Introduction: Three Processes, One Question
Every sheet metal assembly — whether it is a scaffold bracket, an appliance housing, a motor cover, or a structural frame — depends on one thing: a weld that holds.
But “a weld” is not a single thing. It is the result of a specific process, chosen for a specific reason, executed within a specific set of parameters. Choose the wrong process, and you get distortion, burn-through, weak joints, or a part that fails in the field. Choose the right one, and you get a joint that is stronger than the base metal itself.
In sheet metal fabrication, three welding processes dominate: MIG, TIG, and spot welding. Each has its own physics, its own strengths, its own limitations, and its own ideal application.
This guide compares all three — their principles, advantages, disadvantages, costs, and best-use scenarios — so you can specify the right process for your next project, or simply understand what your fabricator is doing and why.
1. Why Welding Is Different in Sheet Metal
Welding thick plate and welding thin sheet metal are fundamentally different challenges. Thin sheet — typically 0.5 mm to 6 mm — behaves differently under heat:
- Heat distortion: Thin material warps and buckles easily because it has little mass to absorb heat
- Burn-through: Excessive heat punches a hole instead of forming a weld pool
- Faster heat conduction: Heat spreads quickly, affecting a wider area
- Tighter tolerances: A 0.5 mm misalignment is a minor issue on a 20 mm plate but a serious defect on 1.0 mm sheet
This is why process selection matters so much in sheet metal. A process that works perfectly on heavy plate may be unusable on thin gauge material — and vice versa.
The three processes in brief:
| Process | Full Name | Heat Source | Filler |
|---|---|---|---|
| MIG | Gas Metal Arc Welding (GMAW) | Electric arc | Consumable wire electrode |
| TIG | Gas Tungsten Arc Welding (GTAW) | Electric arc | Separate filler rod (optional) |
| Spot | Resistance Spot Welding (RSW) | Electrical resistance | None |
Now let us examine each in detail.
2. MIG Welding: The Workhorse
2.1 How MIG Welding Works
MIG (Metal Inert Gas) welding — technically known as Gas Metal Arc Welding (GMAW) — uses a continuously fed consumable wire electrode that also serves as the filler material. The wire is fed through a welding gun at a controlled speed, and an electric arc forms between the wire tip and the workpiece, melting both the wire and the base metal to create the weld pool.
A shielding gas — typically a mixture of argon and CO₂, or pure CO₂ — flows through the gun to protect the molten weld pool from atmospheric contamination (oxygen and nitrogen). Without shielding gas, the weld would become porous and weak.
2.2 Key Parameters
| Parameter | Typical Range |
|---|---|
| Wire diameter | 0.6 mm – 1.2 mm |
| Current | 60 – 250 A (sheet metal) |
| Voltage | 14 – 28 V |
| Shielding gas flow | 10 – 20 L/min |
| Travel speed | 30 – 60 cm/min |
| Material thickness | 0.8 mm – 25 mm+ |
Two transfer modes are common in sheet metal:
- Short-circuit transfer: Low current, low voltage. The wire touches the pool and short-circuits, creating a rapid cycle of arc-on/arc-off. Ideal for thin sheet (0.8–3 mm). Produces more spatter but lower heat input.
- Spray transfer: High current, high voltage. The wire tip forms fine droplets that spray across the arc. Produces a smooth, spatter-free weld with deep penetration — but requires higher currents, making it less suitable for very thin sheet.
2.3 Advantages of MIG Welding
Fast deposition rate. MIG deposits 2–4 kg of filler per hour — substantially faster than TIG. For long, continuous welds, MIG is the productivity leader.
Easy to learn. An operator can become reasonably proficient in MIG welding within days to a few weeks. This makes it the most accessible of the three processes.
Works well on thicker sheet. MIG handles 3–6 mm sheet comfortably and can go much thicker with proper joint preparation.
Excellent for automation. Because the wire feeds continuously and the process is relatively forgiving of gap variation, MIG is the easiest process to robotize. Robotic MIG welding cells are standard in automotive and appliance manufacturing.
Lower cost per meter of weld. Consumables are inexpensive, and the high deposition rate means lower labor cost per unit length.
Good gap-filling ability. MIG can bridge small gaps (up to about 1 mm on thin sheet) that would cause problems for TIG or spot welding.
2.4 Disadvantages of MIG Welding
Spatter. Short-circuit MIG produces spatter — small droplets of molten metal that stick to the workpiece. This requires post-weld cleaning, adding labor and cost.
Less precise. MIG produces a wider heat-affected zone (HAZ) and a less controlled weld bead than TIG. It is harder to produce a cosmetically perfect weld.
Burn-through risk on very thin sheet. Below about 0.8 mm, MIG becomes difficult to control without specialized equipment (pulsed MIG or very low current settings).
Fume generation. MIG produces more welding fume than TIG, requiring proper ventilation.
Not suitable for all materials. MIG works well on carbon steel, stainless steel, and aluminum (with a spool gun or push-pull system), but is less suited to titanium, magnesium, or very thin exotic alloys.
2.5 Best Applications for MIG Welding
- Structural frames and brackets
- Scaffold components and supports
- Appliance housings and chassis
- Automotive body panels and underbody parts
- General fabrication where speed matters more than cosmetics
- Medium to thick sheet (1.5 mm and above)
- High-volume production with robotic automation
3. TIG Welding: The Precision Instrument
3.1 How TIG Welding Works
TIG (Tungsten Inert Gas) welding — technically Gas Tungsten Arc Welding (GTAW) — uses a non-consumable tungsten electrode to create the arc. The electrode does not melt into the weld. Instead, the operator manually feeds a separate filler rod into the weld pool when filler material is needed.
The shielding gas is typically pure argon (sometimes argon-helium mixtures), providing superior protection and a cleaner weld than MIG.
Because the tungsten electrode does not melt, the arc remains stable and the operator has independent control over three variables: arc current, arc travel speed, and filler feed rate. This independent control is what gives TIG its precision.
3.2 Key Parameters
| Parameter | Typical Range |
|---|---|
| Tungsten electrode diameter | 1.0 mm – 3.2 mm |
| Current (DC, steel) | 40 – 200 A |
| Current (AC, aluminum) | 60 – 250 A |
| Shielding gas flow | 8 – 15 L/min |
| Travel speed | 10 – 25 cm/min |
| Filler rod diameter | 1.6 mm – 3.2 mm |
| Material thickness | 0.5 mm – 6 mm (sheet) |
AC vs. DC TIG:
- DCEN (Direct Current Electrode Negative): Used for steel, stainless steel, titanium. Concentrates heat in the workpiece, produces deep penetration.
- AC (Alternating Current): Used for aluminum and magnesium. The alternating cycle breaks up the refractory oxide layer that forms on aluminum, allowing the weld to fuse properly.
3.3 Advantages of TIG Welding
Superior weld quality and appearance. TIG produces the cleanest, most aesthetically pleasing welds of the three processes. Properly executed TIG welds have a smooth, uniform bead with no spatter.
Exceptional precision. Because the heat input and filler addition are controlled independently, TIG can weld extremely thin material — down to 0.5 mm — with excellent control.
Minimal heat-affected zone. TIG concentrates heat more precisely, reducing distortion and preserving the base metal’s properties.
No spatter. TIG produces no spatter, eliminating post-weld cleaning. This is critical for applications where cleanliness matters.
Excellent for difficult materials. TIG is the preferred process for titanium, magnesium, and other reactive metals that are difficult or impossible to weld with MIG.
Superior for stainless steel. TIG produces cleaner, more corrosion-resistant stainless steel welds because it produces less oxidation and preserves the passive chromium oxide layer.
Highest weld quality and ductility. TIG welds typically exhibit superior toughness and ductility compared to MIG welds.
3.4 Disadvantages of TIG Welding
Slow. TIG travel speeds are typically 10–25 cm/min — roughly half to one-third the speed of MIG. Deposition rates are 0.5–1 kg/hr, compared to 2–4 kg/hr for MIG.
Requires high skill. TIG welding requires coordinated control of both hands (torch and filler rod) plus foot pedal control of current. Becoming proficient takes months, and mastering it takes years.
Higher labor cost. Because of the skill requirement and slower speed, labor cost per meter of weld is significantly higher.
Higher equipment cost. A quality AC/DC TIG machine costs more than an equivalent MIG machine.
Less gap tolerance. TIG does not bridge gaps well. Joints must fit tightly — typically within 0.25 mm for thin sheet.
Not easily automated. While robotic TIG exists, it is far less common than robotic MIG because of the complexity of filler feeding and the sensitivity of the process.
3.5 Best Applications for TIG Welding
- Aerospace components and structures
- Medical and pharmaceutical equipment
- Food and beverage processing equipment (hygienic, cleanable welds)
- Stainless steel sanitary tubing and vessels
- Thin-gauge precision assemblies (0.5–2 mm)
- Titanium and exotic alloy fabrication
- Visible welds where appearance is critical
- Repair and touch-up welding
- Prototype and low-volume precision work
4. Spot Welding: The Speed Champion
4.1 How Spot Welding Works
Spot welding — formally Resistance Spot Welding (RSW) — works on a completely different principle. There is no arc, no shielding gas, and no filler material.
Instead, two copper-alloy electrodes clamp two overlapping sheets of metal together and pass a high electrical current through them. The electrical resistance at the interface between the two sheets generates intense heat, melting the metal locally and forming a nugget — a small, lens-shaped weld that joins the sheets.
The heat is generated according to Joule’s law: Q = I²Rt
- I = current (typically 1,000–50,000 A)
- R = resistance (highest at the sheet-to-sheet interface)
- t = time (typically 0.05–0.5 seconds)
Because resistance is highest at the interface between the two sheets, that is where the heat concentrates — producing a weld exactly where it is needed, without melting the outer surfaces.
4.2 Key Parameters
| Parameter | Typical Range |
|---|---|
| Weld current | 1,000 – 50,000 A |
| Electrode force | 1 – 6 kN |
| Weld time | 0.05 – 0.5 s |
| Electrode diameter | 4 – 8 mm |
| Electrode material | Copper-chromium-zirconium alloys |
| Sheet thickness (each) | 0.5 mm – 3 mm |
| Total stack thickness | Up to ~6 mm |
4.3 Advantages of Spot Welding
Extremely fast. A single spot weld takes 0.1–0.5 seconds. A pedestal spot welder can produce hundreds of welds per hour; a robotic spot welding cell can produce thousands.
No filler material required. Consumables are limited to electrode tips, which are relatively inexpensive.
No shielding gas. Eliminates gas cost and the need for gas handling.
Minimal heat distortion. Heat is generated only at the weld interface, so the surrounding material stays cool. Distortion is minimal compared to arc welding.
No spatter, minimal post-processing. Spot welds leave a small indentation on the surface but no spatter. Minimal cleanup is required.
Excellent for automation. Spot welding is the most automation-friendly of the three processes. Automotive body assembly — with 3,000–5,000 spot welds per vehicle — relies almost entirely on robotic spot welding.
Low skill requirement. Once parameters are set, operation is largely push-button. This reduces labor cost and variability.
No surface damage on the outer face. With proper electrode selection and force control, spot welding leaves a clean or minimally marked outer surface — important for visible parts.
4.4 Disadvantages of Spot Welding
Requires overlapping sheets. Spot welding can only join parts in a lap configuration. It cannot produce butt joints or T-joints without flanges.
Access required from both sides. Both electrodes must contact the workpiece, so the joint must be accessible from both sides. (One-sided spot welding exists but is limited.)
Limited to conductive materials. Spot welding works on steel, stainless steel, and aluminum — but not on non-conductive materials or materials with very high conductivity (like copper).
Weld quality is harder to inspect non-destructively. Unlike arc welds, spot welds are internal. Inspection typically requires destructive testing (tear-down) or ultrasonic/other NDT methods.
Not suitable for thick stacks. Above about 6 mm total stack thickness, spot welding becomes impractical due to current requirements and electrode wear.
Electrode wear. Electrodes degrade over time, requiring dressing or replacement — typically every few hundred to a few thousand welds.
Less structurally efficient than continuous welds. Spot welds are discrete points of connection. For applications requiring continuous sealing or maximum stiffness, a continuous weld is preferable.
No sealing capability. Spot welds do not create a leak-tight joint.
4.5 Best Applications for Spot Welding
- Automotive body assembly (doors, panels, structural members)
- Appliance cabinets and housings (refrigerators, washing machines, ovens)
- HVAC ducting and panels
- Metal furniture frames
- Electrical enclosures and panels
- Wire mesh and grating assembly
- Any lap-joint application where speed and automation matter
5. Head-to-Head Comparison
| Criteria | MIG | TIG | Spot |
|---|---|---|---|
| Full name | GMAW | GTAW | RSW |
| Heat source | Electric arc | Electric arc | Electrical resistance |
| Filler material | Consumable wire | Separate rod (optional) | None |
| Shielding gas | Required | Required | Not required |
| Speed | Fast (30–60 cm/min) | Slow (10–25 cm/min) | Very fast (0.1–0.5 s/weld) |
| Deposition rate | 2–4 kg/hr | 0.5–1 kg/hr | N/A |
| Skill required | Low–moderate | High | Low |
| Training time | Days–weeks | Months–years | Hours–days |
| Thin sheet capability | Good (≥0.8 mm) | Excellent (≥0.5 mm) | Good (≥0.5 mm) |
| Max thickness (sheet) | 25 mm+ | ~6 mm | ~3 mm per sheet |
| Spatter | Yes (short-circuit) | No | No |
| Weld appearance | Good | Excellent | N/A (internal) |
| Heat distortion | Moderate–high | Low–moderate | Very low |
| Gap tolerance | Good | Poor | Poor (requires contact) |
| Joint types | All | All | Lap only |
| Automation | Excellent | Limited | Excellent |
| Equipment cost | Low–moderate | Moderate–high | Moderate–high |
| Consumable cost | Low | Low–moderate | Low (electrodes) |
| Labor cost per meter | Low–moderate | High | Very low (per weld) |
| Sealing capability | Yes (continuous) | Yes (continuous) | No |
| Best materials | Steel, stainless, aluminum | Steel, stainless, aluminum, titanium | Steel, stainless, aluminum |
6. Cost Comparison: What You Actually Pay For
Cost is not just equipment price. It is the total cost of producing a finished, quality weld.
6.1 Equipment Investment
| Equipment | Entry Level | Industrial / Robotic |
|---|---|---|
| MIG welder | $800 – $3,000 | $25,000 – $80,000+ (robotic cell) |
| TIG welder (AC/DC) | $1,500 – $6,000 | $40,000 – $150,000+ (automated TIG) |
| Spot welder (pedestal) | $5,000 – $25,000 | $80,000 – $300,000+ (robotic cell) |
6.2 Operating Cost Drivers
MIG: Low consumable cost (wire + gas), moderate labor cost (fast process, low skill), plus post-weld cleaning for spatter.
TIG: Low consumable cost (filler rod + gas), high labor cost (slow process, high skill), minimal post-weld cleaning.
Spot: Very low consumable cost (electrode tips), very low labor cost per weld (fast, low skill), minimal post-weld processing — but higher equipment cost and electrode maintenance.
6.3 Cost per Meter of Weld (Approximate, Steel Sheet)
| Process | Labor | Consumables | Post-Processing | Total |
|---|---|---|---|---|
| MIG | Moderate | Low | Moderate (spatter removal) | Lowest |
| TIG | High | Low | Very low | Highest |
| Spot | Very low (per weld) | Very low | Very low | Lowest (per joint) |
Key insight: MIG is the most economical for continuous welds. Spot welding is the most economical for discrete lap joints in high volume. TIG is the most expensive per meter — but it is often the only process that can meet the quality requirement.
7. How to Choose: A Practical Decision Framework
Work through these questions in order. The answers will point you to the right process.
Question 1: What joint configuration do you have?
- Lap joint with access from both sides → Spot welding is a candidate
- Butt, T, corner, or edge joint → MIG or TIG
- Lap joint but only one side accessible → MIG or TIG
Question 2: How thick is the material?
- 0.5–1.0 mm → TIG (best control) or spot welding
- 1.0–3.0 mm → All three are viable
- 3.0–6.0 mm → MIG or TIG
- Above 6 mm → MIG (or submerged arc, beyond this guide’s scope)
Question 3: How critical is weld appearance?
- Cosmetic, visible weld → TIG
- Functional, painted or hidden → MIG or spot
- No visible weld required (internal) → Spot welding
Question 4: What is your production volume?
- Prototype / low volume → TIG or MIG (manual)
- Medium volume → MIG (manual or semi-automatic)
- High volume → MIG (robotic) or spot welding (robotic)
Question 5: What material are you welding?
- Carbon steel → All three
- Stainless steel → All three (TIG for best corrosion resistance)
- Aluminum → All three (TIG for thin, MIG for thick, spot for high-volume lap joints)
- Titanium / exotic alloys → TIG only
Question 6: Does the joint need to be leak-tight?
- Yes → MIG or TIG (continuous weld)
- No → Spot welding is acceptable
Question 7: What is your quality standard?
- Aerospace, medical, food-grade → TIG
- Automotive structural → MIG or spot
- General industrial → MIG or spot
Quick Decision Matrix
| Your Situation | Recommended Process |
|---|---|
| Thin stainless steel, visible weld, high quality | TIG |
| High-volume appliance cabinet, lap joints | Spot |
| Structural bracket, 3 mm steel, moderate volume | MIG |
| Titanium aerospace component | TIG |
| Automotive body panel, robotic line | Spot |
| Scaffold frame, 2–4 mm steel, long welds | MIG |
| Food processing equipment, hygienic | TIG |
| Prototype, one-off, mixed materials | TIG or MIG |
| Aluminum enclosure, 1.5 mm, visible | TIG |
| Metal furniture frame, 1.5 mm steel, volume | Spot or MIG |
8. Weld Quality: Inspection and Standards
Whichever process you choose, the weld must meet a standard. Here is what to know.
8.1 Common Standards
| Standard | Scope |
|---|---|
| AWS D9.1 | Sheet metal welding (general industry) |
| AWS D1.1 | Structural steel welding |
| AWS D1.3 | Sheet steel welding |
| AWS D17.1 | Aerospace fusion welding |
| ISO 15614 | Welding procedure qualification |
| ISO 3834 | Quality requirements for fusion welding |
| EN 15085 | Railway welding |
| ASME BPVC Section IX | Pressure vessel welding |
8.2 Inspection Methods
| Method | Detects | Destructive? |
|---|---|---|
| Visual inspection | Surface defects, bead geometry, undercut | No |
| Dye penetrant | Surface cracks, porosity | No |
| Magnetic particle | Surface and near-surface defects (ferromagnetic) | No |
| Ultrasonic testing | Internal defects, lack of fusion | No |
| X-ray / radiography | Internal porosity, inclusions, lack of penetration | No |
| Macro etch | Weld nugget size, penetration depth | Yes |
| Tensile / peel test | Joint strength | Yes |
| Hardness testing | HAZ hardness, brittleness | No (usually) |
8.3 Spot Weld Specific Inspection
Spot welds require different inspection methods:
- Chisel test: A chisel is driven between the sheets to pry them apart. The weld should hold or tear out a plug.
- Peel test: The sheets are peeled apart; the nugget size and failure mode are evaluated.
- Ultrasonic spot weld inspection: Non-destructive method that measures nugget diameter and detects voids.
- Macro section: A weld is cut, polished, and etched to measure nugget diameter and penetration.
Typical acceptance criteria: Nugget diameter should be at least 4√t, where t is the thickness of the thinner sheet. For 1.0 mm sheet, the minimum nugget diameter is 4 mm.
8.4 Weld Quality Defects and Causes
| Defect | Process | Cause |
|---|---|---|
| Porosity | MIG, TIG | Contamination, insufficient shielding gas, moisture |
| Undercut | MIG, TIG | Excessive current, wrong travel angle |
| Burn-through | MIG, TIG | Excessive heat, too-thin material, slow travel |
| Lack of penetration | MIG, TIG | Insufficient current, too-fast travel, poor fit-up |
| Spatter | MIG | Wrong parameters, short-circuit transfer |
| Cracking | All | Hydrogen embrittlement, restraint, wrong filler |
| Distortion | MIG, TIG | Excessive heat input |
| Expulsion | Spot | Excessive current, insufficient electrode force |
| Undersized nugget | Spot | Insufficient current, short weld time, worn electrodes |
| Sticking | Spot | Contaminated electrodes, insufficient force |
9. Automation and the Future of Sheet Metal Welding
9.1 Robotic MIG Welding
Robotic MIG is the most widely deployed automated welding technology in sheet metal fabrication. Modern robotic MIG cells use:
- Seam tracking: Laser or vision sensors follow the joint in real time
- Adaptive fill control: Automatically adjusts wire feed speed and current based on gap variation
- Offline programming: Robots are programmed in simulation, not stopped on the floor
- Weld parameter monitoring: Every weld is recorded for traceability
9.2 Robotic Spot Welding
Automotive body assembly remains the largest application of robotic spot welding. Modern systems use:
- Servo guns: Precise force control and programmable electrode position
- Adaptive current control: Adjusts current based on resistance changes
- Weld quality monitoring: Real-time measurement of current, voltage, force, and displacement
- Aluminum spot welding: New electrode and process developments are enabling spot welding of aluminum body panels
9.3 Automated TIG
Automated TIG exists but is far less common. Applications include:
- Orbital TIG: For tube and pipe welding, where the torch rotates around the joint
- Hot wire TIG: Filler wire is electrically heated before entering the pool, increasing deposition rate
- Robotic TIG for precision parts: Used in aerospace and medical device manufacturing
9.4 Emerging Trends
Laser welding. For very thin sheet and high-speed applications, laser welding is increasingly competitive with TIG and spot welding. It offers high speed, minimal distortion, and excellent precision — but high equipment cost.
Hybrid processes. Laser-MIG hybrid welding combines the deep penetration of laser with the gap-bridging ability of MIG.
AI-based weld quality prediction. Machine learning models analyze welding parameters and sensor data to predict weld quality in real time, reducing the need for destructive testing.
Digital twin for welding. Simulation tools predict distortion and residual stress before welding begins, allowing process parameters to be optimized in advance.
Friction stir welding for aluminum. A solid-state process that avoids the porosity and cracking issues of fusion welding in aluminum — increasingly used in EV battery trays and enclosures.
10. Real-World Application Examples
Example 1: Scaffold Frame (Q235B Steel, 2.5 mm)
Requirement: High-volume production, structural strength, weather-resistant (HDG finish).
Recommended process: MIG welding with robotic automation. The long, continuous welds favor MIG’s speed and deposition rate. Post-weld hot-dip galvanizing requires spatter removal, but the productivity gain outweighs the cleaning cost.
Why not TIG: Too slow for volume production.
Why not spot: The joints are not lap joints, and continuous sealing is not required but structural continuity is.
Example 2: Stainless Steel Appliance Housing (304, 1.0 mm)
Requirement: Visible surface, corrosion resistance, moderate volume.
Recommended process: TIG welding for visible seams and corners. TIG produces a clean, spatter-free weld that preserves corrosion resistance and requires minimal finishing.
Why not MIG: Spatter and HAZ oxidation would require pickling and passivation, adding cost and risking surface damage.
Why not spot: The visible corner joints cannot be spot welded.
Example 3: Washing Machine Cabinet (Galvanized Steel, 0.8 mm)
Requirement: High volume (millions per year), lap joints, painted finish.
Recommended process: Robotic spot welding. The cabinet is assembled from overlapping panels, and spot welding is the fastest, lowest-cost method. The small electrode marks are covered by paint or accepted as standard.
Why not MIG: Slower, more distortion on thin galvanized steel, spatter requires cleaning.
Why not TIG: Far too slow for this volume.
Example 4: Food Processing Conveyor Frame (304 Stainless, 1.5 mm)
Requirement: Hygienic, cleanable, no crevices, no contamination.
Recommended process: TIG welding with full penetration where possible. TIG produces smooth, crevice-free welds that can be cleaned and sanitized. Post-weld passivation restores corrosion resistance.
Why not MIG: Spatter creates crevices that harbor bacteria.
Why not spot: Lap joints create crevices; spot welds are not cleanable.
Example 5: EV Battery Tray (Aluminum 6061, 2.0 mm)
Requirement: Lightweight, leak-tight, structural, high volume.
Recommended process: MIG welding (pulsed or AC) with robotic automation, or laser welding for the highest precision. Aluminum requires AC TIG or specialized MIG with spool gun.
Why not spot: Aluminum spot welding is possible but challenging due to aluminum’s high conductivity and oxide layer. Laser or MIG is typically preferred.
Why not TIG: Too slow for volume production.
11. Practical Tips for Buyers and Engineers
If you are specifying welded sheet metal parts, these tips will help you get better results at lower cost.
Design Tips
1. Design for the process. If you specify TIG everywhere, expect higher cost and longer lead time. Use TIG only where its quality advantages are required.
2. Minimize weld length. Every millimeter of weld costs money. Use intermittent welds (stitch welds) where continuous welds are not structurally required.
3. Provide access. Both sides of a spot weld need electrode access. TIG and MIG need torch access. Poor access drives cost up and quality down.
4. Control fit-up. TIG and spot welding require tight fit-up. If your parts have large gaps, MIG is more forgiving — but the resulting weld is not stronger.
5. Specify the standard. Tell your fabricator which standard applies (AWS D9.1, ISO 15614, etc.). This ensures the weld procedure and inspection meet your requirements.
6. Consider distortion. If your part is large and thin, TIG or spot welding will produce less distortion than MIG. Design fixtures accordingly.
7. Avoid dissimilar thickness. Welding 0.8 mm to 3.0 mm is difficult for any process. If unavoidable, expect higher cost and more inspection.
Sourcing Tips
1. Ask what process they use — and why. A supplier who cannot explain their process choice may not be the right partner.
2. Ask about weld procedure specifications (WPS). A qualified WPS demonstrates that the process has been validated.
3. Ask about welder certification. For critical applications, welders should be certified to AWS D9.1 or equivalent.
4. Ask about inspection capability. Do they have ultrasonic, X-ray, or destructive testing capability? Or do they rely on visual inspection only?
5. Ask about post-weld processing. Pickling, passivation, stress relief, and spatter removal all add cost and lead time. Factor them in.
6. Request weld samples. Before placing a production order, request sample welds and inspect them — or have them tested.
7. Consider total cost, not just welding cost. A cheaper welding process may require more post-processing, more inspection, or result in higher field failure rates.
12. Final Recommendation
There is no “best” welding process for sheet metal. There is only the right process for your specific application.
Choose MIG when:
- You need speed and productivity
- Your material is 1.5 mm or thicker
- You have long, continuous welds
- You want to automate
- Minor spatter and cleanup are acceptable
Choose TIG when:
- Weld quality and appearance are critical
- Your material is thin (0.5–2 mm)
- You are welding stainless steel, aluminum, or titanium
- Cleanliness and corrosion resistance matter
- Your volume is low to moderate
Choose spot welding when:
- You have lap joints accessible from both sides
- Your volume is high
- Your material is steel, stainless, or aluminum
- You want the fastest cycle time and lowest labor cost
- Distortion must be minimal
The bottom line: The most expensive welding process is the one that produces a defective part. The cheapest is the one that meets the requirement with the least total cost — including labor, consumables, post-processing, inspection, and field reliability.
Match the process to the requirement. Then specify it clearly, inspect it properly, and hold your fabricator to the standard.
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