argon shielding. Best for cleanroom-grade smooth welds, stainless, thin sheet, and thin-gauge aluminium. Slowest method but highest quality.
- MIG (GMAW): gas-metal arc welding with consumable wire and shielding gas. Faster than TIG, suitable for structural duct and thicker gauges. Rougher bead.
- Handheld laser welding: fibre laser with manual hand torch. Fast (3-5x TIG), low heat input, smooth bead. Increasingly used for cleanroom and architectural HVAC duct.
- Plasma arc welding: occasionally used for thicker industrial duct or where higher penetration is needed than TIG.
Resistance spot welding
Two copper electrodes apply pressure to overlapping sheet metal layers and pass heavy current through the joint, melting and fusing the metal at the contact point. Cycle time 0.5-1 second per spot. Used for:
- Rectangular duct corner joints (where TDF flange meets duct body)
- Stiffener attachment to duct walls
- Internal bracing and reinforcement
- Spot-fixing acoustic baffles in attenuators
Equipment: pedestal spot welder (50-200 kVA), C-frame portable spot welder, robotic spot welder (for high-volume automotive-style production). SBKJ supplies SBYFL-50 (50 kVA) and SBYFLHJ-50 series for HVAC duct spot welding.
Resistance seam welding
Two rotating copper wheel electrodes apply continuous pressure and current to overlapping sheet metal moving through the wheels. Produces a continuous weld seam. Used for:
- Cylindrical duct longitudinal seams (alternative to lockseam)
- Tank-style HVAC plenum boxes
- Custom transition pieces with long seams
Speed 0.5-3 m/min depending on gauge. Slower than spot welding but produces airtight continuous joint. SBKJ supplies medium-frequency seam welding equipment as part of the duct welding range.
Stitch welding
Stitch welding is essentially programmed intermittent spot or arc welding — short welds (typically 25-50 mm) at programmed intervals (typically 100-300 mm). Used for:
- Tack-welding components before final continuous weld
- Joining duct sections where intermittent attachment is sufficient (most non-pressurised HVAC)
- High-speed assembly of repetitive parts
SBKJ supplies SB-ZF1500 stitch welder for HVAC duct fabrication. 1500 mm capacity covers most rectangular duct dimensions; multi-axis programmable for repetitive patterns.
TIG welding (Gas Tungsten Arc Welding)
An arc strikes between a non-consumable tungsten electrode and the workpiece, with argon shielding gas protecting the molten pool from atmospheric contamination. Filler metal added separately from a rod. Used for:
- Cleanroom-grade stainless duct (continuous weld, smooth bead, hygienic)
- Pharma sterile duct (per VDI 6022 or GMP requirements)
- Thin-gauge aluminium duct
- Welds that must be ground flush for architectural finish
- Fillet and butt welds on thin sheet (0.5-2 mm)
Speed: 0.15-0.4 m/min (slow). Quality: highest of any non-laser method. Cost: low capex (USD 5,000-15,000 for inverter unit), but high labour cost due to slow speed and skilled welder requirement.
MIG / MAG welding
MIG (Metal Inert Gas) uses inert shielding gas (argon, argon-helium); MAG (Metal Active Gas) uses CO2 or argon-CO2 mix. A consumable wire electrode is fed through the torch, providing both arc and filler metal. Used for:
- Structural HVAC duct above 1.5 mm gauge
- High-volume galvanised duct welding (where post-weld cleaning is acceptable)
- Plant room duct, exhaust manifolds, and industrial HVAC
Speed: 0.4-1 m/min (faster than TIG). Bead appearance rougher than TIG. Heat input higher — more distortion on thin sheet. Less suitable for cleanroom or architectural applications.
Handheld laser welding
Fibre laser (typically 1000W or 1500W) with manual hand torch and protective gas shielding. The laser beam is delivered via fibre cable to a hand-held welding head. Used for:
- Cleanroom and pharma sterile stainless duct
- Architectural exposed HVAC (smooth bead, minimal post-weld grinding)
- High-volume stainless duct production where TIG speed is the bottleneck
- Aluminium HVAC ducting
- Thin-gauge work (0.5-2 mm sweet spot)
Speed: 0.5-1.5 m/min for typical HVAC weld depths — 3-5x TIG. Heat input low — minimal distortion. Bead appearance smooth and narrow. Capex: USD 30,000-80,000 for 1000-1500W system. TCO favours laser over TIG on high-volume cleanroom or pharma production.
SBKJ supplies handheld laser welding systems (1000W, 1500W) as part of the cleanroom duct fabrication package, with welder training and integration support.
Welding standards and qualification
- AWS D9.1: American Welding Society Sheet Metal Welding code. Primary North American standard for thin-gauge HVAC duct welding.
- AWS D1.6: Stainless steel structural welding code.
- AWS D1.2: Aluminium structural welding code.
- EN ISO 9606-1: International welder qualification (steels).
- EN ISO 9606-2: Welder qualification (aluminium).
- EN ISO 9606-4: Welder qualification (nickel alloys, stainless).
- EN ISO 15614-1: Welding procedure qualification — steels.
- ASME Section IX: Required for pressure-bearing welds (steam, hot water, process exhaust).
- JIS Z 3801: Japanese welder qualification.
- AS 1796: Australian welder qualification.
For most HVAC duct projects, AWS D9.1 (or local equivalent) qualified welders are sufficient. Cleanroom and pharma sterile duct typically requires additional certification — VDI 6022 hygiene compliance for German projects, or specific manufacturer-mandated welder qualification for Samsung/SK hynix/TSMC semiconductor cleanrooms. Aramco vendor approval requires ASME Section IX qualified welders for any pressure-bearing components.
Weld Procedure Specifications (WPS)
For projects requiring qualified welds, the fabricator must produce a WPS document specifying: base material grades and thickness, filler material spec and diameter, shielding gas (composition and flow rate), preheat temperature (if any), interpass temperature, welding parameters (voltage, current, travel speed), heat input, and post-weld treatment.
The WPS is qualified via Procedure Qualification Records (PQR) — actual test welds produced according to the WPS that are then destructively tested (tensile, bend, macroscopic, and for some applications hardness and impact). Once qualified, the WPS authorises production welding by qualified welders. Project documentation packs include WPS, PQR, welder qualifications and weld inspection records.
Post-weld treatment
Stainless steel
- Mechanical cleaning: stainless wire brush (dedicated to stainless, never carbon steel) to remove weld discolouration and slag
- Pickling: nitric/hydrofluoric acid paste or immersion to remove heat-affected zone oxide layer (the dark "rainbow" colour next to the weld). Per ASTM A380.
- Passivation: nitric or citric acid bath to restore the chromium oxide passive layer that gives stainless its corrosion resistance. Per ASTM A380 / ASTM A967.
- Electropolishing (cleanroom highest grade): electrochemical surface smoothing. Reduces Ra to 0.2-0.4 μm.
Galvanised steel
- Touch-up galvanising: cold galvanising spray (zinc-rich primer) over the weld zone where the original galvanising has burnt off. Restores corrosion protection.
- Slag and spatter removal: chipping and grinding of weld bead high points if appearance matters.
Aluminium
- Mechanical cleaning: aluminium wire brush to remove oxide and discolouration
- Anodising: if appearance and corrosion resistance matter (architectural exposed)
SBKJ welding equipment range
- SB-ZF1500 stitch welder: 1500 mm capacity, programmable stitch patterns. Workhorse for HVAC duct fabrication. Product page.
- SBYFL-50 spot welder: 50 kVA pedestal spot welder.
- SBYFLHJ-50 seam welder: continuous seam welder for cylindrical duct.
- SBWKHFJ-45 medium-frequency welder: medium frequency for stainless and aluminium.
- Elbow welder (SBWT-1000, SBWT-1500): dedicated welding stations for round duct elbow segments.
- Spot welder (specialty): SBYFLHJ series for high-volume applications.
- Handheld laser welder (1000W, 1500W): Product page. Cleanroom-grade welding at 3-5x TIG speed.
Equipment selection for typical fabrication shops
- General commercial HVAC fab shop: SB-ZF1500 stitch welder + SBYFL-50 spot welder. Covers 90% of typical projects.
- Hospitality + commercial: above + medium-frequency welder for stainless trim and architectural sections.
- Cleanroom / pharma fab shop: above + handheld laser welder + electropolishing tank.
- Industrial / oil & gas / Aramco-approved: above + ASME Section IX certified TIG/MIG stations + WPS/PQR documentation system.
- High-volume export fab shop: above + automated robotic seam/spot welding cells for production runs.
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FAQ
Which welding method is best for HVAC duct?
Depends on application. Spot for discrete points. Seam for continuous longitudinal. Stitch for tack-weld. TIG for cleanroom/architectural. MIG for structural. Laser for high-volume cleanroom (3-5x TIG speed).
What welding standards apply?
AWS D9.1 (sheet metal, North America), AWS D1.6 (stainless), EN ISO 9606 (welder qualification), EN ISO 15614 (procedure qualification), ASME Section IX (pressure-bearing). Project specs may layer additional requirements.
Do welds need post-weld cleaning?
Yes — stainless requires pickling and passivation per ASTM A380; galvanised needs touch-up zinc paint at weld zone; aluminium requires oxide removal. Cleanroom adds electropolishing.
What welding equipment does SBKJ supply?
SB-ZF1500 stitch welder, SBYFL-50 spot welder, seam welders, medium-frequency welders, elbow welders, handheld laser welders (1000W/1500W). Full HVAC duct welding range.
Why is laser welding becoming popular?
3-5x TIG speed, low heat input (minimal distortion), smooth narrow bead, ideal for thin-gauge cleanroom stainless. Capex higher but TCO favours laser on high-volume cleanroom production.
