All Basics Welding Processes: A Shop-Floor Guide

The joint type is decided by part thickness, access, and load direction. A butt joint puts two members in the same plane and carries tension well. A T-joint loads the weld in shear and bending. A lap joint is forgiving on fit-up but adds thickness. A corner joint closes an edge, and an edge joint is used only for light sheet.

Groove shape matters as much as joint type. Square grooves work up to roughly 6 mm plate. Above that, a V-groove with a 60° included angle gives the arc room to reach the root. A U-groove uses less filler on thick sections. A J-groove is chosen when only one side can be prepared.

Fit-up tolerance drives cost more than the process does. A 2 mm root gap on a 10 mm butt joint may need three passes instead of two, and it raises distortion. If the drawing allows a 1 mm gap, hold it. Welders spend more time fixing gaps than running beads.

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  Butt jointFull penetration possible; best for tension and fatigue loads.
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  T-jointCommon on brackets and frames; check fillet leg size against load.
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  Lap jointTolerant of poor fit-up; adds stack height and weight.
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  Corner jointUsed on boxes and enclosures; watch for burn-through on thin walls.

MIG (GMAW) is the default for steel and aluminium frames because it deposits metal fast and is easy to automate. It struggles outdoors when wind blows the shielding gas away. TIG (GTAW) is slower but gives clean, precise beads on thin sheet, stainless, and titanium, where contamination causes porosity.

Stick (SMAW) still wins on dirty or rusty steel and in field work where no gas cylinder is available. Flux-cored (FCAW) sits between MIG and stick: high deposition, tolerant of wind, but it leaves slag that must be chipped. Resistance spot welding is the standard for overlapping sheet metal in automotive bodies, not for structural plate.

Laser and electron beam welding produce a narrow heat-affected zone and low distortion. They need tight fit-up, usually under 0.1 mm, and the equipment cost is high. For a one-off bracket, that is the wrong tool. For a sealed sensor housing in volume, it can replace a whole brazing step.

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  GMAW / MIGFast, versatile, good for 1–12 mm steel and aluminium.
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  GTAW / TIGPrecise, clean; best for thin sheet and reactive metals.
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  SMAW / stickField repairs, dirty steel, no shielding gas needed.
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  FCAWHigh deposition outdoors; slag removal adds labor.

Heat shrinks metal as it cools. That is the root of every distortion problem. Weld sequence matters: back-step welding, where each bead starts behind the previous one, spreads heat and limits pull. Balanced welding, alternating sides of a T-joint, keeps the angular change symmetrical.

Fixturing holds the part while it cools, but a rigid fixture can transfer residual stress into the weld and cause cracking later. Tack welds should be placed at the ends and midpoints, and they must be strong enough to survive the first pass. A tack that cracks is worse than no tack.

Preheat helps on thick sections and high-carbon steels by slowing the cooling rate. It does not remove the need for post-weld heat treatment when the drawing calls for it. For aluminium, preheat is rarely used; the thermal conductivity spreads heat fast enough on its own.

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  Back-step weldingStarts each bead behind the last to limit heat buildup.
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  Balanced sequenceWeld opposite sides in turn to keep angular distortion even.
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  PreheatSlows cooling on thick or crack-sensitive steel.
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  Post-weld heat treatmentRelieves residual stress when specified.

Match the filler to the base metal, not to the process. ER70S-6 is the workhorse for carbon steel MIG. For 304 stainless, use ER308L; for 316, use ER316L. Aluminium 6061 is usually welded with ER4043 or ER5356, and the choice changes crack sensitivity and color match after anodizing.

Porosity comes from gas trapped in the weld pool. On aluminium it is almost always inadequate cleaning or moisture in the shielding gas. On steel it points to a dirty joint or too long an arc. Undercut is a travel-speed problem. Lack of fusion usually means the current is too low or the joint gap is too tight.

A weld that looks good can still fail inspection. Dye penetrant finds surface-breaking defects on non-porous materials. Ultrasonic testing finds internal flaws in thick sections. For critical parts, radiography shows porosity and inclusions directly on film.

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  PorosityGas trapped in the pool; check cleaning and gas flow.
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  UndercutGroove melted into the base metal and not filled.
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  Lack of fusionSidewall or root not melted; raises current or opens gap.
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  CrackingOften hydrogen or restraint related; check preheat and filler.

Welding wins on large frames, enclosures, and structures where removing material from solid would waste 60 percent of the stock. It also wins when the geometry changes late, because a weldment can be cut and re-welded. A machined part cannot.

CNC machining wins on tolerance and surface finish. A welded assembly typically holds ±0.5 mm after welding unless it is machined afterward. At GreatLight we machine weldments after stress relief to bring critical faces back to ±0.005 mm and Ra 0.8–1.6 μm.

The practical rule: weld the structure, machine the interfaces. A robot arm base can be a welded steel frame with machined mounting pads. Trying to hold a bearing bore by welding alone invites ovality and vibration.

For prototypes, welding is often slower than machining from solid because of fixture time. If you need three brackets next week, a 5-axis machined version may arrive faster than a welded one. Volume changes that math.

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  WeldLarge frames, low stock removal, late geometry changes.
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  MachineTight tolerance, fine finish, small complex parts.
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  CombineWeld the structure, then machine the critical interfaces.